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

[JustinOlson]

Funny thing is that I size control valves at work for large industrial control systems. For a drag car like the honda above, I'd imagine there is very little throttle "modulation". Its likely 0% throttle at idle or 100% throttle when running down the drag strip. In this instance, there isn't a great deal of throttling required. There is likely enough traction since its a NA car to put all of its power down in every gear.

 

The larger the throttle valve, the lower the pressure drop across the valve. What you lose by going to a huge valve is modulation. Driveability goes to pot when you go to a huge valve because at very small valve openings you are flowing tons of air.

 

I size valves so that they flow 100% of the fluid at 80% of their percent open. This gives good control with resonable pressure drop. When you are dealing with a system that is just off or on, you can go to a huge valve because there isn't any modulation control issues.

 

Justin

[thehelix112]

Justin,

 

Surely that assumes linear throttle application. In modern systems, such as drive-by-wire T/Bs, and even cable throttles, its definitely possible to have non-ilnear actuation and the best of both worlds?

 

E.g, if the circular track that the cable pulls on to open the T/B is an outward spiral then I think(!) you end up with an appropriate actuation.

 

The amount the T/B is open O (in degrees) is given by:

 

Let x be the movement of the cable.

Let r be the instantaneous radius of the arm acting on the butterfly rod.

 

O = 180 * x / pi * r

 

Just made that up. Probably incorrect.

 

Dave

[JMortensen]

Porsche 944 has a cam on the throttle body cable track thingy to slow down the throttle opening. Most racers swap it out for a round one.

 

About 3/4 down the page, this one is for a 924, but same thing is available for 944s.

http://www.porscheshop.co.uk/acatalog/porsche_924_engine_induction.html

[JustinOlson]

When sizing a control valve you need to following information:

 

Line Size (inlet and outlet size)

Fluid Type (air)

peak fluid flow

Inlet pressure

Acceptable pressure drop across valve

 

With this information you come up with CV number that allows you to size your control valve. You find a valve that fits your application and flows your required CV at about 80% open. Generally control valves are sized smaller then line size to get good control. I will see if I can find some CV tables for butterfly valves to help with understanding.

 

Having a cam on the throttle will help some in drivability, but isn't the end all solution to an oversized control valve.

 

Justin

[thehelix112]

Cool. I think a cam'd throttle (if thats the correct term ) and throttle-dependent boost limit would help improve the drivability of a lot of turbo cars. Both on the list of future mods for mine.

 

Dave

[JIM73240Z]

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

[JustinOlson]

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.

[thehelix112]

Just for peoples FYI, this site is great.

 

http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html

 

Dave

[JustinOlson]

Very good site for physics!!! I like the charts for navigation.

[proxlamus©]

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

[Canadianz]

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

[BRAAP]

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...

[Careless]

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.

[PresaMax]

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?

[BleachZee]

Use the stock throttle. I had the MSA on my car for a few years. I was tired of the sticky and sensative repsonse. I went back to stock and gained power. I could feel the added torque instantly.

[PresaMax]

Thanks for the advice. I'm new to this website and Z cars. Thanks for all the great information.

[WizardBlack]

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.

[cyrus]

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?

[spork]

I'd rather see a restrictor plate used rather than a chunk of wood under the pedal. Even then without a smooth transition, you may not get as accurate results.

[Tony D]

" 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.