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Thread: RPM Display "Stability". What is normal?

  1. #11
    Join Date
    May 2019
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    Ct
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    319

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    Your tachometer probably has some sort of filter to keep the needle steady.

  2. #12

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    Quote Originally Posted by 496Stingray View Post
    My issue is across the entire range from idle to 6000 RPM. Engine runs great, idles great, but I cannot fully stabilize AFR as the RPM signal and the MAP signal seem to "jump around" more than I'd ever expect. Indicative of high frequency noise coming from someplace. Attachment 10173
    Yeah, I wouldn't be real happy with that either. Although I don't think that's the culprit of the gross misconduct of the AFR.

    I guess I never really payed much attention to the fluctuation in the RPM trace, but all the Sniper datalogs I've seen seem to do it, some worse than others.

    For a quick test, remove your alternator belt, shut off anything that uses electricity that's not necessary, and give it a quick run and see if anything changes.

    Are you sure the engine is not suffering from pre-ignition (knock)?

    What spark plugs do you have in it and the gap?

    Have you checked the health of the engine lately (leak down test)? Are you confident all the valves are sealing?

    This just popped in my head, what was the reason you put the chokes on? Have you tried removing them one at a time just for testing purposes?
    Last edited by a b c; 07-29-2020 at 09:09 AM.

  3. #13

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    Quote Originally Posted by Danny Cabral View Post
    Your RPM trace (datalog in post #8) looks fine, and that's even with no Smoothing.
    OK, thanks man. I just find it odd that it goes "backwards in RPM" as it's clearly accelerating. But if that is normal (I've seen it in datalogs from other cars as well) then I guess it's a non-issue.

  4. #14
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    Dec 2009
    Location
    Connecticut
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    25,963

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    Distributor based RPM signals are like that (less resolution & worse timing accuracy). MSD Flying Magnet 4x crank trigger kits are a lot better. And Missing Teeth (12-1, 24-1, 36-1, 60-2, etc.) type crank triggers are absolutely the best RPM signal traces. When I first viewed how stable my 60-2/36-1 ignition timing was with my timing light, I slapped my timing light because I thought something was stuck. Yes, the timing stability is that amazing. You'll never want to use a distributor again...and I never did again!
    http://forums.holley.com/showthread....3038#post33038 (Detailed Crank Trigger/DIS Conversion Information & Reasoning)
    May God's grace bless you in the Lord Jesus Christ.
    '92 Ford Mustang GT: 385" SBF, Dart SHP 8.2 block, TFS T/W 11R 205 heads, 232°-244° duration/.623" lift/114° LSA camshaft, 12:1 C/R, TFS R-Series FTI ported intake, BBK 80mm T/B, Dominator MPFI & DIS, 36-1 crank trigger/1x cam sync, 200A 3G alternator, Optima Red battery, A/C, 100HP progressive dry direct-port NOS, Spal dual 12" fans/3-core Frostbite aluminum radiator, Pypes dual 2.5" exhaust/off-road X-pipe/shorty headers, -6AN fuel system plumbing, Walbro 255 LPH pump, S&W subframe connectors, LenTech Strip Terminator wide-ratio AOD/3000 RPM converter, B&M Hammer shifter, FPP aluminum driveshaft, FPP 3.31 gears, Cobra Trac-Lok differential, Moser 31 spline axles, '04 Cobra 4-disc brakes, '93 Cobra booster & M/C, 5-lug Bullitt wheels & 245/45R17 M/T Street Comp tires.

  5. #15

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    Quote Originally Posted by a b c View Post
    Yeah, I wouldn't be real happy with that either. Although I don't think that's the culprit of the gross misconduct of the AFR.

    I guess I never really payed much attention to the fluctuation in the RPM trace, but all the Sniper datalogs I've seen seem to do it, some worse than others.

    For a quick test, remove your alternator belt, shut off anything that uses electricity that's not necessary, and give it a quick run and see if anything changes.
    Did that and had no changes

    Are you sure the engine is not suffering from pre-ignition (knock)?
    Yes, I thought of that as well and pulled timing way way down - no changes

    What spark plugs do you have in it and the gap?
    AC Delco R44S- Tested gaps at .030 - .035. 045 no changes.

    Have you checked the health of the engine lately (leak down test)? Are you confident all the valves are sealing?
    Engine is a fresh (maybe 8000 miles) 425 Nailhead (.040 over) well built, balanced, forged, really stout for a nailhead. Cranking compression is 225-230 cold engine is 10.87:1 and calculates to 8.78:1 DCR with my cam profile, the leak down results were 4-7% across all 8 holes

    This just popped in my head, what was the reason you put the chokes on? Have you tried removing them one at a time just for testing purposes?
    We must think alike. Initially when I installed the system I had a new set of Accel 9000 series 8mm wires on the car and just swapped the ends for the HEI boots (which I still had in the box) instead of spending another $130+ on wires. I had MASSIVE EMI issues and began an isolation project to figure out why. Since I have a HEAVY electronics background I had already taken serious precautions with the wiring, the alternator, the main harnesses in the engine compartment and such. I was both surprised and kinda pissed when I saw how bad the ECU resets, traces going back in time, car ran like total crap, etc. Initially I thought no way those 500 Ohms per ft wires are leaking that much. So I literally pulled the entire Sniper harness back out of the car and moved it again, I put the ignition coil and CDI box on the ROOF of the damn car and it was still horrible. Next test. I grabbed the Reynolds Wrap and foil wrapped the spark plug wires and distributor cap. BOOM ZERO EMI! OK, so it's the wires. I added the RFI choke coils and removed the tinfoil and the EMI was about 60% gone. I then changed the wires to MSD Super Conductor Spiral cores and it was 100% solved. Have not had an issue since. From that point I left the Chokes on the car as it runs great. When I noticed the oddity with the RPM traces and the MAP kPa not being rock steady at cruise/steady throttle/steady load I thought perhaps the magnetic field might be causing some inductance and creating those "erroneous" readouts, so I removed ALL of the chokes. One by one and tested after each was removed, no changes. Since they were not hurting, and I had already observed that they were worth a good 50-60% reduction when I had the noisy wires on the car, I reinstalled them for added insurance. Figure why not, in case a wire starts leaking or something later at least the car won't crap out and leave me in a jam.

  6. #16

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    Quote Originally Posted by Danny Cabral View Post
    Distributor based RPM signals are like that (less resolution & worse timing accuracy). MSD Flying Magnet 4x crank trigger kits are a lot better. And Missing Teeth (12-1, 24-1, 36-1, 60-2, etc.) type crank triggers are absolutely the best RPM signal traces. When I first viewed how stable my 60-2/36-1 ignition timing was with my timing light, I slapped my timing light because I thought something was stuck. Yes, the timing stability is that amazing. You'll never want to use a distributor again...and I never did again!
    http://forums.holley.com/showthread....3038#post33038 (Detailed Crank Trigger/DIS Conversion Information & Reasoning)
    Hmm, I wonder if I can "conjure" something up that will work on the nailhead. It's funny you mention that because the problematic GM LT1 Opti-Spark was actually a really accurate system when working correctly for this very reason.

  7. #17

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    Well then, I think my next step would be to retrofit a crank trigger.

  8. #18

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    Just curious, but the HyperSpark Distributor I'm using says "fitted with a crank trigger sensor". I had hoped it would've been more accurate. I'm not opposed to trying to build/engineer some form of crank trigger to the car, but I'm a little annoyed that my points distributor seemed to be a more accurate trigger.

    My "bigger" issue is that at any RPM while cruising etc. the MAP kPa is jumping around all the damn time. I would've expected that to be stable especially on flat ground, constant load, no throttle movement and turning 3000 RPM. Since that MAP kPa is always moving the cells being referenced in the VE% Table is also always moving making a consistent and stable AFR difficult to achieve. I have to "tune" it by trying to keep the jagged line in the datalog traces as "close" to targets as possible.

    Now if I'm flying down the road at 3000 RPM and I set a wide range in the VE% Table to all the same value then the AFR stabilizes, if Closed Loop is disabled of course.
    When tuning a factory system GM/Ford whatever, I have never had so much "variance" when dialing in AFR. It'll usually hold a nice clean linear line unless I change the throttle or load. It makes me think this system is still reacting to "noise". But again if the kPa is moving due to an unstable timing that is just happening so fast I cannot feel it then it would explain the kPa/vacuum signal not being consistent. I'm trying to isolate the cause vs treating the symptom. Cheers and thanks for all the help, advice, feedback. - Rob

  9. #19

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    Rob, it's entirely normal for both the RPM and the MAP to fluctuate even when at a constant speed. In fact, if you were to see a "raw" MAP signal, you would be horrified. The MAP signal is highly smoothed in order to give a steady reading, otherwise, the individual pulses that are generated by each cylinder intake stroke would make MAP readings unusable.

    As for all of this making VE tuning more difficult, well that's just not the case. People often forget that the ECU is constantly interpolating the values between each individual cell in the VE% Table. With a 33x33 table, that is more than adequate resolution for any engine combination. In fact, I’ve seen back to back dyno pulls where the first pull was done using a 16x16 VE table and the second pull was done using a 2x2 table...yes, a total of 4 cells. The power was nearly identical. Now, the 2x2 table probably wouldn't drive very well, but the point was that interpolation between cells is a very powerful feature. Post your GCF. Andrew
    Last edited by andrewb70; 08-03-2020 at 04:38 PM.
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  10. #20

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    OK that makes a lot of sense and makes me feel much better. I did not realize that this MAP sensor was that sensitive. Also explains why the car runs great. Thanks again. I'll grab the latest GCF off the car once this storm lets up.

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