Hi Kelly,
How's the leg test going?
on 3/30/00 11:20 AM, Kelly Cash at [EMAIL PROTECTED] wrote:
>> ...Don't get me wrong, I think coated pipes are preferable to
>> chrome ones due to the frictional argument mentioned earlier,
>> and increased corrosion resistance. I just think the scavenge
>> due to heat retention argument doesn't make any sense.
>
> Well, I'm not a flow engineer. I was basically quoting
> the info from the coating companies. But I've got some experience
> with auto/bike mechanics, and how they explain it seems plausible
> to me. I also know that some back pressure is GOOD.
Umm well, are you sure? Why is that? I agree that in some cases, in the real
world, engine designers design valve timing assuming some back pressure, and
that removing that back pressure can result in poor performance
(particularly for mileage) due to over scavenging and, in effect, moving the
fuel/air mixture through the cylinder rather than having the close of the
exhaust valve perfectly coincide with the arrival of the mix in the
cylinder. This results in loads of unburned hydrocarbons in the exhaust mix.
But that's not because back pressure is good, just that engines are designed
for less than perfect conditions. If you remove the back pressure and alter
the valve timing and intake parameters to match, the volumetric efficiency
of the engine is increased, and the engine makes more power. Motors are just
big gas powered air pumps, after all.
But, more to the point, scavenge is the RPM-dependent science of using the
timing of exhaust pulses from one cylinder to, in effect, suck the exhaust
from another cylinder. That's why an engine with headers can, in most
circumstances, outperform individual pipes. But it's not because
backpressure is good. Scavenge is NEGATIVE backpressure.
If I'm
> reading your posts correctly you're saying that the more you
> reduce back pressure, the better it'll run. Not so.
That's not exactly what I'm saying. I'm just saying that I think increasing
scavenge by insulating the header pipes is a nutty idea, and that increasing
the volumetric efficiency of an engine is always a good thing.
(which is
> evidenced by the fact that the 18" can on a Hindle slightly
> outperforms the 16" can)
Are you sure that the TBR 18" can outperforms a TBR 16" can because it
produces higher backpressure? I doubt it.
>
snip
I do know
> that the Airborn and JetHot people claim about a 1.5 horsepower
> gain after coating the pipes. Considering what we saw in Laughlin
> at the dynos, I wouldn't argue with it.
As I mentioned, I'm not arguing with a horsepower gain after coating. I just
doubt the insulation theory. My own guess is that reduction in
boundary-layer frictional losses is more important. Also, I wonder what the
nature of the horsepower gain is. Is it an across-the-board, good for every
RPM gain, or a peak power gain w/o changing the RPM peak? Or is it a gain at
say 5000 rpm and a drop at peak? Or a gain at peak and dip elsewhere?
Inquiring minds want to know.
>
> Back to the sidelines,
>
> -K
>
Best,
Sat Tara
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