Attached is a dyno run I did last week. It shows the Air Fuel Ratio (AFR) is over 14:1 below 4200rpm (i.e. quite lean) and then drops dramatically to around 12:1 and peaks at 11.5 to redline.

I guess this is not normal. Any suggestions of the likely cause(s)?

sparky
'93 968 Club Sport
'02 Boxster S

 

Sparky:
This is what I have seen in most Fuel Inj. ECU's, (not shure about the 968 ECU, but I assume will work the same way as others).
If you are using the stock ECU, at part throtle the ECU is in "closed loop" operation, this mean it control the AFR to a predetrmined value, ussually stochiometric value (14:1 or close) for fuel economy. When you floor the gas pedal, it goes to open loop mode, with a lower AFR (more gas). How do you did the Dyno run, pedal to the floor all the time? Is your car fuel injection stock? Your HP numbers are low, about 210 crank HP.

 

TurboCab
Interesting, will ask my dyno man about how he did it.
Yes mine is quite a bit down on power, hence I'm looking into it.
What sort of rear wheel power would you expect from a very healthy 968 ? (not crank power)
Sparky

 

Sparky,

Here's mine for a comparison.

 

Tim - that's more like it. Are you running stock ECU / no chip ?

 

Yes, stock ECU with modified air intake (large K&N cone filter) and custom exhaust (stock exhaust headers with a 2.5 exhaust and dynomax shorty turbo muffler; no cat as the dyno is from my track car). I have a hesitation below 3K, above 3.5K and the car runs strong.

I should put my 968 DD on the dyno since it only has the modified airbox, all alse is stock.

 

Sparky:
You should have about 150 kW (rear wheel) with stock engine. It is important to meassure the power at the wheels in 4th gear (1.1 ratio), assuming stock transaxle. This way the power reading will not be affected by higher or lower than 1 gear ratios.

 

Turbocab - my dyno maestro tells me he did the dyno run with foot flat to the floor - I take it this means "open loop" mode all the way? 150kW @ wheels for a healthy 968 is good news - means I'm going to find another 20kW ;-) I've arranged for a mate to take his much healthier 968 to same dyno... will be interested to see his AFR spread.

Lemming - what's a 968DD ?? (I know we think of our cars in affectionate terms, but really ... ;-)

 

DD = daily driver, my white '95 cab.

Attached are my dyno numbers from the same session as the AF above. The engine was purchased off of Ebay from a car being parted. The engine is a '95 and had around 70K miles IIRC. These are RWHP numbers.

As of yet I've not gone into the engine to even tune it up. Yes I did put new spark plugs in her but I'm still running the original plug wires and distributer. I have recently installed a new coil but that was done after the dyno.

 

Tim:
Your HP numbers look good. You must add 15-18% for crank HP, which put your engine at about 232 to 238 crank HP.

 

arrrrgh - this kind of thing makes me nuts - dynos are great things, and i highly recommend using them, but only for tuning one individual car, and not for comparitive purposes to any other car - i know everybody is just trying to help, but i feel like i have to clear up a very common misconception

i continually see people trying to compare one dyno run on one car on one dyno on one day, to another completely different run on a different car on a differnent day, and worse, to a completely different dyno - it cannot be done without a lot of very "laboratory like" control and measuring - it takes hours - there are just too many factors that can easily result in a 20hp difference

you don't just add some fixed arbitrary number for drivetrain loss either - you measure it and then add it in - it could easily be as low as 7%, depending on the differential temp, strap tension, tire pressure, outside diameter, etc - we've seen a wide range of numbers over the course of the day, ON THE SAME CAR! - it changes dramatically as the differential temps change

if you are looking to do comparisons on our own car, it's better to just read the rear wheel numbers with the calculations off, unless you are going to take all of those measurements and recreate every single parameter every time - it's a very time consuming and costly process to do that, and certainly the most accurate, but not terribly practical for the average user - you still have to match temps and such, but by eliminating the drivetrain loss calculation, at least you minimize accumulative error

 

Thanks Tim - clearly the AFR spread in my car is wrong.

Flash - don't fear, I understand the dangers of comparing dyno runs. But I do know my car is way down on power. We've done direct comparisons with a mate's stock 968, both on track and with a "G-tech Pro" accelerometer. Even though my car is a stripped down Club Sport, weighing 100kg (220lb) less than his luxury pack, his consistently beats mine by 0.3 - 0.5 secs on standing 400m, and the G-tech says his car develops 25hp than mine (at wheels). I've also compared these figures using a computer acceleration model, and that confirms my theory - my car's down on power as much as 20kW (~27hp) at the wheels. So an engine refurbish is about to happen....

 

Thanks Sparky for the Clarification. Seems like Flash want to throw complicated arguments where there is no need to!

 

Chris,
You are in good hands with the guys at Road And Track, I wouldn’t worry about weather or not that Dyno operator Knows what he is doing ( he has probably forgotten more than some of the so called experts here will ever know) and he is honest also, how are your leak down numbers and compression numbers, I would start there and see how they look, I could have sworn that you had sent them to me but I cannot find them anywhere, you may just need to freshen up the head a little from memory your car has very little km's on it so the bottom end should still be fine, Raj Datta and a few others have had experience with this and have been very pleased with the end results,

On another note thanks so much for the pics of you in the Classic Adelaide (bastard you just had to tease me with that didnt you) Its is one of my dreams to do that rally just once in my life, In my younger days I have done some of the runs fairly quickly before but not under any supervision, besides the boys in Blue, put it this way the Sarg at Clarendon Police station didnt like me very much, when he could catch me,

Best Regards,
Scott

 

As Scott ststed, you need to start with compression and leakdown to see your engine's tune. If you have too much leakdown, chances are you need to refresh the head. The valve seats overtime pickup way too much crud, especially due to the dual resonance design of the intake manifold which tries to burn off PCV fumes. This over time will coat your intake manifold and your intake ports on the head with athick film of oil and dirt.
You will be shocked at how much you can regain by a simple 3 angle valve job. These engines are pretty much bullet proof for 200K-250K plus and you will see what I mean when you remove the head. The cylinder walls will look as good as new. I think a 968 should be get a top end freshening every 100k if not sooner.
Regards.
Raj