Correcting the WOT RPMS - Ford 351w

[haulnazz15]

Re: Correcting the WOT RPMS - Ford 351w

CharlieB, I think that will be my next move. Unfortunately it's a huge PITA to remove the spark arrestor in my boat due to the engine placement, but it's probably worth the hassle. The dizzy is new Pertronix billet unit with the Ignitor II module. I haven't verified what the max advance is set at, but I do have some additional springs provided to change the advance curve. I am really tempted to buy a new prop anyway though. Since I primarily use this boat for slalom skiing and occasional tubing, I thought about going out and getting a 15x16p 4-blade prop. Iboat's has one for around $140 on sale, so it wouldn't exactly break the bank. I'd also get to see if the pitch is the limiting factor as it should be close to a 3-blade 17p, right?

 

[CharlieB]

Re: Correcting the WOT RPMS - Ford 351w

A four blade prop of the SAME pitch as a three blade prop actually places MORE LOAD on the motor as the total blade area is greater.

Hopefully one of the prop guru's will jump in and spell out the differences in dialing in RPM with prop changes, but changing the number of blades does make an added difference.

Aside from the prop you need to address that distributor and total ignition timing, as this makes a tremendous difference in power and performance. Merc Service Manual #2 list total timing for the 351 233hp Ford at 23 to 27 degrees TOTAL, with an initial timing set at 10 this only allows a maximum of 17 degree mechanical advance.

The Pertronics distributor comes from the factory with 24 degrees mechanical advance, and add your initial 10 you get a total of 34 which could be seriously over advanced and at, or near detonation.

You can try a couple of things and see which works best for your motor.

Set initial timing at 2 or 3 degrees then test run the boat to see how performance responds to the total timing of 26/27. If the low speed throttle response, acceleration and plane out are good, and what your WOT RPM runs out to be. The factory recommends 3800 to 4200

If performance is great you may quit right here, but if you really want to tinker, install the Pertronics advance limiter spacers that came with the kit. Using the 'Blue' limiters will limit mechanical advance to 16 degrees, allowing you to set the initial advance at the factory 10 degrees. Again, test the boat to see how well the motor responds to low throttle, acceleration, time to plane out, and WOT RPM.

I highly recommend using the Blue limitoers and keeping very near the factory recomded 10 degree initial, 16 mechanical, making for a 26/27 degree total.

All this should be done prior to thinking about changing the prop.

 

[haulnazz15]

Re: Correcting the WOT RPMS - Ford 351w

Just an update to the thread:

I tried to take the boat out to the lake at the end of last season, but the lower unit crapped-out on me and wouldn't go into gear shortly after putting in. So I found a used lower and put some new parts on it (water pump, shift-shaft seal, etc) this summer. I also got a 15x16p 4-blade prop as well. Upon, doing some investigation, I noticed that the electric choke was not opening up entirely, which was choking the engine of air at higher rpms. With the new prop, and 1.33 gear ratio, the boat was running 3800rpms @ 40mph, which gives me 8% prop slip. Granted the speed was taken using the factory pitot-speedometer, so I'm sure it has some inaccuracy, but it seemed pretty close. The holeshot was worlds apart from before, and it accelerates well all the way to WOT.

The lower unit I bought also came with a 3-blade, 15x19p, so I have that as a backup as well. Next thing I will check the next time we take it out is our total timing advance, to ensure we aren't going overboard. I don't recall which advance springs were in the Pertronix unit, so I can't attest to it, but I don't want to be running out of factory spec. She really runs well as-is, but 39-40mph is where it hits the wall.