Is there any reason why this engine design came first then, so to speak?
I don't understand how what seems like more moving parts, got designed - having everything up top seems the obvious way of doing it.
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I don't think it came first.... Stationary steam engines were full of pushrods in the 1800s. I'd still go for lubrication as the reason for it's slow take-up - oil simply wasn't good enough [outside of racing] until API SG came along in the late '70s. I recall that the pressure between a particular cam and rocker pad was given as 148,000psi.
Early engines' had no oil feeds; their pushrods/rockers were lubricated with an oilcan. Pushrods work well, are cheap and easy to adjust; early pre-hydraulic tappet OHC's were pigs to do. Remember shims, micrometers, oil-stained pieces of paper with columns of complex 0.001" calculations...
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I suppose pushrod overhead valve engines were a natural progression from the layout of sidevalve engines. Until internal/external gear, crank nose mounted oil pumps became common, many OHC engines also had a intermediate shaft where old fashoined side mounted camshafts were which drove the slower running gear or bi-rotor oil pumps and distributor.
In many cases until comparatively recently there hasn't really been a *need* for overhead cams - and for those who never rev beyond 4000 rpm, there probably still isn't a need for OHC!
Number_Cruncher
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Its because the methods of getting spinning motion to the top of the engine was expensive to engineer and assemble and unreliable in use. Dont forget neoprene and fibreglass toothed belts did not exist. It would have been chains and gear trains.
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TourVanMan TM < Ex RF >
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I can remember working on GM / Opel 'cam in head' engines during the 70's
These had the camshaft in the cylinder head, but used very short pushrods and rockers to operate the valves.
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The technology didn't exist to make reliable lightweight cam belts.
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Couldn't they use chains?
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A reason was that engines needed decoking frequently or valves replacing. Lifting the cylinder head and then replacing it was a relatively simple matter on something like the BMC A Series and the valve timing was not disturbed.
My BMW boxer still has pushrods but this is largely due to preserving the ground clearance on bends.
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I wasna fu but just had plenty.
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A reason was that engines needed decoking frequently or valves replacing. Lifting the cylinder head and then replacing it was a relatively simple matter on something like the BMC A Series and the valve timing was not disturbed. My BMW boxer still has pushrods but this is largely due to preserving the ground clearance on bends.
That makes sense
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GWS
My BMW boxer still has pushrods but this is largely due to preserving the ground clearance on bends.
That's a very good point; engine height.
Old engines were typically much longer stroke than todays over-square ones - adding a cam-box to the top makes them even taller. With higher ground clearance needed under the sump for bad roads, the difficulties of fitting an OHC engine under a sloping bonnet is another factor.
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Yes GWS, ease of dismantling is the reason. OHC engines were always more efficient and therefore preferred for sporting purposes. But not so simple to reassemble properly after decoking or any other procedure involving removal of cylinder head. Of course side-valve engines were even simpler, and even less efficient.
Early Bentley 3-litre (I think) had spur-gear driven OHCs but NO REMOVABLE CYLINDER HEAD. That must have been fun to work on...
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I seem to recall the first toothed belt drive was on an OHC Glas V8 in '38. Unless someone knows of an even earlier one?
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No need for belts or chains. Most early OHC engines were shaft driven.
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A pushrod sidevalve engine AFAIR doesn't need rockers and stuff, so less parts than overhead cam
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My first car, a vauxhall viva HA, had a side valve engine. Pushed it along swiftly enough to keep up with the traffic without getting me into to much trouble and returning around 40mpg
One might ask whether most cars really need the sophistication of OHC or even DOHC engines?
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the vauxhall viva had a 1057 cc engine , this was a pushrod type overhead valve engine , then updated to 1159 cc for the hb viva , and again updated to 1256 cc for the viva hc , also used in the baby firenza , this was not a side valve engine.
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forgot link to info (i did my test in a 66 model) en.wikipedia.org/wiki/Vauxhall_Viva
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Emissions control/better fuel consumption forces better engine combustion hence the move to OHC. Aided by computer controlled manufacture and assembly making manufacture to strict tolerances easier
madf
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I think it is the case that most modern OHC are not really true overhead cams in the sense that they still have rockers? All it means is the cam is on top, not directly operating the valves. True direct-acting OHCs existed years ago - Volvo and Triumph Stag are two I know. The Volvo used a belt, the Stag chains. Adjustment of valve clearance is by varying the thickness of the interveaning shims.
Didn't Riley have a classic DOHC in the 50s? Chain driven, presumably.
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The 1940s/early 1950s Rileys had twin high mounted camshafts operating valves by pushrods. Same basic deign as my 1929 Riley Monaco....
Cams operating directly on tappets are standard for Ford iirc
madf
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Cam followers really..
The Pinto had the cam acting on fingers that pivoted on an adjustable ball joint,blocked oil spray bar was the weakness of this engine.
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The earliest internal combustion engines had 'atmospheric' or 'automatic' inlet valves, where the valve was opened simply by the suction created by the descending piston, with no operating mechanism at all. Now there's simplicity for you...;-)
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We had an early motor bike like that Mike with a sprung inlet valve... also trembler coil ign we fired up with a 9v torch battery.
David
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I don't think there are any pushrod engines left in production, the last being the Ford Ka 1.3 before it was revised a few years ago to OHC, and the Skoda Fabia 1.4 alloy pushrod engine that dated back to the Estelle, now replaced by the 3 cylinder 1.2 design. Also last of the 'A' series in the original Mini.
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Think there are a few left. The Rolls Royce V8 , the generic Ford V8 possibly and the BMW boxer. Any more?
Just because an engine uses rockers it doesn't disqualify it from being OHC. The Dolomite Sprint engine had 16 valve but only had one camshaft which actuated the valves via rockers.
A friends 1936 British Salmson is DOHC - must have a closer look at it.
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I wasna fu but just had plenty.
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Is the Rover V8 still used in the Range Rover? Or the Morgan? That had pushrods with hydraulic tappets.
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A push rod engine had the following advantages in its day: -
A short drive mechanism, commonly, in mass-produced vehicles, chain drive with a simple, non-adjustable tensioner.
A low mounted camshaft giving the advantage that the oil pump and distributor could be driven from it. ( most engines were north/south mounted so a distributor would either intrude into the radiator or the bulkhead if it was driven off the end.
Simple adjustment of valve clearances.
Easy removal of the cylinder head.
Simpler machining of the cylinder head. (No cam follower bores to machine.)
No sleeving of cam follower bores. (Followers run directly in cast iron block.)
Power outputs, economy and emissions were not treated with the same importance as today and in general engine did not rev as high as today.
The major operational problem with push rods was the inability to consistently control the opening of the valves. The number of wear points and the tendency for the mechanism to wind itself up under load almost gave rise to variable valve timing by default. High revs exacerbated the problems.
Driving the valves open, more or less directly with the cam lobes, gives more accurate, consistent operation and improves output, etc. (valve closing is still spring controlled, at the moment, anyway). Improved materials, machining methods, emission legislation etc have made ohc the norm, now.
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Is the Rover V8 still used in the Range Rover? Or the Morgan? That had pushrods with hydraulic tappets.
Range Rovers used BMW V8's while under BMW ownership but now use a V8 based on the one fitted to Jaguars. I think the Discovery was the last model to use a Rover V8 and they stopped using them in 2005. I think Morgan use BMW V8's now dont they?
Do any small specialist companies still build new Rover V8's for use in low-volume production cars and kit cars?
Of course in the US they still build thousands of pushrod V8's. The V8's in Chrysler 300C and Jeep Grand Cherokee are pushrod engines, not built in the UK but sold over here.
Rich.
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Somebody may correct me, but of modern UK (post 1960) 'everyday' production cars the Hillman Imp was probably the first to have an ohc egine. It was a delight to work on, although the shims in the tappets caused a few headaches if you did not have a wide selection of spares in the toolbox.. It gained a bad reputation primarily because the motor trade could not readily adapt to torque wrenches, aluminium and the discipline necessary on what was nearer to racing technology than the cast lumps in general use at the time!
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pmh (was peter)
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There are various push rod bike engines, Harleys, MT01 etc.
The push rod design lends itself to a V engine allowing only one camshaft rather than a min of two for an OHC V.
There is not much wrong with the push rod as a valve actuation method, the issue is on cars that hemi chambers on cross flow engines are better served by a camshaft however on a single or V-twin (and to an extent a parallel twin and V4) it is practical to produce a hemi head cross flow design because there is more flexibilty as to where the push rods are routed than for instance with a V6 or V8.
Tuned push rod A Series could rev to 9000 in race trim.
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The Imp engine was based on the Coventry-Climax fire pump engine. In that application, the engine was coupled to a pump and both were mounted in a tubular steel frame. The unit provided a powerful water pump independent of mains electricity. The use of light alloy for the engine was to minimise weight for manual transport by the Fire Brigade. As you say a very high-tech engine which defeated many mechanics used to insensitve cast iron units.
JS
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