Wtf happened?? Help !
 

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

I am concerned that the diameter of the bravo one is too big- like I said my old MACH stern drive 28P prop, im certain had more slip- pitch seems correct and calculating from 1.3 - 1.5 though, Im correct there, no??!!

I most certainly would agree- however, the XZ drive is a 1.25" shaft, so the Mach prop was fixed at 1' shaft- no hub, besides going from 1.3 to 1.5 calculations say a 32P
prop would be very close-( i was able to get 32P AND 30P Bravo one props at the the same time and tested both, i went all the way down to 28P and got 6,000 rpms,
but barely hit 85mph

probably mid 90's era Bravo one- plus of course, it was converted to Bob's Nose cone. I worked quite well with old set up- that's why im puzzled- like i said- im still feeling like if I made the Bravo One into a 14 1/2 diameter- then slippage would be much closer to the old setup. imho. ( just don want to start chopping up a prop before I'm certain. I have looked everywhere for a smaller diameter prop, with 1.25" capability at 32P something in the gut is listing that way?

I was hoping that might be true, the XZ's dual pickup- does that change anything??

sounds like its narrowing down to a smaller circumference prop, 32P being correct prop- just need more slippage for lighter boat- with the same gearing of sorts- wish it could be that simple. The only other factor- i'm wondering if slightly stepping up REVS on the bm250 is possibly causing to much drag on engine- maybe overthinking that, lol. AND I KNOW at least, I should have an 8-71 ideally- just wanted to get through this summer- and was thinking it would be acceptable- since I was content with the old setup hitting 95ish. when I get the 8-71 than I feel ill be at 100+ and then I'm done/happy. maybe HaHa.
of course providing i can get past this,
I just want to say thanks to everyone pitching in to offer feedback, maybe this one's more fun figuring out than others. wishful thinking- lol.

originally my boat tech/shop calculated i was at 10-11% slippage- with old setup- i believe we were using 5600 @ 95ish MPH

btw, like i said before- im not cavitating at all- out of hole with new setup- however, I did with old setup- manageable - since I was hitting a decent speed.

For some reason I was thinking it was only a 5mph diff. 10 is quite a bit more.

I suspect it's a few compounding issues robning you of your 10mph.

The BM250 at 2.22over is really heating up the charge compared to your slower pulley.
You're not in your max power rpm range.
Using a larger diameter prop at those speeds.

It's always nice to have a air temp sensor in the intake manifold even just for testing etc.

I have a B&M INFO magazine from 1990. I'll dig it up and see if it has info on BM250 speeds. Unfortunately, getting blower efficiency charts is hard to find compared to turbos.

STOP…..go back to what SB said.

you changed way too many variables AND went Up in pitch by 4 in and nearly an inch in diameter.

the calculations called for a 32 pitch, after switching from 1.3 to 1.5, that would not seem to differ much, I tried a 30p and I got my rpms back up- yet still 85mph-

Im beginning to suspect the diameter is too big or too heavy and dragging- not sure- I get you though, hence why I'm here, however, correecting if Im wrong, i would not thinnk the twin carbs would matter much, Im temped to go back to the 1.3 drive and mach stern prop (14 1/2 X 29P) easy enough to do- would confirm engine running
optimum still- just spit balling - your thought's ?

May just have to pop 1.3 drive on and old prop- to settle whether engine changes are issue or not- tempted to customize prop more- but then wouldn't be able to sell prop then this puzzle kinda sucks eh? appreciate the help

btw where is the best spot to install air temp sensor?

I owned an AO 26. I purchased the boat from an owner that installed an IMCO 3 inch raised box and kept blowing motors before he could find a good X-dimension and prop. I was amazed on how different props and drive heights using spacers affected the performance of the boat. Eddie Young built the motor and got Art DiNick on the phone to get some guidelines on where the X-dimension should be and we found the sweet spot. I don’t think it’s your engine upgrade but your drive height that’s causing the problem. Call Art and he can help you as he built these boats for a while and still runs a 3100.

Getting the old drive/prop on would go a long way to isolating losing 10mph.

Of course...check the X dimension before pulling the new drive etc

You put a 30p prop on...got your rpm back up to 5600-5700?

Twin blower carbs have better fuel distribution and more flow at WOT. Blowers don't like vacuum at their inlets, so if the carbs don't flow enough, it will impede the flow/boost potential. B&M tried vac secondary carbs as well. They work, it's all about the airflow potential of the engine.

This is a photo of my 420 Megablower BBC intake. Assuming the 250 intake is the same thing.

https://cimg1.ibsrv.net/gimg/www.off...f94c5847f9.jpg
Not sure this is stock...but there's a boost pressure port at the rear of the intake. You could probably drill/tap anywhere along the rear rinner here. In the center would not interfere with flow as much. Should be short enough to not interfere with the distributor.

Or if you don't have a sprung back fire plate, then you could use that to make up another plate with some meat to drill/tap there.

There may be port on the 250 blower bottom you can use. I'll check my 420...but it's packaged up atm.

Thank you, Ive talked to Art before. will do.

excellent advice, I believe thats the hard truth- I believe it is the blower and carb change- but your right- no since guessing, one or the other improvements will need to be
undone to accurately diagnose- thank you sir!

fyi, I went from 1.35 ratio to 1.5 , reason i needed to go up in pitch, otherwise , still good input- thanks

update to WTF Happened?
 

Ok Everyone; UPDATE; so , i appreciate everyone's feedback, seems we were chasing our tails, my apologies, so I had dual holley double pumper 750 carbs installed by my mechanics, upon recent inspection,I found;
1) a couple fuel inlet fittings loose-(leaking fuel)( re-tightened)
2) found loose linkage fittings(tightened) and adjusted.
3) when engine cover was completely closed, throttle cable was being restricted, Raised engine cover 4-5 inches-and it opened up beautifully, accelerated rapidly, confirming 30 pitch prop to low!
with the labbed 30 pitch Bravo 1, this engine revved freely to 6000rpm( rev limiter setting) so just swapped back to my labbed 32 pitch bravo 1. taking out within couple days- will update accordingly

QUESTION; anyone know what the difference is between a satin finish vs polished.

additionally; I checked x dimension, 1 " low- so will be looking for a -1" Imco lower. or relative lower, also recommended by Art Dnick at AO.

QUESTION; how much additional top-end, ya'all expect it would produce with the Imco lower ?

if after these discoveries AND eventually installing imco lower. if not surpassing 100mph i will be next looking for larger blower, thanks again,
any additional feedback or recommendations, i am open to suggestions.

Boat-On, Rock-on, and enjoy the summer !!!

looking for some addtional feedback
 

Ok so the dual 750 carbs installed, I found out they are NOT Boost Referenced, ??? i was told this is not a problem, seems to be running good and I cant be certain that the lean condition on one bank is caused by this or the
problems I found and corrected, I will return with update in couple of days

Holley 0-4779S Street/Strip 750 CFM Square Bore 4-Barrel Mechanical Secondary Manual Choke New Carburetor

https://cimg7.ibsrv.net/gimg/www.off...e0fe963d35.jpg

AO,

Just to put it out there.... The carb in the pic is not a marine carb. Don't know if the carbs you are using are the same, but those overflow tubes should really be J-tubes. Or so I'm told....

Thanks. Brad.

you can put a rubber hose betw. them and cut a hole in the upper most part of the hose.had that set up for years on my boat,never a problem.

Brad is correct...this likely would not be to USCG spec.

USCG only requires that the excess fuel from the carb would drain into the carb and intake.

Pulling and twisting the bowl vents so the angled cut faces the carb bores, would meet the USCG requirements.

All throttle shafts are sealed...or you would have a vacuum leak.

Insurance providers may have other requirements. But we're talking safety and the legal requirements.

Without boost reference, the power valves cannot determine when to open when the engine requires it.

Modern carbs have cone a long way, dumbing them down by removing power valves and jetting higher is a downgrade of sensitivity to the engine's needs.

It's not difficult to do, there's a few methods. Mine are all internal, but require drilling.

There are less invasive methods.

Either way...if you don't have an O2 sensor setup to tune, it's something that would really benefit you. Too rich means washing out the cylinders. Too lean...you get the picture.

I found the B&M book. I'll see what they say on Boost Ref.

Satin vs polished props?

The satin finish is faster. But people like the polished bling.

One thing you don't expect to have to check from a marine mechanic...is if the fuel fittings are loose and leaking.

One of the most common power losses is throttle not opening all the way.

Glad you tracked it down, amd before you had issues with leaking fuel.

Did you ever smell the weeping fuel from the connections?