K&N filters v trunking and remote filter

[jono]

I took my Plus 2 to Bogg Brothers last week for a session on the rolling road. The engine is mildy tuned QED 360 spec to around 135bhp and is only 6k miles old after a full rebuild.

Dave Bogg is a real gent and a wizard with carbs. I believe he's over 80 now but his enthusiasm is infectious and he clearly still has a love for messing around with old cars and carbs in particular.

One interesting finding is that the trunking and air filter in the nose cone appeared to be causing the engine to run very rich at high RPM (which may explain why it was never willing to rev much past 5.5k.

We pulled off the trunking and the running rich problem disappeared. Whilst the filter itself was a K&N cone 'copy' it did not seem to be blocked or obstructive in itself.

Dave commented that he had experienced a lot of twinks which didn't like the trunking arrangement and recommended a filter directly onto the air box.

Despite having misgivings about the lack of cold feed in this arrangement, I fitted a K&N cone filter (which, with oil, only came to ?33 at my local auto factors) and did a 230 mile navigational rally yesterday with some very fast roads and lots of full throttle overtakes.

The difference is remarkable. The engine is so willing to rev and will easily run to 6.5k before you notice.

It is a little noisier but it a good way and not obtrusively so.

Dave did some re jetting and the car is transformed, expect for the air correction jets needs to go down a size as he only had 135's in stock (and recommends 132's). This 'should' cure a slight stumble on take up.

Has anyone had a similar experience on removing the trunking. The lack of cold air does not seem to be an issue.

Jon

[gjz30075]

So, you basically moved the cone filter from the nose to the entrance to the airbox?

[jono]

Yes Greg, exactly that.

I did use a new K&N cone filter which is better than the one which was in the nose but we demonstrated it was the trunking arrangement which must have been reducing flow at high RPM by simply un-coupling it at the air box and watching the O2 drop on the dyno screen

Jon

[collins_dan]

I have never done a dyno test, but I replaced the stock filter, trunking and airbox with a foam pipercross filter and the difference was remarkable. After a number of years, and outgrowing the boy racer look, I changed back to the stock airbox, trunking and a K&N cone filter as described and was shocked at the difference. I am planning to have some dyno work done on the car and I will have him pull the readings with the stock set-up and with the pipercross and let you know what it reports. Dan

[Stevie-Heathie]

Hi Jon

Congratulations on the mod.

I've always been surprised at the need for the trunking and the filter in the nose. The trunking takes up so much real estate in the cramped engine bay and the whole arrangement seems over complicated.

I would ideally have liked to fit K&N pancakes direct to the webers but of course the airbox is necessary to deal with the unburnt fuel spitting back.

It had never occurred to me to fit a cone filter to the airbox. Best of both worlds?

Please could you post a photo of your engine bay as it is now?

Was it a hot day or a cool day when you had your successful run?

Thanks for sharing.

Steve

[john.p.clegg]

How about a K+N cone filter on a short length of flexi-pipe just to the the radiator ( filter in the cold air ).

John

[Mazzini]

I think I can recall Jimj saying that he similar but reversed experience with an Elan (not Plus 2), whereby air gathered behind the rad was hotter and reduced the power. Cold air from in front of the rad improved the performance.

Apart from the air filter I wonder if the route of the trunking and the trunking material has an effect? I can't remember the path taken by Plus 2 trunking.

Dave tunes my Elan's but we haven't seen the same problem.

[oldchieft]

To get the best of both worlds, have the trunking in place but pointing at the filter on the air box?

Jon the Chief

[The Veg]

The ridges on the inside of the trunking slow down the flow a bit and cause the flow to be not very smooth.

I experienced something similar with my former Esprit. It had a hose from the air metering unit to the inlet of the turbocharger that was made of rubber-coated fabric suspended on a big 'spring' or helical wire that spiraled around for the whole length of the hose. Not a very smooth inside-shape at all, and the fabric between the windings of the wire could contract a bit under acceleration. I replaced this hose with a setup made from aluminium tube and silicone connectors. The difference in throttle-response was quite noticeable, to say nothing of the new setup being much more durable.

[jimj]

Mazzini recalls my having the reverse experience; not exactly. It was just that I noticed a DB5 has a very similar airbox and trunking at Aston Engineering and I asked the question. Brian Jacks, who knows a thing or two, was adamant that on the dyno. they can see several extra BHP with the cooler air drawn from ahead of the radiator.
The ribbed trunking slowing the air is an interesting theory. I`ve often thought that having the filter housing in the nose facing forward would improve air flow at speed, but would you really notice a difference on a road car?
Jim

[jono]

..it's possibly the case, for me at least, that the power loss due to the trunking and filter in nose arrangement was greater than the loss with the filter fixed directly the air box with the attendant sucking in of warmer air.

It certainly goes a lot better though so that will do for me

Jon

[1owner69Elan]

My recent experience with trunking and a remote K&N filter:

On a dyno we found a 7hp drop putting the air box on the engine vs just velocity stacks. From 181hp to 174hp. This drop was consistent whether the box was just open or with trunking and a filter attached. Conclusion was that the trunking and filter in this air box configuration had no effect on performance with the limiting factor being the stock S4/Sprint air box being too small for this engine.

Thus, I had a larger air box fabricated (26R racing version as a model) and intend to attach the trunking and filter to it. Engine still out of the car. No quantitative (or qualitative) results yet.

[jono]

I extended my air box depth by 1.75" to overcome the narrowing near trumpet number 4 when I built the car

[jono]

..photo attached of engine bay with K&N fitted, note I extended the air box to reduce restriction at Nr 4 trumpet

Jon

[rgh0]

My recent experience with trunking and a remote K&N filter:

On a dyno we found a 7hp drop putting the air box on the engine vs just velocity stacks. From 181hp to 174hp. This drop was consistent whether the box was just open or with trunking and a filter attached. Conclusion was that the trunking and filter in this air box configuration had no effect on performance with the limiting factor being the stock S4/Sprint air box being too small for this engine.

Thus, I had a larger air box fabricated (26R racing version as a model) and intend to attach the trunking and filter to it. Engine still out of the car. No quantitative (or qualitative) results yet.

my dyno testing showed very similar outcome.


cheers
Rohan