Thunderbolt 4 vs. Thunderbolt 5

[LazyLightning]

I'm running a Mercruiser 350 Mag / Alpha One in a 1987 Four Winns Liberator. The new ignition amplifier isn't stamped with the timing advance, mine is stamped V822. The wiring harness is for a Thunderbolt 5 mine is a Thunderbolt 4. I believe ignition amps are set for different timings. Its an OEM amp and since I called Merc customer service with my engine number, this is the replacement so I hope it works. Especially for 550 clams. Any thoughts on this?

 

[alldodge]

I'm running a Mercruiser 350 Mag / Alpha One in a 1987 Four Winns Liberator. The new ignition amplifier isn't stamped with the timing advance, mine is stamped V822. The wiring harness is for a Thunderbolt 5 mine is a Thunderbolt 4. I believe ignition amps are set for different timings. Its an OEM amp and since I called Merc customer service with my engine number, this is the replacement so I hope it works. Especially for 550 clams. Any thoughts on this?

The V822 stands for V8 engine with 22 degrees total advance.
Your engine came with a base timing a 8 BTDC using the TB4. The TB5 will use either 8 or 10 BTDC and variations between inboard, sterndrive, EFI and Carb. What gets me is you said Merc customer service used your serial number and said the TB5 will work? As you mentioned the wiring is different and you need to know the total advance curve.