Mercury racing 525 EFI rpm and prop
 

Hello everyone! This is My first post here and haven't even got my boat home yet. As it sits right now It's a 1998 Wellcraft Scarab 26 running a slightly tuned 525 EFI. Top end has been done with larger valves and crower cams. Crankshaft, Rods and pistons are forged and the engine is running Haltech and bore should be 4,5". At the moment I'm probably running a too big prop I think as the top Speed is around 51knots and the rpms were around 4700-4800 I think. I'll get back to you on that when I get the boat.

My questionuestion now is how much RPM can this engine take? I'm getting the boat tomorrow so I can start trying out different props, but I'm wondering about the the rpms. A standard 525 EFI has the revlimiter on about 5400rpms I think. What do you guys with more experience than me think? Precious owner said that it should be OK all the way up to 6000rpm but I don't know.

if it was dyno'd I would talk to the engine builder, if peak power was made at 5400- 5500 it makes no sense to spin it to 6k..........for now Id prop it for 5200-5300 @ wot

Alright. And I'm not using this boat for top Speed all the time but sure would like to make the most of what I have. Then I know where to draw the rpm line in the quest of finding the perfect prop. Think It's running a 26pitch 4 Blade prop att the moment. What do you guys think about that? As I Said, I'll get the boat tomorrow so going to check the prop and outdrive gear ratio then.

We're going to check all the settings in the haltech ecu tomorrow. Does anyone know what the ignition timing should be? At a quick glance today before the mosquitos ate us alive I think it was around 24 degrees at high rpm, sounds low but does anyone know what it should be?

Total timing should be 32 to 34 BTDC.
You need to determine whether your "24 degrees" is in reference to TDC, or if it is "degrees of advance" from baseline. If it is the second, and if initial timing is 10 BTDC, then ADDING 24 degrees of advance to that will result in 34 degrees total.

When making a top end run, make sure fuel pressure doesnt drop off at high revs. And shortcoming or obstruction can "hide" at lower speeds.

Plug wires are one place to look. One cylinder missing results is a loss of 60+ hp.

Finally, are you familiar with drive trim angles and trim tab settings? Trimming the boat to run "free" dramatically affects top speed and wide open rpm.

It's worrisome to hear of a cam swap and 'mildly tuned" without specs on the cam grind and lobe separation and its installed lobe center. Many "performance car" cam grinds are unacceptable for marine performance usage.

Give more details and we will all try to help.

MC

Could be wrong but these heads have the larger 2.25" intake valves already and just 1 cam.

Yeah, of course I meant cam not cams! Talked to the previous owner today and he said that the cam were a CompCams XM284HR. Crank etc. is crower. I'm going to double check tomorrow that cylinder 1 is in it's highest position when I read 0 degrees on the wheel and then check in the ECU if It's the same there, because something is off somewhere.

Just as a heads up. The XM284HR has less duration and alot less lift than the 525EFI cam. My guess (which should be close) is that it will create 30-40hp less than the 525EFI cam (all other parts being equal)...

BTW, speaking of Crower everyone...Bruce Crower died not long ago. :( 2-3 weeks ? RIP Bruce !

Sad to hear about crower :-( Maybe that's One of My problems then. Was thinking of pulling the engine in the Winter and that's even more reason to pull it and go through everything. What's the stock cam specs?