19" pitch/ 21" pitch SAME WOT RPM ??????

TSHELNUTT · Oct 7, 2003

I HAVE A NEW 20' BOAT 350MAG MPI ALPHA 1 WITH POOR PERFORMANCE. I HAVE TESTED A 19" AND A 21" PITCH PROP AND HAVE EXACTLY 4750 RPM WITH BOTH. ANYONE EVER HEARD OF THIS?

Bondo · Oct 7, 2003

Re: 19" pitch/ 21" pitch SAME WOT RPM ??????

You're hitting the Rev. Limiter..... Go to a 23"..... You want to pull it down to 4650/4700rpms.... AND, Loose to Cap-Lock.... It's Rude,+ Hard to read....

TSHELNUTT · Oct 7, 2003

Re: 19" pitch/ 21" pitch SAME WOT RPM ??????

The wot rpm on a 350 mag mpi is 5000 and the rev limiter is set @ 5100 rpm.

Bondo · Oct 7, 2003

Re: 19" pitch/ 21" pitch SAME WOT RPM ??????

Then you've got Other Issues.... Getting good fuel flow ???? Solid Spark ??? Timing ??? Detonation ????? Bottom Growth ???

Northern Eclipse · Oct 7, 2003

Re: 19" pitch/ 21" pitch SAME WOT RPM ??????

are both props made from the same material and manufacture??, can you provide us with more details to what exactly is the performance issues??

Hasbeen · Oct 7, 2003

Re: 19" pitch/ 21" pitch SAME WOT RPM ??????

Was it a smooth 4750 RPM or did it bounce a little at top end? Also was there any speed diference at all between the two?? Was that at full trim? Hasbeen

akriverrat · Oct 8, 2003

Re: 19" pitch/ 21" pitch SAME WOT RPM ??????

tach reading a little low and rev limiter a little off and you bounce off the rev limiter with the tach indicating 'exactly 4750 rpm'. not much else would show zero rpm change. unless tach sticks at 4750. stick to your guns bondo.

TSHELNUTT · Oct 8, 2003

Re: 19" pitch/ 21" pitch SAME WOT RPM ??????

With a 19" pitch prop top speed is 52 GPS, 21" is 56 MPH GPS. The RPM is being measured by a Mercruiser Tech from Stillwater, OK. with a handheld device that plugs into the smartcraft motorola 555 ECM mounted on my engine. My boat Tach actually shows 100 RPM more. Both props are aluminum brand new out of box.The engine has been dynoed @ 5000 RPM and produces 290 HP, 355 lbs. of torque.The engine runs great @ 4750 and we are definitly not hitting the rev limiter as this produces a missfire.

Hasbeen · Oct 8, 2003

Re: 19" pitch/ 21" pitch SAME WOT RPM ??????

Aluminum props are not as much of an exact science as stainless props, and with those torque figures it is intirely possable to have this happen! Not the norm though! What is it you are trying to acheve?? Hasbeen

Doug Durako · Oct 8, 2003

Re: 19" pitch/ 21" pitch SAME WOT RPM ??????

I'm with Hasbeen (now there is a qoutable quote)---put a 21 pitch Mercury Vengeance SS on there and have another go. Are you trimmed to the end of your limit range? Running the standard 250 hp small block Chevy, I hit 62.3 (gps) at 4800 rpm on the dashboard tach. Just me in the boat, full tank of gas, light chop on the water. If you are running a Chapparal boat the hull design may limit your top end somewhat, but a stainless prop should give you 4 to 6 more miles per hour at the same rpm.

flashback · Oct 8, 2003

Re: 19" pitch/ 21" pitch SAME WOT RPM ??????

HC, I have a 19" with some pretty agressive cupping and also 21" that is standard... there isn't a nickels worth of difference in the two..the 19 is a little better out of the hole but top end is about the same, the 21 has just a touch more top end. 1-2 mph.........rpms stay between 4500-4600........I guess it depends on how exact you want to be.......

johnkc · Oct 8, 2003

Re: 19" pitch/ 21" pitch SAME WOT RPM ??????

have the tech put the DMT on it also to monitor rpm,if the DMT goes crazy at wot then you are banging the rev limiter.i know this works, did it today.and did not get the backfire sound that i expected when i banged the rev limiter.

19" pitch/ 21" pitch SAME WOT RPM ??????

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