Prop shaft hp losses with Bravo XR compared to TRS and SSM
01-11-2006, 07:29 AM
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Prop shaft hp losses with Bravo XR compared to TRS and SSM
Let's say you have a 454 with 500hp @ the crank.
Whats the difference in prop shaft hp output with a Bravo XR compared to a TRS with trannys, and to a SSM#3A.
Whats the difference in prop shaft hp output with a Bravo XR compared to a TRS with trannys, and to a SSM#3A.
Last edited by tomas_wallin; 01-12-2006 at 03:14 AM.
01-12-2006, 07:35 AM
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Re: Prop shaft hp losses with Bravo XR compared to TRS and SSM
I was told that the speedmaster uses about 40 more hp than a bravo. Then add about 200lbs of extra weight for the speedmaster/gimble/tranny and moving the motor forward if you have the room changes the center of gravity. ALL BAD FOR SPEED.
01-12-2006, 08:02 AM
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Re: Prop shaft hp losses with Bravo XR compared to TRS and SSM
I do not think there is a published listing for these values, it would have to be an independent shop that has both an engine dyno and a propshaft dyno.
Some time in the 90's Mercrusier switched the HP ratings from crankshaft to propshaft, but this change in rating came after the TRS was out of production.
Example, the 330 HP 454CI became the 315 HP when rated at the propshaft, using Bravo drives.
The 315/330 results in a 95% efficiency for the bravo, I would guess that the TRS would be 93%. I would also guess that the trans would be between 95 to 98% for a total efficiency of 91 % or so. Giving the TRS package a shaft power of 299 HP, if the engine truly put 330 hp out. The problem with that is, did Mercrusier rate these crankshaft powers with other losses such as the circulation pump, alternator, and power steering pump running?
Some time in the 90's Mercrusier switched the HP ratings from crankshaft to propshaft, but this change in rating came after the TRS was out of production.
Example, the 330 HP 454CI became the 315 HP when rated at the propshaft, using Bravo drives.
The 315/330 results in a 95% efficiency for the bravo, I would guess that the TRS would be 93%. I would also guess that the trans would be between 95 to 98% for a total efficiency of 91 % or so. Giving the TRS package a shaft power of 299 HP, if the engine truly put 330 hp out. The problem with that is, did Mercrusier rate these crankshaft powers with other losses such as the circulation pump, alternator, and power steering pump running?
01-12-2006, 08:12 AM
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Re: Prop shaft hp losses with Bravo XR compared to TRS and SSM
i'm no drivr expert and ive never run a drive on a dyno but i can tell you that the very best RACING gearboxes don't get near 95 % ... and in a drive you have 2 90 deg directional changes... sounds very optomistic to me...
maybe true ... but thats a great number if it is and ide like to know how they do it...having been inside my bravos they look pretty plain jane simple and 60's is sort of bearing , materials and clearences technology...
maybe true ... but thats a great number if it is and ide like to know how they do it...having been inside my bravos they look pretty plain jane simple and 60's is sort of bearing , materials and clearences technology...
01-12-2006, 12:29 PM
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Re: Prop shaft hp losses with Bravo XR compared to TRS and SSM
Years ago when my brother used to build his 19 footers with 260hp/alpha combos they radared at 59mph. We built one with the 280hp/TRS package and the best we could get out of it was 54 mph on radar.We thought there was something wrong with the 280 package because it had 20 more horse but ran 5mph slower,so we gave mercruiser a call and they offered us a day with a trailer full of props and some radar shooting.Nothing changed still 54mph out of the 280/trs boat.We learned what a power robber the tranny and trs drive really was.
01-12-2006, 01:28 PM
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Re: Prop shaft hp losses with Bravo XR compared to TRS and SSM
Originally Posted by tommymonza
Years ago when my brother used to build his 19 footers with 260hp/alpha combos they radared at 59mph. We built one with the 280hp/TRS package and the best we could get out of it was 54 mph on radar.We thought there was something wrong with the 280 package because it had 20 more horse but ran 5mph slower,so we gave mercruiser a call and they offered us a day with a trailer full of props and some radar shooting.Nothing changed still 54mph out of the 280/trs boat.We learned what a power robber the tranny and trs drive really was.
