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mirage diffuser / rev 4 diffuser

Old 11-21-2019, 11:06 PM
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Default mirage diffuser / rev 4 diffuser

496 mag ho turns a 25 mirage plus into the soft limiter every time, no problem and on plane acceleration is kind of a wow factor, for me anyway.
there was a fair amount of chine walk at wot actually it would start in the high 60's all thru best ever speed of 73. ?
"theres a pic of that best ever somewhere"

friend says start cutting the diffuser ring off an 1/8 inch at a time and see if it helps, it did and i have it cut down prob 3/8 of an inch now.
chinewalk is all but gone so i can let it run wot but theres that limiter issue again,
plus it can now in some instances blow out at lower speeds

bought a new 25 rev 4, it cured the chinewalk, on plane acceleration isnt quite as hard and so far best speed is 72.6 @ 5130 rpm.
So its right there at the limiter again.

the rev 4 is box stock and the mirage plus was never repaired but sent to fj to verify and it is also stock.

i know less than nothing but it was reasoned to me :
that the flaired diffuser on the mirage lifts the stern and contributes to the chinewalk &
that the long straight diffuser on the rev 4 aids in stability ( maybe something about the aspect ratio)

i have no idea if stability was gained going from 3 to 4 blades or if was the long straight diffuser on the rev 4 ??

before i send the pcm off to get the limiter bumped up a hundred why dont i get a 27 mirage and try it ,,
most likely the ring will need cut down to stop the chinewalk,
so the question is how about having a longer straight diffuser ring added to the mirage ?
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Old 11-21-2019, 11:38 PM
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Originally Posted by outonsafari View Post
496 mag ho turns a 25 mirage plus into the soft limiter every time, no problem and on plane acceleration is kind of a wow factor, for me anyway.
there was a fair amount of chine walk at wot actually it would start in the high 60's all thru best ever speed of 73. ?
"theres a pic of that best ever somewhere"

friend says start cutting the diffuser ring off an 1/8 inch at a time and see if it helps, it did and i have it cut down prob 3/8 of an inch now.
chinewalk is all but gone so i can let it run wot but theres that limiter issue again,
plus it can now in some instances blow out at lower speeds

bought a new 25 rev 4, it cured the chinewalk, on plane acceleration isnt quite as hard and so far best speed is 72.6 @ 5130 rpm.
So its right there at the limiter again.

the rev 4 is box stock and the mirage plus was never repaired but sent to fj to verify and it is also stock.

i know less than nothing but it was reasoned to me :
that the flaired diffuser on the mirage lifts the stern and contributes to the chinewalk &
that the long straight diffuser on the rev 4 aids in stability ( maybe something about the aspect ratio)

i have no idea if stability was gained going from 3 to 4 blades or if was the long straight diffuser on the rev 4 ??

before i send the pcm off to get the limiter bumped up a hundred why dont i get a 27 mirage and try it ,,
most likely the ring will need cut down to stop the chinewalk,
so the question is how about having a longer straight diffuser ring added to the mirage ?
Before you start changing rev limiters and cutting more diffuser rings I'd do more testing.

I'm surprised you pulled to the rev limiter with the Rev 4 25" prop. It has significantly more blade area than the Mirage. Typically with equal pitch Mirage and Rev 4 props the Rev 4 would load the motor more. It has been my experience to drop 2" of pitch when going from a Mirage to a Rev 4. For example Mirage 25 I'd goto a Rev 4 in 23" as a starter.

The longer barrel on the Rev 4 will typically make the drive more responsive to trim and steering, that coupled with the massive blades will hold the nose with less trim. And yes the lack of diffuser will have less stern lift.

IF it were me, I'd try a Mirage+ 27 without mods first. Then perhaps tweak the Rev 4 just a tad to bring the rpms down a tad.
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Old 11-22-2019, 01:03 AM
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What kind of boat?? I would try a stock 27 Mirage + and the have it labbed and diffuser cut down. BBlades has a winter special on labbing
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Old 11-22-2019, 07:47 AM
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Just my opinion but I think it will be a dog with a 27.

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Old 11-22-2019, 09:25 AM
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boat is a 26 x 8 no cabin retired raceboat that i did a rebuild on.

all tests have been calm water, similar to conditions in pic.

the 25 mirage plus would spin into the soft limiter,
chinewalk after the ring was cut down wasnt bad but the combination had me pulling back to keep
it @ 5140-5150 rpm,
not knowing what the mirage could spin up to is why i went 25 rev 4 and it ended up right at the limiter.
tach is a mercmonitor, speed is gps, fuel is 65 gallons so add that to the weight slip.

i will test a 27 before cutting the ring, and i wouldnt be surprised by any results a 27 netted me.

just dont know where the stability came from, the 4th blade or the long straight diffuser


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Old 11-22-2019, 10:53 AM
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Originally Posted by outonsafari View Post
friend says start cutting the diffuser ring off an 1/8 inch at a time and see if it helps, it did and i have it cut down prob 3/8 of an inch now.
If enough of the diffuser is cut off, you may not be able to plane the boat, it will slip ao bad. . Happens with some.

Happened to me with a few of the props I have ended up with.

Ended up blocking the exhaust passage in the outdrive and wallah, perfect grip ! Never would have imagined. I used a rubber PVC cap, but that was not a Bravo.

Tinkerer fixed his diffuser cut off slip induced issues also by blocking his on his Bravos. Forget exactly how he did it. Do a search with his name.
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Old 12-05-2019, 02:24 PM
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Got a 27 mirage plus to try, it appears box stock in really good shape.
i cleaned it up with a purple scotchbrite and see no tool marks or stampings.
its tracking almost perfectly and balanced on the money so it can go right on and get tested.

heres the thing, should i try it as is or should i straighten the diffuser ring first ?
testing on saturday so i got all day tomorrow


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Old 12-05-2019, 10:53 PM
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Originally Posted by outonsafari View Post
Got a 27 mirage plus to try, it appears box stock in really good shape.
i cleaned it up with a purple scotchbrite and see no tool marks or stampings.
its tracking almost perfectly and balanced on the money so it can go right on and get tested.

heres the thing, should i try it as is or should i straighten the diffuser ring first ?
testing on saturday so i got all day tomorrow

Do not cut the ring. Try the prop as is. Then make adjustments, if needed.
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Old 12-07-2019, 04:17 PM
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Tested the 27 mirage plus today best ever speed 74.3 best rpm was 4900 (i will have to check that)
it jumps right up on plane and liked alot of trim.

have to watch video seems like it might have pulled 4900ish becacause i remember thinking it needs another 150 rpm.

chinewalk was almost nil, seemed like tab would induce it and more trim it would stop.

i have alot of video to go thru and digest.

the river was calm, glass on last run, ocean was 1& 1/2 feet @ 9 seconds winds under 12 mph

it ran completely different from the 25 mirage plus.

let me get it sorted out but im looking for direction/suggestions

thanks






Last edited by outonsafari; 12-07-2019 at 04:20 PM.
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Old 12-07-2019, 09:31 PM
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