Subject: RE: To crossover or not to crossover???
Author: Chuck Sherman
Date: Mar 30, 2005, 6:50 PM
Post ID: 1718624244
I know something about this; was involved in fluid dynamics and got a
patent or two in a former life...
The function of the crossover is to allow the exhaust pulse to be shared
by both pipes - thus reducing the overall pressure drop of the system.
In addition, the crossover (often called an H-pipe) allows the exhaust
pulse from one cylinder to pull, or scavenge, the spent gases from the
other cylinder - improving volumetric efficiency and HP output.
Crossovers are good for a 5% improvement in HP on the average and have a
significant effect on the area under the HP curve - especially in the
midrange. Generally speaking, the closer the crossover is to the
exhaust port, the better the effect of said scavenging, which is why
some Yamaha twins had it more or less 'cast-in' to the exhaust system at
the factory.
Seat of the pants dynos are notoriously biased. To prove this, a friend
of mine with a portable dyno ran a test with a two stroke quad - had
guys run it with the muff and stinger - then pulled the stinger and had
them run it again. 100% of the riders said the quad ran better sans
muff and stinger, but the dyno proved otherwise. the stinger actually
adds HP to a 2-stroker; removing it limits the ability of the expansion
chamber to scavenge properly.
Finally - there are limits to how much pressure drop can be removed from
an exhaust system. Remove too much, and you will lose midrange torque -
honest. In modern engines, low tension rings rely on 'backpressure' to
retain ring seal - lose this and the engine starts to burn oil. I've
gone too far on backpressure reduction on my wife's PT - even though it
runs like it has a turbo, it burns a quart of oil every 1000 miles.
Small price to pay for smokin' SUV's, sez I...
dave luby wrote:
Cam,I really don't have a good technical answer for your question.BMW put a crossover on their boxer twins in the 70s.(not sure of the year)I use to have a 75 Sporty that had crossover for the first year.The HD 78 XLCR used a siamese exhaust(crossover in sense)that they claimed really helped performance?Now if you want to go way back,57 Chevys had a crossover on the power pack 283. So maybe somebody who really knows performance exhaust can answer this? Dave
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