Larger exhaust system

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Ian James

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I would be greatfull if some one told me the consequences of fitting a 1 3/8" exhaust system (RGM peashooters ,fluted baffles) to replace the very tatty 1 1/4 " set up currently fitted. 828 20 thou over ,4s cam 32mm mk 1 Amals x2 430 jets at present . The large kit was a gift from a mate after a big bore idea didn't germinate . They look very fine hanging on the wall . Thoughts and experiences please . Thanks in advance Ian
 
Ian James said:
I would be greatfull if some one told me the consequences of fitting a 1 3/8" exhaust system (RGM peashooters ,fluted baffles) to replace the very tatty 1 1/4 " set up currently fitted. 828 20 thou over ,4s cam 32mm mk 1 Amals x2 430 jets at present . The large kit was a gift from a mate after a big bore idea didn't germinate . They look very fine hanging on the wall . Thoughts and experiences please . Thanks in advance Ian
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Are you sure about the 430 main jets?
 
I always use skinny pipes in preference to fat ones. Exhaust systems work on resonance, the gas in a skinny pipe is easier to vibrate than in a fat one. Also flow is a function of velocity and pressure. A big diameter inlet tract suits high revs better, and the same applies to the exhaust. When the valves are open at TDC, there is flow across the top of the piston into the exhaust, which then gets stuffed back into the cylinder. Your exhaust cam timing determines the strength with which the gas in the exhaust resonates. With fat pipes you need a bigger pulse. With a Commando, you do not usually have independently movable inlet an exhaust cams. And the rev limit is 7000 RPM.
 
I have tried a set of the RGM 1 1/2 bib bore pipes several years ago,curiously they are about four inches longer than standard. Contacting RGM the brackets were out of stock (p/n 050302) so they suggested I cut them to standard length! I argued the point why make them that length in the first place? They must know this ‘tuned’ length is better. Anyway they sent me a drawing of the part and I made my own.
Always had a lot comments on the way they look, sticking out the back.
A friend has told me that Norman Whites optimum production race system was standard bore, modified bend and a length from valve to tip of 33 inches.
The RGM kit made way this year for a set from Armours, a nice stainless system but I have a tip for anyone fitting one . The downpipes also come 3 or 4 inches too long to be cut to length but this would only leave a couple of inches of connection, the clamp is part of the silencer.
Two inches inside the silencer is a welded seam which stops the pipe from going in further.
Half an hour on each with a die grinder and the pipes go all the way inside and the joint is incredibly strong. Cheers Paul
 
Ian James said:
I would be greatfull if some one told me the consequences of fitting a 1 3/8" exhaust system (RGM peashooters ,fluted baffles) to replace the very tatty 1 1/4 " set up currently fitted. 828 20 thou over ,4s cam 32mm mk 1 Amals x2 430 jets at present . The large kit was a gift from a mate after a big bore idea didn't germinate . They look very fine hanging on the wall . Thoughts and experiences please . Thanks in advance Ian
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Exhaust pipes are normally referred to by the outside diameter. It looks like you are calling out the inside diameter. Stock Commando pipes are 1 3/8" OD, and the big bore pipes, as LAB pointed out, are 1 1/2" OD.

Ken
 
Ian James said:
I would be greatfull if some one told me the consequences of fitting a 1 3/8" exhaust system (RGM peashooters ,fluted baffles) to replace the very tatty 1 1/4 " set up currently fitted. 828 20 thou over ,4s cam 32mm mk 1 Amals x2 430 jets at present . The large kit was a gift from a mate after a big bore idea didn't germinate . They look very fine hanging on the wall . Thoughts and experiences please . Thanks in advance Ian
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Are you running methanol for fuel? Normal main jet selections for the engine you describe will be somewhere b etween 240 and 280 on gasoline. On my 750 race bike with 32 mm Amals my most common jetting was 260 mains.

Ken
 
With exhaust systems, there are two considerations - performance and noise. The 2 into 1 system on my Seeley 850 works extremely well, but the noise is horrendous. Performance is affected by cam timing and gas restriction, If the exhaust valve opening point is advanced to overcome gas restriction, the 2 into 1 works better - but it is louder.
 
lcrken said:
Are you running methanol for fuel? Normal main jet selections for the engine you describe will be somewhere b etween 240 and 280 on gasoline. On my 750 race bike with 32 mm Amals my most common jetting was 260 mains.

Ken
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If you are using slow taper needles, the main jets become less important, as long as they are slightly too rich. With lean needles, you usually tend to wind the throttle on slower, but the bike accelerates faster. Take note of how long you are usually on full throttle, and how long it takes to get there. With lean needles, the motor will usually spin up much faster. The difference between slightly too rich and just right is very significant. If you have quick taper needles and whack the throttle open, the mixture as your motor spins up is probably richer than the optimum. The taper is there to compensate for loss of vacuum. Getting onto the main jet faster, does not mean faster acceleration. You would not ride a two-stroke that way.
 
Ian James said:
I would be greatfull if some one told me the consequences of fitting a 1 3/8" exhaust system (RGM peashooters ,fluted baffles) to replace the very tatty 1 1/4 " set up currently fitted. 828 20 thou over ,4s cam 32mm mk 1 Amals x2 430 jets at present . The large kit was a gift from a mate after a big bore idea didn't germinate . They look very fine hanging on the wall . Thoughts and experiences please . Thanks in advance Ian
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The RGM 1.5" headers reduce power vs stock separate pipes.
IIRC the 1.5" were just a tad behind 1 3/8" balance pipes on Dyno hill. I believe it was 109 kmh for the big pipes, 110 for the balanced skinny pipes and 113 kmh for the separate skinny pipes.

That was on a stock 850.

Glen
 
Ian - If you're talking about 1-1/2" throat peashooter mufflers instead of the more restrictive 1-3/8" peashooters then yes absolutely they will make a little more power.

But not so the headers. Agreed the bigger headers don't help on a stock bike and are a bummer. Hovever - on a screaming race bike they do work well. Tuners like Axtell and Leo Goff used 1-5/8" headers for their 750s and had the dyno and race results to back it up. I also used 1-5/8" headers and had one of the fastest 750 Nortons on the West coast.

Larger exhaust system
 
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Not sure if this is a question or an answer. Exhaust gases slow as they cool. Wrapping keeps gases hotter (faster) longer. So would a wrapped 1 1/2 pipe flow better in a slightly modified a 850?
 
OK think I got it all and thanks. The measurements I gave are correct big bore is 1/38 id 1/58 od and yes they do go into the muffler around 4" . They will be staying on the wall for now. Thanks Icrken and Baz I will experement with jets. Ian
 
Ian James said:
OK think I got it all and thanks. The measurements I gave are correct big bore is 1/38 id 1/58 od and yes they do go into the muffler around 4" . They will be staying on the wall for now. Thanks Icrken and Baz I will experement with jets. Ian
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Has anyone on here tried stepped headers ? , its quite common in the V8 world , so smaller off the head , and I'm sure there will be a calculation for the ideal length for the cam for the smaller tube length then step to the next size up tube .
 
Flat trackers are experimenting with a 1 1/2 pipe out of the head traveling about 8 inches then expanding to 2 inch(?) for 6 inch length then returning to 1 1/2. Secrets are being kept nobody talks about it, but guessing it boosts torque for coming out corners.
 
Considering it is only a whopping 1/16" increase in the ID radius, and a 1/8" increase in ID diameter, I seriously doubt you'd notice anything significant on a street bike with a 4S cam. You could put some time into tuning rather than just putting them on and testing them without doing anything with the carburetion.

Or as you have decided, leave them on the wall. Might have to polish them occasionally to keep them rust free if hanging on a garage wall.
 
Slightly OT but why did Atlas/Dommie come with 1-5/8 pipes, at least in the 1965/66 era? I note that there are on offer 1-3/8 pipes and as my 1-5/8 pipes are dingy I'm tempted to replace them but with what. It has the kidney-shaped mufflers with the ends of the baffles cut out. As I discovered on another bike, cutting the baffles make a huge difference in jetting.
 
Thomasa said:
Slightly OT but why did Atlas/Dommie come with 1-5/8 pipes, at least in the 1965/66 era? I note that there are on offer 1-3/8 pipes and as my 1-5/8 pipes are dingy I'm tempted to replace them but with what. It has the kidney-shaped mufflers with the ends of the baffles cut out. As I discovered on another bike, cutting the baffles make a huge difference in jetting.
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A mate of mine fitted a megaphone to his 500cc Velocette in place of the muffler , without changing the jetting, and almost immediately burned the piston.
What interests me with the dyno guys - when you use a dyno, can you tell if there is an increase in torque in the middle of the usable rev range. If you get a gain in top end power, the motor still needs to accelerate to get there when it is pulling a load. On the dyno, the rate of rise in torque might be more important than outright top end power. I have found with my bike, that I don't know if I have an improvement until I raise the overall gearing. If you raise the overall gearing, theoretically the bike should accelerate slower - but sometimes it doesn't - the rate of rise of the revs does not change, but the bike is quicker.
To my mind the gearing of my Seeley is miles too high, but with the close ratio box, the revs never drop out through the bottom.
When I first started using it, it seemed very strange. With low gearing the revs do not rise any faster and the bike accelerates slower.
With my short stroke Triton 500, the situation was the exact opposite. If I lowered the gearing it accelerated much faster.
 
Schwany said:
Considering it is only a whopping 1/16" increase in the ID radius, and a 1/8" increase in ID diameter, I seriously doubt you'd notice anything significant on a street bike with a 4S cam. You could put some time into tuning rather than just putting them on and testing them without doing anything with the carburetion.

Or as you have decided, leave them on the wall. Might have to polish them occasionally to keep them rust free if hanging on a garage wall.
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When I tested the large bore pipes I spent quite awhile on carburetor tuning but they just don't produce as much hill pulling power as the stock skinny pipes do.
Dunstall discusses this in his tuning notes.
He was also had input to Commando engine design as a consultant. The separate skinny pipes were his recommendation. Seems to be correct, for the stock motor at least.
The reduction in power isn't huge, 4 kmh on Dyno hill vs skinny pipes. That translates to about 3 bhp loss @ 4500+- rpm.

Glen
 
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