Hi Performance Exhaust Questions

dwyatt · Posted: October 20, 2007, 1:43 am Post subject: Hi Performance Exhaust Questions

I'm trying to find or design a hi performance exhaust for a road racer I'm building and the information on the web is confusing, so I'd appreciate suggestions.

I'm going to run 34mm Mikuni carbs, initially 700cc (CMRA), later maybe 750cc motor (AHRMA), shell #1 cam or equivalent and would like to keep the redline near stock to keep the beast alive. Minton et al suggest 1-1/2" diameter pipes about 30" long, but they don't say if this is ID or OD or discuss adding cans or megaphones. My guess this is ID of the header pipe otherwise I think you'd need the anti-reversion mods from MMM. I'll probably go with low pipes but high mounts would be good too. Form and function are both important here. Anybody have words of wisdom for me?

Jack · Full Member Joined: Mar 15, 2005 Posts: 411

There are several types of exhaust system one could construct but to say which one is best suited for your racing application is anyones guess only your motor can give you that feed back. The greatest restriction in any header would be the radius slowing the exhaust pulses increasing the resistance as the pulses enter the turn. For me the optimum exhaust primarys should excert a pulling effect to enhance the low pressure at the exhaust ports to further assist the scanvenging and initial intake charge during the overlap cycle. As far as keeping the Rs near redline,well thats's a matter of mantaining self control of the throttle Wink

...............Jack

dwyatt

Jack,

I agree with your thoughts, my engine parameters are stressing mid-range torque, not high RPM HP and the header ID and length control this. What I am asking for is some experienced guidances in what others have tried and discovered with different variations. I'm also looking into 2-1 vs 2-2 systems and opinions on silenced megaphones and cans. I suspect I will have Jemco build me an exhaust, but need to settle on design. Any relevent thoughts on this will be appreciated.

Jack · Full Member Joined: Mar 15, 2005 Posts: 411

Pipe length and ID are one of several variables that control Torque and Hp,goes alot deeper than that. The exhaust system I'm building for my XS will consist of a AR insert dumping into a 1 3/4 primary for the first 23" and will flare down to 1 1/2" for total primary length of 36" and the mufflers will be of canister style with a speacial modified baffle to enhance low pressure at the exhaust port as mention above. I would suggest you search the web for building your exhaust system rather than giving your money to a company who's system might not be applicable for your engine combination,you'd gain more knowledge of how your engine responds. There's was a gentle at the XSGarage that use to race,there's no pictures but there's enough detail imformation to get you headed in the right direction,I'll try to paste a links.......................Jack

xs650temp.proboards29....1082131250

Tomterrific

Some of the modern formula cars are using increasing diameter primary pipes. The smaller diameter coming out of the head goes into a larger diameter for the remainder of the length. The thinking is the smaller diameter will keep torque while the larger last section will allow higher rpm breathing. There is a Norton racer that had professional consult on his pipes. They were 36" long with the first 18" 1 3/8" and the last 18" 1 1/2". The lengths are correct but the diameters may be wrong. Triumph and Norton factory found more power as the pipe diameter got smaller. There is also the idea of using a small pipe the size of the port as a port extention to have more pull during overlap before the exhaust valve closes. Everything I've researched over the years has a smaller diameter working harder over a wider rpm range and the power dying with the pipe too large.

I want to try a stock pipe small diameter stub as a port extention with an 18" x 1 3/8" going into an 18" x 1 1/2" for 36" overall for a hot street exhaust. I've also thought of going to 40" in length but the last 20" to 22" inches would space out segments to 1 3/4" diameter giving a megaphone of slight taper effect. Both of these would be a crap shoot. The saver would be the small diameter first section giving strong pull to the exhaust.

If you want to learn about the exhaust read Philip Smith's "The Scientific Design of intake and Exhaust Systems". It is an old book published in the UK. Very readable and will give a formula for the exact acoustic tuning length for intake and exhaust given diameter and cam timing. A perfect hit gives 2psi free surpercharge effect at the tuned for rpm.

Tom Graham

dwyatt

Tom,

Terrific! This was the type of info I was looking for, but once again, do you mean ID or OD when you mention pipe diameters?

Tomterrific

It is a crap shoot because I have not done any testing. I'm pretty much looking at other successful exhaust systems and trying to see how they work. Like I said, the current top experimental systems use small pipe diameter at first and step it up later. Look at the stock XS exhaust and it seems too small but then wind out a good running stock bike and it is darn fast for what it is. Good torque and good power. You won't go wrong with the stock diameter right at first. High compression? 750 kit? Step up the internal diameter one size over stock.

Good Luck,

Tom

Jack · Full Member Joined: Mar 15, 2005 Posts: 411

I would suggest you contact 650Central and talk to Michael Morse,he's been racing and wrenching on the XS longer than any of us and I might suggest you tape record the whole conversation for later reference Wink

Like Tom said it's a crap shot but if you're wanting to address the mid range torque and overall performance you need to start thinking about addressing the exhaust port it self,this is the one area where you'll net most all torque and Hp gains at all rpm ranges regardless of what exhaust you choice to use....................Jack

grizld1

dwyatt, diameters above refer to ID (stock=1 1/4", MAC 2-2=1 3/8" if you want a cheap alternative to Jemco). For the "stub" Tom refers to, cut the forward end with port filler ring (a good thing to use) from a stock pipe, chisel off the outer wall, grind the inner wall smooth, chamfer the inner edge, and peen it into the 1 3/8" header. 1 1/2" ID straight stock is easy to find; you might consider getting pieces that are swaged to slip over the 1 3/8" ID (1 1/2" OD) stock so that you can experiment with different lengths at different diameters without having to break welds.

dwyatt

Thanks guys, that helps. I've chatted with MMM a couple of times and am just doing some more homework before we talk again. I forgot to mention in my first post that porting the head will be one of the first mods done. I'm really just down to finalizing the pipe design before I start my engine work.

I was considering using the MAC 2-1 header with a more open megaphone but hear many reports of grounding the pipes during hard cornering due to their location. I figure I can bend the end of the collector to point the megaphone up further, but still worry about the header pipes. Any experiences with this?

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