Cylinders

[paul19]

What shape of cylinders do most cars use these days? ex. hemi, pentproof hemi, flathead, ect.

[MagicRat]

All the 'cylinders' are round, or 'cylindrical', (hence the name).

Do you mean combustion chambers?

Overhead cam engines almost always have a hemispherical chamber.
Almost all pushrod engines have a 'wedge' shaped chamber, except, most notably the Chrysler Hemi engines. The Hemi engines, whether from the 50's, 60's or the current Hemi v8's all use a semi-hemispherical chamber.

[Evil Result]

Umm i belive A hemi engine was based upon the Hemispherical cylinder head chamber... i'v never heard of a Semi-hemi. OHC engine use a pentroof design because of valve arrangement (2 on either side). Although newer pentroof designs are coming close to the design or performance values of a hemi.

[duplox]

Actually semi-hemi is a term used to describe the Ford Boss 429 cylinder heads. They had a squashed hemispherical chamber to allow some quench space. I don't think the chrysler hemi heads were actually hemisphereical - which means exactly half of a sphere. Its probably more of the top third of a sphere. so technically it would be a semispherical chamber, but hemi sounds cooler. The bad thing about hemis is they have no quench unless designed like Ford's.

[SaabJohan]

Modern engines mainly use pentroof chambers with four valves, they offer the best performance and the lowest exhaust emissions. They also offer a short flame path and therefore reduce the risk for knock, the fast combustions also increase fuel efficiency.

With pentroof chambers the high performance engines usually use narrow valve angles, the combustion chamber is in other words almost flat. Larger angles can be used to get more space for the watercooling in the head.

F1 engines, like this BMW use a modified pentroof design (slanted squish, possibly also compound valve angles) with narrow valve angles


A head like this offers a high intake valve / bore area ratio, close to 40% of bore area is taken up by the intake valves, the intake mean flow velocity is also low at peak power which offers a low restriction. With a good engine a head like this offers a volumetric efficiency of 115-120% at high speed operation (like in F1), lower speeds offers volumetric efficiencies of over 130%.

Modern diesels use a flat head with the combustion chamber located in the piston. More and more are of 4 valve design.

[paul19]

Thanks a lot saab, i understand now

[MagicRat]

Originally Posted by duplox

Actually semi-hemi is a term used to describe the Ford Boss 429 cylinder heads. They had a squashed hemispherical chamber to allow some quench space. I don't think the chrysler hemi heads were actually hemisphereical - which means exactly half of a sphere. Its probably more of the top third of a sphere. so technically it would be a semispherical chamber, but hemi sounds cooler. The bad thing about hemis is they have no quench unless designed like Ford's.

You are correct.
Maybe I shouldn't post after a 14 hour long day at work.
(And to think I have a Ford 429 in my garage)
The modified wege heads, (big block Ford and Chevy, along with the GenII 3.1 V6 from GM) all have splayed valves and are a 'semi-hemi