Importance of airbox/intake assembly?

[MrBonus]

My car did not come with the factory airbox backing plate and cover but rather just a pair of K&Ns sitting on top of the Webers.

I've never noticed any issues with how it ran or power delivery but I'm wondering if I'm going to notice any improvement to driveability by adding this assembly?

I ask as it bugs me that the car isn't complete in this sense, but the cost of the airbox backing plate is pretty expensive.

[bitsobrits]

I've run Elans both with and without the trunking and front air filter and have found no practical difference between the two setups. Sure, it's theoretically better to bring in cool air from the nose, and for racing or in really hot climates maybe you would need to do so, but if you can't discern the difference then why bother to spend the money to source the original airbox?

There is also the early racing trick of drilling two 3" holes (covered in screen) in each of the inner fenders to exhaust hot air into the low pressure front wheelwells. Helps greatly to keep underbody temps low.

[baileyman]

I sure would like to see some aerodynamics results on these little bodies. About the wheel wells, when I see cars in the rain, the spray is usually heading away from the wells, not into. And I wonder what the pressure is on the sill in front of a rear tire, the spot where racers put an intake to diff cooler.

Anyway, if I had no airbox, I'd get a big TTR one to make sure 3 and 4 got fed. In the mean time I do have a spare s2 era box. John

[bitsobrits]

I sure would like to see some aerodynamics results on these little bodies. About the wheel wells, when I see cars in the rain, the spray is usually heading away from the wells, not into. John

Yes, spray heading away from the wheel opening is indicative of the low pressure area I mentioned. Holes in the inner arches take advantage of this pressure differential, helping to move more air through the engine compartment: in through the nose, and out through the side vents, as well as down and out the front of the chassis.

[knockoffnut]

I have always run foam filters mounted directly on the carbs as they sound so much better... I agree that there is no practical, noticeable change in power for the road. I did however, cut those two oval (4"x6") holes, one into the left side wheelwell, and the other into the right, near the rear of the bonnet, way back in 1979 when my S3 was a used car, not a collector's item... So maybe the holes counter any issue with trapping cold air. I also have about 145HP - 150HP, an oversized rad laid forward to the bumper, and an electric cooling fan, in the mix on that car.

[Stevie-Heathie]

Isn’t there also the matter of fire prevention to consider, given how much fuel the carbs can spit out through the trumpets? I had understood that was one of the arguments for an air box as opposed to direct mount filters.

Best
Steve

[Andy8421]

Isn’t there also the matter of fire prevention to consider, given how much fuel the carbs can spit out through the trumpets? I had understood that was one of the arguments for an air box as opposed to direct mount filters.

Best
Steve

Indeed. The airflow through a carb on a one choke per cylinder setup is a complex series of pressure waves traveling both toward and away from the inlet valve. At certain rpm, standing waves can form and a cloud of fuel can exit the inlet trumpet. The effect is called 'stand off' and is clearly illustrated here from 1:10 onward:

https://www.youtube.com/watch?v=RSSYcVTUrT8

There have been many threads on this subject. My view is that Colin wasn't one for wasting money. The early pictures of the Elan show a carb mounted air filter, yet the production car had a far more complex and costly arrangement of trunking, airbox and front mounted filter. Colin obviously thought it worthwhile.

[Stevie-Heathie]

Andy, wow that is quite something. An aerosol rather than a “spit”. You definitely don’t want that cloud in your engine bay, or soaking into foam / paper filters either.

[Old English White]

Thru the years, I ve collected a lot of pictures on the web...
I have a full file of wrecked Elan's. Some caught fire. Always from the carburetors side. This is only a pictures file...
In 2015, during our Club Lotus France Festival at Montlhéry, a very good friend of mine came from far with its superb 26 GTS Elan.
After its track session, spend some time queuing, and when he was stopping in paddock I noticed flames thru the front grid.
With 3 extinguisers, we manage to save the car, by getting close and ready arround before removing the bonnet.
The foam air filter worked as a sponge, as usual, and almost desappear, leaving a few black drops.(see pict.)

With an air box, you still can get fire, but clearly not so often/quickly... the coil, ignition harness are so close... if you like gamble...
Christian.

[Andy8421]

For a bit of history, the thread link below has a scan of the original 1500 Elan brochure. Apart from being interesting in itself, page 6 of the scan shows an illustration of the engine bay, and a bolt on carb mounted air filter.

https://lotuselan.net/forums/viewtopic.php?f=19&t=38412

Interesting that the filter links both carbs - this is often ignored on after market carb mounted arrangements. The carbs on the Elan are flexibly mounted, and unless they are linked somehow (by the airbox in a conventional setup), they are free to move relative to each other. The way the linkage works means that this will upset the positioning of the butterflies between the two carbs and is likely to make setting the tickover a continual challenge.

[MrBonus]

Thank you all for your feedback. When I posted this, I was wondering "is this a stupid question?" (and perhaps it is) but I was completely unaware of the fire hazard risks associated with my filter setup, so I'm glad I asked.

In the two years I've owned this car, I am always amazed by the little quirks (like the tension on the water pump belt or how loose the carbs need to be with respect to mounting to the intake) that one might overlook as a ham-fisted home mechanic with limited prior Lotus experience.

[gherlt]

I would like to add that my car (S3, not electric fan due to historic plate restrictions) had K&N air filters on the Webers. In hot weather (30° and +) with slow traffic the car had uneven idle and was a bit difficult to drive.
That went away after mounting the airbox and the air filter in the nose.