diesel overhead cam or pushrod more reliable setup?
 

Some of us dinosaurs still remember pushrod engines...question; is the pushrod setup, with a gear driven cam, more reliable over the lifespan of a well maintained motor[albeit less efficient], or is the overhead cam more troublefree?
I've recently had my share of woes with a chain-driven overhead cam on a mercedes diesel engine.
I believe many of the large tractors are pushrod...why?
I owned a Volvo B-18,pushrod, gas, years ago...put 500k on the engine, and retired it because the body had metastatic carcinoma[rust].
I kind of miss doing valve adjustments...touching the heart of the iron. Forever listening.

Well, about 99 percent of pushrod engines are still chain driven.. the race engines with gear to gear drive were usually too noisy to be pleasant in normal use.... much is dependant on other factors like the oiling system pressure and routing..and of course the maintenance period and quality after it is manufactured.... so it is hard to compare them in a logical fashion.....I too had a B18 in a 65 p1800....

My limited experience
 

My other 2 vehicles are old Range Rovers (1989) both with, I believe, Aluminum head Buick pushrod V-8s in them. These are from my understanding terrific motor designs from the 1960's and were adopted by Rover. One has 130k and the other has 200K plus with no motor work whatsoever and both run very well (200K plus is my wife's daily driver). I'm not aware of any "timing chain stretch" type issues to worry about on these engines and they apparently run "forever" with oil changes (hope there aren't any of these type issues that I don't know about).

It seems that sometimes a more basic set-up has its advantages.

Would be interested in other's comments on the issue.


P.S. When and if either engine goes, I will consider a Diesel conversion (either by the Rover gurus in Vermont or a MBZ 5 Cyl. Turbo transplant on my own).

uhh, guys, whats the differance??? Also, been wondering this for a while. Why dont engineers use gears to drive cams vs the chains or belts we use? thanks
Ryan

Entertaining only crank gear to cam gear setups with pushrods...vis a vis a direct overhead cam without rockerarms, I have often speculated that, perhaps, with pushrods and rockerarms [given that the trail of transmission of forces is a longer one], that this cushioning of impact, especially by the pushrods and rockerarms, is a crucial difference.
However, would this cushioning exceed that of hydraulic lifters in the typical overhead cam design?
In my vocation, the simpler one keeps things, the better one sleeps.
'85 190e...broken, looking for a 2.3 engine
'85 190d...broken, thinking of replacing the motor[other threads]
'64 Unimog...sold to the four winds
'84 300td...sold
'85 300td...sold
'64 122s Volvo,su carbs...if I could buy that car new, every 7 years, I would be content.

Ryan, gears , unless Herringbone or similar , have produce end thrust and are usually Very noisy compared to chain drive... Greg

In a diesel, I like a pushrod setup. RPM limitations are not an issue like a gasoline engine, and they are noisy enough on their own that you cannot hear any gear noise anyway. My Cummins is a gear driven cam, pushrod setup. The only noise I hear is ignition clatter, same for the MB.

The Cummins Signature 600, which is the current king of heavy-duty over-the-road truck engines, uses dual overhead camshafts. They are certainly gear-driven.

While I don't think that there's anything wrong with pushrods in most applications, an overhead cam setup is definitely more efficient and makes for a lighter valvetrain.

Aaron

Chevy and Ford inline six cylinder engines had gear driven cams and pushrods. They never went out of time and were fairly sweet running things. the chevy earned a nickname; "as reliable as a stovebolt" They used a helical gear to eliminate noise.
For Chevy small block v-8's it is common for hot rodders to upgrade to cam gears because of the ease in which you can change the timing, as well as the accuracy.

The advantages of pushrod engines: CHEAP

There are no other advantages to speak of, since regular maintenance will eliminate timing changes due to chain stretch.

Overhead cams eliminate most of the slop in the valve train, reducing valve float and permiting higher rpm. They also permit steeper cam lobes, and an opening/closing valve train (where the cam actively closes the valve) is much simpler.

While higher rpm isn't necessarily a problem with diesels, Renault makes a double overhead cam diesel that runs about 6000 rpm (they also make a gas racing engine that turns 12,000, or did!).

Dual overhead cams also allow a better hemispherical combustion chamber for gas engines, and better injector placement for diesels.

Incidentally, pushrod motors are more difficult to machine properly -- those lifter bores are a critical as the piston bores -- get them out of alignment and you have different valve timing on each cylinder!

Peter

Peter;
I do not agree.
Pushrod engines should be more expensive to make than OHC engines.

Ed:

Only if you a designing a new one and not using 40 year old machine tools!. No capital investment need to continue to manufacture old designs on old equipment.

Peter

Pushrod engines are easier to work on. I overhauled my Case Diesel tractor ENGINE (it not a motor) and it was much easier to remove the head than an overhead cam engine. No timing chain or cam to remove.

No timing problems when I put it back together. Just torque the head bolts, replace the rocker shaft, put in the push rods, hook up the fuel lines, adjust the valves and its ready to run. It ran without the intake or exhaust manifolds or valve cover so I could warm it up and retorque the head bolts easily.

Since this engine runs about 2500 RPM max, a push rod setup works fine. Very efficient engine, only uses about 1/2 gallon of Diesel fuel per hour. It is about a 40 HP engine.

P E H

Probably the main disadvantage to a pushrod engine is limitations on combustion chamber design. The valve train pretty much eliminates anything except a wedge-head design (although some of us will remember bent pushrods on a Chrysler Hemi in its various guises). For low RPM engines with low specific horsepower output, they are a relatively simple way to make a reliable engine. For high specific horsepower, they are outdated. Seriously.

Peter

PSFRED,

But most Diesels have no combustion chamber design, Its almost metal (piston) to metal (head) to get the high compression ratio.

I had quite a few Chrysler Hemis and never had a bent pushrod. They were a pushrod engine and had the hemispherical combustion chamber (CC), the best CC there is. So why is a pushrod engine limited to a wedge CC?

P E H