HEMI Engines

[Nayr747]

Ok, this isn't DSM specific but I thought it might help people understand engines a little better.

So seeing all the new commercials for cars with HEMIs, I started wondering about hemi engines and did a little research. But to me it seems like they aren't even as good as normal engines. They can only have 2 valves per cylinder and have to have 2 spark plugs per cylinder. The only difference other than that is they have a hemepherical head design. But the normal design in cars is a pentroof design (which is better right?) So why are they putting them in every GM car? Also, why do hemi's make so much hp if they aren't as good? The new Jeep Grand Cherokee has over 300hp. And look at these other #s. Look at the liters compared to hp. The top one is the hemi:

Dodge 5.7 liter V-8 - 345 hp @ 5400 rpm
Ford 5.4 liter V-8 - 260 hp @ 4500 rpm
GMC 6.0 liter V-8 - 300 hp @ 4400 rpm
GMC 8.1 liter V-8 - 340 hp @ 4200 rpm
Dodge 8.0 liter V-10 - 305 hp @4000 rpm
Ford 6.8 liter V-10 - 310 hp @ 4250 rpm

Here's a quote from the site:
"One thing that a hemispherical head will never have is four valves per cylinder. The valve angles would be so crazy that the head would be nearly impossible to design. Having only two valves per cylinder is not an issue in drag racing or NASCAR because racing engines are limited to two valves per cylinder in these categories. But on the street, four slightly smaller valves let an engine breathe easier than two large valves. Modern engines use a pentroof design to accommodate four valves.

Another reason most high-performance engines no longer use a HEMI design is the desire to create a smaller combustion chamber. Small chambers further reduce the heat lost during combustion, and also shorten the distance the flame front must travel during combustion. The compact pentroof design is helpful here, as well. "

http://auto.howstuffworks.com/hemi.htm

[drdisque]

the combustion chamber on the modern hemi engine is not truly completely hemispherical, therefore, they could make a 4 valve one if they wanted, however, that is very difficult with a pushrod engine.

The engine is so powerful because it has lots of displacement, (5.7L) and has a very free-flowing intake manifold as well as a very highly tuned computer. There are other Pushrod engines with similar displacement that make more power though, for example the GM LS6 in the Corvette Z06 made 405 hp with the same displacement.

Also, all of the competing engines you listed are Truck engines designed for large trucks where the main goal is torque rather than horsepower and therefore they are tuned to sacrifice top end HP for Low end torque. The 5.7L Hemi is a universal engine though that will remain largely unchanged between the 3500 lb Charger and the 4800 lb Durango and Ram 2500

[Nayr747]

Ok, but my point is: if the hemi's are not a better design than normal engines of today then why are they being advertised so much and being put in every car in GMs lineup. I mean, I'm sure they were great when they came out and a big improvement over the flat head design but what does that matter today? Is it just advertising hype, trying to capitalize on people's memories of their old hemi powered cars and a way to cut costs?

Originally Posted by drdisque

Also, all of the competing engines you listed are Truck engines designed for large trucks where the main goal is torque rather than horsepower and therefore they are tuned to sacrifice top end HP for Low end torque.

I didn't think of that. Good point. But I think the new Grand Cherokee with a hemi does 60 in 7sec. The magnum, though, with a hemi is very slow i think.

So, other than forcing more air/fuel into the cylinders or increasing the space and therefore allowing more air/fuel to enter (with 14.7 psi of normal atmospheric presure, right?), the only way to increase hp is different tuning or increased efficiency, right? I'm really just trying to get all this straight.

[pro3racer]

The hemi is a Dodge product, not a GM product. FYI. They had to make the hemi an all around motor, i mean, they have it in the magnum, a station wagon. Put that up with a 350 small block, or a 454 big block, and then we'll see the stats, actually i kind want to see that, since the 350 SB is an all around motor.