Why is it so slow?

mossfred · 08-26-2008, 02:00 PM

Finally got my Slingshot to Europe, but a little disappointed by the performance, she maxed out at 87 on the GPS. Running 575sci/bravo xr 32p bravo props turning 5600rpm, with smaller s/c pulley and reflashed ecm supposedly making 630 hp.
I thought the boat shpuld make 95. Any suggestions???

Fred

mwdill · 08-26-2008, 04:30 PM

which one is it ?
and how many hours on the motors?
the 575's usually need a refresh topend at 300 hours or so depending on how it was used

mossfred · 08-26-2008, 11:34 PM

Boat was named Just Push Play and Throttle therapy.

Had new valve springs last year and i just did valve job on both motors. All compessions are good.

jeff32 · 08-27-2008, 12:23 AM

I may be wrong but 32 props at 5600 rpm, 87 gps, not sure about the theorical math of that set up but looks pretty close to reality to me! We often say a 30 pitch is roughly 80 mph, so 32 at 87 I would think it's good!

No?

mossfred · 08-27-2008, 01:34 AM

Sounds like i need bigger props, 5600 rpm is rpm limit, maybe 36p bravos?

Strip Poker 388 · 08-27-2008, 11:10 AM

the factory 500efis was 5250rpm and 30inch 1.50 gear speed varied between 85-86, that was the factory test reports,

My factory test sheet showed 86.9. most 388's ive seen and driven are a 85mph

at 750 hp around 5900 was 99 w/ 28's 1.30 gear ratio

What options and and altitude?

May be past peek HP RPM wise

What speed was claimed when you bought it?

mossfred · 08-27-2008, 02:21 PM

Hi Strip, boat is at sea level, ambient temp 25c and of course i was told mid 90 s! However the tattle tale on the GPS does say 92 but i dont think i would see that with the current setup, maybe the boat has had different props on.
I know from reading the posts on this site that you are extremely knowledgeable when it comes to the slingshot as you have successfully increased the performance of your boat. I do think i may be close if not on the limiters (5600) so perhaps a bigger prop? Do you think 36 Bravo would get me there or do i need to look at something special like Herrings or labbed props?
Fred

mwdill · 08-27-2008, 04:08 PM

i believe the guy who owed it (just push play) had purchased a set of Herrings for it,
I knew the people that owned it before him(throttle therapy) and the best he saw was 97 but it would run 92-93 regularly, and he rarely ran it hard and took excellent care of it.

Strip Poker 388 · 08-27-2008, 04:57 PM

Quote:

Originally Posted by mossfred

Hi Strip, boat is at sea level, ambient temp 25c and of course i was told mid 90 s! However the tattle tale on the GPS does say 92 but i dont think i would see that with the current setup, maybe the boat has had different props on.
I know from reading the posts on this site that you are extremely knowledgeable when it comes to the slingshot as you have successfully increased the performance of your boat. I do think i may be close if not on the limiters (5600) so perhaps a bigger prop? Do you think 36 Bravo would get me there or do i need to look at something special like Herrings or labbed props?
Fred

Fred I am running stock merc 4 bl bravos, ck the props and see if theve been reworked, ya never know what someone els put on it?

As far as 5 bl maximus or herrings 5-6 blade I belive it will have to much stearn lift, and could throw it into a bad chine walk

Most drive settings are at 2.5 to 3. & tabs up at wfo

I usally run a 30pt 59-6000rpm w/ a 1.30, I tried a set of 32 it was getting up on the props,pretty hairy,still ran a 102.1.they need to be labed to get less stearn lift. Really dont thimk anything is left in mine,unless ya sart throwing money and unlimited testing.
Does it have a blower? if so whats the boost presure?

I like buzzing a motor, try a set of 34's, I was told the bigger diamiter like 36 and above wasent as efeciant as 28-32 in a bravo one pump

Do you have any info on the motor were peek HP might be???

also remenber be easy on the bravo drives

Rob

mossfred · 08-27-2008, 05:58 PM

Thanks MWDILL, that sounds like a familiar story. I was told that Herrings (borrowed) had been tried and given 97 mph but i thought it was just sales talk!

Rob,I bought the boat and it had blown both head gaskets on the port motor, the previous owner had fitted a boost kit from Arizona speed and Marine upping the boost on the 575 sci to 6 / 6.5 psi and increasing the rev limit to 5600 from 5200 adding 80 hp per motor (630 at the prop). I guess that the peak power is probably around 5200 and i should try props but which type??

Fred