Prop Advice: Rev4 XP on 383 Stroker

[Kola16]

So I assembled my own 383 stroker build. Roller cam with hydraulic roller lifters and rockers. Aluminum heads and intake. I will not be dynoing it, but I would guess around 420 HP comparing it to similar builds. I also converted to a thru-hull exhaust and EMI Thunder manifolds and risers. Boat is a 1979 Wellcraft Suncruiser 225; 22.75' and 4000 lbs. dry, no engine. With a full 100 gallons of gas and fishing gear, I am probably around 6,500 lbs. fully loaded.

The outdrive is a Mercruiser 228 with a 1.5:1 gear ratio. I know I need and want a Bravo 1, but I do not have the $$$ for that right now. I do have a Bravo engine coupler and I will not be punching it out of the hole.

I am unsure about the max RPM I should be shooting for since I assembled this engine myself. This is not a typical Mercruiser engine. The 383 Mercruiser max RPM specs I have seen are 4800-5200 RPMs, and for the newer ones, 5000-5400 RPMs. So my guess would be to shoot for about 5000 RPMs. Does that sound right? I use this boat for offshore fishing on the west coast in deep swells. The boat is often times loaded heavy. Holeshot and low planing speeds are probably the most important 2 desires, followed by a good cruising speed, but you can't have it all, am I right? Top speed is cool to brag about, and I would love to hit 50, but never really used for more than 2 minutes out of the entire year just to put a smile on my face.

So I have tried 2 props so far. The first is what I had laying around, which is a Michigan Wheel 3 blade aluminum, 14.5x19P. Max RPMs I got was 5400 RPMs @ 51 MPH. 36 MPH @ 3500 RPMs. 21% slip at WOT.
The next was one I bought used thinking that it would put me close to 5000 RPMs. It is a Mercury Rev4 XP 4-blade stainless 14.6x21P. I got a max of 4500 RPMs @ 51 MPH and 38 MPH @ 3500 RPMs. 15% slip at WOT. So I wasn't even close. At the time I thought 1" additional pitch equates to 100 RPMs gain. Now I am reading it is more like 200 RPMs.

I read on other forums that the Rev4 XP performed like it was 2 pitches lower than its listed value compared to the regular Rev4 due to the XP's reduced cup. So a standard Rev4 19P would run at the same RPM as a Rev4 XP 21P. Is that true? I actually really loved the Rev4 XP 21P I put on as far as cruising and top speed goes. It's holeshot wasn't bad either I was satisfied, but I feel like 4500 RPMs is too low. I want to stay with a Rev4 because I did love it. So a Rev4 XP 19P should give me 4900 RPMs roughly correct? And then the Rev4 non-XP I would need a 17P? I am not sure. Can I have my current Rev4 XP 21P reduced to 19P or even 18.5P? Sorry for the boatload of info and typing outloud...

 

[Pmt133]

Couple of things...

First do you know for sure the tachometer is accurate? 15% slip at WOT for a 4 blade seems kind of high.

Second, what are the cam specs? That is going to dictate to an extent where your WOT RPM should be. Ideally a dyno would tell you exactly where it is but if you're not doing that an educated guess is going to be good enough.

I think your 5k max is fine for the drive sake, also I'd avoid getting airborne... IE shock loads on it.

 

[Kola16]

The RPM reading I guess I am not sure. I am in the process of ordering a new tachometer. The RPM reading I am getting is from my Holley Sniper EFI display unit, which is a digital readout, so I assume it would be accurate, but I am not sure how to actually test its accuracy with what I have. It is reading the signal from the negative of the coil. I know that when the old tachometer worked with the old engine before the swap, the Holley Sniper unit and the old tachometer did not match. They were like 400 RPMs off at WOT, with the Holley Sniper unit reading lower than the tachometer that came with the boat in 1979.

The cam I put in it is this one and set the timing to 12 degrees at idle:
https://www.compcams.com/xtreme-marine-218-224-hydraulic-roller-sk-kit-for-oe-roller-sbc.html



So I guess I am good to 5200 RPMs. That is helpful, but would be fine with 5000 RPMs.

The prop slip, I am not sure what all affects that, but my boat is relatively stern heavy I would say and it has a 22 degree deadrise at the transom so it is not the easiest boat to move I wouldn't think. Ya, I will not be getting airborne with this old boat, but thank you for the reminder of what happened the one time I did several years ago haha. Boat was fine, but passengers were disgruntled.

 

[Scott Danforth]

Drive max RPM is 5500 unless you work on the drives oiling

Run a rev limiter , because if you come out of the water (ie airborne), that 383 will grenade the drive when you re-enter the water (BTDT)

Your cam determines your RPM and the cam you have is on the edge of reversion - check for it. The cam will pull a bit past 5500

Regarding timing, what is your advance curve? The 12 BTDC base is fine, however will want about 24 degrees of advance

Based on your numbers, run the 19p and limit your RPM with the throttle since holeshot is important

 

[Kola16]

Any suggestions on a specific rev limiter? I have never looked into that and didn't realize people put them on I/O's. I will not be launching the boat. It takes a lot to do that to this boat and I have only done it once, half on purpose, but never again. Buuuut a rev-limiter wouldn't hurt if they weren't too complicated to install.

I am not sure what my timing curve is yet. I only have 10 hours on the engine, and need someone who can operate a boat to come with me so I can see what kind of advance I am getting. I will check for reversion, thanks for the reminder. The gaskets are expensive, but less expensive than a hydrolocked engine.

Are you saying get a Rev4 XP 19P? That would get me to about 4900 RPMs right? Not that I wouldn't be happy with that, but an 18 or 18.5P would do the ticket if I got the XP model right? The 3 blade aluminum I have was not very impressive to me in all aspects. Not amazing holeshot, not much grip, felt very loose, no "brakes" like the Rev4 had, porpoised only at high trim angles, but the Rev4 didn't porpoise even at preposterous trim angles. The Rev 4 also had a crazy low planing speed even for being overpropped.

 

[Pmt133]

Fun story, I have the "next step up" in comps marine series and that 5800 rpm peak is bang on on my dyno sheet. Pulls about 200 rpm past it. I'd say 5500 is probably about right for being a little beyond the power peak. I think your target of 5000 rpm is safe then and you are lugging it with the 21p. I'd target 5200 rpm no load and set the rev limiter 100 rpm above above that.

Regarding the tach, the holley unit is using the pickup coil, that should be damn near exact unless you are getting EMI on the signal wire. Go off of that for all readings.

I agree checking for reversion.