I have heard recently that, compared to the heart shape wedge chevy design, the HEMI design is lack luster in performance wise. Dont they use a HEMI design in all three classes in NHRA POWERaide series? Everytime I see a pic of a head they always look like a HEMI. So does anyone that knows something about heads have something to say? I am stumped there.

Thanks
Jeremy

 

The engines used in top fuel dragsters and funny cars as well as most of the top alcohol cars, use an engine based on the Chrysler 426 hemi design from back in the 1950's. The hemi head on these engines have huge combustion chambers. Which gives you lots of room for large valves. But this limits how much compression you can get out of them. If your running a blower you are not concerned about having alot of static compression, so they are ideal for this purpose. Also the centrally located spark plug is the best place for it to be.

An good naturally aspirated engine's combustion chamber is as small as possible so you don't have to have a large dome on the piston to get compression. Large domes disupt the flame travel, which kills power.

There is nothing wrong with the old hemi cylinder design. It just is not do-able for todays N/A racing engines that are pushing 17:1 compression ratio. The newer hemi's that Mopar developed for pro stock are much better for N/A purposes but it's not exactly a true hemi head from the pictures i've seen.

My 2 peso's. Terry

 

The hemi's are great for flow but they have NO quench. This means little turbulence and not very efficient chamber design. Add the fact about a LARGE dome just to get any amount of compression and you only worsen flame travel and efficiency.

As mentioned, a blower and Nitro methane will produce its own turbulence and there for the quench is not needed and the better flowing of the hemi head is more of a factor.

The best recipe for most people seems to be use the smallest chamber possible that is unshrouded enough to flow well. Quench pads are a plus as well. Use a flat top (or dish) piston to acieve desires compression ratio. This seems to be more efficient that a large chamber- big dome motor.

At some point the small chamber will hurt flow and high RPM HP so it is all a compromise but for most REAL street cars with a 6500 RPM shift point, the fat torque curve that is associated with the small chamber/flat top set up is more desirable even though the open chamber/dome set up might flow better (10-20 cfm) and make slightly more HP above 5500 RPM.

This is my take on it.

NightTrain66

 

Thanks guys, I was thinking about it last night and realized that you would have lots of tumble in the chambers, but very little swirle in the chamber. I saw a pic of some Euro companies engine, forgot who it was, but they have seem to adopt a HEMI chamber, but one that has the vavle standing mroe straight up then what the old HEMI was. I know that is the deal with the new HEMI compared to the old one. The runners are almost standing straight up bcuz of the tightness of the trucks engine bay.

I am kind of curious what Chuck has any comment on this, since he works for the R&D company, if he has anything to say about this.

Wouldnt the canted valve design of the bbc have this problem too with getting a good compression and enough tumble and swiral in a chamber? It seems to have a similar design to the HEMI, just not as extreme. What about in 9000 rpm NASCAR engines? NASCAR still has the HEMI outlawed, is there something in the race design that gives it a competitive edge in those kinds of racing?

 

Whats the deal with the new hemi in dodges street cars? are they really a hemi?

they sure do get some **** POOR gas milage for a 350