Terry,
1.5" of Hg (Mercury) is approx 315m/s. Depending on the design of the
engine airspeed will vary. Most stock datsun engines run average
airspeeds of about 343m/s at 20deg celcius during the time the inlet
valve opens till it closes. 343m/s is also the speed of sound at 20deg
celcuis. Going much beyond this the valve becomes choked and flow rates
decrease.
Don't underestimate the Hitachi's (SU) carbies. What you have to think
about is the area each cylinder is breathing through. An engine with
twin webers with say 34mm chokes has LESS area to breath through for
each cylinder than the same engine with Twin SU's which are 35.8mm in
internal diameter. It doesn't stop there either. The design of the SU
manifold doubles the amount of resonant pulses each cylinder sees and if
tuned correctly can create a better ram air effect. The result is more
more HP at high RPM for identical area and coefficient SU's versus
individual runner Webers.
The killer with the SU's will be the discharge coefficient. This is
basically how efficiently the carby flows for its area. Because of the
sharp edge of the large sliding piston the airflow becomes turbulent
this helps air/fuel mixing but is not good for flow rates. Look down the
throut of a Weber and everything is nice and rounded and smooth. Webers
are actually best suited to low to mid RPM ranges like rally engines. If
you wanted all out HP at revs a single carby with a good flow rate is
the go on a tuned length manifold.
I remember someone on the list that ran some sort of track racing that
changed from Webers to a Single Holley and improved high RPM power
output. This is a classic example.
Craig.
Terry wrote:
>
> Craig & Errol,
>
> Thanks for the info guys. If I have this correct now, the pressure
> differential at wot on a hypo engine could be more than 1.5" hg? and
> therefore the carbie gets more air/fuel through the same hole. This is where
> a camshaft to match the engine characteristics and fuel delivery can have
> quite an impact, high lift vs duration and compression pressure. A lot of
> engines in the mid eighties were very different in these areas and are very
> difficult to tune out detonation with lower RON fuel (re post on the L24
> recently). I guess if you're head is not flowing at an optimal rate then you
> could play with venturi sizes to get the best out of it but the camshaft
> specs seem to have a huge effect on these engines.
>
> I know from my Ford days that you can have basically identical engines, same
> 600 or 780 carbies etc but the 351 with the 4V head and GTHO cam absolutely
> craps on a 2V with the GT cam. Same thing with a Datto engine and this is
> why a certain jetting & choke setup will not or not seem to work the same on
> 2 similar spec engines.
>
> What started me thinking about this a few days ago other than having a slow
> day at work was why do the US boys love Hitachis so much, other than they do
> provide a very driveable engine that is easier to get good fuel consumption
> out off too, but to get the performance level that even a single DCOE is
> capable of that is anywhere near tuned right will kill the hitachi fed
> engine.
>
> terry
>
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