Anyone remember my discussion a while ago about the exhaust manifolds, and porting? Maybe, maybe not, eh? Well the general idea was that I didn't find too much that could be ported inside the manifold. The walls were pretty thin, and aside from enlarging the ports, there wasn't much that could be done in the way of removing in-the-way metal. I then talked about what I believed was a hefty restriction... the chunk of iron resembling a doughnut between the exhaust manifold & y-pipe.

There's no flange on our manifolds or y-pipes; we've got a conical-shaped chunk of iron in between the two. The cone fits into the y-pipe, and the widest end fits flat against the exhaust manifold. This makes a seal at both ends, and doesn't require a gasket.

Well, this cone/donut had an odd "step" in the bore. Think of it as the letter "Y" and not the letter "V"! At the manifold-end of the cone, the opening was wide. Halfway through the cone is a steep ridge that makes the inside of the cone more narrow. This thinner area goes all the way to the end of the cone. Think of how a quarter looks on top of a dime, and you've got a good picture of how the ridge inside the cone looked.

I had filed a junkyard donut down last year, and took out the ridge. I made the transition inside the cone smooth, basically, changing it's shape from a "Y" to a "V". Yesterday, I scored another one (for free, thanks, junkyard-guy!) from an '87 2.8 Firebird. I filed & ground it down like the original, and went to slide them in. I figured it was a nice easy mod, since I just have to loosen the manifold-to-y-pipe nuts to slide the old donut out and the ported one in.

I raised the car, got underneath, and then promptly snapped off both manifold-to-y-pipe studs on the driver's side exhaust manifold! F#^%! That'll teach me to screw with something, eh? The only lucky part was that I had another driver's side manifold in the basement, along with new bolts. (The new bolts were bought last year in prep for my port job...)

I removed my old manifold- luckily no bolts broke on me. I did discover that the previous owner snapped a bolt off on me- on the driver's manifold, I'm missing the frontmost upper bolt. It's snapped off flush with the head... so vice-grips won't even work. I coated all the new bolts with anti-seize, and bolted the new guy on. As an afterthought, I removed it again, and cleaned the manifold-to-head surfaces by dressing them lightly with a metal file.

I slid the donut in place and tightened down new manifold-to-y-pipe nuts (coated with the anti-sieze, of course). Then I loosed the y-pipe on the passengerside (thank goodness those studs didn't snap), slid out the old donut, and slid the ported one in.

Wow- what a difference! The car seems much zipper all around- off the line, mid throttle, and WOT! Not too shabby for an hours worth of filing and 3 hours of exhaust manifold replacement! This is definately something to check into if you're not about to drop a ton of cash for headers just yet. Each donut was free from the same junkyard, and I used a "rat's tail" metal file for the major work, and an air-powered die grinder with grinding stone for the finish work.


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-Tom P (Hot rodded 1986 Firebird 2.8l) from http://www.f-body.net/mailbag/3rd/3rd_mailbag.html message boards
---Think your car could be pic of the week? Visit http://www.f-body.net for details!

[This message has been edited by TomP (edited July 23, 2001).]

 

That is not a bad thought!
Wish I could see a pic, as I recently had them in hand.
Didn't seem that must air flow loss.
BUT, you sound like you hit a great tip!

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Chat Soon,
KED85
Karl
1985 Firebird 2.8 to 3.4 swap project for Smog Happy LA, CA

 

I really didn't expect to "feel" any gain... but man, I'm not arguing! I'll have to see if I can hog 'em out wider some time...


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-Tom P (Hot rodded 1986 Firebird 2.8l) from http://www.f-body.net/mailbag/3rd/3rd_mailbag.html message boards
---Think your car could be pic of the week? Visit http://www.f-body.net for details!