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Big Throttle bodies.. WHY?!?!?!?

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i as thinking about this on the way to work this morning. stupid question but bear with me. lets say you have a head that flows 200 cfm. easy math. put a turbo on at 14.3 psi. does the head now flow 400 cfm because of the above atmospheric pressure? and what is the actual flow of a 60 mm tb? i have my set up at 3" or 75mm because i did not want any restrictions from the turbo, throught the fmic, tb and into custom intake that matches the same diameter. just trying to understand better.

 

jimbo

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Ideal Gas Law should help your understanding of the relation of pressure and number of moles in the motor.

 

PV=nRT

 

P is the pressure (SI unit: pascal)

V is the volume (SI unit: cubic meter)

n is the number of moles of gas

R is the ideal gas constant (SI: 8.3145 J/(mol K))

T is the thermodynamic temperature (SI unit: kelvin).

 

If you double the pressure you will flow twice the moles of air(twice the amount of air). The problem is the intake temperatures aren't equal between NA and turbo motors.

 

This also doesn't take into account the difference in reversion into the combustion chamber due to backpressure from the turbo. There are a lot of factors that will effect the power the motor actually makes.

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Heres an example..

 

The FIA WRC rally cars have a 34mm restriction.. they produce a low number of 300whp.. but have you seen a Rally car launch and drive? 0-60 in less than 3 seconds. They are NOT limited to torque.. for example the 2001 Impreza has 471nm of torque at 4000rpm.

 

anyway.. im sure with an allowance of more air... they could create more power.. buutt hey a small point

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I understand the concerns that large bore TB's may be overkill for some applications however lets remember its Hybridz and we are all about being over the top. I guess my beef is bench racing only goes so far and it would be pretty cool to see if there is any difference in performance or driveablity if one were to switch from a 60mm to a 90mm. Its just funny how alot of theoretical ideas seem good on paper but just don't work the same in the real world. Matt K seems to be doing nicely on his SR with the 90mm so who knows, and thats only a 2liter. In a few weeks I'm going to have my RB25det running with a 90mm TB on it like a couple other members on this site, so I guess Ill see then if its a pain to drive ...

 

Cheers

Chris

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You must have missed the references with actual dyno figures in this thread pertaining to more than a few HUGE power producing engines running seemingly small throttle bodies. Small throttle bodies have been ran and posted in this thread with dyno results to back it up, not just bench racing or theory. On a Z car engine, whether it is N/A or running 30 PSI of boost, going from a 60mm TB to a 90 MM throttle body isn't going to free up any more appreciable HP, if any at all, nor should hurt power either, though as John C. brought up, an engineered throttle body size taking advantage of the air “velocity” under specific conditions and RPMS, which includes engineering the entire induction tract from the air filter to the intake valve, a smaller throttle body should produce a tad more torque. Personally, I view the 90mm throttle body on any L-series engine on the same level as a 4” exhaust tip on a 4 cylinder Honda. Why bother. HP is air flow, period! You can make so much HP with so much air flow, (efficiencies such as engine friction, pumping losses etc will skew those figures a tad, but in general, air flow is HP), If the throttle body is a restriction, then HP would be down. Now if those references such as a single 2” (50.8 mm) ID OE Datsun throttle body flowing enough air with only 12 PSI of boost to produce over 570 HP recorded HP on the dyno, (I guarantee you that NO turbo L-28 or Turbo RBxxDE on this forum is flowing that much air with only 12 lbs of boost no matter what throttle body they are running), or any of the other examples such as the rally example above, isn’t enough empirical proof, not just bench raced, sorry.

 

As for the SR20DET with the Q-45 TB? Sure, it isn’t hurting his performance, but rest assured that 90 mm throttle body isn’t giving that 2L engine ANY more power over what a 60mm throttle would deliver, N/A or Boosted! As for drivability, that is totally subjective and is dependant on many factors, such as the engines torque curve, whether or not the HUGE throttle body has “cammed” actuation and if so, the details of the camming such as, is it a linear opening rate, or a rising or falling rate, (as viewed on graph, curved)? Throttle response/drivability is a tunable parameter by camming the actuation, and each persons idea of the optimum throttle response/drivability is going to be different as well.

 

As an aside, there is one member on this forum, 280Zforce, that is running the Q-45 throttle body on his boosted L-28. How he executed his entire induction system, visually, it is NOT overly blingy or rice. His engine bay, and his entire car for that matter, is rather stunning.

 

That's my $.02, again...

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i'd have to agree.

 

while there are many other cars on this board that make someone wanna buy the first z car in sight, I feel that 280zforce is the one that did it for me :)

 

 

AND, i'd like to point out that Signal's R34 Skyline with 1060 hp has a 100mm throttle body. that's probably still not a restriction. plenum volume is something I would be more concerned about.

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This is a very interesting thread. After reading all the previous posts, how big of a TB would someone recommend for an L28 N/A engine with a mild cam, header with 2 1/4" exhaust, and MSA intake? What do you guys think about the bigger TB sold by MSA?

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I think it is all about smooth airflow. The factory manifold is basically a cylinder with one end open to bolt the throttle up to. If you go to a bigger plenum, you'd want a bigger throttle that follows the sizing of the plenum. This would in effect allow for, say, 2.5" piping up to a 3" throttle and into a 3.5" plenum (as a more extreme case). In that case, the throttle performs the improtant job of attempting to keep airflow smooth. If, however, you use a surge tank that has a tear drop shape with a taper out from from the throttle mounting plate (like the second version of the manifold in the linked thread), the surge tank is performing the taper for you and the throttle needs be no larger than the piping itself. In that case, the piping and throttle should all be appropriately sized for the engine.

Regarding the above poster who mentioned rally cars. Yes, the restrictor does a good job of keeping the power (flow) down but it doesn't stop them from making monster torque, of course. As has been shown time and again, of course, rules of thumb are merely that. Nothing is going to absolutely stop making more power than a part says it should given the right circumstances.

What I usually do for a turbo car is follow the turbo outlet for IC piping size and then make sure the throttle is a slight step larger (for 'dump' style intake manifolds) or close to the plenum size if it has the tear drop shop. I think keeping it smooth is JUST as important as pipe diameter.

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Ladies and Gentleman...

 

I, being of simple man, propose a challenge. Let us find a owner with the said "Oversized Throttle Body" to dyno his car. Then insert a small piece wood under his throttle pedal, which reduces the throttle opening size by ~10 percent. Then re-run the car. After posting the dyno graphs we will know.

 

Do I get a prize?

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" This engine was breathing enough air to produce this 1100+ HP and was doing so through only TWO 2” ID throttle valves, EXACTLY like the 2” ID OE L-28 EFI throttle valves with full diameter throttle shafts etc."

 

The Nissan VG30 in the #75 IMSA car run in the 90's (Z32) ran smallish throttle bodies, but the really amazing thig was they made 1100 HP, without restrictions. The IMSA rules required them to run 26mm restrictions on the turbos, and with those the engine made 'only' 750, but talking with Steve Millen, he said the Torque was really helped by this restriction, making the car much more driveable, and much more tractable coming out of corners.

 

Where you have the restriction is as important as the size some times.

 

Larger throttle area magnifies tip-in response, and on humongo bodies really requires non-linear response to make it driveable at all. After a certian point, it's just minimizing inlet restriction, but as has been noted velocity and manifold design will dictate how the dynamics work in any system.

 

The Barrel Throttle Bodies available to replace 'Weber DCOE' style throttle bodies show some very interesting design features. The 45mm Barrel Throttles will have the ultimate flow equivalent to 48mm Standard Butterfly setups (thanks to no throttle shaft blocking airflow at WOT), but have tip-in response similar to 40mm Butterflies up to around 15% throttle opening. They taper the opening of the barrels to give tip-in that allows drivability, yet without compromising ultimate airflow.

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" This engine was breathing enough air to produce this 1100+ HP and was doing so through only TWO 2†ID throttle valves, EXACTLY like the 2†ID OE L-28 EFI throttle valves with full diameter throttle shafts etc."

 

The Nissan VG30 in the #75 IMSA car run in the 90's (Z32) ran smallish throttle bodies, but the really amazing thig was they made 1100 HP, without restrictions. The IMSA rules required them to run 26mm restrictions on the turbos, and with those the engine made 'only' 750, but talking with Steve Millen, he said the Torque was really helped by this restriction, making the car much more driveable, and much more tractable coming out of corners.

 

Where you have the restriction is as important as the size some times.

 

Larger throttle area magnifies tip-in response, and on humongo bodies really requires non-linear response to make it driveable at all. After a certian point, it's just minimizing inlet restriction, but as has been noted velocity and manifold design will dictate how the dynamics work in any system.

 

The Barrel Throttle Bodies available to replace 'Weber DCOE' style throttle bodies show some very interesting design features. The 45mm Barrel Throttles will have the ultimate flow equivalent to 48mm Standard Butterfly setups (thanks to no throttle shaft blocking airflow at WOT), but have tip-in response similar to 40mm Butterflies up to around 15% throttle opening. They taper the opening of the barrels to give tip-in that allows drivability, yet without compromising ultimate airflow.

 

I remember you, specifically, mentioning this information about the IMSA car and the barrel throttles in another thread, and I did some research back then. I found this here:

 

http://www.autocar-electrical.com/roller.pdf

 

Looks like a really good system. Almost like a coates roller-barrel cam.

 

But I have a question regarding the actual airflow dynamics. With throttle plates having their shaft in the way, and with barrels having a top side that would create a high-pressure zone, or slow down air-flow (looks like a slight restriction, considering the size of the air horns), why hasn't anyone come up with an aperture blade or iris type throttle body setup?

 

I was toying with the idea of using a system similar to minolta camera blades, in that it has a spring that when pulled on, moves a bearing ring around which causes a pin holding onto the back of each blade to move further away from the pivot point. I think with about 7 to 9 blades, it would work exceptionally well.

 

Infact, I think using the flap design from a jet engine woudl work even better because it wouldnt be a flat set of blades... they'd actually be angled into the trumpet, and it can make full benefit of the bell-mouth and air horn/runner ID size, no?

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Flat Slide Lucas Mechanical Injection EFI conversions are out there as well.

 

An iris would be pretty complex, and hard to seal tightly...as well as being very prone to fouling. Normally for competition, the easier and more robust the mechanisim, the better.

 

But a simple sliding blade (Flatslide) setup does exist, and offers the 'no restriction barrel' at WOT.

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I was looking through one of my last megasquirt datalogs, and I found that at 60% throttle, I get full ambient pressure in the manifold. So that last 40% is doing nothing. This is with a 60mm 240sx throttle body and throttle body flange ported to match.

 

On my 42mm ITB's i'm planning on attaching the 944 throttle cam that everyone gets rid of. It should help with tip-in and low speed driveability.

 

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I dug up this thread to vent about the difficulty I'm having tuning low speed running on my 2jz-powered Lexus. I have a TEC3 and 720cc injectors - not TOO big...and a 90mm throttle body. No, I didn't choose/install it.

 

At low rpm/loads, my bigass TB has such a small opening that the TPS was near the idle setting, intermittantly triggering the decel fuel cutoff with small thottle position changes.

 

I have almost tuned around this problem, working with the decell enrichment settings, TPS/Map blend, along with a smaller idle opening and using timing to set/smooth the idle RPM - but even so that big TB makes tuning so critical that it's a MAJOR PITA.

 

I will probably go to a 75 mm TB sometime in the near future (My intercooler tubes are 3", so any bigger TB is gaining me ZERO).

 

From one who has been there -Resist the manly urge to go big.

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regarding TB sizing, bigger doesn't always mean more airflow. Intake air velocity will have more of an impact with volume of air than shear size opening will. The larger the TB assy on an L28 (even turbos) the less off idle response you will get. This moves the torque curve up the rpm ladder.

 

I purchassed a big throat to augment extensive head and intake work. Camming the butterfly shaft will be in the works.

 

I have to finish the ET upgrade first.

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