Air Trumpet Length

[William2]

My car is fitted with standard height air trumpets and I notice that the one fitted to number 4 cylinder is quite close to the airbox cover. I also have found that I have to wind the idle adjustment screw to a setting of 1 1/4 turns compared to the others which are set at 2 3/4 turns to obtain correct Color Tune blue combustion. Could this be because no 4 cylinder struggles more to suck in air? Having done a bit of research on covers, I couldn't see anyone in the UK who manufactures a deeper cover if that is a worthwhile mod. Carbs are standard DHLA 40s'.

[nmauduit]

I would think the air restriction due to trumpet/air box interference is more likely to happen at full throttle rather than idle.

On trumpet clearance, I've read somewhere that the recommended back plate distance should be 1 1/4 times carbs inlet diameter, so for 40s that would be 50mm.

[bill308]

Hi William2,

When you say you have to wind the idle adjustment screw to 1 1/4 turns, are you referring to the idle mixture screw for the number 4 cylinder?

nmauduit,

It may be that air a flow restriction is present at wide full throttle but can't be seen as a Colortune is not suitable for looking at this condition. A Colortune is only suitable for looking at idle and no load, off idle transients, due to construction limitations. Also, one would likely need a dyno to visually observe Colortune results under load, if the device were robust enough to even support operation under this condition.

Bill

[el-saturn]

trumpet length (just an anecdote): my current cylinder head has high lift cams, better springs, steel cups and 40,5mm intake valves, is ported etc... - made in england by a master - once i build the new engine i showed it to OUR swiss master and he (so to say) "allowed" me to use 75mm trumpets as they were more appropriate - can you imagine the smile that put on my face!!!! sandy