Carburretter Air Intake Box

[Donels]

An article from Motor Sport Magazine on Ron Tauranac talking about the 1971 BT34 FI car:-

“I’m pretty sure this was the first time I’d used an air-box to feed the engine inlets. The intakes were either side of the rollbar because I reckoned you’d pick up more flow there than above the driver’s head, because of deflection of the air by his helmet and the windscreen. You get two effects from the airbox: the first is some supercharging ram. Also, if you ever watch an engine on the dyno there’s some fuel mist standing off the inlet trumpets by about 75mm. If you put a surface parallel to the trumpets a short distance away, it returns that mist and you can control a bit where in the rev range you get your power by adjusting how close it is. Without the airbox that mist just blows away into atmosphere, so you also save petrol. We didn’t have a dyno facility to play with that spacing, but since then there’s been a lot of development on this, particularly in F1 .”

I wonder if the twin cam air intake is designed to take this into account, hence why it appears to be too close to the trumpets?
Rohan we need you!

[StressCraxx]

There were several versions of the airbox. I have only experience with the early one. It's adequate for a std or SE engine. The airbox distance is too close to the #4 cylinder and costs 3-5 HP on a twincam with sprint cams and higher compression and the larger chokes.

The whole intake needs to be considered as a system, along with the exhaust to the tailpipe. An engine is in essence an air pump. The more air you can pack into the cylinder and extract out of the cylinder to the tailpipe is the key. Everything has to work together to optimize performance.

The hose and the air cleaner ahead of the radiator also affect performance. The stock arrangement was mostly for cost and packaging considerations. I cut two 2" diameter holes in the bottom of the outer air cleaner housing because the stock air cleaner really impacted higher RPM. The corrugated hose from the air cleaner was also restrictive. The stock S1-through S3 cast iron manifold and stock exhaust is also very restrictive.

Bottom line for my car was I would remove the airbox cover and run without it when I autocrossed or ran a track day years ago. When I installed the sprint cams, 34mm chokes in the Webers, I installed the Dave Bean street header with the 4 into 2 into 1 collector and the least restrictive silencer I could find without annoying my neighbors. The engine made 100HP at the rear wheels with good torque from 2500RPM on up to redline.

[gjz30075]

The stock arrangement was mostly for cost and packaging considerations. I cut two 2" diameter holes in the bottom of the outer air cleaner housing because the stock air cleaner really impacted higher RPM.

What type of element do you run in the outer air cleaner housing? If K&N, do you have a part #?
Thanks

[StressCraxx]

At the time, I couldn't find a K&N for it. I used a Fram filter for a Fiat 850 Spyder. Later on, I stacked two of them and used a barrel nut and an additional stud, locktited together with the wing nut holding it together.

I have seen a larger 3-1/2" K&N cone filter used, but I have no experience with it. I do use K&N filters on my Formula Ford and my Lotus 23 with twin 45's and they work great.

[rgh0]

There were several versions of the airbox. I have only experience with the early one. It's adequate for a std or SE engine. The airbox distance is too close to the #4 cylinder and costs 3-5 HP on a twincam with sprint cams and higher compression and the larger chokes.

The whole intake needs to be considered as a system, along with the exhaust to the tailpipe. An engine is in essence an air pump. The more air you can pack into the cylinder and extract out of the cylinder to the tailpipe is the key. Everything has to work together to optimize performance.

The hose and the air cleaner ahead of the radiator also affect performance. The stock arrangement was mostly for cost and packaging considerations. I cut two 2" diameter holes in the bottom of the outer air cleaner housing because the stock air cleaner really impacted higher RPM. The corrugated hose from the air cleaner was also restrictive. The stock S1-through S3 cast iron manifold and stock exhaust is also very restrictive.

Bottom line for my car was I would remove the airbox cover and run without it when I autocrossed or ran a track day years ago. When I installed the sprint cams, 34mm chokes in the Webers, I installed the Dave Bean street header with the 4 into 2 into 1 collector and the least restrictive silencer I could find without annoying my neighbors. The engine made 100HP at the rear wheels with good torque from 2500RPM on up to redline.

I saw the same 5 HP loss in a 150 hp enhine I built and dynoed many years ago. This is due to the taper on the airbox depth restricting flow to the rear cylinders. All the standard air box versions ( 1 bolt , 2 bolt , 3 bolt, Elan and Plus 2) have this taper. I found the standard aircleaner on my S4 irself was not restrictive versus the airbox. and when the aircleaner was removed and just sucting through the straight duct it made no difference on the dyno.

I am sure there is some optimum airbox volume and dimensions for a particular engine and state of tunebut what is is would require a lot of work to find it. There is also limited room in an Elan to experiment with different size airboxes. I have run foam socks on my Elan for many years without issues. i have had one small engine fire about 35 years ago back when i was running the standard airbox and fuel collected in it due to a loose venturie in the 40 DCOEs from a weak spring clip and the fuel dripped out the bottom and ignited.

cheers
Rohan

[prezoom]

After doing extensive dyno work, what we found out was one needs is one and one half times the diameter of the carbs through bore, from the end of the air horn to the inside of the air box. So, with a DCOE 40, 60mm is the minimum number. Anything closer to the air horn will decrease horsepower.

[JonB]

There's no taper on the Stromberg air box, as fitted to my S4. I wonder why...