SB

iTrader: (1)

Wait a minute. This engine is not a 525SC. BM 250 / BG Carbs...what else but cam is different than a 525SC and how ?

Let's start from scratch. Tell us everything (parts, real compression, etc,etc) as our answers may + probably will change.

Don Johnson

IMHO you need to dig into why the slip numbers are so high. Given what you have described I am speculating that the TRS lower has hit a MPH point where they are blowing out. Switching to a 28 or 30 may bring the RPM down but the WOT slip issue would theoretically be the same. Call a good prop shop and ask if they have any thoughts on this. I do know from experience that the non step Fountain likes Hydromotive P5 props, did a fair amount of testing on the 42 Fountain we race in the Catalina Ski Race but it has bravo drives on it....

stimleck

**** I love to see smart people give good advice but **** I hate that I am clueless when it comes to this stuff!
No amount of watching Powerblock, Pinks, or reading magazines can replace real experience

SB

iTrader: (1)

On the internet, you can pretend.

Griff

What he said^^^^^^^^^

Your engines are NOT 525SC's. They may be somewhat similar, but from what you have posted, there are also many differences.

Crude Intentions

iTrader: (1)

The point isn't necessarily what the engines are. They are 454/425s with 250 blowers. It was more about prop slip and prop suggestions. Engines are set up right and running good. Some good ideas on timing and afr's and its appreciated but the internals and compression have nothing to do with prop slip and what might be best for TRS drives.

Anyone running TRS in a similar hull may be able to help more.

Griff

26* Total Timing is not set up right. Its one hard run away from tulipping an exhaust valve.
The engines need to be tuned first and then address the possible prop issues.
Bumping the timing 4-6* may add another 300-500rpms at WOT.

What has been posted about the rpms, minimal speed increase and the increase in propslip % does not make much sense unless it is overtrimmed.
Some hook in the hull could also be a possible cause

How is the running attitude of the boat??? Is it running real wet?? Is it carrying the bow well???

Unlimited jd

iTrader: (5)

I agree with griff. No sense chasing prop slip if the motor isn't going to last through your next day out. Dial that in and then dial in the slip.

MILD THUNDER

iTrader: (3)

I agree with the others. 26 degrees total is not a good idea, at all. BUT, from what I have played with and seen as far as timing on blown marine engine, guys get a little over zealous about the amount of power it picks up. I have tried 30, 32, 34, and 35 degree timing on my blower engines over the years. You know how much speed i picked up going from 30, to 35 degrees, about 1 mph. On the dyno, going from 33 to 34, picked up 4HP at 5500. Going from 34 to 35, picked up another 4hp. 2 degrees of timing, was worth 8hp. In my boat, to gain 1, maybe 2mph, you're gonna need about 20-25hp per side.

I would give it a bit more timing. But, without knowing the entire content of the build, I wouldnt want to recommend going to far. A small blower like the 250 makes alot of heat. Throwing alot of heat, alot of timing, and questionable fuel we get at the pump, with an unknown jetting or AFR, is scary. Main reason to get away from the 26 total, is like the others said, the exhaust temps will get very high with lack of lead.

I would probably start with the timing at 30 degrees. Get your fuel right, before going any higher. Is this a single carb, or dual carb application?

As far as your mph situation, its not really adding up. Something isnt right. Are you turning the props inwards, or outwards? They should be turning in, if speed is what you seek, on that hull/drive setup.

MILD THUNDER

iTrader: (3)

I just noticed, you are running a single 825CFM carb on this application!! You are undercarbed my friend.

Food for thought, the 490HP 525SC, used a 1050 dominator, on a 177 blower. I would at minimum, install dominators on what you have, or, better yet, a pair of 750's on each engine. With twin carbs, you will get much more even fuel distribution, a better idle, cooler air intake temps, and should pick up a nice amount of power.

Heres a mathmatical equation for you to figure out carb size on a roots blower. I've found the result, to be pretty much a minimum, of what you need CFM wise. Take everything you've learned about carbing a N/A street engine, and throw it out the window. When it comes to roots, bigger is better (within reason). I calculated a 454ci turning 5500RPM, with 7lbs of boost, and came up with 1066CFM required. IMO the carbs you have would work just fine, IF you didnt have the blower sitting below them, and it was a N/A 454 with a upgraded cam.


The size of carb(s) or CFM required for a given application can be calculated by the following formula A: {(CID x RPM) ÷ 3456} x {Boost ÷ 14.7) + 1} = CFM required. The amount of CFM required will determine carburetor size and quantity. If you try to use a carb with less CFM than required, performance and economy may be greatly reduced. Bigger is not always better when selecting carbs that are 30% over what is required, you may encounter problems in fuel distribution.

Give it more carb, turn those props inwards, and call me in the morning