boatfreak

I have a 28 Thunder with a 496HO. I have a Merc. labbed 24 Bravo and like it. My cruise speed went up a couple but no improvement on top end. I would liked to have seen a little on top but the cruise is more important anyway, isn't it?

rv

Macklin,
Check your PM or email.

Rick

Macklin

Rick, I sent you a reply. Did you not get it?

It would really be nice if Dustin (or somebody else who has dyno'd the 496) would post some numbers here.

rv

Macklin,
No I did not receive it. I checked both my email and the private messages on OSO. I just sent you another private message on OSO with a different email address. Maybe if you just replied to the private message that would work.

Rick

fountain27ho496

Sunsation is a faster hull I believe, maybe lighter as well.
Fountain says this boat runs 71 (on website) never happen.
My performance report says 68, I'm only hitting that on speedo
gps is 66. I'm already at max rpms, can't belive you guys are
running 24's? I'm running 26 bravo 4 blade, 1.5xz. The 24 should put me at 5500 prms? Don't want to spend a fortune testing
props, did that last time. Also have the larger shaft to deal with.Still have a 23 mirage that worked on my last boat but can;t try it. If any one has something that actually works better on this combo let me know. thanks John

CigDaze

I'm not sure if the Merc 496 is much different from the marinized GM 8100, but here's the basic motor curves from GM. Can anyone confirm that this would apply to the merc offering....I don't know if they change anything after receiving the engines.

http://www.gm.com/automotive/gmpower...1mhp_tc_lg.jpg

http://www.gm.com/automotive/gmpower.../81hpspecs.htm

Indy

Fountain27ho496...Your right about the 71 Fountain said you'd get...never happen. Same with my manufacturer, although they gave me a range and I got within 1mph of the low end. Your real world numbers are right in there where they should be without spending a fortune on props and testing, even then, you'd not be looking at huge gains. I almost bought your model boat and the dealer (one of the realistic/honest ones) said 68 tops. I've come to the realization that the only thing that will make me go consistently faster is more hp. Don't forget that you'll get more hp based on the conditions, more so than any prop work. Somebody in my exact model boat got 70 out of it, but it was a 50 degree day!! My speeds will vary 3-4 mph based on temp, humidity, and chop. I don't even bother with WOT runs when it's 90 degrees with 75% humidity.

Regarding your comment of not increasing speed from 5100 to 5250 is because of slip. You're probably trimmed to far out and losing grip. That rpm where start to lose or at least maintain speed is the optimal trim angle for your hull, anything more you are just gaining rpm through greater slip. Didn't you hit the rev limiter at 5150 anyway?

Macklin

Baja Daze, interesting. The chart indicates max hp at 5000 rpm then it levels off. This is consistent with my limited experience with my engines above 5000 rpm. I am thinking that I want my new props to be in the 4900 - 5000 range in normal (not blazing hot) summer temps.

Indy, I attribute the increased rpm more to temperature than trim. Those rpms were achieved last fall with the 5250 day being the coldest. It was just starting to tap on the rev limiter at that point.

CigDaze

Macklin,
Ya, there's some good info there. Looking further, a comparisson shows:

496 HO
GM: 420HP @ 5000rpm
MERC: 425 HP MAX, WOT RPM Range is 4600-5000

496 Base
GM: 375HP @ 4800rpm
MERC: 375 HP MAX, WOT RPM Range is 4400-4800

I would think that going much past 5000 isn't going to net too much of anything advantageous.

Indy

Macklin...following that line of logic, if your rpms increased from 5100 to 5250 and you saw no increase in speed, you've got a big slip problem, or trim might have come into play. I suspect a GOOD prop person would have a better idea, but even with a high slip factor, you should have seen some increase in speed, abiet a small amount. That's why I was focusing on the trim angle. I know in my boat I can hit max rpm if I just trim to the moon and throw a huge rooster, but I acutally lose speed. But hey...I'm no prop engineer