Panzer9

Would someone explain just what makes a hemispherical head- aka a
"Hemi". I believe our cars DO have hemis- am I correct? and how (if at all) do they differ fron the famous Dodge 426 c.i. hemi?

FlyingDog

Hemi = hemispherical. 32V are pentaprism which is basically a 4v version of a hemi.

The old Dodge Hemis have hemispherical heads and nearly hemispherical piston domes. It's an easy way to get very high compression and very high crossflow. 1980s 8v 4-cyl Toyota engines are deisgned very similarly.

Andre Hedrick

Think of pop-up pistons.

blitz928

A lot of cars are "hemis" It's kind of funny. If the heads are arched or a hemispherical design it's a "Hemi"

It's become mainly a Marketing tool for Dodge at this point.

Hell I've got 2 "hemis" my 928S4 and My Dodge Ram 1500 Quad cab.

FBIII

Actually a "pure" hemi is not that great a design by todays standards. It is quite an old design. The big problem with the design is it lacks an effective squish area in the combustion chamber. I think that the 4 valvers with the pent roof were designed to be a big improvement in that area.

ckabee1

I have never understood the term "Hemi" either. If the heads are arched, shouldn't it be referred to as "semispherical" or just Semi for short?

FlyingDog

Half of a sphere is a hemisphere...

MrLexse

"Hemi" is a term copyrighted by Chrysler. 85-86 heads have 4V hemispherical combustion chambers. Later 4V heads changed the combustion chamber design and no longer appear to be "hemi's"































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ckabee1

Sorry, bad joke and I forgot the smiley. I passed geometry in HS, I promise.

Seriously, I am glad this question came up, as I commented some time ago that the 928 motor was considered a hemi and the person I was talking to asked what that meant. I did not have the answer...

928ntslow

The BIG difference is the Dodge Hemi's were/are push rod engines making stupid amounts of torque! The big cast iron block weighed a ton, but put out enough power to create massive amounts of torque to pull a freight train.

largecar379

early Chrysler Hemi's ('50's 315 ci, 331 ci, 354 ci, through the 392 ci) were "true" hemispherical combustion chambers.

later Hemi's ('64 through '71 426 ci) were not "true " hemispherical combustion chambers---the exhaust valves were canted in 10 degrees to allow for inner fender clearance on the heads/valve covers.

the new Hemi's ('06 and newer 5.7L and 6.1L) are hemispherical chambers with a "dimple".

but......that's history.


----Russ

largecar379

and--- an added note:

you can get a late (426 ci style) Hemi from Indy cylinder heads, complete w/ aluminum block and heads, with cubic inches approaching the 650 number, for around $20-$30K, producing around 830 hp (streetable 10.75:1 CR) and weighing less than your standard 32 valve 928 engine..........

605 cubic inches
1000 hp-- on race gas
804 ft lbs torque

( you can upgrade to a 636 and get 1120 hp and 930 ft lbs torque)

And it'll fit in a 928.

---Russ

Panzer9

Regardless of whether it has a true Hemi- When I drive it, I get a Semi.

jpitman2

Originally the idea was that a hemispherical chamber gave the flame front equal distances to bore sides from a central plug, but as soon as compression was lifted by doming the piston, you got a shape like a half orange skin, and no squish, so it wasnt all that efficient....
jp 83 Euro S AT 51k

fabric

A DOHC engine with a cast iron block would weigh even more. Overhead cams make for a heavier engine, all else being the same. The advantage is in better breathin,higher revving, and generally higher fuel economy, but as evidenced by the Corvette Z06 (and to an extend the LS2 engine as well), pushrods are hardly an impediment any longer in those areas.