OMC ford 302 stroked to 347

"Good day all,
This is my fir

"Good day all,
This is my first post here, I just found the page. Up until now the only thing I had to worry with about the boat was time to go to the lake. Made it out for the 3rd trip this year and had some trouble. I blew out the drive coupling. This was only the third trip out this year and since I had the foot worked on (upper bearing carrier seal replaced). Before putting the boat in I noticed that three of the six lock nuts that hold the upper gear case on where gone. My boat stays locked up so this is not a case of theft. I went to a local marina and got three new bolts and tightened them back on. I got the boat in the water and took it out on a quick test drive before heading out. About 2.5 hours and miles from my put in the coupling let go. I have contacted the shop that did the work and they are playing dumb. Is this a case of a drive coupling failing after 16 years of service or bad work? My boat will not be going back to this shop unless they make this repair at a reduced price or free.
So this brings me to the topic. The motor has to come out for the coupling repair, now seems like a good time for a rebuild. I have been thinking about a 347 stroker motor. I hope to get about 300 hp. This would be a huge jump from my current 190. What concerns should be considered, ransom strength, Cobra 800? Really want to hit break the 50 mph barrier. Boat runs GPS 40 mph loaded with 2 barrel Holley, seasoned 302 and 14 ¼ x 19 aluminum prop.
Thanks"

"Ted, don't take this pers

"Ted, don't take this personally but the coupler has just gone bad from age. trying to get the shop to change it for free is unreasonable. I could understand it if they had removed the engine and not aligned it properly but from the post this is not the case. Parts wear and this a case of stuff happens."

"You don't say what year b

"You don't say what year but I'm assuming around 1990 as you said the coupler was 16 yrs old. I guess it could have gone out due to age and usage but more importantly did that shop check the engine to gimbal alignment before putting the drive back on? There's nothing like an alignment problem to take out the coupler. As for the engine HP your drive will not take that much HP that's why OMC brought out their big drive in the late 80's early 90's that bolted on the 7.5 ltr (460), around 340 HP."

As for your 300 hp you will de

As for your 300 hp you will definately find the weakest link.

"Bob:
There is no alignment n

"Bob:
There is no alignment needed for the 800 drive/stringer systems. As for Hp load on the drive, go above 250/4800 rpm and You will be in trouble! Intermediate shafts and upper unit just to mention some."

Morten:

Volvo sold that sam

Morten:

Volvo sold that same unit with a 454.

Jeff

"My comment was generally base

"My comment was generally based on experiences with the 350/351 with 4V carbs and numerous change of lower driveshaft as the splined top broke off. This mainly on boats that was run 'hard'. Another problem is the lack of oil cooling in the upper drive and limmited oil volume for cooling. Some of the above mentined units I resealed EVERY winter after 80-100 running hours, and the seales were almost totally burned, and sometimes I even encountere 'blued' out shaft splines!
How many of the 454 combinations are still running??"

"Thanks for all the great repl

"Thanks for all the great replies guys, I will post some links to some gear sets I found this evening that are rated for 300 hp. Busy day at work today, Thanks again."

"Another thing to consider- is

"Another thing to consider- is the boat going to be safe at 50MPH? If the additional HP is enough to get it over the top (based on the hull design, it may or may not), it doesn't necessarily mean that it should be run at that speed. If you want a speed boat, buy one. Hard acceleration will be the worst thing for the drive- you decide whether it's going to be worth it.

You didn't say what kind of boat it is, either. Not much for us to go by.

You could probably get close to the speed you want by replacing the intake manifold, carb and cam. Then again, "there's no replacement for displacement". Are you going to freshen up your original motor or use a different one? A 351 would be a good alternative. I know of someone who changed the intake and installed a Holley 4150 on his Mastercraft. Definitely better low end and all the top end he needs/wants."

"Morten...my comment was based

"Morten...my comment was based on the fact that I thought he had a Cobra not a stringer (I know that Stringers don't need to be aligned) He said "three of the six lock nuts that hold the upper gear case on where gone"

Ted...maybe you could clarify exacly what type of drive you have, does it have a big black rubber boot around it or does it have hydraulic trim rams on the drive similar to Mercruiser?"

"Guys thanks again for all of

"Guys thanks again for all of the input. I do believe that my drive is a cobra 800 and is not the stringer style. It does have a trim ram on each side and I have never seen the big black boot. To answer a couple of the other questions: The boat is a 23’ Berretta Success bow rider. I will try to find a good picture of it tonight to post. I believe this boat would handle great at 50 or 60 MPH. If I take it out with a light boat it feels great flat out at 40+. The idea behind the stroker kit is if it needs to be rebuilt then why go back with a stock setup. I do not want a speed boat I just want a faster pleasure boat. Money is a concern on this so a repower is not an option at this point.

My trouble with the work that was done is the fact that the three bolts where missing, the only tight ones where on the bottom of each side. The chances that these fell off during my 2 mile trailer trip to or from the lake is unlikely at best. This would suggest that I operated my boat for an unknown amount of time out of alignment (alignment would have to be affected by the fact that the drive was not on properly). I most likely will not pursue the “free” repair, but I do believe that the two are related.

The same shop told me that the compression on all 8 cylinders “looked good” but offered no readings on the compression check I requested. So a rebuild may not be needed it just feels like now is the time to do it. I will be going with a 4 barrel carb and intake (Edelbrock 600 cfm and intake) in the next season regardless of motor displacement. As far as the cam goes, I though there was a lot of concern with changing the cam profile do to the possibility of sucking water back into the cylinders from the exhaust manifolds. What is a good cam to run in this motor?

The big drive that was mentioned, is this the King Cobra? What are the chances that my transom can take this drive? Is the gimbel plate the same?

Have you guys ever played with any of the boat speed calculators? Do you think they work? I know my boat speed(GPS) and H.P. (190) but do not believe my boat is that light. Something looks wrong.
Thanks again"

"OK...please stop calling it a

"OK...please stop calling it a Cobra 800, it sounds for sure that you have a Cobra but the 800 and 400 were models of the Stringer drive. The early King Cobra version of the Cobra for the big blocks just used a beefier lower unit with bigger gears to take the stress, I would be more concerned about your transom taking the torque of the more powerful engine as the rear of the engine attaches to the transom mount which is attached to the transom. If the stern drive was loose on the transom mount then that could certainly affect the alignment and coupler. I can't believe anyone would work on a drive and send it out missing 3 of those nuts."

"Thanks Bob, a little touchy a

"Thanks Bob, a little touchy about the 800 thing arent you. Okay so what i am taking from your post is that i have a regular Cobra drive and that I may be possible to put a King Cobra lower unit on my drive. Is this correct? I will be going back to the boat in a few weeks i will get the model / serial number off the drive at that time.
I do not believe that the drive left the shop with out the bolts i would have noticed that and i to would find it hard to believe that they did not install them. I believe that they vibrated off due to the fact that the 16 year old nylon lock nuts where reused. Improper torquing could have played a factor as well.
I have been looking for information on my hull on the web and did find a cbin cruiser model with a big block in it that looked very simular to my boat. I will try to get a picture of by boat onto my avatar.
the transum has been a concern for me as well. I struggel with the affects of this power and torque being used only on the top end, i would not be blasting out of the hole.
My boat is mainley used for ware sports a cruising. Thew only time it would see WOT is on high speed runs. I geuss what i am asking are the two stresses the same. Faster boat=more power=more stress. If it was operated responsibly do you think it would hold up?

Jim do you have any idea what cam profile your buddy installed?

Thanks for all of the help guys."

"Sorry Ted...didn't mean t

"Sorry Ted...didn't mean to come off that way, it's just that we rely on the info you give us to try and give you meaningful responses and when people mix things together it can get confusing.

I think you would need to put the entire King Cobra drive on, not just the lower unit."

"No good pictures of the boat

"No good pictures of the boat in the water, Maybe next year.
No trouble Bob i was just giving you a hard time. Any body got a King Cobra they want to sell cheap?"

"Well at least we have got the

"Well at least we have got the drive type correct, and anyway perhaps my 'mistake' on top was not wasted if anyone else was on the idea of beefing up their stringers.
To encrease Your present power up to around 260+ should be no problem, but depending on gear wear in the drive it at least may be wise to change the gears and reshim the unit. What even might be a thought is to take the exhaust through the transom and not the drive to reduce back pressure and more important take the heat off the drive."

"Okay guys, I have almost talk

"Okay guys, I have almost talked myself into doing this swap. Below is a link to some heavy duty gears for my current Cobra drive. So here is the plan: preform motor swap, to go on the safe side I will most likely go 331 instead of 347 and shoot for about 275 hp. If i run into drive issues i will install these gears. Morten i did not think about the heat from the exhaust affecting the temp of the unit. Not sure i want all of the noise of the thru the hole and silent choice is so expensive. I will look at doing some type of drive cooler. Thinking more about the transom and drive strength, I came up with a car analogy. You could easily rip the rear end out of a car with a hot motor during a drag style launch, this same rear end would most likely handle a WOT stump at cruising speed. Any imput on the analogy. My goal is a faster boat not a brooken one.
Thanks guys
http://www.sterndrive.info/heavy_duty_gears.html"

A 350/351 will be no problem w

A 350/351 will be no problem with reference to the transom and engine mount even if You tune it up to the mentioned hp range providing the transom is not rottened out and the engine mount stringers are in good condition! You may add a lot more hp before the engine 'passes' your steering possition on a hole shot!
A silencer system and through transom exhaust is probably a lot cheaper than a burned down drive!

"After doindg some more resear

"After doindg some more research on marine ford engines it looks like a 351 might slip right into place for the cost of the stroker kit. Am I missing something here? More cubes less money, not concerned about the weight difrence since we wake board behind the boat any ways. A 351 setup should get me to the power level I am looking for. I am going to search this forum and the web and see if i can find some guys who already did this swap. Thanks guys"

Hi, so its been a few years on this post.. (close to 10 haha,) Driftwood did you ever end up with the 347 stroker upgarde? how is your fuel consumption?

18fourwinns said:

Hi, so its been a few years on this post.. (close to 10 haha,) Driftwood did you ever end up with the 347 stroker upgarde? how is your fuel consumption?

Click to expand...

Ayuh,.... Driftwood hasn't been here since,.....
Sept. 14th of 2006,.....
That's closer to 20 years,.....

ugh ! wow.. I don't know what I was thinking.. this post is history !!!