free breathing mufflers for speed

Someone asked me about exhaust for high speed road work. Heres a tip for all you do it yourselfes.

Cut off the end of your muffler if it necks down.

Use 1/4" hardware cloth and fiberglass heavy weave woven roving (won't unravel and blow out). Fold the hardware cloth over one or two layers of the fiberglass so the fold traps it at the rear and keeps it from blowing out the muffler. Cut to length, Roll this over a tube to shape it, then insert the packing into the muffler. Drill and run a long machine screw through the back end of the muffler and use two nuts to lock it in place (or loctite). Be sure the ID of your packing is larger than the ID of your header pipes. It will be loud but fast.

Also - I think RGM has some peashooter specials with a 1-1/2" inside diameter.

RGM do a full no ballance pipe system with 1 1/2" o/d they will sell it you as a full set pipes and peashooter type silencers. It has slightly longer pipes than standard for better exhaust gas pulse tuning in theory which means you will need their different mount plate of your Z plate and it gets very close to my mk2a swinging arm. It ment i altered the infamous offset to the left of the arm and power train by thinning the RH ISO abutment adjusters an put spacers in left side to move hole power train across. I was going to do it anyway so no realy issue to me or you could as suggested by Roger at RGM just cut the end down wher it fits to the silencer to standard length.
The hole system fitted very will an looks well made an for those not totally fermilier with a standard vike would pass as being the normal exhaust. I got mine about 18 months ago if i remember it was about £220 plus vat. Which i being more used to Japanese bike part prices reckoned to be a bargain. No issues with it myself an sound good without being to loud.

For north americans, http://www.commandospecialties.com/ does a big bore muffler/header kit (1.5") for like $390

Anyone know if the big bore pipe / muffler combo actually has an effect on performance?

Fast Eddie said:

Anyone know if the big bore pipe / muffler combo actually has an effect on performance?

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Yes - especially the top end. Muffler restriction is probably the most ignored problem with performance bikes that put in the hot cam, flatslide carbs etc. The header pipes can stay stock size unless you are going up in displacement with a ported head. Even then you shouldn't need more than 1-1/2" for the street. I put 1-1/2" headers on my hot street 750 and got nothing.
But when you have mufflers with smaller IDs than the headers you have a choked motor.

You can cut off the ends of smaller peashooters or any other muffler (a straight cylindrical muffler works best). Then insert the glass pack as described at the top of this thread. You can re-attach the muffler end - just be sure the exit hole is larger than the header pipe.

Tuning changes have to compared back-to-back on the same dyno, same day to see if it really helps (or makes any difference at all besides higher numbers on the DB meter)
Seat-of-the-pants dynos are notoriously inaccurate.

LUCKY DAVE said:

Tuning changes have to compared back-to-back on the same dyno, same day to see if it really helps (or makes any difference at all besides higher numbers on the DB meter)
Seat-of-the-pants dynos are notoriously inaccurate.

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I agree...

When I posted the question, I was rather hoping that someone might say "Yes, on the dyno I saw this..." or similar.

On my old race Triton (840cc with a Norton crank in a unit Triumph motor) I had 1.5 inch downpipes going into a mild tapered mega (NOT reverse cone) followed by a length of 2.5 inch daimeter perforated tube wrapped with fibre glass in a 'can' on the end.

If I fitted 1.5 inch perforated tube silencers instead of the above set up (ie 1.5 inch downpipes feeding 1.5 inch perforated tube), it knocked 10 bhp off in the mid range and something like 5 or 6 off the peak (on the dyno). On the track this made such a difference that when forced to fit the 1.5 inch silencers at one track (due to noise) I nearly stalled it off the line, as the power was so much less than what I was used to!

So, why say all this? If 1.5 inch pipes strangled my 840 motor (albeit a race motor) then surely small bore pipes and silencers on a even a mildly tuned Commando MUST be strangling it somewhat. I've heard conflicting opinions, and was hoping to hear more here, after all, folk aren't usually shy of voicing their opinions on here...

hi Guys I have posted dyno results carried out on the same day on both my race bike and road bike with no other changes other than the mufflers.
I have three sets of mufflers, although only one set will give me the best results on both bikes.
These are copies that came with the road bike when I bought it, now they share duties on both bikes.
They have lower profile weldments for the mounting holes,( I use spacers here) spaced at a narrower distance that originals, were a metric thread , but now tapped out to UNF to make all mounting bolts common.
They are more of a straight through muffler, therefore a little noisier, they don't rob my midrange like the other two sets and by far make the better power every where. The header diameter on both sets of pipes are standard size, although the race bike ones are upswept.
The other 2 x sets have a welded seam at the reverse cone, these ones have no weld.
Regards Mike

As far as headpipes go I have never seen anything but a loss with larger than stock headpipes on a streetbike.
On a hot roadracer a 1 1/2 inch headpipe has a little advantage.

You do want a much larger core in the muffler. When the core is considerably larger, then the end of the headpipe is the tuned length.
If the core is close to the headpipe size then the core length is added to the headpipe and the tuned length gets much longer. [usually too long] Jim

Dunstall ran high compression and hot cams, tried bigger pipes, then smaller. With the small pipes he went faster than everyone else, so the small bore1 3/8" header became the norm on Dunstall Dominators.

That is why Commandos have the 1 3/8" headers. Dunstall was brought in as a consultant to the general Commando design before the launch. The small bore headers were one of his suggestions. Race proven on the Dommie 650 and 750.
Definitely agree that the muffler can jam up the lot tho. In fact it is hard to come up with a muffler that doesn't suck at least some power away.

Glen

From the folks at Harley (XR 750 dirt tracker).

Here's one of the better options if you have room for it - also very good for roadracing. Downsize the header diameter for your personal tastes if not going for an all out top end screamer.

Thanks for that diagram Jim, I'll try and get some plain comes rolled to experiment with on the Dyno, 1 pair with 3.5" outlet ends and 1 pair with 2.5". No science behind the 2.5", its just what has worked so well in the past for me.

worntorn said:

Dunstall ran high compression and hot cams, tried bigger pipes, then smaller. With the small pipes he went faster than everyone else, so the small bore1 3/8" header became the norm on Dunstall Dominators.

That is why Commandos have the 1 3/8" headers. Dunstall was brought in as a consultant to the general Commando design before the launch. The small bore headers were one of his suggestions. Race proven on the Dommie 650 and 750.
Definitely agree that the muffler can jam up the lot tho. In fact it is hard to come up with a muffler that doesn't suck at least some power away.

Glen

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That sounds reasonable for the street bikes, but Norton experimented on the race bikes with 1 3/8", 1 1/2", and 1 5/8" systems, and the 1 5/8" systems always gave the most top end power. Some of the others were better for mid-range power for tight tracks, and sometimes compromises were used to meet sound regs (115 dB requirement for ACU at the time). I suspect they also experimented with 1 3/4" pipes, but I have no info on those. Straight 1 3/4" pipes with no megas or silencers were used on some of the Norton factory flat track race bikes for the TT courses, but I don't know what sort of testing, if any, that was based on. Axtell did extensive dyno testing on exhausts for 750 Nortons, and settled on 1 5/8" pipes with megaphones to his own specs. I think comnoz is right that there's not a lot to be gained on a street bike with larger than 1 3/8" pipes, unless you're running really low restriction silencers, which would have to be pretty large physically, or else really loud. On the other hand, it depends on what sort of bike you are building. If you're building a race spec engine for the street, and can live with open megaphone exhausts, then the larger pipes will make more top end power, particularly if it's a 920 or larger. I'm getting a little too old to see that combo as a fun street bike, but that's just me. I have ridden that sort of bike in the canyons (in my youth, of course), and it can be pretty exhilarating. Not so much on city streets, however.

Ken

Yes, 1 3/8th pipes produce peak torque on a 823 cc motor at about 5000 rpm which is about optimum on a streetbike.
On a 750 they will peak at a little higher RPM.

Of course as the engine gets larger or if you are tuning for peak torque at peak horsepower like you would do for a drag racer or top speed motor then they will need to be bigger.

Here is an online calculator that works very well for 2 into 1 systems. Jim

http://www.mez.co.uk/mezporting/exhaust_length.html

If one was looking for power, directly, you could go this route:

http://www.coneeng.com/pdf/cone%20list%20PDF.pdf
http://www.coneeng.com/pdf/motorcycle/M ... fflers.pdf

A straight core reverse cone muffler should offer little to no backpressure and take some of the high end buzz off without really affecting flow. You could also get coreless cones if you were really after performance. Cone eng is also nice and cheap :mrgreen:

I suggest it all has to do with how you intend to use the bike. I found a few things with two into one pipes over the years. If you open the exhaust valve before 85 BBDC the noise usually gets louder and the bike goes slower. With race cams, any back pressure is bad. The tail pipe needs to be the big enough to present no back pressure. I always use skinny header pipes the same ID as the stubs in the head, so there are no steps anywhere. However I only use my bike on tight race circuits. The two into one pipe can easily knock 2000 RPM off the top of the rev range, however the torque increase allows higher gearing to be used. With the commando, the exhaust and inlet cams are not independent, so moving the cam to compensate for the exhaust is always a compromise. My own bike is too loud, however it still seems to perform very well.
That situation with the cam is probably the only way in which a Triumph motor is better. For quite a long time I used the hacksaw and welder on the exhaust on my Triton, and changed the exhaust cam timing until it finally got up and went quicker. I kept cutting the collector back and fitting larger diameter tail pipes. I don't believe the length of the tail pipe is critical, however the header pipes probably need to be the tuned length.

Tuned length does not have as much affect as the diameter, as far as determining the rpm for peak torque.

The length pivots the power around the peak torque. IE longer pipes move more power to the area below peak torque and shorter pipes move more power to above peak torque.

Then to make it more interesting everything is dependent on timing and fuel mixture. The exhaust pulses move at the speed of sound and the speed of sound depends on the temperature of the exhaust gas which depends on the timing, mixture, compression ratio and a few other things.

So the best formula only gets you in the neighborhood and from there it is cut and try. Jim

So what is the ideal tuned length for the headers on an 850, stock cam w/ FA head? Or, how would I calculate this?
Also, if going 2-1, what would be the ideal collector size?

I really want to go with the Maney style but given they are now relegated to the "unobtainium" category I figure I would need to get something similar made up.

Dkt26 said:

So what is the ideal tuned length for the headers on an 850, stock cam w/ FA head? Or, how would I calculate this?
Also, if going 2-1, what would be the ideal collector size?

I really want to go with the Maney style but given they are now relegated to the "unobtainium" category I figure I would need to get something similar made up.

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Just fill out the top row on the calculator posted earlier. It works pretty well. Jim

midnightlamp said:

If one was looking for power, directly, you could go this route:

http://www.coneeng.com/pdf/cone%20list%20PDF.pdf
http://www.coneeng.com/pdf/motorcycle/M ... fflers.pdf

A straight core reverse cone muffler should offer little to no backpressure and take some of the high end buzz off without really affecting flow. You could also get coreless cones if you were really after performance. Cone eng is also nice and cheap :mrgreen:

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I used one of those on my project bike and really like it. It is a bit of a Jekyll and Hyde silencer, quite soft sounding at idle and just off idle, but sounds like a fuel dragster when the throttles are opened. At least you can sneak around town fairly quietly. As far as performance, I have no comparable, but it sure doesn't seem to be holding anything back.