$42 in 383 initial tune help

[Fastmax32168]

Sooooo...I had a nearly perfectly tuned and running 350 TBI conversion in my Malibu station wagon and decided I needed a 383. My plan was that a good starting point would be to take my previous tune and make some changes to it that should bring it in the ballpark so I could get started down the right path with my 383, since it was not all that different other than the displacement. The 350 had a stock sized TBI and stock 350 injectors running at 18 PSI, the BPW was set to 115 and all worked great. Based upon the new displacement and a change to a bored 454 sized throttle body with 70lb 454 injectors run at the higher 18 PSI I calculated a new BPW of 86. It seemed to actually run OK just driving it around but was pulling tons of fuel with BLMs under 100. I was able to get the BLMs into a reasonable range by pulling a bunch of fuel out of it, and I am sure I could get them perfect if I kept working with it. However as soon as I step down on it and it goes into PE mode it falls flat on its face, pings and runs like crap so I am wondering if my original BPW calculation was way off. No point in continuing to work with this fueling map if I am going to need to start over. Suggestions?

[Chewy1576]

I replaced the 350 in my K2500 with a mild 383 also. I had it running very well with 80# injectors and 15 psi FP. I swapped out the SB TB unit for a BB one, and I couldn't get it to run very well, really rough and inconsistent idle. I got thinking about the engine and it was built to be able to utilize all of the stock 350 TBI equipment, so the BB TB may be too much for it. My theory was that air was moving too slowly through the BB TB unit to get a consistent air/fuel charge to the cylinders and that it was why it would run so rough. I swapped back to the SB TB unit, same injectors and FP, and it runs a lot better now.

[Fastmax32168]

I replaced the 350 in my K2500 with a mild 383 also. I had it running very well with 80# injectors and 15 psi FP. I swapped out the SB TB unit for a BB one, and I couldn't get it to run very well, really rough and inconsistent idle. I got thinking about the engine and it was built to be able to utilize all of the stock 350 TBI equipment, so the BB TB may be too much for it. My theory was that air was moving too slowly through the BB TB unit to get a consistent air/fuel charge to the cylinders and that it was why it would run so rough. I swapped back to the SB TB unit, same injectors and FP, and it runs a lot better now.

Thanks, but I actually dont have any issue with the idle or low speed running, at least nothing that a little more tuning wouldnt perfect. After the initial run and pulling the fuel out of it, it ran well and I thought, man this was easy, until I tried to put my foot in it. My problem surfaces as soon as I push it past 1/2 throttle or more, as though the fueling there is way way off.

[Chewy1576]

Do you need to adjust the AE and PE adders because you increased injector flow and FP? Could be pushing too much fuel in those situations. I would think they need to be reduced because you're feeding much more fuel than stock injectors.

[lionelhutz]

Do you have a wideband so you know if it's going rich or lean?

Did you make a head change that might require a very different timing curve?

It's quite possible that AE and PE modes might just need tuning once you get the VE table reasonable throughout the operating band.

[Fastmax32168]

I was thinking that if the tune was good on the 350 and I changed the BPW according to the new displacement and injectors, that it would be reasonably close on the 383. But after doing that I had to take probably 20% of the fuel out to get it close at idle and part throttle, so apparently not. I dont have a wideband on the system to tell if it is rich or lean, but I do have an LM-2 innovate unit that I have never used. I guess it is time to get familiar with it and try it out. Because I had to take so much fuel out at part throttle I am probably going to find out that it is really rich.

[Fastmax32168]

Maybe I should reword my thoughts like this. Since when I changed the BPW according to the mathematical formula the a/f ratio was so far off (20% or better) on a tune that was so close before, would I be better off to lower the BPW 20% so that everything was close again and work with that, or should I leave it as is and change all the fueling? Does it matter?

[lionelhutz]

Yes, you could drop the BPW and see what happens.