From stock exhaust to Gills
 

Took the boat out Sunday,(was 60 degrees in Ohio)ran the boat first time since installing a set of used gills with silent choice risers. 2 mph and 200 rpm, couldn't believe it,really didn't expect anything.We had exactly the same load of people as previous top speed(went from 79.1,@5400 to 81.1@ 5600) and similar temp, except water temp was quite bit colder.Boost really didn'y drop off like I expected, still 8 lbs at wot.I had been getting stories from different engine builders, some said didn't need exhaust w/blower some said it would add power.Next in line,maybe lab finish 23p Rev 4 and add a little pitch to get rpm down.

I've been telling people for years that you will gain 200-300 rpm by adding good aftermarket exhaust. Had some guy telling me NO-WAY. They sure make a difference. If you don't have a problem with blowout in turns or coming on plane stick with the 3 blade prop it will be faster than a 4 blade. If you want to keep the same rpm go up 2 inches and have it lab finished.

Tried a 3 blade before, the boat won't even plane, just blows out before it breaks over. Unfortunatly,23p is the largest Merc makes at this time, and of all the props I've tested, I like the Rev4 the best.It may be possible to up pitch the one I have though.Will be checking into it.

27fountain- what pitch prop were you spinning at 5600 rpm to get that 81mph speed?
is your drive ratio 1.36:1 or 1.5:1?

[ 02-01-2002: Message edited by: Luke_Chinewalker ]

Luke, Merc Rev 4 with a 1.36 drive. This prop seems to have a low slip factor,11%.

27, I also run a 23" Revolution 4 with 1.36 gears. Had Chad Hill lab finish, pitch up, and add cup and rake for more bow lift. This worked, the rpm dropped about 300 or so and gave more lift to the nose adding 2 to 3 mph. I think you will like a pitched up lab finished rev 4.

A question that I have yet to have answered is I have Gils, but with silent choice (switchable y-pipe). If I dump that, will taht increase speed? No one seems to know...

Shooter, my boat was originally straight thru hull with Gils. I added silent choice last winter and it made absolutely no difference in speed for me. nuthin, notta, zero, zilch.
Jerry

SFO, What kind of boat are you running the Rev 4 on, have you tried other props.This prop works the best for me and would like to continue to use it.Some prop shops want to steer me towards the Bravo 1 which I would like to try.

As a comparison, I ran a 22 Bravo1 on my Imco 2" shorty and only ran 63. With my Rev4 23" I run 67. Great prop.

I have heard that the water temp will affect speed, colder water = higher speed.

27, it's a 22' Avanti (built in New Jersey for the Atlantic ocean) that needs lots of help holding the bow in the air, great in the rough for a little boat though. A Bravo One prop could be 3 to 4 slower if your boat does not like the stern lift. A Mirage will be faster than the Rev. 4 on a boat that likes bow lift. Most boats need stern lift so the four blade large diameter high pitch props work well. Mine needed bow lift so I changed gears (1.36) to run a smaller diameter lower pitch prop for a net gain of 5.5 mph over a stock Bravo One 26" with 1.5 gears. I have a 23" unmodified Mach Sterndriver on the boat now, just haven't gone out to test it yet, should be even faster than the Rev. 4 (I hope -please-please) :)

Do ou think they will make a Rev 4 in a 25 or so pitch. I run a 26 Bravo now. I gues you really don't want to up pitch more than 1".

27 Fountain, ANYTIME you add a better breathing exhaust system to an engine it's going to help(some more than others)....and the more the engine is modified the more the engine is going to breath better with the better breathing exhausts. That's what helps make the power. Because you're running a blower application, the engine builders you were refering to probably meant more like you "don't really NEED the Gils" vs. "Gil's won't help". If you have increased your speed by 2MPH, I would say that is equal to approximately a 40HP gain just by adding the the Gils. And "yes" they were right, with a blower application you probably didn't really need the Gils, but it certainly helped---and you can't argue with that!

good morning Mark.....your on the board early today!

How do you like your Avanti? I live 5 minutes from where they are built in Palmyra NJ. :)