What type of exhaust for power. (2019)

Thanks gortnipper.

Still more thought and trial and error needed tho. Cos my ‘silencer’ is less silent (shorter and bigger ID) to begin with, the dB killer might have to be more extreme?

I’ll probably have to experiment with different ones. May even need one for the noise test and a less drastic one for the actual riding when on track days etc.

Might even look at getting a new can made up.

Fast Eddie said:

Thanks gortnipper.

Still more thought and trial and error needed tho. Cos my ‘silencer’ is less silent (shorter and bigger ID) to begin with, the dB killer might have to be more extreme?

I’ll probably have to experiment with different ones. May even need one for the noise test and a less drastic one for the actual riding when on track days etc.

Might even look at getting a new can made up.

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Research. Maybe ask the guys on the Ducati.ms Superbike forum about experiences with different designs for track days?

gortnipper said:

Research. Maybe ask the guys on the Ducati.ms Superbike forum about experiences with different designs for track days?

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Already doing that cos my 1200s Monster with Termi cans needs ‘em too. As does my other part time track bike.

I’ll soon have the UK’s largest collection of dB killers I reckon...!

DB killers are funny things. Sometimes those that look like they’ll make no difference really do, and vice versa.

Like you say, research and trial and error required...

These are the ones off my 848. I bought the bike with these out and I havent needed to put them in, so have no idea how effective they are.

Tube is 28mm OD. Oval is 165 long.

acotrel said:

I always have a problem getting my head around the difference between a top end motor and one which has loads of torque. If you ride two bikes which have that difference between them, you find that it really changes the way you can race. Over the years since 1973, many guys in Australia have raced Triumph 650 and 750 engines in historic races. Most of them have blown up after going fast for not very long. The better approach seems to have been , to built the motor to pull harder, rather than rev the tits off it to get more power.
I often wonder about the calibration of the dynos which are being used these days - can they measure the difference between the two types of motor ?

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A dyno will measure whatever is presented to it.
When people refer to a "torquey" motor they usually mean one which is tractable from low revs - i have never ridden one but Harley springs to mind along with standard spec Commandos of course. A torquey motor may or may not produce a higher maximum torque than a race motor but it will produce good torque, and hence good usable power, at revs which us mere mortals can use on public roads.

I consider a torquey motor to be one which does not falter when you turn your bike into a head wind. Two-strokes in the 70s delivered high horsepower, but if you turned into a head-wind, they would almost stop. Later two-strokes had variable timing on the exhaust port and reed valves on the inlets which gave them more midrange - they are faster. You can have a Commando motor which delivers 80 BHP, but will not pull a high gear because there is nothing in the midrange. If you gear a bike with a more torquey but lower ultimate horsepower motor high, you end up in a similar situation- but at lower revs. When you ride a bike, you need a motor which will actually get to the higher revs easily when pulling a load. With a Commando, you keep the heavy crank spinning high, so you are always near the point of maximum torque. With a close ratio gear-box, they are superb. But a standard box is hopeless.
Generally speaking, if you fit a race cam into a standard motor, you get an increase in torque right across the rev range, and even below the point at which the motor comes on song. But if you use a profile, timings and exhaust system which increases the top end at the expense of midrange power, then you might not benefit much.

When I race, I always take note of what the other guys are riding, so I know where they will thrash me and where I can make up on them. A good modern two-stroke of any capacity, will beat any Commando on most circuits. Some historic bikes contain all the goodies. I've seen a BSA Bantam fitted with the lot. It is almost an RS125.

When I built the Seeley 850, I went straight to the exhaust design and cam timing, based upon what I had found with my short stroke 500cc Triumph. The slightest restriction in the exhaust system will kill top end, but with a 2 into 1 you get more midrange. To get the motor going with the 2 into 1, you need the exhaust valve opening earlier. When you do those two things, the noise level is absurd and horrendous. So you either live with the noise or you go a bit slower by fitting a better silencer. My motor pulls from bottom to top and is difficult to stop from over-revving. However with the heavy crank, throttle response is poor - so you definitely need the close box with at least one very low gear for clutch starts.
I was at a Broadford Bonanza a couple of years ago. There was a guy there with an 850 Commando which had twin pipe megaphone exhausts. I heard it and shuddered - that would have been an exciting ride The Broadford circuit is very tight and twisty - that bike would have given big bursts of power everywhere, even in the slowest of corners - and there are plenty of them.
I did not race the Seeley 850 for 20 years because I never believed in it, but now I think it is excellent - You can outride anyone with it and it does not need to go any quicker..

Apologies for disappearing mid thread, apparently when you go racing, the jobs at home that stop getting done need addressing before anything else get done. hmm, nearly caught up now so!!!!

Reading back through this and looking at various dyno charts, I was impressed by how good Snotzo's prediction software is, below is his prediction followed by my dyno chart, Moto55UK then Brooking 850, I realise the figures are different but the shape of the curves are quite close, useful software is that. Not surprising for the actual charts, but the software chart was a surprise for me.

Nigel, Steve and Chris, you all mentioned that the problem might not be with the exhaust and I did have the feeling that the bike was dropping onto one off throttle, I checked the carb balance at the track and found it to be spot on, however, I did have a duff plug, I changed the plugs and the bike would hold steady at low revs and I thought at the time I had found the problem. A small sideways moment at the hairpin proved otherwise and it was just as much of a pain elsewhere. I think I may have just been underestimating just how bad the condition were, they did abandon the meeting after all.

Anyway, the bike hasn't been touched since that day and a couple of days ago I got this off the bench, which is the yellow bike I used to use on track.



and put the Seeley back on the bench ready to strip it to get it ready for the coming season. Whilst I was messing about with it I noticed there is, what I consider to be a lot of slop in the drive through the gears, here is a short vid starting with first gear going through to 5th.

I am thinking the thump the engine would give the rear tire going from off to on throttle with this kind off slop would be the cause of my problem. I will strip the box to have a look but I am guessing this is just how they are and probably why the TTI box is so slick.

I hasn't been a problem before, so like I said possibly just an overly slippy track.

Opinions please !!!

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Sorry, can you see it now mother.

Yes, and it looks normal for an AMC box. If I change down and not adjust the revs and just let the clutch out the back tyre will chirp for a second until the engine speeds up, never done that around a bend but if I did then the back tyre would move sideways. Its an 850 with the same back wheel.

My problem was when I was going back on the power from a closed throttle, the box is a TTI.

Only time I have felt close to losing the backend was on a slow roundabout and opened the throttle too early and the back end stepped sideways, caught it in time though, the engine has good torque at low revs. Tyre was an Avon Roadrunner well before Roadriders.

My 500 does that (rear wheel), it's an AMC box with Manx outer cover and Hemmings internals. Will check my TTI box on the other bike and see if it is the same. I suspect it is.

Moto55UK said:

Hi ralph , the dyno charts from April 2018 are on this forum somewhere along with the video of the wheelie off the start line , Jim schmidt posted them; Jim was concerned as to the length of the inlet tracts, so I shortened them, It never felt as good after that . The NRP pipes were part of the package of mods I made that year which produced a 10 bhp gain , of course Nigel took credit for all of the 10!!!

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I'm usually an advocate of longer inlet tracts. 10-1/2" from the end of the velo to the head (not the valve). Were yours longer than that?
Got any specs, photos, more info of those NRP pipes?

Hi Ralph, Ive just checked mine ,it has about 3" movement measured at the tyre with the gearbox in 3rd. I found having the tickover set to 2000rpm helps at hairpins.

kommando said:

Only time I have felt close to losing the backend was on a slow roundabout and opened the throttle too early and the back end stepped sideways, caught it in time though, the engine has good torque at low revs. Tyre was an Avon Roadrunner well before Roadriders.

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Slightly different on track, I was loosing the back on three corners and the front on two corners every lap at Cadwell, it was a bit wet though.