Theoretical engineering states that with small-diameter, long pipes you
concentrate hp & torque peaks lower in the powerband - this is ideal for
typical daily driver or truck use. Large-diameter short pipes will get you
higher levels (peaks) of hp and torque, but those peaks will come higher in
the powerband - this is ideal for street/strip-racers, drag racers, big-oval
racers.
These concepts hold true for both intake and exhaust. This is why the older
carbed cars would run better with a carb that was a little too small than
one that was too big. The too-big carb would cause the car to bog off the
line, but it would run like a raped ape at the tail-end of the quarter.
In general, lighter vehicles can get away with using larger-diameter pipe on
the street. Heavier vehicles should stick with smaller pipes so that they
can move out easier. OTOH, if you're not concerned with driveability or
mpg, you can use large pipes and launch at high rpms to have a fast vehicle.
Keep in mind that this is theoretical engineering relying on physics. All
engines are different and have their own breathing personalities (based on
cam profile, timing, etc.) so you may find that certain applications respond
to mods to different degrees. This is further confounded by the engine /
vehicle combination.
There are some things you can do to enhance flow without resorting to larger
pipe. Two things that come to mind are 1) always use MANDREL bends in the
pipe, and 2) use as few of bends as possible in the layout and keep those
bends at a large radius (you want to keep the flow in as straight a line as
possible).
Hope this helps.
-----Original Message-----
From: BRAD BAKER <BEASTYTHANG@webtv.net>
To: dakota-truck@buffnet.net <dakota-truck@buffnet.net>
Date: Monday, March 01, 1999 9:08 PM
Subject: DML: RE Guttless 318
The truck still has the 2 1/2" section from the Y-pipe to the stock
2 1/2" cat. I was going to go to a 3" cat and replace that section with
3" pipe. You think that's a bad move I take it. Sure don't want to lose
any more bottom end. I noticed only positive results with the 3"
cat-back and major improvement from 1500 rpm all the way up with the
headers. I kmow the 31's are not helping any, with the already too-high
gear set up; but they didn't slow me down that much. I stil suspect the
omni-pak is part of the low end problem. It runs like it has a flat cam,
lack of power-poor throttle response-picks up rpm very slowly when
revving in neutral. stalls/stumbles if nailed hard and fast.
Thanks Brad
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