Why are we going back to SOHC?

[KiaDiseasel]

stevesingo said:

On the subject of VVC on turbo engines, why when all forced induction does is increase intake air density, would the engine behave any differently in relation to cam timing than a NA engine?

Even if the cams in both engines were the same there would be differences in how the engines reacted to changes in timing. The turbo is primarily affected by anything that reduces exhaust flow to spool the turbo up which is why even on similar duration cams you usually use a wider lobe centreline angle on a turbo cam to reduce overlap flow. When you start talking about long duration N/A cams vs short duration turbo ones the differences become much larger. However yes in general principle more lift and duration will help power with a turbo engine albeit at the expense of low rpm power but I think this topic could easily become too complex to go into in detail.

[Evoluzione]

stevesingo said:

BMW could be argued as the makers of some fairly cutting edge engines, now mainly turbo charged of course.

N54 DOHC VVC Turbo
http://www.e90post.com/forums/attachment.php?attac...
http://www.e90post.com/forums/attachment.php?attac...
http://www.e90post.com/forums/attachment.php?attac...
http://www.e90post.com/forums/attachment.php?attac...

N55 DOHC VVC Turbo
http://www.bimmerpost.com/goodiesforyou/N55-guide-...

B58 DOHC VVC Valvetronic variable lift Turbo.
http://s3.bimmerfile.com.s3.amazonaws.com/wp-conte...

You'll find it hard to find a turbo engine built within the last 10yrs or so that hasn't got VVTI - for very good reasons

stevesingo said:

On the subject of VVC on turbo engines, why when all forced induction does is increase intake air density, would the engine behave any differently in relation to cam timing than a NA engine?

Not really no, it's totally antiquated thinking that suggests otherwise. From a performance point of view you can cam a modern turbo engine just like you would a modern N/A, the more overlap you have on a turbo the earlier it spools and the more power it gives. That's why VVTI and turbos work so well, you can dial out the overlap and duration to get a decent idle and pass emission tests, bring it back in to spool the turbo and give good power.

[hoffman900]

CrutyRammers said:

...And I've never actually seen an example where there was a 2v and 4v version of the engine, and the 2v made more torque. Made peak torque at lower revs, yes, but not more torque. Seems to have been one of those "bloke down the pub things" as a way of justifying having an older, less powerful engine

A 4 valve engine will always make more torque than a 2 valve when developed correctly. When you add up the valve area of a 4 valve vs. a 2 valve for an equal displacement, you'll find the 4 valve will require substantially less cam duration to flow the same amount of air. Less duration makes for a much stronger bottom-end mid range.

Nothing made this more clear than what John Kaase did in the Engine Masters Challenge with the Ford Modular motor a few years back.

http://www.hotrod.com/articles/jon-kaases-ford-fou...

The 2 valve engines have to use some 40* more duration @ .050" lift to compete in the same power band. In a format like that contest (best average from 3000rpm to 7000rpm), the 4 valves crushed the 2 valve engines.

Why people think the 2 valve makes more torque is usually a displacement thing, but given equal displacement, the 4 valve will always outperform the 2 valve. It's simply physics.

Everything that works on a NA engine works on a turbo engine, including variable valve timing. The turbo does nothing more than increase atmospheric pressure. SOHC is simpler and cheaper to build than DOHC engines. It's also lighter. Variable cam timing problems have been fixed with the SOHC in using the 'cam within a cam' type set-ups.

[Zad]

Peak torque, sure, but most people don't spend their commute running through the gears up to the red line. I suspect most of them don't go above 4000 rpm at all. I'm not sure how much truth there is in it, but it was often said that "multi valve" heads suffered from poor turbulence at lower revs, due to the lower gas speed in the inlet valves. 2 valve heads effectively provided a faster and more turbulent flow, which improved the mixing, and thus better efficiency. Obviously that is inherently going to limit gas flow at the higher end.

I know that some manufacturers close off the second inlet valves at lower revs (I'm thinking Ford Duratec V6s) and don't various Honda's VTECs apply a higher lift at higher engine speeds? Yes, it is to provide improved breathing at higher revs, but the implication must be that without it, it would be less drivable lower down too.

[Inline__engine]

4V heads give tumble but often little swirl. Swirl can be used to give good bottom end torque

[hoffman900]

Zad said:

Peak torque, sure, but most people don't spend their commute running through the gears up to the red line. I suspect most of them don't go above 4000 rpm at all. I'm not sure how much truth there is in it, but it was often said that "multi valve" heads suffered from poor turbulence at lower revs, due to the lower gas speed in the inlet valves. 2 valve heads effectively provided a faster and more turbulent flow, which improved the mixing, and thus better efficiency. Obviously that is inherently going to limit gas flow at the higher end.

Two smaller valves with more valve area for a given bore size than a larger single valve for a given bore, will give better velocity. Always.

4 valve heads are tumble heads, not swirl.

A 2 valve head done properly, will have a more efficient chamber than a 4 valve pent roof combustion chamber.