Outdrive replacement requires new prop-gctid399274

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I upgraded my Cobra outdrive to a Cobra SX. The original ratio was 1.41:1 and the replacement is 1.51:1. The engine is a 235 hp 5.8 liter OMC. The boat is a 2155 Ciera Sunbridge. I knew the ratio change would require a new prop pitch and first sea trial yesterday confirmed it. Advertised WOT 4000-4400 RPM. I was able to get 5100 RPM so I am 700-1100 over spec with my 14.25X17 SST Rapture prop. I have found online that 1" of pitch change equals about 225 RPM. I am thinking I need to go to a 14-15 X 20-21 to. I'm not sure how much diameter affects RPM. Any Ideas?

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The best I could find in all of Phoenix was Michigan Wheel 14.25X21. I'll try it out this weekend. I now have a nice used SST Michigan Rapture 14.25X17 if anyone is interested.

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Talon wrote:
I upgraded my Cobra outdrive to a Cobra SX. The original ratio was 1.41:1 and the replacement is 1.51:1. The engine is a 235 hp 5.8 liter OMC. The boat is a 2155 Ciera Sunbridge. I knew the ratio change would require a new prop pitch and first sea trial yesterday confirmed it. Advertised WOT 4000-4400 RPM. I was able to get 5100 RPM so I am 700-1100 over spec with my 14.25X17 SST Rapture prop. I have found online that 1" of pitch change equals about 225 RPM. I am thinking I need to go to a 14-15 X 20-21 to. I'm not sure how much diameter affects RPM. Any Ideas?

Just a point of info here, the 5.7L and 5.8L OMCs were a 260 hp engine. The 5.0 OMCs were 230.

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@ Download Complete

Download_Complete wrote:
Just a point of info here, the 5.7L and 5.8L OMCs were a 260 hp engine. The 5.0 OMCs were 230.

That may be true in some cases but my 1989 5.8L is rated at 235 HP according to all my manuals including my factory OMC manual.

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Download_Complete wrote:
Just a point of info here, the 5.7L and 5.8L OMCs were a 260 hp engine. The 5.0 OMCs were 230.

Wasn't the 5.8 rated for 240hp at a lower RPM? I don't believe the Bayliner's ever had a 260hp 5.8.

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ishiboo wrote:
Wasn't the 5.8 rated for 240hp at a lower RPM? I don't believe the Bayliner's ever had a 260hp 5.8.

Nope. Every Bayliner handbook and manual I ever had was showing the 5.8 with 260 hp, including your 2955. Thats why the 1989 Avantis are desirable...it was really the only way to get twin 260s in that boat.

The 5.8 was optional in the 1989 2450s over the 5.0, and it was known as the big engine with 260. Why would they have a 5.0 with 230 hp, and a 5.8 with 235 hp? Makes no sense whatsoever.