Best Air Filters

Brooking 850 · Feb 28, 2017

Having had a pair of PWK's on for a few years using a series of air filters, I decided to modify the 'hamcan ' original to suit these after market carbs (there is a thread on here where a forum member has done the same)
I have had a great success with the K&N clamp on filter RC-2370 , and thats my go to filter if the 'hamcan' mod fails.
So I cut down the perforated outer gauze and spot welded it into the front plate

I then cut down a new original paper element to suit

Then used some stick on door seal strip to the backing plate to seal the paper element and stop it collapsing when in use

Then I fitted a set of rubber carb boots that are original Norton items to the carbs

Then mounted all on the bike and took it for a 500km (300 mile ) test ride

I took a route that is familiar to me, twisty country roads so I had a good idea where I would be needing fuel.
Lo and behold I hit reserve well before my regular fuel stop. Carried on the ride realising I was running way too rich and once at home did the figures.
Over 300 miles I averaged 10 mpg LESS (imperial gallons) than with the K&N filter which is way smaller than a 'hamcan' original paper element.

Now to find the ideal size K&N element to fit the 'hamcan' and test again

lazyeye6 · Feb 28, 2017

I saw a post a couple of years ago wherein there was a link to a comparison of air filters in regards to their ability to actually filter out micro debris. As I recall, K&N filters ranked very low. It would seem logical that they breath better and result in better gas mileage.. The question is, "at what cost"?

ashman · Feb 28, 2017

I ran K&N filters for over 37 years without any problems at all, clean them and reoil them by the book and they work fine and my Norton has long life on it well over 140k and only been reboared once in 41 years of ownership.

Ashley

Brooking 850 · Feb 28, 2017

I think if I ran the same test with a set of Amals, I would get a different result, it seems the PWk's could be very sensitive to filter types.
When I fitted them originally they came with JS EMGO conical filters , JS did some research and found that they were way too small and I know from my experience , made the carbs run way too rich. His site now offers the RC 2370 items, although for a different model of carb.
I may well fit the K&N inside the 'hamcan', just have to enlarge the front plate holes.
Work in progress
Regards Mike

gortnipper · Feb 28, 2017

looking-for-better-aircleaners-for-flatslide-carbs-t18641-15.html#p237007

comnoz said:
So JS sent air filters for testing. Here is the test setup. 34mm

Here are the air filters I tested at JS request. All were tested dry except the K&N"s which were pre oiled.

From left to right.
First -Open carb- no air filter --------------------------------------- 206.5 CFM
#1 dry unifilter -half taped up to simulate a short filter --------181.6 CFM
#2 dry unifilter - full length--------------------------------------------181.9 CFM
#3 Taiwan K&N copy -dry #150154-----------------------------------181.2 CFM
#4 Emgo K&N copy - dry #150035-------------------------------------178.3 CFM
#5 XS K&N copy -dry----------------------------------------------------195.2 CFM
#6 K&N dual filter -one side taped up-oiled------------------------163.7 CFM
#7 same as above with no tape---------------------------------------195.2 CFM
#8 stock paper filter -non baffled backplate------------------------204.5 CFM

comnoz said:
Here are some flow rates I tested previously for comparison.

FCR 35 with velocity stack-------------------------------228.3 CFM
FCR 35 with small CNW K&N -oiled----------------------206.6 CFM
Mikuni vm34 -open or velocity stack, no change----194.5 CFM
Mikuni vm32 -open or velocity stack, no change----190.8 CFM
Amal 30mm concentric w/offset velocity stack------175.9 CFM
Amal 32mm concentric w/offset velocity stack------188.8 CFM

The Amals loose 15% of flow without the velocity stack. Jim

comnoz said:
gortnipper said:

Jim(s) - I would be interested in what the optimal flow amount into RH10 and/or Fullauto heads is? Ie, at what cfm do you begin to see the flow from the filter/carb combo begin to "choke" the perferformance? Or, are all of these filters flowing at sufficient rates?

Click to expand...

The Xs filter above does flow sufficient air.

Any flow restriction will reduce the amount of airflow through the port. With a port that is capable of flowing 150 CFM it takes a throttle body or carb close to 50mm with a velocity stack before it becomes transparent to the port flow.

A carb that large would not deliver fuel since there must be pressure drop over the carb to draw fuel from the bowl.

I will be using two 42mm throttle bodies on my fuel injected bike to keep the port flow up near 150. Jim

gortnipper · Feb 28, 2017

Based on the above, with using a larger K&N -0990 dual filter, I think that would approach the XS filters' flow.

Brooking 850 · Feb 28, 2017

I had forgotten about those tests, wonder why my paper filter makes it run so rich then?
Its brand new so should be in a healthy condition!
Regards Mike

xbacksideslider · Feb 28, 2017

Good stuff, since I'm running the PWKs too, thanks.

I did a similar thing to get the Amal offset velocity stacks to fit on 932s and inside the ham can with a ham can K&N filter.

Maybe its rich because the air filter is cut down?

Brooking 850 · Feb 28, 2017

Could be although you would imagine 13mm (close to 1/2") wouldnt make that much difference, thats how much I cut off.
I would like to keep the front plate original, so reluctant to modify it to accommodate the K&N filter rubber boots.
Might make another or locate a second hand one.
Any donors out there?
Regards Mike

kerinorton · Feb 28, 2017

mike, i am running the std 32 mm carbs and ham can on my 850. I got slightly better fuel economy [ 7% roughly ] than Blaizes on his 850, and slightly worse than hon Pressies 850 with the single Mikuni. Ie $8.20 per fill compared to $7.80. nearly 60 mpg @ 70 clicks. Bike has pleanty of gwet up an go as well.
DERECK

PS still haven't had to adjust the rear chain yet after 5000 miles. Got the wife to sit on the bike while I checked the play. Its right on the button.

gortnipper · Feb 28, 2017

If this is for your streetbike, then the Amal flow should be 188.8 x .85 = 160.5 cfm. with no velocity stack.

Seeing that the KN RC-2380 is:

Height 1.75 in (44 mm)
Outside Length 6.25 in (159 mm)
Outside Width 4 in (102 mm)

It flows 95% of the bare Amal carb. And, I think it was used by CNW because it was shorter to fit the FCRs where it flowed at 90% of the bare carb.

And the R-0990 is:

Height 3 in (76 mm)
Outside Length 6.125 in (156 mm)
Outside Width 3.875 in (98 mm)

If we do some overly simple maths, the 2380 has 61% of the surface area of the 0990, and it should flow at least 152.5 cfm..

The R-0990 should flow much better than the 2380.

Nater_Potater · Mar 1, 2017

I wonder if you're getting into the realm of turbulent flow at the carb's inlets due to the closer proximity of the backing plate. As the air pushes into the void at the mouth of the carb, it tends to "bunch up" as it comes in from all points of the compass, essentially running into the air coming in from 180' opposite. 2X the inlet diameter sticks in my mind as a minimum in this situation. You could try fashioning a pair of cones to attach to the back plate to help direct the airflow into the throats. Check out how a Braswell air filter is constructed, and you can see where the area around the center hold down stud helps bend the airflow into the carb throats.

Brooking 850 · Mar 1, 2017

I have some ultra fine stainless steel gauze that I will line the 'hamcan' with instead of the paper element and give that a test
Mike

BritTwit · Mar 1, 2017

Nater_Potater said:
I wonder if you're getting into the realm of turbulent flow at the carb's inlets due to the closer proximity of the backing plate. As the air pushes into the void at the mouth of the carb, it tends to "bunch up" as it comes in from all points of the compass, essentially running into the air coming in from 180' opposite. 2X the inlet diameter sticks in my mind as a minimum in this situation. You could try fashioning a pair of cones to attach to the back plate to help direct the airflow into the throats. Check out how a Braswell air filter is constructed, and you can see where the area around the center hold down stud helps bend the airflow into the carb throats.

That's a great concept for an airbox design.
The vortex effect will help organize and direct the airflow into the mouth of the carb.
The old baffled airbox was an attempt by Norton to organize the airflow arriving in the airbox from all directions.
The baffle approach had its own restrictions, but the Braswell design is very interesting.

kerinorton · Mar 1, 2017

I wonder how the braswell design will work with 2 carbs rather then the single entry Holley.

Why not just use trumpets for racing like is usually done. Street bikes dont need to perform that well. Especially our with the low down grunt the engines develop.

gortnipper · Mar 1, 2017

xbacksideslider said:
I'm feeling pretty smart about my old set up - twin 32 Amals with offset velocity stacks into the ham can. Stock rubber bellows reversed, on the inside instead of the outside of the ham can; sealed the ID of the bellows with glue against the outside of the stacks' bells. The set screws on those offset velocity stacks were a joke, so I JB Welded the stacks to the carb bodies.

Brooking 850 · Mar 1, 2017

Thats clever, I may look at something like that. The PWK's have a bigger OD at the mouth of the carb than the Amals.
Regards Mike

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