Pushrod design

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Sun 13 Mar 2016 19:26
Pushrod design - brum

I am completely disillusioned with modern belt and chain driven overhead camshaft designs. Motor manufacturers just cannot seem to design a maintenance free system and often end up designing in disastrous weaknesses, e.g. VAG.

Old pushrod ohv designs were supposedly superseded in the 70's because of the introduction of high rpm short stroke designs.

As newer turbocharged engines are designed to run at significantly lower rpm than those 1970s pioneers, I wonder if pushrod ohv could make a comeback. The main advantage being the camshaft could be simply gear driven from the crank and truly maintenance free.

Or is this just wishful thinking?

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Sun 13 Mar 2016 19:59
Pushrod design - oldroverboy.

Oh for the days when I could lift out my A30/minor 1000 engine on my own, take it to bits change a cltch and do the brakes without a degree ib fluid dynamics.

I would think that friction and wear and tear have something to say in any engine like that, but a larger capacity low compression V8 running a slow revs might fit the bill.

Anyone remember the rover/gm v8 3.5? looked after properly it was a low maintainence engine, and easy to work on too.

Some of the jap and german engines have also been durable, but it is the constant seaarch for lower emissions which is crippling engine design, until someone can economically produce a "proper" electric or hydrogen fuel cell vehicle.

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Sun 13 Mar 2016 20:11
Pushrod design - bathtub tom
The main advantage being the camshaft could be simply gear driven from the crank and truly maintenance free.

I'm working on an engine with exactly that arrangement now.

It's an Austin 7. Cost five hundred quid to get the crank ground and the con-rods white metalled!

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Sun 13 Mar 2016 20:29
Pushrod design - gordonbennet

Brum, you can have good reasonably simple design that works well, but you'll have to join those of us staying firmly in the motoring 90's.

Yes OK some makers managed to make cambelt changes an absolute nightmare, but if you avoid the Germans and Italians like the plague (possibly still good advice) most others, even the French, weren't too bad at all to DIY.

Ironically i think even those makes now getting well deserved reputations to avoid used (though many people still arn't getting the message and have to learn the hard way) can be durable so long as take no notice the makers servicing advice, that'll be the makers who warrant their products for three years max by the way.

Edited by gordonbennet on 13/03/2016 at 20:29

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Sun 13 Mar 2016 23:32
Pushrod design - Wackyracer

Please don't start a petition to go back to OHV. Can you imagine how terrible they would be with new fangled 20,000mile oil changes and oils without ZDDP, you'd be able to hear the mechanical clattering from a mile away.

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Sun 13 Mar 2016 23:44
Pushrod design - focussed

OHV pushrod engines worked ok with iron blocks and heads - are they going to work with aluminium blocks and heads?

Pass the earplugs please!

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Mon 14 Mar 2016 00:39
Pushrod design - brum

Why would they be noisy? You could still have hydraulic tappets and a lot of the latest engines have cam followers/rockers. And whats the problem with aluminium heads/blocks? Some designs iirc had the camshaft quite high and very short pushrods.

Alternatively why cant manufacturers use a geartrain to drive an ohc?

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Mon 14 Mar 2016 07:57
Pushrod design - nortones2

Exactly, Brum. RR used pushrod engines until 1998 (BMW thereafter) and Bentley still do, AFAIK. Seem quiet enough.

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Mon 14 Mar 2016 09:41
Pushrod design - artill

Nothing wrong with LS V8s used by GM in America. Alloy block and head, light weight, compact, and no shortage of power or torque.

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Mon 14 Mar 2016 10:12
Pushrod design - John F

The main advantage being the camshaft could be simply gear driven from the crank and truly maintenance free.

They actually are now almost maintenance free. Chains are expected to last the life of the engine, as do modern belts, e.g. Ford ecoboost. Even our ancient 16yr old Focus has a recommended 100,000m 10yr belt change, which means it will probably last the life of the car as long as the tension pulley doesn't break.

I remember seeing a pre-war engine in a German motor museum with a shaft driven OHC. Might have been a Horch straight eight.

It's a shame we were/are not more forward thinking - sad that cars like RR and Bentley rely on advanced reliable German engineering. The best bit of engineering in a 70s RR was arguably the American gearbox.


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Mon 14 Mar 2016 11:57
Pushrod design - jc2

There were shaft-driven OHC's in MG between the first and second world wars.And as for gear-driven OHV,they were noisy with metal gears and quiet but unreliable with fibre.

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Mon 14 Mar 2016 13:05
Pushrod design - brum

There were shaft-driven OHC's in MG between the first and second world wars.And as for gear-driven OHV,they were noisy with metal gears and quiet but unreliable with fibre.

They were probably simple spur gears, a modern helical cut gear is much quieter.

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Mon 14 Mar 2016 15:45
Pushrod design - Sofa Spud

Going back to pushrod operation of valves would only be a half-measure. Why not go the whole hog and go back to side-valve engines? So much simpler!

But seriously . . . I've never had any problem with cambelt or valve operation on any of my OHC cars. Had cambelts replaced at the recommended interval (or longer), that's all.

Pushrods mean more reciprocating weight in the valve mechanism and less direct operation, so more potential points for play to develop. Also with many modern engines being 4 valves per cylinder, employing pushrods would add complication, not reduce it.

Edited by Sofa Spud on 14/03/2016 at 15:51

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Mon 14 Mar 2016 15:57
Pushrod design - madf

I refuse to comment on this subject as it shows amazing ignorance of mechanical design issues of engines.

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Mon 14 Mar 2016 16:14
Pushrod design - brum

I refuse to comment on this subject as it shows amazing ignorance of mechanical design issues of engines.

Please enlighten us further with your knowledge.

Edited by brum on 14/03/2016 at 16:16

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Tue 15 Mar 2016 06:16
Pushrod design - madf

I refuse to comment on this subject as it shows amazing ignorance of mechanical design issues of engines.

Please enlighten us further with your knowledge.

Modern emission controls basically require precise control of valve opening and closing.

A pushrod operated system cannot do that due to high moving mass - think bounce at top and bottom of cycles.

Four valve per head engines make pushrods expensive, bulky with lots of go wrong.

Variable valve timing and pushrods ? With camshafts set in the block? Complex and bulky.

The suggestion that pushrods are compatible with modern physical requirements is risible.

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Tue 15 Mar 2016 11:42
Pushrod design - brum

Well thank you Madf for that information which is both interesting and certainly difficult to counter argue.

It was only a spurious thought that I thought might be worth discussing beyond the usual dreary topics such as "which car should I buy" (almost always a toyota or "the same as mine" seems to be the standard reply). I realised it would provoke the sidevalve jokes etc. but as an electronics designer in my previous life, I do think it good occasionally to think out of the box instead of the usual humdrum evolve slowly approach.

Any way I could have done without your personal dig at the end of your comment, but you being madf I half expected it.

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Mon 14 Mar 2016 20:10
Pushrod design - Sofa Spud

I refuse to comment on this subject as it shows amazing ignorance of mechanical design issues of engines.

You just did !

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Mon 14 Mar 2016 16:01
Pushrod design - Sofa Spud

The best bit of engineering in a 70s RR was arguably the American gearbox.

Or was it the French 'power hydraulics' braking and self-levelling system?

Edited by Sofa Spud on 14/03/2016 at 16:01

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Mon 14 Mar 2016 16:55
Pushrod design - focussed

Camshaft drives using either internal chain or external toothed belts are brilliant, I've never had a problem with either system since the early 1970's.

Pushrods? No thanks - unless it's the lawn mower!

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Mon 14 Mar 2016 17:44
Pushrod design - Bianconeri

Camshaft drives using either internal chain or external toothed belts are brilliant, I've never had a problem with either system since the early 1970's.

Pushrods? No thanks - unless it's the lawn mower!

My lawnmower is OHC
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Mon 14 Mar 2016 19:52
Pushrod design - Wackyracer

I personally think a good OHC engine is far superior and reliable than OHV engines were.

We had a Viva where the pushrods punched holes through the rockers ( a common fault with them), a friend had a 90's metro that although had duplex cam chain it still managed to break. We had another A series powered car where the rocker adjusting screw came loose and felt out. Luckily it was just laying on the head and didn't cause any lasting damage.

My Citroen has a 10 year 80,000mile belt interval, I'd like to see someone who can honestly say they drove an OHV for 10 years or 80,000miles without having to do something to the valve train.

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Mon 14 Mar 2016 22:20
Pushrod design - brum

Ford Essex v6 engine, in my case the 120bhp 2.5ltr version in a Granada. 60,000 miles before terminal rust forced a sale.

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