What is a HEMI? And, why a TM?

Factual Questions
21

I think the actual situation is more complex than that.

Even the Chrysler 426 Hemi didn’t have perfectly hemispherical combustion chambers. As can be seen from this image, the valves imposed a Pentagon aspect to the chamber: http://www.thehemi.com/pics/Hemi_Cutaway_1.jpg.

It’s probably more accurate to say the new HEMI™ combustion chamber is less hemispherical than before, but it’s still more hemispherical than a pure wedge or pentroof design.

The situation is further complicated since some common usage refers to the new HEMI™ engines as “pentroof Hemi” designs: http://www.stuffo.com/hemi.htm. In that sense the new Hemis are simply an abbreviated version of that description.

Unless Chrysler explicity says every engine denoted as “Hemi” has combustion chambers that are perfectly hemispherical to within +/- x%, then fail to provide that, there’s no basis for false advertising.

And if you say “well, people will see HEMI™ and think it’s an old-fashioned Hemi, so that’s misleading”, in fact the vast majority of buyers have no idea what Hemi means. Likely a significant % couldn’t even tell you what a hemisphere is.

22

I don’t think any combustion chamber on any engine was every completely hemispherical. The valve faces are flat. To be completely hemispherical they would have to be concave.
Looking at the link, I would refer to that engine as a hemi head, as compared to a pent roof, or a wedge.
A modified hemi to be sure, but a hemi none the less.
Just my $0.02.

23

That first sentence will read much better if we change it to:
I don’t think any combustion chamber on any engine was ever completely hemispherical.
:smack:

24

Yeah, your’e correct there. I don’t need to take one apart. Although they don’t have a high failure rate I’ve seen the inside of a couple. But thanks for the links.

25

There was another thread on Hemis a while back, and it got me thinking about what the whole deal with a Hemi is and why the mistique.

The main reason for going to a hemispherical head is so that you can make the valves larger than if they were paralell. Larger valves=more area for gas flow=more power.

Thing is you only really need big valves at higher RPM at the low end, the lower port velocity lowers volumetric effiency and hurts torque.

So an engine that took best advantage of the hemispherical heads would be fairly high reving. BMW made such motorcycle engines, but these were fairly exotic by detroit standards (forged pistons, nikasil bores, aluminum heads, etc.) Valves need to open by about 25% of thier diameter, so those bigger valves need lots of travel. AND they require a more complex and heavier rocker arm design. AND those things do not translate into good high rpm numbers. And since those high-lifting valves are now nearly perpendicular, you can’t have much, if any, valve overlap, as they would run into each other, and that is also not good for high rpm operation.

Also, that hemispherical combustion chamber inevitably ends up having a lot of volume, so to get a decent compression ratio, you need either need a longer stroke or heavier, high domed pistons, and those aren’t good for high rpm operation either.

So what you do, is you design the whole engine for great low rpm torque. Longer stroke, mild cam, etc…never mind that you started out looking at valves suitable for high rpm/big horsepower numbers.

Basically, the choice of the hemi-head forced the mopar engineers down a path toward a torquey, lower reving engine…and that turned out to be a pretty good thing in the real world.