Looked today. The upper has the serial tag with 1.5r on it. Is the upper and lower going to have the same ratio?

That will be the overall ratio of the drive. Here's a link to a prop calculator you might want to play with.
http://www.marksmarineinc.com/prop_calc.aspx

5400rpm's with a 23P and 10% slip would put you right at 70mph. There's really a lot of variables that can determine the prop you would want. Most prop shops will let you test props before you settle on one. Most boats like yours with stock power will have 23's.

Here's the specs of a couple of Merc factory 454's. Notice the max RPM's.
http://www.perfprotech.com/store/art...fications.aspx

uhpguy,

If you don't mind me asking-
What broke when the first motor blew up because of oil problems? Did you use all of the same engine accessories on both motors?

The first engine blew due to oil stavation in my opinion. It had just a standard oil pan with no baffling or anything. I ran it in some rough stuff for several minutes. When I noticed the oil pressure fluctuating on each wave, it was too late. The crank broke on each side of the 5/6 throw. For the second engine, I purchased an offshore pan along with correrct pickup and a good quaility oil pump. Oil pressure would stay rock solid no matter the conditions.

I would take a very good look at your harmonic balancer to make sure it is 100% I went through a couple of engine failures, 1st time- spun main bearing, 2nd time broke crank at that same place- ended up being the bad harmonic balancer I moved from 1st engine to 2nd one..

Can't hurt to look....
Rick

Actually 2 different blanancers and flywheels since I went from gen 5 to gen IV. The first crank was forged and you could tell it was hot and dry as it toasted the bearings. The second engine was the gen IV with a cast crank. Trying to pull another boat on plane and hitting full throttle to try and get him up as it was a long tow. Got him up once and then had to slow down for a set of saltwater locks. Second time didnt go so well...Made a noise and just sounded funny, but no noise. Towed him the remaining 8 miles at about 2000 and everything seemed fine. Got to the dock and released him and actually took it out for a couple high speed passes and it didnt make any noise, but just didnt "seem" right. Brought it back to the water the next day and it made all kinds of noise but yet ran. Loaded back on the trailer and now 8 months later finally got back home to the states and tore it down. It broke right in one of the counter weights. All the bearings are good , though the rear main and the next one forward show a little wear, but nothing spun, nothing was dry.

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