Push Rod Vs DOHC/SOHC

Jim,
I like your motorcycle analogy, but you are also making an unfair comparision...comparing in most cases a 4 cylinder Japanese DOHC engine to a twin cylinder harley. But for sure, it's pretty pathetic how little power comes out of a Harley engine compared to its displacement! (No offense to Harley guys, I have nothing against them, just an observation!)

Birdman

 

Harleys are like Ferraris in the sense that they are both never the best performing in their class, but they are almost always the most desirable as well as the fact that their sound is an important factor to an owner/buyer . Harley also often sells their new models out before the first delievery at over MSRP; in the secondary market as well. As far as production models are concerned, Harley production bikes are a much better "investment" than Ferrari production cars. I bought my 1992 Fatboy new for $11,500 and it is now worth around $13,500. Has the 1992 348, Mondial or 512 increased in value by the same 15%+ ? I think not!

 

I have a neighbor who builds 6000+ HP top fuel dragster and tractor pull engines. He gets about 1000 BHP per liter - from pushrod engines. They are blown and run on alcohol/nitromethane. The water passages in the block and heads are filled with tile grout because water cooling is not needed for the short runs. Several gallons of oil provide all the cooling necessary.

I go hang out in his shop (every automotive machine tool known to mankind) every once in a while because he's a great guy, and very generous with his time. Last week he said he wanted to show me something - a Cadillac Northstar engine with a supercharger he is building. One of the heads was on the Serti valve machine, the other was on the bench. It was a thing of beauty - dual overhead cams, directly actuated 4 valves per cylinder. Very similar to the 4-valve heads off a 308. All aluminum, hardened valve seats, familiar camshaft babbets and caps.

He had done a bit of porting, and the heads looked like they would flow very well - a totally smooth run on the intakes and the exhausts. I didn't look at the aluminum block, but he said it looked like it could take the added HP. It looks like at least one US automaker is keeping up with the times.

 

I don't think overhead cams and belts/chains are complicated. I have 5 vehicles, each with overhead cam design - all total 106 valves. There has never been a problem with any of them. And 3 of them have mileage over 125K. Each has good acceleration in top gear because top gear is reasonable in each.

Anent to Diesels: While they are somewhat more efficient than gasoline engines (innately due to the basic cycle), one must never forget that a gallon of Diesel fuel contains about 23% more energy than a gallon of gasoline. Diesels had better get more miles per gallon. Diesel has a higher density than gasoline so on a per pound basis, the difference is even greater.

 

Lowest price is not always lowest cost. Sheesh. Read back in this thread about power vs. gas mileage and pushrods all of a sudden look a lot more expensive. But that's a cost passed over to the uninformed or misinformed buyer. Likely an AMERICAN. Likely from the south.

Ken

 

Ummm, the modern generation of pushrods are fine on gas mileage(for their power level). If anything, the LSx series performs *better* than comparitive DOHC counterparts (the *closest* NA DOHC engine to the LS6 is the 4.9L V8 in the older M5...miles more expensive).

The LS2, in my mind, is easily the best *engineered* naturally aspirated performance car engine available in a car less than $75K. The power level, torque level, price, NVH, modability, reliability, engine size, etc....all combine to make an extremely competitive package. The best combination of all those for a naturally aspirated performance car under 75K.

 

Funny you should mention Top Fuel dragsters, as there is a team in Australia that is making a billet SOHC 3 valves per cylinder engine and it has been running for a while now. He is the only case of this I know of ... but it is interesting.

Anyway this thread is now close to pointless

Pete

 

Comparing a Top Fuel Hemi to just about any other engine would be like comparing a wrecking ball to a tack hammer. The whole thing is focused around a BLOWER, and once an engine is supercharged, depending on how long you want or need it to last, horsepower is totally dictated by boost, and has NOTHING to do with the architecture of the engine. It could be a flat head Ford or a briggs and stratton, but given enough boost, it would make equivalent power.
Also, how anyone can say a OHC, or DOHC head is more complicated or has more parts than a pushrod engine has never really looked at engines or worked on them. My God, they had DOHC engines 100 years ago when engines were barely making one horsepower per cubic inch. They knew then that it was better. The only reason pushrod engines stayed around was they were simpler to "engineer" and cheaper to manufacture. But they were never considered better in any way shape or form. One thing that may have led to them staying around was metalurgy. They didnt have good chains until after WWII, so they had to drive cams with gears and or shafts. On the Bentleys, they used "connecting rods", three of them articulating to drive the drive gear up top. Very interesting appraoch in the mid 1920's
In regards to the Harley, I used to feel they were stone aged compared to the modern bikes, but then I got some books from the library and started reading. There is a whole design philosphy behind it, and its the reason that 100 years ago, and up through the 50's, the design dominated, and continues. The engine has forked connecting rods, so that both rods not only share the same throw, but are in line front to back with each other. It allows the engine to be narrow. The V twin allows much larger displacement over an inline twin like a Triumph, and offers better balance. But at large displacement, RPM is limited, and it becomes a low RPM high torque engine. The vibrations are found to be low frequency, which allowed the rider to ride longer without being fatigued. So they might shake at idle, but its a soothing shake, not a buzzing that goes through your hands and feet and turns them numb, like say a Yamaha 650 Special would. No, it dont make power like a small inline four, but it was never designed to. Its a long distance road engine.
The goofy loping idle is caused by the fact that it doesnt fire in even intervals. The front cylinder fires first, with the crankshaft rotating counter clockwise as veiwed from the right side of the engine. The rear cylinder fires in the next revolution, but as its a 60 degree V, it fires not 360 degrees after the front, but 420 degrees after. Then the front follows again in only 300 degrees of revolution, followed by another 420 degrees to fire the rear again, and so on and so forth. ka-thump pause thump p-a-u-s-e ka-thump pause thump p-a-u-s-e ka-thump pause thump p-a-u-s-e ka-thump pause thump p-a-u-s-e ka-thump pause thump p-a-u-s-e ka-thump. They do sound cool.

 

The cost of going to overhead cams is that there are more surfaces to machine and that costs money. 4v vs 2v is the same issue. double cam vs single, same problem. Cams are expensive to make so using 4 instead of 1 adds a lot of cost.

I saw a cost analysis a while back when buick first started adding superchargers. They could build a supercharged pushrod 6 and it was cheaper/per, smaller/hp, lighter/hp, and gave better mpg. What more can you ask for? The only reason US makes have started to move off the push design is marcketing. There is a perception that DOHC is better, and ignorant shoppers look for it....so car makers give it to them whether they should have it or not.

For milage, lower rpm is key. The friction forces are much lower at lower rpm. A engine running at low rpm does not require much overlap in the valves, so no chqarge is lost out the exhaust. Also it does not require a lot of flow throw the heads, so 4v is a waste of time, it also wastes fuel since it adds friction surfaces.

On the race track, hp is king. HP=rpm x torque/5252, so spinning it faster will allow more hp. But that means it has to flow more air and not flow vavles. A 4v DOHC does about the best that can be does for both.

So, if displacement is limited, DOHC is the answer, if not pushrods are as good or better.

 

Okay you were going good until the better balance comment. 45 degree engines absolutely suck for balance. Now if they were 90 degrees you would have a point ... but the hackers at Harley never did the maths and thus choose 45 degrees because it looked good. Just about every other twin designer choose 90 degrees because then the balance is right.

This also is complete rubbish. What book did you read?, somebodies gushing view of the Harley. Any vibration causes fatigue, plain and simple, just like noise causes fatigue. That is why millions and absolutely millions has been spent on modern car manufacture to make them smooth and to isolate noise so well ... not just for comfort but to control fatigue. You try cruising a noisy race car ... you get tired real soon.

Agree with never designed to part, but again the fact that they vibrate is just poor design, 'cause they choose the wrong angle.

No V twin fires evenly. BTW Harley engines are a 45 degree V not 60.

Forked connecting rods simply came about because they started with a radial aircraft engine ... very common practise in those engines. Even the v12 Rolls Royce (Merlin?) aircraft engine has this.

Harley's evolved, they were not designed. They have their place in the world and there are actually some that I quite like, but there is NO way good design and Harley engines and gearboxes should and can go together. Even Porsche would not have designed them a 45 degree twin if that had not been a requirement from Harley (refering to the new water cooled engine).

Pete

 

Agree. Even my people mover stupidly has DOHC and 4 valves per cylinder. It would be much better off with 2 valves per cylinder and some real torque.

And small displacement saves fuel in around town driving. If you drove on long straight roads all day then the big engines will do okay, but the reality is 90% of modern driving is around town or stuck in traffic. The smaller the engine is the less fuel is wasted ... simple fact.

Pete

 

FYI, pushrod has never given better tq/L. They only have more torque because of the added displacement.

 

Mr Payne,

Your reading skills are slipping. I never said it should be pushrod!

I said it should be 2 valves per cylinder not 4 valves per cylinder. 2 valves per cylinder engines usually have a torque curve that peaks lower in the rev range ... may not be as much torque but they suit the plugging sort of driving a people mover is normally subjected to. Also from a maintenance point of view ... less is best.

Pushrod versus OHC regarding torque should make absolutely NO difference at all.

Pete

 

Wow, my reading skills do need some improvement. I admit error.

 

No worries mate ... it is probably late over there, while I am supposed to be all fresh after a good night sleep ... yeah right!

Pete