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Thread: Bad tune, or engine at its limit?

  1. #1
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    Question Bad tune, or engine at its limit?

    Last month, I took my '90 Corvette to the drag strip for the first time (if pertinent, it has a 1227727 ECM and runs $8D). I've been swapping parts bit-by-bit, and I've reached a pretty respectable combo:
    • 383ci
    • 9.3:1 SCR (8.4:1 DCR)
    • AFR 190 heads (the old-school ones from before the Eliminators)
    • Lunati 211/219 @ .050" cam (small, I know, but I didn't want to risk failing smog)
    • 1.6:1 roller rockers
    • Super-Ram plenum and runners
    • Edelbrock High-Flo intake base, gasket-matched on the head side
    • Hooker Elite 1 5/8" long-tube headers
    • Accel 33.3 lb/hr (advertised as 36) injectors
    • 58mm Holley throttle body
    • 3" front Y-pipe
    • Corsa cat-back


    I've never had it on a dyno, and I've done 100% of the tuning. It would have been cheaper and easier to pay to have it dyno-tuned, but I wanted to learn to do it myself. I finished the header install just hours before I went to the strip, so I didn't have as long as I would have liked to nail down the tune. That said, the timing shouldn't have needed much tweaking, and by the end of my tweaking my wideband was showing lambdas of around 0.85-0.88 (12.5-13:1 AFR) at WOT.

    While it was a blast at the strip, my highest trap speed was just 106.23 MPH. According to an online calc, that corresponds to 322.38HP. Even taking into account a 20% drivetrain loss (which I've heard is accurate for the ZF6 transmission), I'm barely breaking 400HP at the crank. Based on what I've read, that seems pretty weak for a built 383. Granted, my static compression ratio is a little low, but with such a small cam my DCR is already at the limit for pump gas.

    Do you think my combo just at its limit, or does my tune need work?

    Here are my relevant tables: spark advance, VE lower, VE upper, and PE enrichment and spark advance (if you're not familiar, it adds in the fuel and timing in those tables when I step on it). I've also attached a datalog (not from the strip, but it has some WOT) if you feel so inclined to look at that.
    SR34 Timing.PNGSR34 VE Lower.PNGSR34 VE Upper.PNGSR34 PE.PNG
    Attached Files Attached Files
    Last edited by 90Vette; 10-06-2018 at 04:15 PM. Reason: Fixed incorrect compression ratio numbers from 9.9/8.9 to 9.3/8.4.

  2. #2
    Fuel Injected! woody80z28's Avatar
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    I had a Vortec-headed 355 in my Camaro that trapped the same. Started at 101 and I tuned it to 106. 3300lbs race weight and I figured about 400hp on it. I was less cubes with more compression and more cam (10.55:1 and 230@.050). Seems possible that the extra cam & compression on mine offset the cubes and heads on yours.
    80 Camaro Z28 - 550hp AFR 383 / T56 6spd (Holley HP EFI)
    91 Beretta "SS" - 260hp 3400 MPFI / HM282 5spd (7730/$A1)
    94 Silverado Z71 - 300hp Vortec 357 TBI / NV4500 5spd (7427/$0D) gone but not forgotten
    01 Silverado 2500HD - stock Vortec 8.1 / ZF6 6spd (0411/8322)

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    Fuel Injected! Dirtybob's Avatar
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    do you know what the density altitude was? (it can be approximated from the time/location of your pass)
    could you post the timeslip info?
    400 hp isn't necessarily weak considering that mild cam....
    assuming you were into the low/mid 12s with that mph?
    how many rpm at the shift (you need to rev to make big hp #s)?
    Last edited by Dirtybob; 09-22-2018 at 03:03 PM. Reason: I can't read

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    Looks like the density altitude was 1,697 feet. I'm new to drag racing, but that doesn't sound too bad. According to another calculator I found, with that DA my HP should be 100.5% of rated HP.
    I was actually at 13.4. Granted, that was my first time going to the strip, but on that run I felt like I had a good launch and fast shifts. I'm running street tires and wasn't launching too hard, so that might contribute to the slow ET. Honestly, I was mostly shifting just by ear and feel, but I think I was shifting around 5200 rpm. It's hard to say because my tach is a bit off.

    By the time I figured out the basics, bracket racing had started, so I didn't want to mess with my launch RPM or shift point (no regrets, I somehow ended up second). From what I've read though, 2000 rpm is a decent launch for street tires, and 5200 is right about where a Super-Ram 383 with a small cam should be shifting. Am I off on that?

    R/T .064
    60' 2.104
    330 5.792
    660 8.758
    MPH 83.66
    1000 11.314
    E.T. 13.436
    MPH 106.23

  5. #5
    Fuel Injected! Dirtybob's Avatar
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    I don't think a 1700' DA equates to "100.5% of rated HP". I would expect it to calculate out at around 96%. Are you sure which barometric reading you used (corrected vs. uncorrected)?
    Cant speak to the power curve of your motor but you may be able to squeeze more rpm out of it (good heads & intake might carry the hp past peak for a while), try raising the shift points and see what happens.
    60' times are abysmal unless you're running hockey puck (aka all season) street radials. Get some better rubber and/or practice your launch. You will probably find that your mph will drop (but ET will be much better) with improved 60' times. I would be looking for 1.8 or better 60'.
    When using ET/mph to calc hp, just remember that the numbers are really only good for comparison. Say you put gears/stall converter/drag shocks/slicks on your car and it picks up 7 tenths and loses 2 mph? HP did not change but your numbers did...

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    I pulled that number from this calculator, which appears to use an SAE standard. Corrected for what? Temperature? The linked calculator takes temperature, altitude, barometric pressure, and humidity into account, so I assume it's making corrections.

    I was under the impression that even the old AFR heads are pretty good? And unfortunately the Super-Ram is my only option for a smog-legal intake. Next time I hit the strip I'll try some runs with a bit higher shift points.

    I know I suck at launching, I guess I was just hoping to have a higher trap speed with all the work I've done. But my spark and fuel curves look decent to you?

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    Fuel Injected! Dirtybob's Avatar
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    Most weather stations report corrected (to sea level equivalent) barometer. You need to uncorrect that reading or factor in for track elevation.

    I would say you have good heads, unfamiliar with the superram but it's got to be better tan the stock long-tube tpi. Wouldn't suprise me if your car pulls close to 6k before laying down. My stock (iron headed) LT1 with the wimpy little cam turns 5500+, I went as high as 5750 but no improvement vs. 5500 (shift completion rpm - auto trans).

    13.1:1 afr and 30 deg timing has worked well on my car (apples vs. oranges), yours may like more timing.

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    Hmm. Well, the track was only at 70' elevation, so I can't imagine that correction would have thrown stuff off too much. But good to know for the future.

    The Superram is waaay better than TPI, but it isn't meant to rev high. On a 383, everyone I've seen is shifting in the 5000-5500 range.

    Did you tune that with a dyno, or have you tweaked it with trial and error at the track? I think mine could take a bit more timing, but on the street it's hard to see if changing timing or fuel helped or hurt. I'm also a bit paranoid about knock - as a newbie, I am not familiar with what it sounds like (I've watched videos, but it's not the same). I've been getting lots of knock counts at lower RPMs which I've attributed to false knock, so it's hard for me to tell if knock counts at WOT could be false knock or if it's the real deal. I don't want to risk adding in timing, datalogging a run or two, and hurting my engine from detonation.

  9. #9
    Fuel Injected! Dirtybob's Avatar
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    Quote Originally Posted by 90Vette View Post
    Did you tune that with a dyno, or have you tweaked it with trial and error at the track?
    I baselined it with the butt dyno and then finished tuning at the dragstrip.
    I saw more kr on the street than at the track (varying conditions vs. WOT all the time). Going from light throttle cruising in 3rd gear around 50 mph to full throttle downshift (into 2nd) will still carry a bit of kr beyond burst knock - sometimes. Was able to run timing up to 35 deg with 93 octane and no (WOT) kr, ran the same ET/mph/60' as 30 deg advance on 10% ethanol 87 octane (also kr free). Car is a daily driver so cheap gas is a bonus. Worth mentioning that w/e pos opti I have is probably not indexed correctly from the mfr. - there are some timing light discrepancies. I quit worrying about it and just tuned it for what it wants. Small amounts of kr had no effect on ET/mph/60' but I tuned it kr free to be safe. Progressively leaned out PE afr as far as 13.5:1, sweet spot appeared to be about 13.1:1.
    stock 178k mile motor '95 Roadmaster, 3.42s/posi, ebay headers. 4450# as driven. 14.66 @ 92.5 - 2.045 60', 1800' DA. Needs more power (and a diet lol) plus a real (not a 12") torque converter...
    http://magic-photos.com/2011/Fri%20F...s/DSC_0306.JPG

    I would say if you can't hear it rattle (assuming your car isn't obnoxious loud), and the knock counts are minimal, you are at low risk to hurt anything. Have you tried higher octane to see if kr is reduced?

    Edit: headers will amplify valvetrain noise - but you probably already know that...
    Last edited by Dirtybob; 09-22-2018 at 07:00 PM.

  10. #10
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    So you tune based on knock retard, not knock counts? With your current tune, around how many knock counts are you seeing in a short (say 30min) drive?

    How quickly do you ramp up to your full 30? As you can see on mine, I don't reach the full 34 until 4800 RPM, so at say 3200 I'm only at 27.

    Did you find that optimum AFR just by what was fastest at the strip? Did you play around with timing at all at the different AFRs?

    My car's fairly loud, but not bad. The Corsa mufflers seem quieter than Magnaflows/Borlas/etc. It has a bit of a rattly noise at low RPM and high load, but I'm 90% sure that's just transmission noise (knock sensor doesn't pick anything up). I'd been running it on 91, but since I drove it out to the east coast I've been using 93 octane. I haven't been able to take it to the strip out here, but it seems to run the same. Looking at my datalogs, the occurrence of low-RPM knock doesn't seem to have diminished, and I was/am seeing WOT knock so infrequently it's hard to say if the 93 made any difference.

    Actually, I'd forgotten that, but the low-RPM knock was there before I did the header swap.

  11. #11
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    Quote Originally Posted by 90Vette View Post
    So you tune based on knock retard, not knock counts? With your current tune, around how many knock counts are you seeing in a short (say 30min) drive?
    with the exception of sometimes seeing lingering kr after forced downshift/throttle stab, the only knock counts I see are at startup.
    I am talking about following father time up the interstate on ramp and he wants to merge with 80+ mph traffic doing 45. Stabbing the gas and passing gramps will occasionally generate counts/kr. I think it's an artifact of lugging while the trans decides to downshift plus the burst knock function. I don't drive it like that much and will manually downshift to avoid lugging the motor 99% of the time.

    How quickly do you ramp up to your full 30? As you can see on mine, I don't reach the full 34 until 4800 RPM, so at say 3200 I'm only at 27.
    laptop temporarily down - IIRC it's all in b4 3k rpm.

    Did you find that optimum AFR just by what was fastest at the strip? Did you play around with timing at all at the different AFRs?
    actually tuning for consistency while looking to improve ETs too (and keep respectable mpg). A lot of back and forth on timing and afr in the beginning. Timing first, then afr, then back to timing - lather rinse repeat.
    I bracket race the car so finding that last .05 sec. doesn't mean a whole lot to me. It's slow and I know it, FWIW I don't need this car to ever go quicker than 13.50.

    My car's fairly loud, but not bad. The Corsa mufflers seem quieter than Magnaflows/Borlas/etc. It has a bit of a rattly noise at low RPM and high load, but I'm 90% sure that's just transmission noise (knock sensor doesn't pick anything up). I'd been running it on 91, but since I drove it out to the east coast I've been using 93 octane. I haven't been able to take it to the strip out here, but it seems to run the same. Looking at my datalogs, the occurrence of low-RPM knock doesn't seem to have diminished, and I was/am seeing WOT knock so infrequently it's hard to say if the 93 made any difference.

    Actually, I'd forgotten that, but the low-RPM knock was there before I did the header swap.
    trans or valvetrain noise, mine does the same thing. 99% sure its just valvetrain racket on mine...

  12. #12
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    Quote Originally Posted by Dirtybob View Post
    I am talking about following father time up the interstate on ramp and he wants to merge with 80+ mph traffic doing 45.
    Hahaha that killed me.

    Maybe I'll try bumping up timing leading up to my max. Although it doesn't feel like it needs timing there - torque feels pretty strong until it seems to run out of steam at around 4500.

    How many runs do you do to check for knock? Do you make changes between every run? Because I'll increase timing and not have it knock until I get a single count on maybe my fourth WOT pull. It makes me kind of unsure when I can call a certain timing map safe.

    Also, unrelated but have you messed with startup enrichment at all? Ever since I came out to the east coast, I've been having problems with it running really rough and surging at part throttle (especially when I first add throttle) until it warms up. Only seems to happen when it's dead cold. I'm wondering if it's related to the humidity, because I don't remember ever having this problem out in CA. Unfortunately I don't have time to hook the wideband back up, but I've been playing with startup enrichment to try to make it better.

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    I tuned for knock on the worst 90+ degree, high humidity (crap air) days that I could. Street driving because it doesn't knock at the track. Maybe collect 2 hrs of data over a couple of days then check it. Variety of driving conditions, lots of flogging the car. Trying to generate worst case scenario if that makes sense.
    Haven't touched startup enrichment, haven't needed to (besides the headers/home depot intake the motor is stock).

    Surprising to me that you say the motor "seems to run out of steam at around 4500".

    Back in 1992 I had an '87 IROC Z (305 tpi/auto/3.27 rear gear) that I swapped in a 350 crate motor originally for a 1/2 ton pickup (opportunity swap). Edelbrock headers/3" single and a 204/214 .465 hydraulic flat tappet. Bumped fuel pressure to 50 psi and used the stock 305 injectors. Had a buttload of people tell me it wouldn't work but it did. Ran 14.50s with 1.8/1.9 60's and I drove it everywhere and so did the old lady. Shifted at 5k rpm and broke the pos 7.5" rear three times, would also shred the rear trans/front torque arm mount with regularity.
    Best it ever ran with the 305 was 15.9x...
    paid AS&M for a "custom chip" that killed my gas mileage but did fix a slight 'hunting' issue I would see at idle. No improvement in ET/mph/60', complete waste of time & money.

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    Hmm, I tuned mostly in the evenings, so I guess next summer I'll have to dial in my maximum timing a bit more rigorously. Does false knock from valvetrain noise not happen at WOT?

    I dunno, maybe it's just the nature of a long-tube runner intake and small cam on a stroked engine, but it feels like right when it should really be getting into power it starts losing torque. Granted, I'm shifting at 5200-5300, but it just doesn't feel like it has the top-end oomph it should. Probably just a case of misplaced expectations.

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    Does false knock from valvetrain noise not happen at WOT?
    I don't really know. I'm using a mid length header and my exhaust likes to resonate around 2500 rpm at certain loads (not WOT) and on decel. Shows up on the logs (in green) but does not pull any timing. Never saw it when I was running manifolds. Will see if it's still doing it after I repace the motor mounts.
    I had limited my maximum KR in PE mode to 5 degrees. Getting into KR at the track (when advancing the timing) did not seem to have a whole lot of effect on ETs but I think my WOT AFR is a bit on the lean side. It could act differently with more fuel.
    I found that with mid 12 AFRs my car would speed up more than I liked if the DA started dropping so I set it a bit more lean to (hopefully) make it more consistent.
    I have diffculty dropping ET at the big end of the track when being chased by a car/bike/dragster thats 70++ mph faster than me and I am on a breakout run, for that matter my butt dyno isn't too good at letting me know I am about to break out either... Maybe I should just keep out of trophy and gamblers ;)

    Lunati advertises that cam as 1600-5600 (prolly in a 350). so maybe 5200 in a 383...
    try shifting at 5500 and then 5000 next time out

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