Advancing the cam

If a bike comes on cam at say 4000rpm, if all things remain constant, ie. intake, exhaust etc. does advancing the cam have a linear affect on the torque. For instance if 5 degree advance made it come in at 600rpm sooner does 2.5 degrees advance make it come in 300rpm sooner, if you get my drift.

T.I.A. Paul.

Hehe Good One Paul, real Nortoneers know nothing is linear with Commando's. Your linear logic holds for mechanical geometry but the piston acceleration to speed of suck in - gets less as rpm lowers so the new lower rpm level of 'coming' on cam will be less dramatic than more retarded cam. Above is just reasonable theory so please try it and let us know how it goes. For low down response grunt, w/o hi mixture velocity or flywheel energy or impeller speeds or turbo exhaust flow, there is no replacement for displacement.

brxpb said:

If a bike comes on cam at say 4000rpm, if all things remain constant, ie. intake, exhaust etc. does advancing the cam have a linear affect on the torque. For instance if 5 degree advance made it come in at 600rpm sooner does 2.5 degrees advance make it come in 300rpm sooner, if you get my drift.

T.I.A. Paul.

Click to expand...

I wish it were that simple. Try it and see. Let us know. Jim

Seems to me that as far as a cam goes, what you got is what you got. You may or may not tweak it one way or the other, but if you try to add something on one end you will lose everything on the other or bring physical limitations into the equation. Seems like it needs to be timed to best suit it's design characteristics and not much else.

If you want a different come on point, you need a cam to suit your requirements, I think.

We've plenty of tales on Nortons and plenty more in other engines - me included, about tweaking cam degree for more useful power band character and peak out power. Advancing broadens flattens torque curve while taking some peak off top end power, & retarding visa versa. Can be similar sense of benefit as 1/4 turn of pilot needle or moving a carb needle a groove up or dn or a few degrees of spark timing or taking off air filter or some baffle resistance out muffler. Its been proven on a 2.5% increase in power band can be worth a few 10'th's sec off a 1/4 run, which is quite significant, say 12.24 sec vs 11.92. The few engines I did get completely dialed [trial-error] in by above were delightful to run rather under max out put, like dialing in radio station to remove static but not played WO all the time. Its goes w/o saying at some cam degree change the valves can hit piston.

I have never run my bike with separate pipes or changed from the initial 12 degrees advance I gave the 850 cam when I fitted the two into one pipe. I'm also using methanol and 34 degrees ignition advance. . All I know is that trying to gear for the extra torque is difficult and deceiving. The bike is very quick once it is moving, it comes up through the 4 speed CR box probably too quickly with too little lag after each gear change. What you are playing with is harmonics. If the pipe length changes theoretically you should also change the cam timing to suit. You can really only suck it and see, and make changes in small steps, changing only one thing at a time. The two into one pipe is different again - I use a big tail pipe with skinny header pipes. The ones I've seen on Youtube and in photos have the same diameter header pipes and tail pipe. I don't think that works too well.
You main problem will be getting somewhere to test the bike which gives repeatable situations with no speed restrictions or traffic, after you've made each change. I used to do it when I was a kid by racing every month, and changing the bike gradually after every meeting. With a road bike you are probably stuck with riding your mates' bikes and copying what has been done to one that you like. Alternatively go to a race meeting and pick somebody's brains if they have a fast commando. Sometimes they will talk.

In one of the posts on this forum, somebody reported a higher torque output by advancing the combat cam 6 degrees. I believe that gives an inlet opening of 60 degrees BTDC, that is what I use with the 850 cam. I felt that the exhaust was then opening too early, however with the two into one pipe it works well - an error fixes an error ? The bike is too loud for road use. I recently bought a combat cam be cause that is what I originally intended to use. It will make the inlet closing point considerably later. With me, it is all 'suck it and see' and 'arse beats class'.

I have done experiments with advancing a cam on a 500 Norton twin.

It is not a simple linear result. Obviously since the Norton cam has both inlet and exhasut lobes ground in fix relationship you advance both.

Many tuning books talk about advancing the inlet to gain lower torque and retarding the exhaust to get back some of the top end you might lose advancing the inlet. You cant do that without regrinding the Norton cam. eg A g Bell four stroke tuning.

What I found is that the outcome of advancing the cam is VERY greatly impacted by the type of exhaust you are using. In particular if you are running a megaphone system with or without a reverse cone then advancing the cam say 10 degrees can destroy everything below say 3000 rpm and this only be regained by retuning the exhaust.

So the answer to the first posters question is no. It isnt simple, results depend on exhaust (and inlet tract tuning).

If you starting advancing the cam dont forget to check you are not going to get valves hitting the piston.

A typical thing with Cam Chains , is as they get worn & stretch , and the motor gets loose - is they rev out more - as the timeings later .
But thats mundane car engines , like a 1600 Ford .

HITTING things , advanceing puts the valves closer to the riseing pistons . So one does check .

Typically you want the center of the overlap between T D C and 5 Deg. before T D C , perhaps . It is as well to figure where this is at least .

A three Keyway Sprocket would get you finer adjustments .

brxpb said:

For instance if 5 degree advance made it come in at 600rpm sooner does 2.5 degrees advance make it come in 300rpm sooner, if you get my drift.

Click to expand...

You'd be close enough to right there,except I'd only expect it to come on about 480 rpm earlier with 5 degrees advance.
It will bring the intake valve closer to the piston at TDC,but the intake valve isn't the troublesome one and only needs about 0.030" clearance.

Start worrying if the exhaust valve has less than 0.060" cold clearance to the piston.This clearance will increase when you advance the cam,so there's no problem.
If I've got 0.050" more valve lift on the intake valve at TDC (compared to exhaust valve lift at TDC),I'm happy.That works OK on most engines.

It pays to place a small piece of kids' plasticine under the edge of each valve, in the valve cutaways in the pistons, and rotate the engine over TDC. Section the plasticine with a razor blade and you can see how much clearance you have. I didn't need to do this with the 850, although I always did it with my old 500cc Triton which had extremely long duration cams. The problem is you have to lift the head to do it. You can sometimes put a lever on top of the rocker arms to depress the valves further as you rotate the crank to check for clearance when the valve is at max. lift. I don't believe much in increasing valve lift - the lift rate is important if you want to prevent valve float. My approach is usually to go for longer duration even if the power band is more pronounced. The important thing for me is not to move the power band further up the rev range. That depends in the timings and exhaust configuration. My 850 is not savage, it seems to have good strong urge from about 3000 RPM so on a race track I never notice a kick, however it is difficult not to rev it past 7000 RPM. I don't believe this would be achieved with separate pipes.
I have a strong dislike of megaphones on separate pipes, a two into one is a much better option. It doesn't matter how big a circuit is there always seems to be a tight corner somewhere where you have to slip the clutch if you have a savage power band. In the worst case , if the motor drops off the power band when you are in a corner at 70 MPH and you slip the clutch to bring it back on power to get out of the corner, the bike can go sideways. My 500cc Triumph was always like that, even with a two into one pipe to some extent.- Nobody needs that sort of anxiety.

I might have a play around in the future if a vernier cam sprocket comes my way cheap enough. I'm inclined to agree with Pete's comment "what you got, is what you got". On a similar vane what trade offs do longer inlet tracts bring, seems easier to experiment with than cam timing, I've got some different lengths heat spacers here to try.
I'm about to check my cam timing, I've got the head off and Norvil give a figure for the 4s of 0.1885" at tdc. measured at the top of the pushrods, can anyone verify this as I'm long past taking things Norvil say at face value.