Compression vs. Altitude

[Shuriken]

Hello XSives,

What is the relationship between compression PSI and altitude?

If the manuals state the XS should be at 142 or something like that, is that at sea level?

I just rebuilt a 2-stroke Suzuki 125. Bore/hone, piston/rings. At 6660 feet elevation, the max (cold, no oil drops) compression PSI I get is 120.

Assuming new rings/piston/bore and only getting 120 makes me wonder about my readings.

My 79sf read something like 110, 110, 100, 115 cold, no oil drops at 5700ft elevation.

Assuming elevation affects compression pressure (does it?), are my readings fair? I don't think I'll ever see 142psi at my elevation but I may be wrong...

Ben

[TopCatGr58]

Hey there Ben,

Air Density info from my "old" encyclopedia says sea level=14.7 psi; at 20,000ft it's 6.76psi, ~8psi drop, so at 6,700 feet, about 1/3 of 20,000, then I would expect about 1/3 of 8psi drop, or about 2.7psi. So, 14.7-2.7=12. 12/14.7=~82%. So, 142psi times 82% is around 116psi, so your figures at your altitude sound about right! Anybody else want to chime in or disagree?

[ratbyk]

Compression

I don't think TC has enough to do!
I concur on your reasoning though.
Ken/Sooke

[Shuriken]

That could be encouraging.

I did see some XS11 parts online last week - the seller said they came off of a bike that had 140psi at 8000 feet.

??

Ben

[MartyA]

Consider this. The gauge starts at zero regardless of altitude. So I am thinking that you should get the same compression at 8,000 feet as you would at sea level. The gauge is measuring only what is going on inside the chamber, and not compensating for external relative conditions.

Granted there is less air being taken into the chamber, but the compression ratio is the same, you are taking 120 cubic mm of air and squishing it into 12 cubic mm (whatever the actual measure is, it is rougly 10 to 1.)

Is this a logical counterpoint? I really don't know the answer. Just starting and argument...

[TopCatGr58]

picking up the gauntlet!

Okay, "S"marty,

Yes, the gauge starts at zero, but it responds to a certain amount of pressure. That pressure is developed from the compression of a certain volume of air, BUT at a specific density! The air at 7,000 is appx. 1/5 less dense, so it exerts 1/5 less pressure under the same amount of volumetric compression as a comparable volume of denser sea level air!

Back at ya!
T.C.

[Shuriken]

Good discussion, guys. This is exactly what I wanted. I am well aware of different factors that contribute to the condition of an engine, and compression is just one. But I'm trying to decide if compression by the numbers, at my altitude, indicates severe engine wear or normal wear.

Ben

[pgg]

If the motors' got good response and power, don't worry about all that stuff - you've got even compression - thats all that matters.

[Shuriken]

Normally, that's what I would have in mind - and not worry about the numbers. But since I'm comparing two engines just by the numbers, I can't really do that.

I haven't been able to use the new engine (ride the bike) because I'm still waiting on rear brake shoes and a rear master cylinder kit.

Ben

[TopCatGr58]

Rear Brakes

Ben,

Waiting on the rear brakes!? Who uses the rear brakes??
Seriously, ~80% of the braking force is on the front tire, why do you think they put 2 calipers there? Take it out and ride it, and use engine compression and the front brake so you can test the engine, you're not driving cross country!!!!
T.C.

[MartyA]

alrighty then

I consulted a perfesser (my spelling!) of physics, and a general science perfesser, and both, independently of each other, heard both sides and responded with "I'll get back to you." Figgers!

Analogy (my point):
Put 32 psi in my tires in Portsmouth, I have 32 psi in those tires.

AND

If I fill those tires in Denver to 32 psi, it is still 32 psi.

Anyway, that's what I was thinking.

[skids]

Re: picking up the gauntlet!

At a mile high, Denver, the density of air is indeed 80% of that at sea level (.062 lb/cuft and .076 lb/cuft). I wish I had access to my Ingersol Rand compressor handbook, 'cause I eat this stuff up. Forget about the ideal gas laws because the compression is too much for air to obey those laws. Let me just say that when the 78E had 122 to 130 psi compression, it pulled REAL HARD! :-)

Originally posted by TopCatGr58
Okay, "S"marty,

Yes, the gauge starts at zero, but it responds to a certain amount of pressure. That pressure is developed from the compression of a certain volume of air, BUT at a specific density! The air at 7,000 is appx. 1/5 less dense, so it exerts 1/5 less pressure under the same amount of volumetric compression as a comparable volume of denser sea level air!

Back at ya!
T.C.

[TopCatGr58]

Who's more dense!?

Hey there Marty,

BTW, I just saw your reply to the valve clearance question "After" I finished and submitted "my" reply.....clearly "we" have too much time on our hands!!!!
COUNTERPOINT:
Okay, riddle me this, you take those tires you filled in Portsmouth with denser air to 32psi at sea level, and then you drive/trailer them up to Denver's 7,000 feet, and you'll find the pressure will be much higher.....no accounting for heating affect, cause there is less atmospheric pressure exerting against the outside of the tire vs the density of the air that was used to fill them.

Conversely, if you filled them to 32psi in Denver, and took them down to sea level, there would be less pressure in them due to the increased surrounding atmospheric pressure vs. the less dense air that was used to fill them.

Think about a turbo charger at sea level, let's say it's giving 7lbs of boost. Now the combustion chamber can't get bigger right? So the turbo isn't putting in more volume of air in as much as it is increasing the density of the air....which is a round about way of putting more volume inside the combustion chamber....when the engine takes in it's load of air, instead of getting a certain volume of 14.7 psi dense air, it gets 21.7psi dense air......more air molecules in the same volume! I know racers of all types of vehicles have to adjust for the altitude when they set up their vehicles.

But I am curious to hear what the professors say!!
T.C.

[Shuriken]

I remember once going to San Francisco for a field trip in highschool. I took my football with me. Maybe I was looking for Joe Montana or something...

Anyhow, the ball completely deflated. That was the most curious thing I had ever seen with a football. It was inflated here - at 6660ft.

Ben

[MartyA]

"clearly "we" have too much time on our hands!!!!"
Yeah, and we basically agree on the shim sizes too!

You are correct about the tires and the change in altitude. No argument there.

I will go back to my original point WRT the supercharger. 7 PSI is still 7 PSI. You are correct about racers adjusting for altitude, but that has to do with molecules of oxygen available per cubic meter - a little less at higher altitude. You get say 82% in Denver of what you get in Portsmouth, so you have to reduce the percent of gas mixing to maintain the proper ratio. So 7 PSI boost in Denver will get you less performance than 7 PSI in Portsmouth, but you still read 7 PSI in both places. That's all I'm saying. I think the pressure gauges we are using for compression testing are taking 'relative' measurements, that is they start at '0' and read what the piston does in the chamber at that 10-1 ratio.

I will continue to bother the professors.

Or, lets take our bikes and gauges to Denver!!