original air cleaner system pressure drop data

[Stagmatt]

In the process of revising some po mods on my Plus 2 I decided to check the losses caused by the original air cleaner arrangement. Here are the data.

I bought the car with two big K&Ns installed. Pretty noisy and drawing hot air. Before reverting to the original air box, duct and filter arrangement I thought it wise to measure the pressure loss at max power before finalizing this. The test procedure was:
- estimate air flow at max hp
- set my DIY flow bench to produce that figure (flow measurement by orifice and pressure sensor)
- record the loss across the original filter box with new standard filter (MANN C1833)

Max. airflow I guesstimated: 100% charge at 6000rpm = 100/s x 1/2 x 1.6 litres = 60 l/s. Seems plausible to me. My flowbench is powered by two vac cleaner turbines, yielding a bit more of that figure. If interested in details, drop me a pm.

Results:
A - filter element installed in filter top without bottom half of housing: 21mm of water = .21 kPa
B - complete filter housing WITHOUT filter element: 46mm = .46 kPa
C - housing complete with top AND element installed: 58mm = .58 kPa

System A

System C

System B

The big duct from the box to the filter housing will also contribute, but certainly very little.

See photos of the 3 setups. Sorry, I was unable to rotate the views. Uploading photos is a bit of a pain.

I was surprised by the very low loss of the filter alone. The dominant resistance comes from the resonator pipes on the housing. This is a Helmholtz low frequency filter. But even the complete filter produces less than 1 percent loss at max hp. OK for me. No need for K&N.

Cheers, Matthias

[berni29]

Hello

Very interesting and not surprise me as such.

Thank you for posting!

Berni

[StressCraxx]

Very nice work. Have you done any testing on any of the airboxes? I have been told the airbox takes about 5-7HP on the dyno due to losses at the #3 & 4 cylinders with the early Weber airbox and backplate.

[Andy8421]

Mathius,

A couple of points, the air stalled in front of the radiator at speed leads to a pressure increase relative to the rest of the engine bay, and the air is much colder than air that has passed through the radiator. Both of these effects would suggest that picking up inlet air from this area may compensate for losses due to trunking and filter, and could potentially lead to a pick up in performance over the 'two K&N' solution. I am not sure if it is a racing requirement, but most Elans I see at circuits this standard arrangement (albeit non standard filter and in some cases a larger airbox).

The comment above about the back two cylinders has been mentioned by others, but seems more to do with the standing waves from the carb inlets being constrained by the airbox, than airflow issues per se. I can't find it now, but someone posted a 'rule of thumb' about the distance the airbox should be away from trumpet entry to avoid these issues, and the standard airbox was too close.

For maximum performance, a front mounted free flowing filter (Tony Thompson recommend an ITG cone filter), trunking and larger airbox seem to be the best choice.

edit: Larger airbox on the Elan I race(d).

Andy.

[Donels]

Hi Mathias, a great piece of work. I have not seen science applied to the intake filter before. As others note cone type filters are often quoted as ‘being better’ it would be really good to test one with your rig for comparison purposes. I wonder if anyone will lend you one to test?

[rgh0]

Many years ago I did a series of dyno runs in a developed engine to determine the impact of the filter and airbox. There was no measurable difference in maximum power at around 160 hp with or without the standard air filter versus a open duct in the nose. Removing the air box increase power by 3 to 4 hp due to eliminating the low clearance of cylinder 4 inlet. My rule of thumb is you need around 1.5 times the inlet diameter above the air horn end to avoid this restriction

cheers
Rohan

[StressCraxx]

Many thanks gents. I really appreciate your work and advice.

[Stagmatt]

Thanks all for your replies/additions/comments.
My response of yesterday obviously did not make it to the forum. So here again:

@ StressCraxx and Donels: happy to help with my flow bench. That would mean shipping bits to Germany and back. Tedious, but no problem if desired.

@ Andy: yes, this seems to be the agreed wisdom. One can estimate the dynamic pressure in the nose, but only under the assumption of a closed volume which obviously it is not. Measuring the pressure in the nose at speed would be easier, but that would need to be versus the exhaust outlet, not the engine bay. Interesting exercise, but then there is nothing one could do for improvement.
Greetings to Surrey! I spent two happy terms at Surrey Uni when I was young.
Very nice bay area! Do you take the vent duct from the cam cover to a catch bottle and then out to atmosphere?

@ Rohan: the bigger collector on the carbs is certainly helpful and a simple mod. It also occurred to me that combining adjacent cyls. in one filter housing, as is necessary with the K&Ns might also cause interference, all the more as their pulses occur unevenly. One more disadvantage of filters directly on carb.
My Plus2 is waiting here for a test drive...

Enjoy your weekends -
Matthias

[cliveyboy]

If you are saying the original cannister has little effect on restriction of air flow that would go against my experience.
When I fitted the K&N cone filter onto my Sprint to replace the original cannister.
Whilst no noticeable increase in power etc the engine certainly revved a bit better and cleaner
Clive

[Stagmatt]

Just a quick note to Clive:

If you read my original post you will see that I am saying the same as you. The original canister is restrictive (but not much), due to the silencer arrangement with the two rather small pipes. The original filter element itself is fine. So by putting on an open filter w/o housing you remove that restriction.

Cheers, Matthias

[rgh0]

@ Rohan: the bigger collector on the carbs is certainly helpful and a simple mod. It also occurred to me that combining adjacent cyls. in one filter housing, as is necessary with the K&Ns might also cause interference, all the more as their pulses occur unevenly. One more disadvantage of filters directly on carb.
My Plus2 is waiting here for a test drive...

Enjoy your weekends -
Matthias

Hi Matthais
I arrived in Germany last week. So we must make a time to catch up

cheers
Rohan