IInteresting read for the Pushrod vs OHC debaters :)

[Wobbler]

About packaging, when holden was devolping some over head cam heads in the early 90's they had to go single cam per bank and a twin would be to wide to fit between the chassis rails when they were to be assembled. Engine got scrapped due to gm deciding it was a no go.

"proper" twin cam or DOHC engines don't use rockers

So an sr20 isnt a "proper" dohc motor?

[Nick]

Correct. :D

[GTSBoy]

Clever people at ALFA once had an OHC engine with pushrods. In fact they had two such engines.

Sure they were horizontal pushrods (at least nominally horizontal, or at least perpendicular to the cylinder) and they were very short. But they did their job!!

Other than that application, I'm with Bill.

cheers

[TonyJZX]

www.goatsesengine.com

How come the above url doesn't work?

I don't have a huge problem with the LS1 - it's a cost issue in that you won't see Ford have their V8 down to the same cost of the SS.

I'm guessing you guys have driven an SS - it's not so bad.

About the 5 valves thing, I can only think of Ferrari and Yamaha doing it.

And how come no car manufacturer has done desmodromic valves?

T.

[Nick]

TonyGTSt: You can also add Toyota (I suppose you can call them Yamaha due to them doing most of Toyota's Heads), Audi/VW.

[Captiva Fan]

And how come no car manufacturer has done desmodromic valves?

I'm trying to remember my engineer-speak ... is that when the cam's designed to actually "throw" the valve, meaning the valve can move more quickly than if it were always touching the cam/rocker/whatever? If so, then Ferrari have been using that type of design for a while; and hence I wouldn't mind betting a few Fiat-owned Italian makes have started doing it too.

[GTSBoy]

No Forg,

Desmo is where there are two cams for the valve. One is the reciprocal of the other, so there are no valve springs. The normal profile cam drives the valve open, and the whole time the valve is working up against the inverse profile cam, so the valve motion is controlled. Then after the normal profile cam goes over max lift, the inverse profile cam drives the valve closed against the (now receding) normal profile cam.

Very fiddly to design and set up, and moderately expensive too. Done right, it is a purely sexy engineering solution to the problem of valve control.

cheers

[Nick]

Interesting, seems like a good hi-revving design, Ducati's seem to be using it on Production bikes but as you said cars it doesn't count.

Only downside I can see would be the wear and shimming for it.

[Captiva Fan]

Could it be that car engines are expected to last much longer than bike ones?
Also ... car engines seem to rev to 9k+ without it, and it appears to be only about high-revs valve-control; maybe people generally don't want to drive cars that rev so high (or, more accurately, techsales-marketing folk in the likes of Toyota & Honda don't see a payoff for the R&D cost in terms of increased marketability)?

[FastEHHolden]

SEX...A Commodore with the noise of a Ducati times 4.

I cant wait until solenoid operated valves hit production..infinately adjustable in every possible way, no drag from trying to turn a camshaft (or 4).

Hey Bill...did you try and address the frictional losses of driving 4 cams vs 1?

[260BC]

Another problem with upright twin cams is the high weight of the engine, that is the engine has a higher center of gravity. Plus, for example, the twin cam FJ20 engine is itself considerably higher than a comparable L series Nissan engine and heavier as well.

This is not good from a handling point of view, nor does it help with a lower bonnet line and all the ramifications of that.

So the advantages of twin cam multi valve engines come at a cost and that cost can be quite substantial.

[Wobbler]

:worship: That is a damn sexy piece of engineering

[Beavis]

I remember reading an article with a Ducati engine technician a while ago.. he said that, nowadays, the main reason for using Desmo valves today - i.e. that in the old days, valve spring technology kinda sucked - is that they allow the valves to be open & closed faster than you can with agressive cams & hard springs.

[260BC]

Looks like the relative complexities are at issue. Lets look at an upright inline four, a V configuration is even less favourable for the twin cammer. Use an FJ20 as an example, won't consider the valves, springs, etc..

FJ20.
two camshafts; eight camshaft bearing caps; 16 cap studs, nuts and washers; 16 buckets (various thicknesses required); one relatively long duplex chain with joining link; three sprockets with separate retaining devices; two fixed chain guides with studs, nuts and washers; one pivoting chain tensioner slipper with multi part oil pressure driven tensioner and pivot shaft with circlip and washer; one chain spray bar.

Generic pushrod OHV
One camshaft with retaining device; one short chain; two sprockets with retainers; eight camshaft followers; eight pushrods; eight rockers with integral adjusting devices; one rocker shaft and whatever is required to bolt it on. Missed anything substantial?

Your Lordship I rest my case :p

[Billzilla]

For about the tenth time, I've already answered all of these questions on my site.
http://www.billzilla.org/2v4v.htm

I talk about frictional losses, airflow, packaging, and give realworld examples.

It's abundantly clear that a twin-cam multi-valve engine has huge advantages.