View single post by StevenD57
 Posted: 05-31-2011 12:39 am
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StevenD57



Joined: 04-10-2009
Location: Hubbard, Ohio USA
Posts: 103
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JodyKerr wrote: It's easy enough to go with the 45's instead of the 40's and you can always jet them down if need be. With carbs, providing you're not putting too much on the car, bigger is always better.

Jody
When it comes to carbs and the size of the throttle plate this is an erroneous assumption. Yes, you can fit smaller chokes but if you end up choking a DHLA45 or DCOE45 down to 32mm then you are not got to have the drive-ability you would with a properly sized carb. A 45mm carb is probably ok for a 2.2L engine but it WAY overkill for a 2.0L engine.

Here are a couple of quotes from a guy who has done lots of work with different size throttle bodies fitted to various BMW M10 motors (used in 2002 and 320i):

"Another problem with this setup was the size of the ITBs.  I had ordered 45mm ITBs even though my own research had indicated that 40mm would be a better size.  I allowed myself to get talked into the 45mm TBs by a sales person who told me it would be "fine" and "everyone" uses this size and "EFI does not have the same problems with large TBs as carbs do".  All this was wrong.The 45mm TBs are too big for a moderately modified M10 engine.  This produced a really bad behavior at very small throttle openings.  The throttle was basically "digital" at low engine RPMs.  You only needed about 1/8 of the pedal travel for daily driving, anything over this and you were already at the 100% load point for the engine.  This was extremely fatiguing for a daily driver.  You just rest your foot on throttle and the engine started to race.  This issue also made it extremely difficult to tune, I will describe this on the tuning page"
After he switched to the 40mm throttle bodies he said:

"The 40mm TBs are the perfect size for the engine.  Throttle response and sensitivity are greatly improved over the 45mm TBs.  I can take the engine up to 6500 RPM and still not see any reduction in the MAP signal due to the smaller TBs."

You really should read the following article where he does more analysis of the behavior:

http://77e21.info/mstuning_tbsizing.htm