Thread: headers

guys, i going to build a new header for my race car which has a ve in it, just trying to decide on the primary runner size. what are your thoughts? i was thinking 1 1/2" 4 into 1 then into a 2 1/2" then to a straight throught muffler

2.5 to 4 to 1. Then 3 inch straight pipe

^what?
Are you planning on doing straight primarie tubes or stepped?
I would go 1.75" primaries about 22-23" long 4-1 with a good 15deg merge collector with a 2.25" choke and a 7deg taper on the megaphone out to 3" exhaust. If stepped headers i would probably do 1.63" stepped to 1.75" on a 2L although a lot of guys have run 1.75" stepped to 1.88" with good succcess.
The important part is the choke point, dont be tempted to go too big. you can loose power in a big way, if in doubt err on the smaller side.
Thats my 2c anyway. There is a few guys on here who have done quite a bit of exhaust testing, they will no doubt pop in shortly and give some feed back

Stepped. What type of power do you plan on making

Nick knows what's up. He's read all the threads twice and built his own iirc.

How do you measure primary lenght? From the head/or flande to a collector star or collector slip in?

flange to collector

ok so 1 3/4" primary about 23 inch in length, into a 4-1 then to 2.5" inch as i only have one muffler as this a race car, and limited to space. but also have a noise limit.

ok 2.5 " will be ok, just dont go straight too 2.5 from the collector you will loose power. 2.25" at the collector then taper out to 2.5" piping.
I know 1/4" doesnt seem like much but im serious!
One of the guys on here did a header test with about 4 different headers, One header was exactly the same except the choke point of the collector was 2.25" and the other was 2.38" and the 2.38" lost about 6hp peak from memory and a decent chunk of midrange!
The specs I gave you were based on a n1 cammed 2L revving to about 8-8.5k If you reckon you will spend more time in the midrange and want things to happen a little lower maybe go for around 25-26" and maybe aim to drop the choke to 2.125-2.25" for a bit more midrange.
I guess being an offroad racer you would like quite a bit of midrange?

yes correct mid range from say 4000-8500 would be good. as this is where most of the time is spent mainly in the higher rpm of coarse. but this will be a n1 camed engine as i just picked a set up dam cheap. so yeah guess i looking for the best exhuast setup. we do have some noise restrictions. i all ready have a new 2 1/2" muffler so use that. also dont need to worry about cats. but total exhuast sytem length is max 1mtr i would reckon