Is it worth improving this perromance? Tahoe Q6, 4.3 MPI

gt2003

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I just bought a 2007 Tahoe Q6 with the 4.3 Liter MPI motor. I took a test ride on it before buying and there was significant porpoising and the trim was overly touchy. I bought it and before I took it out I installed smart tabs and changed out the 14.25x21 aluminum prop with a 14x20 Alpha 4 aluminum prop. It now performs SIGNIFICANTLY better! No porpoising and planes almost immediately. I was able to run it WOT today with GPS and got 44 MPH with 4300 RPM. I downloaded a GPS on my iPhone for the speed. The rpm's were per the gauge on the boat. I'm more concerned with hole shot than I am with top end speed. Since the recommended RPM's for this motor are 4400-4800 RPM, would it benefit me to go to a revolution 4 stainless steel in 19 pitch or am I pretty close to perfect right now? That's a whole lot of money to spend for a minimal improvement in performance. But, you all are the experts. Let me know what you think. Thanks!
 

steelespike

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Your numbers aren't quite right.Assuming the gps app is accurate 44 mph at 4300 rpm is only 2% slip Slip more typically is
double digits say 13%.44 seems low as well.Even though relatively young the tach may be off low.
If we assume all is accurate and use 13% slip the rpm would be about 4800.
Describe the Q6 to us.
 
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gt2003

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What info. would help? Boat length, water conditions. It's a basic bow rider with a dry weight of about 2800 pounds I believe. It's listed as 20'6" but that includes the built in swim platform so It's more like 19'. It's got the 220 HP 4.3L Mercury MPI motor. Water was not glass but just had a little ripple on it. I was able to go WOT and trimmed the motor up as high as possible. It's got a 45 gallon tank that had likely 40 gallons in it. Let me know what other info you need and I'll supply what I can. Thank you
 

steelespike

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I just needed to know your boat is a fairly typical model for a 4.3 and not a 23 ft cuddy.
Is 4300 and 44 with a light load?
Going to a 19" will lower you rpm more and slow hole shot.
Using a 1.81 gear ratio ; if we go with a 18" 4 blade same model guessing a reasonable 13% slip would net about 4700 and 38.6 mph and better hole shot.
If we assume 44 is accurate and use 13% slip the rpm would really be about 4800.
I think we need to varify the tach is accurate if so then your app may not be accurate.
I think boats like yours usually run closer to 50. A 16 degree dead rise would slow hole shot a little and may be farther from 50.
Perhaps you can borrow a shop tach to check your rpm.
 

gt2003

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4300 rpm and 44 mph was a pretty typical load. Me, my girlfriend and the usual boating gear and an almost full gas tank. I'll see if I can get my hands on a shop tach and give you better information. I appreciate you taking the time to give me all of the above. Thanks!
 

dazk14

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Just as steelespike has indicated. That boat is running far slower than expected. You've got 220hp.

I'd start by disabling the tabs and trimming it out to max before blow out. check GPS speed and current tach.

Do you have any top speed numbers for the 3 blade 21".
 

jestor68

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I also suggest a run without those smart tabs forcing the bow down, which increases the wetted surface, and reduces the top speed.

Or you could try adjusting them to the "lightest" setting to reduce the tendency to force the bow down.

That boat should run with about a 5-6 degree high up attitude at speed.
 

QC

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No matter what other things are amiss, 2% slip indicates something is wrong with the numbers. One or more has to be wrong, pitch, RPM, speed or gear ratio. I suspect tach, but you could also get another free App and check speed. Speed Box is very accurate once it settles down. Check the back of your tach for a 4, 6 or 8 Cyl setting. I don't care how new, or how expensive, if it is set wrong, it will read wrong.
 
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gt2003

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Dang, that's why I'm on this sight. Ok, let me do some checking on the tach, adjust the smart tabs and run it again. I don't have a speed for the 3 blade but I'll put that sucker back on if it'll help with this project. Thanks again
 

gt2003

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I've lowered the smart tabs to their least resistive setting and downloaded the speed box app on my phone. If I get the chance to run WOT then we'll see what she runs. I'll post back when I get results. Thank you again.
 

jestor68

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I invite you to visit the Mercury Marine web site to view the engine tests of the 4.3L Alpha 1 in various boats like your's.

You will see that all are running 47-50 mph at 4700-4900 rpm using 21 pitch 3 blade and/or 20 pitch 4 blade props.

The only difference I see is that none of them had tabs scrubbing off speed.

Another consideration is the fact that your boat is 7 years old with unknown percentage of full throttle time on the motor. Seven years of use will certainly result in some power lose. These engine tests are conducted with new boats. So it wouldn't be too surprising that your 7 year old motor couldn't match the numbers posted in the tested boats.
 
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Maclin

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I also suggest a run without those smart tabs forcing the bow down, which increases the wetted surface, and reduces the top speed.

Or you could try adjusting them to the "lightest" setting to reduce the tendency to force the bow down.

That boat should run with about a 5-6 degree high up attitude at speed.



Adding to jestor's point, Smart Tabs need some dialing in. They should not cost any top speed according to their specs. If the top speed was higher before their install then lighter setting mounting hole or even switching to next lighter ram may be needed. I think they will swap out rams for you. It will be interesting to see how the mounting hole setting change you just made fares.
 

QC

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it wouldn't be too surprising that your 7 year old motor couldn't match the numbers posted in the tested boats.
No boats ever meet test numbers in my experience. Perfect setup on the test boats compared to boats with wear, added stuff, imperfect setup etc. usually costs 5 MPH. This has been true with every boat I have seen on Boat Test.com compared with real world results.

Also, the ONLY way any trim tab can make a boat any faster than it was without them is by eliminating porpoise with the bow even higher than possible previously. Point is, tabs can allow you to shift more weight aft, trim higher, yet not porpoise. If that is not the case, higher trim capability than without, they will slow your boat down.
 

gt2003

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The lighter setting may be the trick. The water was rough so I didn't get the chance to run WOT. But, I was able to get it up to 45ish which is higher than before. I'm actually VERY content with what my set up at this point. I'll pass on the numbers to you all when I'm able to run WOT on a smooth day. Thanks again for all your help up to this point.
 

gt2003

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WEIRD THING! I ran the 14x20 Alpha 4 prop with above results, 44 MPH with 4300 RPM (yes, wrong but that's what it showed). I hit a low area in the lake and wasted the 14x20 Alpha 4 and replaced it with a supposed "match" of a Michigan Wheel Vortex 14x20 (factory second prop). HUGELY different results. The Michigan wheel 4 blade, 14x20 prop seems so much better as an all around prop. It runs super smooth and hits over 45 mph with about 4700 RPM on my gauges. HOWEVER, the Michigan Wheel Vortex 14x20 prop WON'T PULL MY A** out of the water on slalom like the 4 blade, 14x20 Alpha 4. It gets me very, very close where I feel like I'm about to pop up then just stops there. So, I'm guessing cupping or something else is to blame. Maybe since it's a "second", something is different. Any ideas? I've ordered, per the mercury prop selector website, a 14.5x18 Alpha 4 that will be in Thursday. I'll keep my eyes on the RPM but am expecting this thing to yank me out of the water like nobody's business. I'm keeping the Vortex for all around cruising but damn, it's time I get up on slalom...CONSISTENTLY!
 

steelespike

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The Solas Amita or Rubex (removable hub) have a good reputation.
The Merc. Spitfire 4 blade is said to deliver 4 blade performance without sacrificing top end.Thus a 20" may improve hole shot and top end.
By going to a 18 Your cutting off more top end but hole shot should be excellent.
We used to ski with a 50 hp Evinrude 375 pound boat(18' bare utility). with a 100 lb adult spotter.It would get up a 180 pounder on one ski but if he didn't keep his weight forward it wouldn't even come close.So watch your form.
 

gt2003

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LOL, those were the days Steelespike! I learned to ski behind an old, old kingfisher flat bottom bass boat with a 55hp motor. It would pull me and my buddy both up on two. I remember advancing to "dropping a ski" but don't remember the 1st boat I did this behind. My fondest memories are getting up behind my college roommates older ski antique. I was AMAZED at how fast that boat could yank me out of the water. So, just getting back into it, have gotten up once and was just upset at the performance (for skiing) of the Vortex. It's an incredible all around prop though. Man, it runs SOOOO smooth. I'll definitely keep it for cruising! I bet I'm even more surprised at the 18 pitch Alpha 4. It comes in Thursday and I plan to be on the lake Friday!
 

steelespike

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I spent my summers at the Thousand Islands during my youth.In the 40s,50s and 60s There were a number of outboards in the area powerful enough to pull up a light skier. Yet try as I might I never made it until I tried behind a 15 year old 95 hp 18' 1948 Chris Craft.It popped me up like I wasn't there.
From then on I could get up behind almost anything.
 

gt2003

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I bought the Alpha 4, 14.5"x18 pitch prop and it seems to be the perfect match. It runs me about 45 mph at 4850 rpm's (4400-4800) at WOT. It also has the power to pull my big butt up on slalom! I can deal with the 50 RPM's over recommendation since I very RARELY go WOT. Amazing the difference the proper prop will make!
 

SkiDad

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I'm responding to both posts - that is GREAT news ! I think your out drive must be a 1.62 ratio for your numbers to make sense - I've seen a couple others with that ratio in that boat with 4.3 - it must have been an option. So you have the perfect ski prop and keep the Vortex as your spare/cruise prop. I wouldn't change a thing and now enjoy that skiing - i know I have had a lot of fun this year but its starting to get too cold in Ohio for the Captain to pull me.
 
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