51 B: compression test

Would cranking the engine with a 12 volt battery cause the compression test to read higher than with a 6 volt ? Owner claims 110 & 120 compression, but used the 12 volt. If so, any ideas how far off this would make it read?

Thanks----

Original Comp Ratio of 4.65:1

May be that the pistons were changed..?

Cranking only will raise a compression gage just "so much" and it tops-out..

6V or 12V won't make much difference..

Ron.

Original Comp Ratio of 4.65:1

May be that the pistons were changed..?

Cranking only will raise a compression gage just "so much" and it tops-out..

6V or 12V won't make much difference..

Ron.

110 psig is the correct cranking pressure for the late gas burner A and B.

110 psig is the correct cranking pressure for the late gas burner A and B.

I beg to differ on the relation of cranking speed and compression. Crank a B over on 6 then on 12. It makes all the difference in the world in speed.

Now for the logic on compression increase and speed. Crank a diesel over slowly in cold weather and have it not start. Then charge the batteries and have it crank quickly. It will start.

My 4020 used to have two 6 volt batteries and cranked over so-so. It would start without plugging in or ether assist down to about 40 degrees. I installed two 12V batteries in parallel. It cranks much faster and have never had it fail to start even in the teens with no ether assist and no block heater.

The single and only change is cranking speed. Cranking speed equals increased compression which equals increased heat which means a diesel starts. The same would apply to cranking speed and compression in a gas.

In the end cranking speed has a direct effect on compression.

Thanks for all the comments. I can see logic in each. Guess I will have to see if anyone marked the side of the block if it was bored.

Starting a Diesel is a matter of creating HEAT in the cylinder to Fire the fuel..
Yes, that requires good cranking speed...

In a gasoline (or all-fuel, etc) engine, with the throttle partly opened and with good rings and valves, with only a 6V battery, after 5 compression strokes, you have the cranking pressure of the engine..
The ONLY difference with faster cranking speed would be less time for leakage at the rings or valves..
I feel a Warm engine gives a better reading than a cold one..

Ron..

So I doubt you are an expert in thermo dynamics but what exactly do you think makes that heat in a diesel?? It is an increase in pressure which causes and increase in heat. Cranking speed affects compression.

We are not talking about what it takes to make a tractor run. Where our opinions differ is whether or not cranking speed affects compression.

Much agreed on gas engines. If you have a good spark and a bit of volotile fuel it will fire and run.

You stated that cranking on 6V would not make less pressure than the same engine cranking on 12V, which is not true. If you agree that a diesel needs fast cranking to make heat (incresed pressure) then you must agree that a gasoline engine would make more compression if cranking speed were increased.

My only point is that cranking speed DOES affect compression numbers. If it were not so then Deere would not have specified how the test was to be performed (speed) to come up with meaninful numbers. Some test are at rated speed and later ones are at cranking speed.

yes of course it will read higher on 12. cranking faster brings in more air #1. less time for leakage, which ALL engines have #2. anything over almost nothing will run just fine on a B when ignition & carb are good #3. as for only the pressure creating the heat in a diesel, its just not so. suppose you had ZERO leakage in a diesel cylinder, & brought the pressure up slowly over twenty or thirty seconds...... any rise in temp? NO. the rate at which pressure increases is what really makes the heat. & no it cant all be absorbed into the metal in 20 seconds. do it on a 180 degree engine the results will be the same.

You know, I started to enter another explanation for you..BUT..I can see that you are not going to "Get it"..and I don't have the time to argue with you..go ahead and believe that the compression ratio INCREASES as cranking speed increases...!!

Have FUN with it..Believe it..take it to the TRACK..!!

Good Byeee to you.'Nuff Said..

There is no confusion here. Compression ration WILL NOT change. It is a fixed value based on the CV of the engine.

I made all of these statements because you said,

"Cranking only will raise a compression gage just "so much" and it tops-out..

6V or 12V won't make much difference.."

Which is BOLOGNA. Cranking speed makes ALL the difference in the world. That is my point! Sure cranking an engine for long periods of time will not build any more compression, but increase the speed and Walah... It gained compression. Again that is why compression tests for tractors and automobiles have a given speed for compression tests. Like it or not that's physics at work.

The only reason I give a rat is that some poor soul will read your post when archived and be greatly mislead. It is just wrong. I dont know how to make it any more visual.

This is basic engine theory.

This is great! Of course you know you are both right. But this is a great thermodynamics question for a 3rd or 4th year engineering student.

If we assume no loss of mixture out the valves and past the rings, we have two cases. Slow compression where all the heat of compression is dissipated by the cylinder walls. This is called isothermal compression. We all know the ideal gas law PV=nRT. With T constant then P=1/V. With a compression ratio of 4.65:1 we would have a maximum pressure of 53.65 psig.

But, with the faster compression like with a 12V battery we might assume that there is no time for the mixture to cool down. We call this adiabatic compression. The ideal gas law still applies, but with the temperature increase the resulting pressure will also increase. A 34 Deg R. increase would cause the observed pressure to be 58.7 psig. Not as much as you might think with this low of compression ratio.

But, there is another factor that needs to be considered. With the faster cranking speed, there is more pressure drop through the air filter, carb and intake manifold. Just a slight drop in initial cylinder pressure do to these losses would more than offset the increase do to temperature change.

Increase the compression ratio up to say 20:1 like in a diesel and it is a whole new ballgame. The temperature increase and resulting pressure increase is substantial.

This is pretty easy to prove or disprove. Just crank it with 12 volts , check compression and then crank it at 6 volts and recheck on same cyl. This is one of the very large problems with getting an accurate compression reading, maintaining the same cranking speed during the test on all cylinders.