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I'm currently rebuilding my 180SX track car, which includes an engine rebuild and a new loom. Part of this is to convert it e-throttle. I'm wondering if there is a some way of calculating what size throttle body I should be using?
Some more specs on the car - engine is a VCT SR20, cams are Tomei Pon cams (256 deg and 11.5mm lift), turbo is a GTX2863R, will be running E85, plenum will be standard, staying at 2 litre capacity with 9.0:1 compression pistons. The goal is to make ~280rwkw (previously made 260 on 98 with lower comp @19psi)
The standard throttle body is 50mm, and Bosch throttle I've been looking at are available in 54, 60 and 68mm. I've calculated the percentage increase in area to be approximately 16%, 44% and 85% respectively.
Any advice on choosing a suitable size would be much appreciated!
The other thing to consider is bolt pattern and manifold mount - if you are keeping the existing manifold and simply replacing the original cable operated throttle body, do you want the new one to bolt straight up, are you happy to do some fab work to get it to fit or are you making a new manifold?
I have recently been through the same exercise and came to the conclusion that the STD throttle body size was more than adequate to develop the power I was after so I didnt need a different size throttle body but looking at some of the car manufacturer throttle bodies, they differ in spec from the off the shelf Bosch items (even though they are still made by Bosch and labeled as a Bosch part) - I found that a VAG throttle body from their 20v turbo engines was a direct fit for the bolt pattern on my manifold and was a 2mm larger body making life easy with no mods.
Good points Ben - I hadn't even thought of looking for something that was a direct bolt up. I just figured some fab work would be necessary with either an adapter plate, or welding a new flange onto the current manifold. Neither idea phases me at all, but I guess easier is better! I'll measure up the current bolt pattern and see if anything is a match.
Turns out the 54mm and 60mm bosch throttle body bolt pattern is the same as the stock plenum. I guess that narrows it down to two.
The bigger the 'body the less restrictive that part of the intake tract will be - however that is only one part and really only needs to be as big as the rest of the system and, in particular, the manifold.
The main problem with going larger than needed is that the 'authority', or effectiveness of the 'body in regulating the flow will be compromised at both ends - under light throttle a small change in butterfly angle will make a bigger change in the opening area so reduce driveability and, close to fully open, the other restrictions in the intake system may mean quite big changes in butterfly andle will have little affect in overall airflow.
Personally, as you're looking at a modest increase, I'd leave it stock, or next size up.
I vote 54mm, easier to control.
Thanks for the detailed reply. I'd considered the control issue at low openings but hadn't thought about how it would affect it at WOT or close to it. Very good point and got me sold on going with 54mm