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Thread: LT1 Hard RPM Limit

  1. #1
    Fuel Injected! Terminal_Crazy's Avatar
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    LT1 Hard RPM Limit

    I see in Kur4o's LT1 XDF the usual Fuel Cut Offs:
    Speed:26DC & resume 26D7
    RPM: 26D8 & resume 26DD

    I can also see
    167B4: Max RPM for 0 bpw
    167BA: " " " " " " " must be same as above.

    Max value 255 (0xFF) sets it as 6375 RPM

    I've read many threads about big cams being shifted at 6700+ over the years and LT1's spinning to 7k.
    Maybe their tachos were out?

    If this hard limit is correct (and I suspect so because I can't get this motor over 6400 rpm), can it be raised or deleted in any way?
    I'm still making power at this point.

    Thanks
    Mitch
    '95 Z28 M6 -Just the odd mod.
    '80 350 A3 C3 Corvette - recent addition.

  2. #2
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    167B4: Max RPM for 0 bpw
    167BA: " " " " " " " must be same as above.

    Stock is set at $e6=230*31.5=7245 rpm,

    You are not hitting that kind of limiter.

    The whole routine that runs the engine is cut at 7228 rpm based on low res signal, so it is neither this.

    What happens when you reach 6400rpm, Is the fuel cut or the spark gets too weak.

    I strongly recommend upgrading to ls1 coils for high rpm performance, you can get 20-30 hp increase , solely on this mod.

  3. #3
    Fuel Injected! Terminal_Crazy's Avatar
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    Quote Originally Posted by kur4o View Post
    167B4: Max RPM for 0 bpw
    167BA: " " " " " " " must be same as above.

    Stock is set at $e6=230*31.5=7245 rpm,

    You are not hitting that kind of limiter.
    OK. The XDF shows it as 6375
    Would it be better set back to $E6 rather than $FF>

    The whole routine that runs the engine is cut at 7228 rpm based on low res signal, so it is neither this.

    What happens when you reach 6400rpm, Is the fuel cut or the spark gets too weak.
    Not really sure, it just seems to sit there. It doesn't sound or feel like it's missing, it just doesn't rev any higher.

    Since the dyno run last year, I have upgraded to a 58mm TB from the stock 48.
    Map started dropping at about 4k and was down to 90% by 6k4
    I've also just replaced the 1.5" Mac Mids to 13/4 Speed Engineering Long Tubes.

    I still have a Racetronic 255 & Hot wire kit to fit yet, I think fuel is getting into short supply.

    I strongly recommend upgrading to ls1 coils for high rpm performance, you can get 20-30 hp increase , solely on this mod.
    I've acquired an MSD 8226 coil.
    I've also read the later 96/97 LT1 coils were better than 95-96.
    Or are you suggesting multiple coils & controller like Scotts (Pfautsch) or Torquehead ?


    Thanks
    Mitch
    '95 Z28 M6 -Just the odd mod.
    '80 350 A3 C3 Corvette - recent addition.

  4. #4
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    Or are you suggesting multiple coils & controller like Scotts (Pfautsch)
    That is exactly what I am suggesting. I am sure you might have the controller already, so you need the coils and some connectors with custom wires.

    If the problem was fuel related you will definitely feel something like knock. The problem might be mechanical limit of cam or the spark getting weaker with the less charge time being available at higher rpms.


    The xdf must have used the *25 conversion instead of *31.5.

  5. #5
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    don't you usually want to pull timing at high rpm?

  6. #6
    Fuel Injected! spfautsch's Avatar
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    Quote Originally Posted by tayto View Post
    don't you usually want to pull timing at high rpm?
    Spark timing and energy (the result of dwell time) are two completely different things. With a distributor setup there is less time for dwell (think of it as charging time) between firing events, thus reducing spark energy.

    With that said, I'm relatively sure I've got a log somewhere (please dont ask me to locate it) demonstrating 6700+ RPM back when I was running the stock ignition system with my setup before the stroker build.

    Since the stroker build, which has only been run using the diy-ltcc setup, I've rarely had the opportunity to spin the engine above 6000 or so because the tires and / or clutch can't do anything productive with the torque. I now have solid lifters and shaft mounted rockers so it should be able to rev to the moon and still make power.

    However, I only know of one instance where I might have hit the limiter in 2nd and was unfortunately not logging at the time. I also didn't have a 100% full tank of fuel, so it could have been caused by the fuel pump sucking air. Whatever the case, without finding some drag slicks there's not much point in spinning my setup far beyond 6k unless I'm trying to scare the crap out of myself or an unwitting passenger, or see how fast the chariot will go on a straight stretch.

    I've been wanting to update the controller thread with what I'm doing, but would rather keep soldiering on at it without the distraction of questions. I've been thwarted by tumultuous weather swings and now a viral outbreak. No-one in my tribe is sick, but the widespread panic hasn't been helpful.

  7. #7
    Fuel Injected! Terminal_Crazy's Avatar
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    I was just after confirmation that the PCM was capable of more than 6400 rpm.

    I had been looking at the MSD digital 6a, but more for the 2 step launch & Timing retard built in.
    Kur4o: Could those be patched easliy to EEHack ? :-)

    After a dyno pull last year of 429 hp and 325 to the back wheels i was a bit miffed.
    I later took it racing but could only run over the 1/8 and the car had serious traction issues, far worse than on the street.

    So hopefully I've fixed the shortcomings on the motor that I didn't upgrade on the original rebuild.
    Fixed the manifold leak and gaskets pulling into the ports ( really badly)
    I'm now waiting on some UMI suspension parts for the rear, Adjustable LCA, relocation brackets & Panhard bar and a pair of 15x10 Dragstars.
    I'd got another dyno day arranged for april and now we're on lockdown.

    Hope none of this Corona Virus affects anyone.
    Stay safe guys


    Thanks
    Mitch
    '95 Z28 M6 -Just the odd mod.
    '80 350 A3 C3 Corvette - recent addition.

  8. #8
    Fuel Injected! steveo's Avatar
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    i've had these things running near 6800rpm a lot without issues.

    it does hit a software 'limit' of 6375 in some parts of the code because low resolution RPM is stored in a byte (8 bits), you see it labelled as RPM25 in a lot of places...that's because it's actually raw rpm * 25 (where 255, the maximum value of an unsigned 8 bit integer, is 6375)

    i've seen a lot of functions that store an 8 bit setting (like say an EGR enable RPM or whatever) compare against that value, but those are things that don't happen in upper RPM ranges.

  9. #9
    Fuel Injected! Terminal_Crazy's Avatar
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    Thanks for the replies guys.

    I'll finish my upgrades and see where I am later, after this epidemic has died down and we're allowed back out.

    Thanks
    Mitch
    '95 Z28 M6 -Just the odd mod.
    '80 350 A3 C3 Corvette - recent addition.

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