1978 Scout II 345 Rebuild

[twistedaxle]

Originally Posted by Tim Potter

Must be all that California smog stuff I gotta live with

A bad EGR valve can contribute to detonation.

But as Tom points out, cam choice has a lot to do with it. The later the intake valve closes as measured in crank degrees after the piston reaches bottom dead center, the lower the combustion chamber pressure. Basically as long as the intake valve is open while the piston is swinging up into the compression stroke, you're not compressing the intake charge.

Here's a link to a static/dynamic compression ratio calculator:

http://kb-silvolite.com/calc.php?action=comp

If you want to get more into it:

http://www.amazon.com/Performance-Au.../dp/1934709476

You can't just go by the calculated compression ratio as no two engines are the same so not all compression ratios are created equal. Just by way of illustration you can have two 10:1 engines, one with a small closed chamber head and flat top pistons with valve reliefs (maybe a small dish) the second with a large open chamber in the head and a domed piston. The first engine won't knock because the spark plug is close to the intake charge and you get quick flame propagation and complete combustion. The second engine will knock because with the spark plug farther from the intake charge and the piston dome blocks the spread of the flame from the plug. It's a function of time; the lazier the chamber the longer it takes for the flame ignited by the spark plug to detonate the intake charge so the bigger the chance a hot spot in some other part of the combustion chamber will ignite part of the intake charge. You get uncontrolled ignition; detonation.

You don't have a choice about combustion chamber size & shapes on IHC engines. So you can't play with those variables to promote combustion. But you can reduce the chances something other than the the spark plug will ignite the intake charge by polishing the chamber. It never hurts. You get rid of casting irregularities and sharp edges that can become hotspots, the increased reflectivity seems to even out temps between hot & cold areas, and just polishing the chamber won't appreciably increase its size and lower compression.