After market exhaust pipes

Date: Sat, 1 Mar 1997 18:30:20 -0800
From: Jim Davis
Subject: Exhaust systems

HI Peter--
As promised, a short diatribe on pipes. Motivated by TimJr@MVR's very
biased and spam filled post, but based on experience.
I sent this to John G. Jensen in Denmark, first.

There is NO pipe that will give you 10 HP on a bone stock HawkGT motor. I
have hawks from stock to 75+HP, I have about $2500+ in dyno time, and the
charts to show for it, and I have been collecting pipes to do a
semi-comprehensive test later this summer. I presently have the following
pipes to test: TBR large head pipe, TBR stepped head pipe, MVR stepped head
pipe, SuperTrapp, SuperTrapp with cannister muffler, D&D reverse megaphone,
Kerker reverse megaphone, Kerker White tip, and stock. I will be doing dyno
runs on 3 bikes in several configurations: stock, stock with airbox top off,
stock with K&N filters, stock motor with Megacycle 153X-1 cams (with same 3
airbox configs), +1 mm bore high compression motor with ported heads, big
valves, carillo rods, etc, with 174X-8 cams with stock carbs airbox top off
and no airbox at all, modified stock carbs same two airbox configurations,
and Keihin 39mm flatslides. You can see that there are a terrific number of
variables, and for each of these tests, the bike will have to be rejetted,
so the time invested will be a couple of very full days. I realize that
there are configurations that won't be tested, but I'm being as thorough as
I can afford to be.
The results that TimJr@MVR has been posting are not conclusive, nor do they
indicate that there is any sort of a level playing field. The MVR
advertisement also tells only information that could easily be not
altogether straightforward.
I have had MVR pipes on a couple of bikes, and had reasonable success with
them. I have also had LOTs of other pipes, with reasonable success. I have
some engineering background (BS and MS in engineering), and have done some
calculations on exhaust design, and 1 1/2 inch headpipe diameter (the size
of stock head pipes, and the SuperTrapp, and the starting 10" or so on the
MVR and the TBR stepped pipe design) is the size that keeps the velocity of
the exhaust gases up where it should be, and for a displacement up to at
least 700cc, I am extremely doubtful that there is any benefit from going to
a bigger pipe diameter. Stock Ducati 900ss motors have 1 1/2"
headpipes-with a single exhaust valve (like the hawk, but MUCH bigger), and
Ducati heads are a much better flowing design than hawks. If a big headpipe
system is put on a stock Ducati, low end and midrange performance actually
drops off, until the motor is heavily modified. It would be hard to
convince me that the hawk needs bigger pipes than a 900cc motor, that revs
to 9500 rpm. The same performance degradation in midrange holds true for
hawks, if the headpipe diameter gets bigger than 1 1/2", the exhaust gases
"stall", which is where the infamous 5000-5,500 rpm hole in the power comes
from. The stall of exhaust gases happens farther down the pipe with the
stepped header design, but still happens. You won't get that with the stock
exhaust or a SuperTrapp, and honestly, what rpm range do you ride in on the
street? If you're over 7000 rpm most of the time, and still have your
license, I'd recommend getting your butt to the racetrack post haste!
Besides the MVR has a ceramic coating that looks OK at first, and then
starts to look bad, and then starts to rust. At least the TBR is either
nickel plated, or Stainless now. By the way, Mike Velasco designed the TBR
pipe when he worked at TBR, and they let him go, for legitimate reasons. I
had a long talk with Craig Erion about this a year or so ago.
Gee, I seem to be long winded today.
catch you later,
jim