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Jesper's Avatar
 
Join Date: May 2006
Location: Denmark
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KN filters wich one?

Hello mates

On my 1976 930, I have the stock air intake with a KN element. I was wondering if there would be any difference in changing it into the “Bolt On” KN filter. Maybe some more space in the engine compartment is all? I have the Bolt On filters on my 911T and I am very happy for them. If I should change the air intake, what should I buy, and where to buy, and who has a spare?

Best Regards
Jesper

My stock:



My 911T




Should I buy this type



Or maybe this

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1969 911T "The White Lady" Stainless DANSK
1976 930 "The Phantom" RarlyL8

aka The Beauty and the Beast
Old 12-28-2009, 07:28 AM
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There's too many opinions you'll get from this question on an internet message board to get an accurate answer.

Bigger air filter with more surface area will always flow better and easier with a given intake vacuum and if you modify the motor for more power the little ones are definately too small and they can actually cause a turbocharger to suck oil past the compressor side piston ring oil seal, and once that starts it does not stop.
If you don't believe me and I don't expect you too, just call any professional turbocharger rebuilder like Durabilt turbo at 800-346-8641 and ask them the same question.

From some people's experience you can get by with a little air filter for a while if your motor is stock and the turbo seals are in really good shape but I wouldn't promote or advise doing that.
And you can not group all available small air filters together because some of them are made by K&N, some are inferior K&N copies that don't flow as much but look similar, some are taller and some are oval giving more pleated surface area, and some are paper.

I had a small K&N copy air filter on mine for a while only so I could get at my Brian Leask adjustable WUR to remove it over and over again while tuning around an overly rich flowtech modified USA fuel head without having to remove the intercooler and OEM air cleaner housing to get at the WUR every time.

When I was finally satisfied with my WUR settings I put the origonal air cleaner housing with K&N element back on and on my first boost run I got higher boost pressure than with the small air filter. The needle on the gauge went from a steady 1 bar to halfway between 1 and 1.1bar.
I have the 2bar VDO/NHS mechanical boost gauge where the origonal clock was so the readings up around 1bar are accurate.
Also have a manual boost control valve in the wastegate pressure hose so I turned it down so maximum boost is about 1bar after that.

My car is not stock though. It's got 964 cams, IA modified fuel head, B.L. adjustable WUR, Garretson longneck halfbay intercooler, B&B headers and B&B muffler with some baffles removed,
I have a twice rebuilt and upgraded K27 HFS turbo I have used before sitting in a box, while a tough and reliable factory built borg warner K27 7006 is currently on the car.


The bottom air filter pictured sometimes called a "powerflow" will flow best and you can make it flow even more air easier by putting a little bit longer cone filter on it than the one pictured with more surface area that will still fit in there without hitting the right side hood hinge.

In this turbo failure diagnosis chart a small, faulty, or obstructive air cleaner is listed twice as possible cause for turbo bearing failure because the turbo will overspin under boost while trying to suck air through the small restrictive lawnmower sized air cleaner element and overheat/cook the turbo shaft bearings, thrust bearing, and oil while doing it.
Failure-Diagnosis

You can also call or email Bob at Durabilt turbo and let him explain it to you.
http://www.durabilt-turbo.com/
Old 12-28-2009, 10:11 AM
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Further to the quality info above, I did back to back dyno runs with the "powerflow" (you have it pictured) versus the stock airbox with a KN filter. I got more HP using the stock airbox. After that, I drilled 6x1 inch holes in the bottom of the stock airbox to increase airflow. The engineers at Porsche knew what they we doing when designing all the components to work together. Also, my motor was highly modified (CIS) too.

I also serviced my KN filter each time I changed the oil.

Do a search on this subject as there are TONS of posts.
Old 12-28-2009, 11:18 AM
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JFairman and DonE, thank you so much for your very detailed explanations. Of course, I believe what you are telling me, I’ am very thankful. Sounds like I will keep the stock air box with the KN element. On my 911T engine, the HP was increased by 13 HP with the Bolt On kit. However, I know there is a big difference between the two types of engines.

Thanks.

Best Regards
Jesper
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1969 911T "The White Lady" Stainless DANSK
1976 930 "The Phantom" RarlyL8

aka The Beauty and the Beast
Old 12-28-2009, 11:57 AM
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Jim and Don are referencing high output CIS engines. Completely different animal.
A stockish engine with non hi-flow turbo will not see these issues as they do not require the flow. It is also good to note that these small round filters come in different sizes.

If you measure the surface area of the small round filter and compare it to the stock flat filter they are similar. I did that years ago when cobbling up that breather but have long lost those figures. Easy enough to do. That round hole on the stock mailbox breather is also a much smaller surface area.

It is best for the filter to have as much surface area as reasonable, but not necessarily harmful if they do not. Different flow needs for different applications. I have used one of these tiny filters for years on my 315WHP engine and have done may dyno runs. I cannot see a significant difference in power nomatter what filter is used. The same UMW K27-7200 has been in place for over a decade with no issue.

Removing the stock air cleaner assembly on your non-intercooled engine is easy so I see no reason to change it unless just for looks. It is always good to error on the side of caution, so if you believe a small filter may harm your engine in any way you should not use one.

As a side note, that picture you posted of the round K&N filter is on a 400WHP CIS engine running a K27HFS at 1bar. It is the larger version of the round filter and has more surface area than the stock flat air filter.
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RarlyL8 Motorsports / M&K Exhaust - 911/930 Exhaust Systems, Turbos, TiAL, CIS Mods/Rebuilds
'78 911SC Widebody, 930 engine, 915 Tranny, K27, SC Cams, RL8 Headers & GT3 Muffler. 350whp @ 0.75bar
Brian B. (256)536-9977 Service@MKExhaust Brian@RarlyL8
Old 12-28-2009, 01:45 PM
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Just to put some numbers to it, Garrett has this to say about inlet restriction:

"Depending upon flow rate, charge air cooler characteristics, piping size, number/quality of the bends, throttle body restriction, etc., the plumbing pressure drop can be estimated. This can be 1 psi or less for a very well designed system. On certain restrictive OEM setups, especially those that have now higher-than-stock airflow levels, the pressure drop can be 4 psi or greater."

Most turbochargers are designed to tolerate as high as 4 psi, as this can occur even in a well designed system with a dirty air filter, without excessive compressor seal leakage.

My stock air filter housing achieves below 2 psi pressure drop (relatively clean air filter), but I have EFI (no CIS flapper).

For turbocharged engines, the inlet pressure drop has a much smaller influence on power than naturally aspirated engines. The biggest enemy for our engines is inlet air temperature. It is in this area that the air filter design makes the biggest difference. I found the round filter to be unacceptable because it draws air after it has been heated by the intercooler, regardless of the filter size. For intercoolers that do not take up the full width, I think you will find that drawing air from near the stock location is better than from behind/beneath the intercooler.
Old 12-28-2009, 06:42 PM
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If the option is to vent fresh air to the filter or to the intercooler then vent to the intercooler. Decreasing charged air temperature is the job of the intercooler.

If you have the option to do both this is what I did. I built a shroud enclosing the intercooler with the back left open so that the fan also pulls air right through the area where my little filter sits. I could lay my hand on it at idle and feel the air pulling through the intercooler and the open vent for the air filter. I later added louvers to the other side for use in the hot Alabama summers.

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RarlyL8 Motorsports / M&K Exhaust - 911/930 Exhaust Systems, Turbos, TiAL, CIS Mods/Rebuilds
'78 911SC Widebody, 930 engine, 915 Tranny, K27, SC Cams, RL8 Headers & GT3 Muffler. 350whp @ 0.75bar
Brian B. (256)536-9977 Service@MKExhaust Brian@RarlyL8
Old 12-28-2009, 07:07 PM
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If air goes into the compressor 10 degrees hotter, it comes out 10 degrees hotter, so the cooling the charge air is the "job" for the whole system, not just the intercooler. IMO your arrangement compromises the engine inlet and engine fin temps to get higher flow through the intercooler ambient side at low speeds. I would rather have access to as much outside air as possible. The engine cyl head temps must get quite high if all the air gets pre-heated by the intercooler first.
Old 12-28-2009, 08:15 PM
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Quote:
If air goes into the compressor 10 degrees hotter, it comes out 10 degrees hotter
I understand your thoughts but that statement is incorrect. I went to a professor of automotive engineering for the U of I when designing the above setup. Same guy that designed John Force's dragster rear wing and the same guy that designed the Indy F1 track and also holds a land speed record in the steamliner class. He took me to school on all of this.

If intercooler efficiency is not sufficient to offset inlet temperatures you need a better intercooler.

My wing is a GT2 which has air inlets on both sides. These inlets allow for any residual flow need at high RPM and also create a ram-air effect at speed.

The above setup produced a very significant increase in power by allowing a controlled amount of air to go around the intercooler at low speed and including ram-air at high speed. I've used this setup for a lot of years with no issue.

There are many threads on this subject and I don't want to hijack this one.
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RarlyL8 Motorsports / M&K Exhaust - 911/930 Exhaust Systems, Turbos, TiAL, CIS Mods/Rebuilds
'78 911SC Widebody, 930 engine, 915 Tranny, K27, SC Cams, RL8 Headers & GT3 Muffler. 350whp @ 0.75bar
Brian B. (256)536-9977 Service@MKExhaust Brian@RarlyL8
Old 12-29-2009, 04:11 AM
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At 1.7 pressure ratio, if you go up on the inlet temp by 10 C you will go up on the outlet temp of the compressor by about the same amount.

I think that you are claiming that in effect you have so much heat rejection capacity that it can accomodate modest increases in intercooler inlet temperature (compressor outlet temperature) and still achieve the same intercooler outlet temperature.

If that were the case, you would be achieving ambient temperature at your intercooler outlet, which would indicate that you have "over capacity". Obviously that is a tall order with the space available.

Ignoring the inlet air temperature is terrible advice, for others and yourself. Maybe you misunderstood what you friend was trying.

If you think you are happy now, just get your air filter connected to that blockoff plate, and seal off the back of the intercooler to the wing. You will be died and gone to heaven happy!
Old 12-29-2009, 10:38 AM
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His words were very clear.
It is not that simple. Ten degrees is nothing compared to what a turbocharger driven by 1000* exhaust gas will do.
I also built an air filter housing that was isolated and fed by the GT2 vents. On the street I notice no difference so I took it off as it was a huge PITA to get to anything in the engine bay with those hoses in the way. The GT2 tail is designed for ram air which doesn't do much until you sustain 100+mph.
There are a few folks here on the board who are data nuts and have measured temps in the area of the small filter. It's not significantly different than anywhere else in the engine bay.
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RarlyL8 Motorsports / M&K Exhaust - 911/930 Exhaust Systems, Turbos, TiAL, CIS Mods/Rebuilds
'78 911SC Widebody, 930 engine, 915 Tranny, K27, SC Cams, RL8 Headers & GT3 Muffler. 350whp @ 0.75bar
Brian B. (256)536-9977 Service@MKExhaust Brian@RarlyL8
Old 12-29-2009, 01:59 PM
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Dear Jasper,

do you mind to tell me where the pipe (indicated by red arrows) is going to?

Thanks Christian

Old 06-10-2014, 04:36 AM
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I went with the K&N RU-5163. I did a few mods to it. I cut the rubber lip off the bottom, drilled 2 holes through the top and used long 6mm bolts going to the stock holes. Had to crush a small section of the air cleaner to clear the adjustment screw. Also mounted a L fitting to the top for the WUR hose.
For the i/c I made an aluminum frame with a rubber lip that slightly presses into the grill. I figure this would at least keep the air going into the i/c instead of washing over the top and down the sides. Now if it actually does anything, I don't know.
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Old 06-10-2014, 05:59 AM
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I'm going to try to post a picture of my tiny air filter that flowed enough to allow 21psi/1.4 bar of boost and nearly 129 MPH in a 1/4 mile.

It has a surface area is 2.5" tall, tapered cone with a 6" total OD, like 4.5" in the smaller OD.
Old 06-10-2014, 07:33 AM
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Jesper your air box looks metal...isn't the correct one plastic? Like in this link Porsche 930 Air Box Plastic | eBay
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Old 06-10-2014, 03:46 PM
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Hope fully that he has not been waiting 5 years for todays answers
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Old 06-10-2014, 04:03 PM
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Quote:
Originally Posted by JJ 911SC View Post
Hope fully that he has not been waiting 5 years for todays answers
Lmao..I didn't notice that...
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Old 06-10-2014, 05:22 PM
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Quote:
Originally Posted by speednme1 View Post
Lmao..I didn't notice that...
You know what, that is what is great about this forum, it doesn't matter.

This morning Wayne (Mr PP himself) answer one that was 2 years old. I think the longest revived thread I have seen was 9 years old and it was still current for us, Air Cooled guys.

But wait, once I answer a 2 years old FS adds and "Low and Behold" I got the item. The guy felt so bad that he had forgot about it, he was going to give it to me...

When I do it (revive old thread on purpose), I usually title it "For Future Search" to CMA...
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83 911 Production Cab #10, Slightly Modified: Unslanted, 3.2, PMO EFI, TECgt, CE 911 CAM Sync / Pulley / Wires, SSI, Dansk Sport 2/2, 17" Euromeister, CKO GT3 Seats, Going SOK Super Charger
Old 06-10-2014, 05:56 PM
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