Don Terrill’s Speed-Talk

Walter R. Malik wrote: ↑Mon Jan 15, 2018 10:08 am I was under the impression that this was supposed to be a comparison discussion between an OHC and pushrod technology not 4 valve vs. 2 valve.
There certainly are 2 valve OHC engines and 4 valve pushrod engines; 3 valve and 5 valve. too.

The number of valves does not predicate one design over the other.

And I left that ambiguity in one of my first posts. This really shouldn’t be a long discussion.

I mean, do you have anything to add?

bigpoppapreston wrote: ↑Sun Jan 14, 2018 10:20 pm

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What Are The Advantages of Pushrod Engines Over OHC whether SOHC or DOHC?

There are none. Zero, Nada, Zilch.

Overhead cam, with finger follower will out-perform any pushrod valvetrain.

What Are The Advantages of Pushrod Engines Over OHC whether SOHC or DOHC?

There are none. Zero, Nada, Zilch. Overhead cam, with finger follower will out-perform any pushrod valvetrain.

A too-broadly posited question gets a too-general answer. If one interprets it as "max performance", then Mike, as usual, is correct. As others have mentioned, pushrod OHV engines are more compact, less expensive.

PackardV8 wrote: ↑Mon Jan 15, 2018 11:32 am

What Are The Advantages of Pushrod Engines Over OHC whether SOHC or DOHC?

There are none. Zero, Nada, Zilch. Overhead cam, with finger follower will out-perform any pushrod valvetrain.

A too-broadly posited question gets a too-general answer. If one interprets it as "max performance", then Mike, as usual, is correct. As others have mentioned, pushrod OHV engines are more compact, less expensive.

And over time, DOHC belts drives will need replacement and it's not an inexpensive job on some vehicles...Chain drive may also need replacement and this may also be a lot more expensive than a pushrod cam chain...
In the past some OHCengines had cam and follower wear issues ,I assume this isn't a concern any longer....But I do believe the LS lifter replacement requires head removal? Now that would make a cam or lifter replacement big bucks...

Truckedup wrote: ↑Mon Jan 15, 2018 12:23 pm

PackardV8 wrote: ↑Mon Jan 15, 2018 11:32 am

What Are The Advantages of Pushrod Engines Over OHC whether SOHC or DOHC?

There are none. Zero, Nada, Zilch. Overhead cam, with finger follower will out-perform any pushrod valvetrain.

A too-broadly posited question gets a too-general answer. If one interprets it as "max performance", then Mike, as usual, is correct. As others have mentioned, pushrod OHV engines are more compact, less expensive.

And over time, DOHC belts drives will need replacement and it's not an inexpensive job on some vehicles...Chain drive may also need replacement and this may also be a lot more expensive than a pushrod cam chain...
In the past some OHCengines had cam and follower wear issues ,I assume this isn't a concern any longer....But I do believe the LS lifter replacement requires head removal? Now that would make a cam or lifter replacement big bucks...

the lifter tray hold the lifters in place for a cam change in a LS engine by rotating the original cam to move the lifters out of the way. GM also sell a set up do do this on the original SBC engine.

You ask "What are the advantages of pushrod engine over OHC?"

In the case of an OHC engine with the cam above the valves the pushrod advantages can include:
Superior packaging.
Less weight.
Lower centre of gravity.
Higher mechanical efficiency.
Less difficulty dealing with harmonics in the cam drive.
Cheaper to manufacture.
Easier to build, maintain and repair.
More economic to build, maintain and repair.

It is possible to approach the packaging efficiency of the conventional pushrod engine were one to employ an ohc design wherein there is a single cam per cylinder head actuating valves via rockers (locate the cam low in the head in similar fashion to the Ford Australia E-series six cylinder engine). All the other pushrod advantages remain to a greater or lesser degree however.

The OHC design has advantages in terms of control of the valves since there is less valve train inertia to worry about, hence valve springs can be softer or RPM can be allowed to go higher without the need for heroic engineering solutions! An additional advantage is the complete avoidance of any issues to do with pushrods and ports needing to be in the same location. Hence aspects relating to port design are less troublesome.

peejay wrote: ↑Mon Jan 15, 2018 6:09 am Vee engines have a smaller package for the same power output.

Tht's pretty much it.

Notice that even as GM eliminated all pushrod inline engines years ago, they still are married to pushrod V8 engines, and they only stopped doing pushrod V6s for marketing reasons. The 3.6 is a significantly larger engine than the 60 degree or Buick V6 engines.

No they didn't. They still offer the 4.3l V6 with pushrods. Years ago? You make it sound like they quit producing them 10+ years ago. 2013 was the last year for their High Value V6, which was OHV.

PackardV8 wrote: ↑Mon Jan 15, 2018 11:32 am pushrod OHV engines are more compact,

Not in all cases. With finger follower systems, the cam is no higher then a common shaft rocker system.
Also, without having a cam and pushrods to work around, you can put the ports and intake where you want.
Here's a good example.

Hot-in-the-V turbo engines can really use all that space that is usually taken up by the cam-in-block arrangement. With a twin-turbo hot-in-the-V four-valve cross-plane V8's with super short intake runners and no intake pulse tuning to speak off, one can get a compact engine that runs very smoothly, revs to the moon if necessary, has low fuel consumption, has good emissions, and shreds tires.

The only real advantage to the pushrod design that I can think of would be in a conventional V8 where one cam can be used/less expensive. Otherwise I think an OHC setup can be simpler really with no real disadvantages except maybe in a class like Top Fuel where having a pushrod valvetrain could simplify the R&R of the cylinder heads.

push rod engines were decades in development the newest design that had been developed as OHC and valves in block were the first.
OHV design was wanted for the smaller packaging , cheaper to manufacture, cheaper to rebuild, better low end torq it made better sense in V engines more so than in line engines for some reasons.
a comparison done years ago showed at least in production engines lightened OHV train builds preformed about equal to OHC builds. Also split port OHV engines might do a bit better. High RPM is were most OHC engine have the advantage but didnt make much sense as from the factory in the US 6000 RPM was about the limit.
Have also seen were a V motor about 390 cubes were the aussy 6 banger OHC valve train was used and the engine was so big it wouldnt fit in a HD truck.

Are there any V or opposing pushrod engines that DON'T have interchangeable heads?

Merc marine went to OHC engines because vale train problems were a big cause of OHV marine race engines dropping out of a race

pamotorman wrote: ↑Thu Jan 18, 2018 12:11 pm Merc marine went to OHC engines because vale train problems were a big cause of OHV marine race engines dropping out of a race

most high end race boats valve springs lasted 20 to 50 hours not that much different than NASCAR

turbo2256b wrote: ↑Thu Jan 18, 2018 11:49 am High RPM is were most OHC engine have the advantage but didnt make much sense as from the factory in the US 6000 RPM was about the limit.

You may want to check out the new Cummins Diesel 4V OHC V8 engine in the Nissan Titan XL.
Most of the new engines coming out of Cummins are OHC engines, and diesels aren't what I'd call "High RPM".