benefits of 18:1 pistons in an NA engine

[dieselhumvee]

Doc,

As part of my ongoing effort to make more power and quiet down my Hummer, I got to thinking if lower compression might help on both fronts, and here is why:

it seems to me that the diesel "knock" is made substantially louder by direct injection and timing, since there is less cylinder pressure in an 18:1 engine than a 21.3:1 engine, wouldn't that translate into a lower volume diesel "knock"?

I got to thinking of how a gas engine when detonating sounds like diesel knock but is much lower in volume, and the compression of a gas engine is much less too....coincidence????

Finally, I think lower compression may improve power by lowering cylinder pressure thereby loweirng cumbsution temps and EGT, allowing me to turn up the pump more. Another key is that lowering compression means more piston to valve clearance, allowing a more agressive cam such as the one the diesel depot offers.... do they even reccomend it for high cmpression 6.2's? would the valves hit the pistons?

Just some things I have been pondering, but I was curious if your engine seems quieter since the 18:1 pistons?

[dieselhumvee]

I know most people run 18:1 pistons just so they can run more boost, but what if theroretically speaking, an engine would stay NA it's entire life..... would it still be a good idea to lower compression if it would never see a turbo?

[john8662]

I think that the ONLY reason for moving to the 18:1 pistons would be to run more boost that stock. By running the lower compression ratio you will want a turbo to compensate for the loss of power from lower compression. Think worn engine, and how much better a freshly overhauled engine (after break in) runs. I don't think that it would be worth it N/A for the loss of power, and harder starting. You'll basically find out that at some point you will smother the engine by turning up the fuel. But you are right in that the engine would be quieter, as most of the ol worn out 6.2's that I drive.

[Peter J. Bierman]

Peninsular did go to 18:1 for reasons of wear on cylinders and pistons.
Bench tests showed that lower compression engines running at full power for log periods had little or no wear on these ereas and that normal compression engines running the same time at the same power output performed much less in this field.
theoreticly lowering compression should gain a little power couse the compression takes less energy which can be used to drive the wheels.
Conciddering that 17:1 is ideal, I think you will not loose power.
Main reason for the high 21.5:1 is cold starting performance, so you might find some negative results there.
On the other hand, no turbo is helping to start couse it's not working without exhaust gasses.
I don't see any real objection but there will be not much gain either so it's all in the $$ :D

Peter

[gmctd]

The intense ball pein hammer on cast iron sound of the indirect injected 6.2\6.5 is caused by combustion in the pre-cups, which expands them into the heads.

A direct injected diesel has the tinkling combustion sound, similar to pre-ign in a gasser, which sound is masked by all the little pall pein hammers in our engines.