In Defense of Pushrods

[T-Type]

In Defense of Pushrods
Pity the poor pushrod, the Rodney Dangerfield of engine components.

by Gary Witzenburg
Automotive Industries, May 2004

The pushrod is a simple metal messenger. It transmits dynamic messages upward from a carefully designed camshaft buried in the block to a pivoted rocker atop the engine’s cylinder head. The rocker transmits those messages downward to a spring-loaded valve that precisely opens, then closes, to let fuel/air mixture into the combustion chamber or vent exhaust gases out. It does this effectively, reliably and inexpensively, usually for the life of the engine.

This simple, cheap, anvil-solid overhead valve (OHV) arrangement has served America’s motorists well for many decades and continues to serve where its advantages outweigh those of overhead cam (OHC) designs. One camshaft between the banks of V-type engines drives both intakes and exhausts. Losing the pushrods gains some mechanical efficiency but adds at least a second camshaft and moves them up onto the heads, which adds cost, complexity and engine size.

Larger engines are tough to package under low, sleek, aerodynamic hoods, so OHCs tend toward smaller in overall size, and therefore displacement, which reduces power and torque. They can regain power, and efficiency, by adding valves (three or four per cylinder) and camshafts: one per bank for intakes, another for exhausts. More cost and complexity. While this evolution to OHCs was happening, driven by high gas taxes and punative penalties on larger displacements, in Europe and Japan, the majority of American engines continued with OHVs because there was no need or demand to change them. Then fuel efficiency became an issue with the 1973 fuel crisis, and imported cars from Europe and Japan began gaining market share.

Importing mostly the best of what they sold at home, these makers positioned their smaller, less powerful yet more expensive OHCs as more sophisticated and more desirable than American OHVs. And, in most ways, they were, since the typical iron-block American OHV of the ’70s and ’80s had evolved little since the ’50s. Compared to most import engines of the time, they were noisy, shaky and thirsty.

Thence came the conventional wisdom that “high-tech” OHC engines are smooth, refined and efficient while “low-tech” OHV engines are rough, crude and inefficient. The pitiful pushrod took the blame for the sins of entire old-design engines. But what most media and auto aficionados seem unable to grasp today is that there is a lot more to an engine’s refinement and sophistication than its valvetrain configuration.

Full Article Here

[tgagneguam]

I definitely appreciate those who find and post these types of articles. I find myself wanting in the "how it works" department, so articles like these help to educate. Thanks.

I do agree with the bottom line: "...If it starts instantly and runs reliably every time, provides the performance [i] need with reasonable fuel efficiency, requires little or no maintenance and lasts as long as the vehicle, fine!" That's really what I, too, want in an engine.

Of course, I say this despite the fact that my vehicle has one of those "fancy, high-tech" engines! In my defense, I chose the car for a variety of reasons, least of which was the "high-tech" nature of its engine.

[ponchoman49]

Ahh the ohc vs ohv debate again. I have owned over 20 cars in my 18 years of driving and have owned both. I can honestly swear by the pushrod. Be it the 3800, the 3400, most of GM's older V8's and even Ford's Vulcan 3.0 or chryslers 3.3 and 3.8. These have been generally long lasting motors with good fuel economy, off the line power, no timing belts to replace etc. If it wasn't for the intake issues on GM's pushrods and some of Chryslers, I would call these bullet proof. And not one of my pushrods ever used a drop of oil! Now I have also owned some Dohc motors like a 99 Intrigue with the 3.5 Shortstar, which was a nice engine untill it got about 35k on it and a 3.4 twin cammer in my lumina that had all sorts of troubles. That Intrigue engine would consume about 2 quarts of oil between changes on average and ate 2 pistons that had to be replaced under warrenty. My 02 Intrigue has 60k on it and uses about a quart every 3k and has developed a leak at the rear valve cover! Worse it has also started to develop quite a knock when cold. Driving around I have noticed lots of 98 and up Intrepids smoking oil when moving from a light with there 2.7 Dohc motor and numerous law enforcement Fords doing the same thing with the 4.6Sohc V8's! And lets not forget Mitsu's famous 2.6 ohc silent shaft 4 and 3.0 liter Sohc V6 that went into tons of caravans. Those can leave a cloud of smoke so bad that you will have to turn on your high beams to see. I'm sure there are many good Dohc and Sohc motors out there but I for one will stick to the Pushrod!

[Smaart Aas Saabr]

While OHV engines are pretty good, so are OHC motors...

Essentially, the way a particular motor actuates it's valves is irrelevant, rather the entire design of the motor itself is what is important.

I've driven OHC motors that sucked, a lot! Ford Escort 1.9l 4-cyl can I hear you?

My Saab (with a twincam 2.0 four) makes great power, runs very nicely, drinks like crazy if you really keep your foot in, but otherwise efficient and it's good. I still prefer the 3800 Supercharged in the GTP.

[T-Type]

I'm pleased to see the positive response to the article. It always bugs me when people come to the forum and say GM sucks because they use pushrod motors. Who cares what the motor design is as long as it's competitive in terms of power, efficiency, emissions and so on. The motor could be a flathead design, just as long as it's competitive in the marketplace.

That felt good to get off my chest.

[usa1]

Most of us here know there are advantages to OHV engines in terms of cost and packaging efficiencies. I agree there are benefits to GM for sticking with OHV engines, but consider what the advantages are to the consumers.

If the packaging efficiencies are so great for OHV engines, why hasn't GM used this to their advantage and styled a car in a way that could not be done with an OHC engine?

If OHV engines are lighter than OHC engines, why do GM cars not have a significant weight advantages to their competitors? Maybe it's because most GM OHV engines use heavy cast iron blocks.

OHV engines are cheaper to build, but do you really care when the cars from GM cost the same as its competitors? The cost savings help GM save money to do wonderful things like (cough) fund its huge pension system.

Who here really thinks OHV engines are more inherently reliable that OHC engines as was mentioed early? Anyone got ANY data to support this claim?

Someone mentioned the maintenance cost of constantly changing timing belts on OHC engines. This certainly was the case in the past, but most timing belts live for 90K+ miles now. Some are timing chains that require no maintenance.

Really the cost savings are the biggest benefit for GM using OHV engines. The GM accountants love it, but until GM can demonstrat how this and OHV is better to me, the consumer, I'm not really terribly interested.

Lastly, we also need to recognize that most pushrods are simply not as smooth and refined as a good OHC engine. Compare the 3.5L OHV engines from the Malibu to a 3.0L DOHC engine from a Accord and there simply is no comparision. Can someone name a OHV engine that is considered more refined compared to a good OHC engine?

Mark

[thehemi]

Smooth and refined? I find those to be vague terms.
Can anyone expand on what that's suppose to mean?

[OzoneGTX]

Screw 'em all!!!

ROTARY N-GINS 4 EVA!!!

Just kidding

If it werent for their lack of torque, though...

[Dodge Drivin' Paul]

Although I would tend to agree that an overhead cam design is superior to a pushrod configuration in 4 cylinder engines, the advantages of the overhead cam design start to vanish once you get into a V-configuration engine.

I'd think it's obvious why a pushrod engine would be more durable and outlast an overhead cam engine. A pushrod engine makes it's power lower in the power band than an overhad cam engine. This means that the gearing can be taller, and the engine can sustain speed and provide adequate power at lower RPM's. RPM's are Revolutions Per Minute, which is how many revolutions your engine makes in a minute. Common sense dictates that an engine running all day at 1500 RPM's is going to experience half the wear of an engine running all day at 3000 RPM's. Your cylinders are moving up and down only half as much. Also, all else being equal, a V6 engine running at 1500 is going to burn less fuel than a V6 that needs to run at 2500 RPMs.

Also, pushrod engines are smaller. You can pack in an engine with larger displacement because of all that space saved, which will result in better torque...and the fact that the engine can be geared to cruise at lower RPM's, any fuel efficiency lost by going by a greater displacement is more than made up.

Honestly, I think anyone who can't see the advantages of a pushrod engine design are truly blinded by the advertising hype.

[ibechip]

Originally posted by Dodge Drivin' Paul@May 25 2004, 12:59 AM
Although I would tend to agree that an overhead cam design is superior to a pushrod configuration in 4 cylinder engines, the advantages of the overhead cam design start to vanish once you get into a V-configuration engine.

I'd think it's obvious why a pushrod engine would be more durable and outlast an overhead cam engine. A pushrod engine makes it's power lower in the power band than an overhad cam engine. This means that the gearing can be taller, and the engine can sustain speed and provide adequate power at lower RPM's. RPM's are Revolutions Per Minute, which is how many revolutions your engine makes in a minute. Common sense dictates that an engine running all day at 1500 RPM's is going to experience half the wear of an engine running all day at 3000 RPM's. Your cylinders are moving up and down only half as much. Also, all else being equal, a V6 engine running at 1500 is going to burn less fuel than a V6 that needs to run at 2500 RPMs.

Also, pushrod engines are smaller. You can pack in an engine with larger displacement because of all that space saved, which will result in better torque...and the fact that the engine can be geared to cruise at lower RPM's, any fuel efficiency lost by going by a greater displacement is more than made up.

Honestly, I think anyone who can't see the advantages of a pushrod engine design are truly blinded by the advertising hype.

Yeah I completely agree. An OHC engine is makes much more sense in an I4/5/6 engine. But the DOHC V6 in the Accord? Yes it"s a good engine but is it so much better than a similar pushrod? I doubt the average car driver could tell the difference. And if they could, they'd tell you that there's more power in the lower RPMs. It just doesn't make much sense to me why they'd put one in there other than they might get ripped on for using a "low-tech" pushrod engine.

[stewacide]

If GM could make DOHC v-engines for the same cost as its OHV units they'd do it, all other rationalizations notwithstanding. In fact they've tried to design such engines several times and have failed in all attempts (they still have the Cadi' DOHC family but it's too expensive to stick in most cars).

So I've heard GMs cost advantage is on the order of $500-$1000 per engine compared to their DOHC competitors. This gives them a decided advantage at the low end, although not having a midpiced DOHC family basically writes them out of the midmarket.

For example who would load up a Malibu with it's less-refined, underperforming OHV engine when they could go for one of the Japanese cars with their high-output DOHC vvt v6s?

[BigAls87Z28]

Originally posted by stewacide@May 25 2004, 04:51 AM

For example who would load up a Malibu with it's less-refined, underperforming OHV engine when they could go for one of the Japanese cars with their high-output DOHC vvt v6s?

because the Bu runs on regular while the other cars run on premium.

[Dumb_Ass_2003]

Whoever thinks that valve actuation has any effect on the 'smoothness' of a heavy rotating mass of pistons needs some serious help.

[stewacide]

Originally posted by Dumb_Ass_2003@May 25 2004, 06:01 AM
Whoever thinks that valve actuation has any effect on the 'smoothness' of a heavy rotating mass of pistons needs some serious help.

The lack of cam timeing certainly does, as does the limited (i.e. lower) rev' range.

[paul8488]

are accord buyers car guys (and gals)? i wouldn't think so (though i'm wrong as much as i'm right, it seems!). that's why i'm surprised that OHC apparently matters so much. i don't know if there's a measurable 'smoothness' difference between an accord engine and a malibu engine because of the engine configuration. i'm sure the 'feel' of the car means much more to many buyers than how it works. heck, i'd be fine with an underhead cam engine if it made my car go!