Distributor advance curve for a heavy boat

[Finnerty]

My 26ft Tollycraft is running fine with the single Mark IV 454 on a 2:1 V-drive, but I’m wondering if I can improve performance by adjusting the advance curve on the distributor?

This is a 10,000 lb flybridge cruiser, 26’8” LWL, 10’ beam, runs at 7 knots at 1700 rpm most of the time, or 14 knots on plane at 3200 rpm when we are in a hurry to get across to Vancouver.

It has a Mallory points ignition that replaced the original distributor a while back. I am only now understanding the concept of properly curving the distributor.

With my initial advance set at 8 degrees BTDC at 800 rpm idle, I measure centrifugal advance up to 5 degrees higher than the max spec in the Mercruiser manual mostly at the lower 1700 rpm that I spend 90% of the time at.

So should I dig into my distributor and try replacing springs or just leave it alone as I am not hearing any pre-detonation?

Is there a general rule of thumb that slow heavy boats should use a slower curve, or the opposite?

1977 Mercruiser Mark IV BBC
800 hours on rebuild
Serial #5551947
2:1 Borg Warner V-drive
Mallory Points Ignition
Coil, ballast resistors, dwell all within spec

 

[Scott Danforth]

22 degree or 24 degree advance? Both OK

Your base timing should be set at 650 rpm

 

[Earl Cordova]

Slow and heavy = Slow timing advance.

 

[Scott Danforth]

Load is load, engine doesnt know if its spinning a low pitch prop pushing a big boat or a high pitch prop pushing a small boat.

It only knows RPM and BMEP

What is the max RPM the v-drive is rated for. Because at 3200 RPM, the motor has another 2000 RPM to go

 

[Finnerty]

Scott, what does BMEP mean?

Max rpm for this MIE 330 is spec’d at 3800-4200 rpm in the service manual.
My WOT is 3850 rpm turning a 19x17 3-blade prop on the 2:1 V-drive.

The manual gives a min and max advance curve that gives the range for this engine.

Total spark advance minus initial timing is spec’d at 5 - 9 degrees at 1700 rpm (the engine speed I usually run at). My advance is 14 degrees (so plus 8 initial is 22 total).

At 3200 rpm, spec is 18 - 22 degrees, I’m at 23 degrees (31 total).

At 3800 rpm and up spec is 22 - 26 degrees. (I didn’t test mine at WOT.)

Thanks for the help!

 

[Scott Danforth]

Break Mean Effective Pressure