Quote:
Originally Posted by tom s
here is where I have a problem and maybe im stupid.We put larger cams in our engines to make more HP?I know they bleed off comp at low RPMs BUT they MUST add more in the mid to upper ranges SOOOoooo I guess if you drive a car with a big cam at idle it wont be detontation sen. BUT I seem to drive my cars in the mid RPM ranges where I might have a problem?Tom
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BINGO! Dynamic compression increases with rpm to a point then pretty much levels. There is also a big difference between preignition and detonation. Preignition usually destroys with little to too late a warning. Preignition can be caused by cummulative heat build up and usually occurs damage somewhere around bdc. The mix ignites as the engine is trying to compress it. Sustained detonation can lead to preignition from heat buildup but its seldom the primary cause.
Knock or ping that you hear is detonation. An engine can sustain quite a bit of it. However the duration and intensity affect how long before you kill bearings and eat rings. Preignition WILL burn holes thru your pistons if your lucky and if they dont blow thru first, something else catastrophic will likely occur. (I've burned down the same motor and cylinders 3 times in a 1 month period all 3 were preignition and the problem appeared at seemingly random intervals. Turned out to be compression related though I actually didnt lower the compression ratio till I figured out I had changed dynamic compression ratio. Engine had NOT ONE sign of ping just sudden rapid failure when preignition occurred)
Some more about compression ratio, fuel capability, and ignition needs. Usually "ping" tells us we have spark occuring too soon BTDC. We fire the spark btdc so that the cylinder pressure rise can reach peak cylinder pressure ATDC. Typically PCP needs to occur @15 deg atdc(iirc) or thereabout to reach optimum rod/crank angles for best power. If you are running excess cylinder pressure for the fuel being used it will explode too fast(octane partially controls the speed of this flame travel. Heat of compression also factors into how fast this burn occurs.)
A little too early we hear a light ping of detonation and little else occurs unless its sustained and intense. As we retard spark to prevent the ping we LOSE HP due to the loss of mechanical advantage of rod, piston, and crank positions. It usually ends up excees temp not used to move the piston ends up transfering more heat to your water jackets instead. Often turning your car into a highway boiler at normal road loads when it never did it before you "upped the compression and made compromises to run a less than adequate fuel octane".
I'm in the middle of rebuilding my main pc and there was a related link I'd like to share on there. Hope I havent wiped it.
So I'll summarize that its best to work with known "safe" compression ratios and going beyond them is likely going to require a compromise that overall could = zero net gain or cost you needlessly later.
If your motor is not built yet you can build it with whatever head you chose and tailor the compression to best meet the needs of type head you have and fuel you plan to use. Its good to have safety margin to accomodate the possible bad load of fuel you may get when a delivery truck accidentally starts to pump the wrong fuel in the wrong tank etc.