221 Vision with 496 BBC prop/speed help

[Bstonewilliams]

Hi guys, I’m new to the forum and the speed boat world, but I just got the 1990 221 Vision with a freshly build 10.2:1 496, according to the previous owner the engine is right about 500hp or so. The weird thing is that I can only get it to reach 72mph and the motor is turning about 4800rpm at that speed on relatively flat water. My trim gauge sender isn’t functioning at the moment so I’m not exactly sure where it’s at but it’s in the upper end of the trim for sure, the boat is dancing around pretty good. The boat has a bravo 1 drive, molded nose cone, 27 pitch 4 blade prop, hydraulic steering, big trim tabs etc.


Judging by other folks I’ve found, it seems like this thing should be doing 75-80mph at the top end, any help or ideas?

Thanks

 

[Glenn Kornett]

Hi guys, I’m new to the forum and the speed boat world, but I just got the 1990 221 Vision with a freshly build 10.2:1 496, according to the previous owner the engine is right about 500hp or so. The weird thing is that I can only get it to reach 72mph and the motor is turning about 4800rpm at that speed on relatively flat water. My trim gauge sender isn’t functioning at the moment so I’m not exactly sure where it’s at but it’s in the upper end of the trim for sure, the boat is dancing around pretty good. The boat has a bravo 1 drive, molded nose cone, 27 pitch 4 blade prop, hydraulic steering, big trim tabs etc.


Judging by other folks I’ve found, it seems like this thing should be doing 75-80mph at the top end, any help or ideas?

Thanks

View attachment 89502View attachment 89503
View attachment 89507

View attachment 89506

Nice Rig,
If we assume that the 72 mph is an accurate GPS speed, and the 4800 rpm is an accurate rpm (tach's can easily be out quite a bit). If that was a Bravo prop we would use 26P for the calculation but it has no diffuser ring so it is likely another type of prop. If we use 27P for the calculation the boat has 12% slip using 1.5 gear ratio, 72 mph and 4800 rpm. That is relatively good. A nose cone on a Bravo drive at only 72 mph can often scrub speed, and so can dragging the trim tabs but if 12% slip is accurate I don't think either is hurting you. If the boat is chine walking, and you learn to control the walking by steering and the boat stays straighter it will often pick up a few more mph. A bit of a chop will free it up and pick up a bit as will you alone in the boat and cooler weather. Do you know at what rpm this engine made the most HP? You could try a lower pitch prop if the engine makes more power at 5200 rpm than 4800 rpm. If that is a shorty drive, you could try running a spacer but since the slip is low already, I am not sure it will help. Now if your gear ratio is really 1.36, that changes everything because then the slip is then 20%, which is high. That would mean the drive might be too high or too low, the nose cone is hurting you, you are dragging the trim tabs, or the boat is running with the wrong drive trim angle. Or it needs a different type of prop that will reduce the slip. Do you know what the gear ratio is?

 

[miller]

I too am guessing that motor would be happier with a pitch or two smaller prop and one with more of a bow carrying blade design. Looks more like a stern lifting prop to me. If thats a 496" stroker motor with roller cam and forged internals, they seem to like being in the 5300 to 5600 rpm range to make their power. IMHO.

 

[Bstonewilliams]

Thank you guys for the detailed responses. I do not know what the gear ratio is, it is a balanced roller cam motor with forged internals. It has a Howard cam. Sorry for the rotated photos. I can’t figure that out on my phone. Do you guys have a link to an example of a prop you think would be a better fit ?

From a quick glance the trim tabs are a little less than flush with the bottom of the hull. I’ll have my wife look back and check the wake and see if she can notice any effect on the water from them dragging.

 

[miller]

Maybe a 25" mirage+, 23" rev4 or a bravo 24". Just some ideas.

 

[PA-Checkmate]

Just a idea, maybe it's not making 500hp or engine isn't in the power band where it makes peak HP?

Could the stock exhaust limiting HP output?

Here's a link to summit about the cam.

Howards Cams 120245-12 Howards Cams Retrofit Hydraulic Roller Camshafts | Summit Racing

Free Shipping - Howards Cams Retrofit Hydraulic Roller Camshafts with qualifying orders of $109. Shop Camshafts at Summit Racing.

www.summitracing.com

 

[Glenn Kornett]

Just a idea, maybe it's not making 500hp or engine isn't in the power band where it makes peak HP?

Could the stock exhaust limiting HP output?

Here's a link to summit about the cam.

Howards Cams 120245-12 Howards Cams Retrofit Hydraulic Roller Camshafts | Summit Racing

Free Shipping - Howards Cams Retrofit Hydraulic Roller Camshafts with qualifying orders of $109. Shop Camshafts at Summit Racing.

www.summitracing.com

That is certainly another possibility. I owned a 230SX Stingray with a 375 HP 496. It ran 67 mph at 4800 rpm. Not sure what the 221 Eluder weighs, but it apparently hit 72 mph, which is a tad quicker than the 230SX with a 425 HP 496HO. It certainly could make 20 - 30 additional HP at 5200 - 5400 which would be at least another 2 mph. A good exhaust (if it does not have one) could be another 1 - 2 mph. My best guess would be that the 230SX Stingray would be around a 76 mph boat with 500 HP at best.

 

[PA-Checkmate]

I also owned a 97 230SX Stingray with a 502/415 for one season and never got more than 64-64mph. Another friend also had same and never got the happy 70+ claimed by stingray. Sorry to side track.

 

[PA-Checkmate]

Any update to this?

Howard cams give this RPM window so you may not be at peak HP?
1,800-5,400

 

[Bstonewilliams]

Forgot to get back to you guys. I was able to try a merc bravo 24p prop from a local guy. We got the motor turning 5000rpm at top speed with it, 200rpm higher than before, but it was only doing 68-69mph in the same conditions. Seems odd that it would turn higher rpm but go slower. It also felt like the moto was breaking up a bit so I’m going through the carb and ignition components this winter to make sure that it’s my contributing.

 

[Bstonewilliams]

Also I found out the carb is a 1000cfm Holley to my surprise.

 

[Glenn Kornett]

Forgot to get back to you guys. I was able to try a merc bravo 24p prop from a local guy. We got the motor turning 5000rpm at top speed with it, 200rpm higher than before, but it was only doing 68-69mph in the same conditions. Seems odd that it would turn higher rpm but go slower. It also felt like the moto was breaking up a bit so I’m going through the carb and ignition components this winter to make sure that it’s my contributing.

A 24P Bravo prop is really a 23.3" to 23.4P prop. You only gained 200 rpm but you also went from a 27P (if that was really the pitch) to a 23.4 pitch prop. If you can find a 24 Bravo FS, it will have more bow lift and turn 150 - 200 rpm more than a standard Bravo prop but that would only add another 2 - 3 mph, unless the prop also reduces the slip. Either way if it is breaking down at WOT, you have to address that first like you said. At 10:1 compression with aluminum heads, you will be likely be running at least 89 octane if the timing has decent advance.

 

[Bstonewilliams]

Sounds like the bravo 24 may have more slip then causing lower mph? I’ve got the carb out for rebuild now. I was planning on checking all the plugs and base timing when I get it back together, I’m a fuel injection guy by nature, but the advance is set in the distributor with the weights correct? Any idea what I should be shooting for base timing and advance? Is it typically to follow standard big block automotive info or is it typically different for boats? Thanks for your help

 

[Glenn Kornett]

Sounds like the bravo 24 may have more slip then causing lower mph? I’ve got the carb out for rebuild now. I was planning on checking all the plugs and base timing when I get it back together, I’m a fuel injection guy by nature, but the advance is set in the distributor with the weights correct? Any idea what I should be shooting for base timing and advance? Is it typically to follow standard big block automotive info or is it typically different for boats? Thanks for your help

Sounds like the bravo 24 may have more slip then causing lower mph? I’ve got the carb out for rebuild now. I was planning on checking all the plugs and base timing when I get it back together, I’m a fuel injection guy by nature, but the advance is set in the distributor with the weights correct? Any idea what I should be shooting for base timing and advance? Is it typically to follow standard big block automotive info or is it typically different for boats? Thanks for your help

I just looked up the 500EFI Mercruiser and the WOT timing is around 36* at 3000 rpm. I think they set the initial to 8* at idle but it depends which timing module you have. Or do you just have an MSD stand alone distributor. If you do you can change the weights and tailor the curve. Or you can buy other digital ignition systems where you program the curve. I think they would program a mostly linear curve from 8* at 800 rpm to 36* at 3000 rpm.
At 68 mph, and 5000 rpm assuming a 1.5 gear ratio the prop slip works out to 8% which is very good. So the prop slippage was less than with the 27P prop. So something does not make sense. If the 27P prop was really a 27P, then the engine should have turned this 23.4P Bravo prop to at least 5300 rpm at 8% slip. I can only speculate what could have caused all of this. Some possibilities are the boat was much heavier and had more fuel and people on board, the engine was not making as much horsepower when you tried the Bravo prop, the 27P prop was off pitch and really more like a 25.5P prop, the speedo and tach do not always read accurately. Again, I am just guessing, but going from a 27P to a 23.4P at the same mph, should have increased the rpm to around 5500 rpm but it did not. Due to the lower slip, it should have turned it at least to 5300 rpm. 8% slip is extremely low for that type of boat, drive and prop. Either the speedo is reading high, or the tach is reading low to give you such low slip. I have a lighter 24 ft Checkmate Pulsare / 300R that has a lot of bow lift, and it will only reach around 9% slip with a 24 Bravo FS or 24P ProMax and it is only running 76 mph with the ProMax.

 

[Bstonewilliams]

Thank you Glenn, that is a great bunch of info. I’m guessing it’s possible the tach has the error if anything, but i would imagine the differential between the props should show on the tach even if it’s not exactly accurate, though I am assuming. Truthfully I was worried about driving the boat and the tach needle was jumping a bit, but I wasn’t watching it like a hawk. The Speedo is a new gps auto meter unit so that’s not likely to be the error point. The boat has an msd digital 6al controller. Our lake fills back up in February and I can get the boat back on the water then. I’ll go through the ignition system, very timing and the distributor setting and get the fresh carb on, and go for some more testing with the current 27p and reply back. Thank you again for your help and knowledge

 

[Glenn Kornett]

If you have a digital 6AL then I think you can't program the advance curve. I think you have to buy a timing module from MSD or change the weights and springs in the distributor. Do you have a crank trigger?
I have a Livorsi GPS speedo (I think Autometer makes it for them) I have a digital recall from the GPS speedo. I have compared the speedo needle to my wife's cell phone GPS and up to about 68 mph they are the same or within 1 mph. When I am doing over 70 mph the needle on the speedo starts to read higher and when my recall shows I went 76 mph, and the cell phone confirms it, the needle on the GPS speedo is between 78 - 79 mph. I called Livorsi and they said the digital recall will be accurate but the stepper motor driving the needle could easily be out that much?? Go figure!! However, I agree yours would most likely be out a similar amount with each prop. Perhaps the tach is just very lazy after say 4800 rpm and you did turn about 5300 rpm but it only showed 5000 rpm?? That would make the slip more like the 27P prop and you would have turned it at least at 5300 rpm.
Most people on this forum who have played with props will agree that if you change the pitch by 3 to 3.5" you should see around 350 - 500 more rpm. You had 4% lower slip (if the instruments are accurate) and that would be about 200 rpm less than the expected 5300 - 5500 rpm. Do you know what type of prop the 27P was? If the diameter was smaller than the Bravo it would have more slip (you did say it was a 4 blade). I think what you are doing right now is good, just in case the engine did make less HP due to the carb or timing. If you can feel it breaking down, you are losing some speed.

 

[Bstonewilliams]

No crank trigger. Distributor doesn’t look great but maybe that’s normal for marine engines. Looks like the weights and spring. Only setting on the 6al is rev limit.

Here is the prop

 

[Glenn Kornett]

Was that the distributor that was in the original marine block that cracked? Or is that an automotive distributor? It doesn't look like an MSD distributor but I always had MSD crank trigger distributors on our drag cars. That distributor needs some TLC. New cap and rotor, disassemble the weights, springs and lubricate the pins so the weights fly out easy.
Do you have an advance timing light to check the max timing? If so, check the initial timing and then set the advance timing light to about 20* and have someone slowly increase the rpm in neutral just until the marks stop moving. Then you know at what rpm the timing is maxed out at. Then turn the dial on the timing light until the marks align with 0* on the dampner and then read the scale on the timing light. On some Mercruiser engines around 1990 - 1995 there was a plug you had to disconnect to see the actual timing at idle, if I recall correctly.
Prop appears to be 15" in diameter. It looks like it has a straight barrel (no diffuser) like a Bravo FS. That usually helps lift the bow. It might be a HOSS prop but some of the members on this forum might be able to identify it. I could not really read the numbers on it HC-CL 27R? The number on the barrel appeared to be HE11671RP26. That is not a factory stamped number but looks like the prop could have been reworked and pitched down to a 26P. If it was reworked to a 26P then the rpm values would be a 150 to 200 rpm lower than I anticipated and not out nearly as bad.