Fullauto Head C/R

Wondering what the C/R is for a Fullauto (STS) 750 head. Same as RH1?

You are really asking what is the combustion chamber volume. And indeed the answer is 'the same as a standard Norton head'.

If you want to use one on a Combat you may need to skim it the 0.040" that Combats were skimmed, or drop back to standard compression, which won't suit the 2S cam a great deal.

SteveA said:

You are really asking what is the combustion chamber volume. And indeed the answer is 'the same as a standard Norton head'.

If you want to use one on a Combat you may need to skim it the 0.040" that Combats were skimmed, or drop back to standard compression, which won't suit the 2S cam a great deal.

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When compression ratio is lower - either leaning-off the jetting or advancing the ignition timing can restore the power characteristic, Whatever you do, you end up in the same situation - you need to optimise. If cam timing, or exhaust system are changed, optimisation is still needed.
To me compression ratio and octane rating are pretty much irrelevant. I set the ignition timing to something sensible, then jet to suit. With Amal carbs, the problem is mainly one of adjustment. Most road bikes are probably jetted too rich and probably do not get best power.

How does compression ratio affect the way a 2S cam works ? Cam timings and lift are about getting more mixture into the combustion chamber, With less compression, the other variables are usually adjusted to compensate. In Triumph 650 motors, race cams work just as well at 7 to 1 comp. as at 11 to 1 comp. It is often a mistake to consider variables as single issues. They usually combine to create can effect. I do not usually change only one variable, other than jetting, and expect a better result.
I suggest it is impossible to buy needle jets for Amal carbs which give adjustment fine enough to get the best out of petrol. If you were lucky, you might fluke getting it right. As soon as the weather changes, it would probably be wrong.

acotrel said:

How does compression ratio affect the way a 2S cam works ? Cam timings and lift are about getting more mixture into the combustion chamber, With less compression, the other variables are usually adjusted to compensate. In Triumph 650 motors, race cams work just as well at 7 to 1 comp. as at 11 to 1 comp. It is often a mistake to consider variables as single issues. They usually combine to create can effect. I do not usually change only one variable, other than jetting, and expect a better result.
I suggest it is impossible to buy needle jets for Amal carbs which give adjustment fine enough to get the best out of petrol. If you were lucky, you might fluke getting it right. As soon as the weather changes, it would probably be wrong.

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It’s about the difference between measured static compression and effective (aka dynamic) compression. Cams with greater duration suffer a lower effective compression and need a higher static compression to compensate.

Increasing compression is the route to the best mean brake efficiency coz it makes a bigger bang.

‘Within reason’ being the caveat.

Everything in an ICE is a compromise.

But it’s of little to no interest to the vast majority of classic bike owners who simply want a nice classic bike to enjoy on the back lanes.

Or like me if your favourite type of riding is on the old highways in Canada and the US. Sometimes low Octane fuel is all that is available in the little towns.
On one trip thru Wyoming the first fuel available along the route for the day was at the 125 mile mark, most of a tank on the loaded Vincent. The next fuel was 80 miles on down the road, it would never make it
We arrived at the little town only to find out that the GPS info was outdated, the only pumps in town had been taken out a few months earlier.
The owner of the only restaurant in town sold me some lawnmower gas of unknown age and quality. The bike is at 8 to 1 and it ran fine on it.
Now I'm looking at reboring and those 9 to one pistons are so tempting.... but then one of my mates holed his 9 to 1 bike in a similar situation running on low Octane backwoods gas on another trip.
My bike started life in the Isle of Malta at 5.3 to 1 compression in 1947. I guess the fuel quality there was horrible after the war. The bike was presold to a British military man in Malta, so the factory set the CR for that fuel.
It was at 7.3 to 1 when I received it and then I raised it to 8. I'll probably leave it right there. That would be the sensible thing to do....

Do standard 8.9 to 1 750 Commandos run ok on regular in a pinch?
I have run the 850 on regular a couple of times. It measures right around 8.5 to one , no base gasket and a thin version of the composite head gasket.

Glen

Not a direct comparison but when i bought my 850 unknown to me it had a heavily planed head, deep valve pockets in the pistons and a full on race cam. In my ignorance i ran it on regular petrol for many years with no noticeable issues.
On the basis of that sample of one Commandos seem to be a tolerant engine with respect to fuels.

When this Combat engine was last rebuilt (by Old Britts), Fred replaced the 2S cam with Std., but left the Combat head. I will now have a std. volume head with std. cam. can I still call the bike a Combat?; or is it now a Fullauto? I also now have a set of custom Maney pushrods for a "C" head and std cam.

ntst8 said:

Not a direct comparison but when i bought my 850 unknown to me it had a heavily planed head, deep valve pockets in the pistons and a full on race cam. In my ignorance i ran it on regular petrol for many years with no noticeable issues.
On the basis of that sample of one Commandos seem to be a tolerant engine with respect to fuels.

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Cam timing can have more affect on octane requirements than "Static" CR. "Dynamic" CR is affected by the closing of the intake valve during the compession stroke..

Yakatak said:

When this Combat engine was last rebuilt (by Old Britts), Fred replaced the 2S cam with Std., but left the Combat head. I will now have a std. volume head with std. cam. can I still call the bike a Combat?; or is it now a Fullauto? I also now have a set of custom Maney pushrods for a "C" head and std cam.

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So you're telling us you replaced your "C" head with a STS Fullauto? Seems that information is lacking about 750 heads. Order choices are just for 750, no choice for intake diameter or if milled .040 for 10:1 CR.
Question for STS.
Pictures seem to show 32mm intake ports, which are fine for Combat 32mm carbs/ manifolds. Not so much for other model 750's with 30mm carbs.
If it hasn't been milled .040, your Maney pushrods will be too short. I don't think pushrods matter about the cam, they are shorter to compensate for .040 less distance between lifter to rocker arm.
As far as octane/ CR combability, my experience is that it's more about ignition timing.

Swings and roundabouts!

You may have less compression, but you will have a better inlet port flow. And much improved exhaust porting.

To be honest, in your position, I would just fit a thin (0.020") copper head gasket to raise the compression a little, and well within street fuel parameters.

If you don't like copper head gaskets, no reason not to get the Fullauto skimmed, either 0.020" or the full 0.040". Assuming you had no problems with pump fuel in your locality.

Yes, technically the pushrods are 0.040" too short, but less of a problem than the longer ones in an original Combat were!

My Maney pushrods are as recommended by Steve, 0.090" shorter than standard, with 0.020" off the head, 0.020" off the barrel and a 0.030" gasket! So technically my pushrods are also 0.040" too short

The Fullauto has the 0.042" base that the Combat head doesn't and has 32mm ports. Most like a RH5? The ex valve guides also appear to be the larger 850 type.