How much is too much compression?

[crew84]

I finally compression tested my engine that I now have 19K on it. I tested around 525psi compression around. Now I wonder if that is why I get high EGTs when the pump is turned up so it runs strong? Maybe not enough CCs for the complete burn.

[britannic]

That's excellent compression - but certainly a bit higher than normal! How many puffs per cylinder were used to record that reading?

My manual says the following:

Record compression gage reading while assistant cranks engine. Allow six puffs per cylinder.

Normal compression should be 380-400 psi (2620-2758 kPa).

Repeat steps for each cylinder. Lowest cylinder reading should not be less than 80% of
highest cylinder reading, and no cylinder reading should be less than 380 psi (2620 kPa).

Remove and inspect valves and piston rings for any cylinder that fails test.

I can't say from own experience or research that an extra 125psi or so of compression could influence EGTs - personally I doubt it, but others here may know better.

Factors that I know affect EGTs (by no means inclusive) are:

1. Too high a gear and throttle opening so that there's not enough air flowing into the engine.

2. Restricted air intake and/or exhaust.

3. Overfueling.

4. Retarded cam timing.

5. Retarded ignition timing.

6. Malfunctioning turbo.

7. Restricted coolant flow around some of all cylinders.

8. Overloaded engine.

9. ... etc. ...

[crew84]

I tried to count 'Puffs' to six. The first reading was 550psi when I did not count them. The engine has Total Seal Rings. It has been decked and the piston heights have been adjusted to -.002 to -.005. I am using a NAPA Compression Checker. I do not know how accurate it is. I have the Dual Idler Gear set. I have not tried to open the stock Banks air intake plate in the radiator support yet. I have opened the exhaust before the muffler with no change in EGTs.

[britannic]

Sounds like those total seal rings are awesome and revise what a "normal" compression psi is!

Presumably the adjusted piston heights are matched to the amount of "decking" that was performed and haven't unduly raised your compression ratio?

Do you know where your timing is at? After the exhaust and air restrictions have been taken care of, timing is the next biggest single factor that will influence the EGT. After that is how much extra fuel is being injected after turning up the pump.

[crew84]

I allowed for the decking, but did not cc the heads after they were surfaced. I degree set TDC on the engine and used a KD timing meter using the injection pulse method. I set the timing at different settings, but not more than 5 degreesBTC. The more I turned up the pump the more power/EGTs I had. Man did it run good!! I turned it down to the max/1050 degree EGT. Lost most of the power.

[britannic]

Originally posted by crew84:
I allowed for the decking, but did not cc the heads after they were surfaced. I degree set TDC on the engine and used a KD timing meter using the injection pulse method. I set the timing at different settings, but not more than 5 degreesBTC. The more I turned up the pump the more power/EGTs I had. Man did it run good!! I turned it down to the max/1050 degree EGT. Lost most of the power. What rpm is 5 BTDC at? Does your timing meter compensate for actual combustion moment (the pulse method doesn't actually meter the point the charge ignites)?

Start with TDC @ 1400rpm, check the EGTs and then experiment with TDC @ 2000rpm.