Just made a couple of baseline pulls on my 55K mi bone stock '02 C2. Made 278.7 rwhp and 243.7 rwtq, but has an ugly group of dips from 5250 to 6250 rpm, where it goes fat (11.4:1) . It stays fat to 7K , but power ramps up and the curve is smooth from 6300 to 7K. Any ideas as to what's causing that ? Must be somewhat normal, as there was a stock '03 in a couple months back and his curve mimicks mine (has dips in same range), but his car maintained at 12.4 and made 10 more hp with the better A/F . My A/F is a downhill run from 4700 to 6200, where it levels off at 11.4: 1 .

 

The resonnance flap on the back intake crossover tube is opened at 5250 rpm which increases the length of the intake which in turns causes a sudden and short drop in power to the engine.

 

interesting- is that something you want to mess with or leave alone and just deal with it?

 

Thanks for the post. Not too concerned wiht the short drop-off in power..How should I chase the rich A/F condition ? Will bringing more air in cancel some of that out or will the sensors simply adjust and keep it fat and happy? Should I look at trying another MAF ?

 

I would clean the MAF and check that your fuel pressure regulator is connected to vacuum. If you have a scanner, MAF readings at WOT near redline along with throttle position reading would tell you alot. What year and model car?

 

If you want a smoother pull on the engine, you can bypass this valve activation. However, you would have to decided whether you want less or more area for the air to flow through.

Keeping the valve closed will reduce the area of flow and therefore increase low RPM torque and keeping it open increases area of flow and will help high rpm torque.

Remember, this valve is only present in the NA engine.

 

It's an '02 C2. I was going to install the plenum tonight and re-dyno, but I think I'm going to follow your advice and throw it on the scanner first. As far as the flapper, I'm guessing it is more easily accessible when changing the plenum? Can you give me some info as to what has to be done and will it need a software upgrade or will the ecu compensate for the intake velocity changes at different rpm's? I appreciate your advice.

 

There is an electric changeover valve that controls the flap. The valve is located just above and behind the alternator. It has an electrical connector attached to it along with 2 vacuum lines. One line goes to the flap and the other to the vacuum canister located back near the oil seperator under the left intake.

If you want to keep the flap open at all times, simply disconnect the vacuum line from the changeover valve that goes to the vacuum canister and plug the line. If you want to keep the flap closed at all times, disconnect both vacuum lines from the changeover valve and connect them together. No codes will be generated. No software changes needed.

Jim

 

Yeah pretty much any engine has a short drop off around 5K rpms. This is usually an engine speed that manufactures will open an additional intake valve/runners. I'd leave it alone. Sometimes I've noticed with upgrades like a plenum and intake, it smooths out that drop off. Not sure about American garbage engines but every import engine I've ever dynoed or seen a dyno of, they all have a little dip around that range. It's usually always attributed to a switch of air flow. Porsche engineers know what they are doing, I think someone before hit it right on, having the variation gives you more low end with increased flow top end.

 

+1, If you want to minimize dips... put the plennum and free breathing filter on. It will smoothen the power curve out, as well as raising the HP.

 

Isn't 5250 RPM about where the HP & TQ lines should cross? I recall reading an article about the mathematical reasons for this. You may not be able to entirely eliminate that dip. While I cannot explain it, physics rule...

 

Yes, that is correct. The valley of my dip is 5800 and does not begin level out til 6250 .

 

I've got the plenum and exhaust already, and an intake on the way. But I don't want to start modding til I find out why the A/F is so fat. I am putting the car on a scanner tomorrow to see what the sensors are doing..I just don't believe 11.4:1 is normal...I think there is a minor issue somewhere. Does anyone else agree or am I a bit off ? The only only other dyno pull I have to go on is an 03 like mine, and his curve is choppy, too, but it made 10 more hp with a 12.4: 1 ratio across the rpm range. It was a 27K mile car vs my 55K..
I may dyno all 3 ways on the intake flap once I get things straight and see if there is a benefit.

 

whats the normal whp/wtq for 02+ 996's?

 

I think 260 - 270 is the OE rwhp range.