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502 MAG MPI lacking power

Old 06-05-2013, 10:14 AM
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Originally Posted by 28Eliminator
11% is not bad, but it is not going to be as good as the Mirage+ based on your numbers. You can get better if you lab the 4 blade, but it probably still won't be better than the mirage+. Do the other boats you are comparing yours to have the same trim tabs? My guess is that the strakes are there to help handling, but with a 4 blade prop, it is just more drag in the water
I am looking at other boats and requested pics/info from others. I am pretty sure they have to mount the same due to location limitations.
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Old 02-10-2014, 02:08 PM
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Ok. Update. I went to Green Lake, WI this summer and was able to run on a nice 68ish degree morning with all gear off the boat and about 1/4 tank (15-20ish gal) and a glass lake. I ran with the phone GPS. The boat felt faster and more out of the water than ever. But phone still read 66. Speedo read higher than ever as well. Prior to that it was reading maybe 2 over. When I was running 66 before it was at 68. This time it was reading 72+. I understand you can't trust them, but I was thinking if its off, its at least consistent. But maybe I am wrong. I think RPMs were 4800 this time.

I am going to check compression sometime this winter. Also, I will be trying a 23 pitch prop to see if maybe I need to let the motor spin to hit the 70GPS mark. Mainly getting the prop for hole shot though.
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Old 04-13-2014, 08:46 AM
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If your wanting a better cruise. I suggest staying with the 25! Yes, going down in pitch with get you higher on the top end, but will bring down cruise speed!
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Old 04-13-2014, 10:02 AM
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Originally Posted by 92nsx
Simple test to see the condition of the engine is to take a compression test.
I take one every time I put in new plugs since Im already there and only adds 30 seconds to each plug.

Mercruiser manual shows 150 psi on a NEW, factory engine.

I have 130-135 psi now on freash top end. Last season I was all over the map.. 75-150 psi and the engine "felt" strong... But head gaskets were toast.

Do a compression test. They all should be pretty close to eachother. If you have the tools you can also preform a leak down test, to let you know the condition of the bottom end.
how does a leakdown test tell the condition of the bottom end?
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Old 04-13-2014, 06:48 PM
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Originally Posted by nailit
If your wanting a better cruise. I suggest staying with the 25! Yes, going down in pitch with get you higher on the top end, but will bring down cruise speed!
That is not the proper way to prop a boat.
All that does is lug the engine harder at a lower rpm.
Assume a 50 mph cruiise speed. With the 25 you turn 3400rpms. With the 23 you turn 3700rpms.
The throttle will still need to be pushed forward to the same physical position and the engine will use the same amount of fuel.
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Old 08-21-2014, 03:58 PM
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Update. I put on a 23 Rev 4 and mid range is nice. Plane time is nice too. Both seem to have improved but mainly when running heavy. The 25 rev 4 was such an improvement over the Mirage 25 but going to a 23 rev from 25 rev was not much better. I can stay on plane a little lower speed. But not much. Have not topped it out yet...I did just break my coupler which leads me to think maybe I was not aligned properly which may have created some resistance on the powertrain. So I will see how it goes after it is fixed. But it did seem to hit 60 faster with the 23 (fastest I pushed it this season...been very choppy with a lot of wind so it gets scary.
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Old 08-21-2014, 04:44 PM
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I know the Stingrays are fast hulls with low and moderate power, but I think they start evening up with other V hulls as the power grows. I remember reading of several big power stingrays in the past and while fast, they weren't that much over typical (like the are at sub 300hp levels). I don't think you are that far off personally. My personal feeling is that if the boat liked the switch from the Mirage to the Rev, it may like the Bravo 1 prop even better.
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Old 08-27-2014, 03:24 PM
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Another update. So recently something got lodged in the prop and I blew the coupler. While at the shop the tech noticed my prop shaft was bent. I do not know how I never noticed. It runs out pretty good. So I know this is a contributor to loss of top end...I just do not know how much. I will be taking it in this week or next to be fixed. It must have been there already cause the prop was unharmed from the incident. So must have been pre-existing when I bought it. But I never really noticed any excess vibration. So I am curious what it will do when fixed.
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Old 08-27-2014, 03:26 PM
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Originally Posted by liquidlounge
I know the Stingrays are fast hulls with low and moderate power, but I think they start evening up with other V hulls as the power grows. I remember reading of several big power stingrays in the past and while fast, they weren't that much over typical (like the are at sub 300hp levels). I don't think you are that far off personally. My personal feeling is that if the boat liked the switch from the Mirage to the Rev, it may like the Bravo 1 prop even better.
Manufacturers test shows 72-74 same with magazines with this engine. So 66 is way off.
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