powerguy

Running a 502 gen V with EMI exhaust and K&N air filter on otherwise bone stock 502 magnum. I can spin a 4 blade spinelli prop to 4900-5000 rpm at WOT. I have the stock ignition module (20 deg ful advance) and the initial timing set at 10. The problem is that the full advance of the ignition module does not happen until 5250 rpm so the max advance that I am getting at 5000 rpm is 29.5. My question is whether I should set my initial advance to say 13 or should I get a new 24 deg. ignition module that has the full advance in at 3750 rpm and set the initial timing at 10 to get 34 degree advance above 3750 rpm. Looking to squeeze just a couple more rpm out of the boat. Will this change in advance gime me more high end hp and therefor 200 more rpm and a couple more mph?

blue thunder

Something doesn't sound right there powerguy. 5250 is a common rpm for rev limit but not full advance. What is the recommended rpm range of the 502 mag?

Dave

H2Xmark

all the newer 502mag mpi's have the factory rev limit set at 5050 rpm's, is your boat a carb are MPI engine?

powerguy

1993 gen v 502 with thunderbolt IV and no rev limiter

powerguy

carb motor ( weber 750)

blue thunder

V8-20R Tbolt IV module. 5250rpm full adv. That is what my book says too. In that case, I would reprop to spin the engine 53-5400 and see what you get. I don't know what the torque curve looks like for your engine, but if it runs out of speed spinning it in the 53-5400 range, then your best bet is to go to the 24* module and tune from there with the prop you have.

I personally wouldn't try to get what you want using base timing alone.

Dave

blue thunder

Well that could be another problem, you may run out of carb (cfm) before reaching 53-5400rpm. I don't have a cfm calculator handy to say for sure. Sounds like a different module is quickly becoming the best option. I just personally like to see a forged rotating assembly BBC spinning in the 52-5400 rpm range as long as it has the hp to support that rpm.

Dave

jpclear

Powerguy, Can you give us the #'s from your ign. module? Something sounds very wrong here. I don't know of any non special, hi-perf, Merc. module that wasn't at at full advance before 3500 RPM. And I can't find where Merc. used a 20 deg. "stock, as shipped" module. (But you could order them. I'm running one now.) Do you have someone at the engine with a lite checking the advance at 4900-5000? (Don't laugh! I remove my hatch and do this just to be sure.) I would use this method to set my final advance to 32-34 deg., use 92 0ct. fuel, and let the timing at idle lay wherever it falls. Merc. has all the proper stuff in their service manual for you. Get a hold of one get back to us for shortcuts and other questionable advice. --- Jer

formula31

What JP said, plus, the modules are not that accurate. I have 2, 24 modules and I set my engines at 34 at wot and one module is advancing 21 and the other 25 so set your timing at full in, usually 4000.

Vmax

I have a used Thunderbolt IV V8-24 module removed from my1989 Mark IV 454. I went to the V-6 14 degree module to get a decent idle with a supercharger and ignition retarded. At 6 degrees initial I recorded a steady 28 degrees advance at 3200 RPM on my timing tape with the 24 degree module when I removed it about 3 weeks ago. It may give a couple more degrees at several more hundred RPM did not measure it though. If interested I will sell the V8-24 module at a reasonable price since I have no use for it. If interested let me know.