I am getting ready to fire up the motors in the boat and need to know what the total timing should be set at.
468 big block chevy
8 to 1 compression
twin turbos with intercoolers
factory square port heads
flat tappet hydraulic camshaft
850 cfm holley carb
This is a Mercruiser 475 twin turbo package that originally came in this boat.

I'd consider a msd 6 btm,set base at 34 degrees and pull some out with btm (it's got graduations from 0 deg per lb of boost to as much as 3 degrees per lb boost) then you would get strong idle and clean midrange when not making boost but safe timing under boost,if you make 10lbs boost you could pull timing back to 24 degrees in 1 deg/1lb boost setting,my .02

Opps forgot to say that it does have the MSD 6 box and the boost timing retard box with an adjustable dial. The turbos are set up to make 7 LBS of boost.

What are your jet sizes? I would set your idle timing to 8 this way you will have 32 in total. Run it from there pull some plugs. This should tell you if you can put some more lead in the motor. Now if you do you will also have to put more fuel into her.

The plugs will tell you what you need.

my .02

I would be careful about pulling to much timing out. It will drive EGT higher than most people realize. Are you innercooled? It will make a difference. Just my .02 and I am no expert.

The motors are intercooled. Not sure what jet sizes are in the carbs. This whole motor/turbo combo was set up 10 years ago by the 1st owner. I can check the other motor to see what it's timing is set at, but I have to figure out how keep the msd to show me full advance without it wanting to pull timing out.

Sutphen30 Here is a picture I too last year. nothing has changed with the motors since then. I think next year I will be converting both motors to fuel injection.

To check timing on your btm turn knob to zero and unhook your vacum/boost reference hose,Smitty

Holy Hot Plumbing!

The intake plumbing is simple, It's the water plumbing that is a nightmare. It has water cooler oil cooler, turbo shields, exhaust, wastegate, exhaust manifolds, turbo risers, exhaust pipes, intercooler, pre intake piping, carb plenum. What some people spend on braided fittings and lines for their motor, I can spend on simple hose clamps