Datsun100ATurbo
You got it right. A Graham Bell has a talent for easy explanation of complex concepts with regard to performancee engines. Compulsory reading for any performance engine modifier/builder.

Feral is the Boss but don

What goes round comes round.

This site helps me too so have a beer for all of us.

feral you are a bloody legend, pm me you address and I will send you some beer! this man deserves a prize. You are an endless source of knowledge and its people like you who make this site one of the best clubs on the net.
Thankyou very much!
Quinn

The formula for a tuned length pipe is P=((850*ED)/rpm) -3 where ED is the degrees before bottom dead centre that the exhaust valve opens plus 180 degrees. (ref 4 stroke performance tuning by A Graham Bell pp112)

From the figures I have, with a cam opening 57 degrees BBDC the tuned length pipe for 4 into ones would be between 27.7 to 28.4 inches if the pipe was designed to work between maximum torque and maximum HP engine speed at say 6500 rpm.

For an interference system the primaries would be 16.5" in length and the secondaries 27.7 - 16.5 = 11.2" in length.

The pipe ID for the primaries is calculated from (((sqr root((cc per cylinder/(tuned length + 3) *25))) *2.1 that turns out to be 1.434" ID so an 1 5/8" primary would be best.

The secondary pipe diameter is ((sqr root(ID*ID *2)) * 0.93. This works out to be 1.886" ID for the secondaries, so 2" OD pipes are the go.

Hope this helps.

ok the headers are to go onto a a14 which will be heavilly modified, port, polish, BIG cam dubble valvesprings, large (as big as i can go) valves, modified rocker gear, twin 40s, shaved head and decked block (12 or 12.5:1) 1mm overbore, everything balanced lightened (not to light) and I want a set of headers to suit. I want lots of mid range pull and a good high end, I dont care about low range as it wont be driven on the road.

Quinn
Sorry my friend, but i can't help you with this one. Tuned length headers is a bit of a curly subject.

If someone has a formula, it would need to take into account the fairly narrow speed range that the "tuned" effect comes into play & this will be goverened by your induction ststem, your cam specs, & to a lesser degree, any other mods, including the rest of the exhaust. The actual length will vary with the diameter of the tubes. Some measure of final tuning can be achieved with adjustments to the lemgth of the collector, & best results are achieved with an open exhaust. [Nothing after the collector] & a dyno is needed to tune it that fine.

If memory serves, it's shorter & or fatter pipes for higher RPM power, & longer & or thinner ones for power a bit lower in the rev range with a flatter torque curve. If this is to be a street car, then custom built tuned length headers may be a bit of overkill with benefits that are somewhat muted by an exhaust with a muffler.
If you are flat out racing, then this is the way to go.

I really wish i could be of more help.
Chris

heres a pic of what i want
http://datsun1200.com/modules/myalbum/photo.php?lid=3428
I can get them for cost of the metal.

ive got a set of mild steel headers but I really want to do it properly this time. I want the big sausage headers that all twist arround each other I want PROPER tuned length headers. Does anyone know the actual numbers, feral? dodgeman?

I got a set of macdaddy coby ones for $130, they arent stainless but it beats all the shagging around building them.