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Thread: Max boost on a LT4. Anyone hit 19 psi safely?

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    Max boost on a LT4. Anyone hit 19 psi safely?

    Running a ported 2650 on a LT4 Z06 and we hit 19psi. The car hit knock at around 4k every time no matter how much timing we seem to take out of it.

    Did we hit a limit here? The car is running meth too so octane shouldn't be an issue and the injectors are only at 5.4-5.5 pulse width.

    After it peaks around 5k the boost seems to leak out to. I wonder if the blower pulley is to small (2.75) and we're getting belt slip

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    STEVE.TRAPP.METHL8.008.hptSTEVE.TRAPP.METH08.08.hpl

    Checked to make sure the bypass wasn't opening on the last pull and it was not. So I'm guessing I'm getting some belt slip. Any ideas on the knock?
    Attached Files Attached Files

  3. #3
    Senior Tuner Ben Charles's Avatar
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    I’ve made 23psi on stock lt4 bottom end stuff

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    You're spraying so much meth to compensate for fuel that the air has no where to go. You can only cram so much air/liquid meth through the bricks especially when they're designed for only air to pass through them. Knock is most likely from un even air/meth distribution.
    Last edited by HeavyChevy305; 09-25-2020 at 02:48 PM.
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    Quote Originally Posted by HeavyChevy305 View Post
    You're spraying so much meth to compensate for fuel that the air has no where to go. You can only cram so much air/liquid meth through the bricks especially when they're designed for only air to pass through them. Knock is most likely from un even air/meth distribution.

    I was getting knock in the exact same spot with less and with no meth. The only time we seem to get knock is right there at 4k like clock work. Nothing up top. Also, the brick on this Edelbrock 2650 is HUGE. Amazing intake temps.

    Plugs are all good and gapped to .025 wires are new.

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    After a belt change we are making 20 psi. The car is dying to lay down 900rwhp but continues to get knock around 4k like clock work so we might put the stock balancer back on it.

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    Im no expert, but are you sure its real knock? Like you can visually see puffs leaving the exhaust? Or could it just be a VT issue?

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    Exactly ShaneB. Might be worth the hassle of putting a Plex on it to see if the knock is real.

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    I think it might be false knock. It didn't change when I added meth, fuel and reduced the timing. What can I do to fix it or diagnose it? I don't have a plex system.

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    Post your latest tune if you don't mind.
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    It hasn't changed much since the one I have posted here. Took some timing out where the knock occurs but that's about it.

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    Like I said, I'm no expert tuner, and you're not logging TMA so I don't know if that would also have a value during the KR event, but around 4500rpm, immediate commanded trq is 879lbft and engine trq is 950lbft. From what I understand about the VT system, if its this far off it would cause issues. Maybe you have to up your DD or lower the VT tables. Looks like your throttle is closing very slightly over and over during the pull as well

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    Put the stock balancer back on (car had a 9% overdrive) and it dropped about 3psi. No more knock and we made an easy 850rwhp in 5th gear. We tried to make a 6th gear pass and see if we could hit 900 but the dyno broke.

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    Can you post a log?

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    Here is the finally log I saved. I think we made 836 with this pull. We added some more timing on the next pull but I must not have saved it.
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    Pretty sure that's not the log you're thinking of. Didn't even go above 4k rpm in that one