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compression too high

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dansawyer

Freshman Member
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A while back I had several MG and rebuild several engines. I kept the 'best' for the car I was going to keep. To make a long story short I ended up with too high compression, it is reading 180 accross the board - warm or cold.

I am not sure what this means. It runs great and has 7000 miles on the rebuild. I 'drive' it but I do not stay on it at high rpms.

What is your experience with high compression?

Thanks,
Dan
 
The only thing I could find referring to max compression was for a 1958 Dodge 325, 8.5-1 ratio. Max was listed as 155PSI with the throttle open, no pre-oilinj. This is the best I could do.

BTW My stock 78B runs 150 on all four.

Guinn
 
Hello Dan,

you cannot measure compression by compression figures, contrary as it may seem. There are a few factors that affect the reading.

If the engine runs well, without any pinking then I would not worry.

Alec
 
Hi Dan,
Compression pressure is usually not a very good indicator of compression ratio. One of the biggest variables affecting cranking compression pressure is cam timing. A short duration cam could show some fairly high cranking pressures.

If the compression ratio is too high you will likely get pinging at low & mid range rpm. Any thing over about 9.5 to 1 is likely to require premium gas to prevent pinging. Do make sure that the ignition timing is not advanced too far. Also it wouldn't hurt to check the compression gage.

It's probably not a good idea to stay on it at high rpm, for long times, in any event, unless the engine is race prepped.
D
 
Thanks all for your replies. I tried 2 different compression gages and they were the same to within a couple of psi.

Yes, if I advance the timing a little it tends to ping in the mid range. I notice nothing at higher rpm.

I am not sure what a short duration cam is? Is that a cam with narrow lobs and less overlap? It would seem this design would not breath very well.

Thanks, Dan
 
[ QUOTE ]

I am not sure what a short duration cam is? Is that a cam with narrow lobs and less overlap? It would seem this design would not breath very well.
Thanks, Dan

[/ QUOTE ]
Hi Dan,
Duration is the time that the valves are open, measured in degrees. An engine cannot begin to compress anything until the intake valve is closed. Since nearly all engines do not close the intake valve until the piston has already started up on the compression stroke, the engine does not achieve it's rated mechanical compression ratio until intake mixture inertia at higher rpm, overcomes the rising piston's reverse intake pumping. At this engine speed/mixture velocity, the engine will achieve it's rated mechanical compression ratio.

Depending on the cam design, this process can begin at low rpm, short duration cam, intake valve closes earlier, more cylinder pressure at low rpm - higher compression gage readings at cranking speeds.

A long duration cam would close the intake valve later & lower the apparent compression, cylinder pressure, until the intake inertia overcomes the reverse intake pumping. As you know, this reverse pumping (reversion) can cause a poor idle & lower compression gage readings. The trade off is better cylinder filling at higher rpm thus more horsepower. Longer duration cams usually do have more overlap.

Compression ratio, cam timing, & intake/exhaust ports/valves/plumbing must all be matched to each other & to the intended use of the engine.
D
 
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