American race cars (formerly - OT: new car advice)

[Frank Davenport]

I believe this technology is now common in nearly all F1 engines.

F.


On Tue, 2005-11-22 at 14:14 -0700, Robert N. Eaton wrote:
> Lyndon Tiu wrote:
> > On Tue, 22 Nov 2005 09:05:10 -0700 mike at garfias.org wrote:
> >   
> >> Hmm.
> >>
> >> Did you know that overhead cams are an OLDER technology than pushrods?
> >>
> >>     
> >
> > Really? 
> >
> > I wonder why most cars today use overhead cams?
> >  
> > --
> > Lyndon Tiu
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> >   
> Reciprocating weight. Push-rods and rocker arms weigh more than the 
> thimble + shims that most ohc engines use. The heavier the reciprocating 
> weight, the heavier the valve springs have to be to prevent valve float 
> (which can lead to interference with the pistons, making very expensive 
> noises.) Many modern engines use twin cam,  four valve per cylinder, 
> design (pioneered by Isotta-Fraschini in 1911, I believe. I used to have 
> some pictures of a marvelous old chain drive IF with a gigantic four 
> cylinder engine with just such a layout.)
> 
> At any rate, two valves of half the valve surface area weigh about half 
> of what one valve would weigh and would need lighter valve springs. Less 
> reciprocating weight equals higher rpm without valve float, and that 
> equals more power from a given displacement engine.
> 
> The upper end of the rpm scale is usually governed by a phenomenon 
> called valve spring surge. This occurs when the valve reciprocation 
> equals the natural vibration frequency of the spring, causing 
> catastrophic failure of the spring, resulting sometimes in an engine 
> devouring itself. Various schemes were tried to get around this 
> limitation: hairpin valve springs, torsion bar valve springs (both 
> difficult to contain, requiring huge valve covers.) Mercedes-Benz in the 
> fifties 300SLRs used desmodromic valve actuation, in which the valve is 
> forced open by a cam and forced closed by an "anti-cam." This won some 
> races for them, but was extremely expensive to manufacture and maintain.
> 
> Probably the most elegant answer to this problem was the one devised by 
> Ferrari a couple of years ago, when they set for themselves the task of 
> wringing 900 horsepower from a
> V-10 twin cam multi-valved unblown three liter engine. They figured that 
> if they could get 300 hp out of a three liter engine at 6000 rpm then 
> they would have to spool up the engine to 18 grand to get 900 hp.  No 
> known valve spring would survive that, so they used 150 lbs of air 
> pressure to close the valves. Air has no natural frequency, so the 
> valves could operate at this rate. The only thing limiting the rpm was 
> the reciprocating weight of the pistons and connecting rods. These they 
> had to fabricate from titanium, at a cost that probably made even 
> Ferrari blink. I never found out whether they ever raced that engine, 
> but what a marvelous tour de force!
> 
> Bob Eaton
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