Computing Compression Ratios

[edward ericson]

here’s a fun tech question: is it possible to compute static compression ratio from the PSI reading you get from a compression test? And if so, what’s the formula?

[Royal]

Ed, If “you can handle the truth” here it is as I know it.  Yes, in an ideal world, you should be able to calculate compression ratio from the pressure you get in a compression test. 

BUT, this is not an ideal world! 

There are too many variables: the RPM at the time of the test depending upon the oil weight, ambient temperature, how good of a connection between the battery and the starter motor, and the condition of the battery.   

And the fact that the compression test pressure also varies dependent upon vacuum as the air is drawn into the combustion chamber.  You are not starting with the same amount of uncompressed air at higher engine speeds since that air is actually “under” a vacuum at higher speeds.  (This is the reason that you are supposed to make sure that your throttle is open and the carb choke is not closed when doing a compression test.)  And the intake and exhaust manifold sizing, and cleanliness (porting and polishing) and valve size as well as valve timing effect compression test pressures as well.  

Higher compression ratio gives us more energy per detonation since it is just that much more fuel/air mixture that is being burned.  What we really want is for the compression ration be as high as possible for the given the fuel that is available to us at the pump.  But if it is too high, then the engine “pings” as it detonates because of other factors (heat generated by the compression process eg) and that causes shorter engine life.

So, what we are really looking for in a compression test is a noticeable difference between cylinders as a troubleshooting tool.   

Depending on what you read, a 10-15% difference from one cylinder to another is the point where you need to fix something (valves & seats or rings or both).   

Ed, is there a prize awarded for the best answer?  If so, I can elaborate on the above. 

Gee, I forgot to mention that you are supposed to run compression tests at normal engine temperature, but you are not supposed to remove the plugs from our aluminum heads unless the engine is cool, therefore how could you possibly get a good number to enter into the formula that you are asking for? 

[edward ericson]

AH, OK. I thought not.

And that cool plugs/hot test paradox is just one of several, isn’t it. I ran a compression test a few years back on the Mercury outboard that came with my boat. Supposedly it was a runner; I should’ve done the test before I shelled out the $1,000 for the boat. One cylinder gave zero (0) compression.

I never tested Bridget’s yet. When I bought her I was planning on dumping the engine in favor of an electric motor! Another “Eddie theory” (as they used to say at the gas station where I worked as a teenager). Sometimes my theories pan out, other times, not.

Pretty sure Bridget has decent compression but will give it a (cold) spin some time this summer just to see how close the numbers are. The ratio in there is going to be low. She looks and feels like a stock 1300 with 1500 or maybe 1600 jugs.

And I suppose there’s no what to know how many CCs I have without pulling the heads off and carefully measuring the bores.

[Royal]

     A cold compression test is certainly better than no test at all and would give a good indication of an engine problem that requires top end work. 

     You might not want to test Briget if she is running well.  MiGi was running well but I decided to run a compression test and discovered that #1 cyl was very low.  Didn’t sleep well until I pulled the engine and rebuilt. 

     A stock 1300 with 1500 or 1600 heads will feel like a 1500 or 1600.  The major differences between these (single port) engines is in the oil passages are larger (10mm vs 8mm) on a 1600sp (this is a 1970 only engine before they went to DP).   The carb is slightly different also but you might not notice it. 

     Most people unless you are going for an oversize setup and had some block and head work done would not bother CCing the engine.  I did it because I bought the heads used (they had just come back from an engine shop) and they had just had larger valves and some head work done.  I wanted to make sure that if I installed them that they didn’t cause some ridiculously high ratio. 

     Lets talk about an electric motor for propulsion at Carlisle.  My exceptionally talented brother is in the progress of rigging up a ev Yugo as we speak. 

 

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