One of the characteristic chambers that people are familiar with is the Chrysler Hemi. The engine had a chamber that was like a half of a baseball. Hemispherical in nature and in nomenclature, too. The two valves were on either side of the chamber with the spark plug at the very top. The charge burned downward across the chamber. That approach worked fairly well in passenger car engines but racing versions of the Hemi had problems. 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.

full article
http://www.germanmotorcars.com/Detonation.htm

 

Bah. Propaganda !!!!!!

I will admit it does have the nice feature of unshrouding the valves better, but it still ain't a hemi head. Even the article stated, and I quote, "However, the new hemi is not actually a true hemi as per its 426-inch predecessor. "

Did y'all know dudge couldn't meet EPA with one sparkplug while making the power they wanted, and vice versa, when it did meet epa, it was weak .

Hence the dual plugs.

 

I have a feeling whenever someone cracks the timing tables and starts messing with them there are going to be some detonated to death (dtd) hemis if they release those codes.

 

You are probably very correct, hence the reason Paxton refused to release thier supercharger with good boost numbers claiming the ring lands were blowing off the piston. Also I think they were leaning the engine out too much, If I remember 30 or so years ago you had to run the hemi richer than the wedges to achieve optimum horsepower. I do not remember anyone running 40+ degrees of timing though... but I am sure that some did. It is interesting to note that the piston is dished. Mopar engines have never had much of a quench area and my old 440 Roadrunner detonated it's A$$ off even with a 9.01 compression ratio and good timing. ( in the days after the crash of good gas )
We will not know what the Hemi engine will be capable of until we get the codes and grenade a few... but then is that not the way all engines are done? The envelope has to be pushed and new developments made.

TJ

 

Im interested in seeing what people get out of them. I agree I think with someone that knows what they are doing they are capable of alot (maybe more than an ls1 ) , I just think they are more volitile and wont take the abuse that a small block gm will. we'll see though

 

I got a question I heard chevy originally designed this head but didnt produce it ?

 

Actually, Rolls Royce used them in Aircraft engines in WWII and to my knowledge they were the first.

TJ

 

If you check out the link I think you will find there is a lot more things the new Hemi has in common with the LS1 if you can believe that. The big difference is in the heads, GM really did a good job on the LS1 head, the ports being very tall and narrow is the best of both worlds, you have the displacement of a large port but since the port always flows from the top down to the point of saturation you get the high velocity of a small port due to the narrow design but one that just keeps on flowing.
The block of the new Hemi is loosely based on the small block LA series but the mains are cross bolted and the cam is positioned high in the block, which I think the LS1 is also, both employ roller cams & push rods, the heads are aluminum. The rods are cracked cap ( are not the LS1's? ) but alas any forged internals belong to the 6.1.
I believe that you are correct on the statements you made earlier about the early Hemi's and thier fuel and timing problems, but I for one think I have some answers to those probs but I will have to get to the point where I can experiement to check it out. Those Ideas will have to remain top secret till then!

TJ