"3/4 race" is a relatively unscientific term
If you look at BMC's factory spec cams for the BMC A series engine, the
'Full race' cam for a sprite in the 1960's was the 'C-AEA-648' grind. On a
road car - even the '731' grind was pretty lumpy, with little under 2,500
rpm. Most guys in road cars didn't go much beyond a Cooper 997 grind -
especially in road going 948cc bugeyes.
Anyone who drove a Sprite or Mini with a '648' cam would testify to its
unsuitability to road use in a 1000ccc - 1100cc engine.
The 648 'full factory race' camshaft was a 300 degree cam. The 731 was a 268
degree cam. There was also a factory 320 degree supersprint cam for the A
series
A standard BJ8 cam is a 252 degree camshaft...... and a Cooper 997 is also
252 degrees. A 948cc Bugeye Sprite is only 230 degrees.
My road going triple webered 3000 has a 300 degree cam, and will pull away
while rolling in 2nd gear almost as comfortable as a stock BJ8. Yet in a
Sprite - a 300 degree cam would be undriveable on the road under 2,500 rpm.
And I've got more lift at the cam than a 648 - and 1.62:1 high ratio roller
rockers - so the valves open around half an inch.
Different engines cope with more cam duration differently. Smaller engines
don't have enough torque - so they suffer at low levs - making them
undriveable on the road. What is classed as "full race" in a Sprite is still
quite tractable in a 3000 - due to capacity and torque.
So what is 'race' in a sprite is 'fast road' on a 3000.
Or just quote the spec!!
Chris
----- Original Message -----
From: "John Miller" <healeys@n4vu.com>
To: <healeys@autox.team.net>
Sent: Friday, June 04, 2004 6:38 AM
Subject: Re: Cam Questions / 3/4 ?
> On Thu June 3 2004 3:46 pm, RAHosmer@aol.com wrote:
> > No, because of the "twice-around" facet of four cycle operation, a "360
> > degree cam" would still be open only half the time Not lame at all! In
> > fact, using a 270 degree duration to define a "3/4 cam" grind may not be
> > far off the mark - in fact - it COULD even be right. I'm 67, and that
> > expression was in use when I was a teenager.
>
> Bein' only 61, I was a few years behind you, but the term was very much
> current in the late '50s.
>
> Then, as now, I kinda figured it was a term of art, that is, a made-up
> expression to convey the concept. Reason was, I don't think I ever heard
> "3/4" without it being followed immediately by "race:" "3/4 race." Since
the
> next step up was "full race" it always seemed that 3/4 race was just a
> convenient handle for something between a street grind and full race.
>
> <shrug>
> --
> John Miller
>
> "Even if you're on the right track, you'll get run over if you just sit
> there."
> -Will Rogers
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