Overhead cam VS pushrod design? Which is better?

[jmortensen]

That is an outstanding point. Are OHC's more efficient? Therefore making them easier to meet emission standards? Possibly smoother or quieter? There's got to be a reason.

The 3 or 4 or 5 valve per cylinder heads have a better combustion chamber shape and flow better than a 2V per cylinder head. That means they can operate better at high rpms, and the OHC cams have better control of the valvetrain.

One more time comparing the OHV to the L series specifically just cause I like to beat a dead horse, then some efficiency stuff. The L6 doesn't have more than 2 valves per cylinder, newer DOHC heads have a very big advantage here that the L doesn't. Along with the number of valves the shape of the combustion chamber can be improved. The L also doesn't have the cross flow that a OHV V8 or more modern OHC head has, so it looses out on the scavenging advantage that the cross flow has. On the L6 the exhaust port is lower in the head than it would be in a cross flow head which makes the short side radius necessarily tighter which isn't good for flow.

Now onto efficiency. More efficient doesn't mean less pollution. This was the reason that engines sucked so bad in the 70's. Look at the early V8 or even the early Z heads vs the ones from 75. They went from a shape we would recognize today (the closed chamber with lots of quench) to the "open chamber" in an attempt to cool down the combustion process. More efficient means more NOx which is the main contributor to visible smog. Also take a look at the compression ratios. The V8's were pushing 10:1, back in the hardcore muscle car days you could buy some hipo V8's with 12:1 right off the lot. Z's started out at 9:1, and lowered their compression to the low 8:1 range in the 70's. Lower compression = cooler power stroke = less efficiency = less NOx. EGR was added. Dump some exhaust into the cylinder to cool it off = less NOx. Here is a related link that tells what things happened, but it doesn't really compare the emissions equipment to an attempt to reduce efficiency: http://ca.autos.yahoo.com/maintain/answer.php?catalytic_converteranswer1.html Nowadays the car manufacturers have finally figured out how to get the high compression, powerful engines we want without all the smog. But the efficiency is in spite of the smog regulations, not because of the smog regulations.

[e_racer1999]

...As for which layout is better... How many OEMs do you se changing from a pushrod to an OHC design? And how many do you see going the other way?

i'm not a fan of the pushrod engines, but chevrolet's corvette did. the ZR-1 of ~1995 had a DOHC V8. the new Z06 corvettes have pushrod engines that are more powerful AND more efficient. it all goes into the little disgn elements..

[CoastGuardZ]

With all the money we put in these Zs we probobly could buy a Vette! :cheeky: I dont think i would want to thou.

My sentiments exactly!

[chickenwafer]

You have to rev the wizz out of the civic motor to get power.

That's because Honda designs their motors for low rpm effciency and high rpm output- ala VTEC. That way, you get fuel milage and horsepower.

BTW: No stock Honda Civic motor (B-series, D-Series, K-series, anything) ever reved to a factory-set 9 grand.

And I vote for OHC. Why? Ever head of a little thing called valve float? Plus, I like having my multi-valve cylinder heads.

[=Resurected280=]

First off i'd like to say that I love the Datsun inline 6. I think that this is one of the most fun motors I've ever driven or worked on but if you want to go FAST for cheap you have to admit that a pushrod motor has alot going for it.

For instance I recently saw a 3.0L Rebello stroker for sale on this site and the owner said that he had about $6,000 into the motor and it pulled 200hp on a dyno (with a misfire mind you, but still under 300 hp). Also, a built SR20DET or RB26DETT is not cheap either. On the other hand an Edelbrock Performer Crate Motor goes for $5140 and puts out 320hp and 382 ft-lbs has aluminum heads, water pump and manifold.

I know that this is not the most efficent design but there is a reason that this engine style engin has been used by Chevy and Ford since the 50's and that the chevy small block has been the hot rodder's darling for so long.

I dont know if this has been brought up yet but just my 2 cents.

[e_racer1999]

"a pushrod motor" by way of SBC or SBF is an inexpensive way to go fast. the only advantage that it has is that it's plentiful. that doesn't make it better, just easier to get ahold of. pushrod motors are GENERALLY inefficient. i emphasize the generally because someone will definitely prove me wrong. anyway, if OHC engines like the SR or RB were plentiful enough, they'd be cheap too...

[LanceM]

My $.02 here after reading this debate... First off I think the question is poorly asked, which is better? That's like saying which is better, Ford or Chevy?... For what, how, and when?? High reving, torque, longevity, or simply the easiest to build the most power for the least bucks... This debate will go on forever unless you define the question to a specific point....

[jmortensen]

"a pushrod motor" by way of SBC or SBF is an inexpensive way to go fast. the only advantage that it has is that it's plentiful. that doesn't make it better, just easier to get ahold of.

Um... you just contradicted yourself. Pushrod motor is an inexpensive way to go fast, but that doesn't make it better...

What's not better about going fast inexpensively?

[=Resurected280=]

Another item on that topic, you ever notice that the majority of the high horsepower cars with sr20's, rb26's, etc. etc. are owned by tuner companies that for the most part wont even admit what they spent on the car? In many cases though these cars cost $60,000 or better!!!!

In contrast a friend of mine has a '69 Vette which he bought and built for less than $10,000 and it runs 11.5 in the 1/4. No it doesnt look mint and is not finished yet, but still 11.5 for $10,000..........

I'm just saying that whats better is only better if it is attainable to you.

[e_racer1999]

Um... you just contradicted yourself. Pushrod motor is an inexpensive way to go fast, but that doesn't make it better...

What's not better about going fast inexpensively?

more efficiency per cc and weight?

what i meant was: it's not a better designed motor. the only reason that it's cheaper is because it's easier to get ahold of. if OHC motors were more plentiful (as in comparison to pushrod motors) they would be as inexpensive as pushrod motors.

[jmortensen]

What engine are you talking about Jason? The L series, or an S2000? Because an L28 series put out something like 135 hp out of 2.8 liters in the last version. That's 48 hp/liter. Let's check my truck again as a comparison. 285 hp / 5.3 liters = 53.77 hp/liter. There are many more V8s out there with more power per liter than my truck, so please don't get hung up on new V8 vs old L6.

Jason you could be talking about the theoretical pushrod L6 vs the real OHC L6 like Dave was earlier, but I really don't see why this efficiency argument stands up any better than the reality argument comparing the OHC to OHV liter per liter.

As far as weight are you talking weight per liter or weight per hp or weight per cylinder or what? I'm much more interested in weight per hp, and the V8 whips on the L6 in that regard. The old iron block 350 with aluminum heads and water pump weighs 40 lbs more than an L28. Just 40 lbs, and the weight will be set back further in the chassis if you use the JTR mounting position. The new all aluminum V8 is supposed to be lighter than the L28. I guess the rotary really wins this weight battle though. Point is, L28 is a HEAVY engine. V is the way to go for weight because the block is half as long, and that's why you see most car manufacturers going to the V setup.

[240ZX]

My $.02 here after reading this debate... First off I think the question is poorly asked, which is better? That's like saying which is better, Ford or Chevy?... For what, how, and when?? High reving, torque, longevity, or simply the easiest to build the most power for the least bucks... This debate will go on forever unless you define the question to a specific point....

I tend to agree with Lance......better for what application. As a general trend in the automotive world , I think its obvious which engine design is more efficient! Of course there are exceptions to the rule, but what's new about that? As far as stuffing a sbc or sbf into a Z car....go for it, if that's your flavor. Would be pretty cool to have 400 hp in a well setup Z and for a reasonable price (engine wise anyway). Crap, if everyone agreed on one engine...life would be pretty boring!!!