The engines in question are a pair of 1986 Mercruiser 454 / 340hp inboards (same engine as the 454/330 sterndrives). They're in a much heavier, slower boat than most on here (about 27k lbs and 25 kts at WOT), but I figure this crew is most likely to have the answers I'm looking for.

Currently they're running the original TB IV setups with V8-24 modules which generally work fine, but give very little timing at low RPM. The curve is fine at higher RPM though. We generally cruise at 2 different speeds. Either around 3200 - 3300 on plane, or around 1300 going slow for fuel economy (which we do a lot of). This makes me think we'd benefit from more timing at low RPM for efficiency (as they definitely like more timing at idle, but with the current modules they can't be advanced any further safely).

The original spec has them running 8* initial / 32* total, but I've had them at 10* initial / 34* total for a few years with no issues. No signs of detonation, spark plugs look good, never heard them ping (exhausts are fairly quiet), etc.

I'm thinking of finding a pair of V8-20R modules to swap in and turning up the initial timing, but I'm a little concerned that there may be too much timing in the 1750 - 2500 RPM range where the boat is climbing onto plane. I'd only get about 17* of advance out of these modules as these engines don't rev high enough to see the full 20* (they top out around 4200), so I'm figuring 16* initial would be in the ballpark.

Looks like I can't post the comparison chart I built as I can't post images yet, but for the range I'm concerned about I'd be seeing 20* total at 1500, 22* at 1750, 24* at 2000, 26* at 2250, and 28* at 2500. After that it's the same or less than I'm running now. With the current setup of the V8-24 at 10* initial I'm seeing 16.5* at 1500, 19.5* at 1750, 22.5* at 2000, 25* at 2250, and 27.5* at 2500 (all of those would be 2* less if timed to factory spec).

Any thoughts on whether the V8-20R would be a safe setup or would it be pushing it too much while accelerating onto plane? It looks like some of the EST setups might be at least as aggressive with timing early on, but every other factory setup out there is milder. Fuel-wise it's normally running on either 89 octane E10 or 90 octane E0 depending on what's available. I've found conflicting info on whether Merc originally spec-ed 86 or 88 octane as minimum, so I'm not sure how much wiggle room I've got to play with from the factory setup based on that.

switch to the delco est system it idles aroud 20 we u7se them in a ton of fishing boats that idle for very long periods they stop the flex plate wear and tear much smoother and quieter idle

No issues with the very aggressive timing curve from the EST modules in a heavy boat? It looks like that would bring the timing in even faster than what I was thinking with the V8-20R modules. If it matters, these engines are closed cooling with 170* thermostats, so normal running temp is around 175 - 180*.

no issues at all many are really heavy with 6 guys and gear we get the kits from michigan motorz awesome to deal with

I dobt you will any difference changing modules, but here are the advance curves for all modules