OMC Cobra HP

[xeddog]

I have an '87 Beachcraft 215c (aka Reinell) with the subject OMC Cobra stern drive (NOT the King Cobra) for which I am replacing the engine. The boat originally came with a 260HP 5.7L V-8 and I have replaced it with another 5.7L, but this one will be generating a tad more HP. Not exactly sure of the HP, but it will be significantly more than the 260 it had before. Like more in the neighborhood of 325-350HP. So my question is, how much HP can the Cobra safely accommodate? Think a 30% increase in HP is too much?

I think I am, at BEST, at the upper limit of reliability, but I do think that I have probably exceeded it somewhat. For that year, the King Cobra drive unit came with a 7.5L Ford that was only rated 335HP, so I know I am in dangerous waters.

I am not a hot rod boater and rarely use full throttle, even when getting on plane. so I am not TOO worried about it. I never run at wot because at full throttle that thing really gets thirsty. A couple of hours of that and my day would be over because my 40 gal of gas would be gone. Also, I do most of my boating in the Sacramento/San Joaquin river delta system here in northern California, and the water is usually too dirty with too much debris, or just plane too damn rough to go very fast.

All opinions welcome,

xeddog

 

[wire2]

Re: OMC Cobra HP

Look at it this way; at 260 hp the drive will last xxx hours. If you up the hp a bit, it will wear parts a bit faster and subtract from xxx hours. If you up hp a lot, it will subtract a lot.

But realistically, I ran an '84 OMC 260 to well over 300 with some mods and no power related failures up to 2007. Then installed an Alpha gen 2 on the same engine. Had to bump the prop from 21" to 23" after the mods, was turning 5500 rpm @ 60 mph.

 

[xeddog]

Re: OMC Cobra HP

Thanks wire2, good info. But the 1984 OMC was a totally different animal. Wasn't that the Stringer 800 line? It was completely redone for 1986 and I don't have any idea how much of the technology carried forward. My guess would be . . .not much. If memory serves (and that is getting more questionable every day ) I think it was all reworked again for 1988. My Cobra was only in production for 3 years which makes me wonder.

Anyway, I don't INTEND to use all the extra power anyway so the xxx hours that I have to subtract will hopefully be minimal. I don't intend to re-prop or do any other mods, so if I am cruising at say 3500 rpm, with all else being equal it doesn't really matter how much HP the engine can generate, it will only be using a given amount.

So you may be wondering why I went with so much HP in the first place. The answer is essentially because it was there, reasonably priced, convenient, and I was too lazy to look around. :redface:

Thanks again,

xeddog

 

[180shabah]

Re: OMC Cobra HP

What rpm is that 350hp being made at?
How is this 350hp being made?

Hopefully cam selection took resonance/reversion in to account.

 

[bruceb58]

Re: OMC Cobra HP

The 260HP may have been rated at the prop and your new engine is probably rated at the crank so the 2 engines are probably closer in HP than you think.

In any case, just take it easy on the throttle and you will be fine.

 

[xeddog]

Re: OMC Cobra HP

180shabah - I'm not sure what RPM the hp is made for, and I'm not sure exactly what you mean by HOW it's made. But the people that built the engine have (according to them) quite a bit of experience rebuilding engines in general, and one of them has a lot of experience building boat engines specifically. They both had a "portfolio" with pictures of some of the work they claim to have done. I know that is easy to fake, but they did talk a good talk.

What I can tell you is that the block is bored .060 over now. It was .040 over but there was some water that got in there and it couldn't be helped. Pistons were changed from a dished to flat-top, and the cam was replaced. According to the specs, the cam is a stage3 with a power range from 2000-4000 RPM. Billed as good low end torque and pulling power. Sounds like what I need. But I also said "in the neighborhood" of 325-350 so it could also be less than that. I'm not going to put it on a dyno to find out.

bruceb58 - Yeah, I read another thread that mentioned that just yesterday. I did not know that until then. Good to be reminded. I wish all these manufacturers would get all their excrement combined into a single mass.
So based on that, I have read that OMC started using prop shaft HP in 1986 for their outboards, and I "assume" they would do the same for the stern drives. Especially since the Cobra was brand new in 1986. So my 260 Cobra is probably over 300 crank HP anyway. If that is the case, then I am probably good to go testicles to the wall and not have to worry about it.

Thanks,

xeddog

 

[180shabah]

Re: OMC Cobra HP

What I can tell you is that the block is bored .060 over now.

Gettin' a little thin aren't we? Cam is good for "pulling" - sounds good but how much overlap - what is the LSA? Flat tops, with what heads - what CR - what intyake manfolds is being used?

Your 260cobra is the same engine that merc and volvo used tghat year - it is a crank rating.

 

[xeddog]

Re: OMC Cobra HP

Gettin' a little thin aren't we? Cam is good for "pulling" - sounds good but how much overlap - what is the LSA? Flat tops, with what heads - what CR - what intake manifolds is being used?

Your 260cobra is the same engine that merc and volvo used that year - it is a crank rating.

Yeah, gettin a little thin. If I was more of a "hot" boater I think I would be more concerned, but I am pretty easy with my boat.

For info on the cam, it's an Engine Pro MC2204.

As for the heads, I have lost that info so that makes it difficult at best to give a c/r. If I were a betting man though, I would guess in the ballpark of 10.2 - 10.5.

Intake manifold is stock for the 260 for now. I might do something about that at a later date if I don't get the performance I expect (which isn't a whole lot). As long as I can get my 46 mph at 4600rpm then I'm good.

Now. The HP rating. I have to ask this, but are you sure that OMC was measuring at the crank? Everything else I have read says that OMC started measuring at the Prop shaft in 1986, but that was for the outboards. I would think that they would use the same system for the Cobra since it was introduced in that year. Can you provide a link or something that will help me out with this? I will keep trying to research this and maybe even see if I can contact Volvo directly.

Thanks,

xeddog