: Down Pipe Should be Ported

07-17-10, 02:46 AM
I'm doing another 1996 N* engine conversion in a Pontiac.

I took off the down pipe from the motor and happened to look inside it where the two pipes are welded together. I was astounded! When GM welded the small pipe into the big pipe from the factory, they left an 1 1/4" of piping sticking out inside. That's a significant restriction from my perspective.

Next time you have exhaust work done, you might want to consider port it.
As a result, I'm going to clean it up. Just wanted those of you that care about your exhaust to know about this restriction. Perhaps most of you already know about the obstruction....

07-17-10, 04:01 AM
This has been posted before with pictures and I couldn't believe it. Yet when I saw one with my one eyes off an eldorado in the salvage yard, I still didn't believe it. Nice..........It definitely needs to be worked on for sure. Thanks.

07-17-10, 12:34 PM
That's just plain GM lazy. That should not be, unless they are trying to compensate for the larger pipe flow as compared to the smaller pipe.

I've seen it on other GM exhaust manifolds, such as the Pontiac Fiero. What's with that? You gain 6 HP just from porting the V6 Pontiac manifolds.

I wonder what the N* will gain. Not much but, shoot, everything adds up.

07-17-10, 10:11 PM
I suspect it is more production line efficiency than laziness, but I agree, it should not be. I too remember those pictures and all I could think was :tisk:.

07-19-10, 08:47 PM
Oh I know.... I posted the pics! Just sloppy on GM's part. I wonder if the engine that GM sent for horsepower certification had similar sloppy work. :bonkers: