Compression vs. Altitude

[pgg]

Those tyre pressures won't change unless the tyres balloon - the pressure would decrease. (won't happen). Tyres are too stiff. Only heat changes the inside pressure.

[Jerry]

Guys...it isn't that hard. The basics have been covered...except Denver is the Mile-High city, elevation is 5,280 feet, not 7,000 feet.

Compression is based on number of molecules in a given volume. At higher elevations there are fewer molecules, so pressure (Boyle's law) is lower. When an internal cumbustion engine has fewer molecules to compress, power will be reduced as will compression readings.

We normall use PSI as an absolute, but it isn't. It is PSIA (atmosphere) and is relative to the surroundings. A 40 PSI tire at 5,400 feet would measure less at 1,200 feet because there are more 'outside' molecules at 1,200 feet; the ratio of molecules in the tire relative to the molecules in the air at 1,200 feet is less than at 5,400 feet.

To get around this for precision use, you have PSIG (gauge) readings. PSIG reads a nominal 14.7 at sea level. If you are trying to duplicte a condition, say a scientific expriment originally performed at sea level but you are at 5,200 feet, you need to use PSIG to compensating for different air pressures. The point is, there is PSIA, "normal" pressures for compression ratios and inflating tires, and PSIG, used to describing the differences in air pressures at different elevations.

So how does the pressure drop with elevation? A good chart is here:

Air Pressure / Elevation

Note: The decrease is not linear. The change in the first 3,000 feet is greater than the next 3,000 feet, 1.53 PSI to 1.39 PSI. The curve flattens out at higher elevations.

At 6,000 feet there is a 20 percent decrease in air pressure. (2.92 PSI drop divided by 14.7) If the XS compression figure is 142 PSI at sea level, it would drop to about 114 PSI at 6,000 feet. (142 times 80 percent.)

Turbos, btw, use PSIA...relative measurement. To get the same performance (number of molecules) of 7 psi boost at sea level you would have to set the Denver boost to around 8.4 psi.

So, although we use the same nomenclature - PSI - to describe compression, tire pressures, and other things, in fact we are describing a relative number based on the 'normal' air pressure at our elevation, not an absolute value. 114 PSI compression in an XS sounds low unless it is a reading taken at 6,000 feet, then it is right in line with the numbers in the manual, which are sea level specs. Like the price of gas and insurance, location does make a difference.

[pgg]

I'm going on my scuba basics. Take a 3000psi tank down say 330ft (11 atmospheres) - your tank still reads 3000psi. Conversly, take the tank to 50,000ft altitude - it still reads 3000psi.

[Shuriken]

The real question I have is whether my 110psi readings are good or bad.

As I said, on all of my bikes, including the freshly rebuilt 2-stroke, I haven't seen anything higher than 120psi at 6660ft.

Is 120psi about the most I am going to see on my instrument at this elevation or should I go by the book and interpret 142psi as what I should be seeing on these engines?

Ben

[Jerry]

I'm going on my scuba basics. Take a 3000psi tank down say 330ft (11 atmospheres) - your tank still reads 3000psi. Conversly, take the tank to 50,000ft altitude - it still reads 3000psi.

The difference is in the volume of air represented by 3,000 PSI in that tank. Ask yourself...how many party balloons could you fill with that tank at 330 feet under water? At 50,000 feet up? Quite a few more at the high altitude.

Volume, pressure, and temperature are all part of this equation. We have been ignoring temperture, but you have to look at pressure and volume to understand why PSI figures drop off in an engine while they don't in an air tank. In engines, the volume is constant, so pressure changes with elevation. In air tanks the pressure is constant, so volume changes with elevation. 2 different conditions.

The real question I have is whether my 110psi readings are good or bad.

These are good readings. You are right in spec.

[pgg]

Jerry, you're fired! No No - you're right. That tank of air will only last a few breaths at that depth. Your 20% O2 / 80% N mix will increase to 220% O2 / 880% by volume. Pretty lethal. And you'll fill a fair few balloons with 3000psi at altitude! Bottom line is, at altitude where the O2's thin, the bikes gonna suffer.

[Shuriken]

Did a compression test when the bike warmed up yesterday:
125-130psi on all of them at 5700ft elev. These are now the highest numbers I've tested on any of my bikes.

Highly encouraging results.

Ben

[steelcat]

I'm now getting 137 to 140psi across the board on my bike after adjusting the valve clearance. I was at the 110 range before. I live at 5050 ft. . Elevation affects vacuam (the best vac. I've seen pulled at this elevation is 28 in/hg on a tight a/c, best on a eng. intake is 25 in/hg), but should not alter compression more than a couple of psi. Lapping the valve & new rings etc. on my old girl would bring me back close to stock spec., but I can live with what I've got for now.

[Shuriken]

Steelcat, that's interesting to know. The values I measured varied just a little, but I did see 135 on one if I remember.

I haven't even looked at valve clearance yet because I haven't had the time just yet. I don't think it would be bad at all to just lap the valves and do the rings, but I think if I took the machine down to that level I'd probably just put the big bore in it.

My 80g is probably on the threshold of poor compression. Last I checked there was wide variance and poor numbers. She's smoking now from ring/valve/stem seals wear. I have thought over and over about rebuilding it but got a chance to get a motor with half the mileage. My idea was to start a rebuild on the lower-use motor.

I rebuilt a 2-stroke bike and see only 120 - but if I remember, that was cold, no oil.

Ben

[MartyA]

In a general sense, aren't 2 strokes lower in compression anyway?

[Shuriken]

Well, the manuals state that compression on 2 strokes can be misleading because the crankcase alternates pressure / vacuum states. I honestly don't know. All I can say is I could barely kickstart a CR480 and can kickstart my XL500 fairly easily.

Ben

[steelcat]

Ben
I'd go ahead & check the clearance on your valve shims. It's easy to do. A metric feeler gauge is needed along w/ a new valve cover gasket. Changing the shims is a bit more work but not bad, the biggest problem w/ the shims is getting a valve shim tool that works. I had to do some grinding on my valve shim tool, but after a bit of light grinding it worked good. I had compression in the 110 psi range before changing shims. After sync. the carbs the bike runs great, nice & smooth w/ good power.