Why is DOHC better than Pushrod?

"marshknute" (marshknute) 04/05/2019 at 15:08 • Filed to: None

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I’ve asked this before, and none of you gave me a good answer. Chevy’s smallblock V8's are ALWAYS the exception, without fail. There literally isn’t a single objective benefit to DOHC over a chevy smallblock. I’m not trying to be an asshole, I’m genuinely confused, and haven’t received a compelling answer in the past.

Anyway, let’s compare my Corvette’s 6.2L V8 to the E92 M3’s 4.0L V8:

Size: The “massive” 6.2L LS3 is physically smaller and lighter and has a lower center of gravity than the “tiny” 4.0L S65, hence why an LS-swap is so cliche; it’s the only thing that will fit in a cramped engine bay.

Power: While both engines make similar horsepower, the LS3 makes 43% more torque (Corvette: 430hp/424lb-ft, versus BMW: 414hp/295lb-ft).

Efficiency : My Corvette gets WAY better fuel economy, especially on the highway (16/25mpg Corvette, 14/20mpg M3). In the real world, I routinely get 27 highway.

Reliability: The LS3 is one of the most legendarily reliable V8’s on the market, and the go-to crate engine. The S65 is a notorious maintenance nightmare.

Redline: Ok, you got me here, pushrods don’t rev too well.

Am I missing something here? What’s the point of having a higher redline if you have to compromise size, weight, COG , torque, efficiency and reliability? Are Chevy’s smallblock pushrod engines some sort of statistical anomaly?  

Gerry197 > marshknute 04/05/2019 at 15:14

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I remember a comparison a few years back with the 911 Turbo 3.8L vs Corvette LS7.

Where the Corvette engine was physically smaller, lighter and actually produced more hp and a lot more torque, while being more fuel efficient and cheaper to maintain and arguably more durable.  

I asked myself, so what if the liter/hp was lower, if the actual results are better?

MasterMario - Keeper of the V8s > marshknute 04/05/2019 at 15:16

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Theory vs reality

DOHC is better in theory

Reality means means many compromises must be made to apply said theory which favors the simplicity of the pushrod

jimz > marshknute 04/05/2019 at 15:16

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because people fundamentally misunderstand how engines work and how they’re typically used.  and believe “more is always better” when it comes to metrics.  hp/l may give you bragging rights, but the higher and higher you push the hp/tq peak in the rev band, the less pleasant that engine is to live with day in and day out. 

bob and john > marshknute 04/05/2019 at 15:17

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or, look at it this way. the BMW motor makes similar HP to the vette even though the motor is A THIRD smaller. 

Ash78, voting early and often > marshknute 04/05/2019 at 15:17

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I suspect it’s more about packaging from when DOHC was still a new thing and pushrods hadn’t been improved to help stay competitive. Then once DOHC caught on, nobody really wanted to go back to pushrods from scratch. So I think we’re looking at two competing branches of the same family tree, where neither is really better than the other at this point.

My understanding is that when DOHC started catching on in the mainstream (IIRC, early 70s?) , it allowed for tighter packaging and better revving, plus less clutter in the block . It didn’t help that the US car market was slow to improve on anything around that time or maybe DOHC wouldn’t be so common...

M.T. Blake > marshknute 04/05/2019 at 15:18

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You want to eat steak or lobster?

gettingoldercarguy > marshknute 04/05/2019 at 15:19

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Low lift flow numbers are massively better in a four valve engine compared to a two valve engine. More power, can rev higher, etc . Valvetrain mass is significantly less per valve than a two valve, so lower overall spring pressure, etc. Can rev higher. Easier to control variable valve timing than Chrysler’s method. Can stagger camshaft events to modify cylinder swirl characteristics. Broaden power band, improve bsfc.

Demon-Xanth knows how to operate a street. > marshknute 04/05/2019 at 15:19

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If you are in a country that gets taxed by engine displacement or otherwise regulated, it makes a difference. Italy I believe had a substantial tax on cars larger than 1.2L, hence the number of cars made with special versions with small engines. Kei cars are limited to 660cc, meaning more displacement puts them in a different legal standing. In these cases, DOHC doesn’t come with the penalty that a larger engine has. There is also the issue of smoothness where a larger engine has more reciprocating mass to smooth out than a smaller engine.

That said, a B16A weights in at 318LBs, an LS7 at 450LBs, 400% the displacement, only 50% more weight.

A lot of the turbo RX7 crowd likes to show off how small their engines are by putting a bare block next to a fully dressed V8, but once you add in the plumbing, the turbos, the intercooler, and the accessories, the size difference often quickly disappears.

That said:

vondon302 > marshknute 04/05/2019 at 15:21

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I still can't figure it out either. My Boss 302 engine is huuge compared to my old pushrod 5.0 fox body. 7600 red line is fun though. 

jimz > bob and john 04/05/2019 at 15:23

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yes, but

1) the GM engine is physically smaller than the BMW

2) the fact that the BMW engine is a “THIRD” smaller doesn’t confer any efficiency or economy advantages.

3) the GM engine is much more suited for the kind of driving the vast majority of people do with its flatter torque curve.

actually, as a corollary to 2) the BMW engine could actually be less  efficient in typical daily driving because it’s being operated below its powerband for much of the time. 

Arch Duke Maxyenko, Shit Talk Extraordinaire > marshknute 04/05/2019 at 15:23

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Pushrod engines can still rev just fine, see NASCAR. The ability to rev has more to do with the bore and stroke than the configuration of the cam

Discerning > marshknute 04/05/2019 at 15:23

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Neither package is better than the other and either package can work for your desired goals.

Ive owned an LS6 C5Z and I currently own the latest coyote with port and fuel injection in my F150.

The two engines are similar in power and efficiency, though the new coyote is more on par with the LT1 than it is the LS6. They have similar weights, though the LT1 is narrower and shorter in height.

That said, a naturally aspirated pushrod engine built for power will likely be less smooth than a DOHC. Even when it was stock, my LS6 had a noticeable lope at idle. In contrast, my coyote is very smooth and vibrations are nonexistent at idle. I suppose the C5Z’s stiff bushings likely transmitted more NVH.

That’s not to say that a pushrod can’t be smooth. There’s a reason rolls used them to this day. Though their cam is ultra mild and they rely on boost to supplement the mild valve train.

In conclusion - they both rock. Just depends on what sort of characteristics you want out of your engine. Ferrari could build a mean pushrod v8 most likely (see dodge v8 , which I refuse to call a hemi), but they know it doesn’t have e the characteristics their customers expect.

jimz > Ash78, voting early and often 04/05/2019 at 15:25

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if your goal  is to have a high-revving engine, then yeah, overhead cams are your best bet.  but unless the application specifically calls for a small high revving engine (F1, S2000, etc.) then it’s academic at best.  At worst its just fodder for internet arguments over who is more awesome because of the things they like. 

Discerning > M.T. Blake 04/05/2019 at 15:27

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This is a perfect analogy

bob and john > jimz 04/05/2019 at 15:28

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I’m less concerned with t he outright phsical size as a lot motor sports limit the swept volume, not the motor size (would be inte res ting to see THAT as a restriction though)

2: yes, thats 100% the case. even a honda S2000, 2.0L motor, gets about the same MPG as the vet.

3: also true.


I’m like 90% sure the D OH C  came  out of wants for more RPM and higher HP for RACING applications where swept size was limited, hence crazy RPMS.

marshknute > bob and john 04/05/2019 at 15:28

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But it’s not.

It’s physically wider, taller and heavier, with a higher center of gravity.

It’s like how the iPhone X has a larger screen than the 6 Plus despite having a smaller body . It’s just a better use of the available real estate since the screen bleeds right to the edges .

HammerheadFistpunch > marshknute 04/05/2019 at 15:29

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The ls7 redlines at 7000, thats pretty good.  Not 8400, but with torque like the LS7 has...why would you?

Arch Duke Maxyenko, Shit Talk Extraordinaire > jimz 04/05/2019 at 15:29

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F1 doesn’t use cams...

Chariotoflove > marshknute 04/05/2019 at 15:30

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I feel the same. Each has advantages and disadvantages, but once you work out engineering methods to deal with the compromises, they both seem to  work well.

bob and john > marshknute 04/05/2019 at 15:33

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swept volume is more of a concern for racing then physical volume.

like I said to jimZ, i’m like 90% sure DOHC came from racing where you are limit it X size motor and your HP comes from Revs. DOHC = high revs = more power out of a smaller motor

jimz > Arch Duke Maxyenko, Shit Talk Extraordinaire 04/05/2019 at 15:36

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er ,as far as I know they do. they use pneumatic valve springs.

Smallbear wants a modern Syclone, local Maple Leafs spammer > M.T. Blake 04/05/2019 at 15:39

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The correct answer is “yes”.

LOREM IPSUM > HammerheadFistpunch 04/05/2019 at 15:39

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It might mean one less shift in a drag race type situation. Not the LS per-se, but in general. 

Snailkite > marshknute 04/05/2019 at 15:39

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1) Manufacturers don’t seem to need the better packaging of an OHV engine. It’s still just as long as the BMW engine and the BMW chassis already has to be tall enough to accommodate other engines and give good cabin room . Yeah, you can make stuff like the corvette, but everywhere else the packaging doesn’t seem to be an issue. Look at a GM pickup, the engine seems lost in the huge engine bay.   

2) Is the corvette really a great example for efficiency? It’s light(ish), doesn’t have much frontal area, and has really tall gearing at the top. It should always get better mileage than a big heavy sedan.  Other OHV applications don’t boast better fuel economy than their competitors. 

jimz > bob and john 04/05/2019 at 15:39

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I wasn’t really thinking primarily about motorsports, though. I’m more of a hot-rodder mindset who gets giddy from things like being able to drop a 400+ hp GM V8 into the same space a 4-cylinder occupied...

bob and john > jimz 04/05/2019 at 15:41

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I agree with that. I so want to do a flying miata V8 one day. But I feel like I will forever be spending my money on motorcycles so that wont happen. 

jimz > bob and john 04/05/2019 at 15:42

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bikes, on the other hand, are a perfect application of a higher-revving OHC engine.  well, performance/sport bikes at least.  not lard-assed cruisers. 

bob and john > jimz 04/05/2019 at 15:45

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oh yea. I want to find a true MC22 honda CBR250. or a ZX-2R. 20K redline!

marshknute > Snailkite 04/05/2019 at 15:48

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Don’t the V8 Silverado’ s offer comparable fuel economy to Ford’s EcoBoost V6 F-150’s ? I thought the inherent advantages of the pushrod architecture is why Chevy skipped the turbo-V6 altogether.

DipodomysDeserti > bob and john 04/05/2019 at 15:53

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It has a lower displacement, but it’s physically larger and uses more fuel. So none of the benefits of having a lower displacement engine.

If only EssExTee could be so grossly incandescent > marshknute 04/05/2019 at 15:54

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Most of those revvy DOHC V8s come from countries that tax based on displacement. If you want to get more power without making the car too expensive for people to buy, you need to rev higher. Pushrods cap the revs, so if you’re forced to stick below a certain displacement a DOHC setup makes sense.

OHC engines also offer better response because the cam acts directly on the valve. F ewer moving parts means less mass in motion means you can gain and lose revs faster.

BeaterGT > marshknute 04/05/2019 at 15:54

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How does positrac on a Plymouth work?! It just does.

bob and john > DipodomysDeserti 04/05/2019 at 15:58

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other then you know, if you were racing and had a size restriction

syaieya > marshknute 04/05/2019 at 16:01

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The best i can tell is that pushrods take to limiting factors less kindly than other engine types. Displacement, revs, fwd applications, variable valve timing, unique assembly.

It just so happ ens that the limiting factors for a grassroots e nthusiast application rarely involve the same considerations so the pushrod still shines on.

Beyond that its the silly pride in not admitting that chevy may still have an idea that works.

M.T. Blake > Discerning 04/05/2019 at 16:06

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I try to be perfectly imperfect in everything. 

M.T. Blake > Smallbear wants a modern Syclone, local Maple Leafs spammer 04/05/2019 at 16:07

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Maybe ask what the difference is between the ‘soup of the day’ and the ‘soup du jour’. 

Spamfeller Loves Nazi Clicks > marshknute 04/05/2019 at 16:13

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One, OHC is lighter. Period.

Two, OHC is easier to increase RPM on. Period.

Three, mechanical issues are cheaper to warranty on OHC. Period.

Four, there is no packaging advantage to OHV. None. There is also no packaging advantage to OHC. It’s a series of tradeoffs either way.

Five, OHV is cheaper only when you’re using technology that was amortized in the 1950's and 1960's. Swap cylinder heads, play with some passages and bosses, and you’re done. 

Pushrod does not have any inherent advantages over OHC, and hasn’t in decades. The undisputed GOD of racing V8's, the DFV, is a DOHC V8.

Uses a central timing belt which uses the block’s original cam location as a mount for a sprocket. That sprocket goes behind the timing belt cover to gear-drive four overhead cams.

My bird IS the word > marshknute 04/05/2019 at 16:17

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Rpm is overrated. Low end torque is king.

Discerning > marshknute 04/05/2019 at 16:20

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It’s hard to compare engines in trucks due to so many factors. Actually, it’s hard to compare numbers regardless.

For example:

2019 Silverado 6.2 4WD, crew cab short bed : 420hp, 460 torque, 16 city/20 hwy

2019 Silverado 5.3 4WD, crew cab short bed: 355hp, 383 torque, 16 city/22 hwy

2019 F150 5.0 4WD, crew cab short bed: 395hp, 400 torque, 16 city/22 hwy

The F150 has power figures in between the 5.3 and 6.2, but with 5.3 fuel economy.

That could partially be down to the 10 speed and lower weight, but the 5.0 also makes due without  cylinder deactivation, unlike the 5.3 and 6.2.

But there are countless factors to consider. Like how I own a 2018 5.0 and I only get about 20 mpg on the highway and that’s if I am careful.

Highlander-Datsuns are Forever > marshknute 04/05/2019 at 16:21

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They main advantage of an OHC or DOHC engine is the ability to have a 4 valve head, this enables more efficient passing of fuel and exhaust. Many of these advantages have been reduced through the refinement of the cylinder head and more customized shape of the combustion chambers of the 2 valve head on the LS and other push rod engines.

2-valve head.

4-valve head.

Highlander-Datsuns are Forever > M.T. Blake 04/05/2019 at 16:22

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Lobster.

Highlander-Datsuns are Forever > gettingoldercarguy 04/05/2019 at 16:25

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So far you and I are the only ones to point out the advantages of a 4 valve head vs a 2 valve head. I though this was obvious .

M.T. Blake > Highlander-Datsuns are Forever 04/05/2019 at 16:27

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Did I mention its expensive and smells?

CobraJoe > marshknute 04/05/2019 at 16:43

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There’s a simple reason : Horsepower is partially a function of RPM.

If the airflow is there, more RPM means more horse power. You can get more RPM out of the DOHC due to its lighter valve train, and more RPM means more horsepower.

That means the smaller displacement engine will make as much power as a larger displacement engine, though that doesn’t always translate to a smaller, lighter, or more efficient engine.

Plus, there’s the preference angle. Seems like more enthusiasts prefer power at higher RPM, because the don’t want an engine that feels choked when they’re on a track (or otherwise trying to keep it at peak power).

Personally, I’d prefer the torquey pushrod motor, because they produce a lot more power in the RPM range that most daily driving occurs at. (Though DOHCs can be torquey too, my 5.6L Titan is proof of that, though it revs only as high as an LS motor).

CobraJoe > MasterMario - Keeper of the V8s 04/05/2019 at 16:56

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Theory vs reality

DOHC is better in theory

Reality means means many compromises must be made to apply said theory which favors the simplicity of the pushrod

If theory is that different than reality, either the theory is wrong, or the technology hasn’t developed to the point to fully utilize the theoretical advantage.

I think the reality in this situation is that one engine is a 1st generation DOHC design from a company known for their overly complex engineering , and the other is an evolution of a 50 year old design from a company known for the reliability of their engines (and only their engines) .

CobraJoe > Arch Duke Maxyenko, Shit Talk Extraordinaire 04/05/2019 at 17:01

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Pushrod engines can still rev just fine, see NASCAR. The ability to rev has more to do with the bore and stroke than the configuration of the cam

If racing engines are fair game for comparison, you can find DOHC engines that rev much higher than the 10k RPM NASCAR V8s.

Not sure how many of them are based on a production engine’s architecture, but I’m not so sure you can claim that of modern NASCAR engines either.  

CobraJoe > Discerning 04/05/2019 at 17:04

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In conclusion - they both rock. Just depends on what sort of characteristics you want out of your engine.

It’s not just high RPM vs low end torque either, my 05 Titan has a lot of grunt down low out of it’s 5.6L DOHC, and it doesn’t breathe well at high rpm either.

Discerning > CobraJoe 04/05/2019 at 17:11

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Yup. In Nissan’s case, they knew they wanted lots of torque since they wouldn’t be offering anything bigger than a half ton. They already had the VK lying around, so they figured they’d build a tall deck version and fit it with an intake and cams that emphasize low end torque while sacrificing top end breathing. Cost likely contributed too.

They could easily build it to breath up top and make substantial power obviously, which they later did in the direct injection version. They made even more power in the Australian supercars Altima. 

CobraJoe > My bird IS the word 04/05/2019 at 17:12

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Completely agree.

Though pushrods aren’t a necessity for a low RPM torque monster.  

ihm96 > jimz 04/05/2019 at 17:22

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Ya but M cars are all about revving out a glorious motor

CobraJoe > Discerning 04/05/2019 at 17:27

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Honestly, it is one of my favorite engines in a modern vehicle (though I haven’t driven many newer engines). It really makes me wonder what a VK powered Nissan muscle car would be like. (Or what it’d be like to make my own...)

It isn’t the cheapest engine to build, though I guess nothing from the later generations is.

Jordan and the Slowrunner, Boomer Intensifies > CobraJoe 04/05/2019 at 17:34

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How many 5.8l engines rev to 10k?

nermal > marshknute 04/05/2019 at 17:42

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It’s a matter of designing the engine for the car.

The LS3 is basically a truck engine used in multiple applications and tuned for performance in the Camero / Corvette .

The S65 was designed specifically for the M3.

People that buy new Corvette’s are mostly creepy old guys having a midlife crisis. The LS3 and its power delivery are perfect for them.

People that buy M3s don’t want a retuned truck engine. They want to feel as though they could rev the shit out of it on the Nuremburgererauschwitzring, while shouting Clarkson-isms, and having supermodels fighting over who gets to touch their weiner.

Each is a perfect fit for its intended application. 

aquila121 > marshknute 04/05/2019 at 17:42

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I think part of the reason modern V8s (often shown in truck applications, especially) are able to compete with a turbo- V6 configuration is due to the prevalence of cylinder deactivation technology that GM has pushed to make the larger displacement engines more able to sip fuel while cruising on the highway. Between that, and insanely tall gearing, the LS and any successors are able to put up decent highway numbers for their EPA ratings. If one were to set those elements aside, the V8 would display a couple points worth of trade-off for the power, torque, and physical size/displacement advantages cited over other engines with fewer cylinders and DOHC configuration.

Arch Duke Maxyenko, Shit Talk Extraordinaire > CobraJoe 04/05/2019 at 18:02

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The DZ302 revs to over 7k

The Lingenfelter flatplane crank LS goes to 8250

MasterMario - Keeper of the V8s > CobraJoe 04/05/2019 at 18:07

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I know when I talk about theoretical vs reality I usually refer to theory with the mindset that you remove the manufacturing constraints of the real world. DOHC makes more hp/l so in theory it’s a better design...

There’s a reason most  race cars are overhead cams

aquila121 > marshknute 04/05/2019 at 18:13

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The other thing that occurs to me is that if you throw enough money, engineers, and R&D time at a specific problem with one of these engine configurations , it can often be overcome. “I want a pushrod engine that revs to the moon,” I think one of Andretti’s cars in the 60's or 70's tried that ( because the rules required it)—the team messed around with the rod/stroke ratio, had some ridiculous pushrods actuating something otherwise akin to an overhead cam, four-valve setup. They made it work.

“I want a front-wheel drive hot-hatch of a car that can hang with all-wheel-drive/rear-wheel- drive competition in the corners, and not torque-steer itself into another state when we put 300+ horsepower through just the front axle.” Honda engineers completely re- thought the suspension geometry and setup for the newest Civic Type R. They made it work.

“I want a 16 cylinder engine with four turbos, 1,000 horsepower, and the ability to go 250mph without all the compromises of a one-off racecar." The teams developing the Veyron said 'fine' and concocted a leather-filled cocoon that had ten heat exchangers and a whole boatload of other black magic that I don't understand. They made it work.

bhtooefr > marshknute 04/05/2019 at 19:44

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So, there’s a lot of factors going on here.

I’ll preface this by saying that I’m not an automotive engineer.

Cam-in-block engines are absolutely more compact for a given displacement than overhead cam, especially dual overhead cam, engines, and this can increase total torque at the crank through having significantly more displacement. There’s a couple other advantages here, that I’ll get to, but also some big disadvantages. (Some people have pointed some of them out already.)

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Torque is an interesting one, though. Torque at the wheels - not at the crank - puts a force on the car. And, the work of moving the car is done by horsepower , which is calculated using the formula: (lb-ft torque * RPM) / 5252.

Note that gearing changes both the torque and the RPM - your lower gears (typically 1st through 3rd) exchange RPM for more torque. A gear, typically 4th, is direct drive - torque and RPM stay the same. 5th and 6th are often “overdrive” gears, exchanging torque for more RPM. And then, your differential exchanges whatever RPM comes into it for torque.

All of this means that peak torque doesn’t directly matter - you adjust the gearing to suit what you’ve got.

However, the powerband does matter - where in the rev range the engine is making useful power. Let’s use the Camaro SS’s LS3 as an example, because I found a good graph of that:

So, at 1000 RPM, the engine’s making about 50 horsepower.

Here’s the BMW’s similar graph:

And this is where DOHC starts to have some advantages.

Variable valve timing and/or lift is a lot easier to implement, with a lot more flexibility, in a DOHC engine, which allows the BMW engine to have a flatter torque curve, improving power at lower RPMs than the peak. 4 valves per cylinder can improve swirl, improving combustion efficiency (which means thermal efficiency, power, and emissions performance) at lower RPM.

However, let’s look at that first torque peak at 3500 RPM, where the engine comes into its preferred operating window for the first time. It’s making about 195 hp there.

For 195 hp in the LS3, looks like that happens in the ~2800 RPM ballpark.

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Lower RPM means reduced frictional losses for the same cylinder... but we don’t have the same cylinder. So, let’s approximate the frictional losses by looking at the cylinder circumference times the stroke (the swept area) times the RPM.

LS3: 83.558 m^2/min

S65B40: 76.072 m^2/min

The S65 wins this one, despite the higher RPM, thanks to its technologies to improve low RPM performance that aren’t practical for a cam-in-block engine.

So how does the Corvette get better fuel economy? There’s a couple of factors here.

First one is simply... the Corvette is smaller and has better aerodynamics. A Cd of .28, and a frontal area of 22.3 ft^2 (2.07 m^2), for a CdA of .580, massively beats the E92 M3's Cd of .31 and frontal area of 2.17 m^2, for a CdA of 0.673. 86% of the drag would imply 86% of the fuel burn at high speeds, and there’s a significant portion of your improved fuel efficiency...

...but not all of it, and none of the city efficiency gains. And, of course, there’s the frictional losses I mentioned earlier.

One factor is that the surface to volume ratio of a larger cylinder is better. The surface to volume ratio of the LS3 is 6.048%, whereas the surface to volume ratio of the S65B40 is 7.007%. This means the LS3 loses less heat to the cylinder walls, the piston, and the head, clawing back some of the losses that it suffered from friction.

The city gains (and some of the highway gains) will come largely from the 432 pounds lower curb weight. When you have less mass to accelerate, you use less power to do it, and that means less fuel. A bit of that is actually the engine, but a lot is the fact that the Corvette is just a much smaller car.

But also, GM likes to use very tall gearing.

At the 80 mph used in the EPA testing, the Corvette is spinning 1712 RPM in top gear (manual transmission, base model). Conversely, the M3 is spinning 3426 RPM (again, manual transmission).

Even with the higher technology level of the M3's engine, when the Corvette is literally spinning half of the RPM? That saves losses in two areas: friction, as well as pumping losses.

If your engine is sucking in more air than it needs, you need to reduce the amount of air somehow, and traditionally that’s done with a throttle plate, but the engine suffers losses - frictional losses in the air - from having to suck air past a closed throttle plate. This reduces efficiency greatly . The Corvette operating at a lower RPM means it can have a wider open throttle plate (let’s say half open instead of 1/4 open), and therefore suffers lower pumping losses.

That is, however, a tradeoff - you’re trading acceleration away to get that efficiency (because you don’t stay on the (narrower) power band as long when you go from one gear to the next).

...there’s nothing saying that BMW couldn’t have put tall gearing in the M3, though, and gotten the same benefits (with the same performance drawbacks). Actually, greater benefits with the same performance drawbacks, or reduced performance drawbacks - the wider torque curve on the M3 allows it to suck even less air than the displacement difference would imply.

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Lower center of gravity is one big one, and that improves handling. Lower weight is another factor, helping handling and efficiency (as mentioned earlier).

...and, the lower hoodline provides more aerodynamic freedom in the design, which could’ve contributed to the good aerodynamics and some of those efficiency improvements.

Now, you want a more fair efficiency comparison, between more similary-sized cars?

A Pontiac G8 GT gets 17/25, versus the similar power BMW 550i getting 15/23, both with 6-speed automatics. And no, I don’t have time to deep-dive that, although I will point out that the G8 GT has cylinder deactivation (to reduce pumping losses (although not frictional losses) further , and I suspect they did the same thing with gearing.

marshknute > aquila121 04/05/2019 at 21:14

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Sorta related, but I remember the Corvette engineers saying they tested various smaller-displacement V8's when developing the C7, but got worse fuel economy on the highway.

Apparently , the smaller engines were working too hard to maintain speed while in the super-tall 7th gear, whereas the LT1 had no problem even while operating as a 3.1L V4.

gettingoldercarguy > marshknute 04/05/2019 at 22:29

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Did you get a chance to take a look at this?

https://oppositelock.kinja.com/1833845864

My bird IS the word > CobraJoe 04/05/2019 at 23:02

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I just love v8s, even the smog choked ones are so much better to drive than comparable 4-pots. Obviously the gas mileage isn't.

BoxerFanatic, troublesome iconoclast. > marshknute 04/05/2019 at 23:08

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Overhead cams have an advantage for fewer valvetrain parts, and fewer tolerances between those fewer parts for less friction, and less potential for lash and valve float than pushrod overhead valve designs.

With overhead cams, the cam lobes act directly on the valve stems, or with only a thin shim/lifter between the two, rather than the cam acting on a lifter, which acts on a pushrod, which acts on a rocker arm, which acts on the valve stem or shim.

Each of those parts have mass that reciprocate, with a specific tolerance between them, and have various expansion rates as their material properties heat and cool.

That is before getting into the options for separate cams for intake and exhaust, cam lobe spacing that can accommodate more than two valves per cylinder, anywhere from two to FIVE valves per cylinder depending on design.

Then you start to get into variable valve timing, variable valve lift and other effects that are more conducive to implement with overhead cams, than central cam, pushrods, and overhead valves.

Yes, a block can be smaller, heads can be smaller, and more swept volume per dimensional volume... but that isn’t the only aspect, and before the LS engine design, the pushrod dimensional volume advantage wasn’t as stark, and increasing RPMs by raising the redline was the way to increase power before 21st century technology boom made bigger advantages than old school engine design, as well as manufacturing technology bringing engine design itself into the 21st century, also.

There is a reason that pretty much every modern engine OTHER than the LS family and the sole V10 engine for the now discontinued Viper , have overhead cams, usually two of them. Even every other GM engine design other than the LS V8. Old SBCs and 60-degree V6s are no longer in production.

That is before getting into the Wankel Rotary that is camless and valveless , with only a few moving parts, which GM, Mercedes Benz, NSU, and others aside from Mazda also tried in the 1960s and 70s . Theoretically 21st century technology in terms of engine management, as well as engine construction could do amazing things for the rotary, as well, especially as a parallel/series hybrid powerplant. If you want to talk about output power efficiency per cubic foot of physical dimension, a pressurized rotary leaves all piston engines behind. Arguably it is an engine design before it’s time, before the technology existed to really make it work ideally.

NYankee1927 > jimz 04/06/2019 at 20:32

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Feel free to use a photo of a LS with all of the ancillaries, like the 4 cyl shows

NYankee1927 > marshknute 04/06/2019 at 20:36

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Show me a LS engine with independent  variable valve timing on both the intake and exhaust.

NYankee1927 > bhtooefr 04/06/2019 at 20:38

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This post deserves more stars

jimz > NYankee1927 04/06/2019 at 21:41

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Feel free to explain what “ancillaries” would make a difference. ‘Cos the big ones on the 4cyl are the intake and exhaust, which are in place on the pictured LS.

marshknute > NYankee1927 04/06/2019 at 22:26

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What’s the point?

The S54 is already an objectively inferior engine.  Why add complexity and make it even worse?

NYankee1927 > jimz 04/07/2019 at 10:50

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Alternator, AC compressor, power steering pump. The same ones shown in the 4 cyl photo. People love to show that photo and it is not a apples to apples comparison. It makes the LS look smaller.

In a car it makes a big difference for the serviceablilty and accessibility. Those ancillaries usually get packed in and suck to work on

NYankee1927 > marshknute 04/07/2019 at 11:00

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I thought you were wondering about DHOC cars in general. Independent valve timing allows the torque curve to be flattened over the range. Which means the engine can be both drivable and have top end for Motorsports. The LS is compromised in you can have a truck engine that pulls or a race engine with a hot cam.

The S54 is a straight 6 so I’m not sure where you are going here? It has almost as much power as a period LS in a corvette  with half the displacement and the smoothness of a straight 6. They are two different design applications. 

CobraJoe > MasterMario - Keeper of the V8s 04/07/2019 at 11:44

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Wat exactly are the manufacturing constraints that favor the pushrod? There are tons of OHC engines that make similar power figures and are just as reliable as the LS, the LS just had a head start in development. 

MasterMario - Keeper of the V8s > CobraJoe 04/07/2019 at 16:55

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Sorry, meant design constraints. The smaller packaging and lightweight of the pushrod gives it a big advantage. Not to mention it’s probably cheaper to manufacture simply due to having less parts

BigBlock440 > Highlander-Datsuns are Forever 04/07/2019 at 22:06

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You can do 4 valves with a push rod

BigBlock440 > Gerry197 04/07/2019 at 22:17

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Euro wienies tax on displacement, some places heavily, so it matters there.  Here it does not.

CobraJoe > MasterMario - Keeper of the V8s 04/08/2019 at 13:34

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Fair point, a ohv engine is easier to place in a chassis, but it’s a fairly minimal advantage to an OEM . Most often, the car’s chassis is designed to accommodate specific engines, and the engine bay is consciously designed to fit the larger engine dimensions of a SOHC or DOHC.

Even if the engine bay was not designed to fit an OHC engine, it most likely will still fit. The MN12 Thunderbird was initially offered with a OHV V6, then later a Windsor V8, and eventually a 4.6L SOHC (and a DOHC concept car in ‘97). Plus, most V8s these days are used in trucks or increasingly larger pony/muscle cars. The larger engines do make it more difficult to engine swap a DOHC into an older body, but there are plenty of examples of 4.6 DOHCs in old Ford hot rods or Cobra replicas.

As far as lightness, there’s not a huge difference, a 5.0 Coyote is pretty much identical to an LS1 as far as I can find. (Which makes sense, the heads are heavier, but the block is simpler). As for cost, I don’t really know the exact costs, but I imagine that a DOHC would only be more expensive than a OHV primarily because of the extra valves, but it’s also probably cheaper to implement a variable valve timing on a DOHC than a OHV.

I really don’t have a favorite between these two, just trying to point out that it’s not a “theory vs reality” comparison. Just another “Pros vs Cons for this exact scenario” situation, which pretty much describes all of engineering.

3point8isgreat > NYankee1927 05/03/2019 at 12:13

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I thought people generally considered the LS engines to have a pretty flat torque curve. 

Also independent int/exh valve timing can be done with pushrod. I believe the viper used it. GM decided that they got the majority of the benefit from just varying them together.

VajazzleMcDildertits - read carefully, respond politely > bhtooefr 05/03/2019 at 15:04

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T hank you for this.

Chan - Mid-engine with cabin fever > marshknute 05/16/2019 at 20:52

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Here’s another angle that I came up with after a bit of thought:

DOHC is a revelation for smaller mainstream engines. Such as inline-4s. Having pushrods on those would be a packaging nightmare in modern engines, drive the valves  from above at a much smaller distance, leaving plenty of packaging room for intake and exhaust on the sides of the block.

Given that it works great for inline engines.......guess where most manufacturers start when designing a V8 block?

This is probably not the only reason, but I think it’s definitely a factor in the worldwide migration to DOHC engines in the 80s and 90s.