Hey gang, seeking a little feedback:
26 American Offshore, Gen 6 roller, 496 stroker, BM250,Chiller, CMI, solid blower setup, Eagle crank/rods, total seal, Teague stage 2 blower cam, Johnson lifters, .088heads ported and flowing well as possible) imho. , yada,yada,yada


BEFORE; single Carb. Holley Dominator 1150, 5 1/2 lower pulley, 2 7/8 upper' Bravo one 1.3 out drive with bobs nosecone, 28P Mach prop ,2 persons light load, air fuel ratio sensors approx 12/12.5 both banks at upper cruise, ******( 95mph @ 5600rpms )*****


AFTER; dual Holley 750's , (6 3/8 lower pulley, 2 7/8 upper,) approx 5.5-6.0 boost) NEW XZ outdrive 1.5, Labbed 32P Bravo one prop, barely , 2 persons light load, a/f sensors approx high 11's low 12's both banks at upper cruise ******( 85mph @ 5100-5200rpms )****

Just dont get it, upgraded out drive to XZ, due to bondo peeling and clutch worn out on old drive., got some hunches, however I've not experienced this before- maybe someone here has, and could provide some feedback/guidance-
Much appreciated- fellow speed demons. Ideally looking for an 8-71, hopefully by next year, this year, would be happy with before results

What Teageu stage 2 cam? The Gen V or Gen VI? 236/244 eith 112° LSA?

What was your intake air temps for both scenarios?

The issue I see is the 2nd example is way down on RPM @ 400-500 rpm. Those cams, need the rpm to produce the peak power curve.

The B&M 250 is likely maxing out. It's a big ask to feed a 496, despite the chiller. With the pulley change you may be into a far worse efficiancy curve, heating the air.

1st ratio is 0.52:1. 192% overdriver
2nd ratio is 0.45:1 222% overdriven

What are the slip differences on the 28 & 32 pitch props?

Could be a factor or lower rpm and prop factors.

Wouldn't think the XZ drive would be hydrodynamically less efficient that the Bravo 1. Maybe the X dimension is lower?

I think you might have nailed it, honestly just assumed the shop would have checked X dimension,
Didn’t dawn on me; before I would cavitate if I got on it too much, coming out of hole ( acceptable however) yet with new drive, no matter how hard I got on it, no cavitation, I’ll let you know on the X dimension, maybe like dragging an anchor, lol
btw the way, it was still practically winter when testing, so assuming I let air temps not big factor, I’ll chk it though, have to start looking for that 8-71 ,or do you think 10-71?

one other tidbit of info, bravo one prop is 15 1/4 or 1/2 in diameter, where as the Mach one is 14 1/2 diameter, BTW, gen 6 roller cam,

Slip calcs out pretty close to the same for both setups ~17.5%. If the bigger diameter 32 bravo was ”lowered” in the water, you should have lower slip. Could be the lack of the nose cone that is having a significant negative effect.

I’m a big believer, with any engine changes, in testing with same known prop first. Too many people do engine and prop changes at same time, already get rid of original ‘best prop’ and chase ghost issues for seemingly forever.

A good baseline is a good baseline.

Did I read it correct or was it a typo O?

you went from a 1.3 drive to a 1.5?

I didn’t think a Bravo came in 1.3

All the B1 lowers are the same length except for the -2 sportmaster.

And theoretically a 1.5 if propped correctly is faster than a 1.3.

Hmm, I came up with 15% for the 1.32 drive and 19% for the 1.5 using the 5200 rpm.

https://www.go-fast.com/Knowledge-ba...lip-calculator


With everything else being the same a 10 mph drop is probably equal to 200 HP or so. I had to guess at the input values as I don't really know that boat.

https://www.go-fast.com/Knowledge-ba...eed-calculator

In general 300 rpm should not result in a 200 hp difference in power.

Hmm didnt think of that, I will measure the old drive to the new- still have in storage, will post that when i get it- idid see this morning this front tip of the nose cone is virtually level with the apex of the center sponson, if that reveals anything