OmarSpeedAddict

Guys which one is better for producing power and turbo engines?

I know our Evo comes with a DOHC but i just have this nagging question in my head?! And out of curiousity you know what are the major differences besides the obvious?

The Sohc is lighter right? Which one could rev higher reliably?
These are all the questions that I can come up with right now but I have to get them out of my head cause they are killing me.

Good night and thanks.
Omar

bobaab

iTrader: (5)

i believe DOHC is better in almost all aspects..it's just more effecient with 2 cams rather than one doing the work of two. although an SOHC engine has some ingenious cam designs, DOHC will usually be better for any high performance modifications.

DGS

Engine driven valves place a parasitic load on the engine, in that it takes engine power to push the valves open. This load exists whether the valves are pushed by SOHC, DOHC, or rocker arms.

You could quibble over what creates more parasitic load. DOHC has more belt/chain losses, since it must drive two camshafts, so the belt/chain path is longer, and DOHC has two cams spinning, so there's more rotational mass with more angular momentum. But a single cam has to drive twice the valves - plus added linkages, so it is harder to turn. The net differences are fairly trivial, on a street engine.

Timing belts are, in some quarters, considered to cost less load than timing chains ... but losing a belt on an interference engine results in a lot of damage to the engine. (Losing a belt on a Ferrari means a $15,000 minimum repair bill. But Ferrari still runs belts instead of chains.)

DOHC gives the most precise valve operation, so it's what's used on higher revving engines.

Consider Formula One engines, which rev to 18000 RPM. Valve springs don't cut it, at those speeds. The valves are pushed open by dual overhead cams, and snapped shut by nitrogen gas pressure. A lot of gas pressure. I've heard that the valve train on an F1 V10 consumes well over 100 BHP. But considering the results, the teams obviously find the loss more than compensated by the higher revving output.

The "holy grail" for production engines, these days, are purely electronically operated valves, permitting the ECU to control variable valve timing. But solenoids are too slow, even for street engines. I hear there are some experimental electric valves on racing bikes, but nothing yet that fits the bill for auto engines.

So variable valves today operate by playing with cam geometry or camshaft angle advance -- which makes the cam shafts heavier. Variable valve timing permits more aggressive timing at higher RPM, while still passing exhaust standards at idle, which is a concern on production engines, but not on racing engines, so far. But electric valves would remove the parasitic valve train losses -- at the cost of needing more load to drive larger (and higher voltage) alternators.

There ain't no setch thing as a free launch.

Earl Needham

There's another aspect of DOHC versus SOHC. Yes, two cams weigh more than one, but with a DOHC engine, you have less weight on the valves themselves and they are less subject to valve float. This can POTENTIALLY give you higher RPM.

One thing to remember is that the cams can have any grind you can think of whether you have a SOHC or DOHC engine, so you might have a DOHC with a peak horsepower at 4000 RPM and a SOHC engine with it's power peak at 7000. See http://www.popularhotrodding.com/fea...2phr_afxstang/ for just one example.

Wow, I hadn't heard of using nitrogen to operate the valves, sounds like a lot of complexity to add.

Another type of valve operation that is just great for high RPM operation is the desmodromic setup, in which the valves are closed by either a second lobe on the cam, or else another cam entirely. Theoretically, an engine setup up like this won't have an RPM limit because of the valve operation, but practically, the rest of the engine has a limit somewhere. Like breaking a rod or crankshaft. And desmo is just plain expensive, too.

bahamut

Are you seriously thinking of detuning a 4g63T to a 4g63BT? If so, look at the Aussie version of the starion, try to make power out of it if someone is so concerned about valvetrain weight.


Forget F1 and motorcycle tech (except for the S2K) stuff, at 99.8% of the time, manufacturers are not go to make a car spinning 10K redline.

alleggerita

DOHC will always have a lighter valve train which allows higher revs. Also it allows more degrees of freedom with regard to port design, as it is easier to paly with valve angle when you don't need to worry about a complicated valve train. Think of the port design of the Ducati testa stretta heads, albeit the latter valvetrain is complicated by the Desmo design.

chronohunter

iTrader: (1)

Great post, one very cool piece of hi-tech you forgot is BMW's "Valvetronic" system it has variable intake valve LIFT acting as the throttle on the engine! It is in the current V8s and V12 and will be in all their engines within a couple of years.

DOHC heads were needed back when big valve angles were popular, these days the combustion chamber is much "tighter" SOHC is more viable. Valve angles used to be 90 degrees (or so) in high performance engine, these days it's ~20

ItsStockOfficer

I believe with SOHC its a bit trickier to fit 4 valves/cylinder, is it not?

OmarSpeedAddict

Damn, serously, great answers.

DGS your the man, serously bro. I don't even know how you know these things but your the moder***king man. jaja
later
Omar

**This is what this forum is about**

evo542

that's why they replace the engine every 500km or so

Smogrunner

iTrader: (5)

My thoughts exactly.

timzcat

iTrader: (10)

The Dohc is going to win out just about everytime. Take the same engine with a head design of both. Your not going to flow enough air through the single as you will the DOHC. And the is no way you are going to open 16 valves with one cam unless to considerable lengthen the cam and make one ugly engine.

brentg07

ill just put it plain for u, for the evo dohc is gonna be better than sohc

Fourdoor

iTrader: (11)

All modern Mitsubishi SOHC heads are 16 valve.

Keith

PS: I agree that DOHC is better BTW... just making sure people realize that the SOHC are 16 valve.

chuntington101

Buy far DOHC is better. But pneumatic valves would be even better hehe!