hey all. I have a 81.5mm GSR, 13:1 IB Wiseco's with Skunk2 Stage 3's (tuner, they are a couple years old). I have been searching around on the site and have seen some different opinions on what TB size to run for this type of set up, seems like most people say either 65 or 68mm, but 68mm seems to be what is called for in 2.0 setups. I can already imagine people saying get both and dyno test them, but I haven't found a money tree yet so I need to try and get it right the first time. Thanks for the input.

 

64 or 65.

with cams that large you're clearly aiming for a high rpm powerband, a "smaller" throttle body keeps intake air charge traveling faster than a larger throttle body.

Intake air charge velocity beats intake air volume imo.

 

only companies that I have seen that make a 65mm TB are JG and Professional Products, that sound right?

 

I'm running a 68mm from Professional Products. It has been problem free from day one.

 

By your theory, he then should stick with the stock size. Since it is smaller.

How do you know the 65mm won't be choking the motor out in the top end?

I'd go with a 68mm.

 

also forgot to mention, im running a Performer X manifold that is ported to handle a 68mm TB (thanks Combustion Contraption)

 

i had a 68mm on my stock block and head gsr with skunk mani not portmatched and it had no problems .. belive it or not the car acualy ran pretty good for what it was "in the process of gettin rebuilt"

 

I think many will agree with me here though that there is a difference from "running pretty good" and "performing good", im not thinking about the "butt" dyno, im thinking about actual numbers and what will yield the most power.

 

i only pruchased becasue of motor plans ... but anywho if i where to do it all over i would do a 68 tapperd to 65

 

i gotcha, but my motor is all set and done, only thing I have not touched is the throttle body and its the last piece I want to get to complete everything.

 

maxbore can bore out your stock one to 66mm or more depending which tb you use

 

68mm would be a good size for your setup. I wouldn't go any bigger though. 65-66mm would also be fine.

 

yea my plan was to do a 65mm or 68mm, just trying to get opinions on what one of the 2?

 

If you're going for max power, go 68mm...you've got very nice compression and big cams. It won't be too big.

 

Simple, just calculate out the max cfm for the throttle body flow and then calculate the amount of air used (cfm) by the engine at max RPM. I think you'll find that the stock throttle body can still flow more air than the engine can consume, therefore it shouldn't be considered a restriction.

 

would a 65 potentially perform better throughout the entire power band, where as the 68 would be more for the top end? I am going for more top end power as this is for 1/4 mile setup.

 

Just because it can flow at certain RPM's doesn't mean it has the right velocity.

So yes, it can be considered a restriction. Flow is only 1 part of the equation.

 

i run this as well..

+1

 

Correct.



Essentially yes. It's kind of like switching from a Performer X intake to a Victor X. Both can flow more air than the engine may require but one is more efficient from a velocity perspective at lower RPM's than the other.

Typically a larger throttlebody will move the power peak a little higher in the RPM range than a smaller one but not necessarily add more power over the whole rpm range.

 

Especially since losses to air friction increase exponentially with velocity.

An OEM throttle body will always be able to flow more air than the engine can induct, but that doesn't mean it's the right size for all applications.