BHP vs BS ...

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The torque output of the motor depends on many things. A two into one pipe is restrictive, however if the exhaust valve opens earlier to compensate, a better result can be achieved. With the 850, I advance the cam by 12 degrees, This means the inlet opens at about 65 BTDC, and the exhaust at about 94 BBDC - which is theoretically too early. My bike is too loud, however I have a 2S combat cam which only has to be advanced by 6 degrees to get similar inlet opening, and the exhaust opening should be much better. (LOUD is an indication of lost power)
The problem is that if you improve the torque characteristic, the bike does not accelerate any faster unless you raise the overall gearing. And short-shifting often does not help the acceleration as the gaps between gears are usually too large. The close box allows you to ride the top of the torque curve to get the best acceleration. The top end horsepower is pretty much irrelevant if the motor has little midrange. If you've ever ridden a racing two stroke into a head-wind - the almost stop as soon as the revs start to drop.
 
acotrel said:
The torque output of the motor depends on many things. A two into one pipe is restrictive, however if the exhaust valve opens earlier to compensate, a better result can be achieved. .
Click to expand...

You need to jet up for free-er flowing exhausts.
Advancing the exhaust opening sounds like a half baked solution ?

Norton Inters (cammy ohc singles, longstrokes) have adjustable INLET timing.
Advancing the inlet timing by 10 degrees is to suit an open exhaust pipe, and brings the torque back.
(That otherwise goes out the exhaust pipe ?)

Actually, the timing of each valve in an Inter is very adjustable, but the manual warns that the factory has intensively investigated these things,
and no benefit is to be gained by straying outside the recommended settings.

I had a 2 into 1 pipe on a Commando, an otherwise utterly stock 850, road use of course,
and with only seat of the pants measuring, it was stronger - by at least a small margin.
Refitting the balanced peashooter system later was definitely a step slower.
 
More food for thought. Ran my bone stock '74 850 (at least I think its bone stock, but confess to not yet having been inside the engine) on a Dynojet dyno this AM. I wanted to obtain a baseline against which to compare future engine modifications. Thought the information was sufficiently important and timely that I am posting it here.

For background information, the subject stock bike has points ignition, auto advance, cross-over exhaust header, and peashooters that are straight through, i.e., no intentional restriction other than 3 longitudinal rows of shallow baffle indents that face forward, with 1-3/8" ID and exit. The carbs are stock Amal Mk1 932s with Rowley's magic tube, 4 ring needle with clip in top groove, 106 needle jets and 260 main jets.

I was particularly interested in capturing low speed torque figures, so the majority of runs began at 2,000 rpm, but a series of runs were also started at 3,000 rpm for comparison. All runs were conducted in 4th gear. The throttle was modulated by rolling on over ~ 500 rpm rather than snapping it full open in 0.1 sec. In all cases the torque/HP sweeps, for a given starting rpm (2000 or 3,000 rpm), showed excellent repeatability and stacked atop one another so closely that a given series of runs were all indistinguishable from one another. The plot below shows the results of 2 runs, started at 2,000 and 3,000 rpm.

As others have suggested and shown, the torque curve starts high and remains high over a broad range of engine rpm, in this case over 40 ft-lb from 2,500 to 6,500 rpm. The torque and HP numbers seem to be in the right ballpark - give or take a little. Likewise, given the reported airflow of a stock RH10 head (small 30 mm port flowing ~ 75 cfm at 10" WC), when fit on a well prepared engine (high compression, cammed properly, tuned pipe) should support operation to about 6,000 rpm, thus the HP of my low compression, mildly cammed, restricted exhaust engine maximizing around 5,500 rpm also seems reasonable and in agreement with several of the dyno charts posted in this thread.

In conclusion, regardless of whether you resonate constructively of destructively with the dyno chart, the torque of stock 850s make them really fun bikes to ride.

PS - don't want to rile the 750 crew here as their bikes are equally fun to ride!

BHP vs BS ...
 
Thanks for doing this, and posting it here so we can see.
Seems a fairly strong stock 850, from those numbers.

WZ507 said:
PS - don't want to rile the 750 crew here as their bikes are equally fun to ride!
Click to expand...

The Combat guys are likely to respond that they can outdo that !
And the stock 750 I have ridden was more revvy than an 850, so each bike has its strengths.
They were all quite strong and sporty in their day. - one of the torquiest engines in motorcycling.
 
Rohan said:
Seems a fairly strong stock 850, from those numbers.
Click to expand...

Don't forget it's a Dynojet dyno, so knock those numbers back a bit, e.g., perhaps 51.5 HP * 0.9 = 46 HP, or 51.5 HP * 0.95 = 49 HP, either of which puts us in the right ballpark.
 
From those graphs it looks as though for max acceleration the gear up-change should occur at about 5,500 RPM ?
I think on my 850 the up-change should always occur at about 6,500 RPM for max.acceleration. I sometimes see 7,500 RPM (OOPS !)
It is difficult to watch the rev counter while doing that on a small circuit.
Thanks for posting
 
" Thanks for doing this, and posting it here so we can see.
Seems a fairly strong stock 850, from those numbers. "

+1

That is a very nice set of numbers from a standard 850. You can see why they are fun to ride.
 
'You need to jet up for free-er flowing exhausts.
Advancing the exhaust opening sounds like a half baked solution ?'

In the commando engine, when you advance the inlet timing you automatically advance the exhaust opening. With the two into one pipe, the error probably fixes the inertia problem as the resonance is set up and maintained. As I said - the exhaust noise becomes too loud if you advance the 850 cam a reasonable amount (12 deg.). If the combat cam is advanced 6 degrees , the inlet opening becomes 65 deg. BTDC and the exhaust opening point remains about 5 deg. after 90 deg. BBDC which would be much better than the 94 deg. BBDC the 850 cam gives.. I have still to get around to pulling my motor apart again to fit the combat cam.
I believe the standard commando cams are adequate, however on a road bike with separate pipes they are retarded. I have found from other bikes that if the exhaust opens earlier than 85 deg. BBDC, the noise gets louder and the bike goes slower. My 850 is loud, however still fast enough to win races. An opportunity for improvement ?
As far as jetting is concerned - a friend of mine fitted a megaphone to a finely tuned Thruxton Velo without re-jetting after taking the standard muffler off - and immediately burned a piston. With my 850, rejetting is not a problem. I jetted the bike with the two into one pipe fitted. In any case it is too rich on the mains - which on most of our circuits is not a problem as long as the jetting at lower throttle openings is correct.
 
Incidentally, you used the terminology 'free flowing exhaust'. I think exhaust pipes are more about resonance and mass transfer, 'flowing' is not quite correct. (How does a tsunami work ?) The megaphone simply amplifies the pressure in the wave. They tend to make the wave either be there or not be there with no smooth transition. I suggest a good two into one is just as effective and the bike becomes much easier to ride fast. My power band is almost from zero to 6,500 RPM and above, and I tend to keep the revs within the area of Max. torque by peddling the gearbox. There is no pronounced 'cam spot' or kick when the exhaust starts to work.
 
Interesting.

So, I have a question:

On my 850 Slimline racer, I use PW3 cams, lightened crankshaft, flowed head, standard compression pistons, and Dunstall silencers, Amal 32's with velocity stacks.
The bike goes very, very well, revs very easily and doesn't seem to flat out up to 7500 at least. ( i experienced valve float before any sign of flatting out, the bike was clocked at about 210km/h - 130Mph.

It is EXTREMELY loud. (Annoyingly so, in fact).
I am certain that the timing is 'standard', albeit with the PW3 camshaft, very similar to a JS2 if I am not mistaken.
What could I do to minimize the noise level without loosing performance ? The bike is very torquey and accelerate very well even in top all the way to above 7500rpm in top, with 22 teeth sprocket.

Thank you in advance for your comments!
I wish I could modify the engine 'à la Jim Schmidt', but can't afford it I am afraid...
 
Jagbruno said:
Interesting.

So, I have a question:

On my 850 Slimline racer, I use PW3 cams, lightened crankshaft, flowed head, standard compression pistons, and Dunstall silencers, Amal 32's with velocity stacks.
The bike goes very, very well, revs very easily and doesn't seem to flat out up to 7500 at least. ( i experienced valve float before any sign of flatting out, the bike was clocked at about 210km/h - 130Mph.

It is EXTREMELY loud. (Annoyingly so, in fact).
I am certain that the timing is 'standard', albeit with the PW3 camshaft, very similar to a JS2 if I am not mistaken.
What could I do to minimize the noise level without loosing performance ? The bike is very torquey and accelerate very well even in top all the way to above 7500rpm in top, with 22 teeth sprocket.

Thank you in advance for your comments!
I wish I could modify the engine 'à la Jim Schmidt', but can't afford it I am afraid...
Click to expand...

This is what I 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.

BHP vs BS ...
 
Well...it Is ALREADY loud and fast...I was thinking more in the direction of timing etc...my Dunstall silencers are not much louder than peashooters on identical engines.
 
acotrel said:
I have still to get around to pulling my motor apart again to fit the combat cam.
Click to expand...

There are several kits that allow you to swap cams without pulling and splitting the bottom end, you do have to lift the cylinders to clear the cam followers, but the bottom end can stay bolted to the frame.

Of course the kits require that you split the cases one last time to perform the necessary machining; but after that, life gets easier if you are one that likes to play with your engine by trying different cams.
 
grandpaul said:
acotrel said:
I have still to get around to pulling my motor apart again to fit the combat cam.
Click to expand...

There are several kits that allow you to swap cams without pulling and splitting the bottom end, you do have to lift the cylinders to clear the cam followers, but the bottom end can stay bolted to the frame.

Of course the kits require that you split the cases one last time to perform the necessary machining; but after that, life gets easier if you are one that likes to play with your engine by trying different cams.
Click to expand...

can't find the info, link pls?
 
It sounds like a good modification, however I'm not much into changing cams. I tend to persist with what is in there and change timings until the result feels right. The 850 cam is great, however with that early exhaust opening it is too loud, and I believe there is gain to be had from using the combat cam instead. I am not into building a top end motor, to my mind doing that is futile with the commando engine. My problem is that I am used to racing a Triumph motor where the cams are separate, so changing the exhaust timing is much easier. I'm a bit lazy, so the combat cam has been in my drawer for many months. I've been wondering and hoping that is doesn't have more lift than the 850 cam. I will have to measure it.
I have never run with separate exhausts - does the combat cam give a 'cam spot' in a standard 750 commando engine ?
 
acotrel said:
I'm a bit lazy, so the combat cam has been in my drawer for many months. I've been wondering and hoping that is doesn't have more lift than the 850 cam. I will have to measure it.
Click to expand...

You'll be disappointed. The combat (2S) cam has more lift than the stock Commando (750 and 850) cam. Specs for the stock cam is .330" lift at the cam for both intake and exhaust, and the 2S is .390" intake and .346" exhaust.

Ken
 
Bernhard said:
Various New Track Racing Silencers are available from ;

http://www.acu.org.uk/News/2015/01/New- ... ME-Season/

http://www.acu.org.uk/News/2005/04/HOMO ... SILENCERS/
Click to expand...

Really not sure where you are going with this one Bernard. Homologated silencers are only required in certain forms of controlled dirt track racing.

A homolgated silencer has been available for this type of racing, including Speedway since the mid '70s. I tried one on a road race bike in the late '70s. It weighed a ton. It was designed to be low slung on the right, assuming you were turning left, and it was difficult to support due to the weight. It was quiet working well withing the road race requirements of the day, it was completely over engineered and had a huge amount of wadding in hence the weight.

If you were in the UK looking at a race exhaust I would recommend Nigel at NRP, who is producing made to measure exhausts to meet actual road race requirements. He also does a good line in road bike exhausts of all ages and recently made a system for a 961.

In other circumstances, just get a better quality Norton replica system.
 
If the noise is due to the exhaust valve opening too early, it would take a pretty good silencer to quiet that down. The noisiest four stroke I've ever heard was a supercharged speedway 1000cc Vincent which was fitted with road race cams. The reason for the excess noise is obvious one you know what is inside the motor. My brother rode the Vincent at Murray Bridge Speedway years ago. Said he thought he was going to end up in the river, it was so loud and nasty. If it is that loud, I am certain it indicates power loss, even if the bike is fast.
Have you seen John Renwick's videos on Youtube - his bikes are supercharged and fast without being excessively noisy, and they run straight pipes like an Australian speedway sidecar.

https://www.youtube.com/watch?v=NQIhqazUHg8
 
I agree with Acotrel, and that was the gist of my question. There is nothing wrong with my Dunstall silencers, but the engine is extremely noisy because of the PW3 cams and or the timing.
 
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