question about headers...

[Hook]

So okie, this dude I kno does his own mods (cut cams, build headers, exhaust systems, machine cylinders, mill heads, blah blah blah) and he says he'll build me some intake and exhaust headers for free (my GF's sister's BF...don't ask) provided I supply the materials.
Now, he's been diggin' up in cars since before he started growin' hair in funny places and likin' girls so he knows his shit from experience and his mods aren't made or tuned to spec even tho it works out great for him(well, it's his car so he's free to re-tune and re-build whenever he wants).


I'm a stickler for specs so I wanna know if anyone knows how to calculate header dimensions (length, tube diameter, etc.)...also, I've seen 4-2-1 exhaust headers, some of which are joined 1-2 3-4, then there's 1-3 2-4 and then there's the 4-1 headers.
I'm looking for one to gimme a little more low end juice since I don't care too much for top end (I don't race but I'd prefer a little more pull when starting from zero).

Can anyone help a brotha out?

[alltrac165]

Well, typically for a road engine where midrange is important, you would want a 4-2-1 type header where 2 & 3 are connected and 1 & 4 are connected and then they join together. Dimensions are dependant on displacement, your cam and the rpm range you want to tune to, among other things. If you have a 2 liter 4 cylinder and your cam opens the exhaust at say 60* before tdc and you want to tune to 4K rpm then you would want a header with primary tubes that are 1 1/4 or 1 3/8" diameter by 16" long joining into secondary tubes that are 1 3/4" by 32" long that collect together into a 2 1/8 or 2 1/4" collector/tailpipe. That should give you a place to start testing for what works in your engine. Typically, increasing primary diameter will move your torque peak higher while decreasing will lower the peak. Increasing length will rock the power band down a bit relative to the torque peak, and shortening will increase power a bit up top. These are very general rules taken from a well known rally engine builder and every engine will see things differently. If you can tell me your displacement, exhaust valve opening point in degrees before tdc, and the rpm you want to tune to, I can calculate a starting point for you.