More HP = Nearly Same Top Speed

shockwaveharry · 10-15-2002, 04:25 PM

Last april I replaced my 496 MAG HO with a carbed 540. The 496 is rated at 425 HP and my 540 dynoed at 525 HP and 575 ft/lbs torque, both at 5000 rpm's (7.5:1 compression, no blower yet). Although acceleration is twice what it was with the 496, I only picked up appx. 4 mph. 76 vs 80 on the gps. RPM's are 5000 with a 25 Mirage and 5200+ with a 23 Revolution 4. I expected more MPH with a 100 hp increase. What's wrong?

gmnhra · 10-15-2002, 04:42 PM

how am i to believe anything in your post when it looks like that picture was taken at elsinore and the water looks blue!!!!?????

ha..ha...

good seeing you at piru last weekend.....

i will be interested to see responses, but i think a 5-7 mph increase for a 100hp increase is about right....

if you are thinking about that blower addition this winter....give me an email at gmocnik@refractec.com

my carbed intercooled whipple might be available...possible quad rotor in the works....

see you soon

gary

Whiteknuckle · 10-15-2002, 04:59 PM

80 MPH, 5000 RPM, 25 inch Mirage. I think I'm the one doing something wrong. You gotta fast boat Mister

shockwaveharry · 10-15-2002, 05:05 PM

The water only looks blue from an angle! That boat is floating in a foot of the greenest goo on the face of the earth! Maybe that's whats slowing me down!

Definately interested in the Whipple, lets talk.

Did you get in any trouble there last weekend?

10-15-2002, 05:10 PM

You're also comparing prop hp to crank hp. By the time that 525 hp is routed to the prop, you've lost about 30-35 of them, so now you're at 425 vs 490. I think you are doing good to have gained as much as you did with a 65 hp increase. It took me 150 - 175 more hp to gain 10 mph in a similar sized boat.

Don't worry too much, that future blower will make up the difference.

jdnca1 · 10-15-2002, 05:30 PM

I think your speed pick-up is about right. When I spray a 175HP shot of NOS I gain about 9mph in a cat...

mopower · 10-15-2002, 06:04 PM

The faster you go , you need a lot MORE hp to go a little faster

I've seen it before and done it before. I added 50 or so hp and got 1 or 2 mph , friend went from 260 to 490 picked up about 5/6 mph. My personal best was 330 hp to 625 hp speed went from 61/62 to burying and 80mph speedo. (no gps back then). It also depends on the hull

I would be suprised to see more than 3/4 mph with 100 extra ponies

IMHO

audacity · 10-15-2002, 06:10 PM

hummm...i take it you have 1.36 gears?? but with stock pwr package you get 1.5's...did you change the drive as well to an XR/XZ or something???

welcome to boating

MIKEHTMSR24 · 10-15-2002, 07:56 PM

Quote:

Originally posted by shockwaveharry
76 vs 80 on the gps. RPM's are 5000 with a 25 Mirage and 5200+ with a 23 Revolution 4.

Got to luv those Rev 4 props!

1.36 gear
5200 rpm
80 mph
25 mirage = 8.09% slip
23 Rev 4 = 3.93 % slip
I had a 24 Bravo 1 labbed by merc and my 23 Rev 4 stock was 2.5 mph faster than the labbed Bravo 1.

formula for slip
RPM x Pitch
--------------------- = theorical speed
Gear Ratio x 1056

Theorical speed - actual speed / Theorical speed = slip%