Compression ratio, explanation required, please.

[jenand40]

Could you clever chaps explain how the compression ratio is derived.

I have a Rover 4 litre v8 with tin head gaskets and 14 hole cylinder heads. I recently did a compression check and found they were all 175/180 psi.

Now we all know that atmospheric pressure is 14.7, so 180/14.7 = 12.25.
This cannot be right.
The heads are stamped with 9.35.
So, education required, please.

[stevieturbo]

Compression ratio is a mathematical calculation based on the build.

That is not the same as compression pressure from a compression test.

[garrycol]

Stevie is correct but I think there is a rule of thumb but I cannot find it at the moment.

Also remember in your calculation there is already 14.7 psi from the air before your start so in your calcs - the 180psi already includes 14.7psi so in the formula you used you have to take 14.7 out of it - so 180psi reduces to 180-14.7 = 165.3 then divide by 14,7 but that still does not quite work for the reasons stevie has given.

Garry

[unstable load]

Bear in mind that the stamped CR on the block is a static ratio based on the engine as built, like Stevieturbo said, while the compression test you did is a function of ALL the variables present, including atmospheric pressure, outside air temperature, the restrictions of the air filter, inlet tract, dirty valves, condition of your battery and starter motor etc etc.

What you need in a compression test is for all the cylinders to be within a few psi of each other, rather than a specific amount.

[DaveEFI]

Also, any simple mathematical calculation assumes perfection. And no mechanical device will be 100% efficient.

[DEVONMAN]

Don't think it's been said yet in this thread, but the theoretical static calculated compression ratio is derived by comparing the volume above the piston at BDC to the volume above the piston at TDC.

As has been said already, many factors come into play which will affect the actual compression ratio when the engine is turning. This then becomes the effective CR.

[Denis247]

Could be that with the engine spinning, as the piston is going down, the inlet is drawing air at speed and effectively 'overfilling' the cylinder. So as it starts it's compression stroke the air is already under more than atmospheric pressure.
If you measured just the one stroke from piston at bottom to piston at top, I would expect the theoretical and actual compression ratio figures to be equal. But that's not how it's being measured in practice.
Just a theory fwiw.

[sidecar]

Could be that with the engine spinning, as the piston is going down, the inlet is drawing air at speed and effectively 'overfilling' the cylinder. So as it starts it's compression stroke the air is already under more than atmospheric pressure.

Just a theory fwiw.

What you are talking about is the use of the inertia of the gas rushing down the inlet track in order to keep the fresh charge pilling in even after the piston has started its compression stroke. This is the reason why the inlet valve is kept open even after the piston has reached BDC on the inlet stroke. This only happens at RPM speeds higher than cranking speed, it is all part of the engine 'comming on the cam' Infact at cranking speed some fresh charge will be pushed back out of the cylinder but this does not matter as long as enough charge is left in the cylinder in order for the engine to run! With the throttle shut most for all of the inlet stroke there will be a high vacuum because the throttle butterfly is creating a massive restriction in the inlet system. Of course when doing a compression test the throttle should be wide open.

[jenand40]

Thank you for all your replies chaps, I now consider myself educated (regarding compression ratio, anyway).
jenand40.

[Triumph-V8]

There are two things not to be mixed up:

First we have the mathematical/geometrical ratio like explained above
assumed the air is in the cylinder and combustion chamber
and is than reduced to volume of only combustion chamber
what is the chamber in the head, the gasket & the piston.

Second is the pressure test measured at the spark plug hole
when the starter is pulled.
That has nothing to do with 14.7, thats the ideal AFR.

Mainly we do not have the full piston stroke for compression
because the inlet valve is open when piston pulls upward.
It closes about 50 to 90 degrees after the fun started and the
air pushed away by the piston up to that point is pulled back
to some amount into the inlet.

The air heats up during compression thats some thermodynamics.
This will rise the pressure as the fuel incorporated in the gas will
do a little bit because it can not be compressed and the air around
must do more to compensate for that.

Some pressure is lost by the rings bypass, thats why the test is
normally done. It can detect wear and bad valves, too

So at last although not that topic at starter revs there are some pressure
waves that add filling of the cylinder from the inlet and exhaust side.

So there is no mathematic calculation behind it, its a must to have an
engine with that configuration, especially with that camshaft and really
fresh to become the master and than others can be compared to that
and voted being better or worn or damaged.
Close to it it can be calculated what compression will be achieved with
the real left stroke of the piston when valves are closed and fuel in the
air is neglected as the losses from rings bypass. But that is only nearby.

Easy said: If you change the cam to a wilder one the pressure test
result drops! That is the reason to increase CR with hot cams and also
the reason why you can not step back in cam when perfect because
that will cause the engine to pink badly.

Hope that helps!

[DEVONMAN]

That has nothing to do with 14.7, thats the ideal AFR.

I believe that the previous reference to 14.7 in this thread was a reference to atmospheric pressure and never intended to relate to the ideal AFR.