Please help...

Car - 1991 Vette, Built L98, 383 ci, 8.5:1 compression, P600b @ 5-6 psi intercooled, 36lb injectors (customer claimed, current tune is set for 30lb injectors). No FMU

Tuning w/ TunerCat, Datamaster, TechEdge Wideband

Symptom - Wideband shows A/F ratio going lean (16:1) under WOT starting at around 3600 rpm. Initial throttle application shows 11.8:1 for a about second and then goes lean abruptly. I think it may be misfires, but it does not feel like it. Engine does "lay down" after a couple of seconds, if I keep it at WOT.

Datamaster shows >10ms of injector pulse width as soon as WOT is applied and the first WOT rpm frame shows around 3600rpm.

The current tune is open loop and shows that the target AFR the PCM claims is close to the measured AFR during part throttle operation. I have increased the PE curve 30-70% with no change in WOT AFR according to the wideband. The scanner shows a target AFR of 9.2:1, with the richest PE settings.

Are the 10ms pulse widths the maximum allowed? I don't see them getting any bigger as the rpms increase. Is this due to the way the injectors are fired?

Conversely, The spark plugs look like they are getting fouled. The shop owner claims that an "8" range plug is OK and I have my doubts. These plugs are short reach too.

I am not sure if the ECM is actually delivering enough fuel to run this combo or if the WB is reading lean at WOT due to misfires under pressure. I've run from 17 to 24 degrees of spark advance at WOT with effect.

Please comment, I'd geatly appreciate it...

 

Sounds like you need a computer that can utilize a 2-bar MAP. Currently, your adding extra air thru the boost, but not compensating for it fuel wise as the ECM cant see it. There also could be some limitations in the fuel calcs that would prevent higher pulsewidths. I think there was some talk about that here not too long ago.

FWIW, I have seen lean misfires cause deposits on the plugs. Basically, its not black and sooty, but a grey sort of deposit that has a somewhat distinct look to it.

Thinking about it, it could also be too rich to the point of killing the combustion. Are the plugs coated with an oily, sooty deposit after a WOT run? What kind of condition is teh ignition in? A faulty ignition will also cause a false lean condition no matter how much fuel is added as it just goes right out the tail pipe unburned.

 

first of all you should get the right spark plug, your injectors seem way too small for that combo, i'm running 42lb injectors on my 305, with no fmu and a computer that cant see boost is a recipe for disaster, get bigger injectors and switch to a 7749 or get an FMU

 

Thanks for the replies! The plugs are a probable issue. I am going to have the shop install the correct plugs. I would think this combo only needs one heat range colder.

The current gaps were set around .032". The plugs have a deep black soot build up after just minutes of operation. It is a dry carbon buildup look. The engine runs on an Accel HEI distributor.

I was hoping to tune this set-up like I do LT1's and just add fuel via the PE tables and reduce advance in the 100 Kpa column.

I'll study the 7749 swap and recommend it.

Thanks again.

 

CalCam, why pay to have plugs installed (you said a shop would od it). DIY is my opinion. Regarding FMU, I may have a procharger unit for sale soon....86Z chime in!! (going $59 code with his help) For a spark gap I'd recommend a .035-.037 range. Anything bigger and your boost could literally blow it out.

 

i'm using 1 step colder plug on mine gapped at .035 , using PE for fuel is not a great method, thats what i originally did, 2 things i noticed, it takes PE away from N/A driving which sucks, 2nd you can hit PE without boost and your motor will bog with too much fuel. 7749 is the only way to go.

 

Thanks for the replies!

AC, It's not my car. The car is being put together by a shop for their customer. I do the tunes for this shop as needed. I meant to state that I will tell the shop owner what plug and gap the engine should have and have them put them in. Thanks for the FMU offer.

I have not worked on this car again since I spent a day and a half messing with it last week.

I agree that using PE vs RPM and TPS to enable it is not ideal for this combo. I was confident I would be able to use it and carefully adjust the TPS enable to add enough fuel to accomodate the low boost this S/C is producing (5 psi). I was thinking I could tweak the VE tables to run good AF ratios under 100Kpa.

I am going to go with your recommendations to use the 7749 PCM and do this right.

I think that the "727" SD PCM also has a 255gps limiter in its calculations like the "165" MAF PCM.

FWIW, I have successfully tuned forced induction LT1's and LS1's with the OEM PCM using PE for the additional fuel required.

Thanks again guys. I'll let you know how it works out.

 

If he swaps the '727 for a '749 he's gonna lose half his dash and car functionality. The vette '727s are a WAY different animal than the fbody '730 stuff.

And honestly, after years of running a '749 on a boosted bird, I find it hard to recommend that. The 2-bar approach is a hack in itself, the code is buggy and feature limited, has no TCC on a v8, and the resolution of a 2-bar
map is horrible. Bruce (RIP) tried very hard to tame $60 into a decent animal, but gave up when only a few of us were using it and hacking at it.

Honestly, I'd go with a FMU or other fueling system under boost. Leave $8D to control everything else.

I've done it. Not too reliable, but worked. Problem is you'll get different boost
at different RPMs in different gears, so your AFRs are gonna be up and down.

Different animal. MAF sucks ONLY on thirdgen apps. On everything else, it seems to rock.

-- Joe

 

Joe is right you will lose part of your dash and your cags communication if you go the $58. It may be worth piggy backing a 7730 with the newer updated $59 code (version of the $58) The newer code has some advantages. WWW.code59.org It has grown since Bruce introduced the $60.

-wide band control from 0-100 kpa
-Map able ve tables 0 to 307 kpa/3 bar x 400 to 6400 rpm
-Autotune xls
-It has been patch for complete v8 operation
-xdf and ads file for tunerpro
-Different starter bins for various injectors

I have been using it for a while now and it works great.


ED89

 

Thanks for providing further details and ramifications regarding the "749" swap. I guess the FMU will get the car and me "down the road" with the least fuss.

The injector BPW and duty cycles the Datamaster logs showed were a litlle perplexing. Although the PE curve in the calibration was rising dramamtically the BPW would not increase. The BPW would go to >10ms at around 3000rpm and stay there. The duty cycle calc. would climb, as expected, but the injector open time didn't change. Is this due to the batch firing mode? The injector BPW times two results in the appropriate duty cycle to "on time" ratio. It seems pretty clear that it is but I'd like confirmation.

Thanks again,
Cal