Extra Performance??

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nickjh

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All,

2 Questions...Is it possible to perform performance enhancements on Mercruiser petrol engines? It is common on cars to remap ECU's change carbs, Air Filters, Exhaust and re-work heads etc... I haven't seen anyone doing it to boats, other than race boats of course.
2nd Question, There is a product called stepdrive for the volvo DP290 sterndrive, is saw it at the LIBS, anyone heard of anything similar for the Mercruiser Bravo range?

Thanks

Nick

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Answer to your first question......Yes, but, and theres always a but, how deep are your pockets. Extra horses cost extra bucks......as the americans would say.
The second question someone else answered so I need say no more.

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What would you expect to pay for engine performance enhancements? And what gains can be expected?

I answered the leg question with a description of a product produced for volvo legs, I want to find out if a similar product is available for mercruiser!

Nick

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The leg idea in theory looks good. Ive always wondered, cost excluded, why manufacturers dont have user adjusted gears on boats to aid performance and cruising or better still the old variomatic gearbox idea that was used on some cars years ago.

kevin

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The guts of a PRM hydraulic gearbox mounted in the vertical, as opposed to the more normal horizontal, position. Its configured to give two ratios in the same dirction, rather than ahead and astern, so I suppose you might get it to change up in reverse if you went fast enough ;-) I suppose it must come with a modified mid-leg section to carry it.

Tony Brooks

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How long's a piece of string? You haven't said exactly what engines you have (although your profile says 5.7). Are we talking MPI or carbs? In any case it is a small block chev, you can get after market heads and cams that will perform well out of the box. As Barry said, how deep are your pockets. Say exactly what engine you have and your budget, and we can give you a better guide.

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with tuning of any engine where do you want to start. Ask yourself if you want out and out speed or a bit more low end grunt. You wont get both. The makers have spent 100's of 1000's getting to where the engine is at in its present form.
Most peeps will tune for extra horses and top end power. Its all a balancing act though. Its no good fitting howling great carbs or other forms of induction if the block and exhaust system cant flow the gases. Think of the engine as a pump. The esier it is to pump air in the esier it need to be to pump it out.
Probably the first place to start are the heads. Depending on what block you have will depend where you start with them. The most restrictive area will be the valves in three ways. ie size, lift and duration. Size is the hardest thing to change, lift and duration is all down to cam profiles. Changing this will also effect ignition timing etc etc.
Bear in mind that the higher the state of tune of an engine the reliability will go down some what and the maintenance will go up. F1 engines run very high states of tune but are only designed to run the race before being rebuilt or replaced. Ok its an extreme I know but bear it in mind when your running avgas in your boat for a blast acroos the bay!
Before you get into the costly exercise of replacing half the engine, is your current set up running at its best. It helps if you can get the use of a dyno to really get the thing running at its peak. Not a luxury a lot of us have.
Case history, my volvo engine. I dumped the carbs and cobbled togetjer an injection system from a road car. Along with that I bolted on the turbo from the same road car. Instant whammy whammy acceleration. Pee's all over V6 mercs and gave V8's a good run for the money. Top end was down though. Wrong ignition curve and wrong profile cam to run the turbo with. So I banged on the head from the car with the cam for the turbo engine and the leccy ignition to give me the proper curve. All went swimmingly, only the engine was designed for road use with varying revs and not marine use where revs stay more constant. In short the hp out put was spread across a wider rev range, whereas in the boat it more "peaky" in a narrower rev range. A lot of this cam from the cam profile. Where the engine came onto the Cam" and you could feel when the engine hit its narrower power band.
A success or not? yes and no. Engine run smoother and used a bit less juice, but I put that down to the injection rather than the buckets of carb's that volvo use. No real gains in top end altho the engine was putting out more power, it was in the wrong place.
In saying all that. What prop are yopu using, do yopu over rev or not hit peak revs for your engine? worth looking at
Anyway enough of my babbling, I've gotta get back to work. No, I know your no better off as I've not really answered you question........have I!


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Engine is 2001 5.7 V8 Mercruiser (250hp) running carbs. Bravo 3 leg with twin stainless. With 4 people on board and full tanks I can't pull more than 4000rpm, even with a clean bottom! Best achieved is with half a tank,half water and just me straight after scrub running a 4400rpm flat out on an F0 day in southampton water.(according to mercruiser, max rpm is 4600-4800. Perhaps she is a little over propped? She is either 21 or 23" pitch, I will have to check when she is out in a couple of weeks. Will changing down a pitch or two loose me any top end if I am not pulling max revs? A little more pull away would be nice....Or alternatively get a couple of mods to perhaps overcome it??? As far as I know engine is fine, being serviced by southwater in a couple of weeks so will find out!

Cheers

Nick

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What sort of speed are you getting at the moment?

Ok. Firstly, have you experimented with outdrive trim? If you run it with the leg all the way in, you won't achieve full rpm.

You may well benefit from a change of props, but it will cost you to find out. A good place to start is to speak to Steel developments, they are prop specialists and can calculate correct prop pitch.

I would be happy with 4000 rpm as I don't like the idea of "caning" engines all the time.I deally you should be able to go to 5000 max on that engine, but not for long periods of time.
Is the timing correct? A common fault on those engines is for the harmonic balancer to creep round on the rubber bond, this will give a false timing setting. It is easy to check. Also are the secondary chokes on the carb free? Are they sticking at all?( Note: they won't open just by "blipping" the engine) Engine needs to be under load for them to operate. Again, easy to check (specially on a boat)

For a bit more Pep you could change the cam to something like a competion cams Camshaft. Go for the 260 - Operating rev range is 1500-5500 .440 lift with 1.5: rockers
212/212 deg duration @ .050" lift (This is not too radical, with a broad spread of power) Anything more and you need to then look at other things, carb, manifolds, CR, heads etc:
Fairly easy to change but you need enough room in the front of the engine to allow you to get old cam out / new cam in. Should cost less than £175 by the time you have bought gaskets, lifters etc: (assuming you are doing it yourself)

Is that enough info for now? If you have more questions post them here.

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Without any doubt, the cheapest solution is to replace present engine with a bigger one. As your currently running a carb smallblock, you could replace it with the entire Mercruiser V8 range, from the 5.7EFi up to the 8.1 Mpi.

The other alternative would be to purchase a Hi Perf longblock from the States, and use all your ancillaries from the old motor. You could go down the forced induction route, my favourite is the www.procharger.com now this really is cheap hp!

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