Correcting the WOT RPMS - Ford 351w

[haulnazz15]

Trying to get the WOT rpms up to spec, but not sure if it is a prop/drive limitation or engine issue. Engine runs flawlessly up to this point. Drive has been trimmed up until max rpms/speed is attained. Max rpms per Mercruiser should be around 3800-4200. While this is the same prop that has been on this boat for over 20 years, we didn't recall what the WOT speed/rpms were before repowering. The old engine was the 233HP 351w, so the only major change was a 4bbl carb. Is the 15 1/4" diameter a bit much to swing for this runabout? Looking to see if maybe dropping a tad in the diameter and running a 17/19 pitch would be a better fit. Prop slip calculators put my numbers in the 11-13% slip range, so it doesn't appear the numbers are out of whack.


1) WOT RPMS are ~3200
2) Michigan Wheel SS
3) 15 1/4 x 21
4) 3200RPM @ pitot-measured 42mph
5) Mercruiser 233 (1.32 ratio) 351w w/4bbl carb ~ 265HP. New engine, less than 15 hrs time.
6) 1976 Mark Twain 200VBR. 20' - guessing 3000lbs.

 

[jestor68]

Re: Correcting the WOT RPMS - Ford 351w

That is serious lugging of that motor. Due to your gear ratio, I recommend reducing diameter as well as pitch.

 

[Maclin]

Re: Correcting the WOT RPMS - Ford 351w

1) WOT RPMS are ~3200
2) Michigan Wheel SS
3) 15 1/4 x 21
4) 3200RPM @ pitot-measured 42mph
5) Mercruiser 233 (1.32 ratio) 351w w/4bbl carb ~ 265HP. New engine, less than 15 hrs time.
6) 1976 Mark Twain 200VBR. 20' - guessing 3000lbs.

Just an opinion, but I think the 1.32 drive ratio is the issue. That sounds like a drive "meant for a King" (Cobra 460 that is). Would be hard to make up the diff with a prop IMO.

 

[haulnazz15]

Re: Correcting the WOT RPMS - Ford 351w

Well this is a factory Mercruiser setup a far as the engine/drive combo is concerned, so the recommended WOT rpms are based off of the 1.32 ratio. Again, the boat jumps out of the hole, accelerates, and cruises perfectly, it just didn't go past that rpm of 3200 or so. I suppose we could try a 17 pitch of the same diameter and see if we get any more RPMs out of her.

 

[Maclin]

Re: Correcting the WOT RPMS - Ford 351w

Hauln, yeah, going down in pitch is the only option I can think of with a gearset like that if you are sure the engine power or rpm is not limited somehow.

 

[QC]

Re: Correcting the WOT RPMS - Ford 351w

Even though the numbers add up I'd verify the tach before I did anything. I am always suspect of a combo that "jumps out of the hole" and also has low WOT RPM. Rare to have both.

 

[haulnazz15]

Re: Correcting the WOT RPMS - Ford 351w

You know qc, I thought about getting a cheap tach to verify RPMs. While I don't have cause to doubt the tach, it is over 35 yrs old. However, assuming it was making even the low end of the recommended rpm band, it would put prop slip numbers way out of spec I'd think. I'll have to bring a GPS and get a backup tach next time we go out.

 

[444]

Re: Correcting the WOT RPMS - Ford 351w

The 1.32 drive and a mid 70's ford 351 does not sound like it would have been a great combo even back in its day. What are the specs on the rebuilt 351? Stock replacement mid 70's smogger low compression pistons? Has anyone tuned the carb? Could be rich or lean or secondaries not opening restricting your mid-upper rpm power.

Sounds like you should drop down in pitch but I would verify the carb and distributor are dialed in correctly first.

 

[haulnazz15]

Re: Correcting the WOT RPMS - Ford 351w

Original engine was 233 HP, hence the drive name. The carb is a new Edelbrock 1409, distributor is brand new Pertronix billet unit with ignitor 2. Timing set at 10 btdc. Like I mentioned, she is smooth, fires right up, and accelerates perfectly through the rpm band. She just doesn't appear to get through that barrier just over 3k RPMs. New engine is a rapido marine unit installed late last year. I haven't checked compression numbers, but I haven't had cause to until we got it tuned up this year, so that may be an option too.

 

[Maclin]

Re: Correcting the WOT RPMS - Ford 351w

You may have to put on a 19 or even a 17 and try it just to see if the engine can get past that hump, and if it does then see what RPM it can reach after that.

 

[haulnazz15]

Re: Correcting the WOT RPMS - Ford 351w

Well, just an update:

After running this past weekend, I'm inclined to think we have a vacuum leak of some sort. I put a vacuum gauge on a previously plugged port on the intake manifold and was getting 12-13" Hg @ idle, where it should be above 18"Hg. So, while doing another WOT throttle run, it still tops out around the same speeds, but we noticed a rapid tapping/rattle which developed right at full throttle and ~3K rpms. I'm inclined to think it was spark knock/detonation, so we made sure to keep the rpms lower the remainder of the trip. The detonation only occurred for 5seconds or so before I backed out of the throttle so I don't think any damage was done. I thought it was the latch vibrating on my toolbox for a second!

In any case, I believe the low idle vac is likely due to a significant vacuum leak, while the idle is smooth and the engine runs great until 3K RPMS, it is causing detonation and also not likely enough vacuum to activate the vacuum secondaries @ WOT, thereby limiting the WOT rpms. Does this sound like a plausible explanation?!

 

[haulnazz15]

Re: Correcting the WOT RPMS - Ford 351w

Just a little update: The vacuum leak has been fixed; I should have paid more attention to the open ports on the edelbrock performer manifold. I thought they were just threaded cast-in posts for things like throttle brackets, etc, but they were open ports, lol. Vacuum is not at 18"+ and the detonation around 3K rpms is gone. Runs great still, no hesistation, but it still won't get past ~3200rpms and 40ish MPH (pitot-measured). I can only think of two possible options: 1) the secondaries aren't opened up fully which limits the rpms, or 2)the prop is simply too high-pitched for the 351w to turn any faster.

So if I drop to a 17p prop, keeping the same 15-1/4 diameter, that should get me to around 3600 rpms, right? Still below the WOT spec from Mercruiser which dictates 3800-4200 rpms. I just can't help but think the prop isn't the limitation for some reason since I am so far from the spec.

Any thoughts?

 

[RogersJetboat454]

Re: Correcting the WOT RPMS - Ford 351w

Did you end up verifying that the tach working correctly?

 

[haulnazz15]

Re: Correcting the WOT RPMS - Ford 351w

I have not verified the tach accuracy. I just took it to be correct since the prop slip calculation works with the rpm/ratio/prop pitch numbers and make sense for the speed.

 

[QC]

Re: Correcting the WOT RPMS - Ford 351w

I'll have to bring a GPS and get a backup tach next time we go out.

Ya know we believe the tach because of the speeds, but the speed is pitot . . . Hmmmmm. Seems like we have two data points that are suspect. Like you, I would try and make myself feel good, but from the outside you shouldn't.

 

[haulnazz15]

Re: Correcting the WOT RPMS - Ford 351w

Well, I understand a little animosity about the pitot-speedo, but it seems to be about right, maybe even a little slow. I wouldn't call it "making myself feel good" as I think fixing a possible mechanical engine problem (vacuum secondaries) is a more complicated fix than simply swapping props. I just don't see dropping 4" of pitch to a 17p giving me the 800rpm I need to get into the proper operating range. Maybe I can download an app on the phone that will give me GPS groundspeed. I would just think that 265HP would turn the 21P prop at least to the bottom of the WOT range (3800).

 

[jestor68]

Re: Correcting the WOT RPMS - Ford 351w

Without getting into a lengthy proptology discussion, understand that a larger diameter "power" prop reaches it's peak efficiency at a much lower prop speed than a smaller diameter prop.

That's why these larger props are designed to operate with the higher gear ratios. With a 1.62 gear ratio, that prop would be spinning at the about the same speed(at 4200 engine rpm) as it is now with your gear ratio at 3200 engine rpm.

Since we can't change the prop speed(gear ratio), we need to put a prop on there that reaches it's peak efficiency at a higher prop speed.

The first prop I would test would be a (Solas Rubex ?) 14.5 X 19 aluminum prop. That is the same as dropping nearly 3 inches of pitch; but more importantly, the smaller diameter will be efficient at that higher prop speed. I estimate that your WOT rpm should be about 3750-3800 rpm with that size.

Interestingly, if I put your raw data into the Mercury prop selector(their gear ratios only to go 1.47) it comes up with 17"/18" pitches. The Vengeance SS is 14.5 X 17.

 

[QC]

Re: Correcting the WOT RPMS - Ford 351w

proptology

I usually call it Voodoo. Proptology is much better

 

[haulnazz15]

Re: Correcting the WOT RPMS - Ford 351w

Well, again, this is the prop that has been on the boat since we got it 20 years ago, lol. My father doesn't remember how fast the speed was, or what the WOT rpm was BEFORE we put the new engine in. The only difference in the engines is the new one has the 4bbl carb/intake and center-rise exhaust manifolds instead of the log-style. I can't say as to the reasoning for the 1.32 ratio on the 233 outdrive, but that's just what they strapped to the 351w's in the mid-1970's. If the 21p is overpropping the boat, then the only other thing I can think of is to go down to something like a 17p. WOT per Mercruiser is 3800-4200 for this engine, so I don't think a 19p prop would get me close enough to 3800rpms. Any reason for the diameter change you suggested (currently on a 15-1/4 and it came up with 14.5)?

 

[CharlieB]

Re: Correcting the WOT RPMS - Ford 351w

You may want to take another test ride with another pilot along so you can remove the spark arrester and LOOK down the carb throats to see if the secondary is fully open.

Carefully, as you do not want to get near the belts, or drop anything into the open carb.

And put that spark arrester back on as soon as you see what you need to know.

Then you can decide if the carb needs attention or if you may be searching for another prop.

Edit;

Oh, and find out how much advance is built into the new distributor. Initial advance is fine at 10 degrees, but as the motor speeds up there needs to be more advance, either mechanical or electronic, to bring the total advance more to the range 23 to maybe 28 degrees total by 2500 to 2800 RPM. Retarded total timing WILL limit power and WOT RPM.

Check these things out before spending money on a prop.