Alfa's Hemispherical chambers and the Dodge Hemi - Page 5

Greg Gordon · #61 (**permalink**) 10-22-2006, 05:19 PM

ddouglas 77, I am not smart enough to follow you on this one. What are you saying doesn't exits?

ddouglas77 · #62 (**permalink**) 10-22-2006, 05:20 PM

Picture 1 (A, upper left) shows a wedge design-not a hemi, picture 2 (B, Upper Right) shows a hemi-in case you're wondering, the designs/drawings are from an Automotive engineering file I keep. When the spark plug(s) is between the valves (both vertically and horizontally-centered)-wether 2, 3, or 4 per cylinder, it is a hemi.

Greg Gordon · #63 (**permalink**) 10-22-2006, 05:23 PM

Ok, so what's a Pentroof? That's what I am not understanding.

ddouglas77 · #64 (**permalink**) 10-22-2006, 05:29 PM

I have no idea-you mentioned pent roof-I've been all over trying to find a modern design of one and can't-is that old school?-You mentioned a World War-I'm thinking what you're thinking of has been changed-much like pluto no longer being a planet-I honestly have never heard of a pent roof. Problems you were discussing before about detonation is more a problem with compression (wether it be head displacement, piston compression heigth, ect...). There are times when a valve can get hot enough in those positions to ignition to cause fuel to start the combustion process early, but those designs to both head and valves are so far off now.

joecautela · #65 (**permalink**) 10-22-2006, 05:37 PM

I think you will find that the main consideration of a "hemi" head is that the combustion chamber is of a sperical shape with valves situated for a cross flow movement of air with a central spark. That is where the DOHC arrangement comes to play so well vs. a pushrod mtr. Lower reciprocating mass in the valvetrain with valves normally at a 45deg angle (or less) in relation to deck plane. Charge in, combustion, then exhaust in a straight line. Good examples of these traits are the Alfa DOHC, the Ford V-Tec and the new GM motor, all cross flow heads with a central spark..

ddouglas77 · #66 (**permalink**) 10-22-2006, 05:40 PM

Thank you-you just explained pic B whereas the valves are on the side-but the plug is centered, whereas the rest show a plug on the side but the valves centered or slightly offset. However, hemi doesn't expressly mean dohc or non pushrod engine-hence the Dodge Hemi-and many others-I believe the older pre 80 Volvos had a very nice straight 6 that used hemis.

Greg Gordon · #67 (**permalink**) 10-22-2006, 05:47 PM

Ok, now I understand. I think this is a terminology thing and you and perhaps others are grouping pent-roofs and hemispherical combustion chambers into the same group. As you said, much like Pluto has been reclassified.

While they are similar they are not the same thing, at least not historically. Perhaps automotive engineers have changed these terms, I don't know as I don't work on newer cars.

During WW2 the government brought the top combustion chamber engineers in the country to the N.A.C.A. (that's N.A.S.A. before they went into space) to evaluate the detonation characteristics of various combustion chamber forms. These guys had a nearly unlimited war time budget to work with. They clearly considered "Pent-roofs" and "hemispherical" designs to be two different animals.

Greg Gordon · #68 (**permalink**) 10-22-2006, 05:49 PM

Oh, and yes, there are many many pushrod Hemis. Overhead cams are definatly not a requirement.

tomp · #69 (**permalink**) 10-22-2006, 05:50 PM

One of my first cars was a peugeot 403. It was 1960 mode1500cc four banger with pushrod hemi and a whopping 7.5:1 compression ratio. Not quite as fast as my dads 1750 berlina!
-Tom P.

ddouglas77 · #70 (**permalink**) 10-22-2006, 05:50 PM

Interesting-so exactly what is the difference? And this was world war II? I'll have to study this a bit more-for sure.

Greg Gordon · #71 (**permalink**) 10-22-2006, 06:13 PM

ddouglas, all the naca reports are on nasa's web site downloadable in PDF form for free here: http://ntrs.nasa.gov/ . They did an incredible amount of work with piston engines prior to the advent of the gas turbines. Report number 768 deals with Hemi engines and 655 pentroofs and some others. Keep in mind these two reports only deal with detonation issues as they were trying to extract every last bit of power out of available engines and fuel. There are other reports dealing with just about any aspect on engine design you can think of. While it's old information it was compiled by the sharpest people around with a budget to do about anything. For a simple hot rodder like me they are an invaluable resource.

They are still comming out with reports but they rarely have anything to do with piston engines as turbines and rockets are all nasa cares about now.

I am glad you are here. We need all the engineer we can get.

ddouglas77 · #72 (**permalink**) 10-22-2006, 06:34 PM

thanks for teaching me something new-I mean old, it will definetly help in the future. Todays standards and only definitions are as outlined and attached above, The pent roof was similiar, but had different results-obvisiouly. By the late 60's, Dodges hemis are just as ours, intake, almost 180 degrees away from exhaust as our alfas have the exhaust on one side and the intake on the either, instead of the exhaust on one side of the block and the carb on top. You can have heart shaped (most effiecient-check out the V6 60 degree engines GM put out) and completely faced heads with all the trimmings-the only factor remains is that the plug separtes the valves.

dretceterini · #73 (**permalink**) 10-22-2006, 06:37 PM

Quote:

Originally Posted by Greg Gordon

Stu, I am not sure I understand what you mean. However the old Chevrolet 348 and 409 series had overhead valves with a flat surface where the valves were and the shape of the combustion chamber was built into the piston. That's the only type of engine I have seen like that. Is that's what you are talking about. If so what modern engine is set up like that?

**Gordon Raymond** · #74 (**permalink**) 10-22-2006, 06:58 PM

WoW, Now this IS getting interesting!

Gordon Raymond

Greg Gordon · #75 (**permalink**) 10-22-2006, 08:09 PM

I sure like those old Dodge Hemis. I put a 392 into a 71' Challenger back in the mid 80's. The old Dodge Hemi combustion chamber looks a lot like the Alfa Nord engine. The curretn Dodge Hemi's chamber looks a lot like the Alfa V6.

I do like the G.M. V6. For some reason that motor never gets any respect but it sure is reliable, compact, and simple. It can also take a lot of power. Turbo and Supercharged G.M. 3.8s are always among the fastest cars at the drags.

Stu, that JE piston goes on an engine with a convetional combustion chamber. It's simply dished to lower the compression, almost certainly for some insane amount of boost.