5.0 to 5.7 Rapido rebuilt engine swap

[QC]

Re: 5.0 to 5.7 Rapido rebuilt engine swap

By default, the fastest prop (MPH) will be nearest the correct operating range . . . however, if the tach is off as much as the numbers indicate I am concerned that the 15 will end up above 5000 RPM. May not be a problem, but that would be the risk. Can you find a way to verify the tach before you do more testing?

 

[HT32BSX115]

Re: 5.0 to 5.7 Rapido rebuilt engine swap

At 15% slip you're turning 4800 RPM Could be the pitch is wrong, or the gear ratio.

12-15% is "about right" for most planing type hulls and a single prop drive.

The best I get with the Bravo III is about 8% slip.

I was reading about tachometer error recently and the author indicated that some tachs can be as much as 500 rpm off at the top of the range.

Maybe, maybe not...... You need to connect a known tach to be sure though..........Otherwise you're just spinning your wheels.......Or more accurately spinning your prop!!


That's a big boat that probably did something like 60mph with the original 460 King Kobra. (300 prop hp)

You now have around 230-260 prop hp so you might expect around 45-50 mph at WOT depending on load, trim, sea condx etc....

 

[wil7483]

Re: 5.0 to 5.7 Rapido rebuilt engine swap

I could just be reading everything wrong myself. I looked at each prop and posted the numbers off of them. I was running the 17 in the bay that day and the best the tach showed was 4300 rpms, that I know for sure. The speed was off the gps so I like to think it was correct but I will do it all again Saturday with all 3 props and see if the numbers change any.

If I can pick up a new tach I will and temporarily install it to use for the test.

 

[rad1026]

Re: 5.0 to 5.7 Rapido rebuilt engine swap

I know my tach is low at WOT, I have not tested it on the lake but in the driveway its 200-300 off when the dash gauge is at 2000RPM. I suspect its more when I'm at WOT. So when I hit 4300 I am happy with that.

 

[Cptkid570]

Re: 5.0 to 5.7 Rapido rebuilt engine swap

Don't blow up your new engine while testing props!

 

[QC]

Re: 5.0 to 5.7 Rapido rebuilt engine swap

12-15% is "about right" for most planing type hulls and a single prop drive..

Totally agree, that's why I think he's spinning closer to 4800 RPM

FWIW, Tachs are usually off by a percentage, top to bottom . . .

 

[coastalcruiser]

Re: 5.0 to 5.7 Rapido rebuilt engine swap

I have an old analog tune up analyzer that has rpm and also set dwell, and measure voltage, bet you could find one cheap.

 

[wil7483]

Re: 5.0 to 5.7 Rapido rebuilt engine swap

I dont ever run at WOT just want to be sure that I am getting all the push I can for the horse power at whatever throttle position. Dont want to be wasting revolutions. I will test with each prop and settle for the one with the best numbers within the 4200 - 4800 rpm range.

 

[rafting4fun]

Re: 5.0 to 5.7 Rapido rebuilt engine swap

I hope I'm doing this right! If not plz point me in the right dir.. I am buying a boat with frozen block 305. 2005. It has freeze damage on both sides of block and both exh. manifolds. The motor in question is a GXI/SX vp 305 MPI 270HP> I am considering putting a 350 320HP motor in its place. The folks i'm buying the block from (us engine) say Ihave no worries. The locals here in Boise, say I am going to have huge prob's with Meffi/sp pulswidth etc. The folks selling the block say the 350 has the same cam and the ecm will not know the difference stating they do it all the time.. The boat is very well cared for aside from not knowing any better as in winterizing. I plan on using the boat in local waters and out on the Columbia. I just don't want to shoot myself by trying the motor swap. Lot's of planned skiing, heavy folks with 4-5 people on board max.
I appreciate any advice and If i'm in the wrong place plz help. I did not want to post a new thread.. thanks, John

 

[HT32BSX115]

Re: 5.0 to 5.7 Rapido rebuilt engine swap

I hope I'm doing this right! If not plz point me in the right dir.. I am buying a boat with frozen block 305. 2005. It has freeze damage on both sides of block and both exh. manifolds. The motor in question is a GXI/SX vp 305 MPI 270HP> I am considering putting a 350 320HP motor in its place. The folks i'm buying the block from (us engine) say Ihave no worries. The locals here in Boise, say I am going to have huge prob's with Meffi/sp pulswidth etc. The folks selling the block say the 350 has the same cam and the ecm will not know the difference stating they do it all the time.. The boat is very well cared for aside from not knowing any better as in winterizing. I plan on using the boat in local waters and out on the Columbia. I just don't want to shoot myself by trying the motor swap. Lot's of planned skiing, heavy folks with 4-5 people on board max.
I appreciate any advice and If i'm in the wrong place plz help. I did not want to post a new thread.. thanks, John

Hi John,

Welcome to iBoats btw,

Yes, you are in the wrong place in that you NEED to post your own thread instead of piggybacking on an unrelated thread. You'll get far more people looking at your particular problem.

before you do have look at the ADULTS ONLY sticky at the top of this forum.

CLICK HERE



Cheers,


Rick

 

[Tail_Gunner]

Re: 5.0 to 5.7 Rapido rebuilt engine swap

I dont ever run at WOT just want to be sure that I am getting all the push I can for the horse power at whatever throttle position. Dont want to be wasting revolutions. I will test with each prop and settle for the one with the best numbers within the 4200 - 4800 rpm range.

I cannot speak to absolute's but i can defer to some of the best experts in propping a boat on this site. One is to prop your motor to its peak rpm..aka 4800 rpm. Two since it i/o you have two option's...a stern lifter or a bow lifter. Of those two you will have to make a choice, that is what do you like.

Made simple a strong stern lifting prop which will be a 4 blade with more midrange punch or a 3 blade which will give you more top end speed.

Either prop will hit the top of the rpm band....it is the qualities of the design that you prefer which will make the difference. If your concered about peak rpm and performance.....Marine cams are very weak at above 4000 rpm, something that cant be overcome with a prop.....

 

[Cptkid570]

Re: 5.0 to 5.7 Rapido rebuilt engine swap

I dont ever run at WOT just want to be sure that I am getting all the push I can for the horse power at whatever throttle position. Dont want to be wasting revolutions. I will test with each prop and settle for the one with the best numbers within the 4200 - 4800 rpm range.

Makes sense to me. Thank you for continuing to post your results.

 

[HT32BSX115]

Re: 5.0 to 5.7 Rapido rebuilt engine swap

I dont ever run at WOT just want to be sure that I am getting all the push I can for the horse power at whatever throttle position.

Not a bad policy, except, you NEED to run it at WOT (loaded at what you think will be your average (fuel/equip/people/etc load) at least once to determine if you're propped correctly.

Dont want to be wasting revolutions.

Absolutely. Propping any (stock) engine (inboard, I/O, outboard) so that it turns "RATED" RPM at WOT, helps ensure that you're not lugging at any RPM or "wasting RPMS" at the top end too even if you never run it at WOT.

It's really not like a car that takes very little power to run in overdrive at high speed...

This is why it's extremely important to have a fairly accurate tach for testing.

Cheers,


Rick

 

[wil7483]

Re: 5.0 to 5.7 Rapido rebuilt engine swap

Today on Lake Jackson, Florala, AL Pt 1

Today at Lake Jackson, Florala, AL Pt 2

Okay Rick, heres the results of todays run around the lake. I started off with the 19p prop and the rpms never got above 4200 with a top speed of about 40.3 mph for brief moments. However, the boat shot out of the hole like a bullet and on plane in an instant with very little effort.

I then switched to the 15p prop and I was able to get 4700 rpm's but max speed was 37.2 mph. The holeshot was sluggish but once on plane it cruised out nice.

I finally switched to the 17p prop and I was able to get 4600 rpm's but that was only with the drive trimmed up to the max and you could tell that the prop wasn't biting into enough water to get maximum thrust. When I trimmed down and felt the boat really cruising comfortable the rpms dropped to 4400 and max speed was 39.8 mph. She still came out of the hole quickly and planed out with no effort. Cruising at 3900 rpm she was doing around 32.3mph and the ride was nice.

So I'm tempted to go with the 17p only because it still gives me a decent holeshot and cruising speed. The 15p gets me right up there in the right range for the engine but it is slow coming out of the hole.

 

[HT32BSX115]

Re: 5.0 to 5.7 Rapido rebuilt engine swap

I like those pictures. That Wellcraft is sure a good looking boat!



Did you use a "known" tach?

4400/1.5:1/39.8mph (GPS?) nets about 15% slip.......

I would have expected that engine to turn that 17 a little faster......but I don't know what that boat weighs. you might be slightly underpowered (considering they originally put a 460 in it!!)

That performance might be the best you'll get with the 5.7, which is respectable.


I'm jealous!


The weather here is only just now starting to warm up a little!!!

I think the freezing temps are over for a while!



Cheers,

Rick

 

[Ridemywideglide]

Re: 5.0 to 5.7 Rapido rebuilt engine swap

Based on your post, why would you go with the 17p?

You said first off that the 19 gave you a holeshot bullet, and gave the highest top speed, at the correct rpm.

The others dropped in performance all around from that prop, based on info provided by others anyway.

Why would you not stick with the 19 you started with?

 

[wil7483]

Re: 5.0 to 5.7 Rapido rebuilt engine swap

Based on your post, why would you go with the 17p?

You said first off that the 19 gave you a holeshot bullet, and gave the highest top speed, at the correct rpm.

The others dropped in performance all around from that prop, based on info provided by others anyway.

Why would you not stick with the 19 you started with?

Because everyone here is saying that my engine should turn 4500 - 4800 rpms at WOT. The 19 would only turn 4200 rpms max. So based on that I am not within the range that it should be in so I'd think that I was over propped with the 19. The 17 gets me in the right range and gives me a decent holeshot and cruises at a decent speed. I can keep the 15 as a back up as it puts me right in the correct rpm range albeit gives less than nominal performance.

 

[Ridemywideglide]

Re: 5.0 to 5.7 Rapido rebuilt engine swap

I thought I had read 4200 being where you needed to keep it. Just trying to osmose a bit of knowledge.

 

[krisnowicki]

Re: 5.0 to 5.7 Rapido rebuilt engine swap

You need to buy a new tach. It has to be off... You just spent alot of money a nice shiney engine ... dont you want to make sure it is turning at the right speed so it will last a long time. I bet your put a new tach in and it is turning more then 5000. a 17p is small for that set up.

 

[wil7483]

Re: 5.0 to 5.7 Rapido rebuilt engine swap

I like those pictures. That Wellcraft is sure a good looking boat!

Thanks Rick.

Did you use a "known" tach?

4400/1.5:1/39.8mph (GPS?) nets about 15% slip.......

No Rick, I used the tach in the boat. It is new! All of my gauges are new.

So how do you find a "known" tach? I have read hear on iboats several times that tachs can come out the box bad so how do you keep from pouring money into tachs and not know if they are accurate or not?

All of the speeds posted are GPS speeds. I have a Garmin GPSmap 172 in the boat.

I would have expected that engine to turn that 17 a little faster......but I don't know what that boat weighs. you might be slightly underpowered (considering they originally put a 460 in it!!)

That performance might be the best you'll get with the 5.7, which is respectable.

Well the boat weighs per the literature from Wellcraft 3900 pounds without the power plant. Add in 700 pounds for the engine and leg. 350 lbs for fuel, 275 pounds for me, another 200 pounds for gear and the weight is above 5k pounds easily.

The boat came from the factory with several different power plants. The 260 hp 5.7 was the base engine. As per the literature..........

...........?An Elite 222 can reach top speeds of 49 mph with its standard 260 hp engine.? "It can be made even faster with the optional 350 Magnum or OMC King Cobra engines, or jumbo 7.4 liter MerCruiser of 460 OMC king Cobra power plants."

So I am definitely not reaching my full potential as far as speed and performance goes. I will check the carb over and make sure that it is opening fully the next time I am out. I will also try to make adjustments to the mixture to get the best performance I can but I will admit I will be following the books suggestions since I know very little about adjusting carbs. I still have to recheck the timing while the boat is in the water and under load. I took the timing light with me yesterday but forgot to do it. I also want to change the oil but I need to get a kit so I can suck the oil out through the dipstick tube. Lastly, I am still trying to get the new SEI drive. I am in the middle of moving so I don't want to spend money right now that I might need in a few weeks.

I did have a few questions too. Yesterday I noticed a small amount of oil on the bilge floor. I traced it to coming out of the dipstick tube actually. I think it is when I am turning hard to starboard that it is dripping out. Is this something that I should be concerned about? Plus the oil pressure spiked once for a few seconds but then went back to normal and stayed there for the rest of the day. Is this a potential issue or maybe just a sender or gauge problem? Lastly, should I contact Raul and let him know that I may have a potential issue with the engine since everyone here seems to think it should be spinning a lot more rpm's than it is?

I'm jealous! The weather here is only just now starting to warm up a little!!! I think the freezing temps are over for a while!

Don't be. You will be out there sooner than you think. The water temp here is just 62 degrees so staying in the boat is still the only option for me. The kids want to go to the beach but it is still to cool to get in the water in my opinion.