Hey Ace, before everyone starts hacking up their exhausts, how large a variance COULD there be in balancing exhaust length:absolute power?
What sort of loss (expressed in percentage) could you possibly be looking at?
I really do appreciate your ridiculously hard work on this project.
I realize that at the moment it's all a poker game and you can't be 100% on that figure, but it's that way even with the head at the moment (I'm really looking forward to a real world report on a 535).
Okay, let's back up a bit here, and start at the beginning.
First, I'm not recommending that everyone start hacking up their exhaust pipes. This is in the context of a performance build, not a stock GT.The stock GT cannot support a 6000 rpm hp peak, so there is no point in tuning the exhaust for an rpm range that the engine cannot possibly reach. I'm talking primarily to people using Ace heads here, which we are tuning for higher rpm range and different power curve entirely than stock.
Second, regarding the tuned length, every pipe has a tuned length. It just depends on what it is tuned to. So, it comes down to where you want to bolster the power curve with the reflected waves that the pipe is producing. Any random pipe will give you some wave results, but the idea of tuning it is to get the wave doing what you want it to do. So, your stock pipe, or your regular Carpy pipe, or your Hitchcock pipe, all are going to have some wave activity based on the length and diameter, and they are probably going to be similar, because they all bolt up about the same, and they all have similar length. It's designed around the stock bike. Cutting the header shorter on a stock engine may end up with just a loss of lower end torque, and no increase in higher rpm power because the engine can't reach up there, due to lack of breathing potential in the head and cams. So, we don't want to jump the shark. Don't cut anything until we know what to start doing for the engine set-up that we are working with, and we may not all be working with the same engines, especially when talking about stock head vs Ace head which will be very different. As a general "rule of thumb", the shorter you make the pipe, the higher the rpm that it is tuned to, and the longer the pipe, the lower the rpm that it is tuned to. But every pipe length has the tuning that is associated with its length, sort of like organ pipes with the big long bass pipes, and the short treble pipes.
Regarding silencers, the ones that tune best are open silencers without baffles, such as megaphones. Any baffle stacks in the silencer breaks up the wave action, and make it less useful, which might be a good thing, or a bad thing, depending on what you are aiming at. In a stock bike, it is probably not a bad thing to break up the waves to some degree, because the stock bike is aimed at a very broad powerband that doesn't have much peak to it. Tuning the exhaust wave tends to make a peak higher, but narrower. This is sort of shifting power from one part of the power curve to another, in order to get it where you want it.
So, let's start with the caveat that I'm giving information here based on potential users of the Ace Billet GT Head, and estimated tuning characteristics based on that. Stock GT bikes have different parameters, and different needs. Tuning them needs to be addressed based on the state of the mods on the given bike, and the rpm range that it will be accessing, and the aims of the user.
Regarding your question about "how much difference does it make", it's probably just a couple hp, but it matters where that couple hp happens, If we are hitting 98 mph at top speed, and that's all we can do, having that exhaust pipe tuned to the right spot to give us a couple extra ponies right at that rpm where we are hitting top speed can push us over The Ton, and we have seen exactly that in some previous Fireball applications. So, it's a matter of "how much" AND "where in the rpm range" we put it.