I know you're not supposed to be able to use an early throttle body on a late plenum, but why not? I know it has to do with the IAC passage, but I cant find exactly what it is about the IAC passage that is the problem. Would it create a vacuum leak if I used a '86 TB on a '89+ plenum?

 

opps, no one told me that. I put a TB from a 90 on a plentum from an 86 and didn't have any problem. I don't know what to tell you other than to compare the 2 and see whats different.

 

The '90 TB would be a late TB and the '86 plenum would be an early plenum; so, according to what I've read, you're okay.

 

Nobody wants to touch this one? I've only got an '86 TB but I have the '86 plenum and a '89 plenum. The '89 plenum is cleaner, so I'd like to use it if I can. Forgot to mention, I'm also deleting the cold start injector, if this makes a difference.

Thanks.

 

OK, I dug out the 2 throttle bodies I have. Both are stock, I know 1 is from an 86 and the other is from a 90. The plenum that is not in use is from the 90. With both TBs side by side on the table they look just alike. I did not take the BBK off the car to compare that plenum because
1: Its been opened up to match the bigger TB,therefore it does
not look the same.
2: I'm really lazy.


I even went as far as calling jegs and asked them because they do show an 85 thru 88 part number and an 89 thru 92 part number. All that guy could tell me was maybe something to do with the wiring.
He did give me the phone # for BBK, 1-951-296-1771

Have you looked at the 2 you have to see if they look the same?

Check out www.ChevyThunder.com Thats where I got my start, even before I found out about ThirdGen.org

 

I believe the early plenum has a 3rd hole for the IAC passage just between and below the 52mm throttle body passages. The later plenum does not have this feature. The has to be a passage for the idle air control to get into the plenum. On mine I had to cut the gasket to make it work. Could have drilled the third hole in the plenum.

 

DSmith, the only difference b/w the two plenums is that the '86 has the third passage between the throttle body passages and the '89 doesnt have this. I only have an '86 throttle body; so, I cant compare it to anything.

Acording to chevy thunder the third passage feeds the cold start injector. Does the the IAC feed the plenum differently on the '89+ throttle bodies or is cold start the only reason the site say that you cant use the '85-'88 TB on a '89-'92 plenum?

 

On my 1989 throttle body there are cutouts so that the air can get into the main throttle body channels instead of the 3rd hole in the middle which is not on the 89 model. I believe on the earlier throttle bodies these cut outs are not there.

I bought a Holley 58mm throttle body which did not have the cutouts. I cut out the gasket at the right points so air could get into the main channels.

Also the throttle cable is different between the older and newer units.

 

I just drilled a hole under the 2 bores to where the 88 and prior's are

 

Another question: Does the IAC passage in the early plenum feed into the plenum itself or does it only feed the the tube above the CSI? I was going to use 89+ runners, but if I'll have no idle air with them, it's pointless.