Ian,
I used to run an Offenhauser 360 with good results. If I remember rightly, I
could never attribute a performance increase when I switched from the Offy to
the Huffaker. This was despite the fact that I never matched the Offy manifold
to the heads and the Offy manifold ports would have needed opening up a *lot*
to achieve a match. I never did any flow comparisons before I sold the Offy
though unfortunately.
I cast a 1 inch aluminium spacer that fitted between the Offy manifold and the
carb, forming a sort of plenum, without the dividing wall that the manifold has.
I never proved that this was a better arrangement, but I feel that reducing the
effects of the diviiding wall allows each cylinder to draw through all 4 carb
barrels, making the engine 'see' a bigger carb.
A friend of mine dynoed his engine with an APS sheet metal manifold, an old
Harcourt and an Offy 360 and he said that he got the best results with the
Offy. His engine was very similar to yours. (A high RPM engine may or may not
prefer the Huffaker or Harcourt, but as I said earlier, it didn't make much
difference to my 7400 RPM engine).
A bigger carb may help. Most people would say a 390 is big enough, but I'm not
sure. When the carb manufacturers say it flows 390 CFM, you need to know at
what vacuum to make sense of it. I believe that 4 barrels are flowed at 1.5
inches Hg and 2 barrels at 3 inches Hg.
What this means is your 390 carb will flow 390 cfm when your engine has a
manifold vacuum of 1.5 "Hg, which is quite a high vacuum and hence the air
density in your manifold is not as dense as it could be.
Fitting a bigger carb allows the manifold air density to be higher at WOT
because, for a given air flow, there is less pressure drop across the carb. So
a 650 CFM carb may still only be flowing say 450 CFM, but the manifold air
density is higher, giving more power. The trade off is in low RPM driveability
and response. Only you can make that choice.
1650 lbs?? what you got, a Westfield? Dax rush????
If you want to sell your Offy, let me know. I fancy fitting a blower and it
might come in handy.
Adrian, Halifax, West Yorkshire
---- you wrote:
> Thanks for that info.
>
> Do you have any opinions on whether a Huffaker is OK for an engine
> like mine ?
>
> 3.5 litre +.020, ported heads on standard vitesse valves, Viper Typhoon
> cam (peaks at 6000rpm), Holley 390 on an Offenhauser 360.
>
> I am looking for more power and had considered a Huffaker with a 465cfm. Also
> considered fitting the largest valves that would fit on standard seats but
> was concerned about valve shrouding...
>
> Good luck with the 4.6!
>
> Ian
> (1650lbs + 200HP = 13.2@103 on street tyres, but looking for 12s)
> --
> "Just pass the work I assign you along to somebody else and trust to luck.
> We call that delegation of responsibility." -- Joseph Heller
>
> Ian Crocker ian@btcase.bt.co.uk http://www.geocities.com/MotorCity/9190/
>
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