marton

Ahhhhh you guys with the later models and all these problems

my 81s runs great, no MAF, no flappy, no Hall sensor, no O2 sensor, no cats

Marton

Brett928S2

Hi

I think the problem with "testing" a K&N back to back on a dyno...is the massive hp of our engines...a 1 to 2 hp increase is just not statisically viable on a dyno...

Years ago I had a very carefully built Escort engine that ALWAYS dynoed at 110 hp (plus or minus 1 hp max difference)

I added a K&N ...as air filter was about the ONLY thing that was allowed to be changed in the class I was racing it in...

Dyno....4 hp more plus or minus 1 hp on several dyno runs...

Since then I have used them in ALL my cars, including a Ferrari.....now maybe its just me and my seat of pants feeling...but I "THINK" they work....

All the best Brett

blown 87

Well I have learned somethng today, I never knew that.

heinrich

Here is one of the very few places in 928 technology I actually agree that there is a VOODOO of sorts, in other words the induction tuning of a 928 is different from many other cars, in that the air filter and intake design are synergistic, and changing it almost always results in performance loss.

I believe I know why the stock filter performs better than a K&N in the design our filters have, as opposed to the opposite result for a design that is not as finely tuned ie other cars where a simple cone filter suffices, like Porsche 944 or other cars.

You would do well to heed Louie Ott's advice. He and I tested the K&N vs Stock together first on a dyno and we were very clear on the results, very reproducible. K&N did lose performance vs stock filter. Again I believe I know why. But nevermind that, the point is we tested it and it was reproducible.

I say again: Porsche made these cars and tuned them very carefully. They then added or subtracted some things for US/Swiss Emissions compliance. Anything that was not originally part of the design was a performance loser.

Speaking of X-overs. Porsche added some of the most efficient cats to the system they could find. However, those cats do rob performance, and the X is more than just a cat freeflow replacement. Its design is targeted at synergy in resonance tuning and it succeeds very well.

BUT

AGAIN

Just make what Porsche made run like Porsche built it, and you will be amazed at how it performs. These are cars that ran stock at 186mph at Nardi for many hours at a time. They beat the land speed record when they were new. Do we think they did this with fouled plugs; crusty ignition wires and rats' nests in the intake?

Ispeed

H is right. There have been so many 928ers that quickly think supercharger and cool wheels before they have done all the normal catch-up maintenance. If the car runs as intended with all items functioning under the hood, they are not slow.
New intake gaskets are not sexy, but they drive sexy.
p.s. Does anybody here still think their oem shocks are still good? Or that the oem motor mounts haven't failed? (rare exceptions to this) There's much to do to get the 928 back to 100% function.

John Speake

If you want to make "break out" adaptors to help connect test meters or 'scopes to a knock sensor or hall sensor, I can provide both male and female connector sets which will allow you to make up such tools.

Changing anything in the inlet system like using a K&N will change the inlet flow patterns. The 928 was mapped for fuelling with the stock system. If you change something, then you need to remap the LH to compensate.

blown 87

Are you saying that on a US 87 that the injectors have different pulse widths to compensate for air flow devations?

I had never thought about these systems being sequential.

I guess I am not understanding the "why" here, but then again there are lots of things that i do not understand.

I always thought that one of the great things about a mass air flow system was that it was forgiving up to a point as far as changes go.

I do know on a speed density system any changes you make require a remap, but i did not think that on a LH system that changes to the intake as small as a filter change would require a remap.

You are the acknowledged expert here so I will take your word for it, I am just trying to get a little understanding of the why.

Greg N

Loaded

I think my 89 is supposed to do a 14.3 1/4? Thats not exactly great by todays standards. the top speed is pretty much a rockin number.

I think people get these cars expecting it to be a z-28 slayer or a hotrod. That it aint

John Speake

Hello Greg,
Perhaps I didn't explain very well.....

The MAF hot wire takes a sample of the total airflow into the engine and converrts that into a voltage. That voltage (or more correctly a modified and processed version of it) coresponds to a "load" value on the main fuel map. The other axis of the map is rpm. The map tells the LH how long to open the injectors for (they are all fired simmultaneously).

If you changed the airflow into the MAF, then it may indicate a different voltage at a particular throttle opening than it did before. So this will mess up the fuelling to a greater or lesser extent. A small change may not cause a problem at cruise with a cat equipped car running closed O2 loop, but when you're at WOT, then it's open loop time, and an accurate airflow reading into the engine is critical.

In a perfect world the airflow into the MAF would be laminar, and then it would be easy to change things without upsetting something. But life ain't like that...

The implication of a small drop in power/torque at WOT with a K&N is that it is the result of a weaker mixture than stock. I wonder if anyone has checked this ? Louie ?

Hope this helps.....

Rick Carter

John,
I mistakenly thought at WOT open loop the O2 sensor and MAF were bypassed and it ran from a preprogrammed map. When I previously tuned my car (pre SC) with the piggyback SMT-6, it was tuned with the K&N filter; I had brought the stock filter and changed it out and lost 4 rwhp. I am guesiing if I tuned it with the stock filter I would have had more power with the stock filter and less with the K&N.

John Speake

Hi Rick,
I can't talk about 85/86 yet, but as far as > 87 goes.....

At WOT the LH goes open loop so the O2 loop doesn't fight the extra enrichment required for maximum power.

The LH uses both the "cruise" map and also the WOT map to calculate the exact amount of fuel (injector pulse time) is required at WOT. The value on the WOT map only has one axis - rpm. So it is often assumed that the MAF voltage value was not used at WOT.

During the SharkTuner development we found that the WOT fuelling mainly uses the "cruise" map values at WOT, obviously at the high load values. So the MAF voltage is still used. The purpose of the WOT map is to allow some additional fuelling at various rpm points if selectively required.

Yes, it would be good to find out exactly why it is that the K&N can give a slight drop in power, when used with a standard mapped car. At least the tools are now available to make some accurate measurements of load value, injector on time at WOT, A/F values. The ST data logger makes it easy for the later cars.

sharkmeister85

Given that what you said above is true, John, here's my SWAG. If the K&N filter flows better than the stock filter (as claimed), then that would mean there is more air going into the engine than the original cruise and WOT maps were calibrated for. Therefore, the engine is running leaner than stock design. Could this result in the lower power readings sometimes found?
Glenn

worf928

When I get back from travel, I'll pulll out some references and see what other examples I can come up with. There were 'K' versions of CIS and electronic Bosch FI systems. I don't know which - if any - were ever used on 928s other than the EZK.

EDIT: I just did a quick google. "KE-Jetronic" was bothering me.

I don't think KE-Jet had knock detection.

You didn't learn $ht+t from me except that I can post crap everyonce in a while esspecially when I'm stuck in hotel room...

I will defintely need to research this now just satisfy myself.

I know in the VW world there was CIS-E and CIS-K. The 'K' was a CIS system with knock detection. The Porsche literature maps the 'K' in EZK to knock detection.

John Speake

Hello Glenn,
I see it the opposite :-)

If more air flows into the engine at a particualr RPM and throttle position, then that would move the LH operating point on the map up to a higher value, and would result in more fuel being added.....

Like I said, this is not a perfect world... here's are a couple of K&N theories.

1) The >87 cars run quite rich at the WOT top end - does even more fuel, due to less
restiction make it even richer, lowering power ?

2) Does the K&N upset the airflow patterns through the MAF hot wire so much that in fact the MAF reads lower than it should, weakening the mixture ?

Hello DaveC
In fact the "K" in K-Jet does not indicate "knock" - i found this info.....
QUOTE
"The designation K-Jetronic stems from this fact ("K" stands for the German word for "continuous"). "

heinrich

John my theory the past 6 years has been number two. The mesh over the vanes of the filter causes a mess in airflow.