So going thorugh the engine (mark IV 502) for a tune up and realized that the thunderbolt system is the 20R. With a inital timing of 8 and the 20R module that means a total of 28 degrees, which seems low (was thinking total timing would be around 32-36 not 28). Why in the world did Merc put a module that doesn't reach full advance until 5200+ rpm on an enigne with WOT from 4600-5000?




RBot

Like everything they do....to make it through warranty

Initially, I believe they were also only offered with 1.36 ratio drives to help this too

Just add another 8 to the initial.....as I recall, it was all in by 3k

The 20R module curve sucks. Does not get full advance until at least 5000rpms. I would change to a V8-HP or V8-24 and run 34* of total advance.

That's what I'm thinking, or just ditch the thunderbolt all together. In the mean time would setting initial to say 14-16 or so work?

I would keep the TB IV and just swap modules. TB IV's are good reliable set ups. They often pop up in the Swap Shop for $100 or less.
For now, I'd bump base timing to 12-14*. No reason to run 36* total. 32-34* total is plenty.

Merc used it on the 600 sc and the 800 sc along with the carb 502 and some of the 502 MPI with T bolt IV.

I have a couple of the v8 modules at home if you're interested. I'd do 100 shipped. I'd have to check which one I have.....edit - just remembered I have the v8 module with 20* of advance

If anyone has a 24 or even the 22 module I'd gladly take one.

I never realized how slow that curve is on the 20R. I guess Merc went extremely conservative and wanted some serious longevity out of that 502 mag back in the day.

^^^^ basically that is correct and what above said about warranty or wanting lack of warranty issues as to being so conservative.