JBL930

I've done a search and turned up nothing useful, i have a really simple question. When you have your car on a dyno, and the dyno is taking readings from the exhaust tip, obviously after the CAT, are the readings accurate?
I dyno'd my car recently and was a little perturbed at my AFR readings, i was just wondering whether the lean reading i was seeing was down to the CAT doing it's job or whether i am in fact running too lean. Thanks for any answers, i'm not 100% sure exactly what the CAT does, i know it adds a molecule to something using platinum but it ends there. Doe it affect the over-all AFR reading?

Dr Frankenstein

Post-cat AFR readings are useless. In order to get accurate an AFR reading you must measure it pre-cat.

911/Q45

The whole reason for a cat is to remove fuel residue from the exhaust, therefore post cat readings had better be lean!

JBL930

That's good news then, does anyone know how much leaner the CAT makes the reading? We were discussing Graheme's AFR readings on his GT2 on another thread recently, actually on the same dyno that i used, and nobody mentioned that the dyno AFR result could be faulse due to the CAT so i was just wondering whether this was in fact something to consider. I know that chassis dyno's don't load the engine the same as on the road so there is that also, but it would be good to know how much to correct the post CAT reading by. My readings (after the CAT) were near 0.9 Lambda or around 13.0:1 afr which as we all know is too lean, i just want to know if the "real" reading would be closer to correct?

JBL930

Somebody must know how much a new 100 cell CAT affects the AFR or Lambda reading by, i've read somewhere that it's about half a point, so 13.0 would come down to 12.5. Surely there must be someone who has read the pre and post CAT sensors simultaneously?

srf506

Jonathan,

How've you been? How's the tuning going? Is it getting closer to dialed in? If you're worried about pre- and post-cat lambdas I'd say you're getting close. A brand-new hundred cell cat ought to be pretty efficient. I don't have any empirical data but I've heard the newest technology cats are in the range of 0.6%. Some function of additional scrubbing materials besides just the platinum, better cell shaping so more area is exposed to the flow, and some other "secret sauce" stuff. How are your O2 sensors? Are they new too? If not, I'd probably replace them with new ones to be sure they're running at peak efficiency too. I'd always rather be a little rich than a little lean in an app like yours, but I know you've become a real expert in detonation and melt-down so I'm not telling you anything you don't already know.

I think its time to get a mate to drive for you and you sit in the car with the laptop plugged in and see what it is doing on the road. That's where you'll really get it tuned right especially at WOT.

Keep plugging away mate. Its been enlightening to watch you go through this project. Gotta say I admire you for taking it on.

Cheers for now,
Jim P.

JBL930

Hi Jim, cheers for the reply, advice is a bit thin on the ground here lately. I haven't done a great deal with her to be honest, it's been mapped but it over boosts drastically 1.5bar sometimes and then backs off slowly to a little over 1.0bar, it only goes that high in the higher gears which is strange, i guess it's because there is more load. I am told by Andy Fearns that the problem is the 4 speed box as the Motronic cannot be tuned for it and it will over boost no matter what until i change for a 5 or 6 speed, doesn't make a great deal of sense that one but i'm not the expert.
I fitted new actuators to see if that helped, those little buggers aren't cheap, didn't have any effect either, i am using a manual boost controller running in parallel with the ECU boost control valve (N75) so i can cap the boost to a safer 1.1bar, anything under this the ECU controls but anything over and the manual controller opens and no more over boost which works well, I wish it was mapped properly though!

I have the RSR suspension now, not fitted but it's here, also a set of big red brakes, i just need new wheels to fit them behind, i will most likely go with the Ruf ones. The list goes on Jim, as it always does, I'm biding my time until September when i get everything fitted and all the torsion stuff junked, i'm hoping i have enough aside for the 5 speed G50/52 at the same time but either way the body modifications will be done so the box will drop straight in afterwards, there is a half cage going in too to help stiffen things up, she'll be a different car then.

After that i want a proper map doing, i'd like to be able to get hold of Gunther Mandel and take the car over to Germany but i have no idea how to get in touch with him, or who he is affiliated with, i'll keep digging.

Until then i'm not driving her much, i'm too concerned about going lean and the only info i have is a post cat dyno reading. I have an LM-1 i can have installed from when the 930 lump was in there, but i don't see the point as when it's mapped properly i won't need it. I also have an AutoEnginuity program and OBDII connector that i can read some of the engne sensor data from, but it doesn't read the pre cat sensor in either AFR or Lambda so it's a bit useless i guess, what i can log is

RPM
Ignition Advance
Intake Air Temp
Airflow Rate From MAF
Throttle Position
Calculated Load
Long and Short Term Fuel Trim
Sensor Fuel Trim
Sensor Output Voltage

Not sure if the last ones can be converted to AFR or Lambda?

Hope you're well Jim

Kevin

Why don't you drill another hole ahead of the cat cartridge and weld a stainless O2 bung. Mount the test O2 sensor there...

JBL930

I could do that Kevin, although I was hoping that it wasn't necessary. When you are tuning these cars where do you take the AFR/Lambda reading from? Is there a specific scan tool that plugs into the OBD2 port that reads the sensor data in a way that is useful?
I would have thought there can only be three options, 1) a scan tool that reads the standard sensor data 2) taking a reading post cat and using a corrected figure (obviously not very accurate) 3) actually welding a bung pre-cat, you would obviously need two of these to ensure both banks were running identically right?

I remember being in Toby's car awhile back and he had a Bosch Hammer plugged in (I had no idea what it was at the time) he had me reading the intake air temp off to him as we did a 190mph+ run at Bruntingthorpe, I didn't pay any attention to what the other readings were but I’ve heard it mentioned that it shows boost too which my scan tool doesn't. If the Bosch Hammer shows AFR/Lambda then maybe that is what I need, or does anybody know of another scan tool that does this. I have replaced all four 02 sensors recently.

Sorry if the above questions seam rudimentary, I’m new to 993tt engines and Motronic and believe me I’ve searched for some answers but here isn't much on this subject, cheers for any help

srf506

No worries Jonathan, I know you've seen enough detonation and melted internals to last a lifetime so I'd be conservative too. That's why I was thinking maybe start too rich (if that's possible) and back down from there once you can validate what its doing. Unfortunately I'm pretty ignorant when it comes to the Bosch stuff as we always used Motec. Can you borrow Kevin's Hammer and see what it does do? It might be the answer as I'm sure they're not cheap. I think welding a bung in front of the cat would probably be a good idea too.

As to your boost situation the assessment that its the gearing might be correct in that it has an effect on your issue. Remember, boost is based on flow as well as pressure. The faster you can flow w/o back pressure it'll let the boost "creep" or build. Is it hitting the wastegate often? With a close-ratio five or six-speed its easier to keep it in the optimum RPM bands. The old 4-speed is rugged, reliable and flexible, but you use a much bigger percentage of the RPM range using one of those versus a more contemporary box.

Good luck with her. She should be a killer (figuratively speaking of course!) when you get done with her. She will certainly suprise a lot of other drivers when you blow their doors in!

Cheers for now,
Jim

JBL930

That's the big issue with Motronic, you have to have a remap if you want to change things. If i had Motec i could get in there and play around until my heart was content, I was just hoping to be able to read the stock engine sensors for peace of mind, i would love to be able to log a graph which over-laid RPM, Boost and AFR, it's simply for peace of mind and nothing else. If i knew it was mapped correctly and there was nothing to worry about I simply wouldn't pay it any mind, it's just the not knowing if you know what i mean

mjims

Jon,

Your OBDII reader measures the lambda in voltage rather than converting to AFR, someone on rennlist should know what the bosch lambda voltages equal to AFR, its something like 0.5v=14.7AFR. once you know the conversion its relativly easy to figure out your AFR's.

definately yes to moving the lambda, pre cat, just weld a plug 2-3 inches from the turbo as per standard 993TT.

best of luck

Jim

911/Q45

Engine swaps were so simple 40 years ago!

JBL930

Hi Jim
It would be great if there is a way to convert the voltage to AFR, if you come accross the conversion table then please let me have it.
How are you getting on with yours? Looking forward to seeing it finished, should leave mine in the dust!
Let me know if you need the power steering pump

mjims

Jon,

Get the part No of your Lambda's and I'm sure a quick check on google should bring up some specifications.

Yes please for the pump, no rush.

not started yet, something called work keeps getting in the way

best of luck.