To: TRBILBO@aol.com triumphs@autox.team.net
Subject: Re: Re: TR6 exhaust manifold
You make some interesting points, but I've seen various information
that indicates the opposite conclusion. Biggest item is are we talking
a race car running a straight 2.5" pipe with no muffler at 5-7,000 RPM
or a street/autocross car?
*******THIS IS MEANT FOR DISCUSSION ONLY, DOES ANYONE HAVE FACTS?*******
Does anyone have actual facts for street usage? Something like:
1. backpressure with stock exhaust, single
2. backpressure with stock exhaust, dual
3. backpressure with Monza exhaust (again with crossover)
(show me the numbers, Monza vs. stock, is Monza false advertising?)
4. backpressure with custom exhaust, TRF, etc. (again with crossover)
I was amazed at the number of opinions and the total lack of facts,
i.e. "well, my grandmother's TR6..."
What follows is the results of my research, not much...
>factory HP rating actually went down 4 HP in the '72 and later dual pipes
Believe this is the cam and compression drop, plus new pollution junk.
My understanding was the dual exhaust allowed them to loose ONLY 4HP,
with single exhaust it would be more.
>The dual pipe system does NOT increase horsepower.
What we're looking at is backpressure, forget HP, we're trying to
eliminate backpressure and we'll eliminate HP loss.
Ideal BP being zero, 2.2 psi at WOT trottle being acceptable on the street.
One interesting aside is the pipes themselves. Most of my cars have
replacement pipes of dubious brands and some seem to crimp to about
1" ID at the bends. The Falcon dual SS pipes are a full 1.5" ID the entire
length. The stock single steel is 1.875" ID.
Using Pi*R**2 we get Single is 2.76 sq in versus 3.53 sq in for the dual.
Noting that dual 2.25" ID is an ideal minimum for a 350 cu in, perfect
for a 302 at 7,000 RPM. Note that a 302 is 5.0 L, twice a 2.5 L, so
it follows that for street usage a single 2.25 ID or 3.98 sq in presents
zero backpressure at 7,000, then at 6,000, 3.41 sq in are enough.
(above gleamed from several hot rod books on street exhaust design)
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This lead me to the conclusion that the Falcon SS dual pipes with 1.5" ID
are sufficent for 6,210 RPM on a 2.5 L engine with no noticable backpressure.
This would indicate the stock single pipe starts bogging the engine about
4,855 RPM.
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>The area of the single pipe presents less wall drag (backpressure)
My research on exhaust theory points to backpressure being the result of:
(in order of importance)
1. square inches of area in the pipes
2. muffler design and crossover's in the case of multiple pipes
3. bends in the exhaust flow
4. wall drag
>get mixed as one pipe at the muffler baffle anyway
For optimium street exhaust you want low backpressure muffler and
a cross-over pipe prior to them, so the different cylinders can
"share" the backpressure. The stock muffler design does this, i.e.
this appears to be an excellent design. This crossover also quiets
the engine, hence a win-win. None of the "low backpressure" sport
exhausts except my old ANSI has a crossover pipe, seems this is an
obvious lacking item that would qualify this as non-HP.
The only negative is the dual 90 degree bend in exhaust flow.
Now for the kicker, if you'll look at the Falcon muffler and read
the ad, you'll see it was designed for "stock-like" backpressure.
How do I pull the "plug" out of this 50 lb boat anchor? While the
basic configuration seems fine, its "corked" up to some extent,
ON PURPOSE? Anyone cut one of these open, know the Falcon rep's,
know what the stock ones look like inside as these are supposed to
be copies... Let me know where to pop those extra 1/2" holes!
The more you hear, the less you know, who has the numbers!!!!!
Roger
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