Tony Robinson wrote:
>
> Bombur@snet.net wrote:
> >
> > Having done some performance related engine modifications I am considering a
> > header to squeeze some more HP out.
Peter, what you do with the exhaust depends in part on what you've done
with the rest of the engine, and in what sort of performance range you
intend to operate. In the same way that a camshaft is ground to produce
power within a certain rpm range, tuned pipes are (or can be made to be)
rpm specific.
> > 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.
True. Sort of. There are two main concerns in tuning pipes--sound wave
theory and gas speed. Lengths, arrangement and diameter of the tuned
pipes can affect both. But so can intake length, compression and cam
timing. Tony R. is absolutely right in that regard.
>> 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.
Okay, the firing order of a typical six-cylinder is 1-5-3-6-2-4. The
TR6 engine is typical in that regard. There are two branches on three
cylinders each, #s 1,2,3 into one collector, 4,5,6 into another. Exhaust
valve of #1 opens, gas stream goes down the first pipe. Next to open is
#5, goes to second collector. The next valve to open is #3, to the first
collector... get the general picture? Each pipe dumps into a collector
180 degrees apart from the other... this keeps those streams separate
and apart from each other. This pipe arrangement only gets to be a
problem if the duration of the cam exceeds about 280 degrees... at that
point, gas streams start to go back up pipes they shouldn't. If you've
got a cam that broad, you aren't heading for the street.
> > Can anyone enlighten me?
If that doesn't help much (it might not...), look for this book at your
local library, or through inter-library loan:
_Scientific Design of Exhaust and Intake Systems_, by Philip Smith and
John Morrison, 3rd edition, published by Robert Bentley Press,
Cambridge, MA.
Lots of good basics in it.
Cheers, Peter.
--
My other Triumph doesn't run, either....
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