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Raisin Wake
12-27-2004, 07:04 AM
Anybody know the optimum WOT rpm's for a 7.4 liter Mercruiser V-8?
Thanks

RiverOtter
12-27-2004, 07:07 AM
Anybody know the optimum WOT rpm's for a 7.4 liter Mercruiser V-8?
Thanks
Back when I had one seems like it was 4,700 or so.

TCHB
12-27-2004, 07:12 AM
If it is the small horsepower engine it is 4800 max.

Raisin Wake
12-27-2004, 07:14 AM
I think the horsepower is around 330????

phebus
12-27-2004, 08:16 AM
Here you go, 4200-4600.
MerCruiser 7.4L MPI Specifications
Propshaft Horsepower: 310 (232 kW)
Displacement (Liter/CID): 7.4L/454
Cylinders: V-8
Bore & Stroke (inches/mm): 4.25 x 4.00/108 x 101
Compression Ratio: 8.9:1
Fuel System: MPI
Full Throttle RPM Range: 4200-4600
Ignition Type: MEFI/EST Digital
Alternator: 65/917
Length (inches/cm): 39/99
Width (inches/cm): 32/82
Height (inches/cm): 24/61
Weight (lbs./kg) - Alpha: N/A
Weight (lbs./kg) - Bravo One: 1158/525
Weight (lbs./kg) - Bravo Two: 1170/531
Weight (lbs./kg) - Bravo Three: 1179/535
Weight (lbs./kg) - Blackhawk: N/A
Gear Ratio - Alpha Drive (primary/optional/high altitude): N/A
Gear Ratio - Bravo One (primary/optional/high altitude): 1.5/1.36/1.65
Gear Ratio - Bravo Two (primary/optional/high altitude): 2.0/1.81/2.2
Gear Ratio - Bravo Three (primary/optional/high altitude): 2.0/1.81/2.2
Gear Ratio - Blackhawk (primary/optional/high altitude): N/A

MOBrien
12-27-2004, 08:47 AM
I've got the 7.4MPI (310hp) in my 24' open bow Cheetah turning a stainless 21P 4-blade "Revolution" and get 4650 trimmed up at about 61mph.
What kind of boat is it???
Hope that helps.
-MOB

Raisin Wake
12-27-2004, 08:57 AM
22' Eliminator Extreme. Right now I am trying to prop it correctly. I'm running a 21 pitch 3 blade SS mirage. My WOT rpm was right around 4700. Although it was a little rough when I tested it. I want to move to a 4 or 5 blade prop, because I'm having cavitation issues at throttle up. I was looking at the 5 blade 23' prop. Will my rpm's be too low?

Raisin Wake
12-27-2004, 10:25 AM
I've got the 7.4MPI (310hp) in my 24' open bow Cheetah turning a stainless 21P 4-blade "Revolution" and get 4650 trimmed up at about 61mph.
What kind of boat is it???
Hope that helps.
-MOB
Does a 4 or 5 blade prop of the same pitch, produce the same rpms as a 3 blade prop of the same picth?

phebus
12-27-2004, 11:03 AM
Lots of variables (i.e. diameter, cup, design, etc.), but the general rule of thumb is to drop 1" in pitch when going from a three blade prop to a four blade prop.

Raisin Wake
12-27-2004, 01:10 PM
I just got an email from a prop shop. They said I might want to consider a 4 blade prop. What are the downfalls of a 5 blade prop? I'm not as concerned about my top-end as I am about my pulling power and ability to plane quickly. I still ski and my kids just learned how to ski.

Boozer
12-27-2004, 01:20 PM
I just got an email from a prop shop. They said I might want to consider a 4 blade prop. What are the downfalls of a 5 blade prop? I'm not as concerned about my top-end as I am about my pulling power and ability to plane quickly. I still ski and my kids just learned how to ski.
You will sacrifice torque with more blades. More blades give you more speed but less towing power. I'd try a 23p SS 4 blade and a 23 SS 3 blade. I'm turning a 3 blade 23p with a 190hp V6 and it works fine in Havasu. You have a hell of a lot more power then I do so I think you'd be able to spin a 23p 4 blade no problem.

rrrr
12-27-2004, 01:26 PM
I had a 3900 lb. 24' open bow boat with that motor, traded it in last spring. I was running a three blade 23" mirage and turning 4550 according to the tach. The Thunderbolt V ignition on the 7.4 has a built in "soft" rev limiter, you can't really tell when it's working. I think you are on the limiter with the 21" prop, that might be the "rough" you describe.
Of course it's hard to tell the exact RPMs; dash mounted tachs aren't all that accurate. Borrow a 23" and see what your revs are, usually a prop shop will let you try out a couple of different props.

phebus
12-27-2004, 02:28 PM
You will sacrifice torque with more blades. More blades give you more speed but less towing power. I'd try a 23p SS 4 blade and a 23 SS 3 blade. I'm turning a 3 blade 23p with a 190hp V6 and it works fine in Havasu. You have a hell of a lot more power then I do so I think you'd be able to spin a 23p 4 blade no problem.
I think you have that backwards. You will generally lose top end when you go to more blades, but gain hole shot. (provided you are propped right with the given prop)
The Mercury High Five prop was designed as a ski prop. Great hole shot, but definately not a speed prop.
The new generation of five blade props (i.e. Mercury Maximus, Herrings) are a different beast alltogether.

Raisin Wake
12-27-2004, 03:31 PM
I think you are on the limiter with the 21" prop, that might be the "rough" you describe.
What I meant to type was the water was rough. I'm not sure I was reaching true WOT. I would think I would have maybe 100 or 200 more rpms on smooth water.

rrrr
12-27-2004, 03:51 PM
What I meant to type was the water was rough.
OIC..... :D
I wasn't quite sure what you meant. I'm pretty confident you are on the limiter, and need a couple more inches. I think your boat is a lot lighter than the one I had, and we carried around 200 lbs. of junk in it; boards, skis, parasail, e-z up, and so on.

Sane Asylum
12-27-2004, 04:13 PM
Anybody know the optimum WOT rpm's for a 7.4 liter Mercruiser V-8?
Thanks
For WOT, I believe you push the stick(s) all the way toward the front of the boat............
Try that and see if you get the desired result..... :rolleyes:

dicudmore
12-27-2004, 05:06 PM
For WOT, I believe you push the stick(s) all the way toward the front of the boat............
Try that and see if you get the desired result..... :rolleyes:
i think this guy may be from Beaumont after all :wink:

Raisin Wake
12-27-2004, 06:05 PM
Through all this discussion, I've narrowed it down to the following props:
High Five 23' (http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&tc=photo&item=2496772879&category=26456)
and
Bravo 1 22' (http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&tc=photo&item=4504332182&category=26456)
Now, all I need to do is test these on my boat. Anyone got any spares???;)
Any thoughts on these options?
Thanks for all the advice you all have given me. I appreciate it.

phebus
12-27-2004, 06:57 PM
I would go with the 4 blade. When I bought a new prop for my last boat from Bronson Hill Propellers, I had the same dilema. They talked me out of the High Five and put me in a better all around prop. They said that unless I was pulling all day, the High Five was out.
I suggest calling a prop shop and running your questions through them. Prop selection isn't black magic, and with the proper information, they can get you real close to what you need for your specific application.

98 Vector 21
12-27-2004, 07:05 PM
i think this guy may be from Beaumont after all :wink:
I think its some where about 5300rpm on the7.4l magw/a 27p four blade but Im to busy to look! So just before it lets go lift that seems to work for me.....

rrrr
12-27-2004, 07:08 PM
Try this website:
Bam Marine (http://go-fast.com/boat_speed_predictions.htm)
Using the speed calculator and slip calculator will get you close. Your boat should easily turn a 3 blade 23", and you will prolly be able to cup the prop a bit also. (IMHO, of course....)

Raisin Wake
12-27-2004, 07:30 PM
I would go with the 4 blade. When I bought a new prop for my last boat from Bronson Hill Propellers, I had the same dilema. They talked me out of the High Five and put me in a better all around prop. They said that unless I was pulling all day, the High Five was out.
I suggest calling a prop shop and running your questions through them. Prop selection isn't black magic, and with the proper information, they can get you real close to what you need for your specific application.
Thanks phebus. I plan on calling Bronson Hill Propellers tomorrow. Thanks again for all your advice. You too rrrr. :)

Infomaniac
12-27-2004, 08:14 PM
My rev limiter hits at 4,750

Raisin Wake
12-27-2004, 08:24 PM
My rev limiter hits at 4,750
Which motor?

spectras only
12-27-2004, 08:44 PM
I have the new 6.2 MX [320 propshaft HP] engines in my boat .Max WOT range is 4800-5200 .I run Bravo 1 4 blades 15X26p ,and max rpm is 5000 on the smartcraft gauges.

Raisin Wake
12-28-2004, 07:22 PM
Well, most prop place are closed for the holidays, but, I did talk to a guy at a boat store. He said that I might want to looking at a 19 or 20 pitch prop. That would seem a little too low. Seems like I would be hitting the limiter wioth that. I'll be waiting for some more experienced prop people to come back to work before I make a decision. I'm still leaning towards the 22 Bravo I or the 23 High Five. I just need to test them.