[Notorious B.i.G.]

This debate rages amoung rev head here in Australia.
A Pushrod engine versus a dual overhead camshaft or DOHC engine.
Holden (GM) builds/uses the GM range of pushrod V8s and Ford uses a overhead camshaft. Both have certain advantage and disadvantages. See wikipedia for more detail, http://en.wikipedia.org/wiki/Dohc and http://en.wikipedia.org/wiki/Pushrod

I like the OHC engines. Better uses of Variable Valve Timing (VVT) and the engines can produce more power from a smaller stroke. For example the new(ish) GM LS7 engine or the 5.7lt LS2 vs, the Ford Modular (or Boss as it is known in Australia) 5.4lt V8 OHC. The Ford engine can gain more power from a smaller engine because of it's use of OHC.

What are other peoples take on the Pushrod vs (D)OHC debate?
Is there a debate about this anywhere else outside Australia for that matter?

*Correction, the GM LS2 engine is has a displacement of 6.0lt*

[Thunderhawk]

From what little I know about the distinctions, Im a fan of OHC, Dual for 6+ piston engines.

[GodSpeed]

The GM 6.2 Uses variable valve timing and it is a pushrod.

Pushrod engines generally dont need VVT (VTEC, whatever) because of their nature. Pushrods dont lend themselves to high revs and it is easier to use a single camshaft profile suitable for idle-5700rpm than a single one for idle to 9000rpm. Camshafts are developed with a powerband in mind and there is no one-size-fits all. Higher revs want higher durations and different timing (openings, closing, LSA etc)

Think of it as shoes. It's easier to make a walking/crosstrainer hybrid sneaker shoe than a walking/running/sprinting/cross-trainer.

The BIGGEST advantage of OHC is that it allows for higher revving without valve float. OHV (pushrod)can rev, but hydro-rollers have their limits. You can go to a solid roller, but that is more maintenance and the maintenance is not idiot proof at all. Hence, it is not idea for OEM.

OHC is a bit more "more valves friend" which is nice, but a pushrod engine (or 2 valve) has more room in the head so it can make one big intake/exhaust port instead of say 2 smaller ports. 4 Valves are generally better than 2, but (as shown with GM's OHV), pushrod heads can flow way more air than you need to flow on the street! LS7 heads are something like 370CFM? They support +600hp NA with no changes to the camshaft so yeah, I'd say it's adequate.

With OHC come MASSIVE cylinder heads/valve covers. OHC engines are huge. Compare a Ford Mod motor to a GM LSX motor. A 7-Liter LS7 is the smaller and lighter than a 5.4L DOHC engine.

Pushrod engines are nice because the cam goes in the valley created in the V. They are small, light and fuel efficient. OHC drain more fuel since they are spinning more cams (in most cases) with huge timing chains and the additional weight. All things being equal, the hoodline has to be higher on an OHC engine to fit the taller engine which means more drag.

Has anyone here ever tried to degree the cams on a DOHC V8? What a nightmare. Cam swaps in a pushrod? a few hours and a few hundred bucks. In an DOHC V engine? Christ...

The proof is in the pudding though. GM has by far the best V-8 line up out on the market, less exotic engines/vehicles. They out power the competition, have OUTSTANDING reliability, low cost, great efficiency, tons of potential for hot rodders and low cost. Their small size lends them to some really cool engine swap projects from honda civics to shelby cobras.

OHC is pretty much a marketing tool. It's really only useful if your application warrants high revs. If it doesn't, it's a hindrance to performance in virtually every aspect.

[Thunderhawk]

That was informative.

[GodSpeed]

after re-reading it, I felt I should clarify some ambiguous sentences, hence the edit.

[Notorious B.i.G.]

GM has by far the best V-8 line up out on the market, less exotic engines/vehicles

very very debatable fact.

also
A GM 7-liter LS7 is more of an exotic V8 then many of the European V8's.

OHC is pretty much a marketing tool.

Bit of an exaggeration. Considering that a majority of cars use some form of an Overhead Cam system, rather than pushrod.

It's really only useful if your application warrants high revs. If it doesn't, it's a hindrance to performance in virtually every aspect.

Ford's 4.0l inline DOHC engine has better performance in the lower rav band then the higher. it oozes high levels of torque.

OHC drain more fuel

OHC engines allow for direct fuel injection systems which are more fuel efficient then any pushrod system.

Has anyone here ever tried to degree the cams on a DOHC V8? What a nightmare. Cam swaps in a pushrod?

Good point. It is v. v. easy to work on a pushrod, you've got a lot less stuff to fuck up. But OHC, no need for timing belts. Timing belt goes on a pushrod, bye bye engine.

[hannu]

I prefer non interference OHC engines.

Easier to assemble than a pushrod engine. No pushrods to flex.Nothing breaks if the belt fails.

In general they are just less messy. But I do prefer single cam to double still with 4 valves per pot.

[GodSpeed]

GM has by far the best V-8 line up out on the market, less exotic engines/vehicles


very very debatable fact.

also
A GM 7-liter LS7 is more of an exotic V8 then many of the European V8's.

Where I’m from, facts aren’t debatable. Anyway, who has a more powerful, reliable, cheap, light, fuel efficient V8 line up? No one.

OHC is pretty much a marketing tool.


Bit of an exaggeration. Considering that a majority of cars use some form of an Overhead Cam system, rather than pushrod.

So if a lot of car companies use billboard advertising, it’s not a marketing tool? I don’t follow this logic? Everyone does it, so it must not be marketing?

It's really only useful if your application warrants high revs. If it doesn't, it's a hindrance to performance in virtually every aspect.

Ford's 4.0l inline DOHC engine has better performance in the lower rav band then the higher. it oozes high levels of torque.

Again, I don’t see your point. The powerband of an engine is dependent upon its camshaft profile, not the location of the camshaft. The OHC didn’t make a low-grunter or a high-revver. If you’re going to use an example of OHC+ low end, what was wrong with diesel?
What makes you think that having 4 camshafts driving the valvetrain will make more power or change its torque-curve?T
The point of OHC is to get around the valve float issue that PLAUGES Pushrod.

OHC drain more fuel


OHC engines allow for direct fuel injection systems which are more fuel efficient then any pushrod system.

What does OHC have to do with direct injection?
First off, [modern] direct injection (DI from now on) is a newer thing. With rare exception of some super-new engines, they weren’t designed with DI in mind. To say that OHC engines, 99% of which aren’t DI, are more efficient because of DI requires a serious lapse in logic.

Is it easier to DI with OHC? Sure.
Is true that no pushrod engines have DI- Yes
Does this mean we cant have a DI pushrod engine? Absolutely not!
Keep in mind that there is pretty much only ONE company making pushrod engines today, GM. Sure a few others still have a few pushrods kicking around (I cant think of anyone other than the big “3” though) but no one has them dominating their line up like GM and no one is dumping money in them like GM.
GM hasn’t done DI on many of their engines, OHC inclusive) at much all (I can only think of one). So we are basically waiting for GM to decide they want to do it. Mercedes has their spark plug and direct injection unit built in together. I’m sure GM could find a way to direct inject a pushrod; it’s more a hiccup than a show-stopper.

[Thunderhawk]

Direct injection is a technology 10+ years old in the automotive industry (developed by Mitsubishi) and made open to all users (I believe it was a condition on Japanese Gov't funding). The original concept is from 1940's Germany.

[hannu]

"Direct injection is a technology 10+ years old in the automotive industry (developed by Mitsubishi) and made open to all users "

Not very good either. OK using Japanese fuel but the mixture has to be richened up substantially to run on say european fuels. Defeats the object somewhat.

[Thunderhawk]

To get the most bang for you buck, gasoline needs to be at a certain vapouized percentage (desity, if you will). To low a density and your not producing much power in the combustion. GDI is a form of spraying the gasoline into the chamber in a pattern such that it creates localized ideal concentration under compression without having to fill the whole chamber to that ideal concentration.


If the fuel needs to be "riched up" it is becuase the chamber, piston head and atomizer (vapourizer) were designed for a different fuel. That however is just a matter of fine tunning the specs for different fuels. The concept is sound.

[hannu]

"If the fuel needs to be "riched up" it is becuase the chamber, piston head and atomizer (vapourizer) were designed for a different fuel. That however is just a matter of fine tunning the specs for different fuels. The concept is sound."

OK so you are suggesting Mitsubishi have a different combustion design for each individual market? Don't think they would go for that.

Or are you saying it is acceptable to fine tune the mixture specs.
In the UK they have to alter the specs so much that the mixture is more or less the same as a normal inj/carb engine so GDI has no noticeable benefit.

[GodSpeed]

OK so you are suggesting Mitsubishi have a different combustion design for each individual market? Don't think they would go for that.

It's not that big of a deal to make a "US head"

[Notorious B.i.G.]

Where I’m from, facts aren’t debatable. Anyway, who has a more powerful, reliable, cheap, light, fuel efficient V8 line up? No one.

Where you come from isn’t the rest of the world. Yes GM may have the best line up of V8’s in the United States market. But not here in Australia and not in Europe.

Fact is the Ford’s Boss V8 has 290kw compared to GM’s LS2 of 270kw.
Yes the LS 7 has more, but seeing that engine can currently only be found in the vette z06 and isn’t comparable to the modular Ford OHC V8.

So if a lot of car companies use billboard advertising, it’s not a marketing tool? I don’t follow this logic? Everyone does it, so it must not be marketing?

So because a car company does/uses/produces something it’s a marketing tool? If all OHC engines are marketing tools then fine, by your logic, Pushrod engines are a marketing tool.
I’m not saying that car companies don’t use terms like VVT-I and OHC as marketing tools. But to say that the majority reason for using OHC is marketing is, well wrong.

Is it easier to DI with OHC? Sure.
Is true that no pushrod engines have DI- Yes
Does this mean we cant have a DI pushrod engine? Absolutely not!

So if it is easier to DI with OHC and no pushrod has DI, why waste time, money and resources to develop DI on pushrods? It’s harder (by your own admission) which equates to wasted resources. There isn’t a pushrod DI, so it cannot be said it one is more efficient then the other. But seeing there are OHC engines with DI, currently OHC are more fuel efficient.

Keep in mind that there is pretty much only ONE company making pushrod engines today, GM.

Why is this? It can’t be because pushrods are just naturally better than OHC. If they were there would be other car manufactures producing pushrods for their performance cars.

[Thunderhawk]

But seeing there are OHC engines with DI, currently OHC are more fuel efficient.

Incorrect.
Logically OHC engines have the potential to be more fuel efficient. If they are not using DI, then the fuel saving aspect of DI is not applicable.

[hannu]

Godspeed wrote

It's not that big of a deal to make a "US head"

It may not be, but why bother?

Mitsi just tell everyone how good there engine runs in Japan but fail to tell everyone how limited the benefits are in other country's.

It's worked for them for the last ten years hasn't it?

[Notorious B.i.G.]

Logically OHC engines have the potential to be more fuel efficient. If they are not using DI, then the fuel saving aspect of DI is not applicable.

Yes, sorry, I should have said 'on average they are more fuel efficient.' or 'potentially'

[GodSpeed]

so no one has an explanation for why OHC is better other than
1. Everyone is doing it
2. I saw it on a billboard
3. I hate GM and American cars

forget the size, weight, parasytic loss, math, etc etc- IT WAS ON A BILLBOARD!

[Thunderhawk]

Its difficult, if not downright foolish to say one type is better then the other.

Comparing the "average" Push rod engine to the "average" OHC is somewhat of a red hearing, as most characteristics come from several componants (or the whole motor design) working together rather then then just the valve operation. When your comparing certain characteristics, then you should try to keep the rest of the characteristics the same (or close enough). If your going for fuel savings, compare fuel efficient models from both types in the same litre range, bore size, cylinder #, etc.. Finding such models is usually easier said then done. However, you cannot apply the results from one engine variant (like a fuel economy model) to another variant (like a high HP model).

If you want a generic "which is better" all you can get is people's general feeling/preference, as there is no generic "better" when there are so many variable conditions, characteristics and performance abilities. Like most situations where there are many variables and multiple possible solutions, seeing what is amongst the worst and what is amongst the best isnt too hard, but choosing "the best one" is a judgement call. Even when 2 people have the same knowledge and information they probably weigh the variables and characteristics differently then the other, and will thus have differing opinions (though possibly the same conclusion).

[GodSpeed]

It's fairly simple to understand which will result in higher fuel consumption. It's not magic. OHC is just going to be bigger (requiring taller hoodlines and perhaps wider bodies), heavier and it takes more power to turn a more complicated OHC engine.

It's a science, not an art. Use the right tool for the right job.