ISO Seeley ??

acotrel said:

Why does comp. ratio have any effect on the way the 2S cam works ? I know the combat ran higher comp. - so what ? Is it really necessary, surely timings are about standing waves - harmonics , pipe diameters and lengths ?

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Basically when you install a high speed cam you close the intake valve later on the compression stroke and open the exhaust valve sooner on the power stroke. This means less of the stroke is going to be available to compress the mixture or extract the power.
To make up for the lower cylinder pressure you would then raise the compression ratio to bring the compression pressure back up to an ideal range in hope of making up for some of the power lost due to the shorter effective stroke. Jim

How do expansion chambers work on two strokes ? I suggest on a four stroke motors the standing waves have similar effects. Just because you advance the cam timing it doesn't mean you always get a loss of compression. Triumph E3134 race cams work very well in Thunderbird motors with comp. ratios as low as 7 to one. You get exactly the same cam spot as in a motor on 12 to one, and an overall increase in torque right across the rev range and even below the cam spot - unless you have a restrictive muffler. The pipe length is critical for maximum performance at certain revs - it all depends on harmonics and how much mixture is stuffed back through the exhaust port. Think about how megaphones work. With high compression you might get a bigger bang however the energy cost in compressing the mixture is also greater. Methanol works well in low compression motors despite what Rohan might believe. In all these things WYSIWYG does not necessarily apply. If you have preconceived ideas and don't test the alternatives you might be doing yourself an injustice. In the late 1800s, it was believed that everything that could ever be invented, already had been invented.

Do keep in mind that the need of higher CR for an overlapped valves cam profile is only for the very lowest of rpm operation such as starting and idle and creeping around on low throttle in pits and staging at lights, so once above some rpm mixture velocity threshold, CR don't hardly make a whitworth of difference in power or combustion torque pressures developed. There are charts that ballpark power changes expected going up from say 8>9 as worth maybe 3% and from 9>10 as ~1.5%. I've lucked out to have a few hot engines, from Norton P!! dragster to Ford big block jet boat and propane optimized Chevy small block 406cid that tended to lope down low with sense of stalling on a feather inhaled but >>>only a blip off idle away form tire or prop and drive train overwhelming if not careful down low. No one but no one that's into acceleration attempts or expects much just off idle rpms, unless of course those with the mystical contraversial magic Singh groove like 'ignortant inexperienced' ole Sir Eddie got taken in by.

acotrel said:

How do expansion chambers work on two strokes ? I suggest on a four stroke motors the standing waves have similar effects. Just because you advance the cam timing it doesn't mean you always get a loss of compression. Triumph E3134 race cams work very well in Thunderbird motors with comp. ratios as low as 7 to one. You get exactly the same cam spot as in a motor on 12 to one, and an overall increase in torque right across the rev range and even below the cam spot - unless you have a restrictive muffler. The pipe length is critical for maximum performance at certain revs - it all depends on harmonics and how much mixture is stuffed back through the exhaust port. Think about how megaphones work. With high compression you might get a bigger bang however the energy cost in compressing the mixture is also greater. Methanol works well in low compression motors despite what Rohan might believe. In all these things WYSIWYG does not necessarily apply. If you have preconceived ideas and don't test the alternatives you might be doing yourself an injustice. In the late 1800s, it was believed that everything that could ever be invented, already had been invented.

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If you advance the cam you get more compression, the intake valve closes sooner. Jim

The worst experience can be had by trying to race a four stroke bike with a short stroke top end motor fitted with 4 inch megaphones - if you don't have enough gears. What to you do when you are in a medium hairpin bend doing 70 MHPH, revving at 5,000 RPM and the bike drops off the cam. The megaphones make the power hit with a real bang, so if you even touch the clutch to get the bike back on power to get you around the corner, the bike is immediately sideways.
If you bring the revs up slowly on a race motor, it is possible to sometimes feel two distinct bumps in power - one due to the megaphones, the other due to the cam. I always try to take the edge off the power increase. On a hot Triumph 650 with separate pipes reverse cone megaphones are always better. It is not simply a matter of changing the revs where the cam spot occurs, it is the way the power comes on around the cam spot. Fortunately commandos have the long stroke motor which is usually more forgiving. Two into one pipes are beautiful.

'If you advance the cam you get more compression, the intake valve closes sooner. Jim'

The reason that the inlet valve closes after BDC is due to gas inertia and the standing wave in the inlet tract. If it closes too early and you don't get maximum effect you get less compression. The pulse pressure is higher than the pressure caused by the piston moving up until a certain point. It depends on the inlet tract length. It is difficult to visualize a standing wave giving mass transfer ? What we need is the standing wave to have maximum pressure just inside the combustion chamber, both in the inlet and exhaust systems at particular revs to suit the mechanical limits of the motor. Have a read about Kadency effects.

to produce its maximum power at 5500 rpm and you install a cam with longer duration with no other changes then you will always loose cylinder pressure -ie power -at 5500 RPM. Jim

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Yep exactly what I said, which to continue my unwritten implications, longer duration over lapping late closing cam, may still make as good power at 5500 as prior lesser cam and [*if not over board*] would/could/should provide more torque at a higher rpm peak. I'm building Peel to be over cammed bogger for a toe hold on low rpm low dynamic CR granny 1st operation but other foot in risky hi CR camp. A further corollary to consider concerning our useful boost rpm band pow wow, raising CR and adding low boost can give big block off idle throttle snap grunt and just go up from there as boost builds, till something breaks in pilot or cycle. Hopefully in that order. I do expect decent operation w/o the belt on too. If I didn't have P!! digital traction operation grown into the bone and Graveling horrors or tri-linked power-grip tolerance don't believe I'd follow the path I'm on. Hope ISO Seeley can be worked out to tolerate power to spare. I gave up dreaming about Jim Schimdt's and Comstock's trellis stiffy's and boxed monicoup as I don't think but a few lb to lose and not practical for my daily living additions. Did come across a Norton that was only a long rectangle box from stem to swing arm pivot so narrow could on use volume near stem as oil tank d/t rear volume taken up by the mono shock/spring.

That trellis style frame is interesting. As far as cams are concerned, it is a pity that inlet and exhaust are not independently adjustable in a commando. I tend to base timings and exhaust configuration on experience and I chase midrange (torque) rather than top end, and work with gearing and handling. There are more corners than straights on most circuits. If you get blown off on a straight however can outride the others on every corner and high speed bend, the result might be better ?

acotrel said:

'If you advance the cam you get more compression, the intake valve closes sooner. Jim'

The reason that the inlet valve closes after BDC is due to gas inertia and the standing wave in the inlet tract. If it closes too early and you don't get maximum effect you get less compression. The pulse pressure is higher than the pressure caused by the piston moving up until a certain point. It depends on the inlet tract length. It is difficult to visualize a standing wave giving mass transfer ? What we need is the standing wave to have maximum pressure just inside the combustion chamber, both in the inlet and exhaust systems at particular revs to suit the mechanical limits of the motor. Have a read about Kadency effects.

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This is true. But if you have the motor tuned to creat maximum power at 5500 you are already taking full advantage of the Kadency effect. So when you increase the duration of the cam -even if you alter the tract lengths to correct the timing off the Kadency effects- you end up with less stroke to compress the pressure available and less power.
Of course after installing a high speed cam you may be able to tune the Kadency effects to a higher RPM and end up making more power, or at least the same power at a higher RPM which will allow lower gearing for more torque multiplication. Jim

Al those that make sell off set cranks also have to make off set cams so with just more time and money you can alter intake and exhaust lobes separately.

'Of course after installing a high speed cam you may be able to tune the Kadency effects to a higher RPM and end up making more power, or at least the same power at a higher RPM which will allow lower gearing for more torque multiplication. Jim'

Most hot cams move the top of the rev range up, as does making the ports bigger. I've made a conscious decision not to go there with my 850. Top end motors tend to dictate wider lines in corners and the 'point and squirt' riding style which make you get on the gas later than the smart-arse who turns under you and gets on the gas earlier so gets the run, and is where you want to be before you get there. I know all the theories about big ports, long duration hi-lift cams, separate pipes with megaphones. I'd never try that with the fragile commando motor, they won't cop the revs. If you want to do that start with a Weslake - most of the problems have been solved already. I really like the way my Seeley tightens up it's line in corners, it is not savage enough to step out when gassed hard when laying over. I've got reservations about using it on high speed sweeping bends. I think it needs to be kept on power to hold it's line and not run wide, it might not have enough top end to remain safe on a really big circuit like Phillip Island, even though I usually run very high gearing.
I really like the 850 motor, when I built the bike I didn't believe in the motor so didn't race it until much later. It responds very well to every modification and to my mind it really stuffs all my old theories. I found the steering problem by accident, I never realized it's importance and the effects of torque.

i gots to look up that Kedioscascadian principle but do agree there is pure magic in the Norton twin power delivery. I am a dead man walking ok, so only things about as intense as suicide delays that path [remember ole late Hunter what's his name some say I remind them of, well guess why, ugh] so living through hi times controllablely is, aka:, 'taking care', is my underlying hobby. I've tasted some great power craft on land, water and air, but like the fella with daily 900cc +NOX and 2 GP racers,wrote > Nothing But Nothing Excites me like a Drouin Norton Commando. If it wasn't so smoothly applied power increaser it'd been too scary to enjoy and drive train would vomit out if clutch didn't burn up first or tire just smoke. I got the straight line traction handling grown into my bones from P!! digital daze and THE Dot-Dash Snot a peddle bike can do burn out stunts and fish tailing climb outs, ugh, so at some point just trying to stay inline accelerating becomes identical to on the razors turning edge physics. This coming on cam then falling off torque peak stuff ties in to the power pulse-tire hysteria-break-make grip, which is universally reported to reach maximum with about 10% slippage factor, then suddenly dissolve to shedding grip way below prior no slip grip. The key Al has again and again touched on more than any one else is getting right to the edge of tires grip so entering the whole tire best grip slippage zone but not yet into the few extra percentage pts of slightly more slippage into sonic freq of make/break. Btw any lean state with front turned, [Peel can lean a ways w/o turning front as occurs at times on said climb outs and back out of ditches something pushed me into going rather too fast and room running out faster, what do ya do...] some parts of the patch are not slipping but other parts are. Put palm down and slide it around a turn to get sense of this and tip it some too. Anywho it ya can always be in a state ya can always add more power then you really can about out race a crash. Al senses this by stating gasing it helps it turn in, well doggonit, that's how it should be, but this is also when highest tire loads on frame are vectoring out of line of travel, so somethings got to give to go faster, or if conditions suddenly change, wind gust, rider drafting by, lump chunck, suddenly over threshold of tire grip, frame spring back or CoG leverage with tires lifted to side ways CoG height into bowling ball hazard to others.

If ya develope sense of a cycles hi side tendency to counter with lean then should have sense of how low to go so if ya get into the held sweeper state, ya can goose it to step rear out w/o its regrip launching ya or so low it slips out from under. If throttle wise enough and engine predicable enough then ya just power past the 10% slippage enough to get that extra bit of line up to end up less leaned so more power planting out of there. If ya over do it and realize it ya nail it more into flat tracker style and ease tires back in line to then carry on from wider arc. There can be strong whiplashes through frame and forks and are so close to just falling over with forks so unloaded by thrust they can flip into straight steer actual road following with minimal splippage for tank slapper onset. All mainly based on power response which the newbies have traction control as pure engine curve is too spikey/droppy to predict. Which gets back to the torque vs horse power and gearing character. If hi rpm to make hp and geared lower to multiple the torque, its hard to control rpm multiplication a few 1000's at a time rather than a few 100's and when let off engine drag can spike and skip tire, aka 750 infamous 2nd to 1st hopping over revs and slipper clutches. The Combat power band 2S Peel had and also the P!!, was ability to play tunes on the tires and soften the power cuts so regrip returned predictably quick. The trick to doing it 'safely' is ability to goose throttle to set a tire spin just so much above the 10% and no more and use the instant of break free to reposition line of drive or lean which besides not loosing momentum stores a bit of flywheel in rear that gives ya a nice extra shove in the desired direction till fuel burn takes over. I crack throttle in series of jerks to get around some bends and each rehook up starts the next drag race chirp leap more upright. This is best done right at the scarest limits of both ends sticking and must trust the engine to follow your feelings exactly and needs a steeply rising power curve or limits speed to pull it off. It ya go beyond the slip slick while counter steering then its same as ya see rain racers skewing along and similar to THE Gravel ya just can't lean over much or SPLAT. Al your are mostly doing this with your aggressive rain attitude.