: Perfect exahust flow for 1999 STS?

08-04-04, 04:37 AM
have searched long and hard for the perfect exhaust setup for our Northstars. Nothing substantial, alot of different opinions. You hinted that it really dosen't matter as regards to backpressure, that the 4-valve excells with a lack of backpressure, and the OEM cat is not the bottleneck. Exactly what part of the exhaust system is the bottleneck on the 4.6? What size piping and muffler would you use for maximum performance? I'm looking for a somewhat quiet exhaust system (no droning) but, exhaust flow is more important than anything else. Meaning, I will buy a loud exhaust if it's going effect performance (flow). However I do not want to buy the $1100 Corsa, too expensive. Your replies and others would be greatly appreciated.

Anthony Cipriano
08-04-04, 11:49 AM
I'm pretty sure the real cork ultimately will be the production exhaust manifolds and the "cross through" pipe that goes from the front bank to the rear bank to route the front bank exhuast rearward. The production parts are packaging constrained and designed for emissions performance to light the catalytic converter quickly. That being said, they are not THAT bad but the real hotrod Northstars like CHRFAB is putting into sand rails and such run headers.

I would go with 2.5 to 3 inch pipe from the cat rearward. It will have to be a custom bent setup as I don't know of any avialable sources for the prebent parts. Several people have tried different brand mufflers for low backpressure. Flowmaster, etc.... You'll have to do some searching in the archives for their reports on sound quality and noise. They will definitely be louder but probably tolerable.

There is really no practical way to put real exhaust headers on the transverse mounted Northstar engines due to space, packaging and routing constraints. The idea of a "free flow" exhaust sytem is not lost on the engineers that designed the production car...but....packaging, lack of space, the cross thru pipe design, emissions performance, durability of the exhaust manifolds and system, etc...all got in the way of real headers....

One thing that will help free up a few HP is to get the exhaust manifolds off and have them extrudehoned. There are a lot of sharp edges and corners in the fab'ed manifolds that can be ground , smoothed and radiused. Not a huge gain here and a lot of work to pull the manifolds and such but if you want the most power the manifold rework with the low backpressure system will provide the most. I would say that the ultimate exhaust on a transverse passenger car with extrudehoned manifolds, large pipes and flowmaster style muffler would probably provide 20-25 HP at peak power.

08-04-04, 11:01 PM
Just out of curiosity, what would you think about dual 2" pipes straight from the manifolds to dual cats? The whole system from manifolds to exhaust tips being like 4' long?

Anthony Cipriano
08-04-04, 11:09 PM
Just out of curiosity, what would you think about dual 2" pipes straight from the manifolds to dual cats? The whole system from manifolds to exhaust tips being like 4' long?

With a system that short it will end under the car which is never really a good idea if you are going to drive the car due to the possibility of CO/CO2 intrusion into the cabin. If you look at the crossover/crossthru pipe from the front bank to the rear bank you will see that this is somewhat difficult to do as described. You would have to make the sytems "unequal" in length under the car to make the overall length from each side the same. One constraint of transverse mounted V style engines is that the two cylinder banks are different distances from the rear of the car so it is almost impossible to do a true equal lenght setup (which is most desireable) for each bank that doesn't end under the car (undesireable).

But....it could work OK. I don't think it is worth the effort and expense with the production manifolds and such. The single cat will flow plenty for a relatively stock production application engine and going with a 3 inch cat back system will probably be just as good or better than the true dual system as long as the production exhaust manfolds are still used.

08-05-04, 01:15 AM
I was just thinking restriction wise... I think airflow through a pipe is kinda like electricity through a wire.. More resistance for thinner or longer wires/pipe. The engine I have is being swapped into a hotrod of sorts. I understand what you mean about the unequal length due to the crossover pipe... Is there any specific place that is "ideal" to put a equalizer tube/X or H or whatever you wanna call it for max power or torque? Thoughts are appreciated. Welcome back btw :bouncy:

Wrench Head
04-13-05, 11:16 PM
I have a 94 Eldo with flowmasters and bigger pipes, WOW!!! Sweeeeeeettt!!! I just love the sound of it.

04-14-05, 02:15 AM
I would think that the Corsa system, is probably the best exhaust system for the Northstar. It was engineered specifically for that engine and aplication, and has been dyno tesed and proven to improve performance. I've got it in my car, and I definitely felt some more seat of the pant's power after it was installed. It's a 2.5" system from the cat, and pretty much straight through all the way back. It doesn't drone, and is very quite at idle. Like Bbob said, the exhaust manifolds are somewhat restrictive, but they are better than most OEM's. I was thinking about buying a set off ebay for abotu $50 and just seeing what could be done with them, but I never got around to it. Making a larger crossover pipe for the manifold in the front is pretty much impossible. It has to snake its way inbetween the transmission and engine.

04-14-05, 11:37 AM
My Buick Regal GS had a tube header on one side and a cast header on the other from the factory. There is no law that says they have to be the same. The cast header was a pretty elaborate design so functionally they were probably pretty close.

04-14-05, 12:54 PM
The 'ol 60 Dodge runs great with a big block dodge HP cast manifold on the passenger side and a truck rear exit cast log on the drivers. The frame rails are so close to the oil pan on that side, when we welded in our homemade engine mounts, that we couldn't fit headers or any kind of a center dump or standard manifold in it. It works fine, has lots of power. I'm sure a matched set of HP's or headers would work much better, but we took what we could get. :)