[Spridgets] Question

[Bud Osbourne]

Re: Porsche's methods of cam drive.  It should be noted that, on the engine 
which actually gave birth to the lore & legend of Porsche reliability, the 
cams were driven by a series of shafts & gears.  These quad cam, four 
cylinder units required about 40 man hours to assemble!  This was Porsche's 
war horse from approximately late 1954 through 1964, eventually being coaxed 
to produce 180 hp in Formula 1 trim.  The flat 8 that replaced it (in F1) 
during the 1962 season also featured gear driven cams, as did subsequent 
developments of that engine, including those powering the 907, 908 & 917. 
The type 901 flat 6 engine, which first appeared in the replacement for the 
356, featured a SOHC flat 6, with chain driven cams.  This engine powered 
the 911 right up through the last model year to use air cooling and also 
powered the 911 competition cars along with some 904s, and the highly 
successful 906 racer.  Porsche never hid the fact that they employed chain 
driven cams as a production cost saving measure and that 6 cyl unit consumed 
about 8 man hours to assemble.  However, as anyone familiar with "street" 
911s can tell you, Porsche had some "issues" with timing chain tensioner 
durability.  The failures produced some pretty major engine damage, to say 
the least.  They didn't get it completely cured until the late 70s or early 
80s (and I keep hearing rumblings that it still isn't a fool-proof system).

So, the way I see it, either form of cam drive (chains or belts) requires 
regular maintenance if you want complete reliability.  The more frequently & 
tighter you wind 'em up, the more attention the cam drives require.  My 
research (not the least bit scientific, I might ad) has indicated that 
tensioners, rather than belts or chains, are usually the cause of any 
failure.

Gear drives, while arguably far more reliable, are certainly more expensive 
to produce, which is the main reason you don't see them spinning overhead 
cams on production car engines.  Of course, for "pushrod" engines, gear 
driven valve trains are considered cost-feasible (think all air-cooled VWs).

Bud Osbourne

----- Original Message ----- 
From: "Kirk Hargreaves" <khargreaves2 at gmail.com>
To: <spridgets at autox.team.net>
Sent: Wednesday, January 20, 2010 4:36 PM
Subject: [Spridgets] Question


>A Porsche has it's cams driven by an internal chain.  . the same as I think
> about it as the straight 6 Jaguar engines hailing from the 1950's (and
> possibly earlier).
>
> Many high end cars have their cams driven by an internal chain with a
> tensioner which needs very little maintenance, if any, compared to an
> outside belt drive.
>
> The Ferrari, and I am guessing the Lambo and other super high end cars 
> have
> their cams driven by outboard belts . . which need to be changed out at
> around 30k miles.
>
> With the Ferrari, I understand that it is a big process in that the engine
> has to be pulled to change out the belts.  I have read that it can cost up
> to 10k to have the dealer change out the belts in a Ferrari.
>
> So why would Ferrari, as well as others, go with an outboard belt drive on
> their cams?
>
> Is it because the system is lighter, gives a better response, or?  I am
> guessing that the thought is that outboard belt driven cams enhance
> performance?
>
> Just a question that has been in my head for a while. . I never have heard
> anyone explain the theory behind the out board belt as opposed to the
> internal timing chain for driving cam shafts?
>
>
> Kirk
> Pushrod Drive 1275