compression at higher elevation question

[grumpyone]

Can anyone tell me how much elevation effects the amount of compression on a 1986 R80?? We live at about 6300 ft and have always thought readings were sea level in most books and that elevation effects it quite a bit???
Any help appreciated
Thanks
Jim

[glennhendricks]

The compression is the difference in the cylinder volume at bottom dead center and top dead center. That won't change with the altitude because it is just the ratio between the two values.

However at altitude the air is a lot less dense, on the order of 25% less dense at your elevation and sea level. That means that there is 25% less oxygen in the fuel air mix so engines lose power at altitude. Modern fuel injection measures the air going in so the difference is not as noticeable as with carb engines.

I used to tweak my timing ahead 1 degree per 1000 ft of altitude on my old Cougar in Denver. That was a pretty common fix back in the day.

[skiteach]

My .02. We always used compression as a comparison between cyls. Providing they were near manufacturers specs. Cranking speed plays a large role in most engines. I always found leak down testing to be definitive on suspect cyls.

[dsenn]

compression and compression ratio are two different things. The ratio will not change at altitude but compression will as it is the result of the ratio. Say you have a 10-1 ratio, air pressure at sea level roughly 14psi compressed 10 times gives 140 psi. at higher altitudes air pressure is less giving less compressed pressure.

[glennhendricks]

:doh :doh Had a Homer Simpson moment.

[grumpyone]

So if you spin the engine till the compression maxes out whicnh has always been my way of doing it, it is done and would read the same at sea level or 8000 ft??

[dsenn]

compression at 6300 ft. will be slightly less than at sea level, can't tell you how much less as there are several variables when doing a compression test. A cyl. leakdown test is a better check of engine condition as it will tell you several things with one test, leakdown testing is how it's done on light aircraft.