Upgrading to e-throttle - throttle body size?

Hi everyone,

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 guys.

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

Good discussion this one, I have an N/A 2.0 8v vw, it is getting a microsquirt standalone and the supplier recommended going for an older obd1 throttle body as a first preference. This makes the setup easier as these earlier units have a 3 pin plug, don't have addtional trickery for emissions/idle control I believe.

Anyway the throttle body is easy to get, plenty of oem style 3 pins available, but they are smaller in size (2.25 inch versus 2.5 inch) and I am thinking from the conversation here the reduction on a car like mine should be fine, it has a 272 cam that makes the throttle hard to maintain smooth movement when crawling in traffic in first gear. So I might err to smaller, and pass on going bigger to the V6 obd1.

I'm now trying to find a supplier of an adapter, not a common change, maybe I'll need to get one made, or have a crack at making one myself.

A lot of OEM's run throttle bodies of a larger diameter than is actually needed, for example, on a Toyota 86 with the FA20D engine, opening the throttle to greater than 85% doesn't change the power or feel of the engine. This was seen on both the dyno, and on track blind testing with a couple of professional drivers. This is with the 0% set with the throttle plate fully closed against the mechanical stop, and the 100% set to the blade being at 90 degrees to the airflow.

The other extent that I have seen us the use of throttles greater than 100mm on engines that have no way near that airflow requirement. You end up in a situation where the 100% throttle position is mapped to a physical angle that is 50% of the opening or less, just to be able to get some driveability into the engine.

If 54mm gets you bolt on e-throttle, that seems the best option. It is sufficient for your airflow level based on your power level.