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PaulO
08-28-2000, 11:40 AM
I was originally depressed that the performance of my new 383 looked like it would not fullfill my hopes. This is still true although my attitude has improved. The engine got several hours of hard usage this weekend and it runs good. Carb tweaking paid off yielding an additonal 400 RPMS from the stock settings. I will need a new prop and at least I have determined what I need. My hope of spinning a 25 Mirage up to 5200 will not pan out. I will be attempting to acquire a 23 Mirage or LaserII (any feedback on the LaserII would be appreciated). I went from a tired 260 to a 400HP plus 383 and really would have expected more. I will be borrowing a different handheld GPS ( mine is failing me) for the next set of runs and hope to get an accurate top speed. I think my hopes for 70MPH will be unlikely although, I don't know why. Will keep all posted.
PaulO

CDMA
08-28-2000, 11:43 AM
Paul,

You ever think of trying to get the engine on the dyno this winter for some better tuning and to see what is going on with it?

Chris

Forrest
08-28-2000, 03:01 PM
OK lets see PaulO. The tired old 260 HP Mercruiser turned what prop at what RPM and produced how much speed?

A mild 383 in just about any 18 Donzi will usually run 70 to 75 MPH, even with an old Volvo drive. Paul, you may not be getting the RPM up high enough to produce the HP to get the speed you are looking for. If you turned a 23" Mirage with the 260, at say 4400-4600 RPM, and if you can turn that same prop now at 5200-5400 RPM with the 383, you will be going a whole lot faster. If you go and put a 25" Mirage on that same setup, the HP will fall off quickly as you loose 400 to 600 RPM because of more prop pitch.

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Forrest

[This message has been edited by Forrest (edited 08-28-2000).]

PaulO
08-28-2000, 03:02 PM
Chris,
I tried to call you all weekend, I hope you were out boating!!! Much too much work and expense for a Dyno run. I am pretty sure the engine is doing what it is supposed to do. A little more tweaking may produce some more results but, I think this is just what this amount of power will do in this boat.
PaulO

GEOO
08-28-2000, 05:29 PM
My X-18 ran 66 normally and maybe 68 at it's best with a Volvo 290 with a 383ci (alum. head's, Hydr. roller cam, .520" to 536" lift etc) approx 380 to 400hp. With the speed master it would run 72 normally to 78 at best. An Alpha should run 68 to 70 mph. GEOO

CDMA
08-28-2000, 06:26 PM
Geoo,

Where did you find your Speedmaster and did it really make that much difference?

Chris


Also still all go for the 24th

BERTRAM BOY
08-28-2000, 08:09 PM
Wow....
I knew that you needed a lot more power to get a little more speed, But I didn't think it took that much more to hit 70. I'm running 60 with a 260h.p. 350, Volvo 290 drive 14x24 prop. How much more power do I need to hit 70?
BERTRAM BOY

Oh yeah, it's a '66 2+3

GEOO
08-28-2000, 08:43 PM
Chris, I bought the speedmaster (E http://www.donzi.net/ubb/biggrin.gifrive) used it for a season and sold it for what I paid. Great little drive. It's just the lower unit and it bolts right on to a 289 or 290 upper gearcase. The onlt problem is you can not buy parts. Bolt it on and it gives an instant 8 mph!!! I set my entry form in, just doing some fine tuning.
It takes 100 to 125 hp to gain 10 mph. GEOO

CDMA
08-28-2000, 09:07 PM
Where would I look to find one?

BigGrizzly
08-28-2000, 09:50 PM
PaulO don't give up. I ran into the same type of problems with my boat. it ended up being several things. biggest thing the exhaust manifolds. The shorter the primary hedder length the more problems. Ignition timing may have to be retarded. Yes retarded! For more higher rpm power cam may be retarded. Your new engine probably has higer compression then your old one, so old timming needs to be retarded. Note high compression units need to be retarded. my cleveland has 10.5 (measured) compression, and my total advance is 27 deg. at 32deg. i loose 18 hp on the dyno. you said in a earlier post that you could not afford hedders. my CMI exhaust were worth 700 rpms over the center rise type ones. By the way Forrest they did not cost $10,000. After your post my wife made me pull receipt, then take her to dinner. At $4800 it was 700 rpms i got. i hope you have a holley with front and rear jets. if you have a metering plate get rid of it--too slow. almost forgot, make sure it isn't running too cold. i closed colling systened my to run at 190deg.
Randy

PaulO
08-29-2000, 08:12 AM
Thanks Randy,
After yours and another recent post, I will try to retard the timing and see what happens. Also, I am concerned with the carb. It is a stock Q-jet that I have been tweaking. Maybe it is time to ante up for another carb.
PaulO

Forrest
08-29-2000, 10:42 AM
I never said that they cost $10G. You must be thinking of someone else.

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Forrest