Leaving Power on the Table
 

Got a slight problem here that just might help me pick up a couple MPH once resolved.

2006 Howard Bullet
2006 525EFI With Whipple Stage 1 ECM Program
Teague Platinum Drive with Imco Standard Lower 1.36 Gears
Bravo 28P and Bravo 26P
91 Octane Fuel

With either prop, the motor only winds up to 4900RPM. This is our wall. The guess is we need to swap the gears to 1.50. What are some thoughts on why we can't get full RPM?

Try a un-Molested 555 1st

The gear ratio should not be an issue. The 26 should spin +/- 400rpms faster than the 28 no matter what the gear ratio.

Is the tach reading off a smartcraft or analog guage???

Sounds like a bad ECM program or that you may have some kind of guardian stopping it form revving out.

How old is the fuel? If its detecting knock typically you wont be able to rev.

Who is the engine guy that built it? Id go to him first.

UD

The stage 1 Whipple tune for the 525 EFI nets you 25-30 hp, which puts you around 550-560, trying to spin a B1 28 is tough, I think what your seeing @ 4900 is correct..........I had 625 hp in a 540 cu in and they would spin a 28 B1 5200

Agreed on the fact that the 26P should net 400RPM's. Both props are used with unknown origins and whether they were labbed or modified in any way.

The tach reading is from a Vessel View 4 .

A full tank of fresh 91 octane in the tank (the Whipple ECM upgrade requires 91). The motor was built by Mercury Racing, no other builder involved.

Not sure I agree here. I have had the same exact boat and motor combo in the past, spinning a 28P to 5250 was no problem on a pure stock 525EFI.

The differences between this boat and the last boat are Bravo XZ with 1.50 gears vs. a Teague Platinum with IMCO standard lower running 1.36 gears. This boat
has the ECM upgrade the other boat did not. We are not hitting the rev limiter, getting the same exact top speed with both props (which also tells me one or both have been labbed somehow). So gears are not the problem? What other theories?

That's right off Whipples website, so believe what you want.......change the gear or prop, your trying to spin a 28p with a 1.36 gear with 550 hp, it wont add up in my book

I apologize, I understand what you're saying now.

The 1.36 gears are most likely the culprit here? Is it safe to assume switching to 1.50 gears would get this combo to full RPM range?

Before I deal a lot of money on a .14 gear difference I'd try a 24' and see if it changes. If you're still at 4900, easily tells you have other problems. It not, prop into the gear ratio.

Yup its a its cheaper to put a smaller prop on that to re gear it.

I would think he should be turning the 26 at least a few more RPM than the 28.

UD

The 1.36's are NOT the issue. I have 1.36's with 502mpi's. I spin my 24's to 5100. I borrowed a pair of 26's and spun them to 4700rpms.

Being the props are used and you don't know if they are stock specs, start there.
Are you seeing a speed difference with the 26's and 28's at 4900rpms?? You should see about 4mph more with the 28's.

Griff - your boat has twins - You can spin a bigger prop or run a taller gear than a single engine boat.
I lost 600 RPM when I went from 1.5 to 1.36 ratio. I am spinning a stock Bravo 28 to 6100 RPM but I have 800+ HP.
I just bought a labbed for me Bravo 28 from BBlades to see if I can get on plane a little easier and gain the 300 RPM I need .
I also think that the 1.36 ratio is holding you back. It takes torque to turn the higher ratio.

There is a huge difference from a 1.5 to 1.36...........agree ^^^.............my bud's 35 Ol has 5 blade maximus and 1.36 IMCO's and idles about 12 mph

Agreed, the 1.36 gears is the problem. A 28p prop with 1.36 gears is like a 31-32p prop with 1.50 gears, to compare it to your old boat. You'd probably need to step down to a 24 or so if you wanted to keep the gears you have. To swap gears is ~$1000 I believe. I agree that you should be seeing a difference RPM wise between the two props, but only if they were both stock or both labbed etc. Since you don't know the history of the props, if the 28 is labbed, it will spin the same rpm as a stock 26. Should be about 200rpm between the 2 normally.

Think I'll have to try this route first just to rule out any other problems.

Say I ran a Bravo 24P with the current 1.36 gears, can we assume the RPM goes back to normal 5250? Would a Bravo 24P get the top speed numbers? Sounds like it would be in my best interest to change the gears to 1.5 or trade the drive for a stock XR.

Why don't you find someone with a 1.5 ratio drive that will let you try it.
I lend mine out here in west MI. As long as they pay for the oil change and I go for a ride.
But I am running 1.5 ratio IMCO -2 shorty ( spare ) and a 1.36 ratio IMCO -3 shorty.
So the list of people wanting to try it is real short. Most people can't run a -3.
I was told that I was one of few able to run my prop shaft at 2 inches above the low point of the bottom on a 26/27 Daytona.

Based on gouge numbers, it should pick up to 5100rpm or so with a 24p prop. From what I understand, you should always keep the lowest gear ratio (highest numberically) until you run out of prop, then go to 1.36. With 1.36 gears, the prop is spinning ~350rpm faster at WOT than the 1.50 gears, so there are hydrodynamic effects that come into play with having a smaller prop spin faster. I'd be willing to bet that 1.50 gears and the right prop will net the best speed.

General rules of thumb

2" of prop pitch change = 400rpms

1.5 to 1.36 change = 600rpms

You're right, 600rpm difference at 6000rpm (~10% difference). I was talking prop speed which isn't very useful.

The OP needs to spend some time with the prop slip calculator.

For instance, you said you had this boat with 1.50 gears, 28p prop, at 5250rpm (lets assume 15% slip) would be 79mph.

To run 1.36 gears, 5250rpm, 79mph, and same 15% slip assumption, you would need a 25p prop.

Like I said, these are all theoretical, just math formulas. More comes into play hydrodynamically, mechanically (driveline losses), etc. Post this question on Riverdavesplace.com. Theres several guys with 28 bullets over there that could steer you in the right direction.

I'd bet you're right.

The last boat with 1.50 gears, 28P prop, 5250RPM ran 78.3MPH best. So you're very close.

This boat with 1.36 gears, 28P prop, 4900RPM ran 79MPH.

Just posted on RDP as well. I figured I would start on OSO since it seems there is a larger contingency with 525's. However, not very many with singles. Great problem to have!

As I research new gear sets, I have noticed (3) different parts numbers. Which ones are best in my application?

43-818928A2 15/19
43-8M0104360 15/17
43-840870A1 17/18

Any other options or part numbers out there I'm not seeing?

43-805448A-1 Is another I have seen. Will these work in my application?

An option I have considered is selling the current drive and purchasing a stock XR. Depending on value of each, I may save money instead of spending money.

I agree 1000%! You have two props that are UNKOWN origins, that are giving you the same speed at the same rpms. DING DING DING

Switching gears is crazy, find a bone stock 24 and 26 and see what happens.

I am in search of a 24P to test. In the meantime, I am shopping for gears.

Anyone have insight into the gear part numbers I listed 2 posts back?

15/19 is for a 1.5
15/17 is for a 1.34 (XR's are actually 1.34. regular Bravo's are 1.36)

Got it. I can install the 1.50 gears in the XR correct? (Teague Platinum XR with IMCO Standard lower).

To change ratios, you change the lower gears. The uppers stay the same in all XR's.
All the beefed up XR's by all the different companies still use the same Merc gears.
So yes, they will work in your IMCO lower.

I still do not think the gear ratio change will do anything. Getting a prop dialed in with the 1.34 will yield the same results.