I/O Swap... Omc 800 to alpha 1.

[fabrimacator21]

What all is involved? I priced the I/O itself and it's more reasonable then I thought here... http://www.boatmotors.com/motorparts/index.cgi?pid=7507&flag=2&cyl=8&year=1978&state=ca

But what will be the costs after I get the I/O? Shifting assemblies, gearcase, etc.

Please bear with me as I am a complete newb when it comes to I/O's... I can pretty much figure anything out with some direction though.

Will I be able to use my OMC shift cables?

 

[Bt Doctur]

Re: I/O Swap... Omc 800 to alpha 1.

You dont want much, do you.
http://www.mercruiserparts.com/Mercruiser_Sterndrives.asp
pick one.
steering cable, control cables, rear motor mounts for engine, Y pipe,fiberglassing in the hole (structural work) cutting the new hole in the right place,
or
http://www.mercruiserparts.com/mercruiser_gas_sterndrive_engine.asp
still have the fiberglass work to do.

 

[fabrimacator21]

Re: I/O Swap... Omc 800 to alpha 1.

lol....

little to rich for my blood.... looks like I'll just hot rod the 305 up to 290hp and call it good.


Thank you.

 

[Gary H NC]

Re: I/O Swap... Omc 800 to alpha 1.

Don't forget the time and cost of a transom rebuild,
That big hole from the stringer drive must be filled in...

 

[scottacus1100]

Re: I/O Swap... Omc 800 to alpha 1.

lol....

little to rich for my blood.... looks like I'll just hot rod the 305 up to 290hp and call it good.


Thank you.

sorry for being a newbie, but how can i get my 200hp 305 omc to 290 hp?

 

[Howard Sterndrive]

Re: I/O Swap... Omc 800 to alpha 1.

sorry for being a newbie, but how can i get my 200hp 305 omc to 290 hp?

You can't.
You can get to 230 though with a 4 barrel carb and intake.
It's a nice swap that can be done without pulling the motor, and you'll feel the difference and gain 2 to 4 mph top end most boats.

Any more than 230HP would require more displacement. 305 cubic inches can't make any more at the rpm's boat engines are limited to (<5000 rpm)
350 or 383 swap is the easiest and cheapest way to go if 290 HP needed


I didn't understand this thread from the start.
Swapping to mercruiser doesn't gain any HP.
I don't know how hot rodding the engine is an alternative to going to a Mercruiser drive.
Almost polar opposite objectives if you ask me.

 

[QC]

Re: I/O Swap... Omc 800 to alpha 1.

I don't know how hot rodding the engine is an alternative to going to a Mercruiser drive.
Almost polar opposite objectives if you ask me.

If you have $722 set aside to swap out outdrives and then you find out it costs $7,722, you take your $722 and consider performance mods. Simple there Howard . . .

 

[scottacus1100]

Re: I/O Swap... Omc 800 to alpha 1.

is it hard to swap the 305 out for a 350? and can my 305 outdrive handle a 350? *Sorry for highjacking your thread.

 

[fabrimacator21]

Re: I/O Swap... Omc 800 to alpha 1.

You can't.
You can get to 230 though with a 4 barrel carb and intake.
It's a nice swap that can be done without pulling the motor, and you'll feel the difference and gain 2 to 4 mph top end most boats. Any more than 230HP would require more displacement.[/B] 305 cubic inches can't make any more at the rpm's boat engines are limited to (<5000 rpm)
350 or 383 swap is the easiest and cheapest way to go if 290 HP needed

I take it you don't know much about 305's.... Can't get more then 230hp out of a 305? Heres an article on how to get your 305 up to 370hp using bolt on parts, for a little over a grand (MUCH cheaper then a 383). http://www.popularhotrodding.com/en...hrs_305_chevy_engine_blocks/dyno_testing.html

Click on the link I posted for the dyno numbers of that 305.

As you can see it's 325hp at 5k....

I didn't understand this thread from the start.
Swapping to mercruiser doesn't gain any HP.
I don't know how hot rodding the engine is an alternative to going to a Mercruiser drive.
Almost polar opposite objectives if you ask me.

Well, I wanted to drop in the alpha one so I could take more power without worrying about breakage... the omc 800 doesn't like much more then 260 hp. I was planning on throwing a 350hp 355 in and mating it to my 800 until I found out it would cause the I/O to blow up...

That is why I've decided to wake up the 305... not as much hp or gear crunching torque as the 355, but still hot enough to gain some mph and holeshot without having to change outdrive because of too much power. Plus I don't have to pull the motor and deal with swapping engines... Just some vortec heads, a mild marine cam, edelbrock air gap intake manifold, 600cfm 4bbl, and your up around 300hp at 5k. Thats around 1k bucks...

 

[fabrimacator21]

Re: I/O Swap... Omc 800 to alpha 1.

sorry for being a newbie, but how can i get my 200hp 305 omc to 290 hp?

Vortec heads, edelbrock airgap manifold, this cam... http://www.compperformancegroupstor...=CC&Product_Code=12-236-3&Category_Code=HFTXM

Of course there are little things you can do to make more or less power, depending on cam, windage tray, whether or not you port match the intake, and port and polishing the heads, roller rockers and lifters, etc. The resulting numbers from just stock vortec heads, 262h cam, edelbrock manifold, and 600 cfm carb look like this...

Thats an honest 288hp at 5000 rpm and 330ft lbs of torque at 4000 rpm. That is already a little scary with the stock OMC 800 I/O.

What I/O do you have? If it's the Omc 800 just keep in mind that the recommended max hp is 260... depending on how many hours you have on it, if everything is sound, and how hard you run it, you may or may not be able to get away with 290hp without breakage. I'm thinking of detuning a little to make it more like 270-280ish to be on the safe side. If I wanted to push it I could get 325hp at 5k with some port and polishing and a little higher compression, but I doubt the I/O would like it.

To answer your other question yes you can put a 350 in it but you don't need to... if you know your way around a motor you can get the same tq #'s as a stock 260hp merc 350 but more hp out of a 305. Alot cheaper then buying a 350 also.

I got my answer already so hijack away.

 

[Don S]

Re: I/O Swap... Omc 800 to alpha 1.

Even an Alpha drive is only good to 300 hp, and you better be gental on the throttle at that, and since you are wanting a big engine, I doubt you want to be easy on the throttle.
If you want 350 HP, then move up to a Volvo or a Bravo drive.
OMC stringers are obsolete and not designed for anything with that much HP. Last built in 85 or 86. There isn't any such thing as new housings, and many other parts for the old stringers.
If you want to make a fast boat out of it, get your wallet out. The transom not only has to have the hole filled in, it's also too thin for modern IO's to work in.

 

[fabrimacator21]

Re: I/O Swap... Omc 800 to alpha 1.

I know it... thats why I've decide an I/O swap is not gonna happen. There are people on here running 400hp to apha's though.

Like I said earlier the 350hp 355 isn't going in it... just gonna wake up my 305 to a little under 300 hp. I'm sort of on the fence even with the 305... but I made some more calls today and tq is what kills I/O's according to a few mechanics so I figure a 280-290hp 305 should be fine since tq on a hot 305 is the same as a stock 260hp mercruiser 350 which was bolted to the 800 series from the factory. The hp number is gonna depend on how high I spin it... going off that dyno chart I posted above I'm looking to spin maybe 4500-4600 rpm. That'll put me right around 275-280hp which should be fine for keeping the I/O alive.

It's not that I want a race boat here I just want to be able to dust new 30k ski/wakeboarding boats and keep up with my buddies 350hp jet boat.

 

[mylesm260]

Re: I/O Swap... Omc 800 to alpha 1.

Vortec heads, edelbrock airgap manifold, this cam... http://www.compperformancegroupstor...=CC&Product_Code=12-236-3&Category_Code=HFTXM

Of course there are little things you can do to make more or less power, depending on cam, windage tray, whether or not you port match the intake, and port and polishing the heads, roller rockers and lifters, etc. The resulting numbers from just stock vortec heads, 262h cam, edelbrock manifold, and 600 cfm carb look like this...

Thats an honest 288hp at 5000 rpm and 330ft lbs of torque at 4000 rpm. That is already a little scary with the stock OMC 800 I/O.

What I/O do you have? If it's the Omc 800 just keep in mind that the recommended max hp is 260... depending on how many hours you have on it, if everything is sound, and how hard you run it, you may or may not be able to get away with 290hp without breakage. I'm thinking of detuning a little to make it more like 270-280ish to be on the safe side. If I wanted to push it I could get 325hp at 5k with some port and polishing and a little higher compression, but I doubt the I/O would like it.

To answer your other question yes you can put a 350 in it but you don't need to... if you know your way around a motor you can get the same tq #'s as a stock 260hp merc 350 but more hp out of a 305. Alot cheaper then buying a 350 also.

I got my answer already so hijack away.

Looking at you're torque curve there, there are a couple of things wrong with you're specs

#1. Peak HP is too high (RPMS)
#2. Peak Torque is too high (RPMS)

You're torque curve is for the most part based on you're cam.

You're torque curve is showing peak torque @ around 3800 and peak HP @ 5800.

That basically tells me that you're cam is quite a bit hotter than the stock marine cam. This is bad for several reasons:

#1. You're boat will have no hole shot (you need torque in the 1500-2500 rpm range to get up outa the hole)

#2. You're peak HP will be useless, and if you try to use it, you will fry you're drive and engine in very short order..... 5800 rpms is just too much, especially for an older drive

#3. You will likely suck water into you're exhaust, hydro-lock you're motor, and possibly bend you're rods.

Here's the specs for GM marine's current 5.7, which looks to be about what you're trying to build. Notice the difference in the torque curve?

 

[fabrimacator21]

Re: I/O Swap... Omc 800 to alpha 1.

I don't plan on spinning it over 4800.... the power at 45-4800rpm is already pushing it on the I/O. So are you saying it'll want to keep revving once it hits 4800, even with the right prop? Is it absolutly necessary for peak hp to be at your desired WOT rpm? I've sort of been going off the idea that it'll only spin so many rpm with a certain prop no matter where the peak hp is.

The cam is quite a bit hotter then the stocker but having run ALOT of cams through that program, that one has the best tq and hp numbers from 2000-4800rpm regardless of where it peaks. The "torque" cams don't make as much power through the rpm range. They make another 5 ft lbs at 2000 rpm (but thats about it) then quickly start falling behind... and the hp at 4800 rpm is quite a bit less.

Since the stock 305 probably had somewhere around 200 ft lbs at 2000rpm, 283 will defenitly help the holeshot. I really don't want too much tq as the I/O won't be happy.

Do you have a graph on a mercruiser 260hp 350? Sort of looking for those kind of numbers...

If I had as much tq as the motor you posted I could kiss my I/O goodbuy.

Any cam recommendations? I've tried all the comp cams marine cams and this one http://www.compperformancegroupstor...=CC&Product_Code=12-236-3&Category_Code=HFTXM is the best for low rpm tq and hp at 4800rpm(on par with the torque marine cams at 2000rpm (torque-wise) with alot more throughout the rpm range.

 

[mylesm260]

Re: I/O Swap... Omc 800 to alpha 1.

Well, First of all....

If you cam you're motor for peak hp @ 5800, but never plan to use it, what is the point?

You're making you're engine less fuel efficient and making less torque down low for something that you can never use. There is a REASON marine cam's make peak HP at 4400-4800, because that's where it's useful.


Second of all:
Duration and over-lap are critical on marine cams. If you have too much of either one, you start getting pulses of vacuum through you're exhaust. These pulses of vacuum suck water up the riser and then gravity pulls it down the manifold. Then the next pulse come, and it sucks the water into you're engine.

There is a limit to how much overlap a marine cam can have. If you go over that limit, you will hydro-lock you're engine.


Third:
I don't know exactly what modifications you've done to you're engine on your dyno software, but camshafts alone do NOT increase you're peak torque by my than 5-10%.....

Cams can move where you're peak torque is, which gives you more or less HP, but for the most part, they can't make you're engine make much MORE torque.


These are things that give you MORE torque:
more displacement (new engine)
higher compression (need to run better fuel)
better flowing heads (bigger valves)
better tuned intake / exhaust runners (not much out there for marine)
better ignition curve (Advanced timing) (need better fuel)
A less restrictive Carb (if carbed and if you're carb isn't already big enough for you're engine)

Also, are you comparing prop HP VS flywheel HP by chance?

 

[mylesm260]

Re: I/O Swap... Omc 800 to alpha 1.

Any cam recommendations? I've tried all the comp cams marine cams and this one http://www.compperformancegroupstor...=CC&Product_Code=12-236-3&Category_Code=HFTXM is the best for low rpm tq and hp at 4800rpm(on par with the torque marine cams at 2000rpm (torque-wise) with alot more throughout the rpm range.

That's more realistic, but I Would ask the cam gods in here their opinion. Myself personally, I suspect it might be necessary to run exhaust manifold risers with a cam like that.

Have you read the installation notes? Machine work to the head, a bunch of new valve-train parts, sounds like a lot of work/money for not much gain.

[7] Stock springs cannot be used.
[2] Requires machining on cylinder heads.
[52] Engines with self-aligning rocker arms must use part #1417-16 or 1317-16.



Have you ever considered getting a used vortec 5700 out of a truck and bolting all you're stuff up to that and dropping it in?

 

[Bondo]

Re: I/O Swap... Omc 800 to alpha 1.

That's more realistic, but I Would ask the cam gods in here their opinion. Myself personally, I suspect it might be necessary to run exhaust manifold risers with a cam like that.

Ayuh,... I'd rate that at the Ragged Edge of reasonable for a boat......

Long dry pipes would be a Big Plus....

 

[Howard Sterndrive]

Re: I/O Swap... Omc 800 to alpha 1.

350 or 383 swap is the easiest and cheapest way to go if 290 HP needed

Whether or not I know as much as the guys at Hot Rod Magazine, who's main objective is to start debates and discussions leading to the sale of magazines, I stand by that statement.

No 305 is gonna make 290HP in a boat for less than 3 to 5 times the cost of building a nice, simple, low stress 383 torker.

 

[fabrimacator21]

Re: I/O Swap... Omc 800 to alpha 1.

Whether or not I know as much as the guys at Hot Rod Magazine, who's main objective is to start debates and discussions leading to the sale of magazines, I stand by that statement.

No 305 is gonna make 290HP in a boat for less than 3 to 5 times the cost of building a nice, simple, low stress 383 torker.

If your building from new.... then yes... but if your modifying the already existing 305 then no.

 

[fabrimacator21]

Re: I/O Swap... Omc 800 to alpha 1.

Well, First of all....

If you cam you're motor for peak hp @ 5800, but never plan to use it, what is the point?

As I said above... it makes more power EVERYWHERE in the rpm band... but keeps on making power after I don't need it.

You're making you're engine less fuel efficient and making less torque down low for something that you can never use. There is a REASON marine cam's make peak HP at 4400-4800, because that's where it's useful.

You missed the point... that cam makes more down low tq then ANY other cam I've tried.

Second of all:
Duration and over-lap are critical on marine cams. If you have too much of either one, you start getting pulses of vacuum through you're exhaust. These pulses of vacuum suck water up the riser and then gravity pulls it down the manifold. Then the next pulse come, and it sucks the water into you're engine.

Please explain this a little more... your saying it'll actually suck water in the exhaust valve?

Third:
I don't know exactly what modifications you've done to you're engine on your dyno software, but camshafts alone do NOT increase you're peak torque by my than 5-10%.....

Cams can move where you're peak torque is, which gives you more or less HP, but for the most part, they can't make you're engine make much MORE torque.

As I stated in my previous posts, the engine on the dyno software is a 305 with vortec heads, aftermarket intake manifold, 4bbl 600cfm carb, and the cam I posted.

Also, are you comparing prop HP VS flywheel HP by chance?

The numbers from the dyno software are just flywheel numbers... not quite sure I understand.