Saturday, April 19, 2008

Measuring Compression Ratios

Have you ever considered what the actual compression ratio of your motor is? Are you sure that you took everything in to account or are you going by the rated compression ratio of your pistons?

The simplest way to measure the actual compression ratio of your engine is to have it assembled, at TDC, and then "oil out" (old school term for measuring the volume) the combustion chamber. But to do this you must also be able to position the engine so that the spark plug hole is at the very top of the chamber so that you don't get an air bubble trapped which would throw off the measurement. Pretty awkward to say the least!

Every other method involves measuring separate components and calculating the ratio mathematically. Here are a couple of things to consider when doing so.

  • cc of a cylinder = diameter x diameter x 12.87 x height

This formula is especially handy for an old time Harley guy like me since you enter the specs in inches, and the constant (12.87) converts the answer to cubic centimeters.

  • Head Volume

The normal way to cc a Shovel, Pan or Knuck head is to place a piece of Plexiglas with a small fill hole across the gasket surface. When you do this you will be measuring not only the chamber, but also the space into which the fire ring on the cylinder fits. That means that the volume that the fire ring displaces must be calculated and subtracted from the head volume. Treat the fire ring as a cylinder and use the formula above. To get the cc's of the fire ring it is only necessary to calculate once using the o.d. of the fire ring and then subtract the volume found using the i.d.

  • Gasket thickness & gasket bore diameter

Think of it like another really short cylinder and use the above formula to compute the volume.

  • Piston deck height

If your piston only comes within .010 of the top of the cylinder, it is just like running a .010 thicker gasket.

  • Piston dome volume

This can often be found in manufacturers specs, but not always. You can grease the piston rings, install the piston into the cylinder so that the top of the dome is just below the cylinder top, and then measure the distance down to the deck of the piston. This will serve as the height inserted into our magic formula above, which allows you to use the bore diameter to compute the volume of the cylinder formed from the piston deck to the top (if it did not have a dome in it). Once you have that volume you can oil it out and subtract to get the dome volume.

  • Valve stem protrusion

How far the valves were sunk into the heads from previous valve jobs. Now, if you just finished oiling out your heads, this is of no consequence in computing your actual compression ratio. But it will help explain why your compression ratio is not as high as your piston manufacturer claims. Low and behold our handy dandy little formula comes to the rescue again. If you use the valve diameter in the formula along with the amount that the valve is sunk deeper than the minimum spec as the height, you might be surprised how much compression ratio you loose.

[as an example, stock size valves in a Shovelhead when sunk .050" will increase the combustion chamber size by 4.39cc, lowering the compression ratio by about 1/3 of a point]

This is especially relevant on the older motors which usually have a lot of valve stem protrusion and the valve un-shrouding which often accompanies it. In performance applications, the valves often have to be sunk to maintain valve to valve clearance with the hotter cams that are available, so that is also something to take into account when selecting your cam and pistons.

Now perhaps you think all of this doesn't much matter on your average cruiser. Well, maybe. On the other hand, keep in mind that higher compression ratios make a more efficient engine, and thus better fuel economy and power. On the other hand too high a ratio can lead to hard starting and worse yet, pinging. Another consideration is that all cams have a compression ratio range that they will work the best in.


Anonymous said...

one question here... will a cams overlap actually build compression over the rpm range?

St. Lee said...

I am not entirely sure I get the meaning of your question, but I'll take a stab at it. Keep in mind what I described in this post was mechanical compression ratio. That is the difference in volume between the cylinder/chamber when the piston is at TDC (Top Dead Center) verses the voume when it is at (Bottom Dead Center). This measurement does not take the cam into account whatsoever, but is important in choosing a cam.

Technically cam overlap is the period near TDC at the end of the exhaust stroke when the exhaust valve is not quite closed, but the intake valve has already started to open.

Cranking compression (measured in PSI) is the measurement obtained with a compression gauge when turnig the engine over with the starter. This measurement will be greatly affected by cam timing as well as ring and valve seal.

An engine will only be building compression when both valves are closed and the piston is coming up. By increasing the duration (time the cam holds the valve open) you have decreased the degrees of rotation that the engine is actually building compression. That is why "Hot" cam with a lot of duration needs a higher mechanical compression ratio to run at its best. All of this is related to RPM also, since for a given duration, at higher RPMs there is less time for the air to enter and exit the combustion chamber. That is part of the reason a longer duration cam is needed for high RPM operation; to get good cylinder fill.

So, to maybe answer your question, no one cam will be the best over all RPMs. It all comes down to making the right compromise in cam and compression ratio for the power output required, the riding style, the cubic inches available, the cylinder head flow, the longevity required, the fuel to be used, and probably a bunch of other factors I missed.

Anonymous said...

Thanks, that was what I was refering to...the valve overlap.

Anonymous said...

Overlap has no effect on compression directly, but the correct amount will improve volumetric efficiency at higher speed, which increases the effective cylinder pressure.