RPM limits for operating -4.3 Alpha 1, Gen 1

I'm doing some research on props and performance. I had some issues getting out of the hole with added people and gear.

Question: What is the recommended top RPM rate to factor in for my prop to get this up on plane with a heavier load? And what is the optimum RPM operating range for this motor?.

The pitch of the prop that i have is unknown at this time. this is the clip i'm looking at for prop info

https://youtu.be/Sp0MdKzXaSw

I'm not all that interested in top speed on this boat. If i have more power getting out of the hole, and loose some top end speed, i'm good with that. On the other hand i don't want my motor red lining. there seems to be a balance that needs to be achieved. any help would be appreciated. Kirk

Your engine will be rated 4400-4800 rpm at WOT. You want to be somewhere in there with your typical load, ok to be a tad above(hitting rev limiter) with light loading as you can back of the throttle a touch.

You need to know your current prop pitch and type plus WOT rpm before trying other props as this will guide the selection process. Typically based on what you state as your goal you want to go down in pitch 2”, maybe go to a 4 blade. So if you have say a 19” pitch three blade That isn’t already at max WOT rpm go down to a 17” Or try a 16” or 18” pitch 4 blade. Best bet is to find a local prop shop who will let u try before you buy. Tell them what prop you have WOT rpm and what you want to do, they will make recommendations.

of course you want to ensure your engine is currentLy in good working order, tuned up and capable of hitting reasonably correct WOT rpm.

Excellent !! Am I safe to assume WOT is wide open throttle? And could you explain the rev limiter to me? This is an older 1990 boat that had the engine replaced not long before I bought it by the owner. The previous owner has been less than forthcoming regarding details of this replacement. Only that it was replaced with another 1990 rebuilt motor. The reason I included this info is in regards to the rev limiter you referred to. Is this an electronic devise connecting to the tachometer? How does it work? Kirk

it runs like a top, and is very clean with no obvious oil leaks. Everything looks new as far as hoses, belts, and it has a new carb. I’ve run this a couple times up to 5000 rpms in flat water with speedometer saying 45 mph, but it seemed like I was going faster than that. The next time I went out my speedometer wasn’t functioning correctly and I found the sensor stuck in the up position. That run I used the gps on my phone with a speedometer app and it was showing about 40 mph at just over 4000 rpm. And THAT was too fast for my wife’s comfort level , so I never tested it at WOT.

Actually I can’t see where I’ll ever use it over 40 -45 mph anyway. That’s fast enough for this old man. It’s a fishing boat / pleasure boat to me. I won’t be pulling skiers.

I too use a prop that's under pitched. That helps the boat plane off easier with the entire "gang" aboard, and works just fine when I'm alone. The same cruising speed takes a hundred rpm more to achieve, but the gas mileage isn't that less. All in all, a smart way to go and puts far less stress on the engine.

Jeff

kirkll said:

Excellent !! Am I safe to assume WOT is wide open throttle? And could you explain the rev limiter to me? This is an older 1990 boat that had the engine replaced not long before I bought it by the owner. The previous owner has been less than forthcoming regarding details of this replacement. Only that it was replaced with another 1990 rebuilt motor. The reason I included this info is in regards to the rev limiter you referred to. Is this an electronic devise connecting to the tachometer? How does it work? Kirk

it runs like a top, and is very clean with no obvious oil leaks. Everything looks new as far as hoses, belts, and it has a new carb. I’ve run this a couple times up to 5000 rpms in flat water with speedometer saying 45 mph, but it seemed like I was going faster than that. The next time I went out my speedometer wasn’t functioning correctly and I found the sensor stuck in the up position. That run I used the gps on my phone with a speedometer app and it was showing about 40 mph at just over 4000 rpm. And THAT was too fast for my wife’s comfort level , so I never tested it at WOT.

Actually I can’t see where I’ll ever use it over 40 -45 mph anyway. That’s fast enough for this old man. It’s a fishing boat / pleasure boat to me. I won’t be pulling skiers.

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Yes WOT = Wide Open Throttle . Rev limiter is logic in the igntion module on older carbed model or ECU on newer MPI models that cuts spark when max RPM is exceeded. The engine will miss slightly thus lowering rpm and letting the driver know to throttle back. My 1990 3.0 with DDIS ignition had one that I could tap with a 17" pitch prop but not the 19" prop. Yours may not have that feature being that year as they were just introducing that technology. If your tach is accurate and you pay attention it is not needed.

In dash speedos unless newer GPS models basically are +/- 10-15%. If you are already hitting 5k (verify the tach is accurate vs a shop tach) you will be over rpm if you go down in pitch. Generally 2" in pitch = 400 or so rpm change. You can run like this and get better planing and mid range accelration performance, just need to watch the tach at light load WOT. Alternatively you could try a 4 blade of similar or slightly lower pitch (say 18" 4 blade if running a 19" 3 balde now) and that will help with accelration and loaded planing speed at the expense of some top end. This is why its best if you can find a prop shop to try a couple at least until you get some experience playing with props.

Thanks guys. Between my research and your input I think I got a pretty good take on this now. Considering the only time I’ll be running WOT will be coming out of the hole, sounds like a 4 blade or a 17 pitch might be the ticket. I’ve located a couple local prop shops , and think the best thing would be to take my old prop in to verify the pitch.

another thing that I’ve heard a bunch, is that an OME prop vs a generic economy prop can make a big difference in performance itself. Considering my existing prop is kind of banged up, I may fix the issue just by purchasing a new OME Prop. What do you guys think of that philosophy?

now I need to figure out how to get my hands on a shop tac to check that out, so I’ve got accurate info before pulling the prop. Would this be accurate enough? https://www.harborfreight.com/lcd-automotive-multimeter-with-tachometer-kit-95670.html

kirkll said:

Thanks guys. Between my research and your input I think I got a pretty good take on this now. Considering the only time I’ll be running WOT will be coming out of the hole, sounds like a 4 blade or a 17 pitch might be the ticket. I’ve located a couple local prop shops , and think the best thing would be to take my old prop in to verify the pitch.

another thing that I’ve heard a bunch, is that an OME prop vs a generic economy prop can make a big difference in performance itself. Considering my existing prop is kind of banged up, I may fix the issue just by purchasing a new OME Prop. What do you guys think of that philosophy?

now I need to figure out how to get my hands on a shop tac to check that out, so I’ve got accurate info before pulling the prop. Would this be accurate enough? https://www.harborfreight.com/lcd-automotive-multimeter-with-tachometer-kit-95670.html

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Did you check if there is numbers cast in the end of the hub by where the prop nut holds it on? you may have to remove the washers etc to see it. Yes if your current prop is dinged/bent blades that could make a difference. Depending on how Fubar the current prop is might be worth getting it fixed first to see where you are before playing with pitch and blade count. Again take your current prop in and see what they say, keep h=the fixed original one for a spare.

OME- doe you mean OEM as in original equipment manufacturer? All OEMs and aftermarket prop manufacturers have more basic (affordable) and more highend props. You can spend $125 for a aluminum prop up to $700-900 for a quality stainless steel model. I've gotten excellent reults from aluminum props as well. My previous boat was a 3.0 l powered 17 ft bowrider. Swapping the OEM Merc Blax Max for an inexpensive ($150) Michigan Wheel Vortex 4 blade 18" really helped this underpowered boat. I trialed both a 16" and a 18" -bought both one for crusing on for skiing- I learned a lot going through this so when I bought a new boat I felt comfortable stepping $$$ up to stainless. I still keep an aluminum for spring when there is a lot of wood in the water. I bought them all used on ebay for about half of new. kept a couple and sold off the ones that didnt work as well.

That tach shoud work as would any of the newer digital timing lights like an Innova if you can borrow one. Some autoparts stores loan them out. Just want to verify dash tach is reasonably close.

Thanks for all the info.

I've got it in the shop right now getting a water pump impeller put in it, and the first thing the tech told me looking at it was "You need a new prop." I didn't think it looked THAT bad, but hey.... these guys do this all the time, so maybe he's right....

But i don't think i'll be purchasing a prop i can swap out myself at a merc dealer. Wont be cost effective. I've located a prop shop close to me i'll take my old one in to look at, and go from there.

I shied away from doing the impeller job myself because i'm not sure whether this lower unit has ever been removed, and was afraid of breaking something getting it apart. Next time i'll do it myself. Now they tell me the oil seals are shot too... I started a different thread on that question. Kirk

I thought I’d do a follow up after getting my new prop and let you guys know that it made a HUGE difference. I had an aftermarket 17 degree prop on it that to me, didn’t look all that bad. But the shop said it looked pretty rough. I replaced it with another 17 degree OEM Merc Black Max rather than changing anything. First thing I noticed was the Black Max seemed to have more cup to the blades than my old one.... that and it was all shiny and black....

Same crew went out again with 4 people, two dogs and gear. Only I added a new anchor this trip too. Like I said before HUGE difference. She popped right out of the hole and planed out instantly, and I noticed a lot less vibration at high speeds.... but.... it seemed my top end speed wasn’t as high as it was at WOT.

This makes me want to take another good look at my old prop to make sure that wasn’t. a 19 degree. I can live with less speed, and I love that extra hole power. But every thing I’ve studied on props tells this is a lower pitch propeller from the change in it, and less over all speed would confirm that..... but again.... it never hit the rev limiter going over 5000 rpms.

I think I’m going to have to take it out by myself with less weight and check the accuracy of the rpm gauge, and do the WOT test again.... what are your thoughts on this?

but again.... it never hit the rev limiter going over 5000 rpms.

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Ayuh,..... Yer motor doesn't have a rev-limiter,.... it's to old for that, rev-limiters are only seen on the newer Mpi/ Efi motors,...

Well it not having a rev limiter doesn’t surprise me, I was led to believe it did, but wasn’t familiar with them. But after checking out the way the tachometer was wired and looking the engine over closely I found no sign of any mystery electronics or alarms...... No matter.... I’m a gage conscious operator anyway. That rpm gage and temp gage will be monitored as closely while I’m running the main engine as my sonar is when fishing.

There is a new twist in this story now...... Yesterday I was cleaning fishing line out of my kicker prop and checking the prop seals on that motor , and pulled out my old prop for the stern drive I had just replaced and held it side by side with the new one..... Now I may not be an experienced boat mechanic, but it was plain to see these props were not even close to the same size, and the pitch looked different too.

After taking the stamped numbers off that old propeller and looking it up....it shows it being a Michigan Wheel 13.3/4X21RH. Now that explains the HUGE difference in the hole shot and the higher rpms/lower top end speed.

but ....... I’m questioning why a reputable shop would inadvertently drop down from a 21 degree to a 17 degree prop without saying a word to me about it..... They told me I had a 17 degree on it...... now I’m thinking this might have been too much , and want to try the 19 degree.

i may go down to that prop shop I found with my old one anyway now and see what they have to say. This shop I just went to is leaving me feeling like they sold me the wrong prop. I’m going to call them back and talk to them about it.

don't get me wrong here...I liked the difference having that extra power with a full load on board, but I have a feeling I’m going to have to watch my rpms like a hawk with a light load with this 17 degree.

what do you guys think? Kirk

Two things. One, prop selection is an art, not a science. That is, you may have to give up a little here to gain more of what you want there. Yay now you have excellent hole shot because you more or less "geared down" to the 17. This will be at the expense of top speed. Now if you put the 19 on there you may have trouble getting up on plane but you can cruise faster. Pick your poison. In either case the prop shop should have either tested the boat with you and a similar load until you were happy, or offered you a grace period for you to test and evaluate the prop. Def go back and discuss what your feelings are about this. Just buying any old prop off the shelf and hoping for the best is a recipe for disappointment.
Two the water is your rev limiter. There's no way the engine will ever be able to surpass it's "red line" when pushing a boat. Just can't happen. Physics, y'know...

I can see the trade off philosophy , and for what we use the boat for mostly, good hole shot with less top end cruising speed suits me just fine..... but I just can’t help but think I’m leaving money on the table jumping from a 21 down to a 17 without trying the 19 pitch.

The 21 would get us out of the hole with one to two people on board fairly well, and would cruise faster than was comfortable for my wife at 4800- 5000 rpms, but It really struggled getting out of the hole once we got a full boat with a load on.

My thoughts are that a 19 might be perfect for the two of us giving us both a decent hole shot and a bit higher cruising speed. I personally wouldn’t mind a little more top end on those days the water looks like a mirror. My wife isn’t always going to be in the boat, and it would be nice to wind her up now and then.

The new 17 pitch prop was put on while I had it in the marine shop to have my prop seals and water impeller done. It wasn’t a prop shop. I had no idea what I had on it, and assumed they would match what I had on there, or drop it down a notch....I did mention I was having hole shot issues with a load...... but going from 21 to 17 is two notches. I’ll give em a call this am and see what they have to say about their reasoning for this. It very well may have been an oooops! Hopefully they’ll let me try out a 19 without purchasing another one.... Kirk

Your top speed increase will not be dramatic. between 0 and 5 knots more is about all you should expect. Remember, the engine is hitting a wall at wide open...a combination of its horsepower, torque, wave action barometric pressure, fuel quality, cargo weight, wind resistance, blah blah blah factors that keep it from going any faster.
The trick is to maximize that point at which it can't do any more but still within a rpm that's in that 4500-5000 range at WFO.
If you are happy with the 17, the 19 might not make you more happy, all things considered.

Prop Selection for Dummies:
https://www.westmarine.com/WestAdvisor/Selecting-a-Propeller

kirkll said:

I can see the trade off philosophy , and for what we use the boat for mostly, good hole shot with less top end cruising speed suits me just fine..... but I just can’t help but think I’m leaving money on the table jumping from a 21 down to a 17 without trying the 19 pitch.

The 21 would get us out of the hole with one to two people on board fairly well, and would cruise faster than was comfortable for my wife at 4800- 5000 rpms, but It really struggled getting out of the hole once we got a full boat with a load on.

My thoughts are that a 19 might be perfect for the two of us giving us both a decent hole shot and a bit higher cruising speed. I personally wouldn’t mind a little more top end on those days the water looks like a mirror. My wife isn’t always going to be in the boat, and it would be nice to wind her up now and then.

The new 17 pitch prop was put on while I had it in the marine shop to have my prop seals and water impeller done. It wasn’t a prop shop. I had no idea what I had on it, and assumed they would match what I had on there, or drop it down a notch....I did mention I was having hole shot issues with a load...... but going from 21 to 17 is two notches. I’ll give em a call this am and see what they have to say about their reasoning for this. It very well may have been an oooops! Hopefully they’ll let me try out a 19 without purchasing another one.... Kirk

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A 19" is a typical general purpose prop sold on most boats like yours. typically 17 is considered a power prop so not too surprised they went 4" down. Personlly I like a boat that is a tad underproped (lower pitch) as it makes it very responsive and rarly do you really get to enjoy WOT fot that long...

Ask them if you can try a 19 for a day, I bet you like them both ... for different uses. I used to run a 16" for blade for skiing and 18" for general crusiing on an underpowered 3.0 boat

Well I got out today with a light load and tested this 17” out good. WOT 5200 rpms Didnt hold it there long, but tested it several times. This thing leaped out of the hole with a light load and the rpms came up quickly. It was almost scary how responsive it was.

i called the guy at the Merc shop where I bought the prop, and he said he’d swap it out with a 19” no problem if I wanted to try it out.... I think I will give it a go and check it out. I love the power of the 17”, but didn’t like the high rpms I had to run to get over 30 mph. At 4200 that thing purrs like a kitten but only runs 28 mph. I’d like that thing moving about 35 mph at that rpm level with a top end about 42 mph..... I’m thinking I’ll get the power I need with the 19” and better cruising speed without running WOT with a light load no problem. But with the whole crew in the boat, I may want the 17” back on there.... only one way to find out. I may end up keeping them both. Kirk

Ok.... we had another prop test. The guys at the shop swapped the 17” for a 19”. I went out solo and tested it first. Honestly I’d be hard pressed telling the difference coming out of the hole with the 19”. She popped right up on plane and the rpms spooled up quickly.

at 4200 rpms I was right at 35 mph. A 7 mph difference from the 17”. At WOT it ran up to 40 -41 mph. Maybe a fuzz more after fine tuning trim........ but.... I was still hitting 5200 rpms . After loading the kids and dogs up adding another 450 pounds, I had very similar results. Popped right up on plane, but maybe lost 2 mph at WOT.

So my question is.... should I keep going and get the OEM 21” and try and get the rpm level down where it should be at WOT? That is the size prop that was on the boat when I bought it, only it was a Michigan Wheel prop that was beat up pretty badly. There was a noticeable difference in vibration putting a new prop on. Smoothed out significantly.

im quite happy with the performance level of the 19” . I need no more top in speed and cruising at 35 at 4200 is perfect for our wants and needs. Should I be concerned about the 5200 rpm level ? I have no intentions of running it that hard, but do not want to burn something up getting someone else at the helm that isn’t aware of the situation.

your thoughts would be appreciated..... thanks, Kirk

5200 isnt that much over. You had a 21" before and didnt like it, so I'd stay with the 19" pitch as you're better off being a tad underproped vs overproped easpecially with more people and gear. Just need to watch it when you are lightly loaded and trimmed up - jujst dont go all the way to WOT

If you havent verified your tach might be worth doing to make sure 5200 is really 5200.

The only reason i thought about trying the OEM 21" was that the old 21" was in pretty bad shape according to the shop, and i was concerned about the high rpms at WOT.

But.... I agree with you on being a tad under propped and having the extra power in the hole shot is more important to me than top end speed. I'm quite happy with the over all performance of the 19" anyway. Think i'll call her good. Thanks

I thought I’d follow up on this old post after playing with 4 blade props a bit this last year. I ran the 19”-3 blade for a year and was pretty happy with it, but decided to try. 17”-4 blade for pulling a skier last July. It was very responsive and planed out quicker coming out of the hole, but I couldn’t get more than 4000 rpms at WOT, and top end speed was drastically reduced. I got highs rpms with the 19”-3 blade, and even though the bow took a little longer to come down it came out of the hole quicker than the 17”-4 blade….. I wasn’t happy at all.

so I went back to the prop shop thinking I needed an 18”-4 blade, but the tech told me I was going the wrong way to get higher rpms….
so I ended up trying the 15”-4 blade and it was a very impressive difference. My rpms came up to 4400-4500 , and top end speed was higher. but the coolest part was how responsive this was at low rpms…. I can ease the throttle forward to about 2500 -2800 rpms and that nose comes down and it planes at much lower speed. It turns quicker at lower speeds with what seems like more control. It also slowed the boat down at idle speed making it much easier to bring it into the dock. It just handles like a new boat…..

I thought I would share this info for others. Common sense and prop size and configuration just doesn’t work. You just need to match the right prop for your needs by trying different sizes. I would have never tried this 15” -4 blade myself. These prop shops know there stuff.

Kirk

It's an art not a science. What works on your boat may not work on your buddy's boat who has the same kind as you. As you see, testing a variety of propellers is the only way to get the results u want. That last paragraph is exactly why we keep harping on using the services of the knowledgeable pros.

I absolutely agree with it being an art rather than a science.

Besides that, people have different expectations from their boats. One guy will sacrifice a 5 second lag time getting out of the whole to get higher top end speed. Others like me a prefer more responsive craft at lower rpms. That low end torque and quick response may have cost me 10 mph of top end speed, but when you are running in 6' swells and wind waves in the jaws of a bay, quick response at lower speeds trump everything. This 15" prop is much better for using the main to troll with in heavy current too, and pulling a skier with a boat load of people, the right 4 blade prop is the way to go. (for my boat anyway)

Using a professional prop shop is the only way to fly..... Kirk

I did something similar with my 1988 4.3 4bbl Cobra. Started with a 14.5x19 (rpm too low), went to a 15x17, better then settled on a 15.5x15, perfect will easily hit 5,000 rpm and is much more responsive at low speeds and accelerating on plane.

louc said:

I did something similar with my 1988 4.3 4bbl Cobra. Started with a 14.5x19 (rpm too low), went to a 15x17, better then settled on a 15.5x15, perfect will easily hit 5,000 rpm and is much more responsive at low speeds and accelerating on plane.

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They say that the 4.3 with a two barrel will only put out about 190 HP, and the 4 barrel can get 200 HP or more. That may be the difference between ours and getting more RPMs is that 4 barrel. I considered changing mine over when i got my new carb, but dismissed the idea being too expensive changing out the intake manifold.

What is the weight and hull design on your boat? Kirk

My boat is a 1988 Four Winns 200 Horizon, 20’9” weight is listed as 2600 lbs but I’m sure it’s a good bit more probably 3000 or slightly more. The hp ratings on the older pre Vortec 4.3s was 175 with the 2 bbl and 205 with the 4 bbl. The later Vortec models put out about 25 hp more. I think the 4bbl gives a significant performance advantage.

Good info on HP ratings... I'm unsure what the HP difference would be between a Truck engine and the Marine version, but my boat had a truck engine put in it right before i bought it. I didn't find out about this until i took it to a reputable shop for some work to be done. It took a lot of work and swapping out marine grade parts to make this coast guard approved and safe. Starter, fuel pump, water separator, new carb, and the biggest one was changing the soft plugs in the block for a fresh water/salt water cooling system. I've learned a lot about boats the last couple years. LOL

So i'm uncertain exactly what horse power i'm running, but this engine has plenty of power to do what i want it to do without running it too hard. Going down in pitch to the 15 / 4 blade was an excellent move. My 19' glass boat is running about 3000# full of fuel , no gear. Kirk

kirkll said:

Good info on HP ratings... I'm unsure what the HP difference would be between a Truck engine and the Marine version, but my boat had a truck engine put in it right before i bought it. I didn't find out about this until i took it to a reputable shop for some work to be done. It took a lot of work and swapping out marine grade parts to make this coast guard approved and safe. Starter, fuel pump, water separator, new carb, and the biggest one was changing the soft plugs in the block for a fresh water/salt water cooling system. I've learned a lot about boats the last couple years. LOL

So i'm uncertain exactly what horse power i'm running, but this engine has plenty of power to do what i want it to do without running it too hard. Going down in pitch to the 15 / 4 blade was an excellent move. My 19' glass boat is running about 3000# full of fuel , no gear. Kirk

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truck and marine are basically the same power nit much difference in the cam. Hoepfully they swapped marine head gaskets as well?

I doubt very much that this block has marine head gaskets. what are the difference? I know this engine has cast iron heads and not aluminum. Kirk

Marine head gaskets are made of composite or stainless vs steel so they dont corrode when used in open cooling vs anitfeeze in a truck.

Normally when guys take a truck engine and convert it they strip it down to long block use the marine oil pan and windage tray, swap head gaskets and freeze plugs, use the marine intake carb, exhaust , and swap over all electrical and fuel components, use marine circulating pump

I used a Fel Pro marine gasket set when I replaced the cyl heads on my 4.3. This was a marine spec engine from the start and the originals looked like stainless steel; not corroded when I took it apart in 2017 after 29 years of use. The Fel Pro gaskets are definitely some kind of composite.