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Compression...again...

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A puller

03-12-2003 18:08:11




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HOW MUCH COMPRESSION IS TOO MUCH??? I've heard that anything over 200psi Is too much. Another question is how can you change the psi to the ratio type? Is their any way to figure your approxamite compression, like with your specs? Thanks alot.




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a puller

03-16-2003 19:09:59




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 Re: compression...again... in reply to A puller, 03-12-2003 18:08:11  
what happens if it blows the compression gauge right out and off the hose???



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Rick

03-14-2003 18:51:02




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 Re: compression...again... in reply to A puller, 03-12-2003 18:08:11  
compression over 200 is to much for the fuel any more it will run with out a spark plug most diesels run about 18:1 ratio or higher the best fuel out there i found is 114 octain racing fuel it allows us to build engines close to the 200 mark but we found that the flywheel weight is not enough also to turn the motor at lower rpms making hard to start we perfer 185 or lower for starting and high enough to burn fuels like turbo blue or racing fuels call local fuel company they will be able to tell you more on how much compression their fuel will take

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Ron

03-13-2003 18:49:01




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 Re: compression...again... in reply to A puller, 03-12-2003 18:08:11  
Cam duration, and cam timing will change pressures on a gauge... The more duration, the lower the gauge will read... Actual CC-ing, and measuring is the only truly accurate way of knowing the actual compression ratio...



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lynn

03-13-2003 17:30:24




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 Re: compression...again... in reply to A puller, 03-12-2003 18:08:11  
A puller..friend put together and A over the winter and wound up with 250 psi with JD starter..when we dynoed it head gasket let go on number 2..checked number 1 by pulling it and wound up over 300 psi..probably 13 or 14 to one..helped him measure for 9:5 to 1 which should be around 210..put one together last year that had 11 to 1 and 240 psi and held together through last pulling season..my A has 9:5 to 1 and i think the lugging ability is better with 200 to 210 compared to 240.. lynn

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Mike in Mo

03-13-2003 19:00:46




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 Re: Re: compression...again... in reply to lynn, 03-13-2003 17:30:24  
A LARGE varible that hasn't been in this dicussion is the camshaft duration. I had my "A" at 13:1 (for a very short time) I had 290 psi with a 620 cam and was down to 230 psi with a custom grind cam. The only change was the duration and the intake lobe centerline. A further wrinkle to this is I now have 10:1 compression ratio, same custom grind cam, same lobe centerline,different head,manifold, and carb, and still have 230 psi. Bottom line is you CAN'T figure actual compression ratio by using a compression guage. Just my $.02 Mike

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phil

03-12-2003 19:08:56




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 Re: compression...again... in reply to A puller, 03-12-2003 18:08:11  
'A puller', this equation isn't precise but it will get you close. P = 14.7 (R^1.24) - 14.7
where P is pressure & R is compression ratio. Hard to see in print but ^ is an exponent. Take your compression ratio R & raise it to the 1.24 power. Multiply that answewr by 14.7. Then subtract 14.7. That should get the pressure.

Turned around, it is R = ((P+14.7)/14.7)^(1/1.24)
For 200 psi, you should get about 8.7 to 1.

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1066 pro puller

03-15-2003 12:56:47




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 Re: Re: compression...again... in reply to phil, 03-12-2003 19:08:56  
that equation won't work because it doesn't take cam duration into effect and that is a very big factor on cylinder pressure.



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G Taylor

03-12-2003 19:48:36




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 Re: Re: compression...again... in reply to phil, 03-12-2003 19:08:56  
It all depends. Combustion chamber size/shape. Metal composition and combustion chamber wall temp. Spark plug location, oil consumption, fuel ignition temp (octane) and flame speed. Ignition timing, fuel mixture ratio,size of fuel droplets and air/fuel temp.



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G-MAN

03-13-2003 10:44:07




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 Re: Re: Re: compression...again... in reply to G Taylor, 03-12-2003 19:48:36  
Absolutely right. Phil's formula is an approximation, and seems to be a very big approximation to me, as a Chevy 350 with a compression ratio in the 8.5:1 range won't have anywhere near 200 lbs of cranking compression. The only accurate way to calculate compression ratio is by cc'ing the engine, and the only accurate way to determine comression pressure is by plugging in a gauge. Everything else is basically just guesswork.

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Goldsburg

03-13-2003 13:47:46




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 Re: Re: Re: Re: compression...again... in reply to G-MAN, 03-13-2003 10:44:07  
G Man -

The Ideal Gas Law will be a little better than pure guesswork ;-)

However, to use the Ideal Gas Law, you will have to dust off your Algebra and either take some measurements or make some educated guesses(like T2: temperature after compression). Keyword here is "educated" meaning that you ought to have SOME idea of cylinder temperatures after compression! If not, don't worry! The "corresponding compression ratio" result variation that you get by guessing (oh, say 680 Rankine) will not vary your results that widely: 0.1 to 0.2 "points" of compression.

Just remember to use Absolute units of measure:Temperature (degrees Rankine) and Pressure (PSIA; add 14.695 to "gauge" readings)

You and G Taylor are absolutely right that no equation will predict what you are going to get for a compression ratio (using cranking(?) pressure). The Ideal Gas Law will be able to "estimate" within .3 to .5 points, though!

But my questions is: What use is this information? I find that the actual cylinder pressure is FAR more important than the comp. ratio! The cylinder pressure is the "proof in the pudding!"

Regards,

Goldsburg
goldsburg@att.net

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phil

03-13-2003 17:28:24




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 Re: Re: Re: Re: Re: compression...again... in reply to Goldsburg, 03-13-2003 13:47:46  
Goldsburg, like you say, you need Temp to actually solve the Ideal Gas Law. But, if you can find data on several engines, you can "back" into the equation. For example, the manual lists the 50 gas tractor as having 6.1:1 compression and a cranking compression of 118 psi. And a 50 LP has 8.0:1 & a cranking PSI of 172. So, already knowing the answers, you can solve for the exponent (1.24). What do you use the info for? When I have an old engine, I can look up the ratio on a spec sheet, then calculate what compression I should expect when I put my gage on it. On a good day, my little JD H should have 80 psi.

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G-MAN

03-13-2003 15:57:41




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 Re: Re: Re: Re: Re: compression...again... in reply to Goldsburg, 03-13-2003 13:47:46  
I agree that the final compression pressure is the most important. Being able to properly figure the compression ratio (by cc'ing) would be important when putting together a new combo, where you'd want to be careful about having too much or too little compression. For the average old tractor junkie, cc'ing probably isn't something they'd spend time doing, and it would be particularly difficult with a two-cylinder Deere, as getting the pistons positioned just right in the cylinders (simulating TDC and BDC would require the engine to be assembled, and the disassembled for the cc'ing process. I doubt if I'll take the time to do it on my engine, because I have a rough idea of what my compression ratio will be. And like you said, the real test will be when the compression tester is plugged into the spark plug holes.

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