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How much extra HP to gain 16 mph...?

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Old 12-30-2006, 11:18 AM
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Default How much extra HP to gain 16 mph...?

Hi!

Maybe you can help me.

I mounted two Merc 2.5 EFI SS engines on my ol Phantom 25 feet offshore hull. Hull weight is 3.000 lbs with driver and engines. The boat ran 76 mph with a single Promax 300 spinning a A45 Tempest 25 at 6.300 rpms with 1:1,75 gears. Now the boat is running 86 mph spinning Mirage Plus 27 at 6.800 rpms with 1:1,87 gears. Above 6.800 rpms the props ventilates, and the bow drops.

According to other Phantom 25 owners, the boat should run approx 90 mph +. So I presume the lack in speed its due to props, set up and engine tuning. The engines ignition timing is at this time set conservative, and fuel curve fat (10%) since the engines will be serviced this winter. Both engines will be tuned in the spring to pull approx. 290-300 hp each.

In the 80-ties, the boat ran 74 mph with twin XR2 1985 models. I am not sure how much hp they pulled. 150/175 hp?

90 MPH/74 MPH = 1.216

"cube" 1.216, or 1.216 x 1.216 x 1.216 = 1.798

1.798 X 350 (existing H.P.) = 629 HP to go 90 MPH


If 2 x 150 hp I would need approx. 540 hp in total to get 90 mph. If 2 x 175 hp I would need approx. 630 hp in total to get 90 mph. Or something in between. What do you think?

Cheeeers and Happy New Year!

Toffen G - Norway
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Old 12-30-2006, 12:17 PM
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Default Re: How much extra HP to gain 16 mph...?

Toffen I think you will need to pull more rpm to have those 2.5 s put out max power.
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Old 12-30-2006, 01:29 PM
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Default Re: How much extra HP to gain 16 mph...?

the 2.5 start making power at the rpm you stated up till 7600.also depending on where you heve the height of the bullet on the case. set backs / jack plates will also effect your situation. also your props are not meant to surface hense why their blowing out, a three blade choper or lightning e.t should work better for the set up, or p[ossibly a mazco 3 or 4 bld.
in the rigs i have set up,( twin outboard go fasst) i have usually used those props along with a jackplate to fine tune the rig for optimal speed. essentially i used the 4 to 1 ratio. for every 4 inches the bullet is back from the bottom of the boat you can raise the bullet up 1/4 of an inch.
but for speed purposes i would start as even with the bottom as possible, maybe an inch down, put on the chopers, and try to get the rpm's in the 7500 rpm area. should have at least 12 p.s.i water.
with those engines and the set up just rite you should be hangin and bangin in the 90's.

good luck
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Old 12-30-2006, 02:37 PM
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Default Re: How much extra HP to gain 16 mph...?

Hi!

I have the propshaft 1,2 inches below now. I will rise the engines one hole, that is 0,67 inches, getting the propshaft height around 0,5 inch below. If higher is needed, I can rise it higher too.

The engines will be tuned this spring with correct ignition timing for 98 RON and proper fuel curve. Micheal Werner, my tech, told me that this year the engines have been two lazy dogs. Everything was set very conservative since the engines will be rebuilt this spring. So I have been told that I should gain 30-40 hp extra on each side.

The SS 15 inch midsections have 2 inch setback built in. Other Phantom 25 owners which have tried powerlifts have experienced a too transom heavy hull. Ok for WOT runs, but not suitable in high seas, despite bow tank. So I have to stick to have the engines mounted direct, og eventuelly adding a 2 inch spacer.

I will order 2 blueprinted ET props from BBlades. Brett reccommended this over choppers. I will try to get hold of some choppers as well for testing.

Thanks for your insight and advise.

Cheeeers, Toffen G
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Old 12-30-2006, 04:38 PM
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Default Re: How much extra HP to gain 16 mph...?

for the set back jack plates i meant a manual set up, such as the cmc unit made by cook manufacturing. minimal weight with the infinite ability to fine tune your heights without effecting the ability to run in the rough water. this is the same set up io have used on such boats as a 24 superboat, 30 superboat, 24and 28 skater, 28 ghost, 26 imp, who's owner's cared to run the rough water.

if you have 280 2.5's, unless they're beat to death i would not expect such drastic h.p gains unless you are planing on adding modifications such as hot heads, tuners,intake spacers, or a brain remaping. if you are i would recomend getting in touch with Marty from diamond marine in florida. he is a guru of the 2.5.
if you do tell him Bo from newyork says hi

good luck to you.
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Old 12-30-2006, 05:53 PM
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Default Re: How much extra HP to gain 16 mph...?

Hi!

Thanks again!

The ignition timing is set very conservative and an extra 15 hp on each engine should be possible to get when set correctly. I use PCU's from Brucato, and the top rpms are set rich at the moment (+10-15%). So I expect to gain some hp there too when we lean them out to factory specs. Michael Werner is a known European tech, long time F1 driver and engine builder. So I should be in best hands. Thanks for Diamond Marine tip. I have bough several parts from them in fact!

One guy in UK had 2 x 2.5 EFI SS engines (with 2 inch built in setback) mounted on 2 x CMC SS powelifts. It did not work well in high seas. One option would be to change the SS midsections to 20 inch midsections (with no built in setback), and buy manual jackplates from fx. CMC or Bobs Machine. Then the balance should be better allowing the flexibility of the jackplates.

I will try with the SS midsections, higher propheight and ET's first. And then look at the manual jackplate option .

Happy holidays!

Cheeeers, Toffen
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Old 12-30-2006, 08:28 PM
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Default Re: How much extra HP to gain 16 mph...?

Our 2.5 280s are normally set at 1 5/8 with an 8500 ECU. We also run 30 lab finished 3 blades. That is on a 30 cat. If you go to screamandfly.com, those guys live for outboards.
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