Interview with GM engineer

Thought this would be good reading for the Future Vehicle forum as well.

http://web.camaross.com/forums/showt...hreadid=276954

 

Great read!

 

Pushrod V8s =


The only time I would rather have an OHC engine is when it is an inline engine.

 

Very illuminating. It cetainly dispels some of the myths... too bad they can't/won't parley that info into successfull advertising since the general public is mesmerized by multiple valve and cams, not necessarily because they're better but because they are portrayed as better.

 

there it is again. this article seems to keep poping up every time someone "discovers" it. good article though.

 

For ultimate power per given unit of displacement, there's no beating a DOHC layout. The reasons:

1. A 4 valve per cylinder layout allows central placement of the spark plug, and therefore the most efficient combustion. Compression ratios can be increased because the flamefront doesn't have to travel as far to reach the farthest points of the cylinder and detonation is avoided.

2. Reciprocating mass is reduced, therefore the engine can rev higher (more HP) or the valve springs can be less rigid (less parasitic loss). Less reciprocating mass also usually equates to a smoother engine.

3. Without consideration given to a pushrod going through the cylinder head, more efficient ports can be designed. This is the reason the LS1 has only 4 cylinder head bolts per cylinder versus the SBC's 5 bolts.

4. It's not the valve area that counts on a poppet valve engine, it's the curtain area, or the combined circumference of the valves. 4 smaller valves have more curtain area than 2 larger ones, and thus can flow more air.

Note that I said this is the optimal layout per unit of displacement. If you're going to talk cost, or physical size of the engine, an OHV layout can generally be designed to outperform a similar OHC layout, which is exactly what the LS1 does. I personally think there's reasons and applications for both.

 

How can a single cam and short timing chain have MORE reciprocating mass than 2-4 cams, a long *** chain going all around the back of the motor, guides, and whatever else is back there?

Besides, power per given unit of displacement is about as relevant as voting in the NJ presidential primary. HP/lb or HP/MPG are the relevent performance metrics.

 

Because cams and chains don't reciprocate.

True. I just wanted to give some balance lest everyone think the pushrod is the ultimate engine design. There really are reasons to use OHC layouts besides marketing fluff.

 

That is why I think DOHC is better on Inline engines. Inline engines only need 2 cams to be 4 valve (vs. 4 in a V engine).

 

I wonder how much roller cams have to do with OHV viability. A direct-actuating DOHC system cannot use a roller cam, and thus has much less "area under the curve" on a valve actuation event.

It seems possible that a well-designed roller on an OHV engine could swamp the airflow advantages of DOHC/4V designs. Another BIG disadvantage of DOHC.

 

Good point. We usually just here comparisons in terms of valve area. But a better comparison would take into account lobe design as well. I bet a more agressive roller lobe closes the gap significantly because although there's only 1 intake or exhaust valve it is able to dwell near peak lift longer than a comparable dohc setup.

 

if i'm not mistaken, the Northstar 4.6L user roller rockers/cam.

 

makes you wonder is FoMoCo is having remorse about ditching the fabled 302ci

 

What you're looking for here is rotational inertia, not reciprocating mass. In short, the relative resistance seen by the cam gear at the crank snout by having to move the two systems.

The DOHC layout loses this comparison to the OHV layout.

Important areas:

1) Rotating one cam at high speed requires less work than rotating four.

2) In any equation governing energy, mass is relatively far less important than velocity - velocity is squared, mass is linear. An engine that makes 400hp at 6000rpm is more efficient from an energy standpoint than one of less rotating mass that requires 8000rpm to make the same power. The most efficient internal combustion engines in common use are diesels - and they have MASSIVE displacements coupled with rather low redlines. The secret is to NOT spin the motor if you want efficiency... make gigantic torque instead.

 

Gigantic torque at low rpm is not going to produce a "fast" car - efficient or not.