[928uk] 928 air filters

[jonathan at netvue.co.uk]

Mike, according to John’s ST2 manual (pg 20) the CO pot resistance setting does change the LH injector pulse length across the full rpm range (idle-> cruise->WOT), although the effect of it at WOT compared to the injector pulse length increase from the WOT map would be negligible, and the most effect of the CO pot will be at idle rpm. I agree that contamination of the MAF wire would potentially reduce the cooling effect of air mass and therefore the MAF would signal the LH ECU that there was less air mass than reality, resulting in a lean mixture. That contamination effect may be non-linear though, so perhaps adjusting the CO pot for correct mixture at idle wouldn’t provide a suitable correction across the full rpm range. Without a dyno and air-fuel meter we can only speculate… 

 

It occurs to me that oil seen on the MAF wire and MAF outer screen could perhaps be from the exhaust fumes of other vehicles, but perhaps not. I do understand everyone’s reluctance to use oiled air filters in case they affect the MAF. 

 

As I said, I’ll go with the flow on this one. Thanks for all your thoughts.

 

Best regards,
 
Jonathan

 


 

From: Michael Parris <michaeljparris at gmail.com> 
Sent: 20 February 2023 20:41
To: jonathan at rackowe.net
Cc: 928UK List <928uk at lists.928.org.uk>
Subject: Re: [928uk] 928 air filters

 

Indeed the pot is used for idle CO however the ECU ignores that and uses the sensor for mass air measurement when you demand power with the throttle so any contamination can affect it.

However, there's probably enough oil in the intake to change octane at combustion in a standard engine to not worry about it?

It doesn't help with why your filter fell apart except they (928s) get pretty hot and look what it does to the hoses after 10 years.

 

I have watched people perform back to back runs at the dyno to see if there's a difference and gain increased power and torque but that's on more modern stuff that use different mass air sensors. I'm not sure the gain would be noticeable on the road as it's quite small.

 

Filters, like everything have gone up, I think my filter was about £25 the last time I bought one, in fact I bought two.

 

Best,

Mike

 

 

 

On Mon, 20 Feb 2023 at 20:18, <jonathan at netvue.co.uk <mailto:jonathan at netvue.co.uk> > wrote:

Thanks for that Mike. I have never re-oiled my K&N filter as I do less than 3000 miles each year, so perhaps that is why it has never been a problem in the 10 years that I used it. In my case, I have a non-Cat 928 with a CO pot, so perhaps that meant the calibration point of the MAF was less critical anyway as the CO pot was used for fine setting mixture adjustment.

There seems to be a flurry of views generally against the use of aftermarket air filters on a 928 for fear of affecting the MAF, although I have seen that the oiled filter manufacturers say that, unless their filters are over-oiled, the oil does not leave the filters or affect MAFs.

I think I’ll err on the cautious side and go back to standard Porsche air filters.

Best regards, 
 
Jonathan
 
 
928 GT 1991 Amethyst Metallic

 

From: Michael Parris <michaeljparris at gmail.com <mailto:michaeljparris at gmail.com> > 
Sent: 20 February 2023 17:09
To: jonathan at rackowe.net <mailto:jonathan at rackowe.net> 
Cc: 928UK List <928uk at lists.928.org.uk <mailto:928uk at lists.928.org.uk> >
Subject: Re: [928uk] 928 air filters

 

As a MAF rebuilder I do not recommend using oiled filters as it could coat the sensor.  John said the same. 

The ECU will 'burn off' the hotwire on shutdown (engine 80c+) so that should clean it but I get MAF's internally coated in oil quite often that will change the calibration point just by running brake cleaner through them.  If my test can change it then it won't be correct on the car.

 

However I do not know if the subject cars had an oil type filter or how well it was maintained so it might have been over oiled or it could be oil mist from the engine although that's hard to believe when the top screen also has oil on it. 

 

Regards,

Mike

 

 

On Mon, 20 Feb 2023 at 14:53, Jonathan Rackowe via 928uk <928uk at lists.928.org.uk <mailto:928uk at lists.928.org.uk> > wrote:

I have had a K&N air filter on my 928GT since April 2012. It seems to have
performed okay, with just a shake and clean with a paintbrush as maintenance
each year. I have also had K&N air filters on my MGB since 1985!



Last Autumn I decided to clean and re-oil all the K&N filters using a K&N
re-oiling kit. The MGB ones were fine, but when I came to look at the one on
the 928 I found that the rubberised plastic surround had cracks and was
crumbling in places. As I don't want little bits of rubber-plastic getting
onto the MAF screen, I removed the K&N filter and spoke with the company
about it. I expressed my dismay that the 928 filter surround was breaking up
after 10 years and 26K miles, when the ones on my MG are still good after 37
years and 60K miles! I offered to send them the air filter for inspection
and comment, but as I don't have my receipt from 2012 they weren't
interested.



I see that Mahle LX311 OEM filters, Pipercross PP1211, & K&N 33-2591 or
60131465 filters are available. So my questions to the group are:



Has anyone else experienced the surround breaking up on a K&N air filter?

If you are NOT using the standard Porsche air filter, which aftermarket one
are you using?  

Has your experience with the aftermarket filter been good or problematic?

Best regards, 

Jonathan


928 GT 1991 Amethyst Metallic