DOHC better than SOHC?

[gigabites]

hi all

from what i've heard and red (from http://www.automotiveforums.com/vbulletin/t174446.html), i can tell that DOHC is more superior than SOHC. I know that with DOHC, we can make different settings for intake and exhaust... But how can there is a significant hp increase just by that? And how can it compensate with the added weight of the second camshaft? Should it be loosing hp instead?

Thanks for your answers...


Sorry for such newbie question

[GreyGoose006]

well the reason it is better is that it is easier to fine tune the specific intake and exhaust profiles, and the cams can be timed (degreed) independantly of each other.
it also makes things like v-tech, vvt, and whatever other variations of independant valve timing, possible.
as for the added weight, it is a factor, but as long as the engine is well designed, and you take advantage of the valve timing, it is mostly negated.
however there are some who still believe that OHV engines are superior to all others.
it all depends on what you want and what you are looking for.

[KiwiBacon]

Twin cams also give more freedom in designing the combustion chamber. Meaning you can shape it for better combustion which helps power production.

[GreyGoose006]

twin cams as opposed to quad?
i would think that the DOHC quad cams would pose more of a restriction on design, wheras SOHC, dual cams give more freedom...

[KiwiBacon]

Originally Posted by GreyGoose006

twin cams as opposed to quad?
i would think that the DOHC quad cams would pose more of a restriction on design, wheras SOHC, dual cams give more freedom...

Were we talking about 1 bank of cylinders or 2?
I thought it was one, hence twins. With two banks four is an option.

[MagicRat]

Originally Posted by KiwiBacon

Twin cams also give more freedom in designing the combustion chamber. Meaning you can shape it for better combustion which helps power production.

I agree.
DOHC also allows for optimal valve angles without rocker arms (used in some SOHC motors) so the valve trail is lighter, permitting higher revs without valve float.

[GreyGoose006]

gotcha.
i was going along the assumption that with two cams up on top of the heads, the heads would be huge and there would be compromised room for airflow...

i really wish GM would make a DOHC V8 again.
the LT5 was the future, but the public wasnt ready.
a 4.5-5L DOHC V8 would be Perfect IMO

[UncleBob]

Originally Posted by GreyGoose006

gotcha.
i was going along the assumption that with two cams up on top of the heads, the heads would be huge and there would be compromised room for airflow...

i really wish GM would make a DOHC V8 again.
the LT5 was the future, but the public wasnt ready.
a 4.5-5L DOHC V8 would be Perfect IMO

GM makes a 4.6 DOHC V8 engine.....

[curtis73]

DOHC's main advantage is head geometry. The valves can be placed anywhere with DOHC. With SOHC you are limited to a closer valve placement. 90% of the time SOHC works fine.

Its true that DOHC has the ability to alter intake and exhaust timing separately, but since they are positively linked together by the timing belt or chain, the real-world superiority is not really an issue. If you have variable valve timing, its only as good as the system. Most DOHC variable valve timing systems only alter one cam or alter both cams equally, so DOHC vs. SOHC is a pretty pointless debate.

I don't know if any manufacturer is currently doing this, but the only true benefit comes when you have an independently variable DOHC setup. In this way you can totally alter LSA, IVC, EVO, and other events to suit the load and RPM. Otherwise its pretty much a wash.

BMW did this for a while with their double VANOS system.

[UncleBob]

Honda shocked a lot of people when they went to a SOHC on the CRF bikes. No manufacturer uses SOHC on any of their high performance 4 strokes

But honda's reason? Its lighter. I have a 250 apart at work right now too....extremely impressed how they did it. This is a little thumper motor that turns 12K+ RPM's