If you haven't read this article before, its some GREAT info on detonation and pre-ignition.

http://www.streetrodstuff.com/...I.php
page 1 - 4

http://www.streetrodstuff.com/...V.php
page 5 and beyond.

specifically, on page 5:

Because the chamber was so big and the bores were so large, the chamber volume also was large; it was difficult to get the compression ratio high. Racers put a dome on the piston to increase the compression ratio. If you were to take that solution to the extreme and had a 13:1 or 14:1 compression ratio in the engine pistons had a very tall dome. The piston dome almost mimicked the shape of the head's combustion chamber with the piston at top dead center. One could call the remaining volume "the skin of the orange." When ignited the charge burned very slowly, like the ripples in a pond,, covering the distance to the block cylinder wall. Thus, those engines, as a result of the chamber design, required a tremendous amount of spark advance, about 40-45 degrees. With that much spark advance detonation was a serious possibility if not fed high octane fuel. Hemis tended to be very sensitive to tuning. As often happened, one would keep advancing the spark, get more power and all of a sudden the engine would detonate, Because they were high output engines, turning at high RPM, things would happen suddenly.

Hemi racing engines would typically knock the ring land off, get blow by, torch the piston and fall apart. No one then understood why. We now know that the Hemi design is at the worst end of the spectrum for a combustion chamber. A nice compact chamber is best; that's why the four valve pent roof style chambers are so popular. The flatter the chamber, the smaller the closed volume of the chamber, the less dome you need in the piston. We can get inherently high compression ratios with a flat top piston with a very nice bum pattern right in the combustion chamber, with very short distances, with very good mixture motion - a very efficient chamber.

Look at a Northstar or most of the 4 valve type engines - all with flat top pistons, very compact combustion chambers, very narrow valve angles and there is no need for a dome that impedes the burn to raise the compression ratio to 10:1.


This is an engineer for GM. He is referring to the Hemi and the insane ignition advance it requires. For anyone who has seen the inside of an H23 vs. an H22, you know the piston is flatter on the H23 to achieve the same compression ratio (both the stroke and the reduced valve angle are to blame).. doesn't it make sense the H23 would resist detontation more due to a faster combustion wave, which allows for less ignition advance?

any thoughts?

 

I'm not familiar enough with the differences between the H22 & H23 piston & combustion chambers to contribute much, but I am interested in seeing more conversation on the topic . . . .

How different are the tops of the combustion chambers between the two engines? You mention a different valve angle, I hadn't realized that - is that an adjustment on the H22 to make room for the VTEC accessories? Is it a compromise or an improvement?

 

Very interesting....

In theory, yes, the H23 doesn't need more advance. But I think that the difference in combustion between the H23 and H22 would be nominal.