• Hi Guest!
    If you appreciate British Car Forum and our 25 years of supporting British car enthusiasts with technical and anicdotal information, collected from our thousands of great members, please support us with a low-cost subscription. You can become a supporting member for less than the dues of most car clubs.
    There are some perks with a member upgrade!
    **Upgrade Now**
    (PS: Subscribers don't see this gawd-aweful banner
Tips
Tips

altitude

Share this page

tahoe healey

tahoe healey

Yoda
Country flag
Offline
Hey, this is a good one but maybe I'm wrong. I did a compression test again last weekend and got 125 on all six. So I asked a local mechanic and he said it's the altitude (we are at 6246 ft). He said its ok because they were all the same. But I'm thinking I'm measuring the pressure in the cylinders and not outside like in an airplane. What I really think is the guage is wrong (very cheap at Harbor Freight)
 
I think that your gage is reasonable & the mechanic is correct. Atmospheric pressure is about 14.7 psia at sea level. At 6,000 feet the pressure is around 11.8 psia. This is a decrease of 20% at your altitude.

Since atmospheric pressure is what pushes the air into the cylinder on the intake stroke, there will only be 80% as much air to compress at 6,000 feet altitude. So - you should expect a 20% decrease in compression readings from what the test at sea level would read. A reading of 156 at sea level would only be 125 at 6,000 feet. a 156 reading at sea level is entirely reasonable for your car. Drive it down to the coast & check it. You will be surprised at how much higher the readings are. This is why engines lose power at high altitude.
D
 
Jeff,
Yes we did. I noticed before the forum change to the new server that the archive search function only returned about a month or less of Healey related topics. It's still that way. A shame that we can't refer back to any of the previous info.
D

[ 03-26-2004: Message edited by: Dave Russell ]</p>
 
Thanks again, Dave. My logic was that the pressure in the cylinders was caused by the piston compression unrelated to the atmospheric pressure and altitude. Hey, this is a good thing to be wrong. I don't have to spend money or time.
 
You can have more fun with this. Call the automated weather at S.Tahoe Airport (530) 541-5739 and in the remarks section it gives Density Altitude. That combines Temp, Pressure and Alt. It will give you an accurate picture of whats going on that day. To start with, hot Summer temps thin the air and the higher you go the air gets thinner, that's why intercooled turbos can help piston engine planes fly more efficent at higher altitudes. Lets you pack more air into the cylinders. Best approach is to leave the gauge at home and drive your Healey to sea level and enjoy the beach!!! Shelly
 
I've read that there is an approximate 5% decrease in power for each 1000 feet increase in altitude. Is this rule of thumb true?
 
In general, given that atmospheric pressure - air density (the thing that charges the cylinders) drops about 3.3% for each 1000 feet of elevation increase - and that this causes non computer controlled engines to run richer as the density decreases -- your estimate is pretty close. Note that computer controlled engines compensate the mixture for altitude variations & don't lose quite as much. The above statement does not take local barometric pressure variations & temperature (density altitude) into consideration.
D
 
When we drove out to Tahoe in 2002 several of us chose to ascend Pike's Peak which, I believe, is around 14K. 4x14=56 and there sure wasn't much power left for the final part of the climb!
 
When I used to take a fast two stroke street bike to the higher elevations, around 10,000, I actually had to re-jet the thing to get it to run. Carried three sizes leaner main jets just for the occasion. Two stroke engines are tempermental about fuel mixture.

Let's see, 44% of 140 hp still leaves 62 hp, not much. Imagine what a smaller engine would be like.
D
 
Back
Top