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question about speed full tank vs 1/4 gas?

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Old 03-21-2005, 11:57 PM
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Default question about speed full tank vs 1/4 gas?

I have a powerquest enticer 290 with twin 454 mpi motors I am running around 4400 rpms with 27.5 pitch 4 blade props. I am running around 75 mph with a full tank of fuel. What should I gain in speed once I get down to say 1/4 tank of gas? I am running pro chargers with intercoolers and seeing around 3psi of boost. I was hopping for 4600 rpm with 5psi but I am not there yet I just switched from 30p 4 blade props so I got quite an improvement from 3900 rpms and no boost.Also anyone know how much boost is safe at 4400 rpms I was considering getting smaller pulleys for the pro chargers but dont know if it is safe to run.
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Old 03-22-2005, 12:10 AM
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Default Re: question about speed full tank vs 1/4 gas?

I would try to prop where you can get closer to 5000 rpm or above. You should see a considerable increase in speed. I wouldn't increase your boost on stock motors either. Try playing with props, you aren't even close to your power band yet. Did you try three blade props? If you can get on plane with mirages you will definantly be faster. My 30' Kachina with a single 502 mag mpi, headers and Vortech charger (with the fuel kit and Ecm reprogrammed by AZSM) does 75 with a 28 pitch bravo @ 5100 rpm or about the same speed with a 26 pitch at about 5400 rpm with a better hole shot. I usually run the 28 so I dont rev so high but I like the feel of the 26.
Seems to me you would run a whole lot faster if you could get into the right rpm range with a better prop choice. Granted 26's may seem like you wont run as fast as I would but you should have alot less slippage with twin motors, that should increase your speed.
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Old 03-22-2005, 12:24 AM
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Default Re: question about speed full tank vs 1/4 gas?

on top of the 454 mpi it says wot 4600 rpm I thought that meant wide open throttle not to exceed 4600?
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Old 03-22-2005, 12:33 AM
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Default Re: question about speed full tank vs 1/4 gas?

Seems to me your stock rev limiter should be set around 5100 or 5150. I'm not sure what the sticker on the valve cover says. If you have the stock program in your ecu with the rev limiter set at stock as well you wont have worry about it. It'll shut down when it gets too much rpm automatically. That motor (I'm not an expert though) should be capable of 5500 rpm easily. I had my rev limiter moved up long before I put a blower on. You dont however want to "bounce" off the rev limiter. Its not good for your motor. Play with props a little and then send your ecu to AZ Speed And Marine and let them reprogram it for the blower and give you a little more rpm so you dont "bounce" off the limiter.
I would say that there is no way you are getting close to your max horsepower @ 4400 rpm. Hopefully some one else will chime in and shed some more light on it.............
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Old 03-22-2005, 12:34 AM
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Default Re: question about speed full tank vs 1/4 gas?

By the way, my speed doesn't change much with 100 gallons of fuel or a light load, at least not anything worth getting excited about. I run full all the time.
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Old 03-22-2005, 12:55 AM
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Default Re: question about speed full tank vs 1/4 gas?

Quote:
Originally Posted by enticer290
on top of the 454 mpi it says wot 4600 rpm I thought that meant wide open throttle not to exceed 4600?
Exactly what engines did you start out with????
454/385hp??
454/330hp??
454/310hp??

did you put the prochargers on????

You said you had no boost at 3900 at wot--You should have had some!!!!!!

what were speeds before at what rpm???
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Old 03-22-2005, 06:39 AM
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Default Re: question about speed full tank vs 1/4 gas?

A 454 MPI is either a 310hp or 330hp depending on the year (I believe an MPI is 310hp). If that's the case, then 4,600 RPM's would be WOT and a procharger will kill that motor in short order.
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Old 03-22-2005, 09:36 AM
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Default Re: question about speed full tank vs 1/4 gas?

In my experience the rev limiter actually kicked in about 50 rpm above the ratings above....
7.4 mpi is 310hp at the prop and 4600rpm wot, mag mpi's are 385hp 5000rpm at wot

Here is a link to engine specs for reference....
http://www.outdrives.com/merc/sternspecs.html
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Old 03-22-2005, 10:29 AM
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Default Re: question about speed full tank vs 1/4 gas?

I owned a 2000 29 PQ with 454LX 310 HP I ran 68.6 GPS with a full or quarter tank it did not seem to matter. The boat did like to run in choppy water, the smoother the water the slower I went. If memory holds true my rev limiter was set around 4900. Hopefully that helps.
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Old 03-22-2005, 10:45 AM
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Default Re: question about speed full tank vs 1/4 gas?

Quote:
Originally Posted by enticer290
I am running around 75 mph with a full tank of fuel. What should I gain in speed once I get down to say 1/4 tank of gas?
Here's what you do. Divide the number of gallons in the tank by 6. (6 pounds is the approximate weight of a gallon of gas.)
Multiply that number by .75 (the difference between full and a quarter tank). Now divide that number by 180 (average weight of your friends).
Then simply invite the resulting number of 180 pound friends on a boat ride. Start out with just over a quarter tank of gas and the proper number of ballast buddies.

Once you are up on plane, simply toss your friends off of the boat one at a time and record your GPS speed after each friend hits the water.
Then you will know what your speed difference is at different weights, all the way down to 1/4 tank.

It works great. For me the difference is about 2 mph.
Which is lucky for my friends, or I wouldn't have gone back for them.

Good luck.
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