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  1. #11
    LTx Guru


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    1994 Formula
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    Quote Originally Posted by Fastbird View Post
    If you have MAC headers, they are indeed 1 5/8 and you are certainly leaving power on the table.

    But......I'm confused. If your TQ peaked at 405, then that is your peak TQ. So 429/405. That actually pretty solid for that small of a camshaft. That power curve is beautiful. You have a PERFECT ripping around the street machine IMO.
    The problem is the graph shows FLYWHEEL HP. How do you measure FW HP on a chassis dyno. They told him they measure rwHP, then back-calculate FW HP. With that explanation, nothing makes any sense at all.

  2. #12
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    Mitch
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    '95 Z28 LT1 M6
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    Hi
    Reply from the Dyno guys:

    The lower of the two readouts is the wheel HP, the higher of the two is flywheel HP.

    The other plotted line is indeed torque, which is at the flywheel.


    ... so that clears that up then!
    Thanks

    Mitch

  3. #13
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    Doesn’t really clear it up. Those numbers represent a driveline loss of 18.8%

    He had to start with the 348.4 HP he measured at the rear wheels. The flywheel HP has to be calculated based on an ASSUMPTION of drivetrain loss. Maybe he somehow started with torque at the rear wheels, and backed into torque at the flywheel, then calculated flywheel HP, but it requires an accurate engine RPM. Seems more likely he started with wheel HP, divided by 1 minus the ASSUMED drivetrain loss, and the program calculated the flywheel torque from the ASSUMED flywheel HP.

    Note that drivetrain loss is calculated by dividing the difference between flywheel HP and wheel HP by the flywheel HP, not the way you did it to get 23.1%.

    429.0 - 348.4 = 80.6

    80.6 / 429.0 = 0.188 = 18.8% loss

    Might be close for an auto trans, but with the T56 I would use 13.5% at your HP level - see the next to last paragraph in post #7.

    348.4 / ( 1.000 - 0.135) = 348.4 / 0.865 = 403 flywheel HP.

    You indicate he didn’t actually measure RPM of the engine. Did he ask for the rear axle ratio? Because he can’t derive engine RPM from the dyno drum RPM without the rear axle ratio and the tire rolling radius (and that changes during the pull). Are you sure he didn’t clip an inductive pickup to one of your plug wires? Could the data RR 150 possibly indicate he ASSUMED a tire radius is 15”? Could TN 3.721 be the ASSUMED axle ratio?

    Where did you get the A/F ratio numbers? Typical sweet spot fo peak torque and HP is generally between 12.8-13.3:1. So it’s not necessarily leaning out excessively.

    Maybe it’s a moot point, because either way you need to find out why the engine is making 70 to 95 HP less than the core elements indicate it should be making. Several of the items already suggested need to be improved.

  4. #14
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    Thanks Fred,
    Nope, no tach pickup fitted. I'm pretty certain they attached one to the previous car which is why I quiried it.
    Never asked for gear sizes/tyre sizes or anything else. I just said 4th is 1:1
    RPM: Dyno chart prints out to 6850. I had Steveo's EEHack running on the laptop so it connects & logs when the ignition powers the PCM to communicate.
    All three runs revved out at 6144,6199 & 6275
    Rev limiter set on PCM is 6700 in 1st and 7k other gears, although Kur4o's XDF lists a setting of 6375(MAX RPM for 0 bpw)
    I'd presumed the PCM was limited to 6375 RPM but did log 6795 on the way home.
    I'd never got the motor over 6400 previously.

    AFR was off wideband & connected to EEHack.

    Just looked at another printout of a 5th Gen . RR150 & TN 3.721

    I'll see how I do at the track Saturday.
    It was definately struggling for breath with the map dropping down.
    I guess that's more likely the intake rather than exhaust, so a throttlebody first is the easiest fix.

    Headers are doable but i'd want to do duals and need cats and cutouts, so the cost has just escalated.


    Thanks for the insight sir
    Mitch

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