do I need thru transom exhaust?
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do I need thru transom exhaust?

  1. #1
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    Default do I need thru transom exhaust?

    Hello, I bought a rinker flotilla witch the original owner replaced the small block with a kieth black built 502 that dynoed at 570 horse. The exhaust being routed thru the original bravo three out drive. Out of the hole it pulls like a train but no matter what pitch prop I use I still cant get the prm's over 3800 to 4200. rpm's were confirmed with a laptop off the cpu so I'm sure it's right. I'm thinking the exhaust is getting restricted in the out drive and I need headers and thru transom exhaust. Does this sound right?

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  3. #2
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    Default similar boat

    I have a 97 flotilla that the engine is out of right now, it was a 260hp,5.7lx, Im in the process of installing a completely rebuilt 98 502mpi mercruiser engine, I have already drilled the transom and installed the 4" exhasut tips,engine is going to have a complete imco power flow plus exhaust system, Im also changing to lower end of my bravo3 to a bravo 1 with bullet nose cone, I should end up with a 1.65 final drive ratio and going to start with a 26 pitch 4 blade,Ill let you know what happens Im about 2-3 weeks out for final completion. note the exhuast could have too much back pressure, but installing headers is not going to help your stock manifolds should work fine but was the engine dynoed with stock manifolds or headers, if it was dynoed with full headers and now has stock manifold on it you could be giving up 50+ hp, and im not sure how much power you cam make through the outdrive exhasut, did not previous owner give you any infomarion on how the engine,drive,hull did when he owned it or was he never able to finish it and run it, note there are bravo3 out there with as low as 1.65 ratios maybe even less, do you know what ratio the out drive is for sure?? maybe I can help more once i get mine going, let me know what happens with yours PLEASE its very very hard to find anyone with any infomarion on what these boat do modified,THanks!!!!

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    Default 502 not breathing!

    Quote Originally Posted by 502FLOTILLA View Post
    I have a 97 flotilla that the engine is out of right now, it was a 260hp,5.7lx, Im in the process of installing a completely rebuilt 98 502mpi mercruiser engine, I have already drilled the transom and installed the 4" exhasut tips,engine is going to have a complete imco power flow plus exhaust system, Im also changing to lower end of my bravo3 to a bravo 1 with bullet nose cone, I should end up with a 1.65 final drive ratio and going to start with a 26 pitch 4 blade,Ill let you know what happens Im about 2-3 weeks out for final completion. note the exhuast could have too much back pressure, but installing headers is not going to help your stock manifolds should work fine but was the engine dynoed with stock manifolds or headers, if it was dynoed with full headers and now has stock manifold on it you could be giving up 50+ hp, and im not sure how much power you cam make through the outdrive exhasut, did not previous owner give you any infomarion on how the engine,drive,hull did when he owned it or was he never able to finish it and run it, note there are bravo3 out there with as low as 1.65 ratios maybe even less, do you know what ratio the out drive is for sure?? maybe I can help more once i get mine going, let me know what happens with yours PLEASE its very very hard to find anyone with any infomarion on what these boat do
    modified,THanks!!!!
    Thanks for your input. The original guy avoided staying on the subject long but if I was reading between the line correctly, this was a problem for him as well. It may have been one reason for selling the boat. I say that because he had 28's on it and I couldn't get rpms over 3200. I've spent a lot of money and worked my way down to 22's and I still can't get over 3800, 4200 if it's just me in the boat. I understand 22's are the thinnest pitch props mercury makes for the bravo three and I'm not sure what the drive ratio is but I can't think of why mercury would make a drive for a prop pitch they don't make. It is a curiosity though.
    Your exhaust holes, did you run them under the swim step or over it? Seemed to me there would be no room for them below.
    I don't mind spending the money for a good system if it's going to help but so far this is one question no one has the answer for, how much cfm can a bravo three flow?
    Is there a benefit to the bravo one over the b3? to my understanding of hydrodynamics, the less amount of contact with the water, the better so,,, why to props? I can't complain though, We love to entertain and I can put 14 people on the boat and still pull up two skiers, it's just that top end thing. I know I've got more speed in there and I can't get it out. I'm not so much about making it a 70 mph boat because of all the wetted area of it's trihull design I don't think the flotilla will ever be good speed boats anyway. I just want it to do the best that it can.
    I don't know what manifold were on it when dynoed so I'm no help there but the manifolds, albeit heavy, should flow fine. It's that the drive was (i'm told) never designed for a big block. It's the only thing that makes sense. I was thinking of teeing into the exhaust with a preasure gauge and take it out. Please let me know about your build.

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    Default exhaust

    My exhaust is through the transom below the swim step at about water level nearly as wide as I can go and still be on the flat back of the center "pontoon", I had to reposition the deck drain that was directly in my way of drilling the 4" hole for my exhaust on the left side of the boat, The reason I went to the bravo1 bottom is because I knew that I have been running the B3,2.2 ratio, with 28 props on it with my 5.7,that I was going to be under proped when I up the HPs go to nearly 500 I had to change something Instead of changing upper gears in my gear case It would be easier to bolt on a B1 bottom and give it a try, I at least have alot of prop choices, my friends a me combined I have a 24,26,28 all 4 blade labbed props for bravo1s and a 3 blade 26 also, so I got a B1 bottom for 500$ and Im going to give it a try,Ive heard that Im going to loose some bottom end because the B3 really pulls out of the hole, but Im hopeing the torque of the 502 in a 3700lb boat that it should still have a good hole shot,I also normaly have 8-12 people on the boat.Of course all this is just a GIANT PRAYER, since I have searched high and low and have found NO ONE that has yet to put a big block in a 24' flotilla except you,hopefully we can stay in touch and get both of our "TILLAS' dialed in.THANKS-MATT,PINON HILLS CA -951-288-2451, If you want to give me a call its all good till 9pm pacific time..

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    Default bb floats

    Quote Originally Posted by 502FLOTILLA View Post
    My exhaust is through the transom below the swim step at about water level nearly as wide as I can go and still be on the flat back of the center "pontoon", I had to reposition the deck drain that was directly in my way of drilling the 4" hole for my exhaust on the left side of the boat, The reason I went to the bravo1 bottom is because I knew that I have been running the B3,2.2 ratio, with 28 props on it with my 5.7,that I was going to be under proped when I up the HPs go to nearly 500 I had to change something Instead of changing upper gears in my gear case It would be easier to bolt on a B1 bottom and give it a try, I at least have alot of prop choices, my friends a me combined I have a 24,26,28 all 4 blade labbed props for bravo1s and a 3 blade 26 also, so I got a B1 bottom for 500$ and Im going to give it a try,Ive heard that Im going to loose some bottom end because the B3 really pulls out of the hole, but Im hopeing the torque of the 502 in a 3700lb boat that it should still have a good hole shot,I also normaly have 8-12 people on the boat.Of course all this is just a GIANT PRAYER, since I have searched high and low and have found NO ONE that has yet to put a big block in a 24' flotilla except you,hopefully we can stay in touch and get both of our "TILLAS' dialed in.THANKS-MATT,PINON HILLS CA -951-288-2451, If you want to give me a call its all good till 9pm pacific time..
    Hey Matt, I didn't know you were in pinion hills! I'm just on the other side of the mountain in rancho cucamonga. Yea, lets keep in touch, 951 833 0076 or [email protected].

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    Default almost done

    Well I figured Id give you an update, Im almost done, engine is in, exhaust and all electrical and plumbing is done, working on finishing some small details and then I can put the outdrive on which should not take long, I have ran engine in boat and it sounds good, going to see what happens soon,maybe this weekend, Ill let you know what happens.Matt

  9. #7
    Senior Member EVILFORCE's Avatar
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    Default

    Yes. You either need Captains Call exhaust or run them thru transom.

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    Default Thanks

    Quote Originally Posted by EVILFORCE View Post
    Yes. You either need Captains Call exhaust or run them thru transom.
    Thanks for the fedback, I'm having the exhaust installed now. We'll see.

  11. #9
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    Default results

    Well, hereswhat happen with my conversion so far, first trip,, got the 502 mag mpi with imco power flow exhaust through transom. Im running a 1.65 gear ratio bravo one with bullet nose cone and a 28 pitch 4 blade mercury bravo labbed prop, engine turning 4900 rpm and boat is doing 51 mph now the bad part is the is a problem with either the prop rake or drive depth becaust the boat is untrimable, no matter what you do the front end blows any thing over 1/3 throttle and the more throttle you put to it the more the front drops. you can trim it till its throwing a 5 foot roster tail and water is breaking at the front of the hull .as a result the boat became hard to drive do to tracking BAD. I have contacted several prop shops and there answer was they can change prop rake and it will help but may still not be good enough.. SO second trip out I took the B1 bottom off and went back to the B3 still running the same ratio as when I had the 350 in the boat which is a 2.2 gear ratio Im running a set of 28 pitch props and out to the lake I went, out of the hole acceleration is CRAZY, Now on the top end I have a major RPM issue at HALF THROTTLE Im running 4900 RPM and about 50 mph, at full throttle its on the REV limiter at 5150-5200 right away, how ever the boat is back to being trimable again, with the B3 trim is totally normal you can trom the front up no problem. but RPM is now my factor, I really need to get these RPMs down BAD just not sure how far to go. Im in the process of changing the upper gears in the gear case to bring rpms down, going as far as I can go in the upper end at this time Im going to end up with a 1.81 ratio with my B3 bottom this should lower my RPMs by 20% which Im thinking then at half throttle Ill be at about 4000 rpms instead of the 4900 im at now, Then Im hoping that when I go to Full throttle I end up around 4800-5000, and maybe pickup 4-6 mph on my topend, however so far everyone i have talked to says to go to a 1.50 ratio and prop from there, for me to go to a 1.50 ratio B3 I have to regear both top end and bottom end of drive or find someone selling one with the 1.5 ratio in a B3. I had too get RPMs down and Im going to try the 1.81 ratio first and see what happens you got start somewhere if you know what I mean.. Good luck with yours, all I can tell you is with The through transom exhaust you should easily be able to turn that engine 5200 rpm unless your drive ratio is something crazy like 1.36 and its just overloaded the engine because of overgeared. note I would find out FOR SURE what gear ratio your running.Please let me know how it goes. Ill keep you updated on my gearing problem.Thanks

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