firebird_1995

iTrader: (1)

'm trying to get this new setup running right. I've tried everything available to me. In the dfi PNP calibrations I've tried the sc 383 lt1 and sc 350 files. I've also tried the 2 bar preset and the ve calculator. I have a lm1 wideband with a new wideband sensor in thats wired indo the dfi as a generic input (0=10-5=20) Whats happening now is after it goes into closed loop and starts leaning it out (around 13:1) it starts backfiring through exhaust. I thought the timing might be too retarded so I raised it and it helped a little but no major change. I also tried locking it into forced timing mode to eliminate any changes the dfi was trying to make. I do have a balancer with timing marks on it however theres no pointer on the cover. I've also tried the crank offset at zero and ten degrees. At this point if a basic file will not even get it to idle or run at low speeds im going to have to look into mechanical issues. I have a 2 bar map sensor in. Build is a 355 95 lt1 with optispark, 223/234 cam, 60# smartfire injectors, m122 supercharger, Any suggestions?

Wicked94Z

iTrader: (26)

get a cheap stamped chrome timing pointer at your local auto parts place, cut it down and flatten it so you can epoxy it on the front of your opti spark. Pull plug #1--or all of them to make it easier to turn over-- stick a screwdriver in the cylinder and find TDC. Then epoxy your pointer on with some tape to hold it on the TDC marker on your balancer. Now you can fire it up, run it at 2000rpm or so with a fixed timing number, adust crank offset to line up the timing mark. While you're tuning it I'd run it in open loop so you can see how your changes are working. I found the calibration stuff to be basically worthless, enough to kind of get the car to run but not to drive.

firebird_1995

iTrader: (1)

Didn't think of doing it like that. I will give it a shot. Thanks!

lcvette

iTrader: (3)

well explained, make sure the ecu and mechanical timing are the same before you try anything else. make sure your target afr table is good. I like to stay a little richer with cammed engines, around 13.9-14.2 at idle.

I recommend using the auto cal button to get the afr dialed in quickly so you don't foul plugs or wash down the cylinders.

be sure your base parameters are correct, fuel pressure, injector size, firing order, compression ratio etc... also scale your axis before you get to far into it, not sure what your Max rpm is but you can get some additional resolution by narrowing down the table to work inside your parameters. set max rpm and map value then refine where you want the most tunability. I like alot of room from idle to 3000rpm so my part throttle can be made silky smooth. for race cars you might just need idle and then 4k to redline. you get the idea.... Good luck!

firebird_1995

iTrader: (1)

I got in contact with George Landis who informed me he has checked the offset on an lt1 with a timing light and found it requires an additional 15 degrees to be correct. He also sent me a few base tunes to try that are very close to my setup. The backfire is still present. So I removed the supercharger to eliminate it as a cause, no change. Changed the injectors to a stock set, no change. Installed a factory map sensor, no change. Lifter preload was set on each cylinder individually using 1/2 turn past zero lash. But I was checking voltage drops on grounds and power supplies and decided to check reference voltages at each sensor and found something odd.

I need to know what voltage is supposed to be on terminal D (crank +/reference) at the accel ignition adapter measured to ground. This is the same circuit as the gm optispark terminal A (low resolution signal).

I'm getting 11.65 volts and it looks like the factory pcm should read 5. I would imagine the accel should do the same.

I've got full ignition voltage (12.85) on the accel ignition adapter A terminal / gm optispark terminal C so I think something is up there. Is anyone running an accel dfi with optispark that could tell me what they have on those pins?

ugod02010

are you checking it in ac voltage. not dc. just a thought.

firebird_1995

iTrader: (1)

This was measured on the ecm side, not the optispark side. I was doing the opti test method from shbox.com.

WhistlingZ

Really sounds like you have the timing off. Did you double check the firing order in the config screen? I never had an issue like this when I hooked up my DFI. I am running a crank trigger and cam sync now, so I can't check the opti for you.

lcvette

iTrader: (3)

I run the dual synch distributor so can't help you either, but my question would be why keep the biggest Achilles heel of the ltx platform when you know it's going to give you trouble? Ditch the opti..... grab a dual synch dizzy or crank trigger setup. you will appreciate the elimination of the Opti and simplification the dual synch or crank trigger offers.
1/2 turn past zero lash is more than I like, but it's all relative to what you consider zero lash. if you are tightening until the push rods don't spin ( wrong way) thats likely your problem. if lash is set properly, it may be a burned valve our badly worn valve guides. I had a burned valve that had the same symptom you described.