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Cylinder Pressure

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REA

03-21-2001 08:09:00




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Hi Bama Binder: I have only 95# pressure on my cylinders after a restoration that I did. Isn't that a little low for a new engine job for my super c? Thanks. Bob




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Red Raider

03-21-2001 08:56:51




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 Re: Cylinder Pressure in reply to REA, 03-21-2001 08:09:00  
You may loose a little compression until the rings seat, and may even burn some oil according to the shop manual, although I haven't noticed any smoke after my rebulids. Several piston combinations were available for the Super C, and the difference between the three and four ring pistons may account for some lowered compression. It may be a combination of these factors that is contributing to the 95# values.

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Bama Binder

03-21-2001 08:33:56




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 Re: Cylinder Pressure in reply to REA, 03-21-2001 08:09:00  
The shop manual should have the pressure specs on the specific engine you have. My Super H has almost exactly 120 PSI across the board and I think the compression ratio was approx the same at about 6.5:1. My cub manual states that 120 PSI is normal for the Cub and it too is 6.5:1.

To answer your question, 95 does seem a little low, but there are variables that com into play. Did you put in new pistons? If so were they the same as original. Is the head gasket the same as the original or is it thicker? The rings and valves can be in perfect condition yet you only have 95 PSI due to other things. My guess is you won't notice any real power difference and the lower compression engines actually "lug" better than high compression.

Others will no doubt add to this or correct me if I have erred...BB

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REA

03-21-2001 09:41:13




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 Re: Re: Cylinder Pressure in reply to Bama Binder, 03-21-2001 08:33:56  
You say it will lug better? Thats what I want for the pulls! But, I don't understand why, perhaps you care to imbellish a bit? Thanks! Bob



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Bama Binder

03-21-2001 10:20:50




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 Re: Re: Re: Cylinder Pressure in reply to REA, 03-21-2001 09:41:13  
Just goes to show you should never put anything in writing that you can't back up. But I'll try. Actually what I was loosly referring to is torque versus horsepower. Compression ratio is just one factor out of a bunch of factors that matter. Lower compression ratios are associated with engine built to make lots of low end torque, but there are exceptions.

High compression is normally associated with engines that make a lot of horsepower at high revs. Other factors include size of the carburator (smaller typically helps low end torque while bigger makes more HP at high revs.) The engine bore versus stroke, camshaft, valve sizes, etc all come into play.

I probably made a statement that was somewhat misleading since compression ratio is just one factor in many that help to build an engine that produces more torque at low RPM versus an engine built to make more horsepower at higher RPMs. And there are plenty of exceptions and other combinations that work just as well.

Engineers, hot rodders, tractor pullers, and a few others may disagree, but this is my explaination...BB

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Denny Frisk

03-21-2001 12:13:50




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 Re: Re: Re: Re: Cylinder Pressure in reply to Bama Binder, 03-21-2001 10:20:50  
What He actually wants to say is that an engine with 95 psi cranking compression can be lugged down to lower RPM without damage than one with High compression. The Hi PSI engine would probably detonate and blow holes in the engine. But to make any noticable difference You'd have to increase the compression ratio like from 6.5:1 to 7.5:1 or so. You will reach a point where your fuel will not support the higher heat and compression and will detonate & destroy your engine. With these old engines like the H's & M's you would have to modify the engine for much higher compression, and lug the snot out of them down to like 500 to 800 RPM at full load. The higher comp. pressure actually does give more Horse power.

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Bob

03-21-2001 14:45:55




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 Re: Re: Re: Re: Re: Cylinder Pressure in reply to Denny Frisk, 03-21-2001 12:13:50  

Thanks, that makes my gears spin a wee bit!
Bob



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Bob

03-24-2001 08:03:05




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 Re: Re: Re: Re: Re: Re: Cylinder Pressure in reply to Bob, 03-21-2001 14:45:55  
Thanks Denny: I regreat that I had not taken some of the courses that I would have liked to have taken back in High School. I didn't think that I was prepaired because I wasn't the one that developed good study habits in my elementary years. Bob



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Denny Frisk

03-22-2001 13:18:19




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 Re: Re: Re: Re: Re: Re: Cylinder Pressure in reply to Bob, 03-21-2001 14:45:55  
About a year ago I printed out a section off a web site about "Frequently Asked Questions about Gasoline" 50 or so pages that explained the whole process in detail. It helps to have about 3 yrs of advanced college level chemistry if you want to understand it all. What it amounts to is that the faster you can burn it the less time for detonation you have, but if you run slow & hot you'll quickly exceed the point of uncontrolled combustion of your fuel. I found it interesting most of the research results they sited in the report dated back years ago, the newest test was in the late 1950's. And the results are still valid.

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