40_Jahre_911

Dear "11er" friends,

as far as i know there are just a few 996 with the x51 Package registered in Germany and it seems like the kit is more widespread in the US. Unless you have gathered practical experience with my project/problem i am looking forward to your support.

Intitial situation:

I am driving a wonderfull 911 Jubiläumsmodell with a x51 kit. My car is already listed in the registry of TAZ40Jahre (many thanks for your effort!).

Some months ago i have ordered a full exhaust system of M&M Exhaust GmbH (http://www.m-m-germany.de/ ; http://www.m-m-germany.de/CMS/tinymc...Datenblatt.pdf ; Type: M&M96S022 ; M&M is established in the Motorsport and as far as i know also supplier for Manthey Racing). My Exhaust system includes Mufflers, catalysts and racing headers. Compared to the OEM product (55 mm; 2.17 inches) the exhaust system know has 62.5 mm (2.48 inches) diameter throughout.

Compared to the series 911 (320 hp; vacuum motor) the x51 is characterized by a shifted torque curve. With the new exhaust system my expectation was that the torque curve is going to be further shifted. Thats why i ask M&M to provide me with a x-pipe system (compare the link above: Type: M&M96GT3023) to increase the exhaust back pressure (at cost of max horse power). Some colleagues of mine could confirm with data (dyno reports) that they reached significant torque increase in rotational speed (2000 rpm to 3500 rpm; compare link http://www.pff.de/porsche/board104-p...-gt3-angebaut/ file leistungst.pdf)

I have followed a lot of discussion about adjusting the DME/Motronic. To be honest i dont know if it necessary until yet. There are too many different voices.

Situation after conversion:

After the conversion (and something like 700 miles) i have noticed three different symptoms:

1. The torque curve in lower rpms slightly improved (not proved yet; just subjective feeling)
2. running irregularities appear (proved and reconstructed)
3. Around 1100 rpm a "swallowing" occures in the case of excessive acceleration (accelerator right down)

The symptoms are independently from heating of the engine/oil.

What i have done:

1. I've checked the Motronic/DME with durametric. No failure occured.
2. I've checked the mass air flow sensor.
3. No leaks mentioned.

Some thoughts:

I expect that the problem is related to the exhaust back pressure. Some others in some german forums thinks that its related to spark plugs. I dont think so because all worked fine before the conversion.

I realy appreciate your thoughts. Just let me know if you need anything

greetings from abroad

6ta1

what do you mean by swallowing? a hole in the torque curve?

I have an x51 with afterhours exhaust and stock cats. That's still less back pressure than stock but I do not have the same issue as you.

40_Jahre_911

exactly, it is definitly a hole in the torque curve... if i am going to put the accelerator right down at 1100 rpm the car begins to jump (as if it coughs).

Flat6 Innovations

The hole in the torque curve occurs at 5,200 RPM and is a result of variable valve timing change over. its always there on the dyno, but is more noted with X 51 engines.

40_Jahre_911

I am aware of the hole in the area of 5200 rpm. Since the conversion the "hole" in this are is shifted.

I am more interested about your thoughts/opinion related to my symptioms.

Flat6 Innovations

If these things showed up only after swapping exhaust, it sounds like the typical inefficiencies that we see from aftermarket exhaust. Generally these systems lack specific development, no matter who sells them.

40_Jahre_911

In general M&M does a quite good job and they enjoy a high reputation in Germany.

Even for those guys who are familiar with the exhaust systems of M&M my symptoms are not known. But as i already mentions 996 with the x51 kit are not the majority. I am going to five the manufacturer a call tomorrow.

Found this quote on 6speedonline very interesting:

"I firstly took my car to a local tuner with a dyno to try to figure out what was going on. They discovered that the car was pulling timing (about 5 degrees - this doesn't sound much but the car is only meant to run about 24-25 degrees of timing at full throttle/peak rpm). They also found that the lambda sensorswere correcting the MAF far too much - the long-term fuel trims were at about 7% from memory - and unusually, the MAF was telling the ECU that MORE air was going into the engine than was actually the case. Odd as MAF's usually fail lean. This problem was also causing the car to run too rich at full throttle.

I was really surprised to find out from the tuner that the lambda sensors can be correcting a mis-reading MAF by up to about 20% before throwing a code! He told me that an engine in this condition would be running dreadfully but the ECU would still show a healthy car. In fact, the tuner said that if the MAF is functioning but not accurate, it can even prevent the car from adapting properly and it was likely this that was causing it to pull timing.

A new MAF and a reflash of my ECU with the latest stock map for my car (the latter simply a precaution) sorted the problems and it now runs very well. The long-term fuel corrections are under 2% - well within acceptable tolerances - rather than the 7% with the faulty MAF and the car now runs 24-25 degrees of timing as it should.

So it seems that the MAF sensors are given a little too much latitude to be out-of-calibration before they throw a code - something I think could be a possible reason why some cars just don't run quite right.

The simple way to check this is to hook your car up to an ODB-II tester and look at the long-term fuel trims. (You don't need PIWIS or Durametric for this as fuel-trims are part of the mandated emissions information that has to be readble by all ODB-II equipment). If they're more than a percentage point or 2, this could indicate a faulty MAF or maybe an air leak.

The good news is - to go back on topic - that none of the issues with my car had anything to do with high-flow cats. The AWE system on my car just works as advertised - it was other sensors causing the problem.

Finally, there's a common myth that putting free-flowing exhausts on a car somewhow reduces back-pressure at low rpms and reduces torque. This is not the case at all. There is little or no back-pressure from a stock exhaust at low rpm anyway. Think about this. Your engine can probably produce no more than, say, 90BHP at WOT at 2000rpm. If the exhaust was already restricting the outflow of gasses at 2000rpm/90BHP, it would strangle the engine completely at 6500rpm and 355BHP. In fact the engine would never get to 6500rpm or produce 355BHP! So fitting free-flow cats on their own will leave low rpms where they are and increase power higher up the range when the stock exhaust does become more restrictive.

Changing headers however, can change torque characteristices markedly. Different pipe lengths and diameters change how the exhaust scavenges gas from the cylinders. Another topic for another day!"

Maybe i should focus on the interaction between MAF and Lambda Sensors.

Does anybody know if the 996 x51 Kit includes wideband lambdas?

Flat6 Innovations

Typically these engines will pull timing if someone installs a lightweight flywheel and non-factory engine/ tranny mounts.

pete95zhn

Give a call to http://www.vgs-motorsport.com/ / Herr Ansgar Wiesen. That should solve your possible problems with the DME.