Airbox clearance

[stuartgb100]

Finally tracked down an airbox for my Sprint, but never having used one before, I'm surprised how little clearance there is between the lip of the trumpet and the face of the airbox cover (particularly cylinders 4 and 3).

Is this apparent air restriction an issue in any way?

Regards,

Stuart.

[marcfuller]

Possibly. But you need to measure it.

Passini wrote in discussing the free passage of air to the carbs that the "hard and fast rule" is "The distance of the nearest obstruction must be the bore diameter of the carburettors intake away ... the bore diameter of the trumpet, not the diameter of the entry flare is the deciding measurement." (Weber Carburettors: 1. Theory, pg 47)

On the last two Elan S3 air boxes I have restored, I needed to add almost 20mm to the cover's depth to get the clearance for the bore area of the #4 trumpet as specified by Passini.

[berni29]

Hi

On my plus 2 which has a standard airbox there is very little clearance for trumpet 4. Maybe 1cm. The early airboxes are the worse in this respect I think.

Berni

[1964 S1]

There are different length trumpets. My S4 had short ones and a very shallow fiberglass cover. My S1 has a very deep cover, a lot of guys have built up the covers to make them deeper, it sounds like you need to do this or find a deeper cover.

[rgh0]

Based on roling road Dyno tests I have done. The standard air box in place on my S4 Elan resulted in a loss of around 2 to 3 hp compared to removing it. The restriction is in the airbox itself as I also tried it with and without the standard air cleaner and this made no difference.

Making up a deeper airbox is worth the effort if you want those few extra hp.

Rohan

[stuartgb100]

Thanks to all for the advice.

Regards.

[iain.hamlton]

Interesting.
Are there several types of air-box?

On my plus 2 (bitsa-this-bitsa-that, so not worth quoting a year), I think the clearance between the trumpet on the back weber is Ok with the airbox on, because it is quite deep. My challenge is getting the airbox on and off because of the clearance between the trumpet and the pedal box is small. I was seen +2s with over an inch here, with the cover on; mine only clears by 1/4" - 3/8"

Rohan, I am interested in your rolling road run. I have wondered whether you would get more power from less restriction in the filtering, but the warmer air from the engine bay.... I assume the standard set-up is designed to draw cold air from the nosecone. The standard AC aircleaner "looks" restrictive so I have replaced it with a K&N cone, similar to arrangement Cliveyboy sells.


best regards, Iain

[rgh0]

Iain

To my knowledge there were at least 3 versions of the airbox with different numbers of bolts securing the cover to the back plate

1 bolt versions with a curved backplate up to around 66 approx
2 bolt vesions with a square backplate on cars from 67 to 70 approx
3 bolt versions on cars from 71 on approx

The rest of the system in terms of ducting and air filter were the same.

All versions were a similar depth and were too close a fit to the rear trumpets.

While the standard air cleaner looks restrictive in practice it does not appear to affect the engine hp at least up to about 145 hp which was the state of engine tune I had when I did the tests.

The dyno tests were done with an open bonnet so removing the air box was not affected by hot air supply to the carbs. On the road with the bonnet closed you may end up with hot air around the carb trumpets if you remove the airbox and ducting of cold air from the nose. However the way the pedal box, radiator and blanking plates are setup you will still get a flow of cold air up to the carbs through the hole where the ducting used to be so I dont think it is to great a concern. Read a post once on one of the forums where a guy said he had measured the temperature around the carb inlets on the road and found them to stay around ambient temperature. Never measured it myself so cant confirm.

Rohan