[Fot] Cold Air intake on a Spitfire

[George Harmuth]

Hi all, I'm planning some off season work on the racing Spit (1296 with
SUs, H2 or H4 depending on group I'm running with) and I was wondering what
the group think was on cold air induction. Currently, I have nothing, it's
using the under bonnet air (K&Ns with short velocity stacks inside) so I'm
assuming I'll pick up some "free" power with outside air.

I have a couple of design decisions to make, please feel free to advise me.

1. Is a single 3" hose, fed from a NACA duct mounted on the (cool) side of
the radiator duct going to move enough air? Do I need a double hose? SCCA
class rules prohibit me from using the headlight opening as an air intake
unless I move up a class to FP, or switch to an 1147 engine, not worth it.
I can fit a double hose NACA port there but it will be tight.

2. Closed air box or pipe it near the filters? I have a couple of factory
air cleaners (for both size carbs) with double hose intakes I was thinking
of using as a starting point for a closed air box. SCCA rules make me keep
the inner fender wells so there's not a lot of depth available. Are the
factory air boxes too restrictive to pass enough air at 7500 RPM? If I want
to keep my current filter set up, will I still see a gain by just ending
the duct work underneath and between the carbs? The open setup has an
advantage of access for tweaking the carbs and an alternate path for air to
get in if something blocks the NACA duct.

Any other ideas, suggestions?

thanks
mike h
63 Spitfire


G. Michael Harmuth
Software Engineer
Automated Material Handling Equipment Engineering Services