I have 150 PSI compression. Cold engine, throttle open, all plugs removed. 327 350HP L79 11:1 compression. Now that might seem low, but I am in Colorado at 5000 ft and 80* out. Can someone come up with the sea level, standard day equivalent?

 

It's more like compression testing is a waste of time and effort.

Leak down testing results tells you the actual health of the motor.

In the past I built some solid lifter motors for my self and others. One of the best was a Crane Cams solid lifter 238/248 lower. 500 lift with 114 Lc. You would just need new springs and retainers

Just made 327s and old Lt1s a new machine. Like Jebby I put lots of time into 186 casting heads back in the day.

 

The factor at 5,000 feet is .8617
150/.8617=174

 

That's not what kodpkd is getting at with this semi-troll question.

Is the compression pressure higher or lower at sea level or 10,000 feet, mountain?

 

like melwff says, about 3% per 1000 feet. You may want to use density altitude chart since you are compensating for air density.

https://www.m0a.com/wordpress/wp-con...tude-Chart.jpg

Your DA is 7500ft. so down 22.5% roughly on air density.
150/.775= 193.5 psi.

 

Not to be a "troll" but that's about what I was thinking. No wonder the engine doesn't perform up here at 5000 ft on a hot day.
193 PSI isn't a bad compression number.

 

193 is good. Especially considering the shorter stroke, the age and the old tech of the cam.
People don't really understand it till they experience it. I'm running 197.5 average PSI on my 350 on an 80* day @ 4000 feet. So that's a DA of 6200 feet.
Short roller cam, high valve lift, high CR 10.6:1.

If I was at sea level theoretically I'd blow 242 psi. Don't know if it would, never been to sea level and to get there on density altitude it has to be -12* F. Not doing any compression test in those temps.

 

At Denver altitude, you're down 20% on power from sea level due to density altitude. That power loss is not recoverable without forced induction.

Compression test cranking pressure is generally no indication of engine condition - it varies grossly by cam overlap and other factors. As George correctly stated, skip the waste-of-time compression test and do a leakdown test of you want any indication of engine condition.

 

As has been stated not relevant really, but what is relevant is that they are all close to the same, usually less than 10% variation. For what it’s worth I had a factory z-28 near new at sea level back in the day and it seems to me it was in the 170 range

 

A compression test is relevant on a comparison basis. Comparing one cylinder to another and comparing one period of time to another, all with the same engine.

Sort of like chassis dyno's it's a comparison to measure changes either over time or due to changes in the engine.

A compression test is a relevant indicator of what kind of cylinder pressure your making, pre peak torque. Higher pressures are going to indicative of higher torque in that engine. Mine blows 197 @ 6200 DA altitude and produces good low down torque despite the elevation. If it blew 150 psi I can guarantee that it would be producing less torque down low.
I put a battery charger on my battery on 10 amps in order to get consistent cranking speeds during a compression test.

If I want to check valve and ring seal, and maybe cylinder condition, that's where the leak down test comes in very handy. And is a finer indicator of actual condition.
Problem with a leak down is it's not as easy as a compression test and requires an air supply.

I've done both. Usually I use leak down as a diagnostic tool or to confirm a potential condition that came up during a compression test. Compression testing I use as an easy way to see differences in pressures over time

Despite the shortcomings of the compression test I still find it to be relevant and useful information.

When I was racing motocross, (kickstart 2 stroke bikes) The compression test was the ONLY way we determined the health of the top end. When it got down 10% (5% if racing at a national level) it was time for a new top end.

 

I realize a compression test doesn't tell the entire story, but it is a start. It is interesting how much altitude and heat affects the pressure. My first problem was with the compression tester itself. The gauge to hose connector didn't hold air as solid as it should. I put a little grease on the fitting that helped a bit.
I will do a leak down test today.

 

Finished the leak down test. Engine warmed up. At 50 psi I got between 4 and 6 psi drop. So double that and I get 8% to 12% drop. All the cylinders match the compression test I did. Higher compression = lower % drop. I'm not sure how the altitude of 7200 ft would affect this. At 60,000 miles on a 327 I'm not sure how bad 8 to 12 % really is.

 

Altitude will have no effect on those readings.
Your pressure drop is good, especially given the mileage. No big problems there. Worn a bit, but not a mechanical issue based on drop.
Did you note if the air seemed to be escaping from the intake side (carb) or the exhaust side or just out the valve cover (into the crankcase) ?
Should be just out the valve cover primarily indicating some ring leakage. There are ring gaps so that is going to happen in any case.

Race engines have maybe up to 5% leakage. Normal engines around 10% to 15% Time for maintenance perhaps @ 20%. You'll get differing opinions on these numbers. It mostly boils down to where the air is leaking from and what the use of the engine is. If you run it hard and want more performance from it you may want better sealing. Once the rings leak enough to get to 20% you're probably burning oil by then have lots of blowby coming out of the oil fill cap etc.
It's a bigger issue if an exhaust valve is leaking. Possibly indicating a burnt valve.

here is a decent article on leak down testing.
https://www.motortrend.com/how-to/cy...eakdown-tester

 

One thing I have found useful in the tune of the engine is to run a lot of timing advance at idle for off idle response and initial acceleration.

Between vacuum and initial I'm running 36* of timing @ idle. 800 RPM out of gear 600 in gear. 19* initial and my vacuum advance limited (modified) to 17*.
Not sure how much you're willing to modify things, but there are ways I have found to improve performance at higher altitudes.

 

When I first moved to Denver I was told rule of thumb for timing, add 1 degree per every 1000 ft of elevation as a “ballpark” number.

 

Seemed like there was a consistent hissing from the valve covers. Nothing coming out the side exhaust, other spark plug holes or oil filler, I guess I shouldn't worry about doing a top end on the car? I have the timing set to Lar's spec's

 

Are you a real avi 8 or ? I am a Retired avi 8 or.

 

Currently an avi 8 or. It's what I do for a living. part 121 cargo.

 

I could have used that info a long time ago, lol. sounds like a good rule of thumb though.

 

I was UAL for 33 years.