Push Rod vs. DOHC engines

[Rayme]

cliff note : OHV is good because it's smaller, but you can't be displacement limited because it can't even compete in term of displacement per horsepower vs dohc. Fuel efficiency is subjective and reliability just as much. An it has to be a V layout because pushrods sucks in inline engines. Did I cliffnoted everything carefully?

The 1991 zr1 is the example that always come to my mind. I wondered for years why GM didn't go with a DOHC layout when it was so easy to push out extra horses.

[ZDan]

Originally Posted by ottopilot

cliff note : OHV is good because it's smaller,

And lighter, with lower c.g., and cheaper to build, buy, and mod.

but you can't be displacement limited because it can't even compete in term of displacement per horsepower vs dohc.

Undisputed fact.

Fuel efficiency is subjective

Fuel economy is very much OBjective. When I calculate fuel mileage after every tankful, I do it from objective measurements of distance traveled and fuel consumed. Over the course of many tankfuls, you gain accuracy.

and reliability just as much.

Reliability can also be objectively measured, but really isn't an issue for most modern engines.

An it has to be a V layout because pushrods sucks in inline engines.

SOHC or DOHC definitely the way to go for inlines.

The 1991 zr1 is the example that always come to my mind. I wondered for years why GM didn't go with a DOHC layout when it was so easy to push out extra horses.

It was more expensive and it added a lot of bulk and weight. GM decided that it was better to develop OHV cam-in-block 2vpc for their rwd V8 performance cars. As far as I'm concerned, they made the right choice. For Ferrari, DOHC and high revs are the right choice.

There is no one "way".

[ScionRacer]

Originally Posted by rice_classic

ScionRacer, I'm not picking on you directly but want to speak to phrases like this, "heritage".

When I hear things involving cars and car companies and terms like heritage or tradition, I kind of just check out. Usually when I'm hearing those words it's to disguise a company's lack of innovation, lack of imagination and desire to turn a profit by continually fooling the public into buying a "new thing" that's not actually new; it's just the same-ol' same-ol' wearing a new dress and new makeup. This just isn't a Pushrod vs DOHC debate, but my disappointment with American manufacturers in general.

And here's why.... Nothing by American companies gets done "just cuz" or "just for the sake of". It only gets done for profit. (Unfair and partially untrue over generalization used for making a point) Now living in America, we all think this makes sense, this is "normal". Well, it's normal to us and it also explains a lot. Other countries have a culture where things are done "just because", sometimes.

Examples:
Why is Ferrari... well... Ferrari? Because Enzo didn't make cars for profit... profit wasn't the goal. Ferrari only made street cars to fund grand prix racing and that's pretty much how their game is still played with obviously some changes but...

The "heritage" I speak of is directly reflected in the fact that Corvette has remained "CORVETTE" because it still boasts that pushrod V8. The engine makes the same sound it did in the 50's.That's its heritage,its trademark.Just like Porsche, Ferrari and Lambos are unique for one thing,the sound of their engines.

That pushrod engine and that engine alone has kept corvette alive and desirable in the exotic brands.

I have to agree I am somewhat disappointed in what American manufacturers have to offer,but I think the Corvette is our best and one of the best in the world.The day that GM has to put a VVT and dohc in the vette to "sell more units" is the day that recreational driving is truly dead in the world.

[BRZranger]

Me too... Awkward!

[Dimman]

Originally Posted by ScionRacer

The "heritage" I speak of is directly reflected in the fact that Corvette has remained "CORVETTE" because it still boasts that pushrod V8. The engine makes the same sound it did in the 50's.That's its heritage,its trademark.Just like Porsche, Ferrari and Lambos are unique for one thing,the sound of their engines.

That pushrod engine and that engine alone has kept corvette alive and desirable in the exotic brands.

I have to agree I am somewhat disappointed in what American manufacturers have to offer,but I think the Corvette is our best and one of the best in the world.The day that GM has to put a VVT and dohc in the vette to "sell more units" is the day that recreational driving is truly dead in the world.

Well GM did do that in the 90s. They had aspirations to go supercar hunting, and nicknamed the ZR1 'King of the Hill'. However since then they seem to have learned that what they did was too expensive for American Corvette enthusiasts, and not exotic enough for traditional supercar customers. ZR1 was pretty bad ass in its day, it seems.

Now, instead of a completely different motor they modified their current one for boost and put a supercharger on it. Better development cost to performance compromise.

[ScionRacer]

Originally Posted by Dimman

Well GM did do that in the 90s. They had aspirations to go supercar hunting, and nicknamed the ZR1 'King of the Hill'. However since then they seem to have learned that what they did was too expensive for American Corvette enthusiasts, and not exotic enough for traditional supercar customers. ZR1 was pretty bad ass in its day, it seems.

Yea and where did they go,to Lotus and got a typical euro V8. The ZR-1 was a beast and probably the most exotic for its day but Im glad it disappeared.

Ford moved on with the Mod motors and could never get them right without throwing a blower on them.The 4.6 dohc non boosted engines were just weak.The 4.6 sohc was a joke compared to the old 5.0L ohv. Ford appeared to be getting more tech savy and innovative,but in reality they were just hunting for profit.The new "modular" motors have greatly reduced costs,delivered less performance and costed the consumer more.This is a prime example of what Rice_Classic was making a point of

[Thunderchicken]

Just my .02 Chevy Push-Rod engines are great for trucks due to the low-end torque and relatively low RPM. The ability to stay in a gear allows you to get more work done. I'm not saying that a Chevy Push-Rod motor isn't fun to drive with all of it's super low-end torque, but the sound of a high revving and easily getting the backend to step-out.

[Allch Chcar]

If I may add in a bit.

Originally Posted by ZDan

It's just a brute fact that a high-performance 400+hp 4.0 liter DOHC V8 from BMW gets crap mileage vs. a high performance 400+hp 6.2 liter OHV V8 from GM.

That's because the M3 has very aggressive cam and gearing compared to the Corvette. It's also well known that GM has been purposely trying to game the EPA cycle for the Corvette's MPG for years. Remember the 1st-4th gear automatic shift?

Originally Posted by ZDan

DOHC V-engines have ~4x the cam bearing surfaces vs OHV, and two very long cam chains that have to run along very long guides vs. one very short chain with no guides required on OHV.

4-valve engines have twice the number of valve guides, too, but that's probably more than offset by pushrod guides in the OHV motor.

Of course DOHC multivalve engines can rev higher, and breathe better at higher revs. BUT, for same engine size/weight/power output, the smaller-displacement DOHC engine will *have* to rev higher, and frictional losses go up rapidly with increased revs. More efficient to rev slower with greater displacement for a given power output.

They rev higher precisely because they have lighter rotating mass and less friction. It's also why they're more efficient. Now if you rev them to make more power or gear them shorter gearing for acceleration, they lose fuel efficiency. It's all about balance and compromise.

OHV cannot use 4 valves per cylinder(as @Dimman mentioned) but they also can't use Variable Valve Timing or Variable Valve Lift. That's a huge disadvantage. It's not as much with a 6.2L since it makes plenty of low end torque but a 2.0L OHV would either be absolutely torqueless or weak on power. Most likely both. So for example, Japanese cars where there is a tax advantage for a smaller displacement, it precludes OHV completely.

[ZDan]

Originally Posted by Thunderchicken

Just my .02 Chevy Push-Rod engines are great for trucks due to the low-end torque and relatively low RPM. The ability to stay in a gear allows you to get more work done. I'm not saying that a Chevy Push-Rod motor isn't fun to drive with all of it's super low-end torque, but the sound of a high revving

The 6.0 LS2 in my RX-7 revs to 7200rpm (only 200rpm less than the FR-S/BRZ's 2.0 redline) and makes peak power at 6500rpm (only 500rpm less).

I can assure you that it sounds more compelling at 7000rpm than my S2000 does at 9000

Not sure what point you're trying to make here:

and easily getting the backend to step-out.

It's easy to get the s2000's back end to step out under power in limited traction conditions (cold tires, wet, etc.). This is mainly due to more rear roll stiffness bias vs. other cars and rear toe-change-with-bump suspension shenanigans.
With more than twice the hp and about the same weight, it's *quite* easy to get the RX-7's back end to step out under power in any conditions...

[ZDan]

Originally Posted by Allch Chcar

That's because the M3 has very aggressive cam and gearing compared to the Corvette.

It *has* to have those in order to give competitive performance. Apples to apples for me is comparing similar output to similar output and similar size/weight to similar size/weight. For similar size/weight, the OHV 2v V8 can have much greater displacement. For the same power output, larger-displacement will be at lower rpm vs. smaller-displacement at higher rpm. The smaller-displacement, higher peak-power-revs DOHC engine will *of course* be geared much lower (numerically higher) and be cammed to breathe at higher rpm.

It's also well known that GM has been purposely trying to game the EPA cycle for the Corvette's MPG for years. Remember the 1st-4th gear automatic shift?

Easily bypassed. Certainly isn't operational on my car.
Anyway, forget the Corvette (which is smaller, lighter, more aerodynamic), the Camaro SS (bigger, heavier, less aerodynamic) gets better mileage than the M3 as well. 20% better highway mileage, where 1-4 skip-shift (do they still have that?) is not in effect.

They rev higher precisely because they have lighter rotating mass and less friction.

Lighter rotating mass? Almost certainly, given 2/3 the displacement. Less friction? Maybe not (4x the camshafts, 2x the valve guides, 2x the cam lobes, MUCH longer cam chains with long guides vs. single short chain, etc.). It is certainly not a foregone conclusion.

It's also why they're more efficient.

The M3's 4.0 is clearly *less* efficient in terms of fuel economy vs. hp. I would bet money that it's also less efficient in terms of brake specific fuel consumption at 400hp vs. GM's 6.2 LS3.

Now if you rev them to make more power or gear them shorter gearing for acceleration, they lose fuel efficiency. It's all about balance and compromise.

What I would call "equivalent" gearing would have them both revving at the same fraction of peak power rpm during cruise. The GM cars are almost certainly geared taller even taking that into account. But I would bet that Corvette/Camaro geared as described would still get better fuel economy.

OHV cannot use 4 valves per cylinder(as @Dimman mentioned)

Actually, they CAN, and such engines have been built.

but they also can't use Variable Valve Timing or Variable Valve Lift.

They can and do use variable valve timing. The Viper even has independent VVT for intake and exhaust. Variable valve lift could also be done, but I don't think it has been.

That's a huge disadvantage. It's not as much with a 6.2L since it makes plenty of low end torque

That's right, you don't *need* those features (which add size and cost and complexity and weight) as much with larger-displacement engines.

but a 2.0L OHV would either be absolutely torqueless or weak on power. Most likely both. So for example, Japanese cars where there is a tax advantage for a smaller displacement, it precludes OHV completely.

Obviously, OHV cam-in-block doesn't offer advantages for small-displacement, inline engines (see my previous posts in this thread). But OHV cam-in-block *does* have advantages for larger-displacement V8s.

[serialk11r]

Oh, another bit about the S65. It has a 75mm stroke, and hits 8400rpm. This seems very conservative, as the F20C has 84mm stroke for 9000rpm, and the 2ZZ-GE has 85mm stroke for 8500rpm. I bet BMW could've chosen to use a 81mm stroke for 4.3L displacement and more power. In fact it appears that BMW added 7.8mm stroke for the GTS (S65B44), and gained 30 horsepower by doing so.

Then there's the fact that the ITBs don't actually increase power over a normal single/dual throttle setup, but they add weight. It also has a souped up VANOS system for better response, but it probably increases parasitic losses and weighs more too. Whereas it seems that the LS3s used in Vettes don't even have cam phasing of any sort, and I bet the LS3s going into the SUVs and stuff that do have variable valve timing pack a few more pounds.

The reason the Camaro has good highway gas mileage is because the 6th gear is long, and the gearbox has wide ratios. The BMW gearbox keeps the engine spinning rather fast on the highway, which burns a lot of fuel. However if you get the automatic transmission the fuel consumption looks a bit better I think. If engines were operated at the best load on the highway, these sports cars should be getting 50-60mpg at 60mph. But in the real world, they get maybe a little more than half of that. That means that half the fuel is thrown away spinning the engine faster than it needs to be. Longer gearing easily overcomes increased power consumption due to drag. So highway fuel economy is a VERY bad indication of how efficient the engine is. Just look at a minivan, they have massive drag, but their engines are geared long, so their highway fuel economy is comparable to some sports cars.

Again, you talk about bearing surfaces, but the thing is the cam surfaces on OHV engines experience far higher pressure due to the extra mass of the pushrods. The bearing friction for the valvetrain is pretty inconsequential, because the bearings are under hydrodynamic lubrication whereas it is typical for cams to be in boundary lubrication conditions. The overall forces experienced are higher, so at the same friction coefficients you have more losses.

[ZDan]

Note: I said it isn't a foregone conclusion that the smaller displacement v8 would have less friction. It isn't!

None of these cars would be getting anything like 50-60mpg at 60 on the highway even if geared to the moon (which the Corvette/Camaro already are). Gearing is a big factor, but not anything like THAT big!

I get ~27mpg at 75mph at 1800rpm in the V8 RX-7.

Suffice it to say, a smaller-displacement DOHC 4valve V8 won't necessarily be "more efficient" at making the same power as a larger-displacement OHV 2valve V8, and in this instance it is significantly *less* fuel efficient.

The Mustang's 5.0 DOHC V8 does a lot better in this regard.

Of course I'd rather have an M3 than Camaro SS or Mustang GT. But I'd rather have an LS2 RX-7 than any of 'em...

[serialk11r]

Originally Posted by ZDan

Note: I said it isn't a foregone conclusion that the smaller displacement v8 would have less friction. It isn't!

None of these cars would be getting anything like 50-60mpg at 60 on the highway even if geared to the moon (which the Corvette/Camaro already are). Gearing is a big factor, but not anything like THAT big!

I get ~27mpg at 75mph at 1800rpm in the V8 RX-7.

Suffice it to say, a smaller-displacement DOHC 4valve V8 won't necessarily be "more efficient" at making the same power as a larger-displacement OHV 2valve V8, and in this instance it is significantly *less* fuel efficient.

The Mustang's 5.0 DOHC V8 does a lot better in this regard.

Of course I'd rather have an M3 than Camaro SS or Mustang GT. But I'd rather have an LS2 RX-7 than any of 'em...

Well of course none of these cars are going to get 60mpg, any V8 simply has too much displacement for pulling a small passenger car along the road at normal speeds. What I meant was more if these cars ran at peak engine efficiency while going down the road at 60mph, they would get 50-60mpg. So the actual efficiency of these engines when operated on the freeway is something like 50% of peak or less. Typically (that means not straying too far from the production car engine norms of bore/stroke, rod/stroke, displacement per cylinder) somewhere between 1500 and 3000rpm the cooling loss and friction are at a "good balance", and so mpg potential starts dropping as soon as you go above 2L.

The problem with the big V8s is that you can give them very tall gears and run them well below 2000rpm, but the proportion of heat getting rejected to the coolant is very high even though you save a lot of frictional losses, so they can never get really good fuel economy. Having a very large bore reduces the surface area to volume ratio but it's still a problem.

I would not be surprised if the LS3 has better efficiency at peak power than the S65 since piston skirt friction is much higher than valve train friction, and the piston skirt friction at higher rpm probably eats up whatever advantage in valve train friction the engine had. However if we're talking about efficiency at peak power, then we'd want a DOHC motor that revs lower anyways, and weight would not be a concern. Specific power requires tradeoffs, as Allch Chcar said.

Don't get me wrong, I think for most practical purposes the Corvette formula is a good one, though I sort of question whether SOHC would be that much worse. High power to weight ratio is afterall the goal, and whatever fuel efficiency benefits lower displacement engines have, manufacturers tend to throw away with somewhat silly gearing. It just doesn't give me a nerdgasm the way the more exotic German motors do

[Dimman]

@Allch Chcar I asked in an older thread about pushrod 4v engines, and someone linked to one. Consensus seemed to be that it wasn't that great (losing the complexity advantage of 2v ohv and not being able to rev like an ohc).

The cam timing, I knew that GM had phasing for the whole cam on their last ohv V6, but didn't know that Chrysler had developed a system to operate intake and exhaust lobes separately. But that cam-in-cam sounds very complex, and well... Chrysler.