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Thread: HP shutting down with boost increase.

  1. #21
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    Nov 2014
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    Ont. Canada
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    There will always be unburnt oxygen in the exhaust when there's a misfire. An engine is nothing more than an air pump. The WBO2 sensor does not care how much unburnt fuel is in the exhaust, it only reads how much oxygen is in the exhaust. When you have a misfire, there's always more oxygen in the exhaust than fuel. Think about your AFR, if it had a misfire when it was at 14.7/1, you have 14.7 times the amount of oxygen in the exhaust than there is fuel.

  2. #22

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    I hear what you're saying and it make sense. I was only repeating what AFR meter guy had told me.

    Back to the problem, going to remove inlet to the throttle body so there'll be nothing in front of it other than air. If test shows 6000 RPM, I'll know there was a restriction somewhere.

  3. #23

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    Took the car out today without blower or any piping to the throttle body. No failure. I ran only to 5500, but not a hint of noseover. Sure it would've pulled to 6K, but without the blower things happen slower. At this point I know the ignition is not stressed like it would be with the blower, but no hint of a failure.

    Now I have to determine if there is something about the piping, intercooler, or blower that's causing the intake to see negative pressure at wide open throttle after the failure occurs.

    Has anybody out there seen something like this with a Vortech YSI causing a restriction to the inlet tract?

    Thanks to 81 TransAm and everybody else that responded to make me look at the datalog again.

  4. #24
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    Nov 2014
    Location
    Ont. Canada
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    Blow-off valve? Intercooler? Belt slip? Pulley to blower shaft slip?

  5. #25

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    Intercooler Precision intercooler air-to-water unit, 3 inch in & out.
    Nothing obvious when I removed the tubes.

    Bypass yes, would lose boost, but the thing that's most disturbing is negative manifold pressure at WOT.
    The thing that confirms this is the datalog of the failure where I kept my foot in it, the AFR went down to 8.9.

    I always think of AFR ratio and the amount of fuel being injected,
    but if you shutdown airflow, this will change that ratio as well.

    Belt slip here again would account for boost loss, I can't see it causing negative manifold.
    No indication of any rubber in the pulley area. 10 rib pulley.

    Pulley Slippage on shaft.
    Vortech YSI has keyway that drives the shaft.

    I have to do some more checking, but almost sounds like something being sucked closed. Once again thanks for looking!

  6. #26

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    Quote Originally Posted by jdefina View Post
    I have to do some more checking, but almost sounds like something being sucked closed.
    That was going to be my suggestion. Have seen this many times with silicone elbows on the blower inlet.

  7. #27

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    Well you might have something there. I do have a silicone elbow at the inlet and did so because of the distance to the rocker cover. After this point to the air cleaner it has another elbow, but this one is black truck inlet style, doubt this one is it. Going to rip it all out and inspect.

  8. #28

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    How about a faulty MAP sensor?
    351W, 800HP stock shortblock assy

  9. #29

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    Thought about that and I guess anything is a possibility at this point. I've removed the whole input tract and think I might have discovered the problem.

    Won't say anything more until I test. Have to wait, weather has been terrible here. I'll post results as soon as I know. Thanks for the response.

  10. #30

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    Had the car out today. I replaced silicone elbow into the blower with rubber truck style.

    Problem gone. Apparently when increasing the boost the elbow got sucked shut. This is what caused the negative MAP
    while throttle plate fully open.

    You might ask why did I use silicone elbow, well distance from blower inlet to the rocker cover is very tight. At eight psi, no problem, but when boost went to 11+, it sucked the elbow in.

    Again, thanks to everybody that responded. I chopped the rocker cover and rewelded, but it was still tight.
    Last edited by jdefina; 10-31-2020 at 01:03 PM.

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