alpine003

I suppose you could use some of the resistors you have for the o2 as a temporary option.

Imo000

The EPA bankrupted the company that used to make/sell the O2 simulator a few years ago. The easiest way to fool ECM is to get a spark plug spacer and remove the downstream O2 sensor from the exhaust path. Since less O2 passes through the sensor, the ECM will think all is well. These spaces can be foudn at any parts store and they are about $10 each.

alpine003

Based on the video of OP showing custom exhaust, I'm not sure if that would be an option but sort of hard to tell from the vid.

996-LO

ye we dont hav the secondary 02 sensors in the exhaust path at all. We need to redesign the tubing location to weld in new bungs because there is just no room right now

Imo000

No room anywhere in the entire system?

Chris@Fabspeed

I just tested the K&N intake on our dyno and it caused the same issue you are having - too rich.

A/F's were down to 12:1 and the car was not making good power.

Bolted on the stock airbox, and the power came up significantly.

This is due to the fact that the K&N intake does not have the same cross sectional area as the factory MAF housing - this is causing incorrect airflow readings in the ECU. The ECU thinks there is more airflow than there acutally is, and so adds more fuel.

Here is the dyno graph to show the difference. We dyno the cars with the decklid closed and a fan blowing on the decklid.

On K&N's website they show a 996 on the dyno with the decklid open - no wonder there are HP gains.

We have been guilty of this in the past. There is no aftermarket standard for dyno testing components, and this is the problem - it is exacerbated on 911's, which have the engine in the back.

Flat6 Innovations

Playing with intakes on the flow bench with pitot tubes around the MAF area proved this with hard data for me. It doesn't stop with the K&N system in regard to aftermarket arrangements.

We've been seeing this characteristic for about a decade now with these engines.. I have seen some intakes so bad that the engine would literally shut off due to immeasurable turbulence around the MAF

The other interesting data is gathered when air inlet temps are measured at 5 points within the intake system from the inlet to the plenum. That's when you see that a cold air intake is really a hot air intake.

I love data.

Kalashnikov

So are there any CAIs on the market that actually deliver better performance than the stock air box?

My indy stated that stock air box is the way to go on the 996 as he sees a lot of issues with CELs from aftermarket CAIs.

alpine003

If you read up on intake design, one of the fundamentals of a good design is to have the least amount of bends as to not have any adverse flow characteristics, especially when MAF and NA combo are involved. I'm a bit surprised that some basic thought hasn't gone into the K&N system. I don't know of any aftermarket systems that snake around like that with those radical bends.

Chris@Fabspeed

After watching the dyno video, I have some additional comments:

Why are the dyno runs so short (what gear were they using? 3rd?)

If running in 3rd, the car is still down on power. You should be up in the 260 ft. lb range running headers and without cats, if the exhaust is built right. Especially if dyno'ing in a gear lower than 5th.

The fan in front is extremely insufficient. It would be better if you had 2 snail fans and had them directly in each front grill. Proper cooling while on the dyno translates to more accurate dyno results. If the shop has the OBDII data-link setup, post up some graphs that show what the ECT's are and what the MAF is reading.

On a race car like this, I would build a ram-air box based around the stock airbox and use something like a Fabspeed cold air kit, which allows you to remove the internal resonators out of the stock airbox which increases it's volume. Wrap the whole box in gold foil and block off the holes on the bottom of the airbox that allow hot air in. The car will run consistently well.

Imo000

If the source of the air is cold, the speed that the air moves through the intake will not have time to warm up. Therefore, insulating the airbox isn't needed.

jrgordonsenior

Agreed but plugging the holes on the bottom of the OEM box does make a difference. FWIW, I run the Orange CAI with a stock box and a BMC filter element. No MAF issue for 3-4 years. I run straight pipes from 3.8 manifolds, secondary 02 sensors tied to rear fame, and I dyno in the 305-310 range on a Dynojet, 12.5-13 A/F ratios. Oh and I'm a 3.6 bottom end with 3.4 heads, cams, TB, and 5.2 ECU (GIAC street flash)....

Ahsai

You confirmed my suspicion. My air filter is dirtiest where it's close to those bottom holes. The air intake temp is definitely quite a bit higher than ambient. I think a lot of hot air get sucked it from those small holes.

However, plugging those holes also means rain water ingested by the snorkel has nowhere to go?

996-LO

thanks to everyone for the positive feedback. We did install the stock airbox for the last track day and the car ran great. Probably not with the most possible power but dramatically better than our initial dyno with the K&N.

philooo

amazing how the stock air box is the best way to go...

everytime people check my engine bay at the track they ask me why did I , at least , change the air filter box... I am glad to hear that I was right...All the parts are design to work together and all is there for a reason. Change one bit and you have to do it all.