Don't let anyone fool you into believing that an engine "needs back pressure". This is simply a fallacy. There is a lot that comes into play when it comes to exhaust design, one of which is minimizing back pressure. If you take two exhausts of the same diameter and same length, but one with a baffle and one open, the open exhaust will outperform the baffled exhaust every time as long as the engine is jetted correctly.
The problem comes when people pull the baffles out of their pipe and cannot figure out how to jet the bike correctly. In this case the jetting ends up being wrong and the bike runs like crap with the open exhaust, and so people jump to the conclusion that the bike must need back pressure to run right. Sometimes a different needle taper is needed, which may not be available for purchase and has to be custom made. Sometimes other changes need to be made.
Now you do need to have a certain minimum length to your exhaust or you can have problems. Cutting them off too short can cause fresh air to pull back into the combustion chamber during valve overlap at low RPMs, which will lean out your a/f ratio. Stock CB750 cams have very little overlap though, so this is not a huge problem on these bikes as long as you are running the stock cams. Your header length (along with diameter) will change where in the RPM range the power curve lies. There are calculators out there than can help you estimate about how long (and how big around) you want your headers to be to have peak power at a certain RPM. Free flowing exhausts do not cause burned valves, lean jetting causes burned valves.
The biggest problem with open exhausts on the street is the noise.
Well thats great you have an opinion dude but lack of any back pressure {power pulses and how they work} will eventually damage the engine, will be harder to tune, if possible at all, it will create lean situations throughout the rev range which over time will burn valves, will lose low end power and will increase fuel consumption due to lack of scavenging.
Back Pressure in Motorcycles
A small amount of back pressure is crucial for optimal engine performance. Back pressure in the exhaust system pushes any unburned intake back into the engine. Without this back pressure, some intake is lost, leading to less than optimal fuel consumption.
Another benefit of exhaust baffles and the back pressure they create is the power they provide at lower RPMs. Some individuals choose to ride their bikes without any baffles in order to create the most amount of volume. While this certainly produces the loudest sounding bike possible, it also hinders the overall performance of the motorcycle. This is because the small amount of back pressure created by a baffle increases the horsepower of the bike. Without any back pressure, a motorcycle will not reach maximum efficiency until a much higher number of RPMs, which is not optimal for most street drivers.
We are talking 40 year old road bikes here yes...?
The OP asked about cutting his pipes off at the end of the header, my response was aimed at that.... Nearly everything you said related to older race bike theory, go look at a modern Moto GP pipe and report back...
It's nice that you can find a quote that supports your opinion "dude", but quotes only help if they are from a reputable source. Since you failed to mention where you pulled your quote from, I googled it. It looks like your quote has no reputable source, it is from an ebay buyer guide which appears to have many flaws.
I tend to go with the reputable names when it comes to things like this. In David Vizard's book
How To Build Horsepower Volume 1 on page 110 he says:
"Over the years, I've heard many strange claims from so-called experts. One I've heard more than once asserts that a performance engine- or any conventional four cylinder engine for that matter- needs some exhaust back pressure to optimize efficiency. This pronouncement was most certainly made by someone who had never used a dyno or carefully evaluated how exhaust system flow affects performance."
He then goes on to say:
"I have found that reducing back pressure always improves power and fuel economy, providing, of course, the air/fuel ratio and ignition timing are carefully optimized both before and after exhaust system back pressure is increased."
Now this is from a guy who is one of the most well known and well respected engine builders in the world.
A good performing exhaust system creates a scavenging effect with the negative pressure wave that follows every exhaust pulse. Back pressure is the exact opposite of this. You can never reach 100% volumetric efficiency for an engine if your exhaust has back pressure, and yet there are naturally aspirated engines around running over 115% volumetric efficiency.
The back pressure fallacy comes from old timers that had the "bigger is better" attitude and went for large diameter headers only to lose power. They thought it was because the larger headers didn't provide enough back pressure. however, the real issue was that when the exhaust diameter becomes too large you lose exhaust velocity, and when you lose exhaust velocity you lose the momentum that each exhaust pulse has. This momentum creates a negative pressure wave behind it (a vacuum). If the exhaust valve opens while this vacuum still exists in the pipe, the spent gases in the combustion chamber will actually start to be sucked out into the pipe. This is scavenging, and it is how an exhaust system makes power. Remember, a negative pressure wave is completely different from back pressure.
Please explain to me how a free flowing exhaust will damage valves if the jetting is correct? If this were true then all of the dirt bikes, trail bikes, race cars, and harley davidsons made in the last 50 years would have burned up valves. Free flowing exhaust will never damage valves, lean jetting due to poor tuning or too short of exhaust headers will. There is a difference.
The fact that these are 40 year old engines does not matter, this is basic exhaust physics, it was true in the 1960's and it is true today. These bikes are no different than any other engine out there in the world, despite what many people on this forum believe. Your moto GP analogy doesn't work either, as they run mufflers to meet the 130 dB sound limit in the rule book. If you want to look at the most technologically advanced form of racing, look at a Formula 1 exhaust system, you won't see any baffles there.
