Subject: [Fot] Cold Air intake on a Spitfire

From: George Harmuth <[email-masked]>
Date: Wed, 6 Nov 2013 15:14:27 -0500 ZX853FP2HF5|February, 2013) at 11/06/2013 15:14:30

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
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http://www.fot-racing.com

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From: Tony Drews <[email-masked]> us.ibm.com>
Date: Thu, 07 Nov 2013 11:40:49 -0600

I don't see any other responses, so I'll jump in with partial info - 
I can't help with "one hose or two hoses".  However, the closed box / 
open box...  If you do a closed box and it manages to build up 
pressure, it will change the fuel mixture in the carbs.  Most folks 
get cold air to the carbs but have the cold air box / plenum open at 
the back so pressure doesn't build.

I did something of the sort on my TR-4, but don't have any real way 
to measure whether it does any good or not.

Tony Drews

At 02:14 PM 11/6/2013, George Harmuth wrote:
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From: George Harmuth <[email-masked]>
Date: Thu, 7 Nov 2013 13:04:15 -0500 ZX853FP2HF5|February, 2013) at 11/07/2013 13:04:21

Thanks Tony, I did get many replies off list. I noticed the factory air box
has an open slot on the bottom, I assumed it was for make up air if there
was a flow problem ( or to let gas / oil drip out?) but it could very well
be there for pressure relief.

mike

G. Michael Harmuth
Software Engineer