WOT on 4.3 merc

[20002760]

Re: WOT on 4.3 merc

QC-
Love the boat. Wide beam and tall. Has the cabin of about a 33 footer. Went to the lake this morning for a few hours. Had a cocktail while watching TV in the A/C. Then went swimming and took a hot shower. Doesn't get any better

 

[QC]

Re: WOT on 4.3 merc

See I think you are getting close to 4500 RPM unless the pitch is wrong. Go here and you'll start to get it. Slip of less than 10% is not really possible with a boat like this: Prop Calculator

 

[ingalp01]

Re: WOT on 4.3 merc

You have a lot of boat for a 4.3...Your gearing needs to be posted to get close

I think he said he has twin 4.3's

At least I hope that's what he said, cause my 25 footer only does about 27 mph on a single 5.7

Before I started the refurbish on mine, I also had issues propping my boat. Still not sure I got it right before hauling it out last year.

 

[Tail_Gunner]

Re: WOT on 4.3 merc

I think he said he has twin 4.3's

At least I hope that's what he said, cause my 25 footer only does about 27 mph on a single 5.7

Before I started the refurbish on mine, I also had issues propping my boat. Still not sure I got it right before hauling it out last year.

Yes i know..iit would take twins to push that much boat our of the hole. Im with QC something is still amiss and it sounds like tachs. Sometimes in cases like this it can be frustrating it could be as simple as tach sticking at or not working properly across the rpm range or the props are not 19...A WAG puts the right props between 15 and 17..Good luck in your advenure....

 

[mrchev]

Re: WOT on 4.3 merc

I have a 22' doral aft cabin that weighs around 4500lbs plus gas.it has a 4.3lx with a 14.5 17p prop on it . with me and the girlfriend on it it runs about 40mph at 4200 wot, if i add three people it will barley get on plane at 4000 wot. so i changed to a 15p prop and am putting it in this weekend i will post my results.

put it in today with 15p prop. results are 4500 wot and 40mph. comes up on plane alot quicker.

 

[20002760]

Re: WOT on 4.3 merc

look closely. That post above is not mine. Problem is still alive and well. Advice greatly appreciated

 

[Bondo]

Re: WOT on 4.3 merc

I am seeing 39mph at 4000rpm. I would just like to get it in the WOT range of 44-48. Speedo and GPS are accurate. Even ran next to a buddy and compared speeds. I don't know what it will do but I would like to see WOT. Just flushed Port motor on land and exhaust is not coming out of the lower unit it is coming out below the transom. May have plugged exhaust.

Ayuh,.... Have ya read This yet,..??

 

[Bondo]

Re: WOT on 4.3 merc

look closely. That post above is not mine. Problem is still alive and well. Advice greatly appreciated

Ayuh,... Donno Who mrchev is talkin' to, nor what about,..??......

 

[180shabah]

Re: WOT on 4.3 merc

19" X 1.81 X 10%slip @ 39mph = 4300+rpm

One number is wrong, no way around it. If the drives are original(never rebuilt) and the speed is certain - that leaves the tachs (both reading the exact same error???) or the props.

Are you sure the props have never been "worked" and are now behaving like a 21 instead of a 19?

 

[20002760]

Re: WOT on 4.3 merc

There is no sign that they have been worked. At this point anything is possible.

 

[Robbabob]

Re: WOT on 4.3 merc

How about you take the props to a shop and have them confirm pitch. What ever they are today, you need to pitch down 2-3" to see your WOT within the max engine range.

 

[QC]

Re: WOT on 4.3 merc

What ever they are today, you need to pitch down 2-3" to see your WOT within the max engine range.

Not if the tachs are wrong! Any bad piece of data and all of the results are FUBAR. Speculation rarely works with technical diagnosis . . .

 

[Robbabob]

Re: WOT on 4.3 merc

Just sayin' OP says tachs have been checked twice. How about having the pitch checked and evaluating from there?

 

[QC]

Re: WOT on 4.3 merc

I agree ^^^^ just noting that with any doubt on data we can't make any recommendations. Heck, gear ratio could be wrong, and that would account for the slip being so low (too low) and then the tachs could be right and then the pitch would obviously be the solution. Annnnnd the engines could still be suspect. But as long as there is bad data ( I believe there is), then all else sucks.

Still wondering about Altitude although I think 39 MPH is respectable, but don't want to leave any stone unturned.

Edit: And I still believe these are 190's not 220's. The 1.81 ratio is key to both that and my concern about the tachs.

 

[Tail_Gunner]

Re: WOT on 4.3 merc

I agree ^^^^ just noting that with any doubt on data we can't make any recommendations. Heck, gear ratio could be wrong, and that would account for the slip being so low (too low) and then the tachs could be right and then the pitch would obviously be the solution. Annnnnd the engines could still be suspect. But as long as there is bad data ( I believe there is), then all else sucks.

Still wondering about Altitude although I think 39 MPH is respectable, but don't want to leave any stone unturned.

Edit: And I still believe these are 190's not 220's. The 1.81 ratio is key to both that and my concern about the tachs.

That boat is not that far off..1.81 19p 4800 rpm 42mph gets 12% slip... Actual 39 gets 20% 40 mph @ 4800 rpm.. in a 28' boat with dual 4.3's...10000lbs with a few people sounds about right there v6's not 8's and weak on the top end


But the 4000 rpm does not work in any scenario....

 

[20002760]

Re: WOT on 4.3 merc

 

[QC]

Re: WOT on 4.3 merc

DonS has stated that your SN = 1.81 gear ratio. From at least 1996 - 2002 the 4.3s that were coupled to the 1.81 ratio were 190 hp. If the tachs are right, and I do not believe it unless the tech used a photo tach or other method of confirming actual vs. indicated RPM, then the props are not 19s or the engines have been modified or the gear ratio is wrong. 39 MPH and 380 total horsepower (less at 4000 RPM) is good if you ask me. If the tachs are right, you need to try 17s, but I don't believe a good running pair of engines that only achieve 4000 RPM could even plane this thing, let alone happily run up to 39 MPH. I am sorry to report, but this is actually reasonably fast

The other thing you could do is convert them to 205s (there was no 220 4.3 from Merc in the same time period).

 

[20002760]

Re: WOT on 4.3 merc

Would 14.25 x 21p result in about the same performance? Mine are 15.25 x 19p

 

[QC]

Re: WOT on 4.3 merc

You're considering a prop change before confirming the data? Something is wrong. Plus, if you're data is right, upping the pitch will result in LOWER WOT RPM. Going backwards . . .

 

[20002760]

Re: WOT on 4.3 merc

No. I met 2 other owners running 14.25 x 21p. I was curious. I thought the decrease in diameter would negate the higher pitch