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I recently bought a '97 20 footer with 5.7 carb alpha one (GEN II) with 1.47 ratio. It does about 50mph with 800lbs of people and gear with 19P alumn prop at 4400 rpm. I'm looking for more, but fear the alpha one won't take much more hp. Also, max rpm range is stated at 4200 - 4600. Seems kind of low. Can I expect to gain much top end by pushing the rpm with lower pitch prop? How much hp can this outdrive take? Any recommendations for more hp with this drive and engine?
Hey stevebee,
I don't know much about that Alpha, but I am familiar with small blocks.
50MPH is a solid number, if you are hitting it. Your prop slippage with those numbers is only 7%. Very low. Is your speed GPS? If it is, then I don't think you'll get much more by spinning the motor higher, as I am guessing you are right in the top HP range for that engine. However, with an aluminum prop, I'll bet those numbers are suspect. http://www.goldenplasma.f2s.com/foru...confused14.gif
As for horsepower with that drive, I think the general rule of thumb is 300HP, however, I am pushing over 450HP through my Volvo (which the Alpha was designed to the same specs). IMO, a driver will kill a drive faster than horsepower (or more important, torque). http://www.goldenplasma.f2s.com/foru...s/scared04.gif
To make more horsepower at higher RPMs, you'll have to make that engine breathe easier. If you change the cam, go with some good heads and manifolds with through transom exhaust, you'll be able to spin that small block higher (more HP). For a starting point, I would look at the carb and manifolds.
http://www.goldenplasma.f2s.com/foru...es/happy25.gif
You might want to go with a 21 Stainless Steel Prop. You should top out about 53-55 mph with that set-up. (Thats assuming your 5.7 is with a 2bbl carb) You might get a few more mph's with a 4bbl & new cam.
Seems that my rpm would decrease if I go to 21 prop (from 19), and maybe my speed would decrease too?? I was thinking of going to a 17 prop to increase my rpm in hope of grabbing a few more hp on th upper end.
Originally posted by tahoe22:
You might want to go with a 21 Stainless Steel Prop. You should top out about 53-55 mph with that set-up. (Thats assuming your 5.7 is with a 2bbl carb) You might get a few more mph's with a 4bbl & new cam.
Thanks for your reply.
I checked with GPS yesterday, and surprise, I am not going 50, more like 46-47. sigh....
I might try a 17 stainless prop for kicks to rev a little higher. 4200rmp seem low for the 5.7, seems like it should be happy close to 5000 (from my automotive experiences with that motor)
Originally posted by Havasu Hangin':
Hey stevebee,
I don't know much about that Alpha, but I am familiar with small blocks.
50MPH is a solid number, if you are hitting it. Your prop slippage with those numbers is only 7%. Very low. Is your speed GPS? If it is, then I don't think you'll get much more by spinning the motor higher, as I am guessing you are right in the top HP range for that engine. However, with an aluminum prop, I'll bet those numbers are suspect. http://www.goldenplasma.f2s.com/foru...confused14.gif
As for horsepower with that drive, I think the general rule of thumb is 300HP, however, I am pushing over 450HP through my Volvo (which the Alpha was designed to the same specs). IMO, a driver will kill a drive faster than horsepower (or more important, torque). http://www.goldenplasma.f2s.com/foru...s/scared04.gif
To make more horsepower at higher RPMs, you'll have to make that engine breathe easier. If you change the cam, go with some good heads and manifolds with through transom exhaust, you'll be able to spin that small block higher (more HP). For a starting point, I would look at the carb and manifolds.
http://www.goldenplasma.f2s.com/foru...es/happy25.gif
If your in So Cal you should talk to John @ Ultra (619-443-1100) He's done a lot of testing with different prop set-ups. I think he might even let you test some props out.
BTW, what kind of boat do you have?
Thanks for the tip. I've got a '97 Bayliner 2050SS.
Originally posted by tahoe22:
If your in So Cal you should talk to John @ Ultra (619-443-1100) He's done a lot of testing with different prop set-ups. I think he might even let you test some props out.
BTW, what kind of boat do you have?
I must be confused. My understanding, and it worked on my boat was the lower the pitch the more holeshot you had but less top end speed. The higher the pitch the less holeshot and higher top end speed.
I have a 20 foot Larson with a 5.0 Volvo setup which came with a aluminum 21 and I was running 56 to 57 on GPS. I switched to a Michagin Wheel Ballistic stainless 21 prop and now I am running 59 on GPS with full tank of Gas. I have not had a chance to run it with less gas on GPS yet but I expect I will hit 60.
Am I confused?
Originally posted by RBook:
I must be confused...Am I confused?
On boating? No. http://www.goldenplasma.f2s.com/foru...confused14.gif
The pitch refers to the number of inches a propeller would travel in one revolution with no slip (a perfect world). Therefore, if you have the horsepower, you can prop up, and increase your top speed. The key is to keep the top end RPMs in the peak HP range. http://www.goldenplasma.f2s.com/foru...confused46.gif
However, to your point, your holeshot will probably suffer a little, as it is like starting in a higher gear.
Too much prop will drop you out of the peak HP RPMs. stevebee is attempting to get his motor into the peak HP range. By spinning the engine higher, he hopes the engine will get into the HP zone. I'm not familiar with the dyno specs on those motors, so I can't help you where the peak torque and HP RPMs are located. I think he's got his work cut out for him. http://www.goldenplasma.f2s.com/foru...confused09.gif
For example:
4,400 RPMs x 1.47 gears x 19" pitch @ 11% slip = 48MPH
or...
4,800 RPMs x 1.47 gears x 17" pitch @ 11% slip = 47MPH
or...
4,300 RPMs x 1.47 gears x 21" pitch @ 11% slip = 52MPH
My motor pulled top HP @ 5,200 RPMs, so that's where I try to prop my boat at.
Your aluminum prop was probably flexing on the top end, so moving to the stainless gained you an inch or two, just by not flexing. http://www.goldenplasma.f2s.com/foru...es/happy04.gif
Hope I didn't confuse you too much.
http://www.goldenplasma.f2s.com/foru...es/happy25.gif
HH - excellent explanation!!! Don't think you can get much more clear than that.
Just thought I'd add my .02
I have a 98 20 footer with 102 inch wide beam, which doesn't make for a very fast boat. It has a 5.7L carb alpha1 250hp, rpm range 4400-4800. I believe in 97 they were 210hp rpm range 4200-4600.
That said, with the stock 19p alum. prop I was running 53-54 mph and bumped the rev limiter at 4950 rpm .Went to 19p stainless and run 57-58 mph at 4700 rpm.
My humble advice would be to find a prop shop that sells and customizes props. Start with a 19p stainless steel and work with the shop to get it tweeked to run right at your max of 4600 rpm. I talked to Bronson Hill of Hill Props at the LA boat show and I know his shop is one of many that offer this service.
I have a 94 21 foot Kal Kustom with a 5.7 magnum alpha one. I run a 23p stainless and get 57MPH recorded by a police radar gun. I posted asking about more HP on tha alpha drive. with the higher HP I hope to get to 70 MPH with a 24p four blade. With a diff prop you should push 57 to 60 MPH. Anyone know if 400 HP is too much for the Alpha??
Larry S.
[This message has been edited by larry smith (edited September 11, 2001).]
I have seen the experts of the motor guys turn small block chevys to 4,900 rpm -light load -at boat builder dealer meetings to get the best top speed.
I have seen boat hulls that go no faster at 4,900 versas 4,250 rpm using different props.
I have seen very few boats that do not gain 2-4 mph going from a alum. to a stainless prop.
For the sake of comparison:
My previous boat was a Wellcraft Eclipse 21'
bowrider (1994) with a 260HP Merc 350 Mag,
Alpha 1 and 21 pitch SS prop. It also had
Corsa switchable thru-hull exhaust. This was
a carbureted (Carter AFB), flat-tappet engine
and it would go 59 MPH on the GPS at
4,900 RPM.
For the sake of comparison:
My previous boat was a Wellcraft Eclipse 21'
bowrider (1994) with a 260HP Merc 350 Mag,
Alpha 1 and 21 pitch SS prop. It also had
Corsa switchable thru-hull exhaust. This was
a carbureted (Carter AFB), flat-tappet engine
and it would go 59 MPH on the GPS at
4,900 RPM.
I have a 98 5.7gs 2bbl carbed; I switched my alum 14 1/4 x 21 3 blade to a 14 3/4 21 SS 3 blade and gained a solid 3MPH. The best $350 I spent while waiting for my log dinged alum prop to be refinished. The grab was like night and day and the top end is very sticky compared. The alum has never been back on the boat.
Originally posted by larry smith:
I have a 94 21 foot Kal Kustom with a 5.7 magnum alpha one. I run a 23p stainless and get 57MPH recorded by a police radar gun. I posted asking about more HP on tha alpha drive. with the higher HP I hope to get to 70 MPH with a 24p four blade. With a diff prop you should push 57 to 60 MPH. Anyone know if 400 HP is too much for the Alpha??
Larry S.
[This message has been edited by larry smith (edited September 11, 2001).]
Larry - Good rule of thumb. Anything over 300hp will kill an alpha1.
I HAVE A 20FOOT CENTURY CUDDYCABIN 260HP 5.7 ALPHA1. IHAVE A 19 PITCH HIGHFIVE AND AM LOOKING FOR MORE TOP END.WILL A 21 PITCH HELP OR NOT NET WIEGHT IS 2500# THE HIGHFIVE IS TURNING 46-4700 WOT WITH OUTDRIVE ALL THE WAY DOWN.I ALSO HAVE THRU HULL IF THAT HELPS.
Originally posted by monte:
IHAVE A 19 PITCH HIGHFIVE AND AM LOOKING FOR MORE TOP END.WILL A 21 PITCH HELP OR NOT
IMO- get rid of the High Five- that's a bottom end prop. Perhaps a 21" SS 3-blade- you're probably going to have to experiment with the diameters and blades.
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