Actually, as I'm sure you know, that's not the usual reason offered for why
multiple valves work better than single valves. While the theoretical area
for a single valve is larger, in fact you can pack more valve area into the
available space with multiple valves. And the flow is not a function of the
total valve area, but rather the opening of the valve, which imitates at the
perimeter and starts off as almost a linear function, transitioning to an
area function as the valve opens (look at some of the new three dimensional
flow diagrams--there was a very cool discussion of this in an issue of race
car engineering last summer). Extremely complex flow--none of it works like
we visualize it. Initial intake flow at higher RPMs is explosive--almost as
much so as exhaust. Small valves also have very small stems and narrow
guides, which block the flow less even in proportion to the head size.
Finally two valves are individually much lighter than one, so they can be
controlled by their spring much more easily at high RPM
-----Original Message-----
From: Michael Porter [mailto:portermd@zianet.com]
Sent: Tuesday, June 14, 2005 9:51 AM
To: Bill Babcock
Cc: 'Jack W. Drews'; fot@autox.team.net
Subject: Re: 225hp
Bill Babcock wrote:
>But one lesson that YOU learned is that bigger ports aren't necessarily
>better. Possibly if you can change the other tuning factors to take
>advantage of the increased velocity, you might have a gain.
>
>
And, this might be one of the reasons why two (or more) small intake valves
generally produce more power than a single intake/single exhaust bathtub
style chamber. High velocity combined with equivalent volume to a single
larger port (although port angle--particularly with a pent-roof chamber--has
something to do with the flow). I've always had the feeling that additional
velocity is good for mixture turbulence, but there're compromises to be made
when one only has one intake valve per cylinder to work with. That may be
why there were so many experiments with long ram intakes in `60s engines.
Cheers.
--
Michael D. Porter
Roswell, NM
Never let anyone drive you crazy when you know it's within walking
distance....
|