pitts1313

My slip numbers are really low, about 3% with some labbed mirage 23 (non plus) at 5450 rpm @ solid 76 mph (sometimes 77.5) gps. Prop vendors say no faster because my slip is so low. I am questioning my slip being so low. This led to running the numbers on my Fountain Factory Test sheet. The WOT speed doesn't match up with the gear ratio. I came up with no slip! Drive serial numbers are 0k093696 and 0k093724. Do I have a lower gear ratio number than the 1.5 stated on the test sheet? can you direct me to where I can find gear ratio from serial number? What are your thoughts?

pitts1313

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mcollinstn

You've got 1.36:1 drives. Period. The engine RPM and prop pitch and hull speed all point straight to 1.36:1.

The factory sticker has the ratio printed on it, but yours certainly could have been changed at some point in the past.

Nothing wrong with 1.36 ratio, though, so don't sweat it. There are people who swear that their boat picked up speed with 1.5 drives and a certain pitch, but I've also seen boats that ran faster with 1.36 and a certain pitch.


MC

pitts1313

MC thanks for input on this. So I believe fountain flubbed by putting the incorrect 1.5 on the test sheet instead of the 1.36 that would match up with the 22 pitch props the boat tested and came with. All the formulas work out with average slips with the 1.36 ratio used.

I am testing 23 pitch hydro quadIV in the morning. I have spent so much money on engines with a goal of a solid 80mph. I hate to miss out on any mph gain. I am having to buy new props due to the increase in power. Thinking since I have come (spent) this far, why not get the 1.5 gears.

I read several threads on 1.36 vs 1.5. Seems like consensus was below 90mph 1.5 was the choice. Anyone else want to chime in?

lakematdude

Both you're serial numbers we're definitely MERCRUISER BI-ROTATION BRAVO 1 1.50:1 when new, But like said they could have been changed at any time.

Another way to know is to count gear teeth, Starting on page 3A-9 Drive Unit Gear Ratio Identification.

http://www.boatfix.com/merc/Servmanl/11/11a3r5x.pdf

Take the cover off the top of the driveshaft housing and count the teeth on the top gear. If the gear has 30 teeth, it's a 1:65, if it has 32 teeth it's a 1.5 and if it has 29 teeth, it's a 1.36. You only need a 3/8" twelve point socket to remove the 4 bolts. Some oil may leak out though.

pitts1313

Lakmatdude,Thanks for the drive serial info and how to count the teeth. Will post up the teeth count tomorrow.

With 12% slip and using the 22 pitch props @ 4900 rpm @ 72 mph, calculates out to a 1.26 gears!Previous owner order boat with carb engines with intent of supercharging. Think fountain failed to write in the correct gear after they changed it?

mcollinstn

I've seen people swear 1.5's are the universal first choice.
I don't buy it.

Your best bet is to finish tuning your motors, find a buddy wth 1.5 Bravos, take a truck full of props and have a swap festival. Find what works best on the 1.36 then swap drives and see what works best on the 1.5's. Compare your results and decide from there.

My guess is that you'll get more bang for the buck by bore-notching your 454's than you'll get swapping ratios..

MC

Griff

I would also bet that you have 1.36 drives. I have 1.36's on my 32 AT(502mpi's) and spin labbed 24 Bravo's to 5100rpms for about 73mph.

pitts1313

I read 1.36 vs 1.5 till fell asleep last night. Was on the lake this morning at sun up to test props. One engine would just starve for fuel and die at 4800 rpm and up. Fuel flow senders showed have flow on this engine dying. Fuel pressure was good while idling. Checking anti siphon valves in the morning, so little too must to count teeth. But will post up my finding.