# Why does the GX motor miss out on 15 horses?



## Flyingsheep (Apr 24, 2008)

My 85 coupe has the GX engine and I've always known it outputs 85hp, but recently I looked at a chart in the bentley which had the details for each engine code, and the GX engine seems to be the only one with 15 less hp than the rest of the 8v engines. I can't really figure out why. I have CIS-E fuel injection and the later engine models have 100hp and still run on CIS-E.
Here's the chart I'm talking about:








I know the PF engine has 5 additional horsepower because it has the dual exhaust mani/dual downpipe, and of course the PL engine is 16v. The only thing I can think of is the compression. I don't know a whole lot about compression, so please don't call me dumb, but does it make that big of a difference? If that's the reason, is there anything I can do to raise the compression in my motor? Any reason why the GX motor has lower compression?
Thanks for the help.


_Modified by Flyingsheep at 11:49 PM 9-30-2008_


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## B4S (Apr 16, 2003)

*FV-QR*

It's the compression, and cam profile. It was introduced as an economy engine, basically.


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## BillLeBob (Aug 10, 2004)

*Re: FV-QR (B4S)*


_Quote, originally posted by *B4S* »_It's the compression, and cam profile.

Bingo.


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## Flyingsheep (Apr 24, 2008)

If it's an economy engine how come I'm always getting 26mpg in town and 30mpg on the highway? Not that it's bad but what I usually hear on these boards is higher than that from newer golfs/jettas. Correct me if I'm wrong but the GX engine isn't as common.
If that's the case then I'd be all over raising the compression to get that extra power... but is that determined by the head? The bore and stroke sizes are the same, which I don't get.

_Modified by Flyingsheep at 11:20 AM 10-1-2008_


_Modified by Flyingsheep at 11:24 AM 10-1-2008_


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## B4S (Apr 16, 2003)

*FV-QR*

The fuel economy figures are measured at the speed limit, so unless you're glued to the limit at all times, it's not going to get better. The economy is also figured out mathematically, not actual real-world driving. The auto box doesn't help your figures much at all, even if it is the long ratio box they only used for the 85-86 model year.
It is a Rabbit GTI engine with a low-hp cam, and hydraulic head. By switching to the camshaft from a RV/PF/etc cam, you can get a bit more power...but ultimately, not too much can be done to bump up the compression unless you want to swap the internals out, or get the head shaved down by quite a bit. The later high comp engines are a bolt-in deal, and readily available, so find one of those and swap it in http://****************.com/smile/emthup.gif.


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## snowfox (Apr 4, 2002)

*Re: FV-QR (B4S)*

You are losing both power and efficiency due to the 8.5:1 CR. You may also be losing some hp to small intake valves. Do you know if your head has 38's or 40's? You may also be losing some mpg's because you have to flog the spit out of it if you want to get out of your own way...


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## The_Hamster (Jul 31, 2000)

*Re: FV-QR (snowfox)*

The GX has the small valves, single outlet, and low compression with a mild cam.
The MZ has 9:1 in other literature I have seen, but still has small valves and single outlet.


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## 84_GLI_coupe (Apr 3, 2001)

*Re: FV-QR (The_Hamster)*

My bone stock MZ with 350,000km on the odo pulled ~87whp with the stock unmodified airbox, engine never been opened up and doesn't use any oil. With the upper portion of the airbox lifted up by hand, it pulled 90whp. It doesn't seem to have any balls above 4000rpm, despite the 90hp rating at 5500rpm.


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## The_Hamster (Jul 31, 2000)

*Re: FV-QR (84_GLI_coupe)*

What kind of dyno was that on? Sounds a little odd to be pulling that much whp on that engine. I have an MZ myself, great motor just not a powerful one.


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## EuroKid83 (Jan 26, 2005)

*Re: Why does the GX motor miss out on 15 horses? (Flyingsheep)*

Interesting, I figured the GX engines would have the bigger valves. I'm gona pull the head off mine (factory re-man GX) and measure the valves.


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## The_Hamster (Jul 31, 2000)

*Re: Why does the GX motor miss out on 15 horses? (EuroKid83)*


_Quote, originally posted by *EuroKid83* »_Interesting, I figured the GX engines would have the bigger valves. I'm gona pull the head off mine (factory re-man GX) and measure the valves.

Why would you figure they have the larger valves?
The first few years the larger valves were GTi/GLi only (mk2).
Then with the digifant they came standard.


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## EuroKid83 (Jan 26, 2005)

*Re: Why does the GX motor miss out on 15 horses? (The_Hamster)*

The JH code engines came with 40/33 valves in 1983. This why I assumed the GX would have the larger valves.


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## Flyingsheep (Apr 24, 2008)

*Re: FV-QR (84_GLI_coupe)*

Ah, thanks for clearing things up guys. Which head do you think would be the best to swap with? Please don't suggest 16v. Do digifant heads work with CIS-E? I might end up doing this someday because a 15hp gain would be nice. Then I would do a dual outlet swap and a cam upgrade. I know you might say I might as well do an ABA bottom end swap, and I originally had that in mine for a while, but just recently I'm kind of leaning more towards the old school 8vs. Don't ask why but I kind of feel like keeping it old school.









_Quote, originally posted by *84_GLI_coupe* »_My bone stock MZ with 350,000km on the odo pulled ~87whp with the stock unmodified airbox, engine never been opened up and doesn't use any oil. With the upper portion of the airbox lifted up by hand, it pulled 90whp. It doesn't seem to have any balls above 4000rpm, despite the 90hp rating at 5500rpm.

I'm not sure what you mean by "lifting" the upper portion of the airbox, but I do know that drilling holes = BAD. I took a good look at my airbox recently and spotted drilled holes all over the place. Some previous owner must have had the brilliant idea that it would increase airflow thus increasing performance. It probably does increase airflow but it's pulling in hot air next to the engine. I duct taped the holes for now, and I know this sounds crazy but I have gained serious amount of torque. The powerband is WAY more consistent and the throttle response is substantially better at higher speeds like 50-60mph. I used to have to almost FLOOR IT to maintain my speed at 60mph when going up a rather easy hill on the freeway. Now the thing wants to keep accelerating past 65mph IN 5TH GEAR and I'm not giving it much gas at all. It's crazy but the only thing I can think of was bringing the airbox back to how it should be.


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## EuroKid83 (Jan 26, 2005)

*Re: FV-QR (Flyingsheep)*

The best head would be any of the big valve solid lifter heads.


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## Flyingsheep (Apr 24, 2008)

How can I find the information for that? Sorry to keep asking everything there is to ask but I'm really not sure.


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## EuroKid83 (Jan 26, 2005)

*Re: (Flyingsheep)*

Any head from a '83-'84 GTI/GLI and all Cabriolet up untill they switched to Digifant will have 40/33 valves and solid lifters. All JH engines had big valve solid lifter heads.


_Modified by EuroKid83 at 10:59 PM 10-1-2008_


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## Aurora_GL (Aug 10, 2004)

*Re: (EuroKid83)*

in the small 4 cyls .5 compression = nearly 5 hp at the crank


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## 84_GLI_coupe (Apr 3, 2001)

*Re: FV-QR (The_Hamster)*


_Quote, originally posted by *The_Hamster* »_What kind of dyno was that on? Sounds a little odd to be pulling that much whp on that engine. I have an MZ myself, great motor just not a powerful one.

























The red line is what I started with, after playing with fuel and timing on the street (89 octane). Blue line is after a bit more fiddling to get the A/F dialed in, timing advanced right for the fuel, and raising the upper half of the airflow meter (flow plate open to atmosphere, no filter or lower box). Lifting the airflow meter (kept level) brought me up from 86.4whp.
It's pretty darn peppy for what it is. The lack of a catalytic converter from the factory probably helps. It's had noisy lifters for the whole 2 years I've owned it, but I did put in a 3.94:1 R&P when I rebuilt the blown trans. Despite the engine not using a drop of oil, I'm sure there's carbon buildup that's boosting my compression ratio.


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## Campbell (Nov 9, 2004)

Switching to the big valve head (solid or hydr) will give you roughly 5 hp, changing pistons and picking up the 1.5 pts of compression will give you the other 10 hp. The 8.5 compression pistons are the bigger issue. good luck


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## allanmulwee (Jan 24, 2006)

*Re: (Campbell)*

I'm running a gx in my '85 that has had the head shaved and it has really good power.
i haven't run it on a dyno but once I got my fuel issues straightened out and advanced 
the timing it runs great!


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## dubbernutter (Feb 23, 2003)

*Re: (allanmulwee)*

I've got a GX motor on a former auto. Im actually surprised of what 85hp puts out now that it has a 5spd. I got the tranny from a 90 GL, cruises at 70mph running 3000rpm and gets 30mpg at that rate. drop the average hwy speed to 65mph/2800rpm and get a 10% increase in MPG to 33+. Drop the average hwy speed to 60 and get, well I dont know maybe 35/36+. I hit 105mph at 5200rpm, doesnt seem to wanna go much faster than that, and thats when the HP takes over.
Ive been wanting to increase the HP by way of shaved head .5mm and adding a GroupA headgasket. Not too sure exactly what the increase in Compression will be, probably 1point to 9.5:1. But the headswap will also entail a G60 head converted to CIS, dual outlet manifold and Cat. free exhaust. That seems to me to be the easiest way to squeeze more HP without rebuilding the bottom end. No cam planned at this time, but will probably get one eventually. I'm hoping for 105hp at the crank, or at least get that 85hp down to the wheels.
Head swap planned for this winter, dyno in the spring.
Also going to tape up the PO holes in the airbox and see what that does right away.




_Modified by dubbernutter at 9:29 AM 10-5-2008_


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## Flyingsheep (Apr 24, 2008)

So the GX bottom end has different pistons than the bigger valve motor bottom ends? If so, then geez might as well swap in an ABA it'd be less work than swapping the pistons to get 10:1 compression.
Also, I thought about it for a bit and figured it'd be easy to get to 110-115hp with the 1.8 8v motor if you have one with 10:1 compression. 100hp stock right? Well, put in a dual exhaust manifold and an autotech 270 camshaft and that's easily like 10-15 more horsepower. Then you'd be hauling butt...
Let me know how the duct taping of the airbox goes. I'm really curious.

_Modified by Flyingsheep at 2:14 AM 10-11-2008_

_Modified by Flyingsheep at 2:15 AM 10-11-2008_


_Modified by Flyingsheep at 2:17 AM 10-11-2008_


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## B4S (Apr 16, 2003)

*FV-QR*

The PF 8v GTI motor is a plain jane digi-2 motor with a dual downpipe, and it makes 105 hp IIRC. Toss in a nice cam, upgrade the exhaust, and you'd have a pretty fun car to drive.
Or put a digi-2 motor in a Mk1...helluva fun ride then







.


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## Flyingsheep (Apr 24, 2008)

See, that's what I'd like to do as a beginning project... but I don't want to get REALLY heavy and pull out the whole motor since it IS my one and only daily. What can be done with the GX motor without any modifications to the bottom end to wake it up?


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## B4S (Apr 16, 2003)

*FV-QR*

Everything you do will be a waste of money, really. 
Easiest option, just enjoy it for what it is, and try to find another car to rely on while you take the GX off the road.
Middle option, get a digi-2 head, shave it down as much as possible, re-install. Then begin to deal with a high comp motor without a knock sensing ignition, which will never perform to it's potential at all unless you use premium and cross your fingers.
Hardest option, full digi-2 swap.


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## Flyingsheep (Apr 24, 2008)

Hmm, digifant kind of turns me off because of the common complaints I always read about it. What would be really cool is if I threw out the fuel injection for dual carbs. But yeah the higher octane fuel would be a pain in the butt. What do you mean by crossing your fingers?


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## Campbell (Nov 9, 2004)

The only bolt on you can do is putting on a GTI big valve head, you would tell a difference. That would give you the same engine that is in the Scirocco; big valves and 8.5 compression. If you ever do a ring job switch the pistons to the 10:1 GTI pistons - if you then start to hear engine pinging as B4S mentions, up the octane level (buy higher octane gas) till it goes away. Have fun


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## B4S (Apr 16, 2003)

*Re: (Flyingsheep)*


_Quote, originally posted by *Flyingsheep* »_Hmm, digifant kind of turns me off because of the common complaints I always read about it. What would be really cool is if I threw out the fuel injection for dual carbs. But yeah the higher octane fuel would be a pain in the butt. What do you mean by crossing your fingers?

Well, without the knock sensing ignition, you don't have any protection against detonation. To get the maximum power out of the setup, you'd need to advance the distributor manually and keep an ear out for any bad noises. I'm not advocating knock sensors as a tuning tool, but it is a great safety device should something go awry.


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## EuroKid83 (Jan 26, 2005)

*Re: FV-QR (B4S)*


_Quote, originally posted by *B4S* »_Everything you do will be a waste of money, really.

That's not entirely true in my humble opinion. All you would need to do is swap a big valve head onto the GX short block (as mentioned previously) and you have the equivalent to the JH motor. If your installing a solid lifter head you could go one step further and add a 1.6 cam (obtainable from a salvage yard for less than $20) for ten degrees more duration and almost .050" more lift over the later solid lifter cam.


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