jjwithers wrote:While we are on the topic of mufflers, i am thinking of adapting a set of mufflers to a /5 engine from an old BMW R12 (from the 40s). It will be more of a stylistic decision vs anything else.
Will the difference of back pressure be a cause for problems when it comes to tuning a /5 motorcycle with pipes meant for a smaller engine?
I can see it adversely affecting the horsepower a bit...?
Thanks
-josh
Theoretically, given that the
ports and pipes match, and all else being equal,
a smaller diameter pipe (within defined practical limits) will increase exhaust velocity,
the engine will begin producing more torque at a lower RPM,
and the torque peak RPM may be lowered somewhat.
Conversely (as torque and HP delivery are related) HP, or rather, top end speed may suffer
by virtue of the volume of gasses exceeding the capability of the pipe to carry them
and the possible adverse effects this situation will have upon exhaust scavenging.
These statements grossly oversimplify a very complex situation
and should not be considered as a replacement for testing –
Exhaust system changes need to be carefully calculated
and then objectively quantified with comparative dyno testing.
The term "Back-pressure" will frequently be misunderstood in any real productive / performance sense.
The "Crossover pipe" is intended primarily to modify the torque delivery curve and torque peak delivery RPM,
i.e. provide more torque at a lower RPM to enhance drivability,
and place the torque peak at a reasonable and usable RPM for street driving.
Later dual-crossovers were added with essentially the same intentions
and with the purpose of compensating for other engine changes (necessary to satisfy the EPA)
while retaining consumer performance expectations.
A piece of hose connected between the vacuum ports on the intakes
will do an effective job of "balancing" (vacuum) differences between cylinders.
