Compression Ratio
- Thread starter DaveK
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Dave,
stock was .040 shaved off the head raising compression to 10:1 ratio.
The pistons had a problem during production with crown separation if I remember correctly.
The combat had stressed the crank bearings and the pistons.
Domed may have been placed in during the swap.
A compression gauge should tell you what its pushing.
Marshal
stock was .040 shaved off the head raising compression to 10:1 ratio.
The pistons had a problem during production with crown separation if I remember correctly.
The combat had stressed the crank bearings and the pistons.
Domed may have been placed in during the swap.
A compression gauge should tell you what its pushing.
Marshal
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DaveK said:
I've never seem domed commando pistons but wouldn't domed increase and dished decrease compression?
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Commandos all used the same pistons. Combats got their higher compression by cutting the head. Powermax [domed Hepolite] pistons were used in the P11s and CS15s . Using powermax pistons and a Combat head gives compression over 11-1. Not of any use on todays pump gas. Jim
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I'm not sure a compression check is going to tell you much re compression ratio. I have had the compression all over the place on my 850, simply depending on how it's kicked from as low as 80 to as high as 140. Someone with more experience with Norton motors will have a more relevant comment but with many auto engines of the same era as the Commando, the high compression/performance versions can actually show lower compression at cranking speed than standard engines due to camshaft overlap. But the dynamic compression in the powerband is higher. It's my understanding that the Combat motor had a different cam so it's possible that its "higher" compression doesn't show on a compression check.
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I meant to post this in a new thread, but this is a good place.
Last night I pulled the head off my '72 Combat engine. This engine has over 18K miles on it; I expected to see oversize pistons, but found standard OEs. The Combat pistons have flat tops and cut-outs for thr valves. The Combat engine gets its higher CR (10:1, if you believe the marketing folks) from a cylinder head that has been reduced in thickness where it mates with the barrel.
I am anxious to see what bearings I find in the lower end of this engine, which appears to be unmolested.
RS
Last night I pulled the head off my '72 Combat engine. This engine has over 18K miles on it; I expected to see oversize pistons, but found standard OEs. The Combat pistons have flat tops and cut-outs for thr valves. The Combat engine gets its higher CR (10:1, if you believe the marketing folks) from a cylinder head that has been reduced in thickness where it mates with the barrel.
I am anxious to see what bearings I find in the lower end of this engine, which appears to be unmolested.
RS
Ron L
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Combat motors had the same piston design as all other 750 Commandos. I believe the term "Combat pistons" originated with the Hepolite pistons that had deeper valve reliefs and later with no slit behind the oil ring. These were all flat-top. Compression increase was from a milled head and is readily visible when compared to an earlier head.
As to P11's and G15's using PowerMax dome pistons, the four P11 motors I have all had flat top Hepolites with the one-piece oil ring and no valve reliefs, just like the Atlas. I'm sure that adding domed pistons was common hop-up at the time.
Weren't domed PowerMax pistons part of the Dunstall kit for 750's?
As to P11's and G15's using PowerMax dome pistons, the four P11 motors I have all had flat top Hepolites with the one-piece oil ring and no valve reliefs, just like the Atlas. I'm sure that adding domed pistons was common hop-up at the time.
Weren't domed PowerMax pistons part of the Dunstall kit for 750's?
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Ron, I don't know how many bikes came with powermax pistons, but I know my 68 N15 had them when I bought it new. They were a common hop-up item also. I suppose they could have been installed at the dealer but I wouldn't know why. I just presumed that they were a standard item. Jim
Ron L
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Maybe G15/N15's were different? Just checked the parts books and it shows the same part number for all Atlas/N15/P11 pistons, but that doesn't mean a lot.
Seeley920, GPM, of course, your right. I should have remembered that!
Wasn't PowerMax part of AE as was Hepolite?
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MarshalNorton said:
The CR is a theoretical value and doesn't really have as much to do with the actual compression as one would expect. A worn out engine with bad rings but a high CR will show less compression than a low CR engine with nicely run-in rings etc. Volumetric efficiency also plays it's part so checking the compression by kicking or pulling and engine through with the starter is only useful for a conclusion of the mechanical state it's in.
The CR is the ratio of the total volume in the cylinder at BDC divided by the volume of the combustion chamber at TDC. The total volume can be described as swept volume plus combustion chamber. If you have an engine with unknown CR there is more or less only one really good way to figure it out: Put the engine at TDC and tilt it so that the spark plug hole is the highest point. Run oil from a measuring device such as a burette into the combustion chamber. This will give you the volume of the combustion chamber and from this it's a bit of math.
Tim
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