Push Rod vs. DOHC engines

[brillo]

I'm sure I'm inviting hell with this question but this issue has been bugging me for a while.

First off, I'm engine agnostic. I've owned virtually every engine type and layout on the market. I currently drive a rotary as my toy car, but if you could provide me a hamster wheel that is powerful and efficient I would consider it. I really don't care about the engine design so long as it’s small, lightweight, efficient and powerful.

The new GM Corvette engine is an amazing package in terms of power, weight, reliability and efficiency. Given all that GM can get out of a push rod V8 chocked full of technology, why do Ferrari, BMW, Audi etc... all seem so wedded to DOHC engines in their performance cars that seem to be larger, less efficient, more expensive/complex and physically larger?

Are push rod engines dirtier in terms of emissions? I get that it’s hard to build a small push rod engine (i.e. a I4 replacement), but in terms of larger performance engines, I’m just not seeing the advantage of DOHC.

All engines have advantages and disadvantages, so what are the disadvantages of a high tech (direct injection etc…) push rod engine?

[7thgear]

because it works

it's reliable and i you want more power just add displacement

GM is a cheap company that didn't want to innovate since they made their first car, focusing on re-using existing technologies hoping that consumers still think that anything American is the best in the world.

As for efficiency.... Japanese engine builders have been making reliable engines which produce over 1 hp per liter for the last two decades, what has GM made?

[cobrabyte]

Originally Posted by 7thgear

because it works

it's reliable and i you want more power just add displacement

GM is a cheap company that didn't want to innovate since they made their first car, focusing on re-using existing technologies hoping that consumers still think that anything American is the best in the world.

As for efficiency.... Japanese engine builders have been making reliable engines which produce over 1 hp per liter for the last two decades, what has GM made?

I certainly hope it's 'over 1hp per liter'.

100hp/liter?

[Yruyur]

Originally Posted by LSxJunkie

What has GM made? Torque that you can actually use on the road.

Good for hauling all us overweight Mericans! I kid I kid.

Sent from my flux capacitor using Taptalk

[7thgear]

Originally Posted by LSxJunkie

What has GM made? Torque that you can actually use on the road.

GM made a lot of money by swindling their customers with outdated technology

then came the big hammer but somehow the laws of capitalism didn't apply, so they took even more money from the people and continued to produce crap

now somewhere somehow someone up in GM management got the idea that maybe it's time they build something that's not shit


torque you can use? yes lets mask all of our inefficiencies with engines that kick people back in their seats, that should keep them docile and satisfied.

herpa derpa

[gmookher]

The Oldsmobile based pushrods is rock solid but not an optimal race motor imho

[ZDan]

OHV cam-in-block makes a lot of sense if you aren't displacement-limited by racing class or vehicle tax structure.

Obviously, they won't make the same power/displacement as DOHC multivalve engines. But they do make comparable or superior power/physical engine size and power/engine weight.

And for a given power level, larger displacement OHV gives better fuel efficiency than smaller displacement DOHC multivalve.

Also, OHV cam-in-block engines will have a lower center of gravity than DOHC.

There's a certain elegance in the simplicity of large-displacement OHV 2-valve engines. I like 'em so much there's one in place of the rotary in my RX-7

I also like high-revving DOHC multivalve engines, there's one in my S2000 and another in my SV650

[rice_classic]

It's just a different way of moving valves up and down. Doesn't matter much. The design of the ICE hasn't changed much.

It's all still suck/squeeze/bang/blow.

[serialk11r]

I know there's a lot of LSx fans in here but to say that higher revving engines of the same power get worse fuel economy is like saying some fatter people eat less because they're trying to lose weight.

The gearbox is the biggest determinant of fuel efficiency, and the higher revving engines have traditionally lacked a true cruising gear. Additionally as you probably know the EPA city test is more a test of gearing than anything else.

An LS3 probably idles at 0.75 gallons per hour or something like that. 0.75 gallons per hour is the fuel flow rate required to keep a typical car moving at 35mph. Think about that for one second.

I'd be willing to bet that E90 M3s get the same mileage as Camaro SS in the real world, probably better. No, I have not looked online to see what mpg people are actually getting.

What I think the big American V8s should be admired for is how they manage to be so light and pack so much displacement. As @LSxJunkie points out, an LS3 is lighter than an S65, which is already extremely light (the S54 was heavier). GM managed to make a 6.2L lighter than a 3.2L BMW engine. That is impressive. However, you definitely cannot argue that they are fuel efficient. Around town they guzzle gas like nothing else, but on the highway they are okay since GM is willing to suck up their pride and widen gear ratios, whereas BMW insists on short close ratios, same with the Japanese manufacturers. Of course, part of that is the "broad powerband", aka early torque peak.

[7thgear]

Originally Posted by LSxJunkie

Swindling people by selling them outdated technology that they specifically knew they were getting?

haha okay, because car buyers are 100% informed consumers right?

If people actually knew what they were getting from GM then GM would have been out of business a long time ago with the carolla being a staple econobox of every american household.

[ScionRacer]

Ford tried the OHC route in the 60's with the 427 sohc.It made better than 100+hp over the best push rod boss 429 of its day,but was expensive to produce.

Instead of GM spending a ton on new technology,they spent a little here and there refining the old.The one thing i can give them credit on is keeping the corvettes heritage in tact.The new vettes still feel and sound the same way they did back in the day because of that pushrod motor.

[Rayme]

4/5 valves per cylinder will always be more efficient than 2. Take any DOHC engines and remove 2 valves, and try to keep the same Horsepower, good luck. You need more displacement for offsetting those drawback. As far as I know, pushrod engines are limited to 1 intake and 1 exhaust valve. It's easy to see why DOHC engines are so popular, you get get alot of horsepower(and higher RPM) more easily out of them at the added cost of weight (Which isn't a big deal at all).

It doesn't matter though, those pushrod GM is pumping out are nice engines, they are just not impressive as far as engineering goes. There's nothing better than the other. :happy0180:

[Draco-REX]

Just to add some perspective here on the old vs new engine tech argument:

The first production Dual Over Head Cam engine was in 1924.

The first production Over Head Valve engine using pushrods like the corvette engine was 1949.

So technically, today's pushrod engine is newer tech. Not that it really matters. we're talking about the 20s and 40s here. So the "higher tech" argument is pointless, and actually rather silly.

[ZDan]

Originally Posted by ottopilot

4/5 valves per cylinder will always be more efficient than 2. Take any DOHC engines and remove 2 valves, and try to keep the same Horsepower, good luck.

Depends on what you mean by "efficiency". Normally, it would be BSFC, brake specific fuel consumption.

The most efficient designs in terms of BSFC won't be the most power-efficient in terms of power/displacement, that's for sure.

Fewer valves and fewer intake tracts will generally give better BSFC, as for a given flow area/volume, you will have less of the flow near the tract walls. Reduced losses drawing the charge into the intake.

Also, more cams, chains, and valves => marginally greater frictional losses.

You need more displacement for offsetting those drawback. As far as I know, pushrod engines are limited to 1 intake and 1 exhaust valve.

You could do a multivalve OHV engine.

It's easy to see why DOHC engines are so popular, you get get alot of horsepower(and higher RPM) more easily out of them at the added cost of weight (Which isn't a big deal at all).

Weight is a very big deal. So is size. For a given size/weight, you can make as much or more power with an OHV V8 vs. a DOHC V8 (which would have to be of MUCH smaller displacement to be the same size/weight).

It doesn't matter though, those pushrod GM is pumping out are nice engines, they are just not impressive as far as engineering goes

Only if you judge "engineering" by counting valves and camshafts. The engine engineering at GM is top-notch.

There's nothing better than the other. :happy0180:

There's more than one way to skin a cat. DOHC/multivalve is required to get maximum performance out of limited displacement. If there is no displacement limit, the simpler, lighter, smaller, cheaper OHV/2vpc design of greater displacement is as good an engineering solution (superior on some fronts) to achieving the same power/performance.

To disparage the LS-series V8s because of their lack of valves and camshafts is to be too fixated on the means, rather than on the end result.

All said, I'm a big fan of diversity, and LOVE higher-revving DOHC multivalve engines as well.

But when it comes to figuring what's the best engine to get maximum power in a small, lightweight package, the large-displacement OHV V8 is very hard to beat...