Push Rod Vs DOHC/SOHC

The term better is subjective, better for what. Push rod motors have proven themselves to be very cheap and reliable workhorses. They have also shown that with a lot of development and expensive parts they can make pretty passable race motors. Winston cup or whatever they want to call it this week is making those things last 500 miles spinning around 9000 RPM. Old group 7 cars shook the earth, and went real fast. Add a large blower and a lot of nitro methane and their ability to propell a car down the quater mile or across the Bonneville Salt Flats is unmatched.

Over head cam motors have traditionally been more expensive to produce and as a result that until recently they have been less common. In more recent times with the need and demand for more efficient motors, they are becoming far more common. They have been good reliable race motors since the first decade of the last century. The fact is that the cylinder head designs that are allowed by moving the pushrods out of the way of the ports, being allowed to put any number of valves you want, anywhere you want, allowing any combustion chamber shape you want, without the penalty of all that reciprocating weight of a pushrod motor gives you the ability to design far more efficient more powerful motors.

In all but a few types of racing where overhead cam motors are allowed they are almost universally used for the reasons I listed.

Both engine designs are works of genius, both have their place, both are best at something, one however is going the way of the dinosaur.



For your friends that don't seem to like anything that ain't Merican Iron introduce them to a great motor design, the Ford 427 SOHC. Those things make a lot of horsepower, just too expensive to build. I saw Charley Markley running one at Bonneville on fuel last year. The smell and the sound was something, and it had big blue ovals on the valve covers.

 

If they think the Cobra beats a V12 warn them about going side by side with a 550.

If they don't like the high reving Ferrari motors bring them to the Reno air races and let them listen to a flock of P51's (V12 Rolls OHC 4VALVE) going by at 3500 RPM and about 120 inches of manifold pressure.

And if that don't work they are just stuck in the past, closed minded and ignorant. Tell them you heard of a great job available for them at the buggy whip factory.

 

The only argument that pushrods are better is in the cost area, end of story.

You do not have to design a overhead cam engine to rev high, that is really dictated by the bore and stroke ... in fact in Australia you can now buy a 4 valve per cylinder twin overhead cam V8 (in a Ford Falcon) that produces MORE torque than the old pushrod and revs about the same.

The difference is that in this case it would be CHEAPER to modify the overhead cam V8 to REV ...

Some will argue the physical size debate ... and my point of view is they are very limited in their thinking. Who said an engine had to be a 90 degree V8 in the first place ... thus you can probably make the same torque and same power with a smaller CC engine with overhead cams and since we are free to choose whatever layout we like, physically smaller as well.

Pushrod engines are dinosaurs and yes there have been some beaut ones, but there have also been some great drum brakes, separate chassis'ed, beam axle vintage cars, but that does not mean we have to continue life with our head stuck in the 'no modern advances' sand.

Overhead cams allow:
- Better porting ... no need to make access for pushrods.
- Better head cooling ... as above.
- Better valve control and timing ... as cam profile is not compromised by bendy pushrods and too many components in between.
- Just about any valve angles you want ... can get tricky with a pushrod design.
- Less valve clearance movement ... you hardly ever have to reset them, compared to constant checking with pushrod engines.
- etc.

Pete

 

As I recall, most the world pays a lot more for gas than we do. Aditionally, many "furin goviments" tax the holy sh*t out of displacement. Cheap gas and constantly changing body design driven by "THE LATEST FASHION" has conspired to drive the resale value of american used cars way into the dirt, cuz nobody wants to drive an old american car. So who wants to invest in a pricy asset that drops in price like a nikel rocket. Who wants to buy a clunky, heavy, inefficient vehicle??? Education comes slowly to a surprisingly large body of consumers, who are enticed by horrific simple minded advertising.

Gasoline prices, taxes and racing, (in the rest of the world), led to a faster evolution of the otto cycle engine. Here, the big three respond only very slowly to competition - whitness the massive and constant drop in gm, ford and chizler sales.

Here in the relatively isolated land of cast iron, we raced what we had to work with. Being separated by oceans, very few intruded into our own separate reality. You get over going fast on a short bit of road very very quickly. Turn left all the time, i don't think so.

Do not misunderestimate the inertia of cast iron.

Enter the sophisticated Japanese and european cars with variable cam timing and intake manifold runners, multipul valves and ocasionally plugs per cylinder.

What are the high school kids driving???
Damn little detroit iron. Pity, We have the engineering skill and talent to do better.

VIVA ENZO

 

Oscillating a body or lever consumes engine power -- the DOHC valvetrain system wastes less energy than a pushrod valvetrain (all other things being equal -- which they usually aren't). IIRC, they effectively had to ban the 333SP from its class by imposing an RPM limit.

But you need a broader definition of "better" -- if you can't appreciate well-designed versions of both configurations, that's your loss IMO.

Sincerely,

(still) an american muscle nut head

 

Never a truer statement made.

Just because an engine may have twin overhead cams and say 4 valves per cylinder does not mean it is not a dog! ... or actually better than an equivalent pushrod engine. It should be, but ... somebody got some details wrong

Pete

 

It can be argued the the push rod engine in some instances has less moving parts to rob horse power then an over head cam engine.

1. You only have 1 cam shaft to move
2. You do not have in some cases 4 timing chains to turn.
3. You do not have numerous idler and tensioner gears to turn sometimes as many as 6 or even more.
4. You only have 2 valves to open and close as opposed to as many as 5 per cylinder
5.Newer push rod engines never need there valves adjusted..as they are hydraulic and most are also roller along with the roller tipped rocker arms. Such as in the the late 80's to early ninties Mustangs, and Corvette's.
6. The engines cna be made to breath quite well hence the 9000RPM Nascar engine's.
7. Mutipule cam engines can have 32 valves per cylinder to as many as 40 valves per cylinder as in the 355. Each on of those valves has it's own spring that must be compressed by the lifter under the lobe of the cam. Now depending on the weight of the spring this can lead to alot of power consumption.

When you sort of break down the internals of a push rod engine you find there is not near the number of compents comparied to an overhead cam engine that makes the same power. Over head cam engine are more efficient because of the benifits of cylinder head design as has already been stated. But overhead cam engine are big and tall engines compared to a pushrod engine. Just campare the new ford cobra engine's cylinder heads to say the regular 4.6 ohc engine and you will see my point.
Push rod engines are also lighter. If both unit's are cast in the same material the push rod engine will always have the weight advantage.

Wether or not one is really better then the other..I really can't say but they both have there avantages and disadvantages..cost being the main one. I for one much prefer the overhead cam engine's, but sometimes you just can't beat the torque or the sound of an American V8.

 

Those engine will not run hydraulic lifters, and thus need constant valve clearance adjustments.

When big revs are required, hydraulic lifters are not used ... and the same can be said for overhead cam engines, unless technology has marched right passed me

Pete

 

Lets see, what other naturally aspirated DOHC engine can match the current the LS6 at that price point.

 

We have already agreed that manufacturing costs is the one BIG plus for pushrod engines ... not the only one but a substantial one.

Pete

 

And cost is *always* the number 1 priority for any economic endeavor. Seeing GM's "success" (take that for what you will) with the LSx series engine, I wonder why more manufacturers haven't tried to copy it. Massive HP at low cost.

 

If you read just a little bit of history, you learn that it all started out with a cam driving only the exhaust valve, and the intake was automatic, just a light spring and the downward piston would "suck" it open. Very early on as engine speeds increased, they made the cam work both valves, and by the turn of the century many racing cars were moving into hemi head designs and four valves per cylinder. By WW1 any airplane that could fight, was a hemi head making a horse per cubic inch.They had gear driven and shaft driven dual overhead cams and many had 4 valves per cylinder. The Alfas of the 20's were double over head cam engines, as were many other cars.
Meanwhile, Ford was busy dumbing down everything. Splash oil, flat heads, word of the day was cheap cheap cheap. And GM and the gang followed along with mass production while europe developed even better engine technology. And dont forget the Offenhausers that tore up race tracks all over the USA and at Indy, uhmmm, DOHC!
Just remember, your trying to open and close a valve to get air in, and exhaust out. An american V-8 has a heavy lifter, a pushrod, and a heavy rocker arm that all have to be slung back and forth to keep the valve opening and closing. So the valve spring has to not only be heavy enough to close the valve, but also strong enough to push all that train of weight back against the cam. Now imagine all that junk moving back and forth 4000 times per minute.
The Ferrari, and most any other twin cams of later years, have none of those extra parts. The cam is directly over the valve, and the only parts between it are the shim, and the light weight bucket follower. Plus the twin cam allows a much better combustion chamber and better ports into and out of the head. Where as your push rod motor has rather crummy combustion chambers, except in the case of hemi heads, and Chrysler wasnt the only pushrod hemi. Actually a old Harley knucklehead from the 30's was a pushrod hemi! And while your thinking about that, remember that FORD dumped $$$$$$$$$$$$ into Cosworth to build them a Formula One V-8 racing engine and it was (horror) DOHC. No, the only reason US cars had any balls was just out of sheer size. Think of it, an 8 liter Dodge V-10, and it only puts out 450 HP. Yeah it can make more, but so can a Ferrari, and with a lot smaller engine.
And I also dont think the cam drive matters all that much comapared to the valve train loss. belt or chain, one cam or four, it probably evens out.

 

The Ferrari F355 uses hydraulic lifters and revs to 8500 rpm.

 

I get into this a lot with my dad, who is a Corvette guy. I have always felt that the DOHC design was better than the pushrod and could develop more power at a lower displacement, which means a smaller engine. But after reading a bunch of articles about the new Corvette engine, I'm not convinced. GM uses large displacement, low revs and pushrods. They get over 20 MPG highway and make 400 HP. Take a look at a Ferrari engine...the 360 gives us 400 HP but nowhere near the same fuel economy. I'm not sure how the weight of the two engines compares, but I've been told they are actually fairly close in weight. So whether you get your performance with displacement or revs, if the result is an acceptable level of performance and weight, who is to say which way is better?

Birdman

 

It uses hydraulic roller lifters, as opposed to
the hydraulic "flat-tappet" lifters the other
guy was apparently referring to.

Still, you're right in that the 355 doesn't require
valve adjustments either, which pretty much
kills his argument.

 

Corvettes have a very tall top gear that is pretty much useless for anything other than highway cruising. That accounts for the highway mileage. Ferraris use a much lower final gearing.

 

I believe pushrod engines have been in the fastest accelerating cars for decades, i.e. Top Fuel dragsters.

 

This only happens since the engine can make so much torque at low rpms. There is no point in revving high if you dont have to. Sometimes the most simple designs are the most elegant. The first thing i learned when studying mecahnical engineering is KISS. Keep It Simple, Stupid. While i do think the complicated valvetrains of ohc engines are fascinating, i like the simplicity of pushrod engines. The way i look at it is the following- you have two black boxes and you have no idea whats inside. Lets pretend box one has an LS6 and box two contains the engine from a 360. You put less fuel into box one and get out more power and torque, nevermind the fact that the power and torque come lower in rpms. However, the LT5 from the zr1 was a 32v dohc V8 (which lotus designed).

However, looking at engines from a strictly thermodynamic standpoint it becomes painfully obvious that we should all drive diesels, even though they dont sound good or rev that high.

 

A good example to enter into this conversation would be to compare japanese (kawasaki, suzuki, yamaha) motorcycle engines and the power they put out naturally aspirated (adding a turbo can get these bike over 450 h.p.), v.s. a pushrod motor of the same size. The power ratio isn't even close. Yeah a harley will put out about 75 pounds of torque on a stock motor, but the displacement is 13400cc's and up. A kawasaki/suzuki/yamaha will put that out with a 1000 c.c. motor and 180 horsepower. Not saying one is better than the other, but if people are saying that the pushrod motors are better than dohc engines, they really don't know what they are talking about.

 

I only drove my 360 once on an extended freeway trip, but averaged 18.9 at 90mph down I-5 from SF Bay area to Buttonwillow and back. Even with my normal driving (canyon/mountain/twisties) I average 16 or so. Drops to 9mpg at the track, but that's actually better than the 8 I used to get with my M3.

Gary