Hi Tom,
Thanks for the response, but I still don't get it. I'll try to explain why.
on 3/28/00 11:19 PM, TC at [EMAIL PROTECTED] wrote:
> Sat,
> I am going to try to take this on because you make very good points
Aw shucks... (looks at feet - kick scuff)
> .... I hope this makes "some" sense. I was just at an exhaust coating
> site and of course one of the advantages they claim ... quote "The
> coating also contributes to faster exhaust flow and better cylinder
> scavenging."
I might actually buy this, but not because of the heat retention argument,
but due to the reduced frictional coefficient resulting from the INTERIOR
coating increasing velocity through reduction of boundary layer frictional
losses.
Of course keeping the heat out of the engine compartment
> is a biggie for them in regard to automotive applications as doing so
> contributes to lower intake temperatures etc ...... possibly an
> advantage for bikes too, (especially if you have body work), but thats
> another story ...
right
>
> However ... the claim for gains as a result of increased exhaust
> velocity is true. Headers that radiate less heat, (everything else
> being equal), will make more power .. because as exhaust gases cool
> they lose velocity.
I don't think I buy this. Any reduction in velocity due to reduced volume
should at least be offset by 1. concomitant increase in density which will
tend to retain velocity and 2. Reduced backpressure due to the
aforementioned reduction in volume.
Drawing exhaust into an area within the exhaust
> system that is rapidly condensing may produce a scavenging effect of
> it's own but sooner or later even the condensed gases have to exit the
> exhaust system ... and if they aren't doing anything anymore they
> usually have to be pushed out of the exhaust system.
But there are less of them (volume wise) to push. It's a winner either way.
The insulation
> helps move this "event" to the end of the exhaust system by delaying
> the temperature loss. Reducing the speed at which heat is lost
> maintains the velocity of the gases moving through the exhaust system.
This simply isn't true, for the reasons stated above. If you fill two
exhaust pipes with a balloon each, for example, which will be easier to
"blow out", the balloon filled with 1/2 cubic foot of air, or the one filled
with 1 cubic foot?
> An improvement of the velocity that the exhaust gases maintain,
> (everything else being equal), will produce an improvement in it's
> efficiency and the "increased scavenging" will occur as a result of
> this "increased velocity".
This is partially true, the "everything else being equal" is the sticky
part.
The heated gases that are expelled by the
> engine do represent energy and the insulation provides a means to
> utilize some of that energy as work that is beneficial to the engine.
Not so, unless you're using a turbocharger.
> Not to mention the fact that Kelly is much less likely to be burned as
> severely as he would using a pipe with poorer insulation properties.
This is definitely a good thing, although pipe/skin heat TRANSFER
characteristics are probably more significant than radiant. Kelly... want to
give us a comparison of how much it hurts holding your bare leg against 1. a
coated pipe and 2. a chrome pipe for 1 minute each?
> So .. just like intakes .. velocity is important and the "pulse of the
> adjacent cylinder" to which you refer does indeed help the next guy ..
> but only because it helps maintain the speed of the exhaust flow
> ........
right
>
> Would I lie ??
An old Turkish saying: "I trusted my father, and look what he did to my
mother."
> campbell
>
Perhaps the best refutation of the "heat retention is good" argument (other
than turbocharged, of course) is a practical one. If insulation really
increased exhaust scavenge you'd expect to see every formula 1 car with a
silica aerogel shroud around the exhaust system, or expect to see something
similar in the bike grand prix and superbike winner's circles with big
whooping fiberglass condoms around the pipes. Seen any lately?
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.
Best,
Sat Tara Khalsa
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