Compression ratio - Ballpark

[dusty]

Folks,

I have a head which has a depth of 4.635" (5 thou down on standard) and a block running +.040 pistons. Can anyone give me a ballpark on the compression ratio this would give? Not very scientific but i'm only after a rough idea before i consider skimming the head.

Cheers
Jon

[andyelan]

Hi Jon

As I understood things, Big Valve heads were skimmed 0.030" which upped the compression ratio from 9.5:1 to 10.3:1 You can maybe extrapolate from this.

0.040" was regarded as the maximum skim recommended for a standard head and hence 0.010" the max for a Big Valve head

Andy

[dusty]

Does the fact that my pistons are +.040 oversize effect the CR?

[paddy]

Yes, because the swept volume is bigger whilst the combustion chamber remains the same size. All other things being equal, the compression ratio will go up by 2.7%, eg from 10:1 to 10.27:1

Paddy

[dusty]

Ok, what's the easiest way to measure / work it out scientifically (using normal workshop tools)

Cheers
Jon

[andyelan]

Hi Jon

Paddy's is quite correct. The term "compression ratio" is actually somthing of a misnomer, it's really nothing to do with compression but actually a volume ratio.
Its "swept volume+clearance volume : clearance volume"

("clearance volume" is effectivly the combustion chamber volume for the Twin Cam)

If the cylinder head was simply a flat plate across the top of the block, this would be a simple calculation, with a semi hemi head though, like the Lotus, it's almost imposible. If you want accurate answers you have to actually measure the volume of the combustion chamber using a glass plate and burette

Andy

[dusty]

I though so, but was hoping there might be a simpler method.

It's made more complicated by the fact that the piston isn't flat either!

[paddy]

I did a spreadsheet to calculate compression ratio for various amounts of cylinder overbore and head skim.

The two boxes you change are the ones highlighted, taking values in thou.

Putting in 40 thou skim gives a CR of 10.32 for the standard "big valve" configuration.

If you have the cylinder bored at +40, then you need a total skim of 38 thou (ie a further 33 on your head) to get to a CR of 10.5.

Disclaimer: the "base clearance volume" figure is based on that reported by others on this forum (and I guess corresponds to standard gasket thickness). Check your actual "clearance volume" accurately before machining anything. It's not my fault

Paddy

[dusty]

Excellent, thank you.

One question, I can see how it's relatively easy to measure the volume of the combustion chamber in the head using water, but how do you take into account the volume added/taken away by the top of the piston and the cutouts for the valves in the top of the pistons?

Would obviously be simple if the pistons were flat but they aren't...

Cheers
Jon

[paddy]

Don't ask me I'm sure the racing boys have ways to work it out precisely.

With the head off I suppose you measure the chamber in the head itself with oil and burette, and guestimate the various other constituent parts as in this post:

post42361.html

With the head on I guess you can measure the entire volume directly - turn the engine to TDC (measured with dial gauge through plug hole) on compression stroke and measure how much oil fills the chamber.

Paddy

[dusty]

And then of course different makes of pistons might have different cut outs!

I'm off to bed now before I cause any more head scratching

[rgh0]

Measuring the precise compression ratio requires careful measurement of a lot of components -most can be done with simple measurement tools but a few "tricks" help.

I do it as follows with the required mathematics and measure bore and stroke of the engine

1. combustion chamber volume - insert the actual valves and spark plugs you are going to use and invert the head and put a glass plate with a small hole in it over the combustion chamber and seal with grease to the head - fill the combustion chamber through the hole with kerosine from a burrette or other suitable volume measurement device ( eg syringe, pipette) until all the air is expelled.

2. Head gasket volume - measure the bore inside diameter and compressed thickness of the gasket you will be using. If you dont have a used gasket of the type and thickness needed a good estimate is the gasket will compress to around 60% to 75% of its uncompressed thickness. calculate the gasket volume from these measurements

3. Piston clearance volume and intruder volume if any can be done a couple of ways.

a. Directly by inserting the piston in the bore either at its measured top dead centre position or at a measured dimesnion below this so that the piston is totally below the block top deck - seal the piston rings with grease and use the glass plate to measure the volume in the bore.


b. Indirectly - measure the various piston dimensions and the volume of the cut outs and calculated the piston clearance volume. The valve cut outs are a difficult shape to measure especilly in a high compression piston with an big intruder. I do this measurment with plasticene or blue tack and then remould the needed bluetack into a cylinder or cube that I can calculate it volume from

cheers
Rohan