> Having done some performance related engine modifications I am considering a
> header to squeeze some more HP out. I once read that the only effective
> headers are those which are harmonically tuned to the exhaust pressure
> waves. If correctly tuned they take advantage of the low pressure areas
> between the high pressure waves to greatly enhance engine breathing. In
> looking at TRF catalogue, I see that the headers listed have the front three
> cylinders flowing together, and the rear three likewise. Given the firing
> sequence of the 2.5L I can't see how such a header design would do much.
> Can anyone enlighten me?
>
> Peter Firla
> 74 TR-6
Yes, tuned length makes a good bit of difference. However, you
generally don't see them tuned like this, so you don't know what you are
or are not getting. Lemme see if I can make some sense out of it for
you.
On an individual pipe you have a standing wave frequency. This
fundamental standing wave will give you the strongest "tune". You of
course have others, your second octave, third and so forth. However,
you promptly start getting into all sorts of complex acustical problems,
especially with fifths, and the whole thing just becomes a mess.
Headers work with this, or try to if they can. Ideally you'd want that
fundamental wave, and you'd want it tuned to the rpm where you want the
power increase maximized. Problem with a street car is you'd need a
header that ended out at your rear axle roughly, which has all sorts of
practical problems of course. The higher the rpm range you want, the
shorter the tuned pipe length you need. Which is the fundamental reason
why when kids you out and buy a short pipe header for their street car
they often end up with less power then when they started. The
interferences wreak havoc on the system.
So what's a tuner to do? Use fractional waves. That 2nd or third
octave. Which is why when you look at header flow rates and power
curves after installation you see an oscilating line. That's where it's
hitting those various octaves, and a few other acoustical mysteries.
Now, This is the way for the bigest bang for the buck, but it's very
peaky. Works better then the rest, but in a very narrow range. The
classic 4-1 header personality. Hence, the 'tri-y" headers. They use
two different header lengths, giving far wider ranges, but not quite as
clear a "tune." They can also be far shorter then a straight 4-1 header
for the same range. Now, what you want to be carefull of here is your
pipe firing orders. You want them to come into the collector on equally
spaced pulses, not lumps.
For example blat...blat...blat... is good
blatblat......blat... is not good.
So check your firing order and compare it against the header collection
system. It's not critical, but it very much makes a tuning difference.
So can headers work at all on the street? Of course! For often times,
even untuned, they simply flow better then the factory manifold, and
that itself is a benefit.
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