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Discussion Starter #1
I was just playing with the zeal compression calculator and relalized that I had never entered the elevation where i live. I'm at roughly 3800 ft. when I enter that in it drops my "effective" CR from 10.15:1 to 9:37:1.

If you saw my previous post I'm trying to figure out how much compression I can pull on a motor that will run 87octane gas with a stock p28 ecu. (no tuning available around here).

General feedback I'm getting is that 10.5:1 is safe, and some people are even telling me that 11:1 is doable. (keep in mind i want to be able to run this without retarding a bunch of timing mechanically).

So how does this "effective" CR play into this? Or is the static CR the number I need to pay attention to?
 

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Discussion Starter #2
Anybody??? If elevation is reducing my compression ratio that much then there's a lot I can gain by factoring that in.
To run on pump gas, stock ecu, I should be safe at 10.5:1 compression. If the effective compression ratio is the number i need to pay attention then at my elevation to achieve that I would want my static compression ratio at 11.29:1.

But.... I don't know if there are other factors I need to consider!?!?
 

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Elevation will reduce cylinder pressures, not your compression ratio.

Cylinder pressure is the force that creates torque about the crankshaft, thus power.
 

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Effective compression should be what is important

Although at higher static compression will start to get into the area where clearances between the piston and the valves, head, and spark plug will be getting pretty tight.
 

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Discussion Starter #7
soo.... and I may ask my question over in the engine management section. So a stock ecu can effectively manage a engine with a not too radical compression change. (like has been done time and time again with many mini-me setups). I figure the MAP sensor will read the altitude. Will the ecu perform the same on a car with a 10.4:1 static CR as it will a 10.4:1 effective CR?
Do you understand what I'm getting at?
 

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93 Legend L Coupe.
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The stock ECU can actually handle a lot more than most people suspect because it is tuned ridiculously safe from the factory. However, relying on that for a build is silly.

All OBD0 Hondas sold in the US came with a barometric pressure sensor, at least to my knowledge. This allows the ECU to compensate for a wide variety of relative pressures. The tuning will not be very much different anywhere as long as you keep the Baro sensor functioning.

All this "effective compression" crap really needs to stop. That has way more to it than just elevation. There are many times where the ambient barometric pressure changes so much in one day just from a storm front moving in that you would see the same swings in power as driving up a couple thousand feet.

Stop fusing about calculated numbers and build something. Tune it and be happy.
 
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