omc cobra o/d question

4.3sunbird · Sep 9, 2005

hello, Im new on the board . I go by Red I got a 92 sunbird corsair 198 cuddy It has a 4.3 in it main question is It has had the out drive replaced before and the tag on it says 3.0 engine would this be geared different than a 4.3 one ? I got it with a cracked block and just finished an engine build for it. I built it up some by using a roller cammed block and going with an eldelbrock hydrolic roller cam , I raised compression slightly by putting flattop pistons in it , I also went to 96-up vortech heads and matching 4 barrel intake. running a q-jet carb I put it in the water for the first time today with the 13 1/3 x 17p prop the last owner had on it for pulling ski's in a small lake with it I was turning way too much rpm

so I went in and put the factory alum 14x21p prop on it and Im still turning too much rpm at wot (keep in mind it makes alot more power than stock) at wot with the 14x21p Im turning 54-5500 rpm by the speedo Im showing 58 mph I/d like the boat to run 60+ at say 5000-5200 rpm so Im wondering if the o/d may have more gear reduction in it than factory or is that speed / rpm about right for 21p Im looking to put a 23p on it now but was wondering about gear ratios btw: I launched it a couple times wot from dead stop and it spun the rubber in the factory prop

just for more info it had a hydrofoil on it ..I tried it out with it then removed it and the boat likes it on it .. comes up on plane much better with it and there was no mph change at wot with or without it can anyone help me out with this .. Im new to boats .. this is my first one

thanks, RED

whywhyzed · Sep 10, 2005

Re: omc cobra o/d question

You are bang on.. That drive's ratio is for a four cylinder boat. You will need a V6 ratio drive. You can change gears to change ratios, but earlier cobras used lighter duty lower units. Basically a "400" series lower. So you'll want to make sure that drive isn't an early one. 400 lowers use 13 spline props instead of 15 spline like the heavier duty lowers. You might consider a V8 ratio drive as you obviously have some serious HP. Don't expect to see any higher than 58 mph though, at 5500 your valves were probably not yet floating so you saw your max power there, but you will keep from blowing the engine up by changing ratios. No wonder you were spinning the prop hub with that gearing... that's monster torque to the prop...

Lou C · Sep 10, 2005

Re: omc cobra o/d question

The answer is yes the gear ratios were different, you probably need to go with the 4.3 lower ratios. Overall ratios are: 3.0: 1.71:1 4.3: 1.68:1 upper ratios 3.0: 21:18 4.3: 21:19 lower 3.0 13:26 4.3 14:26 In addition to that the lower is slightly larger on the 4.3 models, it takes 64 oz of gear oil intead of 60 for the 3.0. I have a 14 1/2 x 19 prop on mind and it pulls 4500 rpm at full throttle which is just about right.

4.3sunbird · Sep 10, 2005

Re: omc cobra o/d question

thats what I was afraid of .. for now Im trying the cheaper route I have a hustler hub for it so I ordered a 23p prop for it and going to see if that helps I dont think it'll have a problem picking up more speed ..I built it with a power band that runs out by 5500 so Im basically just out of my peak power at the rpm its turning now It pulls real good up to around 5200 then you can pretty much feel the power falling off of course it will rev more but just not making power it went to around 6500 when it spun the stock prop but of course way out of its power range Im use to building high powered v6's (6-700hp)but they are normally turbo charged

all the bottom end in this 4.3 is deburred , shot peened ,arp fastened , and balanced to try and add some durability to it I guess I'll try to make out this season on the 23p prop then do some outdrive hunting over the winter months I may get my buddy to let me try his cobra outdrive off his v8 boat .. his is running low 60's (mph) with a 19p turbo prop I'll definatly let you'll know how the 23p hustler prop works out thanks for your reply's RED

whywhyzed · Sep 10, 2005

Re: omc cobra o/d question

I can't imagine the look on people's faces when a Sunbird cuddy passes them at 60mph+...

4.3sunbird · Sep 10, 2005

Re: omc cobra o/d question

I did run side by side with a big scarab

I heard his at the dock and it sounded stock and was about 6 ft longer than my lil boat cant wait to try the 23p prop I went out tonight with a hustler 19p on it but was out with the family watching a fireworks display so I didnt get to lay into it I'll probly be back on the water tomarrow to try it out .. gonna have to go to more inland water tho ..with that storm brewing off the coast the waters getting a lil too rough by the coast I did hit it one time to shoot it up on plane .. mainly wanted to make sure it was my prop that spun and not the engine hub .. it was fine on this prop so thankfully my engine hub is ok

4.3sunbird · Sep 11, 2005

Re: omc cobra o/d question

I'll be glad when my 23p prop gets here was on the water today with the hustler 19p was showing ~55mph at ~5700 rpm It was pretty rough out today and only going to get worse till this storm goes thru this week which hopefully it wont be a direct hit here and can go away by next weekend

Im not far from the yellow dot in the bottom right corner of virginia http://www.hurricanetrack.com/Java/westatl.html so far looks like the outer banks of n.c. is going to get hit then hopefully it bounces out again wow Im sunburnt

walleyehed · Sep 13, 2005

Re: omc cobra o/d question

Red, #1, I haven't seen any real good numbers on Hustler props. If you want top speed, you're gonna need to get set up with a Turbo made specifically for sterndrives. These props, as produced, will run in the 8000-9000RPM range on outboards and I don't think spinning the hub will be an issue...your 3.0 ratio is too close to my 200 Rude to make me believe the hub will spin. A Turbo in a 21 will bring the R's down some compared to the Hustler, and should show better top-end numbers...the efficiency will be much higher.

4.3sunbird · Sep 16, 2005

Re: omc cobra o/d question

well since I spun the hub in my hustler set-up .I bought the new design hustler set-up .. I dont think I'll be able to spin this hub I got the 13 1/4 x 23p prop to go with it .. bad part is it came in today (friday) but I cant get out on the boat to try it till sunday

problem being is the 3.0 outdrive with the 1.71 gear reduction a 19 pitch with no slip would be 58mph at 5500rpm but no prop has 0 slip I'll post back on the hustler 23p sunday eve a buddy of mine has a turbo prop on his boat and I show about 95% efficient doing the math on his .. so far I've seen 94 - 94.5 % efficiency with hustler alum ... not bad huh

Don S · Sep 17, 2005

Re: omc cobra o/d question

I could be wrong, but wasn't the 3.0l drive smaller (bullet dia.) than the 4.3 and 5.7L engines? And much lighter built gears?

Lou C · Sep 17, 2005

Re: omc cobra o/d question

I don't know about the uppers for sure but they look the same to me but with different gear ratios for the different engines. The lowers are defineitly different, the one for the 4 cyl motors is smaller, and the drive for the 4 cyl motors holds less oil as well (60 vs 64 oz).

lakelivin · Sep 17, 2005

Re: omc cobra o/d question

4.3- you obviously know your way around engines, but since this is your first boat, just a reminder that there are some components (mostly electrical) where marine applications requires different parts than automotive applications. It may well be that you're more aware of the what and the why of this than me, but since the outcome of using some auto parts in a boat could result in the 'big bang effect' (I mean true explosion, not just trashing the engine), thought it worth the risk of offending someone who is obviously much more knowledgable about engines in general than me to provide a reminder. Good luck. Like hystat said, I'd love to see the look on the faces of skippers with big $ boats when you blow by them in your small '92 Sunbird, lol.

4.3sunbird · Sep 17, 2005

Re: omc cobra o/d question

I still have all marine electrical items on the boat , starter , alt , ignition , etc the only thing external on the engine not marine is the carb I posted pics in the show your boat thread yesterday as far as outdrives from the research I've done so far the uppers are the same size wise the lower unit is the small (400) series with the 4 1/4 gears and 13 spline shaft so far so good with the outdrive but Im sure its a matter of time.. for now I look at it as an advantage since I have a smaller lower unit to push thru the water

Im having fun with my lil boat

one of my buddys said " you must think you have a big jet ski"

dont worry about offending me .. your right I know engines but all this other boat stuff is new to me

4.3sunbird · Sep 17, 2005

Re: omc cobra o/d question

ha couldnt wait .. dropped it in the water awhile ago but it was dark out and at speed its hard to play with trim button and watch for stuff in the water alot of crab pots and stuff out were I went I did pull it up to 59 by the boat speedo ...I think I can get it trimmed out and get my 60 goal less fuel would help also I was right at full tank I need to go weigh the boat ..It feels mighty heavy towing it for its size ..Im starting to wonder if it has water in the foam ..may not be the case but it sure feels heavy another reason Im thinking that is under my side cushions in the cuddy it has storage area which is suppose to be dry well I opened one the other day and it had about 5-6 inches of water in it

when not in use I keep the boat covered so it didnt get rain in it I'll find out for sure once it gets cool out Im gonna cut the floor up and redo it .. and move the under floor fuel tank forward alil so I'll be ready if I deside to swap out to a 350 motor right now Im about 3/4 inch shy of enough room for the crank pulley on a v8 hopefully the weather will be nice tomarrow and I can go play with it sommore Is there any way to check for wet foam like drill a hole in the floor and stick something down in the foam and see if it comes out damp ? If so I'll do that then just plug the holes till winter time

vicsponjr · Sep 18, 2005

Re: omc cobra o/d question

How is your tongue weight? Should be 5-7 % of total weight, boat and trailer. If it isn't right it might make it feel heavy while trailering. Besides, with a 20ft cuddy doing 59MPH, you seem to have it licked and need to find something else to fix.

lakelivin · Sep 18, 2005

Re: omc cobra o/d question

Sounds like you've invested quite a bit of time/$ in your boat. You aren't keeping the exterior dirty & scuffy looking, pulling one of the plug wires and limping around till you find a Mark in a big, new, expensive boat, and then goading them into racing you for $ are you, lol?

4.3sunbird · Sep 18, 2005

Re: omc cobra o/d question

that sounds more like the street scene

well I just came in from drilling holes in my boat

I was right sad to say I went in the bildge by the engine and drilled thru the sides into the foam cavities and I now have water coming in the bildge from both sides

well one possitive note is that once I fix the water logged issue which wont be till winter . the boat will be lighter and faster

being as I have holes at the bottom of the foam cavities at the rear I drilled a hole in each side up in the cuddy at the top of the foam cavites so Im goin to attempt to put alil compressed air in from the front and see If it will help push water out the rear...I know I cant dry it out but the more I can get out the better

Im still going out on the water later this eve

4.3sunbird · Sep 18, 2005

Re: omc cobra o/d question

yeah finally seen 61 by boat speedo

my buddys boat only runs 40 .. he set his dead on 37 mph and I was only showing 33 mph

he has ran his boat up beside several jet fleas and his has shown the same as the jet flea speedo's so Im really wondering what Im really going when my speedo says 61

guess I need to find someone with a gps I was turning around 5200 rpm when It went 61 and was at 1/2 tank of fuel with no prop slip it would be 66.23 mph so Im showing about 92.5 % efficiency with a hustler 13 1/4 x 23 prop if the speedo is right

vicsponjr · Sep 19, 2005

Re: omc cobra o/d question

Did you try chucking the seats? And that damn anchor has got to weigh about 8-10 lbs.

omc cobra o/d question

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