Bravo XR Performance Difference
 

Ok, so long story short, I have an uneven throttle issue I have been trying to debug for a few years. I've pretty much ruled out everything on the engines (complete ground up rebuilds with almost everything that could possibly impact performance replaced). Tried multiple sets of props. The only known difference is in the drives. Both are Bravo uppers and IMCO lowers. Both are 1.5 ratio. The difference is one is a XZ with 32/27 helical gears and ones is an XR with 19/16 straight gears. The straight gears seem to be faster. Anyone have any experience or testing on this? The mathematical difference between the two ratios is about 0.2 MPH. Before I drop almost $2000 on a new set of gears, I want to try and get some insight. Any thoughts?

Swap the drives from side to side to test your theory.

Allegedly, it was already tested and found to not be the problem. But at this point, we have not eliminated every possible cause and yet we still have the symptom. The boat is out of the water right now and the drives are open for maintenance so it seems like the perfect opportunity to end this if indeed it is the problem. Swapping the drives again, splash, haul, open one back up again is a fair bit of the way back to the cost of the gears.

What is the actual RPM diff at full throttle? Have you verified the tachs? What exactly do you mean by uneven throttle?

For reference, since the problem started, both motors were fully torn down. Full rebuild, balance & blueprint on the internals. Heads went out to Hardin for CNC porting (stage 3 upgrade on 525EFI) which also included new camshafts. All new valve train including rockers. Fuel injectors flow tested and rebuilt or replaced. New mechanical fuel pumps, all new filters, every electronic sensor replaced, new coil packs, new wires new plugs. Only thing not changed on this major rebuild / upgrade was the wiring harness and the high pressure fuel pumps.

Hmm,it's been 2 years, I did not put it in the water last year because I was focusing on a new business and wedding.

The difference appears to be linear. With the throttles even, the rpm difference starts to show at minimum planing speed and grows larger to wide open. At WOT, if I recall correctly, it was something like 5350-5400 on the port and 5100-5150 on the starboard.

Tachs match the rpms on the laptop scanner. No codes, no noticeable difference on the data logs. The engine noise (harmony) is even when the tachs are even so I don't believe they are the issue. Throttle cables have been replaced since, no change and of course WOT verified at the stops.

Propellers were also changed to a new set.

Spoke to Merc Racing tech this morning... absolutely a difference in the parasitic loss, the XR is more efficient than the XZ. Anyone have a lightly used set of XR 16:9 gears?

I have the same setup. No difference in RPM between the drives. A 300 rpm difference would be around 50hp. I don't see that much extra loss in X gears over XR gears.

Whoa wait a minute. So you are telling me the XR is more efficient than an X or standard bravo?

I thought the XR was 1+ MPH sometimes 2 MPH slower than an X or bravo drive.