Potentially Stupid Question - Overhead Cam Engines
Discussion
Max_Torque said:
It kinda is though. Because the reciprocating part of the drive is more compact it is fundamentally stiffer, and hence you can reach significantly higher valve accelerations than with a push rod system. (OHV has a short rotary path, and a long reciprocating one, the opposite is true for PHC). As such, you can successfully use valve event profiles that increase either efficiency or power, or both, profiles that would not be realisable for an OHV push rod architecture.
That isn't an efficiency issue, it's a power potential limitation. In a typical mildly tuned car engine, and given everything else remains the same, any efficiency differences between OHV and OHC would be negligible.Hydraulic lifters pretty much neutralise the valve clearance issue in production cars.
OHC and then DOHC opened the door for a raft of clever mechanisms to optimise valve and ignition timing improving torque spread, power and efficiency.
Was there ever an all-aluminium OHV engine? If not, why not? I would imagine that keeping the valve lift accurate when there is a chunk of aluminium and a steel pushrod between the camshaft and the valve would be a bit difficult. Lightweight construction obviously all-important these days for efficiency.
OHC and then DOHC opened the door for a raft of clever mechanisms to optimise valve and ignition timing improving torque spread, power and efficiency.
Was there ever an all-aluminium OHV engine? If not, why not? I would imagine that keeping the valve lift accurate when there is a chunk of aluminium and a steel pushrod between the camshaft and the valve would be a bit difficult. Lightweight construction obviously all-important these days for efficiency.
HustleRussell said:
Was there ever an all-aluminium OHV engine? If not, why not? I would imagine that keeping the valve lift accurate when there is a chunk of aluminium and a steel pushrod between the camshaft and the valve would be a bit difficult. Lightweight construction obviously all-important these days for efficiency.
RV8 immediately springs to mind, plus, I think, the LS series engines, but might be wrong on that.I think that one of the constraints, as others have said, was oil supply to the camshaft. In OHV it lives in the sump and has an easy life. An OHC needs quality oil and a reliable high pressure feed. Early OHC designs, especially on highly stressed applications like motorbikes, could knock out cam bearings within 20k miles.
Very early engines were very commonly side valve, like the 3L example shown earlier. These were remarkably simple, the cylinder head is simply a cover. I've spoken to a driver of a 30s Austin who carried a spare head gasket and was able to change it at the side of the road within half an hour. Try that with a DOHC 16V engine.
Very early engines were very commonly side valve, like the 3L example shown earlier. These were remarkably simple, the cylinder head is simply a cover. I've spoken to a driver of a 30s Austin who carried a spare head gasket and was able to change it at the side of the road within half an hour. Try that with a DOHC 16V engine.
Super Slo Mo said:
HustleRussell said:
Was there ever an all-aluminium OHV engine? If not, why not? I would imagine that keeping the valve lift accurate when there is a chunk of aluminium and a steel pushrod between the camshaft and the valve would be a bit difficult. Lightweight construction obviously all-important these days for efficiency.
RV8 immediately springs to mind, plus, I think, the LS series engines, but might be wrong on that.
HustleRussell said:
Was there ever an all-aluminium OHV engine? If not, why not? I would imagine that keeping the valve lift accurate when there is a chunk of aluminium and a steel pushrod between the camshaft and the valve would be a bit difficult. Lightweight construction obviously all-important these days for efficiency.
I think Skoda used all ali OHV engines before the VAG takeover in the Estelles and Favorits?Not quite. Bizarrely the Estelle and I think the Favorit that followed it had an ally block and a cast iron head. I kid you not. OHV with pushrods, too. They were famous for blowing head gaskets, part of the problem was the acres of pipework got porous and lost water, so unless you were forever on the case with the fluid levels you cooked them.
What people over here (Europe) don't realise is that the OHV is the biggest selling single engine type in the world, the Chevy small block. Simple, reliable and will last forever. Granted, when pushing on they drink like George Best, but when cruising they sip lightly and can achieve a decent mpg.
battered said:
Not quite. Bizarrely the Estelle and I think the Favorit that followed it had an ally block and a cast iron head. I kid you not. OHV with pushrods, too. They were famous for blowing head gaskets, part of the problem was the acres of pipework got porous and lost water, so unless you were forever on the case with the fluid levels you cooked them.
That's a weird one. I can't think of another ali block and cast iron head although I'm sure someone here will know of one or several.Was it to do with reducing weight hanging out behind the back axle?
Weird indeed. Unique AFAIK. As far as thought process from Skoda of the time, I doubt they had the means in the days before the Curtain was lifted. I thought the things were bolted together with whatever they had to hand, so God only knows how they ended up with what they did. Does anyone with a long memory and a penchant for weirdo 70s Eat European tat remember what was in the S110R?
R8VXF said:
What people over here (Europe) don't realise is that the OHV is the biggest selling single engine type in the world, the Chevy small block. Simple, reliable and will last forever. Granted, when pushing on they drink like George Best, but when cruising they sip lightly and can achieve a decent mpg.
Thats the same for a lot of larger capacity V8's though. I know in stock guise my Mustang would easily get 30MPG on a motorway @70mph on cruise (not bad for a 1600kg car with a 4.6L V8 and a slushbox), but push on and it would go into low double or high single figure MPG.Granted similar figures out of a larger capacity engine (LS1 5.7) is even more impressive.
R8VXF said:
What people over here (Europe) don't realise is that the OHV is the biggest selling single engine type in the world, the Chevy small block. Simple, reliable and will last forever. Granted, when pushing on they drink like George Best, but when cruising they sip lightly and can achieve a decent mpg.
Is the Chevy small block really the best selling engine in the world? I wouldn't have thought it was even in the top ten!kambites said:
Is the Chevy small block really the best selling engine in the world? I wouldn't have thought it was even in the top ten!
Given they have made over 100 million of them I would imagine it is well up there yes. Don't forget these engines aren't just sold as part of a car they are sold in crates in everything from short block form to fully dressed engines.PanzerCommander said:
kambites said:
Is the Chevy small block really the best selling engine in the world? I wouldn't have thought it was even in the top ten!
Given they have made over 100 million of them I would imagine it is well up there yes. Don't forget these engines aren't just sold as part of a car they are sold in crates in everything from short block form to fully dressed engines.Whether you consider it to be true historically depends on how you define an "engine" I suppose. I'm sure Toyota have sold an order of magnitude more inline fours than Chevy have small block V8s but I guess we're somehow counting each different Toyota engine separately but the GM ones together?
I'd have thought the number of crate engines sold would be insignificant, although I may be wrong.
Edited by kambites on Monday 27th July 07:49
kambites said:
Has anyone ever produced an OHV engine with hydraulics replacing the entire pushrod setup? I'd imagine that could solve most of the issues with fine valve control?
This is the basis of the Fiat Twin Air engine, albeit it's all done in the cylinder head. No doubt it could be achieved by a cam in block approach instead, and the technology allows for a huge amount of variables with regards to cam timing, variable lift etc.It would be a complete re-engineering of the engine concept though, whereas the Twin Air approach uses a conventional block and just bolts the new head onto it.
HustleRussell said:
Hydraulic lifters pretty much neutralise the valve clearance issue in production cars.
OHC and then DOHC opened the door for a raft of clever mechanisms to optimise valve and ignition timing improving torque spread, power and efficiency.
Was there ever an all-aluminium OHV engine? If not, why not? I would imagine that keeping the valve lift accurate when there is a chunk of aluminium and a steel pushrod between the camshaft and the valve would be a bit difficult. Lightweight construction obviously all-important these days for efficiency.
I remember seeing a 'plastic' engine at the design museum as student in the late 80s. Seems the concept is still alive.OHC and then DOHC opened the door for a raft of clever mechanisms to optimise valve and ignition timing improving torque spread, power and efficiency.
Was there ever an all-aluminium OHV engine? If not, why not? I would imagine that keeping the valve lift accurate when there is a chunk of aluminium and a steel pushrod between the camshaft and the valve would be a bit difficult. Lightweight construction obviously all-important these days for efficiency.
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