Compression questions

[BAMF87]

I have maybe 15 hrs on a new top end with hjr dune port and milled stock head for pump gas. I'm at 2500asl and both cylinders are at 120psi. Shouldn't I be higher on compression?

 

 

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[uman1030]

As you your altitude increases the air thins giving you a lower cranking compression. this doesnt change compression ratio but just effective compression. at 2500 ft asl your cranking compression reading will be about 93% of what you would be at sea level. also other factors that come in to play like air temperature as well

[BAMF87]

Doesn't that still seem low? I'd like to be in the 150 range at sea level, guess it's time for a cool head

 

 

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[uman1030]

depending on port timing also it can show lower numbers during cranking compression test cause ports overlap letting some compression leak out. also air temp changes density so 93% would be at 59F wich is what most altitude compensation charts use.

 

general rule of thumb is about 4% per 1000 ft

[BAMF87]

Ok thanks

 

 

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[thomas h.]

I got a little more at sea level with my HJR port. I though I wanted more too but the crap 91 we get here makes me Change my mind.

[uman1030]

I got a little more at sea level with my HJR port. I though I wanted more too but the crap 91 we get here makes me Change my mind.

whats your cranking compression at sea level? and is it a dune port also?

[thomas h.]

D port 4 mill 127

[Surfrjag]

Could also have wore those rings out.

[uman1030]

D port 4 mill 127

then with a 93% altitude compensation compression adjustment that puts him not far off then with in a couple psi. according to the chart he would be at about 128 at sea level.

 

close to the same assuming you both have similar combustion chamber volume.

 

Compression Test Altitude Compensation Factors

Altitude Factor

500 0.987

1500 0.960

2500 0.933

3500 0.907

4500 0.880

5500 0.853

6500 0.826

7500 0.800

8500 0.773

[thomas h.]

I told HJR I'm looking for longevity not race

[thomas h.]

Did you get a fresh bore?

[thomas h.]

Or a second comp gauge with wot ?

[BAMF87]

Mine got a fresh bore, new pistons and rings. One dune trip on them. I bought the gauge at oreilly auto parts

 

 

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[hercalmighty]

I had a no name compression checker and it read about 15 psi compared to my craftsman one. Its best to do a compression check when you first rebuild so you have a baseline to read.