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Throttle Body Size?

Tony de Wonderful
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Tony de Wonderful Veteran

Tony de Wonderful

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I'm trying to decide what throttle body to run on a Plazmaman plenum.

It is my RB25 GTST and aiming for 300 rwkw.

I am assuming the stock throttle body will choke the engine a little and would a 72 mm be adequate?

I know Q45 bodies are around but I think they need an adapter flange welding on?

I don't want to run a 72 mm if it will lose me power.

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Tony de Wonderful Veteran

Tony de Wonderful

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e.g. Proengines as per above - 364kw with standard t/b (can't edit my posts atm).

Yeah but you might have got 374kw with a 90mm TB.

I just don't know.

I'm thinking of settling on a 72 mm TB. My pipe work is all 2.5" so can't see 72mm being a restriction as such.

My main issues is bolting it on to the Plazmaman. I know the Plazmaman takes the stock one, and I know the 72mm fits the stock plenum. The 90mm TB's need an adapter I think? So the 72mm would just conveniently bolt on w/out a need for an adapter?

Tony de Wonderful Veteran

Tony de Wonderful

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Y r you wasting money. Heaps of rb25s out there making over 300 with stock tb the amount of restriction I would make is nothing plus you will loose some mid range power

How is it going to lose mid range? I think you mean throttle sensitivity?

89CAL Grand Master

89CAL

Posted
Posted

Stock throttle body will be fine. Saw in an imports magazine the other day a silvia with an RB25 and 500+ hp with a GReddy plenum and the adapter to fit the stock throttle body. No problems with stock throttle body unless your building a show car or making rediculous power

Tony de Wonderful Veteran

Tony de Wonderful

Posted
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Thanks for the responses.

There are some calculators on the web in places.

I might play around with them at some point, but I am thinking to just stay stock, mainly for cheapness and modulation.

Calculator

  • 1 month later...
  • 3 months later...
Roy Grand Master

Roy

Posted
Posted

Thanks for the responses.

There are some calculators on the web in places.

I might play around with them at some point, but I am thinking to just stay stock, mainly for cheapness and modulation.

Calculator

Interesting, that calculator suggests 72mm when I enter my engine variables. I also run a plazmaman plenum and interested in trialling the throttle body. Hoping they will do me a return policy on one in case the performance doesn't match their asking price

34GeeTeeTee Grand Master

34GeeTeeTee

Posted
Posted

Interesting, that calculator suggests 72mm when I enter my engine variables. I also run a plazmaman plenum and interested in trialling the throttle body. Hoping they will do me a return policy on one in case the performance doesn't match their asking price

I went will the Plazmaman throttle body for looks and that the stock NEO throttle body is about 8 foot long... Haha... I chose to stay with a near stock size tho of 66mm... I guess for street use the throttle response was important...

GTSBoy Grand Master

GTSBoy

Posted
Posted

Get this straight.

Throttle response will be greater with a larger throttle. This is only because a larger throttle will give you a more rapidly increasing throttle flow area as you open it. ie, more mm2 of area opening up per millisecond.

Throttle controllability is maximised with a smaller throttle - well, at the bottom end of the opening range, anyway. If you need to be able to modulate small openings, then you need a smaller throttle rather than a larger one, for the same reason as given the he above paragraph. A larger throttle gives yuou bigger changes in open area per degree of rotation of teh shaft - ergo, less sensitive.

Apart from any restriction put up by the throttle at WOT, a different throttle cannot really cause a loss or gain in midrange power. If it is small enough to cost top end power, then it will cost mid range a bit too. If it is so big that it is not any restriction at all at the top end, then it will be even less of a restriction in the midrange.

/obvious.

Roy Grand Master

Roy

Posted
Posted

So, does that mean you have seen someone leave everything else the same except change to TB and realise a change for better/worse or no change?

I understand people make perfectly good power with a certain TB. The reason I am considering it is the TB is obviously sized for a certain lbs/min of air. When you go from 110rwkws to 310rwkws I dont think its out of line to wonder what affect that has on pressure drop through the throttle body....or more to my thinking what its doing with regards to the velocity of air through it and in turn the pressure drop you have when it goes into the larger plenum and cylinder filling

34GeeTeeTee Grand Master

34GeeTeeTee

Posted
Posted

So with what your saying, the smaller throttle body is still better for daily driving and throttle control on street?

I did a fair bit of asking about all this when I bought my plazmaman kit, but happy to hear others opinions....

GTSBoy Grand Master

GTSBoy

Posted
Posted

So, does that mean you have seen someone leave everything else the same except change to TB and realise a change for better/worse or no change?

I understand people make perfectly good power with a certain TB. The reason I am considering it is the TB is obviously sized for a certain lbs/min of air. When you go from 110rwkws to 310rwkws I dont think its out of line to wonder what affect that has on pressure drop through the throttle body....or more to my thinking what its doing with regards to the velocity of air through it and in turn the pressure drop you have when it goes into the larger plenum and cylinder filling

If you get up to the 300rwkW area (or possibly even less) then I have no doubt that you'd free up a few kW by putting a bigger TB on. If you calculate the gas velocity through things like the TB at the temperature and pressure of post intercooler air you'll be pretty frightened by how fast it's going (compared to industrial gas flows where we try to keep stuff below about 30 m/s if we can, to prevent excessive noise and pressure drop). The very high velocity leads to high pressure drops along lengths of pipe and through fittings and valves (like the TB). Upsize it, lower the velocity, significantly lower the pressure drop, gain a psi maybe, which is a psi that the turbo doesn't have to make, and drive that the turbine doesn't have to provide and hence less back pressure on the engine.

But you have to realise that the cross sectional area increases with the square of diameter, so you don't need to go from, say, a 66mm butterfly all the way up to a 90mm. That's 85% more flow area, which you probably don't need. But what you certainly don't need is the first degree of rotation off closed to give you 185% of the flow area that the original TB would have, if you're trying to drive it normally.

When you say that a TB is designed for a certain lb/min of air flow, that's not quite right. When we design a valve to act as a flow control valve, we start out with a few numbers. Those are, the mass or normal volume rate of the gas, the starting pressure, and the amount of pressure drop that we're willing to accept over the valve when at full flow and 100% open. The designer has freedom to choose that last parameter based on his needs. But importantly, when boosting up an engine, we are not just changing the mass rate of air, we are changing the starting conditions as well. Let's say we double, or triple the boost from the original 8 psi or so. At the same gas veloctiy we're flowing a lot more gas through the valve straight away. Even if we end up with the same pressure drop as the original design, we'll still be flowing bucket loads more air than originally. My point here is that control valve design is complicated - and the application in question here, being automotive compressed intake air, makes it harder to mentally estimate what is going to happen when you make changes (say to boost). So one of the best guides to what to do, is to look at other's experience. That's an engineering basic - copying those that have been successful before, avoiding what those who have not been successful have done. Use their experience to decide which numbers become important.

So with what your saying, the smaller throttle body is still better for daily driving and throttle control on street?

Absolutely. I wouldn't be tempted to go much bigger than 80mm under any circumstances where the car is still supposed to be a street car. Probably even smaller.

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