little 4.3 that could...25p Prop or get a 1.47 ratio drive?

kpforce1

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I am stuck trying to figure out what I should do. I have a 20' 1994 Maxum 2000sc with a 1.8x ratio drive and a custom built 4.3 (edlebrock carb and intake, bored .030" over, Keith Black pistons, healthy roller cam, worked Vortec heads, larger SS valves, comp 918 springs, Magnum Pro rockers, ARP studs everywhere) and supporting exhaust tube, y-pipe, and Marine Power manifolds/risers. My machinist and I estimate that it is probably in the neighborhood of 320hp. Boat will primarily be a cruiser as my kids are to young to do much atm. It will also serve as a speed boat for the wife and I to play with friends that have 60-80 mph boats.

After taking her out with a full tank, a 200 lb friend (i also weigh 200), 200 lbs worth or tools/spare parts and a Highfive 21p prop, I am confident it makes power lol. 45mph if I dropped the hammer it will still put you in the seat and was on plane in less than 2 seconds. With the current H5 21p prop, I could easily blast past 6k RPM if I was willing to and was doing ~55mph at 5800-6k RPM.

The only way to keep my RPMs at ~5500 at WOT would be to more than likely swap to a 25p H5 lol or swap to a 1.47 ratio drive. What would be the better method to achieve a proper RPM threshold keeping cost out of the equation?

Thanks,
Kp
 

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Scott Danforth

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switch to a 1.62:1 drive

make sure you check for reversion. the V6 at 220hp at the prop (270 at flywheel) is on the raged edge of reversion.

sounds like a fun little motor
 

alldodge

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Every way I run your numbers in the prop slip calculator its bad, over 20% slip.

That light of a boat does not need lift and the high five is going to give you that. I would look at the 1.45 ratio if you can get it to try. This will get you more in the range so changing props gives you more choices.

I would suggest 3 blade props until you get close to where you want. If you were able to get to 60+ you need to be careful of chin walking with that short a boat. Going to 70 will be real scary and hope you have full hydraulic steering (really need it above 60 with the boat IMO)
 

Bondo

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I would look at the 1.45 ratio if you can get it to try.

Ayuh,..... I'm thinkin' this should say a 1.65:1,...... Which I completely agree with,.....
 

89retta

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Get rid of the High 5 , small diameter at 13.5". My stock 4.3 GXI would hit the rev limiter when I tried a 21" High five. Try a 15x22 3 blade prop
 

porscheguy

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A high5 is meant for snatching up skiers, it’s no high speed performance prop. Don’t Make the mistake of thinking a high 5 is a small version of the mercury Maximus.

320hp will get you into the high 50s, maybe 60. I’d go for a 1.62 ratio. One final thought. No matter how you build it, the 4.3 ain’t a high rpm engine. If you want it to last, I suggest you keep the tach limited to 5k.
 

jimmbo

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To make 300hp a naturally aspirated 4.3 is going to have to spin close to 6000 rpm, maybe a bit more. whether not it can hold together for long is another story. Would have been easier and cheaper to use a 5.7 or a 6.3 to get 300hp, would have more torque, which is more important in a boat anyway
 

Faztbullet

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Only way that hull will see high 70's is behind a good truck. The X height is to deep on drive...find a Alpha SS drive and that will be good for a 5-8mph bolt on.
 

porscheguy

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I read that about a year ago and wondered how much he spent on that project. Although I was greatly impressed with the mods he made to the drive, I also think it’s an excellent example of proving that speed costs money, and you can solve any problem if you throw enough time and money at it.

Getting the geometry right on that drive took serious skills.
 

kpforce1

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Update guys, I put a TP Hustler 4 blade 14" 21P on it. Added a K&N spark arrestor as well. Loaded weight was around 3500 lbs and we were running averahe of 51 and touched 53 mph at 5300 RPM trimmed out. I still have some tuning to do on the carb but seems to be running ok.

I guess I am a bit disappointed but I dont know much about hull design and I think my Maxum 2000SC V hull isnt exactly a fast hull.
 

alldodge

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Assuming you have the 1.65 drive your at 17% slip so your still loosing a lot of power

Now if you still have the 1.8 then your at 9% and that's good
 

jimmbo

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I think you are expecting too much from a small engine. 50 mph on 20ft boat with a 4.3 is doing very well. Quicker holeshot and higher top speed are going to require more displacement. Myself I would be looking at a 7.4, or one of the new 6.2 liter MerCruisers
 

kpforce1

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Assuming you have the 1.65 drive your at 17% slip so your still loosing a lot of power

Now if you still have the 1.8 then your at 9% and that's good

as far as I can tell it is the 1.8 so prop slip appears to be on point.
 

QBhoy

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A lot of this doesn’t make sense to me but I’ll chip in with two things.
With about 300hp suggested and only getting 53 at all those revs and that ratio seems way short to me.
My friend has a similar hulled maxum 1900 open bow with a normal 4.3 4 barrel and presuming the same ratio...I can tell you with certainty that achieves 50mph with a 21” laser two. Pops on the limiter running light and trimmed way out.
Lastly. You will need way over the HP you have to make that boat do 70 never mind late 70’s.
I have around the same HP in my 18 ft Chase with a more performance orientated hull and there is no way it’s getting to 70. Best it’s done is just under 67. Runs best all round with the most recent of many props...the incredible enertia.
 

QBhoy

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Every bit of me is interested in this post and can’t wait to hear how you get on, but every bit of me also thinks the numbers don’t add up. I haven’t gone as far as some and looked at prop slip or that, but as mentioned a similar boat well known to me. The 1900 with 4.3 and presumably 1.6 or 1.8 with 21” laser gets 50 or just over gps at about 4800/4900. If you are seeing only a few mph over it with so much more revs it seems strange.
Mots probanly been mentioned but hi 5 props are useless at the quick end of the business. Easily drop 6/7 mph compared to a good 3 blade.
 

jimmbo

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I would like to know the specs of the camshaft. Odds are it is most likely a grind that is usable in a car/truck, but leaves more than a bit to be desired for marine use. To get the HP numbers that were quoted in the original post, a 4.3 will have to spin over 6000rpm. To do so, it is going to have a very weak low rpm performance. Dropping pitch or regearing to get the engine into it's best rpm range will of course lower the mph for any given rpm, but hopefully once the engine gets to the optimum range it will have enough torque to spin the lower pitch fast enough to actually show some speed increase.
 

kpforce1

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I would like to know the specs of the camshaft. Odds are it is most likely a grind that is usable in a car/truck, but leaves more than a bit to be desired for marine use. To get the HP numbers that were quoted in the original post, a 4.3 will have to spin over 6000rpm. To do so, it is going to have a very weak low rpm performance. Dropping pitch or regearing to get the engine into it's best rpm range will of course lower the mph for any given rpm, but hopefully once the engine gets to the optimum range it will have enough torque to spin the lower pitch fast enough to actually show some speed increase.

Super late reply but here are the entire specs:

Engine specs:
-Balance shaft block .030" over
-Proper quench distance/area attained by: a.)block deck .005" piston in hole b.) .040" Cometic gaskets c.) KB 12cc dished pistons (quench area matches Vortec chamber)
-ARP fasteners on rods/mains/heads
-Vortec heads with stainless valves (2.02" intake and stock sized exhaust), appropriate radius and valve job, milled to ~61cc, port matched to intake manifold, static compression ratio is ~9.8:1
-Comp beehive 918 springs and appropriate retainers (heads machined for them and over .500" lift cam)
-ARP 3/8" threaded rocker studs (my heads had the M10 sized threaded holes)
-Erson Hydro roller grind: ADV Duration = IN 282*/EX 286*; Duration @.050" 214*/218* Lift .480"/.510" Lobe Center 112* w/4*adv.
-Comp 1617-12 Ultra Pro Magnum 1.52 roller rockers (self aligning)
-Polished tall aluminum valve covers to clear the rockers
-.080" wall pushrods
-Melling stock pressure/volume oil pump and pickup
-light weight Coyles timing set
-Performer 2114 intake
-Edelbrock 1409 carb (endedu up needing .0950" secondary/.0970" primary jets, .065x.057 metering rods/pink springs, .043" pump nozzle) and heat soak preventive gasket
-K&N flame arrestor
-Dual oil catch can
-Thunderbolt IV with new cap/rotor and Moore Performance E-core coil and plug wires (I run 10* base timing, 24-25* total)
-Mercury Remote oil filter kit custom mounted (because it makes life easier lol)

Exhaust:
-Marine Power high flow exhaust manifolds with 3" riser spacers to provide some addition distance to combat possible reversion
-newer style Merc y pipe with better flappers and longer pipe elbows (cobra headed looking type) for the 3" riser spacers
-exhaust tube vs bellows

Cooling:
-I wanted a full closed cooling system with the new motor and $$$$ manifolds/risers. I was able to get a Monitor Performance full system to fit with the manifolds, risers and the goofy positioning of the thermostat location on the Edelbrock intake.
-Johnson 10-24805-01 (F6B-9) crank mounted sea water pump
-Indmar inline sea strainer
-Removed water pump impeller blades in lower unit and modified the pickup holes so I pickup raw water via the Alpha 1 but the Johnson pumps it.
 

QBhoy

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Some money has went into that ! Great work. But you may have been a fair bit cheaper to drop a bigger engine in it. I admire the work though.

as said previously. To run over 60, you will need at least 300 odd HP and at least a performance oriented hull. I think you’d be lucky to see much over a genuine 60 mph GPS.
It certainly won’t be close to 70. That’s pretty much a fact. Then there is the whole reliability issue. But still love what you have done so far. Keep us posted. Ditch the high 5. You’re now into the world of good props and keep it to a 3 blade too.

good luck.
 

jimmbo

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On a 5.7 V8, a Cam with those specs would add about 15 - 20 hp to what the Stock GM Marine Cam would produce, with peak HP around 4600 rpm. Since a 4.3 is basically 3/4s of a 5.7, I expect that grind to make about 11 - 15 more power than the stock grind, I am assuming GM just lopped 4 Lobes off the same grind for the 4.3. I've never Degreed a 4.3 cam
 
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