gears

1. ## gears

if i have a engine reving at 7,800 to 7,900 rpm with 60 gears and a 11/15 prop, and i go to 66 gears, its a cassale with nitro gears, how many rpm should that come down. dean,

2.

3. Originally Posted by culprit294
if i have a engine reving at 7,800 to 7,900 rpm with 60 gears and a 11/15 prop, and i go to 66 gears, its a cassale with nitro gears, how many rpm should that come down. dean,
you can goggle merc prop slip calculator and run you info and changes, you will have to take the inverse of the gear ratio since its a overdrive

4. Originally Posted by culprit294
if i have a engine reving at 7,800 to 7,900 rpm with 60 gears and a 11/15 prop, and i go to 66 gears, its a cassale with nitro gears, how many rpm should that come down. dean,
7800*1.60/1.66=7518
7900*1.60/1.66=7614 This is assuming that your propshaft rpm stays the same.

5.

6. ok thanks, but the guy i got the gears off, is saying 600 rpm, i am confused, he is a marine shop in arizona. dean

7. Originally Posted by Carnivalride
7800*1.60/1.66=7518
7900*1.60/1.66=7614 This is assuming that your propshaft rpm stays the same.
X2

Originally Posted by culprit294
ok thanks, but the guy i got the gears off, is saying 600 rpm, i am confused, he is a marine shop in arizona. dean
Some people have a hard time with math .

8. Originally Posted by culprit294
if i have a engine reving at 7,800 to 7,900 rpm with 60 gears and a 11/15 prop, and i go to 66 gears, its a cassale with nitro gears, how many rpm should that come down. dean,
Heres my best math Dean. 7900X1.66 W/11X15 prop= Freek'n fast! Its simply fast for a drag boat. Its crazy freekin fast for something that has to turn! And do it over and over! You guys are a show! Be safe and good luck with the new boat!

9. Originally Posted by culprit294
ok thanks, but the guy i got the gears off, is saying 600 rpm, i am confused, he is a marine shop in arizona. dean
I'm not sure what to tell you here 1.60 vs 1.66 is 6% so the faster you spin the bigger the difference.

6% at 2000rpm is 120
6% at 4000rpm is 240
6% at 6000rpm is 360
6% at 8000rpm is 480
6% at 10,000rpm is 600

The only time I've seen a big drop in rpm was over gearing a NA combination and pulling it off the powerband at the flash. Usually my max rpm stays within a 100rpm of the math depending where it fell in my powerband.

Loren

10. I think you are talking about the drop in RPM going from 60% over to 66% if your engine will only pull
7800 flat out and you change to the 66's your RPM should drop to around 7200. I have found RPM will go up or down around 300 RPM per gear change. 18's to 22's will get you 300 up or down so your 60 to 66 is two gear changes 60, 63, to 66, so around 600 RPM. hope this helps j.j.

11. Originally Posted by beaverfab
I think you are talking about the drop in RPM going from 60% over to 66% if your engine will only pull
7800 flat
out and you change to the 66's your RPM should drop to around 7200. I have found RPM will go up or down around 300 RPM per gear change. 18's to 22's will get you 300 up or down so your 60 to 66 is two gear changes 60, 63, to 66, so around 600 RPM. hope this helps j.j.
If the engine will only pull 7800 flat out and say you changed to 55's isn't it the mph that changes?
My deal runs about 6400 with 22's and 40's.
Wouldn't it be better explained if you said there would be about 6 mph difference @ 7800?

12. Originally Posted by SoldHondaBoughtHondo
If the engine will only pull 7800 flat out and say you changed to 55's isn't it the mph that changes?
My deal runs about 6400 with 22's and 40's.
Wouldn't it be better explained if you said there would be about 6 mph difference @ 7800?
He is not asking about speed he is asking about RPM, what I think there wanting is to slow down the RPM of the engine at WOT.
SoldHondo you have a very strange boat if it will pull a set of 22's and a set of 40's to the same RPM flat out.

13. 7900 RPM w/ 66's is over 13k prop shaft rpm.... DAYAM that's fast even by drag boat standards. Don't know anything about you guys BAD boats, is there a reason you run a 15" pitch? Seems another 1" of pitch would move the gear/prop shaft RPM down and open up some different gear options....

14. Originally Posted by beaverfab
He is not asking about speed he is asking about RPM, what I think there wanting is to slow down the RPM of the engine at WOT.
SoldHondo you have a very strange boat if it will pull a set of 22's and a set of 40's to the same RPM flat out.
'Strange' ok lol.......same rpm, it just takes longer to get there with the 40's. Never dynoed the motor but the seat of my pants says the
cam is all done at 6400....(.600x250/260@.050/108)ran that rpm with a 1050, 2 660's and 3 different props. Crower recommended .680x280@.050....looking for another 4-500 rpm.
What do you think? I'm plenty new to this boating deal.No drag racing....more like early morning flat water and 10 miles in 6 min.

And daum...7900 with 60's and a 15 prop is blazzzing! Has the OP tried a 16 prop to see what it did with the rpm?

Be kinda funny if the OP picked up 7 mph on the big end and ran the same rpm.

David 519
How did you know i was going to mention prop pitch before i even posted it?

15. ok, this is the deal,we have a new boat the old boat had 62% gears and same prop it ran 7200 rpm with 22 to 24 lbs of boost, so i thought ok i will put 60 gears in the new boat and get 7300 to 7500 rpm witch i would of been happy with but no, as it turns out this boat is more efficient and is turning 7900 rpm with 38lbs of boost , but wait i had 35% over on the old boat, and this new boat i have 15% over on the blower. so here we r so thats why i am going up in gears and wanted to know how many revs it would come down because this thing is outa control and so am i ps gn7 let me tell you it is a weapon at the moment and i cant drive it. dean syd australia.

16. Originally Posted by culprit294
ok, this is the deal,we have a new boat the old boat had 62% gears and same prop it ran 7200 rpm with 22 to 24 lbs of boost, so i thought ok i will put 60 gears in the new boat and get 7300 to 7500 rpm witch i would of been happy with but no, as it turns out this boat is more efficient and is turning 7900 rpm with 38lbs of boost , but wait i had 35% over on the old boat, and this new boat i have 15% over on the blower. so here we r so thats why i am going up in gears and wanted to know how many revs it would come down because this thing is outa control and so am i ps gn7 let me tell you it is a weapon at the moment and i cant drive it. dean syd australia.

Holy Crap 38ibs of boost! I take it you guys run Alky? How long do the motors last? I`m headed to alky next season & trying to imagine how fast I could go with 38ibs of boost. Sorry for high jacking your thread. You BAD guys got it going on thats foe sure.Tom

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