DOHC and SOHC

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Whats the advantage of an engine with two cams when comparing it to a single cam engine with the same number of valves and the same displacement, and the same compression ratio. Thanks.

posted by  Accord_Man

The ability to time the intake and exhaust separately. That's it.

posted by  ChrisV

What chris said...and in the real world it usually means a smoother powerplant that can possibly rev higher.

posted by  thunderbird1100

Thanks, valve timing was the only thing I could think of.

posted by  Accord_Man

not to mention that SOHC designs arent as balanced as DOHC (in terms of the camshafts, not the engine itself). its all due to the position of the camshaft in order to operate the valves. cant put it into the right words right now.

posted by  Inygknok

DOHC is heavier than SOHC :mrgreen:

posted by  nissanTFsx

more moving parts :laughing:

posted by  TurboLag

how do you say DOHC? do you say DOC or like letters D-O-H-C?

posted by  CarEXPERT

Just say dual overhead cam but you say it D-O-H-C :thumbs:

posted by  99integra

Generally they breath better and rev higher.

DOHC generally have multiple itake and exhaust valves/pot, but of course there are three valve heads and even two valve (slack engineers and accountants). For a two valve there is less deflection of the camshaft and better angles of attack through canting and pent roof designs.

Tackling four valve and higher:

There is are a myriad of advantages, but consider you have two valves doing the job of one. This reduces the displacement (acceleration) of the valve and mass. Both these factors are major considerations when looking at force (f=ma).

The valve is less likely to flex or bounce so the rpm redline is typically higher, thus increasing your top end power capability. The radius of the valve is smaller so less likely to deform and more likely to seat when under duress.

The valve train losses are less because of f=ma. Flanking and ramps of the cam lobes is more moderate. Spring tensions are less also, unloading the camshaft and resulting in less shaft deflection and wear. Point load is more evenly spread.

The head itself has a reduced localised stress, both through reduced valve train stresses and stress distribution via two camshafts instead of one. The mass of the individual camshaft is less, reducing rotating mass.

posted by  Wally

Well, yes, Wally. But there are a number of SOHC 4 valve heads, SOHC 3 valve heads, and quite a few DOHC 2 valve dsigns (of course, most of those are older, like the Jag 4.2 liter in my old XJ6 that had been around in one form or another since the late '40s, and the DOHC 2 valver in my Fiat 124)

Since the difference between DOHC and SOHC isn't necessarily predicated on the number of valves, it really comes down to the ability to tune each bank separately (which is hard to do with an SOHC setup, or single cam-in-block layout). While you might lighten each small valve, you still have double the frictive surfaces and up the total mass, using up power to contend with, as well as the added cost and complexity (especially in the mass and location of the cam drive assembly, be it belts gears or chains).

posted by  ChrisV

Serves me right. I actually wrote more than what you see and realised I had explained why SOHC was better than OHV and followed it on with the DOHC. I then deleted out a few paragraphs including the cam timing, rockers, buckets, etc before running off down the coast.

posted by  Wally

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