Compression test on Massey Ferguson 35 petrol Standard 87mm engine

[FordnMassey]

Hi,

I am working on a misfire on a 1962 Massey 35 petrol (gas) 87mm Standard Motor Company engine and am considering compression issues. It's the low compression 6.0:1 ratio.

Does anyone know the expected or "correct" compression results, dry and wet if possible, for this motor or similar?

Thanks in advance,

FordnMassey

 

[greg.currants]

i am working on the same model tractor with the same problem and am looking for the same answer. i am going to remove tappet cover and check valve clearances next

 

[rScotty]

Hi,

I am working on a misfire on a 1962 Massey 35 petrol (gas) 87mm Standard Motor Company engine and am considering compression issues. It's the low compression 6.0:1 ratio.

Does anyone know the expected or "correct" compression results, dry and wet if possible, for this motor or similar?

Thanks in advance,

FordnMassey

I ORIGINALLY WROTE: A good place to start is just multiply the compression ratio time the atmospheric pressure to obtain an expected wet reading.
Roughly 6x14.7=88psi. That is the MAXIMUM theoretically possible. Expect it to be less.

BUT LATER I got to thinking and that's not right. It ignores the heating of the charge as it is compressed. And here the heat causes more pressure. Aw shucks.... and we don't know real piston velocity and we also don't know how much heat is lost to the surroundings as the piston goes up. ugh.... it's gettng ugly.

So I should have said that "6x14.7=88psi. That is the MINIMUM theoretically possible. Expect it to be MORE. I'm not sure by how much.

The rule of thumb is that whatever the reading, all of the cylinders shouldn't vary by more than one atmosphere. Even better, they should be within half an atmosphere. THAT's STILL TRUE

Good Luck,
rScotty

 

[greg.currants]

hi
I have replaced points and all the leads, I have taken carb apart and given total clean. its as though she is only running on 3 cylinders. my compression test came up with 118,119,121 and 97 for no. 4 cylinder so i am heading to the valves to check clearances although knowing that the old girl does use a small amount of water with no obvious leak ( head gasket ? ) Just trying to eliminate everything possible before heading down the path of removing head etc

Greg

 

[rScotty]

And it runs good on three of the four? I have no idea on the water - you didn't mention that! But you might be able to see water to the cylinder as a difference on spark plugs if doing a 15 minute or longer spark plug test. I bet a "little bit" of water would be so subtle it would take brand new plugs or plugs cleaned in a air/abrasive plug blaster to see anything.

Yes, it could also be a valve - especially as it is #4. Although if it is not a valve, it then has to be spark to #4. I'd probably swap the #4 lead and plug with one of the other cylinders just to eliminate the lead and spark plug & cap completely. Then replace the distributor cap because they are cheap and no way to test that part. But it is sounding more and more like valve & gasket on #4. You'll just be getting lucky if it is not. But we always give luck a chance.

Is there is some high idle no load that it will just run at without misfiring? Under what conditions does the misfiring get worse or is it always about the same?

BTW, you probably tried squirting some oil into #4; did doing that change the compression?


rScotty

 

[SCOOBIEGUY]

Here were my reading on a z134 before a rebuild (actually ended up only changing the rings) the valve clearances were checked before.
The rings on #3 and #4 were really worn down but the sleeves were within specs so I only used a cylinder hone and changed the rings.
The engine had 6000+ hours.

The service manual says it should be 6.6 to 1 compression pistons 145PSI @0ft altitude and 140PSI @1000ft alt

Results Dry / Wet
Cyl 1 - 120 PSI / 120 PSI
Cyl 2 - 130 PSI / 130 PSI
Cyl 3 - 90 PSI / 100 PSI
Cyl 4 - 100 PSI / 105 PSI

 

[greg.currants]

i have replaced distributor cap. the water i spot on new set of plugs just put in after doing compression test and it doesn't want to run that well at idle but will keep going at full revs but not sounding a well lady

Greg

 

[rScotty]

Sounds like you are going to be pulling the head. Well, that's not so bad on these old gasser industrial engines. Almost fun for a mechanic to get to do something simple and at a convenient height.

BTW, I just went up to my reply in post #3 and edited it. I did a poor job of explaining compression ratio and am trying to fix it up a bit.
rScotty

 

[greg.currants]

thanks rScotty will let you know the next find hopefully this weekend

Greg

 

[Jerry/MT]

Engine compression ratio is a volume ratio not a pressure ratio

I ORIGINALLY WROTE: A good place to start is just multiply the compression ratio time the atmospheric pressure to obtain an expected wet reading.
Roughly 6x14.7=88psi. That is the MAXIMUM theoretically possible. Expect it to be less.

BUT LATER I got to thinking and that's not right. It ignores the heating of the charge as it is compressed. And here the heat causes more pressure. Aw shucks.... and we don't know real piston velocity and we also don't know how much heat is lost to the surroundings as the piston goes up. ugh.... it's gettng ugly.

So I should have said that "6x14.7=88psi. That is the MINIMUM theoretically possible. Expect it to be MORE. I'm not sure by how much.

The rule of thumb is that whatever the reading, all of the cylinders shouldn't vary by more than one atmosphere. Even better, they should be within half an atmosphere. THAT's STILL TRUE

Good Luck,
rScotty

Engine compression ratio is actually a volume ratio not a pressure ratio. The equation I use to calculate the compression pressure is based on the isentropic equation pV^gamma=Constant (eqn 1) where p is the pressure, V is the volume and gamma is the ratio of specific heats of air.

Sparing you all the gory details, the equation for what you see on a compression gauge;

delta p = p1 x [(compression ratio)^gamma -1], (eqn 2)

where p1 is atmospheric pressure, gamma is 1.4 and delta p is the compression gauge pressure

Now the isentropic equation assumes constant mass in the cylinder but because of valve timing the mass in the real engine is not constant. so K is a constant that adjusts the theoretical value of compression gauge pressure to account for this.

Eqn 2 then becomes delta p = K x p1 x [(compression ratio)^gamma -1] (eqn 3)

Gamma =1.4. From experience K is between 0.70 and 0.75.
So a compression ratio 6.0 engine would have compression gauge pressure at an atmospheric pressure of 14.7 psia would be 124.6 psig (K=0.75). This is what you could expect as a maximum for a engine in good mechanical condition.

I hope this helps you.