ITB size for a 2.0L 4 cyl engine

shauer · Post by **shauer** » Thu Apr 06, 2006 2:49 pm

I'm trying to replace a set of Weber 40 DCOE carbs with DCOE replacement ITBs. I've been waiting 5 weeks for an order with 40mm ITBs and was just informed that it will be another 6 weeks to get them. At this point I don't think they really know if/when they will be made so I'm looking at other options.

I've read everything I can find on ITB size and I thought 40mm would be correct for my engine (see below for specs)

My question to anyone who is running ITBs is would 45mm ITBs be too big? Any guess on how much off-idle response I would be losing between 40mm and 45mm throttles? The intended use for this car is spirited daily driving, not racing or autox.

Engine:
- BMW M10 (2.0L 4 cyl inline, single overhead cam)
- 290 degree cam
- header with stock exhaust
- Redline 6500
- Expected HP 150-160

Thanks!

Mueller · Post by **Mueller** » Thu Apr 06, 2006 4:33 pm

I'm building a set of 45mm ITB's right now for a motor with very similar specs to yours http://www.msefi.com/viewtopic.php?t=6627&start=40....one good thing about going too big is that it's easier to fix than too small

(cam on the throttle linkage or throttle stop)

Karl_Skewes · Post by **Karl_Skewes** » Tue Apr 11, 2006 4:37 pm

40mm throttles are meant to do up to about 250hp

check out toyota 4AGE 20v engines. This is what I used on my old 2lt 3SGE, and on current 4AGE 1.6
You can get 40mm and 44mm butterfly sizes.
They also have a variable TPS, so perfect for MS.

Relatively easy to mount on a standard webber sidedraft manifold.

greggearhead · Post by **greggearhead** » Thu Apr 20, 2006 4:05 pm

If you are doing the M10 engine, Ken Danielson had the idea to use Alfa Spica throttle bodies on the lower 77-79 intake manifold, making a simple flat adapter plate. Looks fairly straightforward. I actually have some Spica throttle bodies, they are 40mm and bore centres basically line right up.

Bill Shurvinton · Post by **Bill Shurvinton** » Thu Apr 20, 2006 11:22 pm

Based on the cross section of all the people I know running ITBs (which is the first upgrade to lotus 7 type cars). 40 is ideal for up to 200HP. 45s are a little large, but work fine. Lots of people run them. Your only issues will be sorting out the AE settings as you will get a bit change in airflow for a tiny change in throttle angle. Get a progressive quadrant and all will be fine.

shauer · Post by **shauer** » Fri Apr 21, 2006 4:02 am

Thanks for all the replies. I just received my 45mm bodies last night.

I expect to run into a little trouble tuning the off-idle response but it looks like others have done it so it must be possible...

I'm keeping a close eye on the status of v2.6 code with XTau in the Beta forum as well.

The bodies came with a traditional Weber style center pull linkage. I'm looking into modifying it to use a progressive linkage.

Thanks again for the input.