Piston Deck Height

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comnoz said:
An analogue Boyer is likely to make the motor less sensitive to fuel octane.

The advance curve from an analogue ignition is going to be lazy through the midrange where the motor is going to be most sensitive.
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Lazy pretty much describes me. And my analog Boyer apparently.
 
jseng1 said:
If you want to adjust compression use a thicker copper head gasket so you don't have to remove the barrels.

Annealing your copper head gasket won't make any difference because it won't crush or change dimension. If you want to make sure it doesn't leak oil then use pliobond (nothing else) and apply it to each surface (4 surfaces) because it must bond to the metal (not just bond to one surface and clamp against another). Use a loop of .005" diameter copper wire around the pushrod tunnels and oil return hole.
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So don’t anneal to resoften the copper?

Where do you get the copper wire?
 
The squish band in a Norton is not tight enough or wide enough to have much affect on on the burn rate or octane requirement of a Norton with a pump gas compatible compression ratio.

Now if you were to go down to .040 squish clearance or less you might see some benefit but buy that time you will be into the race fuel only area with a flat top piston.

So lowering the compression by increasing the squish measurement will make the engine less sensitive to octane and make it run cooler.

Dropping from 10.5 to 9.5 will drop the cylinder head and oil temp by a considerable amount.

The engine will also vibrate less.

Of course using a Combat cam will also lower the cylinder pressure throughout the sensitive midrange rpm of the engine. That is because the intake valve closes much later on the compression stroke so less mixture is trapped to be compressed. Therefor you can get away with a higher compression ratio if you are running a big cam.
 
I don't believe this stuff about race cams and high compression. Once the cam is operating within the rev range of the power band, the effect of the piston increasing the compression earlier is probably defeated by the ram effect of the incoming charge. The simple fact is that most guys who fit race cams to their motors, also increase the comp. ratio. An increase to the comp. ratio is not a necessity. A race cam works just as effectively at 7 to 1 comp. as it does at 12 to 1 comp. It provides exactly the same power band. What raising the comp. does is increase combustion pressures, so the temperatures rise and you are able to increase the fuel to air ratio in the incoming charge. That is where the extra go comes from - the fuel economy changes.
With my 850 motor, I use standard comp. with methanol fuel, so the jets are much smaller than they would be at 12 to 1 comp. But in effect, the right mixture at 9 to 1 comp. is more effective than running rich at 12 to 1 comp. If you jet lean at high comp. , you are probably asking for trouble.
With the 2 into 1 exhaust systems, the power band is not so pronounced. So what you lose low down in the rev range, is probably not so noticeable.
 
If you fit an extremely radical cam into any motor, the power can come on with a bang regardless of the comp. ratio, especially if you have separate pipes and megaphones. Great in theory, but the bike can become impossible to ride smoothly enough to get decent lap times. My 500cc Triton was powerful enough to scare anybody shitless, but when it had 4 inch megaphones it went nowhere fast. After I changed the exhaust system, the limiting factor was it's gearbox. I rode it in later years when it had 5 gears and decent tyres. It was not so bad, but it still went sideways in some corners.
 
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On the subject of piston height/ deck height- in order to get the squish band to work at 50 thou on a standard 850 with RH 10 head I calculate the CR will be 12.2 to one.
I get 13 to one for doing same with a 920?

Are my numbers correct?

RH10 combustion chamber, head upside down and levelled on work bench then filled with fluid to 50 thou over top the squish area.
= 37 cc
828/2 = 414 cc +37 cc = 451 cc

451÷37 = 12.2 to 1
Not gonna work.

For the 920 it's
37.5 CCS in the combustion chamber as the squish area gets widened out to 82 mm or so.
My pistons are 80.5 mm, so it's actually a 910 cc motor, not a 920.

455 + 37.5 = 492.5

492.5 ÷ 37.5 = 13.1 to one CR.

Really not going to work.

Nigel must have gone through all of this already with his 920.
I recall discussion of dished pistons in order to get a close squish while keeping a workable CR.




What is the cubic capacity of the 750 combustion chamber? It might be good to do the calculations.

Glen
 
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worntorn said:
On the subject of piston height/ deck height- in order to get the squish band to work at 50 thou on a standard 850 with RH 10 head I calculate the CR will be 12.2 to one.
I get 13 to one for doing same with a 920?

Are my numbers correct?

RH10 combustion chamber, head upside down and levelled on work bench then filled with fluid to 50 thou over top the squish area.
= 37 cc
828/2 = 414 cc +37 cc = 451 cc

451÷37 = 12.2 to 1
Not gonna work.

For the 920 it's
37.5 CCS in the combustion chamber as the squish area gets widened out to 82 mm or so.
My pistons are 80.5 mm, so it's actually a 910 cc motor, not a 920.

455 + 37.5 = 492.5

492.5 ÷ 37.5 = 13.1 to one CR.

Really not going to work.

What is the cubic capacity of the 750 combustion chamber? It might be good to do the calculations.

Glen
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The 750 and 850 combustion chamber is the same volume from the squish band up. Just the cut of the gasket surface and the diameter of the squish cut varied over the years.

The only way to get a useful squish band on a pump gas Norton is to use cupped pistons.

Here is the 9.2 to 1 pistons in my motor. They run at .020 squish.

Piston Deck Height
 
Wow 20 is close. You probably have forged rods and gods knows what else in there to make it all work.
I think I recall a discussion about Al rod Commandos with standard Flexi crank needing about 50 thou squish to be safe?

Glen
 
I calculate that Dave's bike will have a 9.9 to 1 Cr with flat top pistons, no valve cutouts and a 20 thou compressed fire ring head gasket. This is using Dave's measurement of 43 thou piston above cylinder deck.
With a 40 thou copper head gasket instead that Cr should be right around 9.4 to one.
Actual Cr numbers should both be just a little
lower than this as I see Dave does have cutouts for the valves.
So maybe 9.7 with 20 thou head gasket and 9.2 with the thick one. There are also some ( maybe most) composite gaskets around 30 thou.

Glen
 
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worntorn said:
Wow 20 is close. You probably have forged rods and gods knows what else in there to make it all work.
I think I recall a discussion about Al rod Commandos with standard Flexi crank needing about 50 thou squish to be safe?

Glen
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Yeah, Carrillo rods a stiff crank [2 inch journals] and long skirt pistons so they don't rock too much.

I have run standard Commandos with full skirt pistons at .040 without contact.
I would consider that the bare minimum with a stock crank and aluminum rods and everything tight.
 
worntorn said:
I calculate that Dave's bike will have a 9.9 to 1 Cr with flat top pistons, no valve cutouts and a 20 thou compressed fire ring head gasket. This is using Dave's measurement of 43 thou piston above cylinder deck.
With a 40 thou copper head gasket instead that Cr should be right around 9.4 to one.
Actual Cr numbers should both be just a little
lower than this as I see Dave does have cutouts for the valves.
So maybe 9.7 with 20 thou head gasket and 9.2 with the thick one. There are also some ( maybe most) composite gaskets around 30 thou.

Glen
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Take off a few more points for the extra deep valve seats.

CR probably only tells part of the story so don’t get too wrapped up in it.
 
My last 2 rebuilds were both 750. One was a 69 the other was a 70. My pistons on both motors were flush. Hell if they stuck any higher I would never be able to kick them over.
 
swooshdave said:
Take off a few more points for the extra deep valve seats.

CR probably only tells part of the story so don’t get too wrapped up in it.
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Yes only a part of the story, but it's a critical part. I've seen expensive custom engines destroyed by building with the wrong CR for that engine and usage.

Thats why I calculate before building.

Glen
 
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