# at what point do motorcycle ITB's become restrictive?



## kenny_blankenship (Feb 19, 2005)

I have 42mm haybusa TB's, but I have heard various estimates of what you can wring out of motorcycle TB's, but no dyno sheets







. I am trying to figure this out so I know how much is too much on the P&P and how big of a valve I can go with over stock, or if stock flows more than enough. 
thanks in advance


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## X K R O M X (Jan 19, 2006)

*Re: at what point do motorcycle ITB's become restrictive? (kenny_blankenship)*

It's my understanding that they are to restrictive from the get go,granted I have no actually data to support my theory ,However if one takes the overall size of the original engine that the TB are fitted with you can see that this would be the case.


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## ValveCoverGasket (Mar 20, 2002)

*Re: at what point do motorcycle ITB's become restrictive? (X K R O M X)*


_Quote, originally posted by *X K R O M X* »_However if one takes the overall size of the original engine that the TB are fitted with you can see that this would be the case.

those engines rev a fair bit higher...
the air flowing through a 1000cc motor at 12000rpm, versus the air flowing through an 1800cc motor at 8000rpm...
not saying youre wrong, but its more than just the displacement


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## X K R O M X (Jan 19, 2006)

*Re: at what point do motorcycle ITB's become restrictive? (ValveCoverGasket)*


_Quote, originally posted by *ValveCoverGasket* »_
those engines rev a fair bit higher...
the air flowing through a 1000cc motor at 12000rpm, versus the air flowing through an 1800cc motor at 8000rpm...
not saying youre wrong, but its more than just the displacement









I guess my statement was to general,What you are saying is also part of the equation I was referring to,amongst many other variables.


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## VWralley (Mar 30, 2002)

*Re: at what point do motorcycle ITB's become restrictive? (ValveCoverGasket)*

my bodies have a 42mm plate...carbs also use this size on 16v's and they are not restrictive as far as i know...


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## X K R O M X (Jan 19, 2006)

*Re: at what point do motorcycle ITB's become restrictive? (VWralley)*

smaller diameter stacks create more velocity for the air to travel which is good for a high revving engine,our motors do not rev so well.


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## kenny_blankenship (Feb 19, 2005)

what does all of this mean in sheer numbers?


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## X K R O M X (Jan 19, 2006)

*Re: (kenny_blankenship)*

Hard to tell really you are the guys with the bike ITB's I could be wrong about this but I have yet to see any real info on anyone running them,Hey rally whats your doing for power?


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## VWralley (Mar 30, 2002)

*Re: (X K R O M X)*

my car is down again cause some kids decided to help out my weight reduction by smashing out my pass. door window an popping one of my tires http://****************.com/smile/emthdown.gif 
i chose to small of injectors (stock digi) so im getting larger ones and goin to re-tune an then take it for a baseline number. a friend with a stock rebuilt 9a w/ jenvey (45mm i believe) used my .msq and spent an hour on the dyno (only tweaked fuel) an made 131whp. im assuming that i am right around there. my motor is completly stock w/ well over 150k on it.
this may not mean much, but pretty much anyone that i have taken for a rip has said its one of the quickest 16v rabbit they have been in...


_Modified by VWralley at 10:07 AM 3-9-2007_


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## X K R O M X (Jan 19, 2006)

*Re: (VWralley)*

this may not mean much, but pretty much anyone that i have taken for a rip has said its one of the quickest 16v rabbit they have been in...

_Modified by VWralley at 10:07 AM 3-9-2007_[/QUOTE]
right until they go for a ride in mine.........


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## VWralley (Mar 30, 2002)

*Re: (X K R O M X)*


_Quote, originally posted by *X K R O M X* »_
right until they go for a ride in mine.........
















shut it.


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## X K R O M X (Jan 19, 2006)

*Re: (VWralley)*


_Quote, originally posted by *VWralley* »_
shut it. 









Sorry it was like a drunk and horny girl @ a party,plain old easy.


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## kenny_blankenship (Feb 19, 2005)

*Re: (X K R O M X)*


_Quote, originally posted by *X K R O M X* »_

right until they go for a ride in mine.........

















I thought you didn't have a passenger seat


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## X K R O M X (Jan 19, 2006)

*Re: (kenny_blankenship)*


_Quote, originally posted by *kenny_blankenship* »_

I thought you didn't have a passenger seat









Depends on the situation and mt mood.


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## LHP (Sep 10, 2003)

*Re: at what point do motorcycle ITB's become restrictive? (kenny_blankenship)*

Your restricting the 16v engine with 42m/m throttles.
A starting point for throttle sizing is the crossectional area of the intake port in the head at the intake manifold interface, 
which is "45m/m" for the 16v vw head.
The farther out you mount the throttles the larger you can go as you want to taper the intake tract/manifold out to open air
And running multi throat "carbs" are restrictions as well,
as that's how carbs work, via a venturi which pulls in fuel by the pressure drop across the venturi restriction.
But the end use of the engine will also be a consideration, 
low end power, mid range or top end power etc.
Most M/C throttles are to small for real power making, unless you get some larger V twin stuff, like suzuki TL1000, and honda RC51 etc.
LHP 
Hayward Performance


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## Wraith04 (Jun 24, 2004)

*Re: at what point do motorcycle ITB's become restrictive? (LHP)*

Just out of curiousity what is the effective size and flow of a factory TB? Very few people use an aftermarket TB other than ITB's.


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## rivethead (Nov 27, 2004)

*Re: at what point do motorcycle ITB's become restrictive? (LHP)*


_Quote, originally posted by *LHP* »_Your restricting the 16v engine with 42m/m throttles.
A starting point for throttle sizing is the crossectional area of the intake port in the head at the intake manifold interface, 
which is "45m/m" for the 16v vw head.


So depending on how well your head flows how large a TB could you go with a VW 16V? 48mm? 50mm? 55mm? Running the standard 5 degree taper?


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## VWralley (Mar 30, 2002)

*Re: at what point do motorcycle ITB's become restrictive? (LHP)*

so from a 45mm ("starting point") to a 42mm which i have now...do you have any idea how much hp we're talking? i wont have a chance to dyno my motor stock for comparison as its now coming out an i have a new head w. cams to go on. id like to get an idea of how much flow is being restricted so i can then figure out which method of intake suites my needs. thanks for the great info


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## LHP (Sep 10, 2003)

*Re: at what point do motorcycle ITB's become restrictive? (rivethead)*

The closer to the head the smaller, relative to intake port crossection.
The farther out from the head the larger you can go, 
But you have to be realistic as well.
example: K20 RSX intake system I make, 
To clear the power steering etc,
I had to add length to the manifold,
so as to move the throttles out board to clear the engine parts,
and to keep a tuned length and taper,
the popular throttle size for the system is 52m/m
with just a 40m/m tall airhorn.
Basically the throttles are at the end of the intake tract.
These RSX 52m/m intake systems have made 280 to 300 hp at wheels and good street drivability with the ivtec.
LHP
HaywardPerformance


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## rivethead (Nov 27, 2004)

*Re: at what point do motorcycle ITB's become restrictive? (LHP)*

Lance,
Sent you a PM.


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## ens (Dec 14, 2006)

*Re: at what point do motorcycle ITB's become restrictive? (LHP)*

usrallyteam.com
Throttle response is greatly improved over single TB designs by placing multiple throttle plates immediately before the intake valves. The air charge must only travel a few inches rather than a foot or more with a conventional manifold.
High-rpm power is greatly enhanced due by eliminating flow-restrictions and special timing of intake air pulses. Upwards of 200hp can be made in all out 16v or even 8v engines. Forced induction ultimately delivers more power for the dollar. However, ITBs are much simpler, lighter, and compact. And, then there's that wicked sound! 
Systems Feature: 
1. Equal-length intake sections.
2. Straight air paths to intake valves for superior breathing.
3. Intake passages are each smaller in volume, yet larger in total volume. Port velocity is increased for enhanced response and torque. Total flow is increased for maximum horsepower. 
4. Velocity stacks can be custom-tailored to match cams. Torque/power peaks can be tuned for a target rpm. A supercharging effect can also be produced within a narrow rpm range. 

Each cylinder fills more efficiently with air/fuel. Greater port velocity and improved breathing means a broader/flatter torque curve across the entire rev range -especially in the midrange. A properly-set up ITB'd engine can be very tractable in "normal" driving while delivering a screaming top end. This is a major improvement over regular manifolds. 

Acceptable idle quality may be retained even with wild cams since individual cylinder's air pulses do not affect adjacent cylinders. Those converting from CIS (Bosch mechanical fuel injection) get a double benefit in that the restrictive airflow meter plates is eliminated. 








/usrallyteam.com


_Modified by ens at 5:56 AM 3-15-2007_


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## WolfGTI (Jun 16, 1999)

*Re: at what point do motorcycle ITB's become restrictive? (LHP)*


_Quote, originally posted by *LHP* »_Your restricting the 16v engine with 42m/m throttles.
A starting point for throttle sizing is the crossectional area of the intake port in the head at the intake manifold interface, 
which is "45m/m" for the 16v vw head.
The farther out you mount the throttles the larger you can go as you want to taper the intake tract/manifold out to open air
And running multi throat "carbs" are restrictions as well,
as that's how carbs work, via a venturi which pulls in fuel by the pressure drop across the venturi restriction.
But the end use of the engine will also be a consideration, 
low end power, mid range or top end power etc.
Most M/C throttles are to small for real power making, unless you get some larger V twin stuff, like suzuki TL1000, and honda RC51 etc.
LHP 
Hayward Performance

Lance know what he's talking about - good guy to deal with - can't wait till my 48mm Jenveys get here from him - should be Monday.


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## silvervdubs99 (Oct 7, 2000)

*Re: at what point do motorcycle ITB's become restrictive? (ens)*


_Quote, originally posted by *ens* »_usrallyteam.com
Throttle response is greatly improved over single TB designs by placing multiple throttle plates immediately before the intake valves. The air charge must only travel a few inches rather than a foot or more with a conventional manifold.
High-rpm power is greatly enhanced due by eliminating flow-restrictions and special timing of intake air pulses. Upwards of 200hp can be made in all out 16v or even 8v engines. Forced induction ultimately delivers more power for the dollar. However, ITBs are much simpler, lighter, and compact. And, then there's that wicked sound! 
Systems Feature: 
1. Equal-length intake sections.
2. Straight air paths to intake valves for superior breathing.
3. Intake passages are each smaller in volume, yet larger in total volume. Port velocity is increased for enhanced response and torque. Total flow is increased for maximum horsepower. 
4. Velocity stacks can be custom-tailored to match cams. Torque/power peaks can be tuned for a target rpm. A supercharging effect can also be produced within a narrow rpm range. 

Each cylinder fills more efficiently with air/fuel. Greater port velocity and improved breathing means a broader/flatter torque curve across the entire rev range -especially in the midrange. A properly-set up ITB'd engine can be very tractable in "normal" driving while delivering a screaming top end. This is a major improvement over regular manifolds. 

Acceptable idle quality may be retained even with wild cams since individual cylinder's air pulses do not affect adjacent cylinders. Those converting from CIS (Bosch mechanical fuel injection) get a double benefit in that the restrictive airflow meter plates is eliminated. 








/usrallyteam.com

_Modified by ens at 5:56 AM 3-15-2007_

^^^^take this with a grain of salt, it is purley speculation absolutely no research or development went into this eloquently worded sales pitch







^^^
listen to lance, knows his isht real world instead of bench racing http://****************.com/smile/emthup.gif


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## ens (Dec 14, 2006)

*Re: at what point do motorcycle ITB's become restrictive? (silvervdubs99)*

simply adding on


_Modified by ens at 10:36 AM 3-15-2007_


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## [email protected] (Jan 13, 2005)

*Re: at what point do motorcycle ITB's become restrictive? (silvervdubs99)*


_Quote, originally posted by *silvervdubs99* »_
^^^^take this with a grain of salt, it is purley speculation absolutely no research or development went into this eloquently worded sales pitch







^^^

Ian, if you'd like to play critic, go for it. But, be specific about what you're talking about. What in that "eloquently worded sales pitch" do you take exception to? All those claims are of ITB systems in *general*. Do you have any hard data or other information that contradicts what we've claimed? If so, lay it on us. If you don't... then... how should we interpret your statement?


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## ValveCoverGasket (Mar 20, 2002)

*Re: at what point do motorcycle ITB's become restrictive? ([email protected])*


_Quote, originally posted by *[email protected]* »_
All those claims are of ITB systems in *general*. Do you have any hard data or other information that contradicts what we've claimed? 

i was just about to reply to his post in the same manner...
all of that seems pretty accurate of ITB setups in general http://****************.com/smile/emthup.gif


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## WolfGTI (Jun 16, 1999)

*Re: at what point do motorcycle ITB's become restrictive? (silvervdubs99)*


_Quote, originally posted by *silvervdubs99* »_
^^^^take this with a grain of salt, it is purley speculation absolutely no research or development went into this eloquently worded sales pitch







^^^
listen to lance, knows his isht real world instead of bench racing http://****************.com/smile/emthup.gif 

I think you are somewhat misguided in critiquing USRT's information. Granted they have no dyno charts or anything like that. But they are not lying or misinforming
anyone. Everything they say is true about ITB's, with the exception they do not qualify the filling of each cylinder. ITB's lose a little in the low end rpm due to less port velocity than a plenum setum, but how often do you really putt putt around 1000 - maybe 2000 rpm.


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## vw_stu (Mar 18, 2007)

i`m not so sure about 40-42 mm ITBs being that restrictive, you`ll still flow 200-220 through them on a 2 litre. Plus they`ll provide better torque figures for the same ouput than a set of 45s will.
As for 48s and above...well...theres no point really is there, unless you are going to run top-side of 10k rpm (unrealistic in a vw 16v), or try to make more than 250bhp (again unrealistic due to head design)


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## WolfGTI (Jun 16, 1999)

*Re: (vw_stu)*


_Quote, originally posted by *vw_stu* »_i`m not so sure about 40-42 mm ITBs being that restrictive, you`ll still flow 200-220 through them on a 2 litre. Plus they`ll provide better torque figures for the same ouput than a set of 45s will.
As for 48s and above...well...theres no point really is there, unless you are going to run top-side of 10k rpm (unrealistic in a vw 16v), or try to make more than 250bhp (again unrealistic due to head design)

My 48's are going on a 20v with some large cams, it flows quite a bit more air than a 16v.


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## Jetta2dr (Feb 19, 2001)

FWIW, I think VWMS used 48mm TB's on their mk3 16v kit car that made something like 260hp.


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## vw_stu (Mar 18, 2007)

you could make the same power using 45s and gain more torque than with 48s, bigger is not always better with regards to inlet size.
if you use throttle that are too big you "overthrottle" the engine, which is effectively when you accelerate at half throttle, then floor it and find that there isn`t that much difference in acceleration. this is because the engine is already sucking in as much air ( or pretty close to) as it can at any specified RPM.
theres some interesting info at the jenvey site in the advice and FAQs section
http://www.jenvey.co.uk/


_Modified by vw_stu at 10:45 AM 3-18-2007_


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## WolfGTI (Jun 16, 1999)

*Re: (vw_stu)*


_Quote, originally posted by *vw_stu* »_you could make the same power using 45s and gain more torque than with 48s, bigger is not always better with regards to inlet size.
if you use throttle that are too big you "overthrottle" the engine, which is effectively when you accelerate at half throttle, then floor it and find that there isn`t that much difference in acceleration. this is because the engine is already sucking in as much air ( or pretty close to) as it can at any specified RPM.
theres some interesting info at the jenvey site in the advice and FAQs section
http://www.jenvey.co.uk/
_Modified by vw_stu at 10:45 AM 3-18-2007_

I've done a fair bit of research - and I am presently running 45's on the 20v, I think the 48's will give me what I am looking for.


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## WolfGTI (Jun 16, 1999)

*Re: at what point do motorcycle ITB's become restrictive? (kenny_blankenship)*

FYI - I got my Jenvey 48mm's from Hayward Performance (vortex id - lhp). The setup is 1st class - everything works perfectly and fits perfectly. As far as the size being too large for my 20v - it's awesome, the pull in the high rpms has increased, I've checked the acceleration versus portions of the road I drive everyday and the car is definitely quicker.
Can't wait to put the header on and tune it finally.


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## LHP (Sep 10, 2003)

*Re: at what point do motorcycle ITB's become restrictive? (WolfGTI)*

Ahh, - Another happy customer.
Bigger is better, and too much is just enough.
LHP
haywardperformance


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## Fast929 (Nov 13, 2004)

*Re: at what point do motorcycle ITB's become restrictive? (LHP)*

The factory zuke TB's support 220-240whp on the gsxr motors in race trim. I'd speculate that we should see similar results on our motors from an airflow supply standpoint.
Larger TB's make throttle modulation more difficult. My guess is, 45-48mm is about perfect for an all out VW motor even spinning 10k. Much larger is just hampering intake velocity and low end torque.


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## WolfGTI (Jun 16, 1999)

*Re: at what point do motorcycle ITB's become restrictive? (Fast929)*


_Quote, originally posted by *Fast929* »_The factory zuke TB's support 220-240whp on the gsxr motors in race trim. I'd speculate that we should see similar results on our motors from an airflow supply standpoint.
Larger TB's make throttle modulation more difficult. My guess is, 45-48mm is about perfect for an all out VW motor even spinning 10k. Much larger is just hampering intake velocity and low end torque.

Granted, however remember these bike motors are less than 1200cc's in most cases, so even though the rate if flow is there due to the rpm's - the difference volume of air in a bike motor @ 17,000 rpms and a VW 2.0L motor @ 8000 rpm would be interesting to see.


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## Geoff Rood (Apr 30, 2001)

(cu in * RPM * VE)/3456 = CFM
3456 is a constant, it's really supposed to be / 2 * 1728. 1728 is the constant and /2 is there because it's a 4 stroke motor.
600cc (37 cu in) Honda motor:
(37 cu in * 15,000 RPM * 1.10 VE ) / 3456 = 177 CFM / 1.6 CFM/hp = 110hp
2.0L (122 cu in) VW 16v motor w/ cams:
(122 cu in * 7000 RPM * 1.00 VE) / 3456 = 247 CFM / 1.6 CFM/hp = 154hp
Misc. Info:
1588cc 8v water cooled motors used in the supervee series had 40mm ITB's and made 160hp @ 7800 RPM back in the day. Newer versions of these motors with EFI see anywhere from 185-200 flywheel hp.


_Modified by Geoff Rood at 4:31 PM 3-30-2007_


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## [email protected] (Jan 13, 2005)

*Re: (Geoff Rood)*

Geoff, how are you arriving at those VE estimates? The bike motor has 1.10 and you've got the VW at 1.00. What's the rationale behind that? Just curious.


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## ABA Scirocco (May 30, 2001)

*Re: ([email protected])*


_Quote, originally posted by *[email protected]* »_Geoff, how are you arriving at those VE estimates? The bike motor has 1.10 and you've got the VW at 1.00. What's the rationale behind that? Just curious.


And another thing, where are you getting 1.6 cfm/hp? If you work backwards from 1.6 cfm/hp, you get a brake specific fuel consumption of approximately 0.57 that's rather high, especially for a well turned race engine with typically run closer to 0.45 BSFC or better, that would give you about 1.26 cfm/hp.
For those not familiar with the term _Brake Specific Fuel Comsumption, BSFC_ Here's a brief explanation from Edelbrock

_Quote »_Brake Specific Fuel Consumption is the ratio of fuel consumed (in lbs. per hour) to horsepower produced. This ratio is a direct indicator of how efficiently the engine converts fuel into power. Most factory gasoline type engines run approximately a .50 to .55 Brake Specific Fuel consumption (BSFC) range while a highly efficient normally aspirated race engine operates at approximately a .40-.45 BSFC.


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## Geoff Rood (Apr 30, 2001)

I figured 100% for the VW because it's cylinder head and OE manifolds are only mildly tuned together. More benefits could be seen. 110% for the bike because they're usually factory tuned for just about everything you can get out of them, tuned intakes and headers. Current bikes use current technology, VW 16v's are late 70's and early 80's designs.
I got 1.5-1.6 CFM / hp from 'Maximum Boost' by Corky Bell, same place I got the formula's. I have used these formula's for motors that had dyno charts, and my results were always pretty spot on. And we're not WORKING with well tuned race engines. We're working with street cars using rather basic fuel injection systems such as megasquirt.


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## ABA Scirocco (May 30, 2001)

*Re: (Geoff Rood)*


_Quote, originally posted by *Geoff Rood* »_I got 1.5-1.6 CFM / hp from 'Maximum Boost' by Corky Bell,

That makes sense now, that's a reference for boosted engines and the bsfc is typically higher for boosted engines than it is for naturally aspirated engines and that translates directly to a higher cfm/hp, for a regular N/A street engine a better number would be somewhere in the 1.35-1.45 cfm/hp range


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