JBravo

My buddy had a 502-525hp. built for his 1992 25ft. Sunsation Aggresser. The engine builder told him with his "formula" it should run about 78mph give or take a few! I dont the specs. but it does have lightning exhuast, MSD ign, new barvo1,1:50:1, 5000-5400WOT. We tested 4 props last night and look at this joke test.

23P mirage....5360 @ 69.0..GPS
24p bravo1....4950 @ 68.1
25P mirage....4980 @ 70.0
26P hydro......4930 @ 69.5

Man is he PISSED!!!!!! not even close! His old 454mag-365hp ran 62-63GPS with 595hrs. on it, and he only gained 6-7mph with a new $11,000 502!! I think he got boned big time!!! Whats this frickin "formula" hes talking about 2 get this speed?? Anybody know??? or isnt there one?? Bring on the input guys! ......HELP!!!!

Oh, by the way, he didnt have it dyno'd either, its a insurance project from hitting something last fall so he didnt have the xtra cash. Just the word of the builder and he gonna talk with him again with the results!!

Any thoughts guys................john

BGIII

iTrader: (3)

Your buddy really expected 16 MPH from 160 horsepower gain? Using 20 horsepower to gain 1 MPH as a general rule I've seen many people use, I think he got exactly what he should have expected.

fund razor

I'm with BGIII

16 mph was unrealistic.

JaayTeee

The rule of thumb is that it takes
20 h.p. to gain 1 mph above 60 mph,
525 - 365 =' s 160, 160divided by 20
= 's 8 mph.

Not too far off, IMO

fund razor

Insurance project yields a higher horse motor and 6 to 7 mph increase does not equal "getting boned" IMO.

What do I need to hit to get new motors?
I need two.

I want in on that.

mopower

Yeah that certainly sounds good.

I got to agree. For 160 hp gain he got just about what I would expect. The faster you go , the more hp you need to go faster(if that nakes sense).I would venture to say the builder was rather optimistic in his estimate
I went from 660 hp to about 1250 and picked up about 25 mph

OLDRAT

In that boat, 7-9 mph is about the best for the combo
described.

Sounds like he has something that will give decent power
and last a while. The price is not that bad considered
what everything costs these days.

I would recommend he enjoy it and don't jack with the builder.
He may need him again in the future or if some minor
problems develop.

Good luck,

OR

GOODT

80 MPH + 29 FT BOAT + SINGLE 525 = FOUNTIAN

tomcat

Based on your "before" numbers, hull efficiency and prop slip (I assumed 23 Mirage @ 5000RPM) are quite reasonable. If you really had 525 HP now vs. 365 HP before the boat should go 75 mph spinning a 25 Mirage @ 5500 RPM. If you can't spin a 25 Mirage fast enough to go 75, either you don't have the HP, or boat weight and drag have changed.

Your top speed now suggests about 450 HP, but think about when the before speed was measured versus now. Changes in temperature and humidity have a significant effect. The prediction of 78 might have been a bit optimistic, but don't kill the engine builder yet, some investigation and more tuning and testing may be required.

Also according to your before numbers, your hull curve requires about 12.7 HP/MPH in this speed range. The reason that rules of thumb like 15-20 HP/MPH are used is because engine HP figures are often optimistic dyno header figures that don't reflect the actual installed HP with marine exhaust, or the richer jets that were installed just before the engine left the dyno. If the engine is really only 475 HP installed and your before speed is the best number ever observed on a cold day with no fuel, then more of the difference can be explained without sentencing the engine builder.

JBravo

Thanks guys! Keep'm coming!

I did 4get 2 tell u when he bought the boat 8yrs. ago it did go 72GPS with 220hrs. on it with a 23P Turbo. I understand the hp. ratio after 60mph but, he didnt gain anything with an xtra 160hp. Thats where were lost???

How does a 454mag out perform a fresh 502???
And the GPS doesnt lie. Theres a guy that has a 1994 25ft Aggresser with a 454mag that runs 73-74GPS respectfully around town that will whoop his 502-525hp???? and my other buddy has 500 Bulldog 525hp. that runs 77GPS that i have "driven" in a Checkmate 253 Convincer??!

I dunno?