rpm vs speed vs altitude question.
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rpm vs speed vs altitude question.

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    Default rpm vs speed vs altitude question.

    I gpsd my daytona at 4900 ft at pueblo res here in colorado. 74 mph at 4650 rpm spinning a Bronz b agressor. Down in Missouri I can get 5200 or so rpm out of the same combo. I was just courious about how fast it would be going. I am guessing 82. I was also thinking of getting the b cut to get more rpm. The motor makes 600 at 6000 but I can't get it there with the b agressor. How much would I have to have cut to gain 500 rpm and how bad would it effect cruising speed at lower rpms. Thanks.

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    Seriously off center slowride's Avatar
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    You said the same combo, so I assume you're not jetting it down at higher altitudes. You're leaving some rpm on the table, but look at the bright side..... the air is thinner, so less resistance!

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    Senior Member bp298's Avatar
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    Quote Originally Posted by jungledave View Post
    I gpsd my daytona at 4900 ft at pueblo res here in colorado. 74 mph at 4650 rpm spinning a Bronz b agressor. Down in Missouri I can get 5200 or so rpm out of the same combo. I was just courious about how fast it would be going. I am guessing 82. I was also thinking of getting the b cut to get more rpm. The motor makes 600 at 6000 but I can't get it there with the b agressor. How much would I have to have cut to gain 500 rpm and how bad would it effect cruising speed at lower rpms. Thanks.
    4900' altitude is one attribute, but what your engine cares about is d/a (density altitude). many decades ago, i lived in csprings for a year - weather runs the whole gamut there. if you know the date and time your engine ran that rpm, it's not too difficult to figure out what d/a was, at least in the ballpark.
    same goes for missou. if you know the date/time, and the nearest town where you got the 5200, a ballpark d/a can be calc'd. that will tell you more than just the altitude diff.
    what size (ci) is the engine?

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    Seriously off center slowride's Avatar
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    Quote Originally Posted by bp298 View Post
    4900' altitude is one attribute, but what your engine cares about is d/a (density altitude). many decades ago, i lived in csprings for a year - weather runs the whole gamut there. if you know the date and time your engine ran that rpm, it's not too difficult to figure out what d/a was, at least in the ballpark.
    same goes for missou. if you know the date/time, and the nearest town where you got the 5200, a ballpark d/a can be calc'd. that will tell you more than just the altitude diff.
    what size (ci) is the engine?
    Point being, until the jetting is corrected to compensate it's premature choosing a cut without knowing what it CAN turn.

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    Quote Originally Posted by slowride View Post
    Point being, until the jetting is corrected to compensate it's premature choosing a cut without knowing what it CAN turn.
    Ya think if I picked up 600 rpm or so I might know how to tune? I go from to 900 to 4900 to 3700 several times a summer.

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    Quote Originally Posted by bp298 View Post
    4900' altitude is one attribute, but what your engine cares about is d/a (density altitude). many decades ago, i lived in csprings for a year - weather runs the whole gamut there. if you know the date and time your engine ran that rpm, it's not too difficult to figure out what d/a was, at least in the ballpark.
    same goes for missou. if you know the date/time, and the nearest town where you got the 5200, a ballpark d/a can be calc'd. that will tell you more than just the altitude diff.
    what size (ci) is the engine?
    Brodix headed 11:1 460 bbc. Ya I know all about the weather on an hourly basis, Colorado Springs and the local lakes suck but I am stuck here for 4 more years . I don't really have problems figuring my tune just was wondering what would be a good cut for a few more hundred rpm and how it would effect cruising. I have put over a hundred miles on the boat in one day on Tablerock so I like to cruise.

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    Dave I found a couple articles that might help you Tom
    (ps I still want the Mopar mts )

    "Holley carburetors come from the factory calibrated for operation at sea level and an inlet air temperature of 70 degrees fahrenheit. Use Google to determine the altitude of the location where you live or race. For every 2000 feet of increased altitude, you can reduce your jets by one size. Racers can take this even farther by using temperature, humidity, dewpoint, and barometric pressure to determine the corrected altitude above sea level."
    and This :
    http://www.hotrod.com/techarticles/e...et_change.html

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    Once again, I am not having tuning problems. Just curious about having an impellar cut and what I would lose for cruising. Tom , I will send you the BBD stuff this friday, I have been putting up power lines from our last blizzard for two weeks, real fun stuff if you don't care for sleep.

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    you should be okay to cut your aggressor to a B/C cut , and not lose much if any of your cruise effiencey or hole shot if you aren't hauling a lot of extra weight in the boat . 6 5/8" small dia. on the cone with 32.5* angle and leave the shroud on ( large dia.) and cut the vane away from under it . if you have a machinist to work with , you might want to just go to 6.7" first and see how that works . once you get closer to where the engine is getting ahead of your impeller load curve the rpm will increase more each time you cut it down . hope that helps some , I could e you a picture of mine if like .
    Quote Originally Posted by gn7 If your ran a cammer your odds of finishing WERE ZERO.

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    Senior Member jetboatperformance's Avatar
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    Your Aggressor (if original cut) is closer in characteristic to a Berk/Dom "A" Still have a hunch your loss is directly related to Altitude but a quick expirement would be to swap out the current for a true B Tom

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    Quote Originally Posted by Canuc View Post
    you should be okay to cut your aggressor to a B/C cut , and not lose much if any of your cruise effiencey or hole shot if you aren't hauling a lot of extra weight in the boat . 6 5/8" small dia. on the cone with 32.5* angle and leave the shroud on ( large dia.) and cut the vane away from under it . if you have a machinist to work with , you might want to just go to 6.7" first and see how that works . once you get closer to where the engine is getting ahead of your impeller load curve the rpm will increase more each time you cut it down . hope that helps some , I could e you a picture of mine if like .
    Quote Originally Posted by jetboatperformance View Post
    Your Aggressor (if original cut) is closer in characteristic to a Berk/Dom "A" Still have a hunch your loss is directly related to Altitude but a quick expirement would be to swap out the current for a true B Tom
    Tom, I know I lose at altitude, about 24 or so %. I would like to pic up 500 rpm here and down in Missouri. thanks

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    Quote Originally Posted by Canuc View Post
    you should be okay to cut your aggressor to a B/C cut , and not lose much if any of your cruise effiencey or hole shot if you aren't hauling a lot of extra weight in the boat . 6 5/8" small dia. on the cone with 32.5* angle and leave the shroud on ( large dia.) and cut the vane away from under it . if you have a machinist to work with , you might want to just go to 6.7" first and see how that works . once you get closer to where the engine is getting ahead of your impeller load curve the rpm will increase more each time you cut it down . hope that helps some , I could e you a picture of mine if like .
    Ah ha that is what I was wanting to hear. I was hoping I wouldn't lose much as far as cruising goes. I know it hard to have it both ways. Thanks.

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