Pushrods vs DOHC

[pietran30]

I've recently seen a couple people on this forum knocking the fact that Dodge and Chevy use pushrods motors.

I also recently posted this question on another forum, and had some good responses so I thought I'd ask here.

Can someone give me the benefits of a DOHC motor vs a typical pushrod engine? Why the the mustang use the 5.0?



From reading online, I've found that typically a DOHC makes much more hp per liter of displacement but:



They're typically much larger in dimensions

They are heavier

They are more expensive to produce

They make power by revving to higher RPM's

They produce less torque



So why does Ford use it? Is there a benefit?

I know Ford switched in the early 90's when it first looked like pushrods were gonna go extinct. I'd imagine their main reasons would be that it would cost too much to go back, and that they can meet their power needs with a DOHC engine.

I understand why many other automakers overseas use DOHC motors, since they are taxed on liters per displacement. Smaller displacement = lower taxes.

Is there a true performance benefit?


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[Mustang1260]

Try Google.
You will find many scientific/engineering based articles discussing this topic.
No right or wrong answer.
One is not better then the other in every circumstance, design or use.
If your trying to find "proof" you bought the better engine/car- you simply won't find it.
Performance cars with either engine both provide more performance then you have the skills to use to the fullest so don't worry about it.
Decide which one is best and you get in it. A professional driver (road course or drag racer) gets the other one. You lose every time.

 

[Todd15Fastback]

Only thing I believe are the DOHC motors are better for boosted applications. This is coming from a non-engine expert. Someone can correct me as well

 

[Daytona Coupe]

It is because it is impractical to have more than two valves per cylinder in a pushrod engine. All of the most efficient production engines use 4 or even 5 valves per cylinder. It's very difficult to do that without overhead cams. Two cams makes it easier to have independent valve timing. It's not just about make by power, its power combined with drive ability, emissions, and economy.
It can be done with pushrods, but you're at a disadvantage from the start.

 

[kn7671]

The question is, Are there true performance benefits to an overheard cam engine?

Yes, absolutely, but there are trade-offs in all engine designs. The largest benefit is improved breathing with multiple valves plus increased rev range though direct actuation of the valves without the need of a heavy hydraulic lifter, pushrod, and rocker arm assembly.

I would not think a DOHC motor makes less torque to an equal displacement cam and pushrod engine, but the peak torque is typically at a higher rpm since these engines designed and setup to make more power at a higher rpm target.

 

[BmacIL]

DOHC engines don't make less torque than OHV (pushrod) engines. Typically for similar power outputs, the OHV engine will need to be a good deal larger displacement than a DOHC engine (6.2L Chevy vs. 5.0L Ford, for instance) because it is less volumetrically efficient (breathing) and has more friction (pushrods and corresponding valvetrain components). That larger displacement is what makes more low-end torque than the smaller DOHC engine.

 

[OppoLock]

The question is, Are there true performance benefits to an overheard cam engine?

Yes, absolutely, but there are trade-offs in all engine designs. The largest benefit is improved breathing with multiple valves plus increased rev range though direct actuation of the valves without the need of a heavy hydraulic lifter, pushrod, and rocker arm assembly.

I would not think a DOHC motor makes less torque to an equal displacement cam and pushrod engine, but the peak torque is typically at a higher rpm since these engines designed and setup to make more power at a higher rpm target.

Torque largely correlates to displacement.

 

[JimmyTwoTimes]

Torque largely correlates to displacement.

For naturally-aspirated cars. And even then, not necessarily. There are some MASSIVE flathead engines that can't match the torque of an OHV or OHC engine.

 

[Sinister]

The question is, Are there true performance benefits to an overheard cam engine?

Yes, absolutely, but there are trade-offs in all engine designs. The largest benefit is improved breathing with multiple valves plus increased rev range though direct actuation of the valves without the need of a heavy hydraulic lifter, pushrod, and rocker arm assembly.

I would not think a DOHC motor makes less torque to an equal displacement cam and pushrod engine, but the peak torque is typically at a higher rpm since these engines designed and setup to make more power at a higher rpm target.

The trick is actually getting that OHC to the displacement of the OHV engines.

 

[Trackaholic]

IMO the differences between the two are mostly about the feel of the power delivery, rather than one being overall better or more advanced than the other.

A DOHC engine will have 4, smaller valves per cylinder and less valve train mass. This will allow higher RPMs (due to smaller valves and less valve train mass) and better breathing at the top end (due to having more available valve area for a given bore). These factors will allow the engine to have higher specific horsepower (HP/liter).

A pushrod engine will require more displacement to make the same peak power because it won't be able to rev as high due to having two larger valves per cylinder and having greater valve train mass due to the pushrods and other components. This limits the breathing at high RPM and therefore reduces the specific power capability. However, the larger displacement allows greater torque, so at equivalent RPM the pushrod engine will usually be more powerful.

Because the pushrod engine has a single cam shaft in the center of the block, the heads are smaller (no cams above the valves) and the CG is lower. The overall weight is often less as well, due to the compact external size of the engine (especially in terms of overall height).

Not sure how an engine like the LS3 or LT1 compares to the Coyote in terms of length, but I think the two are relatively comparable in weight, while the SBCs are lower in overall height.

In general I'd say the Coyote will feel less strong at low RPM but will also seem to rev more freely due to the smaller displacement and the way it pulls harder at the top end (torque does not drop as quickly as it does on the pushrod engines).

The pushrods will feel stronger at the low end due to their higher torque, but will feel a little weak up top. They will also feel a bit slower to rev (like when rev-matching) due to their larger displacement.

I drove the C7 at a driver's school and noticed that the engine's pull seemed to fall off up top. I haven't had a chance to drive a Coyote in the same way, but from what I hear (and see on dyno charts), it pulls a bit more consistently to redline.

I personally want the GT350 due to the Voodoo engine, which I think highlights the main advantage of the DOHC architecture, which is making high N/A horsepower while maintaining emissions compliance. I really like GMs LS7 engine, but sadly they were unable to make an updated version for the C7 Z06. Instead they needed to revert to a blown engine, which is not to my liking for a track car (due to extra weight over the front of the car). GM has said that they were unable to reach the power goals in a normally aspirated motor while meeting emissions targets. Ford looks to be able to achieve that with the Voodoo (admittedly, Ford's power goals may be quite a bit lower than GM's).

Still, for most applications, I think the main difference between the pushrod and DOHC setup is about the feel of the power delivery, rather than anything else (at least from a driver's perspective). There may be some minor tax advantages to the DOHC in certain countries, but most are now taxing based on CO2 rather than displacement. And there may be some packaging advantages to the pushrod design, but some of that is offset by their larger displacement.

EDIT: 1000th post. BOOOYA!

EDIT: Wanted to mention one thing regarding emissions: DOHC has an advantage due to the independent valve timing afforded by having a separate intake and exhaust camshaft. I think Dodge has some amount of independent control on the Viper engine (and possibly others), but I don't think GM does on their pushrods. So, that might not be an inherent limitation to the pushrod design, but rather a limitation based on how GM decided to allocate their budget and feature tradeoffs.

-T

 

[alexx205]

Torque largely correlates to displacement.

That is correct. But one more important thing is the ratio of bore/stroke.
An engine with a long stroke should have more torque than an engine with equal displacement but shorter stroke (and wider bore) because of the longer distance of the crankpin to the crankshaft centerline (leverage effect).
This is one reason why diesel engines usually have relatively high torque compared to gasoline engines. (Diesel engines are usually designed with a rel. long stroke.)

 

[pietran30]

Thanks for all of the responses guys. Here's one more question for you. Hypothetically speaking:

The coyote is a pretty large engine in terms of it's size and weight.
Dimensions: 27" H X 26" D X 28" W
Engine Weight: 430 lbs.

If we were to build a pushrod motor with the same dimensions, would it be more powerful than the coyote?

Obviously Ford would not do this, because of the cost. Also, this is just a hypothetical situation. I am not trying to troll here, just genuinely curious.

 

[dreicher]

Another advantage of OHC engines is that with multiple intake and exhaust valves the geometry of the head can be better optimized for both volumetrically efficient and position of the spark plug.

 

[Brent302]

Here is a neat picture showing these high displacement ohv engine are actually small compared to a dohc

 

[cosmo]

Thanks for all of the responses guys. Here's one more question for you. Hypothetically speaking:

The coyote is a pretty large engine in terms of it's size and weight.
Dimensions: 27" H X 26" D X 28" W
Engine Weight: 430 lbs.

If we were to build a pushrod motor with the same dimensions, would it be more powerful than the coyote?

Obviously Ford would not do this, because of the cost. Also, this is just a hypothetical situation. I am not trying to troll here, just genuinely curious.

The LT1 already weighs more than the coyote because of technology adds mainly. But an LT1 build to those dimensions would weigh probably ~550lbs and put out a disgusting amount of power. Pushrods are smaller dimensionally, but typically weigh similar/less than a DOHC.

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