Quote:
Originally Posted by BongoBennie
I'm interested in running higher compression on 87. TSX engine is 10.5:1 which seems possible. what about higher?
Also FWIW I fully expect less power than my current setup and I'm cool with that, I have other cars for that.
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The engine in my previous GMC Canyon had, if I recall correctly, 10:1 or 10.5:1 compression. It ran well on 87 octane. The best ambient temperature for gas mileage was about 70 deg F. It lost about 1 MPG per 10 deg F at lower temperatures. The mileage did not improve at higher temperatures. I think that the knock sensor was retarding ignition timing at temperatures above 70 deg F.
I saw timing as retarded as 2 deg after TDC at high manifold pressure and low RPM, and as advanced as 45 deg BTDC at low manifold pressure and higher RPM. That engine would slowly and smoothly accelerate from 21 MPH in fifth gear, which was about 600 RPM.
Your higher compression should be possible. Pay careful attention to ignition timing vs RPM, intake air temperature, engine temperature, and manifold pressure to prevent detonation. You may need a knock sensor to get your timing fully dialed in. I would not be surprised if getting smooth running at low RPM required ignition timing a few degrees ATDC at higher manifold pressures.
If you are happy with less power, try a camshaft with delayed intake valve closing to get some Atkinson cycle effect.