Well exhaust sizes have been discussed over and over on here but i couldnt find anything on extractors, anything usefull anyway. What brands, primary/secondary sizes etc are best.

Ive been planning to use genie 4into 1s that i have for my race a15 (will be under 8000rpm). Will these flow enough? I can only assume they would.

Will this be an open exhaust ?

good question i've thought about it before but just figured you're limited by what the manufacturers sell .... unless you've got the $$ to get a custom set made up
not hijacking the thread but maybe helping by chasing a little more info ..........

what benefits do you gain from 4-1 or 4-2-1 extractors ??

in 4-2-1 sets how do primary and secondary tubes lengths and dia's vary the performance ??

and ... in using the orig 'GX' exhaust manifold what secondary tube diameter and lengths would be beneficial ??? and any idea on what the original ones were ???

any input would be fantastic

No worries boofhead im interested to know aswell.

As i understand it
4-1 equals better high end flow
4-2-1 equals more midrange torque

In saying that im sure it could all be varied once again by primary and secondary sizes.


Exhaust will be muffled( slightly too loud without) Dumped before the diff.

You want to use 4-1 matched length.

You do the length on where you want the power.It hard to say with knowing the engine specs.

you also knowing the CFM of your ports helps(exhaust flow)

Not saying i am that up to date with the knowledge of gas flow/exahust things but last year my metal work teacher taught me alot about gas flow and sizes of exhaust gas in relation to the size of the engine so ill give it a shot!

What he told me was that the size of the ports, valves and total capacity of the engine has alot to do with the diameter of the primaries/secondaries of the extractors.
Of course tuned length extractors are ideal as then each cylinder or exhaust tract has the same length to travel and same gas scavenging (that the word?)affect and also does not have a difference in pressure.
To know how long each primary should be it all depends on space but also the engines performance. I think he mentioned that professionaly high performance engine builders run the engine with the tube attached and when the pipes turn a different colour that is when they know to chuck a bend or widen the pipe. Dont quote me on this as its been a while since i talked to him and he tended to exagerate a fair bit!

Gas supposedly travels on the outside of the wall so some indentation of press bent extractors aint a big deal but of course mandrel/sand bent extractors are ideal but only when trying to scrape everylast hp out of the particular engine.

When i was desgining my set, we came up with the figures that the primaries should be 32mm and secondaries (as i was going with 4-2-1's) should be round the 39 - 40mm mark for a nice set. This was on a std A15 head and even these are pretty big! he said that this set of extractors should provide enough flow for up to a 150hp engine as the bends came out pretty nicley & flow well.
A set of 4-1's should probably just use the 32mm or 34mm primaries still and say a 2" collector. These will provide enough flow for a nice engine too.

A wall of about 1.2 - 1.6mm is ideal for sand bending (which i had a go at) so im guessing that it should be fine even a bit thick for pressbending as it was really hard to not put dents or fracture the pipe at the tempuratures that was needed to bend the pipe round the mandrel i made. I got as far as making all primaries but not having enough time to finish them off

Thats about all i can think of atm!

Cheers
Dave

Yeah its all about getting the headers to the specs of your engine.

Quote:

If only it was that simple - Unfortunately gas flow at the wall = zero = drag.

Every "off the shelf" item will be some kind of compromise and purpose built items are very expensive because they have to be manufactured and tested then usually remanufactured to suit the individual engine characteristics - only really practicle for big budget teams