# The Weaknesses of the 2.5L Discussion Thread



## @[email protected] (Aug 29, 2003)

So there was some talk about this in the GHL exhaust thread. We went off topic without realizing it. So I decided to post this here for anyone who is interested.
Would like to here from the intake specialists about how poor this design is from a performance standpoint (even though this is an economy motor) and how we can improve such things as it.
We know what some bottlenecks are on this motor.
Intake System
Exhaust Manifolid
Cams
If someone can chime in a explain why these things are rather crappy that would be appreciated too. I'm trying to learn more about whats under that engine cover.








_Our horrendous excuse for an intake system_


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## Silver__DUB (Oct 3, 2006)

1st of we cant tell just from standing there that the intake system isnt well designed but it is. What I mean by that is that it keeps the engine bay very clean. there is no major tubing going around to an external box holding a filter. its all in one single part but evrything gets so hot that we loose a lot of performance. the intake manifold it self doesnt look very strong. I can't wait to see whats going to happen to this once someone starts pushing some serious PSIs.
On the exhaust side, the manifold is somewhat restrictive but this is what we need for that engine. i am afraid that we might loose power if we open up that area. remember that if you put a full catback 2.5inch, you will be loosing power. 
and the cams, i got no clue, maybe someone knows a little more on that subject.


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## mrshl13 (Jun 12, 2006)

*Re: (Silver__DUB)*

Restriction isn't needed. Scavenging is. Scavenging can be greatly improved with a proper separate-runner header and catback...even with 2.5" piping. Torque will be improved when scavenging is increased and thusly power will be improved when restriction is reduced.


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## huevosrancheros (Dec 13, 2006)

*Re: The Weaknesses of the 2.5L Discussion Thread (@[email protected])*

Tell you the truth, I am a new VW owner, so don't know too much about VW motors. All I know is that the torquey and can handle a nice healthy shot of nitrous(Cast Iron Bottom End







) I use to be a honda head.
There is one thing for sure that damn intake system set up is restrictive, but it does clean up the engne bay really nicely. I am running a K&N drop in right now(Increased throttle response). I am definitely waiting for AEM to finish development on their CAI. VW seem to gain HP well from swapping in a CAI, which makes me happy. NOt like hondas, where you don't get that much.
As far the exhaust is concerned, a better headder design will probably open up some more torque up on the top end, depending how the headder is designed.
As far as the cams go, of course they suck, they are econ cams.
Just wating for a decent exhaust system to be developed and a nice CAI and maybe a GIAC chip upgrade and boom Iam done.
Still contemplating a 50shot of nitrous, who knows.


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## Giancarlo (Aug 10, 2000)

*Re: The Weaknesses of the 2.5L Discussion Thread (huevosrancheros)*

Maybe we can get somebody here that is developing a turbo kit to tell us where they are finding the weaknesses, ABD tried a turbo on the jetta, for example.
Another thing that is interesting is where is the true red line and cams will be another thing everyone will want to know how good or bad they are.
This is a very interesting post so hopefully we will get the people with knowledge to chip in.


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## Silver__DUB (Oct 3, 2006)

the engine it self will hold high revs, but the valve train doesnt like it. mine is already making weird noises and i have to get it checked out before i swap in my hairblower. i am pretty sure that it needs avalve adjustment or something similar


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## ~kInG~ (Jun 8, 2005)

*Re: (Silver__DUB)*


_Quote, originally posted by *Silver__DUB* »_the engine it self will hold high revs, but the valve train doesnt like it. mine is already making weird noises and i have to get it checked out before i swap in my hairblower. i am pretty sure that it needs avalve adjustment or something similar

you're talking about the ticking noise??? many 2.5s have this (at least mine ticks and there are threads over @ the mk5 forum where others have the same issue)


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## vr_vento95 (Nov 26, 2004)

*Re: (~kInG~)*

i think they all tick, it's just how it is


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## Giancarlo (Aug 10, 2000)

*Re: (Silver__DUB)*


_Quote, originally posted by *Silver__DUB* »_the engine it self will hold high revs, but the valve train doesnt like it. mine is already making weird noises and i have to get it checked out before i swap in my hairblower. i am pretty sure that it needs avalve adjustment or something similar

Silver_Dub, what revs are you taking it too? Do you have any idea what part of the drive train is not liking it?
How is the hairblower project going?


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## @[email protected] (Aug 29, 2003)

*Re: (Giancarlo)*

The ticking, isn't that the fuel lines rattling?
I got that from day 1, I can hear it coming from the passenger side dash under the car it seems.
Very good contributions so far http://****************.com/smile/emthup.gif 
The intake is a big issue, there's way too much realestate to cover before hitting the intake mani!
As for the opening up the restricted exhaust post, I agree, anything past 2.75'' would seem to work against us. The stock set up is 2.25'' but I don't know if it's mandrel bent or crush bent???








I remember the best bolt on I ever put on a 16v was the cams. 18hp and 13tq for $300







The good old MKII days. I would lov to see how this motor reacts to a cam upgrade. Maybe even get the adjustable cam gear and advance it to max and put ALL the cam power on the bottom end making your power band flatter. This motor is OK down low but I find it pulls really well midrange and dies out at top end. If we get a cam and a CAI (which improves top end) and even a chip that can give us more fuel and air on the top end, our powerband will be quite fun and strong all the way around.
We need cams!


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## mk5vw (Dec 28, 2006)

*Re: (@[email protected])*


_Quote, originally posted by *@[email protected]* »_The ticking, isn't that the fuel lines rattling?
*you are either hearing the cat eating up the unburnt fuel, or you are hearing the injectors ticking one of the two...*
The intake is a big issue, there's way too much realestate to cover before hitting the intake mani!
*agreed and it should be the last on most lists unless you go ITB*
As for the opening up the restricted exhaust post, I agree, anything past 2.75'' would seem to work against us. The stock set up is 2.25'' but I don't know if it's mandrel bent or crush bent???








*all oem products are mandrel bent, just remember that NA cars need some back pressure to optimise HP, this isn't a turbo car or a really well breathing beast, so 2.25 is probably plenty, 2.5 max most likely will show a tiny gain in the top end...* 
I remember the best bolt on I ever put on a 16v was the cams. 18hp and 13tq for $300







The good old MKII days. I would lov to see how this motor reacts to a cam upgrade. Maybe even get the adjustable cam gear and advance it to max and put ALL the cam power on the bottom end making your power band flatter. This motor is OK down low but I find it pulls really well midrange and dies out at top end. If we get a cam and a CAI (which improves top end) and even a chip that can give us more fuel and air on the top end, our powerband will be quite fun and strong all the way around.
*this motor needs to have more HP up top, it's got plenty of tq down low by the design of the motor and displacement! I am not sure how controling the motronic system is, but an adjustable cam gear can help as well if you want some added top end. Of course cams would be nice also, but somebody has to test these, can't just slap anything on and does anybody make cams for this motor, I know it's been in europe for a few years already...*


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## @[email protected] (Aug 29, 2003)

*Re: (mk5vw)*

Sorry man but this motor has NOT been in europe ever. Its strictly our motor.
Nobody has cams for it, but i'm currently talking to Autotech and Schrick about this, i'll update if I get any interesting news back. http://****************.com/smile/emthup.gif


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## mk5vw (Dec 28, 2006)

*Re: (@[email protected])*


_Quote, originally posted by *@[email protected]* »_Sorry man but this motor has NOT been in europe ever. Its strictly our motor.
Nobody has cams for it, but i'm currently talking to Autotech and Schrick about this, i'll update if I get any interesting news back. http://****************.com/smile/emthup.gif 

I must be getting it confused the the VR5...then...
*150/170 hp 2.3 VR5*
configuration 
water cooled 2324 cm³ VR5 
head 
aluminium, DOHC, four valves per cylinder, 10.8:1 compression 
block 
cast iron, wet sumped, six bearings, 81 mm bore × 90.2 mm stroke, 0.9 ratio, 465 cm³ per cylinder 
aspiration 
variable intake manifold, tubular exhaust manifold 
fuel 
electronic injection 
output 
125 kW (170 hp) at 6200 rpm, 220 N·m (162 ft·lbf) at 3300 rpm from 2001 
110 kW (150 hp) at 6000 rpm, 209 N·m (154 ft·lbf) at 3200 rpm from 1997 

*150 hp 2.5 (US)*
engine code 
BGP 
configuration 
2480 cc (151 in³) inline 5 
block 
bore 82.5 × stroke 92.8 mm, cast iron 
head 
aluminum, [DOHC], 4 valves per cylinder (chain driven) actuated by low-friction roller cam followers, build camshafts, intake camshaft adjustment 
crankcase 
aluminium 
aspiration 
cast aluminum two-position variable intake manifold 
engine management 
Motronic, regular unleaded fuel 
output 
110 kW (150 hp) at 5000 rpm, 225 N·m (170 ft·lbf) at 3750 rpm


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## @[email protected] (Aug 29, 2003)

*Re: (mk5vw)*

Wow thats very close...the only thing that stands out is how much higher the VR5 rev's...a lot of specs are close. I can see why you got em mixed up.


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## VolksRacer2 (Aug 6, 2002)

*Re: (@[email protected])*

And note: The VR5 has more HP/less torque.
European engines are traditionally designed to run higher RPM (more HP) while the U.S. specs are for lower RPM (more torque) for a reason...Germany has the Autobahn (unlimited speeds in some areas) and people who generally aren't as interested in 0-60 as much as they are in top speed, while the U.S. is home to muscle cars (lots of torque for stop-light drags but pretty low speed limits). As a result, most European manufacturers tailor the power-band of their engines to the mass market for the area they're headed.
As for opening this thing up:
1> The actual _length_ of the intake isn't the biggest problem, it's all of the bends and turns it takes along the way. #1 mod. in my opinion is the CAI to let more air IN to the engine.
2> The actual piping of the exhaust system is fairly well designed, it's the mufflers that present the biggest restriction once you're past the manifold. Therefore A 2.5" Cat. back is my choice for #2 mod.
3> The exhaust manifold itself APPEARS to be very poorly designed (from a HP standpoint) as it has small primary runners merging into a single collector. This is good for torque (what this engine was designed for) but plays hell with high RPM breathing (where you'd find HP). My #3 mod is a header.
4> Cams determine the RPM range where the engine will perform the best. No matter what else you do, you're never going to see the full benefit of your other mods without some cams that are designed to run in the RPM range you're targeting. I've seen cams advertised to deliver "12hp" gains actually show improvements of 20+ HP when installed on an engine with other mods rather than a stock engine. #4 mod...cams
5> After you've addressed all of the above "issues" THEN it's time to get a properly tuned ECU upgrade. You've allowed plenty of air to get in to the engine, given it an easy path out and told it when you want it to be wherever...now you just need to re-map the fuel and timing to take full advantage if it!
I'm not saying 
_Quote, originally posted by *GOD* »_THIS IS THE ONLY WAY TO MOD YOUR CAR!
 but it's the best way to get the most out of your engine.


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## @[email protected] (Aug 29, 2003)

*Re: (VolksRacer2)*


_Quote, originally posted by *VolksRacer2* »_And note: The VR5 has more HP/less torque.
European engines are traditionally designed to run higher RPM (more HP) while the U.S. specs are for lower RPM (more torque) for a reason...Germany has the Autobahn (unlimited speeds in some areas) and people who generally aren't as interested in 0-60 as much as they are in top speed, while the U.S. is home to muscle cars (lots of torque for stop-light drags but pretty low speed limits). As a result, most European manufacturers tailor the power-band of their engines to the mass market for the area they're headed.
As for opening this thing up:
1> The actual _length_ of the intake isn't the biggest problem, it's all of the bends and turns it takes along the way. #1 mod. in my opinion is the CAI to let more air IN to the engine.
2> The actual piping of the exhaust system is fairly well designed, it's the mufflers that present the biggest restriction once you're past the manifold. Therefore A 2.5" Cat. back is my choice for #2 mod.
3> The exhaust manifold itself APPEARS to be very poorly designed (from a HP standpoint) as it has small primary runners merging into a single collector. This is good for torque (what this engine was designed for) but plays hell with high RPM breathing (where you'd find HP). My #3 mod is a header.
4> Cams determine the RPM range where the engine will perform the best. No matter what else you do, you're never going to see the full benefit of your other mods without some cams that are designed to run in the RPM range you're targeting. I've seen cams advertised to deliver "12hp" gains actually show improvements of 20+ HP when installed on an engine with other mods rather than a stock engine. #4 mod...cams
5> After you've addressed all of the above "issues" THEN it's time to get a properly tuned ECU upgrade. You've allowed plenty of air to get in to the engine, given it an easy path out and told it when you want it to be wherever...now you just need to re-map the fuel and timing to take full advantage if it!
I'm not saying but it's the best way to get the most out of your engine.

Excellent post, couldn't have said it better myself. http://****************.com/smile/emthup.gif


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## huevosrancheros (Dec 13, 2006)

*Re: (vr_vento95)*


_Quote, originally posted by *vr_vento95* »_i think they all tick, it's just how it is

maybe they need to adjust valve lash???


_Modified by huevosrancheros at 7:46 AM 1-10-2007_


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## mrshl13 (Jun 12, 2006)

*Re: (mk5vw)*


_Quote »_just remember that NA cars need some back pressure to optimise HP
Please don't quote the Midas Muffler tech. NA cars need scavenging as I posted earlier, not backpressure. Back pressure does not help performance. The scavenging effect helps to draw the low-speed gases out at a higher velocity which makes room quicker for new spent gases in the area previously occupied by said gases.
The reason a pipe too large in diameter actually hurts low-end performance is that at a certain point the volume of exhaust gases at low engine speed does not have enough initial velocity to fill the tube and force the rest out in the path of the pipe so it starts "bouncing around" in the pipe before exiting, effectively slowing the exit of said gases. You're likely to hear that too as evidenced by the endless supply of fart-canned Civics on the road.
A larger pipe will, however allow for the speedy exit of high-velocity gases at higher engine speeds. This hurts low-end performance but helps on the top end, if you can reach an engine speed that will force the gases out fast enough. VW has determined that ~2.5" is the ideal compromise between the two for this engine and the 2.0T, though the latter does not need so much help scavenging when the turbo is forcing the gases through.
Think of a garden hose. Put 20 PSI of water through it and it squirts out at a pretty good rate. Put the same amount of water and pressure through a dryer hose and it will be much more of a trickle. Then crank it up to 100 PSI. The garden hose can't put much more out than it did at 20 PSI but the dryer hose will allow for a much larger volume of water to exit the open end.
Race cars usually have larger pipes and large-/long-tube headers to optimize high-engine-speed power but street cars usually need to keep some low- and mid-range torque. Some header companies build stepped headers that start with smaller primaries and increase in diameter one or more times toward the collector in an attempt at "the best of both worlds" but rarely does it do much for the bottom end.
You will almost certainly sacrifice the good low-end torque this engine makes when increasing the horsepower. The only things I really see helping a bit on both sides are improving exhaust scavenging a bit and smoothing out the intake side. Based on another thread with pics of a removed OEM exhaust system, it appears to be smooth-bent. I'm guessing improvements will come from straight-through mufflers to replace the stock mufflers (assuming they're turbo-style but don't know for sure) and a header (which will not be emissions-legal).


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## travis3265 (Nov 15, 2003)

*Re: (mrshl13)*


_Quote, originally posted by *mrshl13* »_Please don't quote the Midas Muffler tech. NA cars need scavenging as I posted earlier, not backpressure. Back pressure does not help performance. The scavenging effect helps to draw the low-speed gases out at a higher velocity which makes room quicker for new spent gases in the area previously occupied by said gases.
The reason a pipe too large in diameter actually hurts low-end performance is that at a certain point the volume of exhaust gases at low engine speed does not have enough initial velocity to fill the tube and force the rest out in the path of the pipe so it starts "bouncing around" in the pipe before exiting, effectively slowing the exit of said gases. You're likely to hear that too as evidenced by the endless supply of fart-canned Civics on the road.
A larger pipe will, however allow for the speedy exit of high-velocity gases at higher engine speeds. This hurts low-end performance but helps on the top end, if you can reach an engine speed that will force the gases out fast enough. VW has determined that ~2.5" is the ideal compromise between the two for this engine and the 2.0T, though the latter does not need so much help scavenging when the turbo is forcing the gases through.
Think of a garden hose. Put 20 PSI of water through it and it squirts out at a pretty good rate. Put the same amount of water and pressure through a dryer hose and it will be much more of a trickle. Then crank it up to 100 PSI. The garden hose can't put much more out than it did at 20 PSI but the dryer hose will allow for a much larger volume of water to exit the open end.
Race cars usually have larger pipes and large-/long-tube headers to optimize high-engine-speed power but street cars usually need to keep some low- and mid-range torque. Some header companies build stepped headers that start with smaller primaries and increase in diameter one or more times toward the collector in an attempt at "the best of both worlds" but rarely does it do much for the bottom end.
You will almost certainly sacrifice the good low-end torque this engine makes when increasing the horsepower. The only things I really see helping a bit on both sides are improving exhaust scavenging a bit and smoothing out the intake side. Based on another thread with pics of a removed OEM exhaust system, it appears to be smooth-bent. I'm guessing improvements will come from straight-through mufflers to replace the stock mufflers (assuming they're turbo-style but don't know for sure) and a header (which will not be emissions-legal).

that all makes sense logically....but the point is.......? 
not sure what exactly you are getting at. what exactly should we be looking for in an exhaust system to best suit our cars?


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## @[email protected] (Aug 29, 2003)

*Re: (travis3265)*

Ok boys! Talked to Autotech today! We're getting a cam later this year!
"Mr. Michael, 
Thank you for your inquiry. Your email was forwarded to me by one of oursales staff. We have every intention to develop a sport camshaft profilefor the 2.5L 5-cyl motor, as well as for other engines in the VW family. Weare currently testing/developing a sport profile for 1.8T motors, and alsointend to develop and manufacture applications for the 24V VR6 & 2.0T motorsin the near future as well. We ask for your continued patience, as the 2.5L development will commence inthe future, I just can't confirm when. Proper testing & development cantake up to 6 months, and our production leadtimes are about 3 months so Iwouldn't expect a camshaft profile from Autotech to potentially be availableuntil late in the year. As with our other hydraulic lifter camshafts, thecamshaft profile will be made CARB (California Air Resources Board) legaland will not pose a problem for emissions testing."
Sincerely, 
Ralph B. Hollack
Director of Operations
Autotech Sport Tuning Corp.


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## ~kInG~ (Jun 8, 2005)

*Re: (@[email protected])*

Great News!!!!
but two things I dont like
1. We are last in the list of developments
2. 
_Quote, originally posted by *@[email protected]* »_"Mr. Michael, 
...We ask for your continued patience, as the 2.5L development will commence inthe future, I just can't confirm when....


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## lancGTI (Nov 5, 2006)

*Re: (~kInG~)*

Any chance someone could swap in Gallardo cams? Of course, donations may be mandatory!


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## Giancarlo (Aug 10, 2000)

*Re: (~kInG~)*

So the question is do we get a chip now and then see what a chip and cams can do to pay for another chip, or wait for the cams + chip together?
I really wished they started developing the cams for the 2.5 already. Are the guys with the 2.0T really going to be a bigger market for cams, they already get huge boost in power with just a chip, how many go beyond that? If the gains are big with a cam and chip for the 2.5 then maybe they would have a lot more clients for these than the 2.0T cams. Although as the base engine maybe most people that buy this car will never do anything except gas and oil and that is why we are pushed to the back of the develoment line.


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## vw_rabbit (Nov 9, 2006)

*Re: (Giancarlo)*

2 questions , why would engineers would make a high restrictive intake ?
and second
Since there's a MAF sensor... getting more air wont bug ECU ?


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## travis3265 (Nov 15, 2003)

*Re: (vw_rabbit)*


_Quote, originally posted by *vw_rabbit* »_why would engineers would make a high restrictive intake ?

emmissions, gas mileage, but mainly to rate the HP at 150 so that it didnt compete with the GTI. we all know that with CAI, the rabbit is making 170-180hp (also due to the factory underrating the power). so people wouldnt really feel justified buying the GTI for 10-18k more and only get 20-30 more hp.

_Quote, originally posted by *vw_rabbit* »_Since there's a MAF sensor... getting more air wont bug ECU ? 

its been taken care of by the intake manufacturers supposedly.


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## @[email protected] (Aug 29, 2003)

*Re: (travis3265)*

That makes me laugh though...lol
With just one $200 mod, a Rabbit which is 10k less then a GTi (in canada) will be 13hp shy and +1tq compared to the GTi.
I wonder if the companies who put out exhausts tested it with a CAI and saw more then 8hp? Something tells me that a good Intake system (not that crumby stock setup we have) will do wonders for other mods.


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## osteor10 (May 15, 2006)

*Re: (@[email protected])*


_Quote, originally posted by *@[email protected]* »_That makes me laugh though...lol
With just one $200 mod, a Rabbit which is 10k less then a GTi (in canada) will be 13hp shy and +1tq compared to the GTi.


this is not hating on you or anyone ( just a example) You do raise a great point that the 2.5 is an amazing and affordable engine. But just as we don't like people bagging on the 2.5 as a 'slow engine' ( which its not http://****************.com/smile/emthup.gif ) I believe we shouldn't do the same to them (not suggesting you are bagging on them, i know you are just posting facts). 2 different cars for 2 different reasons. Also I'm to lazy to check your math, but the 2.0t is under rated also.


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## @[email protected] (Aug 29, 2003)

*Re: (osteor10)*

Nobody is hating on anyone man, just makes me giggle when I think that VW (if true) detuned this motor to NOT put out as much power as it originally was plan so that it won't compete with GTi's power...and a $200 mod pulls you somewhat close.


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## Giancarlo (Aug 10, 2000)

*Re: (@[email protected])*

I don't know why VW underrated BOTH engines!!!!!! I mean at least it should have it rated as 200hp in the US and not 197 for marketing reasons.
But hopefully they not only underrated the 2.5 but also left a lot of power for the aftermarket guys to find.
PLEASE START DEVELOPING SOMETHING FOR US!!!!!!!!!!!!1


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## Spotdog (Jun 10, 2005)

*Re: (@[email protected])*

....not sold to Europeans, but a few of us have brought a 2.5 from the US. You are correct in that the European market wouldn't want this engine, after all the 2.0 FSI (non-turbo) engine develops the same power and torque and uses less fuel. The majority of the Golfs here have either the 1.6 FSI or the 1.9 TDI....Jettas are rare (unless you count the Skoda version). Acceleration is only important here if you drive a real sports car (Porsche and such). As for the 2.5 performance, it is much better designed for the Autobahn than the US highway - it is very well behaved at 110 mph and the fuel consumption at speed drops only 1 or 2 mpg. Acceleration from 80 and up is quite acceptable. I doubt that I would ever mess with the current setup....even the max speed at 130 is about the most the existing suspension and brakes could handle.


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## Erik04gti (Sep 28, 2004)

*Re: (@[email protected])*


_Quote, originally posted by *@[email protected]* »_Nobody is hating on anyone man, just makes me giggle when I think that VW (if true) detuned this motor to NOT put out as much power as it originally was plan so that it won't compete with GTi's power...and a $200 mod pulls you somewhat close.

even with a intake , a gti would still crush a rabbit or jettAr


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## VolksRacer2 (Aug 6, 2002)

*Re: (Erik04gti)*

VW underrates all of their engines. They dyno them using the minimum octane fuel that they deem "acceptable" (along with other factors to produce "worst-case" situations) so that they are sure that the engine will meet the advertised ratings when checked.


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## the s is silent (Jul 17, 2004)

*Re: (vw_rabbit)*


_Quote, originally posted by *vw_rabbit* »_2 questions , why would engineers would make a high restrictive intake ?
and second
Since there's a MAF sensor... getting more air wont bug ECU ?

They use the cover to heat the intake air to improve fuel vaporization. Shift vapor liquid equilibrium and use less fuel. It also means that you're decreasing density so you can't fit as much air in the cylinder...but I guess it's a trade-off they were comfortable with.
Anybody with a CAI...how has your gas mileage been affected?
As long as all the air making it into the engine goes through the MAF, then you're all good. The ECU can compensate.


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## kaptinkangaru (Aug 17, 2006)

feul economy hasn't been too badly affected, maybe 2-3 less mpg.


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## osteor10 (May 15, 2006)

*Re: (kaptinkangaru)*


_Quote, originally posted by *kaptinkangaru* »_feul economy hasn't been too badly affected, maybe 2-3 less mpg.

ur mods?


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## the s is silent (Jul 17, 2004)

*Re: (kaptinkangaru)*


_Quote, originally posted by *kaptinkangaru* »_feul economy hasn't been too badly affected, maybe 2-3 less mpg.









Not affected too badly? 10%? That's a pretty big hit to fuel economy.
This just goes to show that the restriction of the intake is not what is holding back the power...it's the heat. If it was just the restriction, then fuel efficiency should go up.


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## weitaro (May 12, 2005)

*Re: (the s is silent)*

shouldn't the mpg get better with CAI?


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## @[email protected] (Aug 29, 2003)

*Re: (Erik04gti)*


_Quote, originally posted by *Erik04gti* »_
even with a intake , a gti would still crush a rabbit or jettAr

Who ever said they'd lose to a Rabbit with a CAI. I said you'd be somewhat close to the advertised power ratings of a GTi. Sorry if you didn't understand me.
Everyone knows the GTi is the fastest car ever built.


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## the s is silent (Jul 17, 2004)

*Re: (weitaro)*


_Quote, originally posted by *weitaro* »_shouldn't the mpg get better with CAI?

If you're only removing restriction...yes.
But like I've been saying, if the air going into the engine is cooler, then you aren't going to be able to vaporize as much of the fuel. This is why a lot of cars in the 80's recycled some of the exhaust back into the intake (EGR valve). It was to use the heat to vaporize fuel. By doing this, you're able to lower emissions and increase fuel economy.


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## @[email protected] (Aug 29, 2003)

*Re: (weitaro)*


_Quote, originally posted by *weitaro* »_shouldn't the mpg get better with CAI?

His MPG is probably dropping because he's driving more spirited with the CAI on because he's excited. That would be my guess, and prob same thing i'd do too when I get one.


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## the s is silent (Jul 17, 2004)

*Re: (@[email protected])*


_Quote, originally posted by *@[email protected]* »_
His MPG is probably dropping because he's driving more spirited with the CAI on because he's excited. That would be my guess, and prob same thing i'd do too when I get one.

It could have an effect...but I doubt it's 10%. Changing the temp from 180oF (assuming the cover is that efficient of a heater to get it to -10 degrees of the operating temp), then bringing in 70oF ambient air will net you a 40% increase in density. That's huge. More air (mass, not volume...volume is the same due to the nature of positive displacement) = more fuel.


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## @[email protected] (Aug 29, 2003)

*Re: (the s is silent)*


_Quote, originally posted by *the s is silent* »_
It could have an effect...but I doubt it's 10%. Changing the temp from 180oF (assuming the cover is that efficient of a heater to get it to -10 degrees of the operating temp), then bringing in 70oF ambient air will net you a 40% increase in density. That's huge. More air (mass, not volume...volume is the same due to the nature of positive displacement) = more fuel.

Ok...Explain to my why K&N advertises you actually increase your MPG? I know it increased with me after getting one (but not for the first week because all I was doing is ripping it to hear the sweet noise) surely the K&N sucks in more air when it's an exposed cone filter then the standard airbox, so how does that make sense?
Give me his car with the standard airbox and i'll drive the same way I do if I just got a CAI 3 min ago, i'll bet the bank I can worsen his MPG by driving like a kid in the candy store by 10%.


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## the s is silent (Jul 17, 2004)

*Re: (@[email protected])*


_Quote, originally posted by *@[email protected]* »_
Ok...Explain to my why K&N advertises you actually increase your MPG? I know it increased with me after getting one (but not for the first week because all I was doing is ripping it to hear the sweet noise) surely the K&N sucks in more air when it's an exposed cone filter then the standard airbox, so how does that make sense?
Give me his car with the standard airbox and i'll drive the same way I do if I just got a CAI 3 min ago, i'll bet the bank I can worsen his MPG by driving like a kid in the candy store by 10%.









I see what you are saying. By removing some of the restriction that was there with the paper filter with a K&N drop-in, you're increasing the volumetric efficiency of an engine. The engine now doesn't have to work as hard to bring the air in. Think of the piston moving down, drawing air into the engine...any restriction is going to fight the downward movement of the intake stroke and reduce economy. You aren't sucking in more air, you're just removing the restriction to pull the same amount of air in.
With a cone filter on a CAI, you're removing more restriction, but you're also throwing a lot more air mass (same volume) into the cylinders by bringing in colder air. That's why you aren't seeing an even larger increase in fuel economy. If you put 180 degree air (or however hot stock is) into the CAI (effectively eliminating the C), I guarantee that the fuel economy would increase over stock (like you say it will).
Now, you're right...I was assuming everything else constant, including driving style originally. Him railing on his car could have something to do with it. BUT, you're still going to lose economy on this one. Such a huge increase in density, and the shift in the VLE from colder air all points to more fuel.


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## Erik04gti (Sep 28, 2004)

*Re: (@[email protected])*


_Quote, originally posted by *@[email protected]* »_
Who ever said they'd lose to a Rabbit with a CAI. I said you'd be somewhat close to the advertised power ratings of a GTi. Sorry if you didn't understand me.
Everyone knows the GTi is the fastest car ever built.

sorry i think i did read into it the wrong way


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## @[email protected] (Aug 29, 2003)

*Re: (Erik04gti)*


_Quote, originally posted by *Erik04gti* »_
sorry i think i did read into it the wrong way

That's alright man, half my posts are retarded anyways because I write em at 3am...


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## @[email protected] (Aug 29, 2003)

*Re: (the s is silent)*


_Quote, originally posted by *the s is silent* »_
I see what you are saying. By removing some of the restriction that was there with the paper filter with a K&N drop-in, you're increasing the volumetric efficiency of an engine. The engine now doesn't have to work as hard to bring the air in. Think of the piston moving down, drawing air into the engine...any restriction is going to fight the downward movement of the intake stroke and reduce economy. You aren't sucking in more air, you're just removing the restriction to pull the same amount of air in.
With a cone filter on a CAI, you're removing more restriction, but you're also throwing a lot more air mass (same volume) into the cylinders by bringing in colder air. That's why you aren't seeing an even larger increase in fuel economy. If you put 180 degree air (or however hot stock is) into the CAI (effectively eliminating the C), I guarantee that the fuel economy would increase over stock (like you say it will).
Now, you're right...I was assuming everything else constant, including driving style originally. Him railing on his car could have something to do with it. BUT, you're still going to lose economy on this one. Such a huge increase in density, and the shift in the VLE from colder air all points to more fuel.

Thanks for explaining that confusion. http://****************.com/smile/emthup.gif 
It makes a lot more sense now. I still think him driving the car more spirited due to the new CAI is atleast half the reson why his MPG is lower... Just think about how you'd react to a new go fast mod? I'd def be looking for the nearest icecream truck to race.


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## vw_rabbit (Nov 9, 2006)

*Re: (@[email protected])*

A feature of the 2.5L exhaust manifold is its
isolated air-flow design. The exhaust
manifold features a protective plate that
serves as a source for heated intake air.
The engine cover on the 2.5L engine
includes the intake air filter and part of the
system that delivers heated intake air to the
engine. The engine cover’s configuration
also lowers engine noise.


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## kaptinkangaru (Aug 17, 2006)

i'm guilty......lower MPG is definitely caused by my driving. if i baby it and stay fairly conservative, i can get the same mileage i used to get, but where's the fun in that?


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## @[email protected] (Aug 29, 2003)

*Re: (kaptinkangaru)*


_Quote, originally posted by *kaptinkangaru* »_i'm guilty......lower MPG is definitely caused by my driving. if i baby it and stay fairly conservative, i can get the same mileage i used to get, but where's the fun in that?
















We only live once, it's alright to drive more spirited once and a while. http://****************.com/smile/emthup.gif


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## Mauler_2.5 (Aug 23, 2006)

*Re: (@[email protected])*

Indeed. I didn't buy a fun car so I can just say that I have a fun car and not actually enjoy it! Geez, it was hard not going over the 3/4 the max speed during the wear-in period of the car...I just wanted to know what it could do!! The Rabbit definitely likes the highway...


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## Viss1 (Dec 1, 2003)

*Re: (the s is silent)*


_Quote, originally posted by *the s is silent* »_
But like I've been saying, if the air going into the engine is cooler, then you aren't going to be able to vaporize as much of the fuel. This is why a lot of cars in the 80's recycled some of the exhaust back into the intake (EGR valve). It was to use the heat to vaporize fuel. By doing this, you're able to lower emissions and increase fuel economy.

I thought the primary purpose of EGR was to reintroduce unburned hydrocarbons back into the combustion cycle to reduce emissions. The heated intake air was addressed by heat risers (pipes leading from the exhaust manifold to the air cleaner housing).


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## the s is silent (Jul 17, 2004)

*Re: (Viss1)*


_Quote, originally posted by *Viss1* »_
I thought the primary purpose of EGR was to reintroduce unburned hydrocarbons back into the combustion cycle to reduce emissions. The heated intake air was addressed by heat risers (pipes leading from the exhaust manifold to the air cleaner housing).

You're recycling some unburned HC's, but the stream is so small compared to the full exhaust stream, that it would have almost no effect on emissions. If the exhaust has 5% unburned hydrocarbons, and you're bringing 5% of it back to the combustion chamber, you're only getting a quarter of a percent of the total uncombusted fuel. (I could be wrong on both the amount of unburned fuel and the size of the stream the EGR recycles) It doesn't make sense to recycle it unless it shifts the VLE of the fuel to result in a better burn.
I'm no car expert by any means. This is just how I was able to reason through it. I've never seen a heat riser, so I'm not sure. The old 2.2 TBI dodges used a water jacketed intake manifold that circulated engine coolant to do the same thing.


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## VolksRacer2 (Aug 6, 2002)

*Re: (the s is silent)*

Viss1 is exactly correct. The EGR valve _is_ an emissions control device (although, as "S" notes, a fairly ineffective one). Its primary purpose is as Viss1 stated above and your car will get better mileage (and performance) without it. The problem is, it's "illegal" to remove emissions components from a car in most areas of the country and all cars after 1996 have OBDII systems that will go nuts if it's altered. Preheating the intake air is used as a means to promote combustion in a COLD engine. Once the engine is warm enough, most of these systems close automatically so that the engine isn't pulling in warmer (thinner) air and hurting performance.
The reason for the intake routing on the 2.5 is simple...that's what fit, looked good and met the noise/performance characteristics that VW was looking for. Remember, this engine was not intended to be the ultimate performer from the factory, it's stated mission is to be an "entry-level" engine with good in-traffic response. The biggest gains for this engine are to be had with breathing assistance (intake/exhaust) provided first. This is what will clear the way for the larger gains to be had from such things as cams and ECU tuning. 


_Modified by VolksRacer2 at 8:10 PM 2-1-2007_


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## the s is silent (Jul 17, 2004)

*Re: (VolksRacer2)*


_Quote, originally posted by *VolksRacer2* »_Actually, the EGR valve _is_ an emissions control device. Its primary purpose is as stated above and your car will get better mileage (and performance) without it. The problem is, it's "illegal" to remove emissions components from a car in most areas of the country and all cars after 1996 have OBDII systems that will go nuts if it's altered. Preheating the intake air is used as a means to promote combustion in a COLD engine. Once the engine is warm enough, most of these systems close automatically so that the engine isn't pulling in warmer (thinner) air and hurting performance.
The reason for the intake routing on the 2.5 is simple...that's what fit, looked good and met the noise/performance characteristics that VW was looking for. Remember, this engine was not intended to be the ultimate performer from the factory, it's stated mission is to be an "entry-level" engine with good in-traffic response. The biggest gains for this engine are to be had with breathing assistance (intake/exhaust) provided first. This is what will clear the way for the larger gains to be had from such things as cams and ECU tuning. 

I never said that it wasn't an emissions control device. I've been saying all along that it is.


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