JimE's AS&M 58mm Throttle Body Mod


  After reading LJ's website about modifying his AS&M throttle body, I decided to do the same thing to my AS&M TB.

The reason that I wanted to do this, is because my 95 Trans-Am was experiencing a sizable "Split BLM" condition at idle, due to the reversion of my relatively large camshaft.

LJ details how to use a piece of brass tubing and cut a special hole shaped notch into it, in order to isolate the IAC air from the common plenum. The hole shaped notch in the tube aligns with hole in the TB, to direct the IAC air only into the tube, which hooks into the special IAC passageways of the intake manifold.

After reading LJ's site, I modified the brass tube and put an o-ring on there, but I wasn't quite happy with the way that it felt, when installing the TB onto the intake. I decided to machine a scrap piece of aluminum to completely fill in the space between the throttle bores, with a hole for the brass tubing to still go through.

I did this with a hand drill, some files, a dremel and a bench grinder. The most difficult part was holding the small piece while drilling the holes. At the time, I didn't have much of a work area, or a drill press, so it was a real pain in the butt.

Here is a picture of the stock, AS&M 58mm throttle body. It is a beautiful piece of CNC machined, billet aluminum. Mmmmmmm.

Next is a picture of the front of the intake manifold.

Here is a picture of the piece that I machined to fill the gap between the throttle bores, and still leave room for the brass tube. For illustrative purposed, the piece is not installed in the gap here.

Here is a picture of the machined piece and the brass tube installed in the throttle body.

Although I installed this setup with the machined piece and the brass tube, it appears that I could have omitted the brass tube, if I adhere the machined piece to the throttle body. I will also have to do something about the TB gasket material not going around the IAC hole too, and that's why I didn't do it the first time. But, I believe I will try it without the tube the next time that I take the TB off, because it was a bit of a pain to carefully install the throttle and guide the brass tube into the manifold's IAC hole.

Another thing about split BLMs: LJ mentions how adjusting the TB screw to get the IAC counts to move around, and also the BLM numbers. Another thing which changed the BLM counts at idle for me, was modifying the timing advance at idle. I was set for 30 degrees at 800 RPM, and noticed a big change when lowering it down to 20 degrees. I also noticed a huge improvement in the smell of my exhaust when I lowered the timing. I was no longer wilting at idle (and yes, I do have a catalytic converter installed on the car.)

Although this modification helped my split BLM condition, it didn't solve it. I was having mechanical problems (clogged cat converter and other stuff) that was the main source of my problem. In the end, I just decided that a CC306 cam is just a little too big to expect an nice stable closed loop idle, so I modified my tune to run a PE (Power Enrichment) idle and all is well now.


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Page last updated on 02/26/2006.