info on building a performance exhaust?

I'm going to attempt to build a custom 2-1 or 2-2 exhaust for my 750twin project, and have been looking for some reading on how to design an exhaust. I think i have the basics figured out on how to construct it, but i am looking for some more scientific info on how to determine the optimal diameter of the headers, collector size( if 2-1),muffler length, and so on. i would like to either maintain stock performance or increase it slightly if possible. Can anyone point me towards something to educate myself?

Heres something to get your teeth into;
www.epi-eng.com/piston_engine_technology...ystem_technology.htm
Good luck with it.

Here is a link for you Doc. www.coneeng.com/designing_motorcycle_header.html

heavy stuff in that first article. Since the motor will remain stock for the moment my plan right now is to use the dimensions of the stock header as the basis, but maybe go with a 2-1. From what I have read elsewhere, and what was reiterated in the first article, to help scavenging the exhaust needs a step in area before exiting. The sudden change form the primary to the collector creates a reflection pulse back to the valve. The stock headers do this with a crossover. If i design it so the primaries are separate from the collector and muffler, components could effectively be swapped out to tune the exhaust, without a complete redesign. I image getting the collector size and length is going to be crucial here and trial and error will be the route. I do love the way a 2-2 looks though so maybe i will go with a crossover? Need to do some more research...

Seems like somebody else around here went through this on a 900. OMR maybe? You might try searching the old threads.

Vic,
These are the mufflers I will be using in my system. 1 1/2 in. ID from Comando Specialties. They are about $200 delivered. I want to try a pair of anti reversion inserts, the XS650 builders use. I will be shooting for a header length of 29 inches with a crossover in the stock location. I am doing one side in mild steel to develop the form, then I'll do a complete system in stainless.
I think there is a substantial performance gain to be had just by using a large volume pair of free flowing mufflers. I may try some additional crossover ideas if I can. I have a good dyno shop here in Cincinnati and plan to use them to optimize
the overall performance when I'm done fabricating.
Please keep us up to date on what you are doing for your exhaust, this is interesting!

Good to know a smart guy like you is tackling the same problem Jon. Are you planning any engine/carb mods? I had never heard of those anti-reversion ports before, good info. I found this interesting article on designing a custom XS650 exhaust with reversion inserts.
www.xs650.org.au/Technical%20Info/exhaust_port.htm

I would imagine that this info is very practical for the kz since the two bikes are similar. The guy in the article they talk to, Mike Morse, owns Vintage Brake and i talked to him extensively about my one-off brake setup for my KZ1000; he is a very friendly, knowledgable guy. I will definitely give him a ring and pick his brain about this sometime. If you make some anti-reversion inserts i happen to know a guy that would be interested in a pair.

I was looking at the 12" or 18" straight core from Cone Engineering. depending on how i'm doing financially I would love to get something exotic like a Racefit muffler.

this is a pipe designed for 750cc XS650. I would assume this would be a bit large for a street bike, as it was designed for his modified race bike, but a good start.

I have also spoken to Mike Morse, a friend of mine is a successful vintage racer and Vintage Brake setup his front brake. Mike is a very interesting person with lots of good information.
I have several sets of FJ1200 carbs I am going to try to use on my 750b. If those don't improve performance I may try a pair of Delorrto 36mm carbs. I also plan to balance all or part of the engine and I will be using an exhaust cam on the intake. A 3 angle valve job and some port work should take care of the head. I am about to order the C5 ignition system.
I plan on using 2 or 4 of the 12 in long Cone Cylindrical Mufflers on a project Goldwing 1000 I just started. I think a 4 into 4 system on a first generation wing could be cool.
I will be doing some frame bracing too and I will be researching a set of close tolerance engine mount bolts. I have a Pingle engine mount kit on my KZ1000 project so that may be a start.
There will always be something to build on these old bikes!

Wow you are going pretty much all out. thats gonna be a sweet motor. Very interested to see how the exhaust cam and FJ carb swap goes. Megacycle does offer a street and a race grind but thats $$$. Have you considered an over bore? I saw some posts over at the twin forum with a guy claiming he got 910 in his twin.

I will wate untill someone has custom pistons fabricated and then I buy in on a minimum order. I am also looking at pistons from other motorcycles. The hold up is the pin diameter, it's big. I am also looking into having the rods modded to accept smaller more common pins. I've been lucky at finding 750b engines on Craigslist, I have 4 so I can build one and keep one mostly stock If I find an easy way to go big I'll let you know.

Looking at most factory pipes on these using the same exhaust gasket sizes from the 400 to 1100 fours, it seems sizing is not too critical as long as your measurements are not too far out.

If you are going for a high state of tune with longer duration on the cams and a worked head, then the engine becomes a lot more sensitive to pipe design and measurements. Then decide is it peak rpm power, or stronger bottom and midrange you want? shorty loud pipes generally better fpr top end power, quieter and longer pipes better for low end power...usually.

The articles linked to are interesting but this is a controversial subject and it seems there are some differing opinions about the science behind exhaust design.

I'd recommend looking at '4 stroke performance tuning' by A Graham Bell there is a chapter on extractor mainifolds plus some useful formulae. I made a custom end pipe straight through for my avatar bike , calculating tailpipe length from this. (it needed to be long hence the bend, tuned for rpm power at about 9k) You can even tune as a function of the exhuast valve setting. It works great but I think putting baffles in it blunts it as it reduces the scavenging power of the reflected pressure wave from the open pipe end. Importantly to me it sounds really nice.