Hot Rod Forum banner
1 - 6 of 6 Posts

·
Registered
Joined
·
671 Posts
Discussion Starter · #1 ·
Hi all- getting ready to install my 383.. im going to be running 2-1/2 pipes mandrel bent (self weld package I bought for 140 bucks)..

Currently my headers are 2-1/2 long tubes with a very short (and pathetic) collector length (4-5") - im going to use the 20 inch collector approach as its been almost proven to be superior to short collector lengths -

Now since im running a 2-1/2 system.. how does the collector sizing work since the diameter is the same?

Should I run the headers straight into a 3 inch collector and taper it back down to the 2-1/2? I always presumed the collector for primaries should be larger than the exhaust system?

Thats why I want to run the primaries into a 3" 20" long collector and then taper it near the end back to 2-1/2..

Or is someone here going to mess with me and tell me to go 3" now :/
 

·
Registered
Joined
·
15,298 Posts
The primaries are the tubes off the flange to the collector. At 500 horsepower they will need to be at least 1-3/4qtr inch and can go as much as 1-7/8ths. Step tubes in this range are also fine. Given the need to package those tubes at the collector it sounds like the collector need to be 3-1/2 to 4 inches of inside diameter, for the street perhaps 3 inches with some stretch to accommodate the primary’s will work best. After all when you get rid of the nonesense that pervades header theory at the core it’s a syphon tube and if this is not running at redline all the time the smaller collector will speed the mass flow which will pull harder on the primaries. Collector length is a variable on what the engine likes, 20 inches is a common minimum but this can be as much as 48 inches. For a street engine a gradual taper cone to your 2-1/2 pipe will be best which for a 500 horse engine probably should be 3 inches. The collector should be bigger than the exhaust pipes otherwise they are just an abnormally long collector, that said some engines respond to a 4 foot collector. Another common way of terminating the collector in terms of length is with a cross over. Space being what it is under the vehicle this may force the use of a H pipe immediately after the collector. Your going to have to get under the machine with a tape measure to see what you have space for, including road clearance.

500 horsepower is moving a lot of exhaust, the system sizes to the engine power more than its displacement. The reason is power depends on the volume of mixture flowing through the engine more than the displacement. So 100 or 300 or 500 horsepower requires an X weight of constituents which has a volume that is unrelated to displacement. So the exhaust system for a 500 horse 383 needs be sized as it would be for a 500 horsepower 454.

Bogie
 

·
Registered
Joined
·
671 Posts
Discussion Starter · #3 ·
thanks bogie - well honestly 500 max, but ill be there at maybe 10% of the time anyway.. mostly between 2500 and 5500 or 6k - just a red light to red light car..

I have my eye on a set of 1-3/4, but would like to start smaller and go from there.. I think a 3" 20" collector tapered down to the 2-1/2 will be my final exhaust setup - I might use a slip connector up front in case I change headers to 1-2/4 or bigger - that way I can slide them in without rewelding - until I settle on a nice medium setup.

Too many variables in the game but at least I have an idea of how to approach it - and go by real world results :)

Thanks again!
 

·
Race it, Don't rice it!
Joined
·
8,813 Posts
Most legit header companies will spec 500hp as needing 1 5/8 primary on a primarily street driven car.
With a cheap header design, you may want to go up in primary size to compensate for the poorer flow design.
I'd personally pick a stepped header.
 

·
Registered
Joined
·
671 Posts
Discussion Starter · #5 ·
thanks - well was planning on using the 1-5/8s and see how it runs.. might strike the best balance for a street vic jr- i generally try follow vizards advice for general collector size/length setup - previously had 2-1/4 on my 400hp 350 and it was rippin fast.. going 2-1/2 for the 383 now with a mandrel bent kit..

Im wondering why/where anyone would use those (stupid?) 180 degree bends? wouldnt 3 45's be better over the rear axle than such a tight bend?
 
1 - 6 of 6 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top