What are the disadvantages of a push rod engine?

Senior - BHPian  
Join Date: Apr Location: Pune Posts: 2,625 Thanked: 1,157 Times Are Pushrod engines really that bad? GM/Ford/Chrysler take a lot of flak from people all over the world because they continue to offer pushrod engines in their mainstream models. But are they really as bad as they are made out to be?
Cars like the new GM offerings in Chevrolet Impala, G6, a host of Buicks and many other trucks and SUVs continue to offer these engines. Infact, the Impala is said to have an "all new" 3.5L OHV V6 which produces 211hp.The "" V6 has been offered on almost all GM sedans (Even "premium" offerings like the Buick Park Avenue) and continues to be on some current and upcoming cars as well. Why do they so steadfastly stick to their OHV designs? Some supporters claim that they are better than most overhead cam engines.

Some major disadvantages of these "old tech" engines I have read about are :

- Huge displacement...Mediocre output.

- Low revving nature.

- Only 2 valves/cylinder layout.

- "Sounds" dated and low-tech!


Addition of things like variable valve timing and electronic fuel injection have boosted their output figures a bit.


Some advantages I have read about are (DOnt know for sure though, can someone please confirm?) :

- Compact dimensions

- Simpler design

- Better low end torque

- Less weight

- Lower lower reciprocating mass and fewer moving parts than comparable DOHC engines. (Hence better FE?)

eg : BMW 90 degree V10 cc
500hp at rpm
383lbft at rpm
528lb dry weigth

LS7 90 degree V8 cc
505hp at rpm
470lb/ft at rpm
458lb dry weight

Which one of the two types is better? From what I have been reading, the (D)OHC layout is more suited to high performance applications. So pushrods are better for towing/hauling applications?
Is their any crudeness/loss of refinement in OHV engines? I have not yet experienced any of them, hence asking. How does a modern LS2 "feel" compared to some high revving Jap/Euro engine?

Mods : Been searching for a thread like this, but cannot find it. Hence created a new thread. Last edited by adya33 : 22nd July at 19:45. Senior - BHPian  
Join Date: Mar Location: Bangalore Posts: 10,409 Thanked: 1,734 Times Well most of what you have posted is true. Just some points.

• Pushrod engines dont have to be necessarily coarse or harsh but they usually are because most of them havent changed in 10-15 years.
• OEMs have been selling them as base engines. Just yesterday I was driving the company minivan with a 3.3 Chrysler pushrod and it was fine till you mash the gas and rev it.
• Biggest problem is not being able to fit 4 valves although there are some with 3 valves per cylinder using a forked rocker.
• DOD (disp on demand) has given a new lease of life to pushrods. It much easier to deactivate a cylinder by collapsing the lifter in a pushrod.
• Now Ford which had given up on pushrods is going to design an all new V8 with pushrods.
• Pushrods really make sense in Vee type engines where you need 2 sets of timing drives and cams with OHC setup. With a pushrod, you can get away
• with one set. There arent any inline 4 pushrods anymore.
• For a super sports car, I wouldnt pick DOHC anyday. I really dont know what GM is trying to prove with the Corvette engine.

BHPian  
Join Date: Dec Location: Cockpit Posts: 911 Thanked: 67 Times Well, looks like all the comments here are good info..

Ever wonder why they have such a restricions...
Major reason is that the pushrods have wieght, and like pistons have to be moved. So does DOHC, what big deal you may ask!

Answer here is they move like pistons, unlike cam shafs, which spin in one direction, these reciprocate like pistons, and being lage metal part they have sizable inertia. which result in going out of sync during hi-rpms, which simply means the cam follower to which the push rod and the valves are attached are no longer following the cam. This result in harshness in the engine and rpm gets limited. They also waste a lot of energy.

One other interesting thing, the then engineers tried is this. Rotary valves. Which completely eleminates the inertia problem, rather uses it to its advantage. The technology did not progress because of the lack of material which can perform well inside a combustion chamber and yet providing sealing charecteristics. Present valves have self sealing properties against the pressure.

Read this some time back, can't remember the source. BHPian  
Join Date: Mar Location: pune / Bahrain Posts: 488 Thanked: 7 Times simple eg is that of RE 350cc 18hp and a pulsar 180cc with 15 or 16hp. But u will find there is a huge difference in torque.

One thing for sure is that pushrods are not capable to revv high and mostly will get out their peak in 3to 4k rpms.

Ive seen the Saudis and Bahrainis feel so much humiliation after their big capriceSS, 5.7ltrs and 6ltrs + engines get smoked by my 2ltr engine. these engines are best to lug and tow boats, caravans and portacabins.

at todays oil prices these American Engines are just guzzling fuel without actually going any further. high time they shelved them.

Being an EngineLabs reader, you are probably more familiar than the average bear with how and why engines work. You probably also know most of the generalities associated with different engine types. One of those generalities is that pushrod engines aren&#;t meant to be RPM monsters. While the statement is mostly accurate, first, it&#;s relative, and second, it&#;s a generalization.

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However, Jason Fenske of Engineering Explained fired up his trusty video camera and dove deeper into the reasoning behind the statement that &#;Pushods can&#;t rev high.&#; Fenske starts with a cool overview of a disassembled single-cylinder engine, and breaks down how the rotating assembly and valvetrain work in unison, Barney-style. He then translates that into automotive V8 pushrod engines.

Uncontrolled Valve Motion

As Fenske explains it, there are three major drawbacks when trying to get a pushrod engine to perform at high-RPM. The first reason is uncontrolled valve events &#; also known as valve float.

&#;The entire [valvetrain] assembly has to reciprocate back and forth very fast. You have a lot of mass in the system that is trying to change direction very quickly as you get into higher RPM,&#; Fenske explains.

&#;The reciprocating mass can start to outrun the spring. There comes a point where parts of the system lose direct contact with one another, and the valve movement is no longer directly related to the camshaft&#;s movement.&#;

Once you encounter valve float, it can be as minor as simply a loss of power, as the valves are no longer doing what they are supposed to do, when they are supposed to do it, or it can be so severely out of sync, that things start impacting one another.

&#;Stiffer springs will help you keep everything in contact in the higher RPM ranges,&#; says Fenske. &#;However, [the stiffer valvesprings] result in an efficiency loss, as it takes more system effort to compress that stiffer spring.&#; That increased parasitic loss can negate any benefit you might see by spinning the engine higher, unless you have a system that is specifically designed to operate in the higher RPM range.

Another factor in the equation is pushrod deflection in high-RPM applications. &#;The length does play somewhat of a role. If the pushrods are not strong enough, they can bend, meaning you won&#;t have proper valve opening and closing,&#; says Fenske.  &#;I think this is an easier problem to address, by simply developing stronger pushrods.&#;

The main component eliminated in an overhead cam engine is the pushrod, and removing that variable is one of the key factors in allowing more control of the valve at higher rotational speeds for a variety of reasons.

&#;DOHC engines have four valves per cylinder rather than two, so this means the valves are smaller, each with their own spring,&#; says Fenske. &#;There&#;s also no pushrod, so you&#;ve eliminated a good amount of reciprocating mass.&#;

This is where the &#;generally-speaking&#; and &#;relatively&#; statements come into play. Afterall, we can list off several forms of racing where pushrod engines spin to the moon. Some of them are designed to live for 500 or so miles without being torn apart, while some are lucky to make it 10 miles before needing new components, but they do perform quite well. And as Fenske points out, &#;Then you see Formula 1 engines spinning to 20,000 rpm.&#;

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To accomplish that feat, the dual overhead cam engines used in F1 have done away with coil valve springs altogether, instead using pneumatic springs.

&#;By using a pneumatic spring with air pressure instead of a steel coil spring, the valves are easier to control, and you don&#;t run into harmonic vibration issues,&#; says Fenske. &#;Generally speaking, dual overhead camshaft designs, especially using a pneumatic valve, will be able to rev higher than a traditional pushrod setup in the same application.&#;

Airflow Restrictions

&#;At high RPM you need lots of air,&#; says Fenske. &#;While it is possible for a pushrod engine to use more than two valves per cylinder, it&#;s quite a complex design.&#; That  design not only adds cost and complexity to the system, we&#;d have to imagine it would also add significantly more mass to the valvetrain, further exacerbating the previously discussed issue. &#;If you were to go to a 4-valve setup, you&#;d reduce the size of the valves so you would be partly improving the situation, but you&#;d be eliminating one of the key advantages of a pushrod engine which is its beautiful simplicity.&#;

&#;[A two-valve-per-cylinder] design generally limits airflow at high-RPM,&#; Fenske says. &#;At lower RPM, you can actually benefit from the two larger valves, as it will increase velocity of the intake charge and promote better fuel mixing. However, an engine revving at 10,000 rpm vs 5,000 rpm, ideally, will be pulling in about twice as much air.&#;

By increasing the number of valves in the chamber, even though they are smaller valves, a simple area calculation shows that your total valve area is increased. While there are chamber and camshaft designs out there to maximize the two-valve high-RPM flow, they are still outmatched in the upper-RPM range by a multiple-valve-per-port design. &#;Ultimately, a four-valve system will be able to flow more air through it at higher RPM,&#; says Fenske.

Valve Timing

Another interesting subject Fenske broaches in his video is adjustable valve timing and how it affects engine performance. While most domestic V8 race engines lock out any variable timing features, he raises the point that not being able to control each side of the equation (intake and exhaust timing) independently and dynamically is a hindrance on a pushrod engine.

&#;Having a single camshaft controlling everything limits your control. All that can be changed is when the valves are opening and closing in relation to camshaft rotation,&#; Fenske says. &#;While Variable Cam Timing is present in some OHV V8s, that only changes the timing of the system as a whole. You can&#;t change duration of the intake or exhaust independently of one another, adjust overlap, or lift.&#;

All of those variables can be adjusted independently on a dual overhead camshaft engine, and with modern controls and designs, adjusting them on the fly is reality, not fantasy. &#;You&#;re able to control the intake and exhaust valves independently of one another, as well as have multiple lobe profiles for lift and duration on each camshaft,&#; says Fenske. &#;That, along with the ability to adjust valve overlap allows for optimal airflow efficiency in any RPM range.&#;

So while it&#;s obvious that an overhead camshaft arrangement is advantageous in a high-RPM application, it&#;s bit of an oversimplification to simply say that pushrod engines can&#;t rev high. Sure they have some inherent challenges to overcome, but like anything in this world, with enough effort, knowledge, and money, anything is possible.

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