Yes, your basic understanding is correct. For a given engine speed, a bigger
port cross-section area will have a lower velocity than will a smaller port
at the same rpm. If the velocity drops too low, the atomized droplets of
fuel will tend to aggregate and drop out, giving an inconsistent mixture.
There are a bunch of factors that need to be "coordinated" in order to make
an engine run right - valve diameter, port size and shape, carb size, cam
profile and engine rpm. It's a complex balancing act.
The spring pressure should only be enough to keep the valves from floating,
and that's a factor of cam profile, rpm and component weight. Any more than
that is just draining power and accelerating wear.
> -----Original Message-----
> From: owner-datsun-roadsters@autox.team.net
> [mailto:owner-datsun-roadsters@autox.team.net]On Behalf Of Daryl Smith
> Sent: Monday, July 19, 2004 9:47 PM
> To: datsun-roadsters@autox.team.net
> Subject: Stroker R16 head flow
>
>
> Thinking about the ability of the R16 head to provide air to a stroker
> motor.....
>
> With regards to "port velocity" vs "flow". Am I correct in
> thinking that the
> velocity will be chiefly determined by the size of the port
> (diameter) vs the
> volume of air being "sucked" in by the piston (too small-restricting, too
> large-loss of velocity), and flow being chiefly determined by the
> shape of the
> port (smooth, no sharp bends) and the ability of the air to get past the
> valve?
>
> Using this logic, is it possible for a valve to be too big
> (assuming it fits
> the combustion chamber)?
> I am just trying to get a basic understanding, I know it is probably more
> complicated.
>
> I was at a head shop Saturday and was told the Intake port is quite well
> shaped, but the exhaust port is probably too small and restrictive. Can
> anybody confirm this?
>
> He also checked with valves he had in stock and says small block
> chevy valves
> can be very easily modified to fit, along with the springs and
> keepers. This
> would make some very good flowing (oversize?) valves available to us
> relatively cheaply off the shelf.
> A point that datsunmike brought up though is the spring pressure
> of the SBC,
> How much is too much? I guess damaging the cam or lifters is a
> possibility?
>
> Anything else to think about on this subject?
>
> thanks
> Daryl
> '66 1600 - Soon to be "Molested"!!
> She doesn't have a name yet, but she's got a theme song....
> "She ain't Pretty, She just Looks that Way" !!
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