in the latest issue of HOT ROD magazine there is a very interesting article about why GM chose to make the Gen. 3 a pushrod engine instead of a OHC or DOHC engine. One of the biggest reasons was felt torque production. What the engineers did was create two nearly identical motors and put them in two seperate covettes, then a bunch of GM exec's went and drove them both and decided hands down that the pushrod engine was the way to go and gave the thumbs up to begin design of the gen 3 engine. You should pick up a copy of HOT ROD, or buy the book that HOT ROD takes this article from (i believe it is some new book about modifying and the history of the Gen 3 engine)...either way PUSHRODS RULE!

 

Call me crazy, but how can a pushrod generate torque?

I know it's not directly related to the pushrod, but possibly
the drag of spinning a chain/belt rather than rolling a lifter
on the eccentric?

All else being equal (forget about cost, engine bay space, etc.),
where/how is the torque being generated in a pushrod motor?

My thoughts are piston surface area (bore), rod length, crank
stroke and cylinder pressure (VE%) are the criteria for making torque.

Wouldn't a cross flow head produce higher VE values due to the
air path being straighter?

Again, all factors being equal, a motor spinning between 1000-
6000 RPM, no V. V. T. involved, runner volume and valve sizing
shared.

Let's not get into economical, or 'real estate' issues.

With a lighter reciprocating assembly and fewer parts, how can
the pushrod be more efficient?

Most of the replies I've read in this thread describe differences
between manufacturers, but not the same short block using
an overhead cam against an overhead valve (LT1 vs LT5)

 

SOHC and DOHC motors use followers, OHV have rockers weight's about the same I'd imagine for comparable engines.

While OHV engines have roller lifters, a single cam (for our discussion at least) that runs the length of the block, and two timing gears and a timing chain... DOHC motors have 4 cam shifts that run the length of the block, FIVE timing gears (at least) and a chain.

So I guess the comparison is this: is the weight of 32 lifters, 32 pushrods more or less than the weight of 4 more gears, 3 more cams, and a longer timing chain? I personally have my doubts.

I think that the very reason that DOHC cars came of age is that it's EASIER to design than a 32-valve OHV design. Yes, you may get the same hp out of a 32 valve DOHC as a larger OHV V-8 can make, but understand that as always, the power's in the heads. OHV cars are NOT making more hp by using head-based camshafts becasue of huge reductions in friction or weight... their typically doing it by increased head flow (DOHC is awesome for variable valve timing it can allow).

OHV will be around for a long time... they run reliable, package big displacements in smaller engine bays, and head flow is more than adequate for street-driven grocery getters.

SOHC/DOHC engines are nice to rev higher, but the packaging in a V-bank design usually mandates them smaller displacement... resulting in minimal hp gains, loss of low-end torque (due to smaller displacement and the increasing reliance on rpms for ponies) and increase development/production/maintenance costs.

Inline engines suffer less from the packaging than V-bank layouts, but it's still there in taller, sturdier heads and increased timing chain/gears (in the case of DOHC at least).

It all depends what you're development goals are... theoretical hp ratios, or total performance and functional packaging.

 

read the article or buy the book, it will explain all the answers / explain why the gen 3 rules so much (from a technical standpoint)

 

2 valve heads are a little mor efficent at low lifts and the combination of displacement helps to generate low end. Also 1 larger valve flows more at low lift than two smaller valves..

 

From reading the article, I'm not so sure the subject engines were 'nearly identical'. My best guess is that one was the prototype 5.7 LS1 engine and the "prototype DOHC engine" might have been a smaller displacement, Northstar-like DOHC (4.6). Remember Ford was then (1992) headed for the mod engines, and GM was well aware of that.

So, even if the smaller DOHC had similar peak power (let's say 325-340) it would be somewhat down on torque from a 24% larger LS engine, especially in the low-mid range. I'll bet this test was with auto trans (up to 70% of Vette production some years), so off-idle performance is more impressive with more low end torque. The current 320 hp/ 315 lb-ft North-South Northstar is mated to a 5-speed auto which fits it's torque curve better than the 4L60. With the LS2 we'll have 400/400 which will take a blower to equal on the Northstar.

It wasn't the valve train architecture that "created more torque" but the entire engine size and package. IOW the shape of the torque curve and area thereunder is what was felt.

Drive an automatic 4.6 Mod Mustang and an automatic LS1 Camaro back to back and see which you prefer on the street.

My $.02

 

HP per L. is the worst way to measure an engines performance. You have to look at the torque curve.

exp:
LS6 5.7L = 405HP
LS2 6.0L = 400HP (probably underrated)

But If you look at the dyno of both engines you will see that the LS2 is more powerful at just about every RPM: http://www.andyseipos.com/albums/album50/P4237723.jpg

 

I think they're referring to HP/L as the potential and efficiency
of the design rather than the performance characteristics of
the motor.

It looks like the LS2 has a higher average torque over any window
of 1000 RPM than the LS6.

LS2 is two intake and one exhaust valve/cylinder correct?

Anybody have cam specs for the LS2?

 

The cylinder head is based around the LS6 design... still one intake and exhaust valve, 2.0" and 1.55" respectively.

Haven't seen anything published on cam specifics yet.

-Mindgame

 

OldSStroker- I beleive the V6 Vortec V6 heads will flow exactly like the V8 versions (on a per-port basis, anyway). I had them side-by-side on a bench. Same valves (1.94/1.50), same ports (decent right out of the box), same chambers (at least they looked identical to me). Not sure why they would flow any less than the V8 version.

The early stock non-vortec castings were a joke by comparison. They definitely didn't have a chance at making any power without a lot of work.

 

I LOVE my GM pushrods, and hope GM keeps it up...

My 3800 is smooth and torquey while getting a combined 27 MPG...try that in a 24V Maxima. Yes, it may not sound quite as nice, but the reliability and MPG makes up for it. And my LT1?? Well, I dare say I don't miss the extra cams there, either...

I applaud GM keepinng the value of pushrods around, and not giving up on them merely to supply Joe Car Buyer with something that, well, he doesn't typically know about anyways...

 

Am I blind, or did I see a photo of GM head with 3 valves?

Was that not the LS2?

Did GM ever build a proto-type head for the LS2 featuring two
intake valves?

 

the 3 valve head is NOT the LS2 head. There was a prototype head shown a while back though.