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Compression Check on 50 Tractor

120psi

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50man

03-30-2002 19:20:25

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I just bought my new comp. tester and checked it out on my 50 and got 120psi each cylinder. Any reason to shave the head a little for a little more power?? How much comp. is too much on todays pump gas? (If you read this Jack, I guess I'm in the ballpark compression wise with your friends 50 tractor-125psi)

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Well Anyhow

04-01-2002 17:40:14

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Re: Compression Check on 50 Tractor - 120psi in reply to 50man, 03-30-2002 19:20:25

Hook it up and try it.I think you will be impressed.Have fun 50Man.

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ChuckW

03-31-2002 16:59:15

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Re: Compression Check on 50 Tractor - 120psi in reply to 50man, 03-30-2002 19:20:25

had a 520 gas and installed hi-altitude pistons, compression 175 P.S.I. Think I had a lot of power, think again, the engine ran but with the increased air flow I could not calibrate the carburator and engine ran unusual, can't quiet explain. Seemed to turn at higher RPM but compression must have drastically changed torque range. Anyway, pulled those out and went standard equipment and was very pleased.

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G Taylor

04-01-2002 09:28:14

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Re: Re: Compression Check on 50 Tractor - 120psi in reply to ChuckW, 03-31-2002 16:59:15

Changing to high altitude pistons will not change airflow in any way shape or form. All high compression does is allow the hot pressurized gases to expand more/longer while the piston is closer to TDC & mechanically extract more energy before the exhaust valve opens. If power was lost it was likely due to detonation from too low octane fuel, too advanced timing, too hot/too lean mixture or a sharp hot edge pre-igniting the mixture. It's also possible something odd happened such as cam timing was unknowingly off a tooth but corrected during the low compression assembly. The 520 usually had more compression with 4.85AF,7.0 gas & 8.75 LP. The 50 made less power with 5.35AF,6.1 gas & 6.6 LP. High altitude pistons were about 1/2 way between gas & LP ratios. Pullers wouldn't up the compression if it didn't make more power.

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Jack

03-31-2002 05:37:44

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Re: Compression Check on 50 Tractor - 120psi in reply to 50man, 03-30-2002 19:20:25

Try some 93 octane and just see how it acts.Put her in granny and watch her go.Get a weight bracket on front because if it hooks it's comin up.Have fun 50Man,later Jack.

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Ron

03-30-2002 21:08:21

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Re: Compression Check on 50 Tractor - 120psi in reply to 50man, 03-30-2002 19:20:25

Originally, the 50 Gas had the same compression as the Late "B"...Less than 6:1.. Either your 50 has had the pistons changed, or it is heavily Carboned up... At standard apmospheric pressure of 14.7 8:1= 117.6 lbs..... ..without any restrictions like carb, or air cleaner.... You are OVER 8:1 now...!!!!!

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Ron

03-31-2002 07:09:17

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Re: Re: Compression Check on 50 Tractor - 120psi in reply to Ron, 03-30-2002 21:08:21

I see that there are some differences in books by different writers...!!! STILL, the Compression is very close to the Late "B"....assuming "Gas"..... I just looked, and I was wrong....it IS 6.1:1

Increased the HP from 24.62 for the "B"..GAS, to 27.5 for the "50" Gas, at the drawbar...Not all that much, actually...a "B" with the same ratio would have the same increase....( Give or Take ).. The bigger increase came with the "20", & "30" series, with the re-designed Head....

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Frank-in-Tallahassee

03-31-2002 07:04:30

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Re: Re: Compression Check on 50 Tractor - 120psi in reply to Ron, 03-30-2002 21:08:21

Ron:

I think you have to add the 14.7 back into the product. Since it's a ratio, say 5:1, means both sides get the atmospheric pressure factor of 14.7. So, 5x14.7 : 1x14.7 = 73.5psi : 14.7psi . Then there are those who add in other correcting constants, like an additional 5 psi, and even then only state a qualifying range.

Here's some info I found on the web that might be helpful:

So how do we determine the proper compression pressure for this engine at this altitude? We first must start with the engines compression ratio. Then times the compression ratio by the atmospheric pressure (this is the mean atmospheric pressure for the area (14.7 psi @ sea level)) Next we add the atmospheric pressure and a Mathematica constant of 5 psi to correct the equation. This gives us the theoretical compression pressure for this altitude , however, we must consider that the actual atmospheric pressure will vary with temp. humidity and altitude. So to give us an operating range we allow a variance of + 15% to the equation.

Link

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Sam in Georgia

03-31-2002 05:19:27

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Re: Re: Compression Check on 50 Tractor - 120psi in reply to Ron, 03-30-2002 21:08:21

Well Ron I hope you don't work on airplanes for any reason. The service manual lists the 50 gas tractor as having 6.1:1 compression and a cranking compression of 118 psi. The 50 LP is shown as 8.0:1 with a cranking PSI of 172. Maybe you torqued the head too tight on your compression ratio equation.

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Ron

03-31-2002 06:57:15

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Re: Re: Re: Compression Check on 50 Tractor - 120psi in reply to Sam in Georgia, 03-31-2002 05:19:27

The book shows 2 sets of compression pressures.... one at cranking speed, and another as a running pressure ( Maximum, at speed, warmed up) It is posible to get a compression pressure HIGHER than the Compression ratio will indicate, because of Volumetric efficiency...Usually at the engine's Maximum Torque RPM... That is why you must be careful not to put too much in as a mechanical Compression ratio...

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Sam

03-31-2002 07:56:24

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Re: Re: Re: Re: Compression Check on 50 Tractor - 120psi in reply to Ron, 03-31-2002 06:57:15

Ron;
You making this stuff up as you go along or you just don't have a clue. Mechanical compression ratio is what is being measured not "Volumetric efficiency" or running pressure. I would like to see the "book" that gives 2 sets of compression pressures for a JD 50. It is possible to have a higher compression pressure at some higher rpm due to cam timing and the use of tuned headers but I doubt a 50 with restrictive air cleaner restrictive exhaust and torque producing cam will improve with rpm. So far you are really impressing us with your tractor knowledge.

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Ron

03-31-2002 17:55:44

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Re: Re: Re: Re: Re: Compression Check on 50 Tractor - 120psi in reply to Sam , 03-31-2002 07:56:24

OK, Sam.....

I will give you my source for the Comp. listing....say for the "20" series...You May Learn Something..... . There are 2 listings;.. Compression Pressure ( PSI at Rated RPM) Compression Pressure (Motoring W/Electric starter, or cranking engine)... Page 26, TWO Cylinder Magazine, Jan.-Feb. 2000...for the "50, 60, 70 Series " Page 12, TWO Cylinder Magazine, July- August, 2000....for the "20" Series... Since you need to look it up, DO SO... I will not bicker with you...Looks like that is all you have to do...I give info to any who are in need of it...and have no need to waste my time here, except to provide what ever help I can... I will not even try to prove a point with you on "Details"... YOU know where I was going, and TAKE IT for what it was.... Maybe you could be providing some positive input, instead of trying to start some hate and dis-content..... Perhaps you can hold the adolescent comments, and HELP these folks, since you appear to think you are so very smart.....

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G Taylor universal gas law leakage

03-31-2002 08:22:37

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Re: Re: Re: Re: Re: Compression Check on 50 Tractor - 120psi in reply to Sam , 03-31-2002 07:56:24

The extra pressure looking like 8x instead of 6x above atmospheric is due to the air/fuel mixture being heated & trying to expand while it's still being compressed. Hence the main reason for cranking compression also being much lower than fast idle compression pressure. Same reason why a diesel has to be cranked with sufficient rpm to start, more rpm in cold weather. Higher rpms also allow less time for leakage & improves volumetric efficiency slightly even on modest tarctor engines. Compression pressure does give a indication to octane requirments vs. just calculating mechanical ratio alone due to valve timing & port efficiency.

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50man

03-31-2002 03:29:05

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Re: Re: Compression Check on 50 Tractor - 120psi in reply to Ron, 03-30-2002 21:08:21

Forgot to mention my owners manual states the compression ratio on a gas 50 to be 6.25:1(All-Fuel 5.35:1)

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50man

03-31-2002 03:03:27

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Re: Re: Compression Check on 50 Tractor - 120psi in reply to Ron, 03-30-2002 21:08:21

I thought the only way to calculate comp. ratio was to take the stroke of the motor and divide it by the distance from the piston to the head (at the top of the stroke)-in other words the head would have to be off to check it. Are you sure you can correlate compression in a cylinder with compression ratio?

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Ron

03-30-2002 20:59:03

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Re: Compression Check on 50 Tractor - 120psi in reply to 50man, 03-30-2002 19:20:25

Generally speaking, it is accepted that anything over 9:1 will change the way you can pull the engine.... You will not be able to "Pull it Down", without pre-ignition--without using some type of additive to raise the Octane level..( expensive)... At 9:1, you will not be able to "lug it"...and will have to stop and change gears, or let it "Hammer", and ruin the rings, Bearings, cam, valves, Pistons.etc... Pick what you want, but the original Compression ratio for "GAS" was set fairly low to allow the engine to Labor, and not hurt it.... I personally like to hear the 2 Cylinder JD's "Reach for their Last Breath", and keep on going..... I don't like that metallic "Clank"....one bit...!!!!! !!

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Ron

03-30-2002 20:58:37

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Re: Compression Check on 50 Tractor - 120psi in reply to 50man, 03-30-2002 19:20:25

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Ron

03-30-2002 20:57:35

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Re: Compression Check on 50 Tractor - 120psi in reply to 50man, 03-30-2002 19:20:25

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