Reply With QuoteIt could be that pushing extra weight and the boat sitting deeper in the water = more friction to overcome.
I swapped my 350 for a 454.
drive is an Alpha 1.5 ratio,,prop is an Alpha 4 blade aluminum 14x20p
the 350 ran 52-53 at 4100-4200rpm
the 454 is running 47-48 at 4500 rpm
same prop,same drive,same everything..
I cant figure how I can be spinning higher rpm and going slower..
what the hell???
It could be that pushing extra weight and the boat sitting deeper in the water = more friction to overcome.
Still doesnt equate..
rpm x pitch-slip = speed
If the engine is spinning higher it should go faster unless the prop is
slipping/cavitating.. It doesnt make any sense.arghhh.
Its 200 extra pounds since I didnt go with the heavier Bravo
and I went from steel manifolds to aluminum shaving about half of that..
Yes, the extra weight is not helping, but the extra torque is "flexing" the prop. Your boat must have a problem with it's riding surface. You have enough torque and drag to "bend" your prop to a lower pitch. Your 20" pitch is (under extreem load) acting like a 16" pitch. Try purchasing a nice stainless prop. PS, the old "Magnum 454's with Alpha" had 1.32 gears. This put MUCH LESS STRESS on the drive. YOU WILL BLOW UP YOUR DRIVE with 1.50/1.47 gears!!!
bottom has not been checked but it hasnt changed either so I imagine if it had
a problem it would at least be a constant.
I thought about the prop flexing, that and a wierd igintion box problem (sending a weird/incorrect tach signal) were the only two things I could think of.
I have a Mirage 21P I can try.Ill do that this week.
As for the Alpha,,yes I know,,everyone has been telling me Im
gonna frag the drive.. :skull:
Could be that most BB's run a larger diameter prop for same pitch. The bigger motors have more torque and can thus run a larger diameter.
I have to assume your BBC is healthy and at a minumum the 330hp Merc 7.4L.
Talk to prop shops like Throttle Up Propeller's.
Edit: Just noticed you are using an aluminum prop. Good time to change to modern SS. A good Prop shop will help you out what do get.
Engine is healthy,great compression in all cylinders,no blowby,runs cool,etc..
http://www.hotrodlincoln.org/bravo454.jpg
just an observation... but that is alot of engine with very little air in there. you may be robbing it when it's warmed up.
So far I have been running it with the little door over engine raised up about an inch..
It would be constant, if it were constant! You have added (2 big people) a lot of weight at the transom, and at the lower RPMs the 454 has much more torque. I hope the bottom hasn't changed, if it has you have major problems. Normally when the bottom "bulges" with excess engine weight, the rocker it creates, causes the boat to Porpoise at all speeds. Also look at your crankshaft and prop hubs. I hope you only have prop slip! Good luck.
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