Hey, lets see if we can inject some technical discussion here. :-)
I've been meaning to ask about this for a little while, and Tim Hawbaker's
recent post reminded me of it...
What I would like to know about is exhaust backpressure and just general
exhaust design as it relates to horsepower and torque. As Tim mentioned in his
post, he has heard from people (and we have all heard this) that some
backpressure is needed or you will lose power. I have especially heard this in
relation to the V6 and there are 6ers on the list who (I think) have
experienced losses from a 3" exhaust which was gained back when they went to a
2.25-2.5". We have also heard about the folks who have installed headers and
driven to the exhaust shop basically with open headers (or at least an open Y
pipe) and have said the truck ran like a dog.
I have several of the SA Design books, and in the "How to Build Horsepower"
series by David Vizard, there is a section on exhaust design. Vizard
specifically mentions how people say that an exhaust should have some
backpressure and he goes on to explain in no uncertain terms that this is pure
BS. According to his personal experiences (thousands of dyno runs), decreasing
exhaust backpressure also increased horsepower (99.9916030267125f the time); something
along those lines. Now, he DOES say that just changing a component CAN lead to
power loss. For example, changing to a low restriction muffler without a
corresponding adjustment in the fuel or timing curve could cause a loss of
power. But its not because of less backpressure, its because the system is no
longer tuned as a whole. Unfortunately, in the entire exhaust section, I don't
see any references to torque, just HP. Perhaps free flow does increase HP but
at the expense of low end torque? If so, this is something that the haulers
and offroaders need to know about. As we all know, power is more than just a
peak HP number; you also need to consider torque, and not just as a peak
number, they need to be considered as a band.
I guess the least backpressure would be had by open headers. The musclecar
and hot rod crowd will always "uncork the headers" to get the best times at the
strip. If this didn't work, I suspect they'd stop doing it. :-) However, as
I mentioned before, we have personal experiences from folks on the list that
show open headers on our trucks really drains the power. Is this merely
because the system is no longer tuned as a whole, or is it really decreasing
the power potential?
Related to this would be the whole "exhaust tuning" area. Carefully measured
full length headers, carefully placed H & X pipes, tri-y headers, etc.
Obviously there are things you can do to an exhaust system beyond the basics
such as single or duals and pipe size. These items obviously also have an
effect on performance. Unfortunately, except for the few formulas available
from the usual sources, this whole area appears to be a black art.
So... Does anyone out there have any experience in this field? Any
theories? I think it would be really cool if we could shed some light on this
whole situation.
---Jon-
.---- Jon Steiger ----- jon@dakota-truck.net or jon@twistedbits.net ------. | Affiliations: AOPA, DoD, EAA, NMA, NRA, SPA, USUA; Rec & UL Pilot - SEL | | '92 Ram 150 4x4 V8, '96 Dakota V8, '96 Intruder 1400, '96 FireFly 447 | `------------------------------ http://www.cs.fredonia.edu/~stei0302/ ----'
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