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Lost 8 MPH??

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Old 08-08-2010, 02:59 PM
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Lost 8 MPH??

I've posted before about getting a new engine and some engine problems I've had with my old engine, and now I'm stumped

We used to have a 454 Mercruiser, had some major issues with that and got the 383 MAG Stroker. The first day we went out on it, got it up to 70+ MPH. Which was awesome because we could only get it up to around 62-64 with the 454. Haven't gone WOT in a few weeks (about 15 hours) and now i go WOT and it only gets up to 62...how did i lose 8+ MPH? I changed the oil after 10 hours of running the engine. Keep in mind that this is all during the break in process of the engine. (20 hours will be broken in)

Last edited by bimini run; 08-08-2010 at 03:12 PM.
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Old 08-08-2010, 05:44 PM
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RPM the same?
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Old 08-08-2010, 05:55 PM
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If you slip your boat, I'd look underneath it for the growth. You can lose 8 mph easy with that crap.

New install, could be bad plug wires, bad plug, loose wire.
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Old 08-08-2010, 06:49 PM
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Was the 70mph with gps
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Old 08-08-2010, 07:47 PM
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@Expensive Date - No the RPM was lower, but when i did get it up to 70 the RPM was pegged all the way up and the tach wasn't working right.

@VtSteve - We don't keep it in a boat slip, we trailer it and run it with fresh water every time we go out. And the wires installed with the new engine look brand new.

@firehawkcat - No the 70MPH was recorded with the speedometer. However it's obvious the boat's going slower because when we had it at 70, the boat was chinewalking a bit
and this time it wasn't. Not only that but we went WOT 3 runs. Went down the river, turned around tried again, and then turned around and tried again. Each time 60, 60, 62.


Could it be possible that 1 extra person, some chairs and a little more gas could be doing it? I know when we had the engine first put in, it was water tested (without the engine cover on) and he said he got it up to 74+. Now i know we lost that 5 or so MPH from the engine cover, but that thing weighs a ton.

Last edited by bimini run; 08-08-2010 at 07:56 PM.
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Old 08-08-2010, 07:56 PM
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Gas about 8lbs per gallon person and gear, guess it could be a mph loss how much torque in that 383 vs. big block
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Old 08-08-2010, 07:56 PM
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Quote:
Originally Posted by bimini run View Post


Could it be possible that 1 extra person, some chairs and a little more gas could be doing it?
How about the river current? Is it variable at all? You mentioned going up and down river, that would be a good reference. Are you maxing out the trim?

I doubt that 1 extra person and gas would impact it that much in that boat. Mine is lower by 2-3 max with 1 extra person and a full tank and that's with a small block.

Is that a new engine? Check the cap and rotor for any corrosion.

Also, check out the tach, and get a GPS just to be sure.
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Old 08-08-2010, 08:02 PM
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@firehawkcat - I lost 8 MPH in the small block. With the 383, i got it up to 70, but now I'm only getting it up to 62. Not comparing the 454 to the 383 at all, just giving some background. At 70, had maybe a little less than half. At 62, between 1/2 and 3/4s.

@VtSteve. Well, going from the bay to the dock, going down the river, got it up to 60. Then we turned around, heading towards the bay again, 60. Turned around once more, 62. And yes this is all with the 383. And I'm not sure what's up with the tach. I know it wasn't working like the first 3 or 4 times we went out, then i played around with some wires (after we got it up to 70) and it was working fine.

Last edited by bimini run; 08-08-2010 at 08:05 PM.
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Old 08-08-2010, 08:04 PM
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torque vs HP = a bigger loss in speed when you add the extra weight.

Maybe?
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Old 08-08-2010, 08:06 PM
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@jeff1000man - Nope...it's the same engine. I'm losing 8 MPH on the 383.
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