bobl

I picked up an SC lower that I'm thinking about trying on my Heat. I know CB-BLR has a lot of experience with them, but he has never run a standard Bravo lower. Is it worth messing with? I've got several drive spacers so I can move it up or down. It's a -1 so I can stay where I am or go 1 or 2 inches shorter. Guess what I'm asking is there an advantage to this drive over a standard XR lower, both with the drive height optimized at speeds in the mid 80s? Is an IMCO SC faster than a standard Bravo? I know I've seen a lot of people try them and lose speed, but I assume it is setup issues. Thoughts?

Steve Zuckerman

Hi Bob,
In general, the faster you are currently running, the more they help. I did a same day test with my XR lower versus a standard length SC (same xd/propshaft depth).
I only gained 1 mph, but I was running 80 at the time.
I think at 85mph, you'll see a small improvement in wot mph, and possibly handling. You will need to get your propshaft spaced down to your current xd/propshaft depth to get a fair back to back test.
One thing to watch out for is a big increase in water pressure with the SC. They ram a lot of water into that slot pickup. I blew my water pump housing on the first test pass. I even blew a head gasket at a later date when the boat was running faster. I am now taking water off the motor in several places, have the Mercury 30 PSI blowoff valve, and mine is still a little high.
My effective xd with a one inch spacer and standard length SC lower is 18.25"s.
I think the people who have reported lower speeds and/or poor handling with SC (or other nosecone lowers) are not running fast enough to need them, and the increased water flow to their propeller actually loads it more, limits the slip % and RPM they are turning with a stock blunt lower unit (which is deflecting some water away from the propeller at speed), or their xd is too low, and the longer lower causes extra drag and handling issues.
Good luck and let us know how it goes.
Best Regards,
Steve

CB-BLR

Hi Bob.

I have tried a standard length XR lower against a standard IMCO SC lower when my boat was going around 89 mph...and found the IMCO lower to be about 1.5 mph faster on the same day...having said that...The SC lower also made the boat handle better.

I have done a lot of outdrive depth testing on several Heats including mine and the Ilmor 710 boat...for a mid 80's boat...they seem to like about a propshaft depth of 3"...which would be the -1 lower with a half inch spacer.

There seems to be water turbulance coming off the bottom of the boat right around the 3.5 inch stock depth that can be fixed by either going up or down from there...with faster boats definately liking the shallower outdrive depth... and definately a SC lower.

The reason the SC lowers sometimes slow boats down is that they funnel more water to the prop...which loads it differently and harder than a XR.

I have found that my SCX lower funnels even more water to the prop than my SC lower. I was running the SC lower at a 3" prop depth with my benchmark 28 Bravo prop and 1.35 gears right at 13% slip on the top end. I am now at 14% slip with that same prop with my propshaft depth being at 2". My overall top speed with my Merc Lab 30 has dropped from 105 mph to 103 mph...but the boat handles the best it ever has...and I have yet to even see any metal on the magnets while running the SCX lower....And no more shreded lowers!

Hope all this helps.

Try running the 1" shortie SC with a .5 spacer...and I think you will be happy.

Chris

Crazy Dayz Skater

Well this is what i know,I am the guy that throttles the 28 ss nordic that ran the 148 on sunday at loto shootout.Last year when we finished the boat we were runnin scx upper,stock sc lower and the best we ran with that combo was 141 and we could not get more than 4 hrs out of a set of gears in that lower, so we got a scx lower and sent it to wilson to have it blueprinted, we took out a half in spacer we had in with the sc and installed the scx lower and we ran 3 shootouts,and 2 poker runs with it this year without any gear failures but the best speed we could get this year with this setup was 141 also(didn't think that was bad to stay the same speed with the extra dependability). Then we sent out the sc lower to wilson and had it blueprinted,on sat at loto we ran the scx lower and that is when we got our best speed of 141, then we pulled the boat out sat nite and put the blueprinted sc lower on and did not put the half inch spacer in, it got on plane great and we ended up runnin 148 on sunday.. so what i can say is the sc lower is no question much faster,yet weaker..
Louie.

bobl

Thanks for the input. The only caveat is I have ITS, so the engine was installed much higher than normal. That's what got me playing with spacers years ago. Guess I'll try it at the same prop height and go from there.

Bob

CB-BLR

Louie,

Which SCX lower did you run...the rounded nosed one or the pointed nosed one?

My SCX lower is also blueprinted by Wilson.

Thanks for the info

Chris

Crazy Dayz Skater

I would have to say its the rounded nose, the nose does not stick any more forward than the rest of the leading edge.I didn't no there was 2 different castings, have you tried both? is one better than the other?
this is some good info,
thanks,Louie.

Steve Zuckerman

Louie,
Thanks for the information. Nice work with you and Buck. There were several great handling tunnels at LOTO, single engines like Buck's and some twins, but the instantaneous set that boat takes, and the midrange and top end handling and set were the best I saw.
What kind of prop were you running....maybe several?
Congratulations........
Steve

Steve Zuckerman

Hi Bob,
Its true there is no perfect XD/propshaft depth for our boats because of the variables involved: hull weight (changed over the years), weight distribution (crew and gear placement and storage), air flow (hard deck vs open bow), factory set XD, and obviously horsepower level.
For a lot of us however, 18" to 18.25" effective XD seems to work pretty well. It may not be perfect, but it's a good place to start.
One other item worth mentioning, if your crew/gas/carry on load varies a lot (mine does), the perfect "light" XD may not work with your "heavy" load. As you approach the perfect light XD, you may loose too much bow lift for your heavy load. The 5 blade props do help alleviate this. The four blades I've run were all weight sensitive with my current setup.
I am looking forward to see how the testing goes.
Best Regards,
Steve

Crazy Dayz Skater

Thanks alot for the kind words Steve,,we have tried just about every brand/blades/diameter of props out there and i would have to say because the drive is so high it likes bigger diameter prop so we have had our best luck with merc maximus . I do believe the biggest thing on this kind of hull/setup is not to over trim the drive,(even thru take off and midrange) it don't like much more than a couple bumps past neutral,
Louie.