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Benefit(s) of larger Throttle Body


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3 replies to this topic

#1 Marck

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Posted 25 October 2004 - 11:48 PM

This thread is in response to WJM's XT6 throttle body modification:

I have a question or more like an opinion about putting a larger throttle body (like the XT6 one) on a turbo car. Don't get mad (I know WJM is really trying hard to make this work), but I don't think adding a larger throttle body will do much if anything to a turbo car. As long as the throttle body is the same size or larger than the turbo intake outlet (so that the throttle body is not the bottleneck), then I don't see any advantage to a larger throttle body. Sounds to me like the pressure will also decrease by increasing the size of the throttle body. Don't you want more pressure to gain more power on a turbo car? Also, the turbo would have to work harder to keep the pressure up.

I remember a long while back when people were discussing intercoolers, they said (and this is from memory) that the benefit of adding an intercooler is to lower the intake air temperature, which in turn makes the air denser, which allows us to cram more air into the combustion chamber. By cramming more air (with more fuel too) into the combustion chamber, we can make more power. Now the downside (I think) to the intercooler is that it decreases the pressure because we have increased the volume. So, to fix this, we just turn up the boost! We increased the volume of the intake which is the downside (and which can be remedied by increasing the turbo boost pressure) to gain more power by cooling the air (upside).

When adding a larger throttle body, it seems to me that we are increasing the volume which decreases the pressure which does not benefit anything (It certainly doesn't make the air any cooler/denser).

Is there something wrong with my thinking? I don't know too much about this subject, but I am just trying to use common sense. You got any links to some good articles that covers throttle bodys on turbo cars or please explain how a larger throttle body is supposed to add power?

#2 Numbchux

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Posted 26 October 2004 - 12:01 AM

I don't know much, but the power is from cfm (cubic feet per minute), or how much volume you cram in the chamber, psi is an easy way to measure it, but 20 psi through a tube the size of your finger is less effective than 10 through one the size of your leg! cooling it off just makes it possible to get more air into the chamber WITHOUT that much pressure, specifically.

As far as the throttle body, if it's not the bottleneck, than no, it won't help that much, but that's just speculation based on my limited knowledge of how cars work. Guys, please correct me if I'm wrong.

#3 NorthWet

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Posted 26 October 2004 - 12:20 AM

The term "bottleneck" really is not accurate for a compressible fluid such as air. More accurate is that it is a restriction. Restrictions are additive in their resistance to flow of a compressible fluid... the more restrictions, the more power needed to flow a given mass of the fluid.

So it does make sense to reduce a restriction such as the throttle body.

As far as volume affecting pressure, this is not really an issue. The throttle-body typically outflows into some sort of plenum, so the increase in volume causes by the TB is unimportant. And the volume does not really affect the pressure, just how long it takes for the pressure to change. And when you are talking in terms of 100-200 CFM (at STP) a volume change of a couple cubic inches is pretty negligible.

The intercooler shouldn't lower the pressure due to cooling per se. It will make the engine "look larger" to the turbo, requiring it to be able to flow more air at a given pressure. I think the real argument against I/Cs is the restriction to flow that they can present. That is mostly a matter of engineering and a little thought given to the application. You wouldn't use a WRX I/C on a Cummins Turbo-Diesel...

Anyway, that is my 2 cents as I see it.

#4 WJM

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Posted 26 October 2004 - 06:56 AM

My reasoning for the larger TB comes from several angles....

1st one I noticed one day that them chamber in the spyder is huge compared to the size of the TB...odd me tought... :confused:

ALL EJ engines have a TB as big as the XT6...hmmm...SUBARU must have had a really good reason for this.

The WRX IC intake side is the same size as the turbo out...and the IC out is the same size as the TB....go figure! :confused: (must ahve something to do with the higher density of air after cooling for the TB/IC exit to be the same size)

TWE has produced a TB even larger than the stock SUBARU units...

Then I was thinking...hot thin air is rushed from the turbo to the IC...cools...and then flows out the IC...but then has to compress again to fit thru the TB...that doesnt seem right...so why not make it the same size?

It may not increase the PRESSURE of the manifold...but it will increase the VOLUME of air which enters the intake manifold. With the cams in the engine this should yeild a much higher engine output. More air in=more power out.

Plus, since it turbo, more air in=more air out to spin the turbo. Since Ive got TWE peices to go on....exponential fuction. The only restrictions left to deal with is the teeny turbo, air flow meter/intake, Spyder legs and its "feet" (heads).

I am hoping to see 200 hp at a min from all this stuff. If I cant get 200 at this point, I will be sourly dissapointed...as I put alot of $ into the effort....




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