> Just a quick thought. The biggest header primary pipes don't always
> equal more power. Varying diameters of primary pipes can change the
> location (within the rev range) and size of the power/torque curve.
> The biggest diameter header pipes may not always be the best for your
> needs. At least, that's what I understand is the case with naturally
> aspirated engines. Does this change when forced induction is added?
>
> Tim Holbrook
> 1971 TR6
Yes. I followed your thinking, and sought previously a fast road header
(Triumph Tune Extractor). However, once reading what Ganahl's book and other
references have noted that a supercharged/turbocharged motor should use the
largest primary pipes possible. Exerpt from Ganahl's book below...
"Supercharged motors do not like backpressure. It's very simple. If you are
pumping a greater quantity of air into the engine on the intake side, you've got
a greater quantity that has to get out the exhaust side and you don't want the
plumbing backing up (duh! - me)." ... "A supercharged engine picks up
significant power when it is uncorked."
"For the street, however, the object i to keep the exhaust system as unreliable
as possible. A set of headers would be a good choice, especially on
higher-boost blown motors. In the case of a supercharged engine, though,
typical header "tuning" theory does not apply. In other words, the length,
diameter, and collector style of the headers is of little consequence. The rule
of thumb for blown motors is to use big tubing, since exhaust-gas temperature is
higher in a blown motor and it will expand more..."
Ganahl goes on to say that the rest of the exhaust system should consist of 2.5"
tubing all the way through... in one of the earlier chapters on supercharging
theory and fact, he mentions as the cylinder pressure (intake stroke) of a
supercharged motor is much higher than on naturally aspirated motor, that the
pressure on the exhaust stroke is also proportional (ie. also very high). The
more exhaust you still have in the combustion chamber, as the next intake stroke
begins, the less fuel/air mixture you'll be able to force in there.
The Kirk Racing header uses primary pipes only marginally larger than the other
ones, I think it is 2 3/8" for the Kirk versus 2 1/4" for the Triumph Tune
headers. Both are similar in design... the Kirk header also being the cheaper
of the two, and I think ceramically coated right from Kirk.
Makes sense? Kinda? Well whatever, I made my decisions before based on common
sense regarding supercharging the TR6 motor... fortunately Ganahl and other seem
to back up my thinking, and in other areas they have made me aware of things
I've overlooked.
We'll find out this spring how the whole set-up works!
Kai
P.S.: Those of you wondering more about this, keep an eye out for another post
in a few days...
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