Negative timing and low rpm after cam swap 2018 6.2 L6

Installed a BTR 220 cam in a L86 Sierra and the idle is getting dragged down by negative timing. If I rest my foot on the pedal and keep the rpm at 800/850 the truck runs great and chops a little. If I let go the map pressure builds to 80/90 and the timing drops deep negative and pulls the idle down. Then it bounces back up for a second and repeats. We smoke tested it and it had no vacuum leaks. Haven't driven it yet but the pedal seems fine, revs free, engine sounds good too. But it doesn't want to idle. Stock throttle body too, only changed the cam. I've tuned tons of LT1s but not very many L86s. Any big differences there?

Same methods apply…

I almost feel like something might be wrong with the throttle body for some reason. I'm gonna see if we have another one laying around we can test out.

The one thing I noticed in the stock tune file that was different was the Idle external load. The stock Lt1 numbers are much lower then the L86 numbers. Should I be lowering these instead of raising them? I usually set them around 150-200 in the idle range. On this truck everything was 500 where it originally idled. I have the idle around 200 now but usually that doesn't do much anyway

Sounds like VT tables need adjusted in the idle area.

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Originally Posted by JASON11WS6

I almost feel like something might be wrong with the throttle body for some reason. I'm gonna see if we have another one laying around we can test out.

The one thing I noticed in the stock tune file that was different was the Idle external load. The stock Lt1 numbers are much lower then the L86 numbers. Should I be lowering these instead of raising them? I usually set them around 150-200 in the idle range. On this truck everything was 500 where it originally idled. I have the idle around 200 now but usually that doesn't do much anyway

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What your adjusting is the peak allowed torque "limiter" if you will. Not what you need to be adjusting. You need to get torque close to bring timing and rpms back in line then adjust ALL of your fueling controls, so torque and fueling stays pretty close in line from cold start idle to hot idle. Otherwise timing and fueling will be ALL OVER the place.

Also to literate on this some more since everyone just kind of "blindly" does this, you don't want to "just" lower or pull out torque like most advise to shift or rather force the timing to come back up - everyone seems to watch that goatrope video and go straight to it. Sometimes if things aren't "far" off you're better off to do this cause it keeps the stock slope and curvatures, but most of the time you're better off to "twist" or "work" the idle areas until timing is right and torque is positive. Having torque negative will stabilize blower setups fairly nicely, but in "cam only" mods you're better off having it positive. Takes work, but you can get it.

furthermore on what greg mentioned, you need to get your airflow "MODELS" dialed in before touching VT