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Thread: MAP sensor as knock sensor.

  1. #1

    Default MAP sensor as knock sensor.

    I recently developed a strange vibration in my twin turbo LS2. While reviewing some datalogs I noticed some strange spikes in my crankcase pressure sensor (1 bar MAP sensor). I think I have read somewhere people are using MAP sensors to detect knock/preignition. Does anyone have any info on this or possible examples of what a datalog might look like showing preignition? Spark plugs look like it might be a little advanced, but nothing crazy.

  2. #2
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    That's only for engines with a vacuum pump (LINK & LINK).
    May God's grace bless you in the Lord Jesus Christ.

    '78 BRONCO: 508" stroker, TFS heads, 11:1 comp ratio, Dominator MPFI & DIS, cold air induction, Spal dual 12" fans/aluminum radiator, dual 3" exhaust/Magnaflow mufflers, Moroso vacuum pump, Accusump, engine oil & trans fluid coolers, 100HP progressive dry direct-port NOS, A/C, LenTech Strip Terminator wide-ratio AOD/2500 RPM converter, 3:1 Atlas II, modified Dana 44/60-lockers-4.10s, hydroboost/4-disc brakes, ram-assist/heim joint steering, Cage long radius arms, traction bars, 4" Skyjacker lift, 35" mud tires

  3. #3

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    Thanks Danny, I have read those links. My crankcase vent system pulls a fairly steady 39-40 kPa (similar to vacuum pump), and I have never seen these spikes before in a datalog. So I'm fairly certain I'm detecting something odd. I had never heard of using MAP sensor like that before, and happened to be using one for a crankcase sensor. After reviewing some older data and comparing to new data, there's definitely something going on, considering I have this new weird vibration/resonance I can't figure out.

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    Quote Originally Posted by vette09 View Post
    My crankcase vent system pulls a fairly steady 39-40 kPa (similar to vacuum pump)...
    That's about 18 inHg of vacuum (LINK). It may be too much, so try lowering it and see if the problem stops.

    ...and I have never seen these spikes before in a datalog. So I'm fairly certain I'm detecting something odd.
    Ensure the ignition timing is still synchronized.
    May God's grace bless you in the Lord Jesus Christ.

    '78 BRONCO: 508" stroker, TFS heads, 11:1 comp ratio, Dominator MPFI & DIS, cold air induction, Spal dual 12" fans/aluminum radiator, dual 3" exhaust/Magnaflow mufflers, Moroso vacuum pump, Accusump, engine oil & trans fluid coolers, 100HP progressive dry direct-port NOS, A/C, LenTech Strip Terminator wide-ratio AOD/2500 RPM converter, 3:1 Atlas II, modified Dana 44/60-lockers-4.10s, hydroboost/4-disc brakes, ram-assist/heim joint steering, Cage long radius arms, traction bars, 4" Skyjacker lift, 35" mud tires

  5. #5
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    Quote Originally Posted by Danny Cabral View Post
    That's about 18 inHg of vacuum (LINK).
    I think your math is skewed. Backwards there. 30-40 kPa of vacuum is only about 9-11 inches of vacuum.
    Which is still a little bit much unless your engine was built for that kind of vacuum (rings need to be done differently).
    I would try to limit an engine not specifically built for that much vacuum to somewhere around 8 inches of vacuum maximum (or 27 kPa of vacuum).
    Last edited by S2H; 01-12-2018 at 06:51 PM.
    -Scott
    Don't forget to check out progress on my Race Car:
    Project Blasphemy - 8.07 @ 171
    Low 8 Second Street Car

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    Quote Originally Posted by S2H View Post
    I think your math is skewed. Backwards there. 30-40 kPa of vacuum is only about 9-11 inches of vacuum.
    No, you're thinking of "inHg Absolute", as opposed to "inHg Gauge": LINK. (I.E. 100 kPa = 0 inHg.)
    He's using a 1 bar MAP sensor, which measures engine vacuum, the same as in the EFI software.

    Which is still a little bit much unless your engine was built for that kind of vacuum (rings need to be done differently).
    I would try to limit an engine not specifically built for that much vacuum to somewhere around 8 inches of vacuum maximum.
    Yes, I agree.
    May God's grace bless you in the Lord Jesus Christ.

    '78 BRONCO: 508" stroker, TFS heads, 11:1 comp ratio, Dominator MPFI & DIS, cold air induction, Spal dual 12" fans/aluminum radiator, dual 3" exhaust/Magnaflow mufflers, Moroso vacuum pump, Accusump, engine oil & trans fluid coolers, 100HP progressive dry direct-port NOS, A/C, LenTech Strip Terminator wide-ratio AOD/2500 RPM converter, 3:1 Atlas II, modified Dana 44/60-lockers-4.10s, hydroboost/4-disc brakes, ram-assist/heim joint steering, Cage long radius arms, traction bars, 4" Skyjacker lift, 35" mud tires

  7. #7
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    He just said he's using a MAP sensor, but it does not mean he's using it in engine MAP scale of no vacuum being 100 kPa.
    He's talking about using it as a vacuum sensor where 0 kPa represents no vacuum.
    He's not talking in 30-40 engine kPa where 100 kPa is WOT and 0 kPa is full vacuum.
    He's talking 30-40 kPa of vacuum, an absolute starting at ZERO vacuum and numbers getting larger as vacuum increases.
    Meaning 0 kPa is Zero vacuum and 50 kPa would be half vacuum and 100 kPa would be full vacuum.
    Or to put it easier, 0V would be 100 kPa of vacuum and 5V would be 0 kPa of vacuum.
    -Scott
    Don't forget to check out progress on my Race Car:
    Project Blasphemy - 8.07 @ 171
    Low 8 Second Street Car

  8. #8

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    I had 40 kPa as 11.8 inHg, but after further review I think the sensor is bad. Went to verify it with handheld vacuum pump and sensor fell apart when I pulled the vacuum hose off. I think all the heat from the turbos killed it. Hooked standard vacuum gauge up to engine and it maybe pulls 1-2 inHg. Still don't know what is causing this vibration/resonance. Double checked timing sync and all is good (LS engine). Inductive Delay is slightly off, but nothing that should cause it to resonate at idle. Went back to old known good tunes, verified every coil is firing, changed to a different set of injectors (this balanced A/F ratio bank-to-bank way better), found & fixed some small exhaust leaks and even put my old turbos back on, but still vibrates/resonates. Seems to be worse at idle with no load. Feels like something is out of balance, more than a misfire.

  9. #9

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    UPDATE: The old sensor was definitely detecting something, and I had it scaled incorrectly. It was scaled as 1 bar, but was a 3 bar. I'm sure if that's why it was reading 40 kPa. I installed new 3 bar MAP sensor as crankcase pressure sensor, and I'm still seeing something strange. Seems like it's detecting too much timing under some load/boost conditions. I datalogged a drive on a back road and could see under boost at higher RPM the crankcase pressure goes very noisy. Under same boost at lower RPM, I see much less noise. I've been using this info to tune my timing map and thing is running much better. Engine is 403 twin turbo LS2 with 10.1:1 compression. I think the compression is a little much for the pump gas race fuel mix I've been using. Please comment, I could use all the help I can get☺ Global File & datalog attached.
    Attachment 2790 Attachment 2791

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