How did they increase the compression ratio in the Combat?

Hi there. I've been reading the site for a while and just decided to sign up because I have a question I was hoping you guys could answer. I've got little expertise so I haven't been able to answer any questions, but now I have one of my own.

I'm working on a '72 Commando Combat, and I've read a lot about the engine, but I don't understand one thing. What's the physical difference that causes it to have a higher compression ratio? The bike has the same displacement, right? Is it a change to the intake pressure somehow? Does the piston travel farther? Doesn't that result in a higher displacement?

The only theory I've been able to come up with is that the cylinder barrels look shorter. At least, the space between the top fin on the barrel and the bottom fin on the head is much smaller. Is that the only physical difference? So piston travel distance is the same, but there's less space at the top of the cylinder? Could you make a Combat engine by grinding a head and set of barrels down a fraction of an inch? Do the non-Combat pistons have the little recesses for the valve clearance on their faces?

Thanks for your time, guys, and congratulations on a great board.

Re: How did they increase the compression ratio in the Comba

The head is shaved, if you look the bottom is almost eaven with the fins.

Re: How did they increase the compression ratio in the Comba

RH3 Combat head is just a standard 32 mm port 750 head with .042" skimmed off the mating surface.

Ken

Re: How did they increase the compression ratio in the Comba

Shaving the head is a nasty way to increase comp ratio. It is probably OK doing it to a Norton head, there is so much of the casting to keep it from warping, however I think it is generally a bad practice.

Re: How did they increase the compression ratio in the Comba

acotrel said:

Shaving the head is a nasty way to increase comp ratio. It is probably OK doing it to a Norton head, there is so much of the casting to keep it from warping, however I think it is generally a bad practice.

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Very common practice in the car (auto) world.

Saves having different versions of pistons, with varying size bumps on top of them.
And when you have a head/port design as clean as a Norton, why clutter up the combustion chamber with lumpy bits sticking up into it...

Re: How did they increase the compression ratio in the Comba

Beside the squaty head job, Combat pistons stick up .050" above cylinder surface and almost all if not all top cylinder surfaces are out square to the base by .003" but i forget which way its slopped. Correcting this small out of square issue also adds a bit of CR. Various gaskets and base plates also alter CR along with the valve train geometry. Combats came with a base plate which I now shun w/o any detonation on cheap gas w/o babying the throttle loads. Gasket clearance matters on Combats. My past Ms Peel Combat ran sans base plate but with a thin FlameRing gasket, so a bit over 10 CR, but I've not been able to find any as robust as the 10+ yr ago variety so best wished keepping current crop of flamering gaskets intact.

Re: How did they increase the compression ratio in the Comba

They have often done some really strange things with car engines. Years ago In Australia, we had a class for Holden Toranas which had a capacity limit, Some of the guys used long stroke cranks, and at TDC the pistons used to stick up out of the block, just looking like high comp pistons in a normal stroke motor. When the motors were measured the officials used to just measure from the top of the bore to the piston at BDC. The bit of piston which stuck out of the block wasn't counted as part of the stroke. Part of the head was actually used as an extension to the cylinder. As far as machining bike heads is concerned whenever we wanted more go it has always been better to machine the barrel or change the pistons. Triumph heads in particular won't cop much machining without warping or cracking.

Re: How did they increase the compression ratio in the Comba

Yes, the standard non-combat 750 pistons have reliefs for the valves.



Dave
69S

Re: How did they increase the compression ratio in the Comba

Zafner,

I think you were asking what compression ratio is? if you can imagine the space above the piston, in the combustion chamber with the piston at TDC, as high as it can go, versus the larger space with the piston at BDC the piston as low as it will go. If the compression ratio was 10:1 the piston at TDC would yield a combustion space 1/10th what it gave at BDC.
And like the other guys said either the head or barrel is shaved, and or high dome pistons are fitted.
Hi compressin is good up to a point, more power but more heat and pressure on the engine. Better fuel economy. But there is a limit, the fuel and the engine design.
Low compression was favoured for trials bikes in the old days, makes for a softer less harsh engine.
Just bumping the compression on it's own wil not help much, breathing through changes/improvements compliments more compression. Then you have static compression and cranking compression, but well leave it there.
Cheers Richard

Re: How did they increase the compression ratio in the Comba

A bit more CR is gained by going sans head gasket but then may have to trim piston rim to avoid clash with head squish bands.
Hardly anyone but me does trail or trials play on a Combat and I didn't find an issue with its low down grunt control taking throttle on good slopes w/o increasing engine speed or spinning out, yet could blip throttle a bit to kick rear around some to hug a tree and miss a rock avoiding drop offs into creeks or ravine sideways as front ways is part the off road fun. Norton can stand hi CR just not hi rpm so fuel octane is main limiting factor.

Re: How did they increase the compression ratio in the Comba

Perhaps its worth stating that compression ratio is the volume of piston displacement plus the volume of the combustion chamber divided by the combustion chamber volume alone.

As an example, for a 500cc engine with a 50cc combustion chamber volume, that is
500 +50 / 50 = 550/50 = 11:1

ie you are squeezing 500cc of capacity and 50cc of combustion chamber volume into that 50cc combustion chamber, so 11:1 compression ratio. Math is a bit more fiddly with Combat engines, since the numbers are not quite so neat.

Re: How did they increase the compression ratio in the Comba

acotrel said:

They have often done some really strange things with car engines. Years ago In Australia, we had a class for Holden Toranas which had a capacity limit, Some of the guys used long stroke cranks, and at TDC the pistons used to stick up out of the block, just looking like high comp pistons in a normal stroke motor. When the motors were measured the officials used to just measure from the top of the bore to the piston at BDC. The bit of piston which stuck out of the block wasn't counted as part of the stroke..

Click to expand...

With a stroker crank, unless the pistons were (substantially) taller, they should have gone deeper down the hole at BDC ?!!

Re: How did they increase the compression ratio in the Comba

There is also some increase in CR by a couple of 10'ths just by the over bores increase in swept volume in repaired barrels.