mesurements for trough hull exhaust

[schlosser1]

i just got a 84 diplomat with a 3.8l i/o i am putting a 350 in and want to know where to drill the exhaust holes in the hull for the tips? any pics on diplomats or sportfires would help thanks.

 

[wca_tim]

I have an 88 diplomat with a small block in it and assuming that dimensions are the same. it can be somewhat of a challenge to get things to fit well - the exhaust being the major issue. depending on how you're building the mounts for the engine in and aligning the engine and drive, I would recommend waiting to cut holes in the boat for exhaust until you have the engine and drive in and exhaust manifolds and risers you're going to use at least bolted up. I might have made a big mistake if I hadn;t done it this way. Space can be tight especially if you're not using factory exhaust. I'm headed out for a quick run down the river, but will try to dig up some pics to show you what i did later this evening. I used two soft 90's on each side with a piece of tubing between them to come out about a foot or so outboard of where my risers were pointing. I'm running imco powerflows and longer risers than factory, so this was a litle different than you'll presumably have to deal with. if I had squeezed between the power steering ram and the cap, I could have barely made it out through the rub rail. I thought that was a worse option than the way I did it. Note that i also had to cut away part of the engine compartment hatch and the cap where it fits down in the back. it is not noticable once the hatch is closed. Dave on here went with bent s-pipes and exited right at the waterline I think / if I remember correctly.

the main obsticle is the power steering unit on the starboard side.
cheers!

 

[wca_tim]

I just realized that i am not paid up on here and can't post pics. In the meantime pm me an email and i'll send you some of the way i did the exhaust in my diplomat...

 

[schlosser1]

awesome some pics would help. i will wait until its installed to cut the holes for sure. how do these boats handle the small block and higher speeds i just sold my ambassador and got this so i haven't had any time in this yet. thanks.

 

[wca_tim]

I think it handles fine at "higher speeds" Mine has a pretty healthy 383 in it that was built to run hard and I do some days. So far have been deep into the 70's (and predict 80's soon) with not a hint of unpredictable or scary handling. note that it's still a fairly light 17 1/2 foot boat and things get way too exciting if you hit a big roller coming off the back of a wakeboard boat with any speed, but I run it in open water on the sound and wide part of the river and up in the skinny parts of the river. I've taken a number of boats off gaurd. even some bigger baja's and a donzi off gaurd when I hit the throttle blow by them. of course it's an i/o, the really fast little boats around my neighborhood are outboards and make me look like I'm standing still when they get serious. but thay aren't suited for an afternoon of pulling teenagers on wakeboards, skis and tubes either...

I took these the other day to send to a friend and figured i might as well post it. a couple short clips of running in the driveway where the lid is off so you can see the exhaust and then a couple looking out the back running down the river at speeds that are comfortable to hold for miles on end when it's nice out. I apologize for the quality... not much of a camera man and besides, i was paying attention to driving... would suck to hit a snag running that fast. turning 5000 - 5500 rpms running a 27" mirage plus. I can turn it to 6,000.

ps. leave room in te budget for some kind of trim tabs - makes a big difference in planing speed and bowrise with the extra weight in the back...

sorry embedding didn't work, here's a link...

http://www.youtube.com/watch?v=8Izi3v75jTs

 

[schlosser1]

wow those are high rpms for a 27p do you have a 1:81 ratio drive?

 

[wca_tim]

wow those are high rpms for a 27p do you have a 1:81 ratio drive?

Yes on 1.81 drive... the builder who did the engine recommended with a wink that I leave the gear the same... he was right on target (at least until now when I'm making enough power that I can turn a hydromotive quad 4 29" to well over 5500 rpms). I'm getting ready to change ratios, not sure if I'm going to go to 1.65 or 1.47 though. I've got a set of hardened 1.65 gears sitting on the counter and 2 spare upper drives (both 1.47) in the garage... right next to a very nice land and sea sternjack... Am thinking I'll build an upper and cryo treat all the internals, and try a few other things people have told me along the way, but am waffling over gear ratio still. My first thought is to go with 1.65 gears. with best props being up to 29" pitch (not the hydro, but something smaller diameter that I can turn faster).

am thinking I want to go with more cam and have the heads ported next, also have a hundred pounds or so that will come out of the but end of the boat and a holley carb on the bench I just finished setting up / putting together. Also another intake or two to try. I'm probablt somewhere between 400 and 450 horsepower now with a peak a little over 5000. Am thinking I'm right on the verge of really getting the hull up out of the water on the pad... so don't know and am thinking I might be able to turn a 27" to 5000+ if I get the right one...

the props in the 27" - 29" range have been the best performers and even running a smaller blade diameter, I was slower with a 1.47 gear ratio. If you get yours set up with a decent small block, drop a line, i've got several mach 4 sterndrivers, one a 27" that might be great on it. the fastest / best al around performing prop I have right now is a mach sterndriver 28" that's been labbed and had the diffuser ring cut off. The vids are with a stock mirage 27" but I bet I could turn a labbed 29" as fast. will just have to see...

First I need to think about nose cones... I'm pretty sure that I'm starting to see gear case blowout right about 77 mph... sometimes is like it just spins a few hundred rpms faster all of a sudden and slows down just a little bit, one time felt sort of like hitting a pothole in the water...

sorry to ramble, would rather be on the river than working this morning..

 

[schlosser1]

i just had the dip out for the first time with the mini 3.8 and it went pretty good considering the size kinda scary trimmed up when you hit waves but its still fun. small boats feel faster. my ambassador went write through waves this jumps them

 

[schlosser1]

3.8 is out and 350 is in just have to hook up all the goodies

 

[daren34]

bigdave went with a 1.47 drive with his 350 still had 3 inch risers so had 3 inch aluminized pipe bent in an s to clear the p/s

he was getting 67.8 gps at almost 6000 with a 20 mach sterndriver 4 blade

mostly they use the boat for skiing and wakeboarding needs trimtabs for slalom though

the 1.47 is way better for a v-8 imo the only reason for more ratio is is you don't have the horsepower to pull a decent prop

daren

 

[schlosser1]

my turd 350 would explode at 6k know, im thinking 5200. but thats a good idea for the exhaust thanks i might use stainless for the s bend. if i kept my y pipe and had a splitter to through hull do you think i could put rubber caps over the tips when i want it quiet or would they blow off ? cheap bastard quiet choice lol.

 

[daren34]

soup cans with holes drilled in the end put in closed end first use set screws to hold them in place

cheap baffles

leave enough room for the water to flow around them

oh most alpha's don't like engine speeds over 5000 rpm
no forced lube for the bearings

daren

 

[wca_tim]

bigdave went with a 1.47 drive with his 350 still had 3 inch risers so had 3 inch aluminized pipe bent in an s to clear the p/s

he was getting 67.8 gps at almost 6000 with a 20 mach sterndriver 4 blade

mostly they use the boat for skiing and wakeboarding needs trimtabs for slalom though

the 1.47 is way better for a v-8 imo the only reason for more ratio is is you don't have the horsepower to pull a decent prop

daren

that depends on what the goal is. best props for speed once you're talking about 50 or so and over have pitch of about two times diameter. for props with enough (but not too much) blade area and diameter for alpha and bravo sterndrive boats, that's prop pitches in the 27 - 30+ inch range. I tried both 1.47 and 1.81 gears in mine.

The boat pulls well out of the hole with either gear / prop combo and is just plain all around faster with the higher gear ratio and any of the props I have in that range. I have several 27 and 28 inch mach sterndrivers (one 28 labbed by bblades with diffuser ring cut off one 27 with more cup added and a 27 and 28 stock), a 27 mirage, a 27x14.5 spinelli, and some others. I ran the 1.47 with a 22 hydromotive, a 23 laser 2, a 21 solas 4blade, etc... we're talking at least 3 or 4 miles an hour difference at wot. Also, if you run a higher gear ratio, it spins the drive slower at a given speed... the trade off is more torque on the lower gear set in particular. but with a light little boat, as long as you don't stiff arm the throttle or catch air (and not throtle back before prop catches again) it is not as hard on it.

note also that a higher pitch prop spinning slower will usually go to highe rspeeds before blowing out as well.

You're probalby talking about running something in the range of a 25" prop with the 1.8 drive.... and runing low - mid 60s? wag based on limited info regarding your engine set-up. whatever you do about gears, run a good gear lube...

 

[wca_tim]

my turd 350 would explode at 6k know, im thinking 5200. but thats a good idea for the exhaust thanks i might use stainless for the s bend. if i kept my y pipe and had a splitter to through hull do you think i could put rubber caps over the tips when i want it quiet or would they blow off ? cheap bastard quiet choice lol.

y pipe with splitter = corsa quick and quiet (with mufflers out the back, splits exhaust lowers backpressure and quiets a decent amount too). have one in the garage attic, wasn't room to rny it with the risers i wound up using..

if you need tubing... I order 4" od 0.065 wall 304 stainless from these guys once in a while.. it comes raw but shines right up...

http://www.speedymetals.com/

good luck with it!

 

[schlosser1]

thanks guys. i will fool with props when it makes it to the water. my 1:84 ratio is leaking like crazy from the top yoke, looks like the guy before me reused the outdrive gasket and the sides were leaking water but i have a drive off a 305 with no ratio stamped into it so i will try that one for know. also witch one works better cambles chunky or regular? lol.

 

[Big Dave]

We pulled a 3.7l 4cyl out of my Diplomat. Had to make stringers for the v8 because the 3.7 mounts like a 3.0. Daren34 made a block-off plate for the stock exhaust in the transom. I had a 228 Merc (305 v8 4bbl) out of a boat we scrapped. I used cardboard to mock up the pipes below the steering and had a local muffler shop bend 3" inside dia aluminized pipe. used a 3 1/4 holesaw and then beveled the holes to accept 1/8 flax packing and 3m 5200 sealer just like the old wooden boats.
make sure to put 2-part epoxy inside the cut-outs to seal the transom wood. L'il Dave blew up the 305 so we put in a forged internals 350 with way too much cam and a holley 650 double pumper with both power valves blocked off on an edelbrock intake. with a 1.5 drive and a 22p Mach Sterndriver 4 blade L'il Dave backed out of it at 69.41mph at 5700 rpm due to rough water. last year we had to rebuild the drive using the HD gear set and all new bearings top and bottom from redline marine due to too many rpm. the wife said no to a speed prop!

 

[schlosser1]

hey big dave any close pics of the exhaust and pics of the motor? i just had the checky out tonight with the v8 and it seemed great for 5 min till i ran out of gas and had to paddle back across the lake with my uncle tomorrow i will fix the gauge and try again with gas in the boat then when my tips come a cutting i will go.

 

[Big Dave]

click on the pic's above and you can zoom in.
I thought I had motor pic's but I can not find them
I will try to take more next time I get home.