High altitude

[craigca]

hello all---I've been off the forum for a few years....I am running a stock M37 at 8k-12K feet in Colorado. During the summer, I vapor lock, so I have solved this problem by rerouting the fuel line and using an electric fuel pump.

Now, i find a loss of power at the high altitudes, so timing advance, leaning the idle screw and removing the oil bath intake helps restore the power. I am thinking of removing the stock elbow intake from the carb, and using some kind of after market air cleaner directly into the the carb base. Anyone have a recommendation to repace the oil bath and elbow intake assembly and substitue a shorter intake---"make it easier to breath?'

Thanks

[ZGjethro]

Hi Craig. John here. Are you my neighbor here in Mountain Valley? If so, you have seen my truck and the air cleaner and fuel line routing I have done. I have a short piece of straight pipe with a K&N filter on it. I have some of the same pipe left over if you want to try it out, or you could bolt on my current intake to your truck for a test drive. Let me know. It would be pretty easy to weld up another intake

[JimC]

On a standard day, your power is ranging from 25% to 44% of sea level power. Less on a hot day. Have you considered a repower with a larger displacement engine?

[craigca]

Hi john, yes I am your neighbor in MV.....I would like to have a section of your pipe, and try it with an after market air cleaner I have.....I chose to use a electric solenoid fuel pump because it seemed like a better idea to "push" fuel in a vapor lock situation, rather than "pull" the fuel. Bypassing the mechanical pump did solve the vapor lock problem.

Your rotary vane pump , pressure gauge and regulator seems like it might be a better set up for high altitude. The higher I go, the "richer" the air/fuel mix, and the less power I the truck has. Can you reduce fuel pump pressure/flow with your set up?

[craigca]

On a standard day, your power is ranging from 25% to 44% of sea level power. Less on a hot day. Have you considered a repower with a larger displacement engine?

Hello Jim.....it feels like I am running 25-44% of power....how did you figure the power drops to that range?

[ZGjethro]

Hi john, yes I am your neighbor in MV.....I would like to have a section of your pipe, and try it with an after market air cleaner I have.....I chose to use a electric solenoid fuel pump because it seemed like a better idea to "push" fuel in a vapor lock situation, rather than "pull" the fuel. Bypassing the mechanical pump did solve the vapor lock problem.

Your rotary vane pump , pressure gauge and regulator seems like it might be a better set up for high altitude. The higher I go, the "richer" the air/fuel mix, and the less power I the truck has. Can you reduce fuel pump pressure/flow with your set up?

Craig, I can adjust the pressure. I have it at 4lbs now I think. Tinkering with the pressure is the reason I have a pressure gauge permanently installed.

I used my digital calipers to measure the inside diameter of the carb intake, and bought some 1/4" wall thickness pipe with the same inside diameter. I also have some steel plate to make an intake flange with if you want a straight pipe intake. You can borrow my current intake to see if it makes a difference. I have the distributor vents and oil filler vent hooked up to it, so it will probably fit your truck. No brake or gas tank vents though.

[JimC]

Craig, 25% and 44% are the density ratios of the standard atmosphere for the two altitudes you mentioned. I ignored temperature deviations from standard atmosphere and ignored humidity.

[ZGjethro]

Hi Jim. I was a little puzzled by the number you posted so I looked up the pressures using this converter http://www.altitude.org/air_pressure.php. At 8000' we have 76% of seal level air. At 12,000' there is 65% of the oxygen present at sea level. Is this a different way of saying the same thing you mentioned? We do not have 25% or 44% of oxygen, but we might be down that amount at 8000 or 12,000 feet

[JimC]

No, it isn't the same thing. I said density ratio. You are reporting pressure ratio.
% oxygen doesn't change with altitude. It remains about 20.95%.

And I screwed up. Got off on the wrong line of the chart. Wish I had said more so I could take it back too.


8000' density ratio 0.7861
12,000' density ratio 0.6933

Since I know perfectly well that my planes generate 75% power at 8400' DA, that's proof I had my brain out of gear. Sorry.

[ZGjethro]

You are right. I knew the oxygen ratio didn't change, but the pressure does, along with the amount of oxygen in a given volume.

I ride a 300cc two stroke dirt bike, and it can be a handful up here at 8000'. I always wondered what bikes like that are like at sea level, with a third more horsepower than I am used to.