Combat?
- Thread starter pierodn
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pierodn said:
Hi Peiro,
You seem to me missing a digit in the "n". '73 750 would start 220xxx. No matter, there were no factory build combats in the '73 model year. There was a high compression 750 that had the standard camshaft. My 220627 was one of these.
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illf8ed wrote: "There was a high compression 750 that had the standard camshaft"
Are you referring to the RH6?
I have a 1972 Roadster with front disk brake, that I am pretty sure started life as a Combat. Currently it has an RH6 head on it. I also have a nice spare Combat head with new valves and guides. At some point when I open up the engine to do the valves, would it make sense to put the Combat head on it? I have no idea what cam is in the bike, which probably makes the question difficult to answer.
Stephen Hill
Are you referring to the RH6?
I have a 1972 Roadster with front disk brake, that I am pretty sure started life as a Combat. Currently it has an RH6 head on it. I also have a nice spare Combat head with new valves and guides. At some point when I open up the engine to do the valves, would it make sense to put the Combat head on it? I have no idea what cam is in the bike, which probably makes the question difficult to answer.
Stephen Hill
Ron L
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Stephen Hill said:
Stephen,
The only difference in RH6 and Combat heads is the RH6 is milled 0.020" while the Combat head is milled 0.042". I would not change heads unless you are certain the bike was a Combat and you want it as original as possible. The RH6 is more tolerant of today's 93 octane fuel.
Good point to remember that higher compression engines may prefer premium 91+ octane fuel.
Depending on how the bike will be ridden, the engine builder should consider available fuel choices.
Ethanol E85 is becoming available at the pumps in the Midwest US now. Would not try in a Norton.
My 1992 Ford fuel pump sure likes the no-ethanol gas available at the Quick-Trip stations better than E10!
Depending on how the bike will be ridden, the engine builder should consider available fuel choices.
Ethanol E85 is becoming available at the pumps in the Midwest US now. Would not try in a Norton.
My 1992 Ford fuel pump sure likes the no-ethanol gas available at the Quick-Trip stations better than E10!
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illf8ed said:
That's a very technical explanation. Did you have issues with the valves tangling with the pistons ? I'm interested in the possibility of using a combat cam in an 850 motor. 'did not do well with my Combat head' does not say much. To my mind, the lift of the Combat cam is too much for mechanical reliability. I often wonder about comments which relate choice of cam to comp. ratio. Over the years I have used race cams in motors of various comp. ratios. The cam will work to give the same power band at any given comp. ratio because the points in the rev. range at which the power comes on strong is a function of the valve timing and exhaust system configuration. - Not comp. ratio.
Fast Eddie
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acotrel said:
If I understand correctly, a Combat head gives a measured 10:1 CR. Which is considered by many to be too high today. An old head that's been skimmed over the years will give a higher CR. Personally, I run 10.5:1 on the street without issue, but I do use the higher octane pump fuel available. And I use a cam with more duration than std (a JS1).
A Combat head fitted to a motor with a standard cam will still give a measured 10:1 CR, but its effective CR when running will be higher due to the std cam having less duration. I have no idea how you calculate or even estimate effective CR (maybe someone does?).
A road user doing this would, I imagine, have a bike that was more difficult to kick start, may 'ping' and run hotter, and might feel more 'harsh' as a result of the higher CR.
You may be OK doing this Al as yours is a race bike AND you use methanol, which can run extremely high CR without pre ignition occurring and avoids the issue of excess heat as it runs cooler. But the crank and cases will be under increased stress. As to whether or not this is too much stress, well, who knows?!
Here's a link to an old thread that discusses Commando compression ratios.
maximum-streetable-compression-ratio-t17472-60.html
Hobot posted a chart which gives measured percentage gains from increased compression. The increases are a lot less than you might expect. For example, increasing compression from 9 to one up to ten to one gives an increase in HP output of 2.9. % or about one and a half horsepower for a Commando.
It's an informative thread to read thru. In the middle of the thread Jim Comstock reports on seizing his 10 to one engine when running on some regular fuel, the only stuff available at that station. He also mentions that on the same trip his brother's Combat cases split horizontally about an inch below the barrel joint.
Nothing like real world testing to remind one that power modifications to these old bikes sometimes results in destruction.
I'm glad I reread it now, it slightly changes my plans for the project bike. Was mulling a higher number, possibly 10 to one, now 9.5 to one is going to be the goal. Will just have to live without that 3/4 of a HP!
Glen
maximum-streetable-compression-ratio-t17472-60.html
Hobot posted a chart which gives measured percentage gains from increased compression. The increases are a lot less than you might expect. For example, increasing compression from 9 to one up to ten to one gives an increase in HP output of 2.9. % or about one and a half horsepower for a Commando.
It's an informative thread to read thru. In the middle of the thread Jim Comstock reports on seizing his 10 to one engine when running on some regular fuel, the only stuff available at that station. He also mentions that on the same trip his brother's Combat cases split horizontally about an inch below the barrel joint.
Nothing like real world testing to remind one that power modifications to these old bikes sometimes results in destruction.
I'm glad I reread it now, it slightly changes my plans for the project bike. Was mulling a higher number, possibly 10 to one, now 9.5 to one is going to be the goal. Will just have to live without that 3/4 of a HP!
Glen
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