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Stephane
08-02-2004, 05:51 AM
I need help ?

My OMC 350 260hp with a 15.5 x 15 prop run 4600 rpm wide open trottle and with a 15.5 x 13 prop it run at 4600 rpm also ! It seem to me a abdormal situation. I think the engin should overrev with a smaller pitch.

The cylinder compression are from 150 to 155 psi the point is new, the spark plug and the fuel filter also. The dwell is adjust at 30 deg ad the advance is at 8 deg BTDC

How can i reach more RPM at low budget ?

MOP
08-02-2004, 07:03 AM
The RPM is right at spec for your engine, you could go a bit higher but doubt if you would gain much. I don't understand it not changing with the reduced pitch, were the conditions the same, weight, temperature etc?

Rootsy
08-02-2004, 07:25 AM
if that truly is the case, all else being equal then that 15P is slipping a lot more than the 13P.. what is the condition of the prop? try a 17.. see what you get...

what gear ratio do you have in that drive?

and are you sure the tach is accurate?

David Ochs
08-02-2004, 08:07 AM
That's good advice, take a look at the tach. I know that the one in my boat it not to be trusted. I sent mine away to a guy that replaced the inards of my original "Stewart-"Warner". I know that it is consistently "off".
Since it's easy to check, it might be worthwhile.

MOP
08-02-2004, 11:12 AM
All good points above, it should be "speed" not RPM you are after. 4200 to 4600 is the Op range for that engine, prop slip or a tach that won't scale up properly may very well be the causes. You can not expect a 5.7 to give blistering performance in a 23 Ragazza, to me the 5.7 is a nice middle of the road engine giving decent economy and fair speed. I have a hunch about 50 at WOT is the best you can expect without dumping fairly big bucks. You may get better speed with a higher pitch prop by getting into a better torque range.

Stephane
08-02-2004, 12:29 PM
The test with the different prop was with the same condition.

With the 15 a took 43mph at 4600 and with the 13 a took 37 mph at 4600 but i never seen more than 4600 rpm.

I am gone a redo the test with both prop and i will test a backup RPM to be sure of the reading.

Both prop are Michigan aloy 3 blades.

I am looking for a stainless 3 blade 15 x 17 but... with the performance i have with the 15 of pitch i think it will be stupid to upgrade to a 17" of pitch.

RickR
08-02-2004, 02:19 PM
You might be hitting the Rev-Limiter.
Do not trust your tach accuracy :rolleyes:

Doug G
08-02-2004, 04:29 PM
I've got the 350 Merc with Alpha drive in my 23 Ragazza. I'm running a 4 blade aluminum, 14 1/2 x 19 prop and am turning 4400 at WOT. That gets me about 44mph with a full tank of fuel. When I've run a 3 blade aluminum, 14 1/2 x 19 1/2, it runs about4700-4800 at WOT. I got better speed with the 3 blade, about 48 mph, but the 4 blade gives better low to mid range performance. Don't know if any of this helps, but I thought I'd throw in my $.02 :biggrin:

Stephane
08-02-2004, 09:28 PM
Many thanks for comments.

I am gone a make some test again to be sure of the rpm reading and speed

I will post result on the forum after