compression 750 S 1970
- Thread starter Peerless
- Start date
Dellortoman
Hero member
- Location
- Tasmania, Australia
What engine?
What compression ratio?
What camshaft?
What compression ratio?
What camshaft?
Said it in the topic header, Cam. 
9-10 bar/130-140psi would be nice.
Just stripped a triple that managed all of 3bar/42psi. Wasn't too hot... Piston/bore clearances around 0.20mm/0.008", ring gaps from 0.4-1.0mm/0.016-0.040", rust marks in 2 bores, scoring from the piston in the third. Valves leaking. Well and truely knackered.
piet
9-10 bar/130-140psi would be nice.
Just stripped a triple that managed all of 3bar/42psi. Wasn't too hot...
Piston/bore clearances around 0.20mm/0.008", ring gaps from 0.4-1.0mm/0.016-0.040", rust marks in 2 bores, scoring from the piston in the third. Valves leaking. Well and truely knackered. piet
chrisk
Hero member
- Location
- Brisbane, Australia
Dellortoman said:
Cam, I fear the bump on your head has taken it's toll ............... how does a different camshaft profile vary the compression figure?
Simonr501
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- Location
- Lympne, Kent. UK
chrisk said:
I'm not a mechanic
But I will have a stab though! Logically if the valves open and shut for different periods then the amount of gas trapped inside the cylinder might be more/less as it gets compressed and thus slightly different pressures?
SimonR501 said:I'm not a mechanic
But I will have a stab though! Logically if the valves open and shut for different periods then the amount of gas trapped inside the cylinder might be more/less as it gets compressed and thus slightly different pressures?
correct, cam timing also has a big effect on compression ratio
chrisk
Hero member
- Location
- Brisbane, Australia
SimonR501 said:
redax5 said:
:-[ I knew that (bolt)
Dellortoman
Hero member
- Location
- Tasmania, Australia
redax5 said:
I know what you meant to say Red, but you chose your words poorly. Cam timing has no effect whatsoever on compression ratio.
For the benefit of those still sctatching their heads, the CR is the ratio betwen combustion chamber volume and cylinder volume, and is constant for a particular engine.
It's the compression measured on a gauge that is affected by valve timing. The easiest way to imagine it is to think about the inlet valve. The inlet valve is open to suck in air as the piston descends on the intake stroke. But it doesn't close until some time after BDC so it's open the first part of the compression stroke. Until the valve closes, the piston doesn't start to compress the gas. It might be a third of the way up the cylinder by then.
Dellortoman
Hero member
- Location
- Tasmania, Australia
If the throttle is closed, the engine will pull a vacuum in the intake manifold. When you compress air that's at a low pressure to start with, you'll get a lower compression reading than if you start with air at atmospheric pressure. To get a meaningful test, always do it with the throttle open or the carbs removed.
Compression is generally higher on a hot engine than a cold one. Engines are designed so that the clearances are optimal when it's hot, so you gt less leakage past the piston. Also, the heat in the engine will heat up the fresh charge of air while it's being compressed, increasing its pressure.
For the sake comparison, compression tests are normally done on a cold engine. I suppose the reason is that an engine being worked on may not be in a condition to be started, so cold compression is the only option available to all engines.
Compression is generally higher on a hot engine than a cold one. Engines are designed so that the clearances are optimal when it's hot, so you gt less leakage past the piston. Also, the heat in the engine will heat up the fresh charge of air while it's being compressed, increasing its pressure.
For the sake comparison, compression tests are normally done on a cold engine. I suppose the reason is that an engine being worked on may not be in a condition to be started, so cold compression is the only option available to all engines.
Ernesto
Hero member
- Location
- Hamburg, Germany
Hi, Cam.
I'm not with you about that. Whole literature (and internet) says "on a warm engine".
Ernesto
tyelav13
Hero member
- Location
- Wales (gods own country)
Have always done compression tests at working temperature - warm engine. Am I incorrect? I thought you would only get a correct reading when everything has expanded to it's correct operating size etc.
Laverda SF
Hero member
I agree with Red - Your thumb can tell just as much unless you preform a Leak Down.
In which case you have to lock each cylinder at a specific degree with valves closed and pump at least 150 psi in the cylinder and determine when the rings stop fluttering then use a lower initial PSI load say 130 psi and time the leak down interval say to 80 psi for each cylinder. It takes a rather informed mechanic to perform a Leak Down and he or she can tell whether you have a broken ring (Weak rings or scratched Cylinder wall), leaky valve or blown head gasket ... etc.
You can perform a Leak down yourself if you have a descent compressor, 2 Valves, 2 PSI Gauges, a Stop Watch and know what your doing.
Just blowing air into a Cylinder with valves closed can tell you most the story just listening to the Intake, Exhaust and Crankcase or if your water cooled - bubbles in the rad.
In which case you have to lock each cylinder at a specific degree with valves closed and pump at least 150 psi in the cylinder and determine when the rings stop fluttering then use a lower initial PSI load say 130 psi and time the leak down interval say to 80 psi for each cylinder. It takes a rather informed mechanic to perform a Leak Down and he or she can tell whether you have a broken ring (Weak rings or scratched Cylinder wall), leaky valve or blown head gasket ... etc.
You can perform a Leak down yourself if you have a descent compressor, 2 Valves, 2 PSI Gauges, a Stop Watch and know what your doing.
Just blowing air into a Cylinder with valves closed can tell you most the story just listening to the Intake, Exhaust and Crankcase or if your water cooled - bubbles in the rad.