Spitfire airbox

[cliff.b]

Just refitted the airbox on my stock 1500 Spitfire & noticed that the air inlet pipes are smaller than the carbs at about 1.25". 

So it got me thinking, is this the correct airbox and if so, why does it not restrict the carbs?

I'm sure there's a simple explanation 🤔

[KevinR]

Firstly, have a read of this thread - 

 

Then get in touch with @spitfireJAGS and do a swap 

[cliff.b]

10 minutes ago, KevinR said:

Firstly, have a read of this thread - 

 

Then get in touch with @spitfireJAGS and do a swap 

Ok, I can see that I have asked a silly question as the outlet holes match the 1.5" carbs, so must be right.

But I'm still wondering why the inlet tubes (that the corrugated pipes fit on) are only 1.25" each?

Why are they not at least the same diameter as the carbs? Is it because you never have maximum flow through both carbs at the same time?

Doesn't really matter, just bugs me when I don't understand something 🙄

 

[Colin Lindsay]

I reckon there was probably sufficient airflow through the pipes at that diameter, and the smaller size may have helped keep dirt and other contaminants out when driving. More than likely it was to do with the 'looks' of the unit, a better proportion with the final dimension.

[Pete Lewis]

to get rid of the corrugated tubes we made up some stock plastic drain pipe for  the Vit6  

better diameter and less turbulence

 

[johny]

I think Triumph probably just wanted to use the same pipes as the earlier models with smaller carbs😄 Its not ideal as there will be a pressure drop and reduction in flow at full throttle openings but not enough to worry about. Obviously the best would be open bell mouths bolted directly to the carbs...

[Pete Lewis]

but youre best  to keep cool air for the intake not hot under bonnet air from stubs or pancakes 

or youre heading for mixture /running problems when in traffic jams  etc 

Pete

 

[johny]

If youve gone to open bell mouths youre not going to be sitting in traffic jams but driving full bore on a race track🤣 

[cliff.b]

17 minutes ago, Pete Lewis said:

to get rid of the corrugated tubes we made up some stock plastic drain pipe for  the Vit6  

better diameter and less turbulence

 

Looks like you have put bellmouths on them as well. Did all this make any noticeable difference?

[Patrick Taylor]

So is it worth me replacing the (newly painted!) MkIII Spit airbox from my GT4s 1300? With intake slots in the bottom edge of the airbox, it draws hot air off the exhaust manifold all year round. Which explains the dreadful tickover in  traffic on hot days... Presumably Spitfire drivers suffer the same thing?

[Pete Lewis]

58 minutes ago, cliff.b said:

Did all this make any noticeable difference?

not measurable   on its own   but all the small things add up to something good  she was a flier

a good supply of dense cool air has more molecules of 02 than that of hot expanded air 

Pete

 

[PeteH]

Didn't I see/hear somewhere that some vehicles had "air flaps" which allowed warm(er) air when cold and then went over to full cold air once the engine warmed up? (Audi? Maybe?)

Pete

[johny]

1 hour ago, Mathew said:

Mk4/1500 boxes don't have intake slots at the bottom of the boxes, only the holes for the flex vent pipes that go to the side of the radiator. 

I always assumed the bottom slots were to allow the fuel to run out if the Strombergs float chambers overflowed. Not necessary with SUs I suppose...

[Pete Lewis]

come on  SU flood and overspill the same as any carb can when floats valves are playing up  has nothing to do  with the makers  

and the slots were the only air intake they do not have the cold air tubes at all 

Pete

[NonMember]

1 hour ago, PeteH said:

Didn't I see/hear somewhere that some vehicles had "air flaps" which allowed warm(er) air when cold and then went over to full cold air once the engine warmed up?

Quite common on later carburretor cars - 1980s and 90s

[Pete Lewis]

a thermac flap in the filter   intake  operated by vacuum via  a temperature sensing valve in the filter intake  the flap blends incoming air with heated air from the exhaust to 

keep emissions  more constant  they controlled  the intake to the carb at around a steady 70F ( in old money)

Pete

[johny]

10 minutes ago, Pete Lewis said:

come on  SU flood and overspill the same as any carb can when floats valves are playing up  has nothing to do  with the makers  

and the slots were the only air intake they do not have the cold air tubes at all 

Pete

Never had SUs so I assumed their remote float overflowed elsewhere and not into the air filter housing like Strombergs. Does seem strange that my Vitesse housing has bottom holes and air intake hoses...

[Steve P]

My SU`s just overflow on to the exhaust manifold,or they did before i fitted heat shields,now fuel drips onto the side valances.

S

[johny]

I think that means Strombergs = holes in base of airfilter box and SUs, holes not required, unless somebody has a standard set up to the contrary?

[Peter Truman]

I always thought the holes in the bottom rear of the air filter box on my Vitesse were to assist air flow to the rear Stromberg carb, as the rear carb air flow is badly restricted by the front carb air filter, ie its not like the later Spits that have the inlet pipes joining the air box between the 2 carbs air filters.

Never thought it was to vent overflowing fuel, that was a by product, and does it effectively, you learn something every day!

[johny]

Interesting idea, although would be really poor design from Triumph, however heres an SU airbox with end pipe connections and no 'drain' holes:

REDUCED Triumph Dolomite 1850 Air Box Air Filter Housing | eBay 

[Patrick Taylor]

17 hours ago, PeteH said:

Didn't I see/hear somewhere that some vehicles had "air flaps" which allowed warm(er) air when cold and then went over to full cold air once the engine warmed up? (Audi? Maybe?)

Pete

My 1980 TR7 had that arrangement.