Higher compression ratio, for higher altitude.
 

Hello. I understand that at a higher altitude there is less air density, so compression in the engine reduces, and this in theory, should give less mpg. Which is why I think my car is not getting stock mpg.

I am at 5000 feet above sea level and on a compression test I get 150 psi all across, at sea level that would be 180 psi. Which is good compression for my engine (d15b7).

Now my engine is all stock, no mods, almost every sensor new, and engine just rebuilt a year ago.

I am thinking of milling .040 off the head, and use a d16xx head gasket. This should give me a higher compression ratio. from 9.1:1 (stock) to about 10.1:1.

Now at my altitude, those numbers would be about 8:1(stock) and 9:1(milled). So it would be like getting back the power I have lost at this altitude.

This project is for better gas efficiency. Is this idea good? It is a low cost project that in theory should give me better mileage.

Any suggestions? Am I wrong in thinking it will help mileage?

Well, actually, the engines compression ratio does NOT change, just the density of the air going into the engine is less than at sea level...so the engine is compressing a less dense AF mixture to start with.

Supercharging (continuous engine driven) or turbocharging (variable exhaust driven) systems and turbocompound (both) technologies were developed initially for aircraft, but have been also encorporated into automotive vehicles, most notably: (a) diesel engines (turbos) and (b) drag racing engines (superchargers).

You didn't mention (in this posting) which model/engine your car is. Is it fuel injected? Can it run on ethanol-laced gasoline, E15-E85, etc.?

Running at 5000ft will have the same effect as dropping nearly 2 full numbers off your static compression ratio.
If I knew I were going to stay above 4000ft I wouldn't hesitate to go above 11:1.

It depends on your drive.

For a gasser the altitude should not have that much of an effect.
The density of the air/fuel mixture is primarily governed by the throttle plate. The only real difference is at idle (needs to be set wider at altitude) and WOT.
WOT at altitude would be the same as just below WOT at sea level.
At altitude you'd just push the pedal down slightly deeper.

Lower atmospheric pressure reduces pumping losses. You could get better mileage in the mountains than is possible at sea level, as long as you stay level. That's usually a problem at altitude. Steep uphills kill FE and steep downhills just waste potential energy.

Carburetors may act differently at altitude; the fuel mixture may become too lean. With modern engines and injection the ECU will command the right mixture.

For a diesel, raising the ratio could help because diesels don't regulate power with a throttle plate; and they use a higher compression ratio.

Thanks for the reply. I know it does not lower compression ratio, it just acts like it has a lower compression ratio due to less air density. I get 150 psi on a compression test here, and at sea level it would give me 170 psi, so its not affecting the engine only the amount of air.

I dont want to invest much in this car, so a turbo charger is way too much. But do you think higher compression ratio will help?

My car is a 93 civic lx, d15b7 engine. Here in mexico we do not have ethanol gasoline, only 87 and 93 octane without ethanol.

Any way to go even higher like you say, without doing huge mods? it is a 93 civic d15b7.

The only way to go higher than 11:1 with out a ton of mods is blend E85.

Ok, well do you know how much I can mill the head without having valve problems? I cant find the stock piston to valve clearance. It is a D15B7. Never been milled.

Hey guys.
I am getting varying answers regarding the following numbers on a 92 Honda Civic VX with the dz engine. This engine has 270,000 miles on it, but I brought this car from North Carolina and acheived about 48 mpgs the first two months at 5280 elevation.
1-128, 2-129, 3-120, 4-130. Assuming it was measured properly at the dealership during a full diagnostic. They said my engine new would expect to measure at 150.
The original service manual says 135 min-180 max. , but doesn't mention readings at elevation.

Bump it to 10.5, but I don't think it will help mileage. Use premium if you go down to sea level.

regards
mech