Valve actuation

While I was thinking about heads, and cams, and all that sort of thing, my
thoughts turned valves, and more specifically, the opening and closing
thereof and how that's driven.

There are 4 main systems in use these days.

Pushrods.

Chain driven OH cams.

Gear driven OH cams.

Belt driven OH cams.

Ok, so what are the advantages/disadvantages of OH cams vs pushrods?
I can think of, so far.....
Push rods will have more flex in the system.
Push rods are likely to be heavier.

Also, what are the advantages/disadvantages of the different methods of
driving those OH cams?
I can think of, so far.....
Gear - less wear, no need for adjustment/replacing of chains and
belts, however when it does break, bugger me is it going to be nasty.
Belt - It's external to the engine, and it's stretchy, so it's
easier to work with.
Chain - ???

 

IIRC they arent as accurate and have trouble with highspeed operation ?

true but it really does last just about forever ,VFR for 1

i would be gusessing that it is more expensive and is heavier , but would be
accurate
as dont get chain stretch

yeh easy to change and cam timing is easy to change , but also stretches so
must be
changed more often at a cost , but quick to change , prob cheaper than
chains

guessing longer life but thats a myth based on plenty of models out there
cheaper than gear drive

 

I think there's a clearer division into SOHC and DOHC.
SOHC generally have rockers. DOHC generally don't.

Adjustment: Screw-and-locknut, shim under bucket, shim on bucket,
hydraulic (automatic)

Springs vs Desmo.

Larger tolerances due to thermal expansion, I think. Depending on
the engine layout, can be quite efficient ... V-twins in particular
can have one camshaft between the cylinders and rods to each head,
rather than two lots of camchains, tensioners, cams etc.

OHC make much more sense on a par twin / inline four design.

chains: cheap, narrow, lasts longer than belts, runs well in oil
(no need for oil seals on the ends of the camshafts). But if they
do break they fill the engine with shrapnel.

gears: last forever, but they're whiney and there's a lot of gears
in there. You can't change the camshaft height (eg: plane the heads)
either.


-----sharks

 

I think there's a clearer division into SOHC and DOHC.

I don't know about that. I have 2 DOHC engines (GPX250 and ZX10)
that both use rockers.

 

On 29/6/04 5:44 PM, in article

They do?

Incredulous Hammo

Even the 84 1100 didn't.

 

Yeah? Colour me baffled. Why? What do they look like? Gotta piccie?

(I think 'generally' is fair, still ... I always say that because
someone always comes up with the SOHC shim-under-bucket Spagthorpe
or something)

------sharks

 

My TX500 Yamaha had twin OHCs with a total of four lobes (two on each)
operating eight valves by means of forked rockers. Adjustment by screw
and locknut.

My previous Yamaha had no cam shafts.

Theo

 

I don't know about that. I have 2 DOHC engines (GPX250 and ZX10)

Yep. And a beautiful thing it is too!

 

When I actually manage to get my hands on a decent digital camera, I will
take some pictures (particularily the nifty system on the ZX10, where the
rockers slide out of the way so you can change the shims - bloody genius
that!!!) and post a link.

 

The R259-series BMW engines have high cams, not OHC but not traditional
pushrod design either

 

VFR400 has the same system

http://www.mc99.co.jp/mvp/member/new/honda/11mr8kj6/pic/004.html

beats taking cams in and out

but gotta be careful not to lose the shims down the tunnel

 

It does sound good indeed. Well, I take it back then ... only
_some_ DOHCs have direct actuation.

-----sharks

 

Why didn't they bloody copy that idea to the 800? - bastard bloody
Honda

John

 

the 800 was designed by a different department

 

Buy the 400 and bore it out to 800.

 

Pushrods can be replaced with a bit of welding rod or summat, on the side
of the road...

Gary

--
Wealth without Work
Pleasure without Conscience
Science without Humanity
Knowledge without Character
Politics without Principle
Commerce without Morality
Worship without Sacrifice

The Seven Deadly Sins of M.K. Gandhi

 

The GPz900 that arrived in 84 had DOHC's and used rockers.

 

buy 2 and whack em together , make a V8

 

came with used rockers ,was there a recall ??

geez they really are a classic

 

Bolt one in upside down and make an X8... (Or would that be parallel radial
4s?)

Postman Pat