SLEEPER 86

trying to get my head around the velocity thing as it relates to my combination.
my 383 has a longer street cam, vorec type heads,etc. (see my sig).
I missed the boat when i built my exhaust, but I'm not sure what the effect will be. basically, I ordered my 1 5/8" primary coated hooker shorties and y pipe that came with an "offroad pipe". i got a cat back system from jegs for a great price. What I missed was the 2 1/2 o.d. of the primary exhaust and the 3" o.d. of the cat back. after that is the 3" in dual 2 1/2" out flowmaster type exhaust
I have seen stepped headers that raise the velocity of exhaust by narrowing the diameter
of the header tubes along their length. In theory the velocity stays up as the exhaust gasses cool andcontract.
I managed to adapt the system with a flowmaster reducer reversed at the beginning of the cat back as a quick fix, so in effect I have a reverse stepped system.
My question is if my primary exhaust system can handle the flow of the engine, will the exhaust gas flow be impeded by the increase in secondary exhaust diameter? have the gasses slowed/ cooled enough to make it unimportant?
As always, I welcome all opions!
Thank's!
Eric B

UnderCover89TBI

iTrader: (25)

I was going to say thier is a big backpressure and velocity thread jus a few posts down. Now on your problem. exhaust actually expands as it goes down the pipe. its more compressed when coming out of the heads. So I really dont think it'll be a problem for you. More of a looks thing, rather than a performance thing.

TA

iTrader: (1)

You are correct, as the gasses cool, they contract, so you will find stepped headers go from bigger to smaller to maintain velocity. The items that jump out at me are that you are running a carb'd 383 into 2.5 primary exhaust, then 3" "after cat" without a cat. The 2.5" primary is probably restrictive on a warmed-up 383, and the 3" after cat is designed to handle the flow after your cooled exhaust is super-heated by the cat(s), so without a cat your exhaust flow is probably slowing tremendously when it hits the step from the 2.5 to the 3" section, but I have no experience running without a cat, so I can only guess. I would recommend going to 3" primarys, and seeing how the car reacts to that. You will find that you can also open up the "holes" in the Hooker shorties' collectors with a grinder to better match the primary pipes, as they make them pretty small just in case there are misalignments and it will still seal. If you bolt up your primaries with wet paint, and remove them, you willl see you can open up the collectors quite a bit, and probably even more with the 3" primaries, and pick up power there.

Good luck

SLEEPER 86

that's what i was thinking TA.
The Hookers already have a 3" collector, now to find a 3"y pipe. Don't think Hooker makes one.

TA

iTrader: (1)

The diameter of the collector is 3", but on my Hooker shorties, the actual hole that the exhaust has to exit was only 1 7/8! This is HORRIBLE on a 383!

One of the reasons that our cars seem to sustain a reputation for not responding well to bolt-ons is that all the bolt-ons were designed in 1982 to fit and work on the most common engines, which were 150HP 305s, which they had little effect on, and thus were not popular. When the comparatively rare HO and TPI Cars became available, the aftermarket companies would not justify developing new products developed for those packages, because the small stuff bolted right up and was (slightly) better than the stock components.

When you compare this to similar year Mustangs, if you bought a GT or an LX with a 5.O, it came with the same engine/induction system across the board and it did not change for years, thus all parts were developed for exactly that package. The Ford 5.0 bore/stroke was much more rev-happy than the SBC 5.0 as well, so just increasing airflow throughtout the system brought good results pretty easily. Thus by giving us so many options, designing an engine package for torque and mid-range, and creating a relatively scarce supply of higher performing engines, GM really did the aftermarket and the hot rodding community a disservice.

Blah blah, unless you want to pony up for Dyno Dons' larger headers and y-pipe, you will probably have to fab up your own system. Everyone I know making large power has done one or the other.

TA

SLEEPER 86

760 bucks for 1 3/4" headers and a 2 1/2" y pipe that needs welded and isn't coated? think i'll fab something.

Gumby

http://www.pontiacstreetperformance....p/exhaust.html

SLEEPER 86

Very interesting write up Gumby!
I saved it to my archives.

UnderCover89TBI

iTrader: (25)

for the record stepped headers do not start big then go small. They start smaller and go bigger.

TA

iTrader: (1)

You are absolutely right. I stand corrected.