A Controversial K&N Filters are Sh!t

[oldtuckunder]

So we take a 3.5 Rover V8 with a Webber 500/Holley carb and one of the big pancake filters that sits on the carb in the V above the engine. Its a good K&N one and appears to work fine apart from on hot days when all the thing is doing is sucking hot air from inside the Wedges engine compartment, not in anyway ideal for good performance.

 

So we get a nice plenum from one of the V8 tuning specialists to sit on the carb, a length of 3" duct to run to in front of the radiator and a nice big Cone K&N filter to go on the end of the duct.

 

Starts up nicely, cruises nicely, plant your foot in the floor and at about 3.5/4K rpm it starts to stumble unless you ease off.

 

Bugger this isn't good what's the problem. Quick test reinstall original filter, problem goes away, so something in new set up is choking engine.

 

First suspect is the new smaller plenum chamber, so just fit that with out duct and filter, works perfectly.

 

OK must be the meter long ducting just wont flow enough air, so fit plenum and duct, works perfectly.

 

Fit K&N filter to duct, problem comes back!

 

OK so a few measurements show that the original pancake K&N has a surface area of around 110 sqin and the new big round one has a surface area of about 85-90 sq in. This is just visible measured surface area not including all the convolutions which must at least double the total area.

 

So a 3" round duct with a area of approx 7sq in will flow more than enough air, but a massive K&N Cone Filter with a surface area of 85 sq in (and probably double that) just wont flow enough air to keep up! Always wondered why those V8 pancakes were so big, thought it was bling factor not that the filters were useless.

 

So going to make my own long tubular one with a cover of KTG filter foam, and I'll bet it will work a treat.

 

So new useless £45 K&N filter heading for the bin! unless any one want's one.

 

Alan

[RogerH]

Hi Alan,

you cone filter of 85 sq" isn;t all air - it has filter element in there.

The 3" duct is all air. - no/very little restriction.

 

You may need to double up on the cone filters.

 

Roger

[Hamish]

Forget the filters.

 

Real racers dont use filters.

;-)

[oldtuckunder]

 

You may need to double up on the cone filters.

 

 

Hang on this thing has 1.5 times the area of the K&N recommended for a 2.5 TR6, and we are saying that 85-90 sqin (possibly double that area if we take the corrugations) won't flow the same air as 7 sqin pipe.

 

Interesting that the K&N site now has no technical details at all about flow rates etc, the most technical detail you can find is that "they are guaranteed to deliver more horsepower" nothing on their site (which I'm sure there used to be) to size filters to cfm etc.

 

Interesting that their recommendation for a TR8 is two small cone filters, which I just calculated have jointly only 2/3rds the area of the big single one.

 

Alan

[RogerH]

Hi Alan,

the answer could be that the corrugations are not flat to the airstream but on edge for the best part.

Thus the airflow is seriously compromised.

A slow (sub 4000rpm) airstream may well work but a faster airstream simple bungs up.

 

eg - you can see light through a sheet of normal white paper looking at it square onto the page. Turn ot edgewise and you get nothing.

 

Roger

[oldtuckunder]

Hi Roger

 

I know you know way more about airflow than I do, and I hadn't considered that airflow over the surface of the filter could actually slow its passage of air.

 

I had always worked on the theory that induction worked by air rushing from atmospheric (high pressure) to fill the low pressure in the cylinder, but your implying that airflow over the the filter surface may be in effect reducing the atmospheric pressure around the filter, but that when in a container (filter housing) or out of the airflow this effect wouldn't happen.

 

Wish I had a regular paper filter the same size to try in the same place. Actually Know what I will try before we junk the K&N is to mount it in the engine bay and try it, if problem goes away then it would be interesting result. Still wouldn't solve the problem that the last place we wan't to take air from is under the bonnet, but at least might explain the cause of the problem.

 

Alan

[RogerH]

Hi Alan,

I did not say that.

 

The filter will have a restriction due to the filter material. No matter how big the surface area of the filter body the filter material will reduce it.

 

Airflow (for whatever reason) will be allowed through the filter up to a certain level (with all the various pressures working on it)

 

As the air pressures/suction remain fairly constant there will be a point where the filter resistance will be too much to allow all the airflow to do its job.

 

This is proven by you pragmatic approach of removing the filter. 3" diameter is far better than a high quality filter that is x7 bigger.

 

So you need a bigger filter or more of the ones you already have.

 

Roger

 

PS - you need to get hold of the data for the filter litres/min airflow. through the filter compared to that being required by the engine.

Edited August 15, 2018 by RogerH

[ChrisR-4A]

Hi Alan,

I did not say that.

 

The filter will have a restriction due to the filter material. No matter how big the surface area of the filter body the filter material will reduce it.

 

Airflow (for whatever reason) will be allowed through the filter up to a certain level (with all the various pressures working on it)

 

As the air pressures/suction remain fairly constant there will be a point where the filter resistance will be too much to allow all the airflow to do its job.

 

This is proven by you pragmatic approach of removing the filter. 3" diameter is far better than a high quality filter that is x7 bigger.

 

So you need a bigger filter or more of the ones you already have.

 

Roger

 

PS - you need to get hold of the data for the filter litres/min airflow. through the filter compared to that being required by the engine.

Or the airflow of the large pancake one that actually works and compare it to that.

[oldtuckunder]

PS - you need to get hold of the data for the filter litres/min airflow. through the filter compared to that being required by the engine.

 

Unfortunately K&N now provide absolutely zero technical information on any of their filters, other than what size they are. Probably enough to make me avoid them in the future. K&N now seem to think that having a filter that will do a million miles is the most important technical attribute, and any performance claims probably derive from replacing a clogged up old paper. From my results a very expensive piece of bling with no technical merit.

 

Interestingly a couple of TR6 competition cars in our Championship running with K&N's in the original Air filter box, who for the timed runs always disconnect the pipe from the plenum at the filter end, swearing the car always went better. As with all these things there was always a suspicion of superstition, but I'm now starting to believe it.

 

Alan

[ChrisR-4A]

I fitted K&Ns to my 4A about ten years ago and they have easily paid for themselves compared to the oval paper ones I was replacing each year at £16+ a pair..

[Waldi]

Hi Alan,

If you can measure MAP that would help to better understand the issue.

Waldi

[Motorsport Mickey]

Heres an under bonnet photo of my 3.9 Litre Rover V8 engine which I hope was maybe giving more HP than where you are now, notice its fed from the original Rover air filter chamber but it now has 2 x KN filters inside the box. The air feed in was taken from the front of the bumper from either side, a handy Bartol drainpipe ( water and air flow is very similar ) 90 def bend with flow direction marked on it takes the air into the standard plenum. Worked fine up to 7000 revs which is probably more than you want to use ?

 

Roger is correct, its access to flow which is needed, double up on the air filters. Sorry replying from my I phone and cant rotate the photo.

 

Mick Richards

[RobH]

You may find this interesting:

https://ac.els-cdn.com/S2212017316301475/1-s2.0-S2212017316301475-main.pdf?_tid=95e1131b-94ec-4932-b90e-cab25731f5bd&acdnat=1534341002_e9b97f65fc892e1e4c6e6e989939ba39

[oldtuckunder]

I fitted K&Ns to my 4A about ten years ago and they have easily paid for themselves compared to the oval paper ones I was replacing each year at £16+ a pair..

 

Unfortunately in this situation we are looking for effectiveness not cost efficiency.

 

If you can measure MAP that would help to better understand the issue.

 

 

If tha'ts "Mad And Pissed off" then it reading high

Heres an under bonnet photo of my 3.9 Litre Rover V8 engine which I hope was maybe giving more HP than where you are now, notice its fed from the original Rover air filter chamber but it now has 2 x KN filters inside the box. The air feed in was taken from the front of the bumper from either side, a handy Bartol drainpipe ( water and air flow is very similar ) 90 def bend with flow direction marked on it takes the air into the standard plenum. Worked fine up to 7000 revs which is probably more than you want to use ?

 

Roger is correct, its access to flow which is needed, double up on the air filters. Sorry replying from my I phone and cant rotate the photo.

 

Mick Richards

 

Thanks Mick, yes we have a limit of 6K on a 3.5, so not quite the same demand. Plan currently is to take duct from plenum round to infront of radiator (and yes already found the correct drain pipe fittings!) and then create a long transverse round filter tube across the front above the air dam but hidden from sight by the bumper overhang. KTG have nicely worked out the flow rate required and the area required and which of their filter foam grades to use, so when it arrives will construct and test and see what the results are.

 

Alan

[oldtuckunder]

You may find this interesting:

https://ac.els-cdn.com/S2212017316301475/1-s2.0-S2212017316301475-main.pdf?_tid=95e1131b-94ec-4932-b90e-cab25731f5bd&acdnat=1534341002_e9b97f65fc892e1e4c6e6e989939ba39

 

Interesting and whilst one has to add a fudge factor into all "scientific" papers, it basically says K&N are crap at virtually everything K&N claim they are good at except for lasting a long time. Expensive Bling then!

 

Alan

 

PS.That should be ITG above not KTG but no doubt due to fear that we are all going to end up in court soon I am now no longer able to Edit it!

[TR NIALL]

The KNs avalible for the Spitfire 1500 strangle the Power also,the Original Filter flows the Air better.

Edited August 15, 2018 by TR NIALL

[Waldi]

Manifold Air Pressure Alan

[oldtuckunder]

I know Waldi, just joshing can datalog that stuff on the Vitesse, but the most sophisticated measurement available on this TR8, is a Temperature Gauge that always under reads, a Speedo that already allows for 10% + 2 on the speed limit, and a petrol gauge that will dump you somewhere unknown if you venture too far below 1/4!

 

Seat of the pants, if the filter is on its choked!

 

Alan

[Peter Cobbold]

Hang on this thing has 1.5 times the area of the K&N recommended for a 2.5 TR6, and we are saying that 85-90 sqin (possibly double that area if we take the corrugations) won't flow the same air as 7 sqin pipe.

 

Interesting that the K&N site now has no technical details at all about flow rates etc, the most technical detail you can find is that "they are guaranteed to deliver more horsepower" nothing on their site (which I'm sure there used to be) to size filters to cfm etc.

 

Interesting that their recommendation for a TR8 is two small cone filters, which I just calculated have jointly only 2/3rds the area of the big single one.

 

Alan

K&N are a pain for not publishing flow rates or approx hp.

I tried to find the old data but failed.

Peter

[oldtuckunder]

probably too embarrassed

Alan

[ChrisR-4A]

" Unfortunately in this situation we are looking for effectiveness not cost efficiency. "

 

Alan, whilst not disagreing with you I think the point I was trying to make was that if an engine is fitted with the K&Ns which are specified for that engine then they work fine and are not expensive in the long term.

However if you have what is in effect a very customised installation then it is up to you to determine what you need. Having said that it is bad that K&N do not publish sufficient technical information for you to calculate your specific requirements.

[oldtuckunder]

I think the problem may be "who" specified the K&N's for the engine, as Niall pointed out it appears that the K&N replacement for the Spitfire 1500 probably isn't up to the job, as I reported the TR6 replacement looks as though it has similar problems.

 

How many K&N's are specified on the grounds they fit the space available rather than that anyone has actually sized them correctly. And given that it appears that there is no technical data on the current flow rates of K&N filters, or filter material, it can only be done by trial and error, and even if someone did it 20 years ago, would they bother to retest with new K&N filters or just assume that nothing has changed?

 

Of course for the average road car its not very important if they are choked a bit at high rpm's. But it does appear that bolting on a set of K&N's replacements far from releasing horsepower as they claim may actually do the opposite, from what the research paper showed.

 

Alan

[Tom Fremont]

I've run 2-1/2" deep K&Ns on my DCOE'd '250 for 22 years/ 125K miles, cleaning and re-oiling them 3 or 4 times only. No doubt you could graph the change in airflow restriction over time between cleanings; my guess is it would be an integer multiple at WOT high rpm. Publishing values for them is thus fraught with risk for the mfr. It's been years since the last cleaning and I've lost an mpg or 2 so it's next on the list.

 

The ones that make me really skeptical are the trumpet screens - these have to exact a terrible toll at high rpm as they reduce the opening area of a naked trumpet by more than 50% if my eyes don't lie.

 

They tell me that in rainy climes no filters are necessary .

 

Cheers,

Tom

[Nick Jones]

Interesting....... I have a really quite small K & N cone on fitted directly onto the throttle body on my Vitesse, because there isn't room for anything bigger....... Perhaps this is the cause of the loss of enthusiasm at 5.5k rather than the 54mm TB itself..... Easy to check!

 

Less easy to permanently fix though as no, zip, zero room for a bigger one........

 

Nick

 

PS. Didn't David VIzard swear by (rather than at) K & N filters and even do flow tests on them?

Edited August 15, 2018 by Nick Jones

[stephen cooper]

I have x3 K&N filters on my triple webered TR6. They are quite old and dirty. Back to back rolling road sessions with them fitted and then removed showed a difference of less than 2bhp. (car makes 160bhp at the rear wheels for context).

 

Thanks

Edited August 16, 2018 by stephen cooper