Mercruiser 6.2 Upgrades
 

I recently picked up a 2003 Cobalt 226 runabout for cruising around my lake. Obviously, not the typical boat for this site, but fiigured this place would be the best spot to get some questions answered.

The boat is equipped with a Mercruiser 6.2 BIII motor & drive; that's totally stock, with the through-prop exhaust. I was looking for a boat with the 496, but ended up settling on the 6.2.

Ideally, I'd like to get a little bit more power and sound out of the boat; so was wondering what the logical steps would be to achieve that. Would headers / aftermarket manifolds do much if I retain the stock exhaust exit? (I'm a bit apprehensive about cutting holes in the transom). Would doing a captain's call setup add much if I retain the stock manifolds? Are there any other basic performance upgrades that people tend to do to this engine?

A lot of Cobalts will have silent choice out the side exhaust but really not much gain there - basically just the noise level. No offense as Cobalt makes a really nice boat but they are not fast nor have a fast bottom plus weigh a lot. You have a Bravo III on top of in which is not the fastest outdrive either but works very well on heavy pleasure boats especially if they have lower power levels. Also the work well on Cobalts.

IMO silent choice and your best bet would be to make things lighter in your boat plus maybe reposition things for better balance. I would enjoy your boat and leave it mostly stock plus in most cases stock will give you the best longevity if properly maintained. Cobalt makes a really nice pleasure boat and that's what it is intended for.

http://www.youtube.com/watch?v=M1xCNjj8z_s

You can make them loud, strait through with no baffles, but you have to make sure your exhaust is high enough. Won't go any faster, and with the 6.2 other than a whipple or a pro charger you are better off building a new engine and selling the 6.2. Plus you will make the boat worth less if you modify it.

We have a 2000 Cobalt 226 with a stock 7.4mpi Bravo III and Captains Call exhaust. With a 1/2 tank of fuel we hit 63 mph-- which isn't too shabby. From my experience the Captains Call exhaust adds about 3 mph to the overall speed. I agree that this boat will never be a speed demon when compared to some of the big guns on this site- but I will say that the quality of workmanship and smoothness of the ride is exceptional.

Regarding Captains Call on our boat - we have both the molded and extended swim platforms. At idle there is a lot of resonance with the extra swim platform - but once on plane - oh does it sound nice.

Safe boating

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Is that GPS speed? So far mine's topped out at 52.6GPS, with a fairly light load.

Obviously, I'm not really looking to go 70mph, moreso just gain a little bit of power without sacrificing reliability. The boat is a pleasure craft that I also ski behind (so captain's call is a definite requirement if I do cut into the transom).

Would doing headers / manifolds, while maintaining the stock exit location give me much (or anything) in the way of power? or just lighten the load in the transom?

Every runabout I've owned with Captains Call (both big block boats) there was no noticeable difference in speed with the exhaust opened or closed. Only add CC if you want the noise. Contact Bob Lloyd at Full Throttle Marine if you want options/pricing on mods to the 6.2 MPI that net real world speed gains. Be prepared to open your wallet.

I will only echo what most others have said. You have a great boat in it current form. Adding Captains Call or an equivalent system will gain you absolutely zero performance increase. It will add noise and cost money. I had a small block with cut outs and it performed the same open or closed other than the sound difference.

For any measurable performance increase you would need to open up the motor or bolt on a power adder (supercharger). You can make a potent 6.2, but it will take a fist full of cash. I'd run the boat until the motor dies then modify it.

Best of luck.

Yup all good advice here. I have potent 6.2s but it took a fist full of cash as mentioned.

I do have my silent choice for sale if you want it though. PM me.

Will add your usual Cobalt buyer for resale is a stock boat buyer so hot rodding a Cobalt will devalue it and possible a very very hard sell down the road if you ever wanted to get rid of it. Also 70 % of Cobalt owners when looking to upgrade for a boat will buy another Cobalt. Just throwing that out there. At one time I had a 1998 Cobalt 220 BR that went 64 -65 mph. It had a 300 hp HO MAG 350 MPI.. I put silent choice in it, played with the Bravo III props along with taking some weight out of the boat.

do you remember what props you put on? Currently I have a set of 28P and we have been getting 63mph 1/2 tank of fuel and not much else in the boat. Ours is a 7.4 mpi


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Admittedly, I'm not overly concerned with resale. The price I paid for the boat was something that i was very satisfied with; and I plan on keeping it for a long enough time that the difference in depreciation is not something I'm considering.

What about something like headers/aftermarket manifolds? As far as I've seen, a kit can be purchased for ~$1500, and as a bolt-on, something that could be removed should I ever want to revert back. Would they offer much of a performance increase while maintaining the stock exhaust exit?

Out of curiosity, what did you do to your 6.2s?

on the Cobalt 220 BR - bravo III - do not remember about what prop changes I did but I did do some filing on the edges to sharpen them up on top of. Also did a lot of reshaping of the skeg and believe it or not that help as well. I could try to find some pics of the boat and send them to you if you like. The boat did handle very well & pretty fast for a family rec boat especially for a Cobalt.

IMO the placement of weight and some taken out really helped the boat. I think I gained 6 or 7 mph with props, running thru hull with silent choice, playing with weight, and messing with the lower unit skeg. No other mods nor to the engine were done. I was going to put aluminum exhaust on it to save even more weight but sold the boat beforehand. I remember the people who bought it were very surprised how fast and how fast the boat accelerated on the test drive.

http://www.offshoreonly.com/forums/d...d-project.html

There's more than you could ever want to know about building 6.2s

BUP thank you so much for the response -- I thought about taking a file to the edges of the prop just to knock the "dirt off" but I was afraid that I would screw something up-- old adage if aint' broke don't fix it! Regarding get tee up and go - I can't brag enough about the performance - Our son pulled me skiing 225 lbs and believe me I was up and running before I could blink - also the wake isn't much -- that extended running surface design really helps.

thanks again

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Headers will not add any power or speed for a small block. Stock exhaust is not extensively limiting for SBC applications. If it were a big block you might see some gains.

Yes, that was done with my sons GPS- don't forget the 7.4 mpi is 310 hp and the 6.2 isn't that 300 hp? Also, I obsessive about cleaning the bottom of the boat - we store it on a lift and I've hit it with cleaner already twice this year

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The Merc 6.2 L - 377 CID is 320 hp. The 383 CID is really a 6.3 L and is not the same motor. The 6.2L has a longer stroke and at times it has caused some piston failures over the years. longer stokes in the cylinder bore are harder on pistons. I am not saying that every 6.2 L has piston problems but there has been some from Merc / GM. I am sure if you do a search on the net or even up here maybe some might pull up or least maybe mentioned about some form of it. I have seen 2 cases of it in person in the past 10 or 12 years..The 6.2 does not even come close in sales to the common 5.7 L - 350 in marine apps.

The 350 HO MAG is 300 hp coming from the sterndrives and in the towboat series engine at one time it was 315 hp from Merc. FYI

The only difference between the 6.2 (377) and the 6.3(383) is one has a standard bore and the latter is .030" over. Both use stroker cranks. Longer stroke is only hard on the pistons if the rod is too short causing excess piston skirt side loads.

I would gather the piston problems are due to them being Hyperyou'llcrackit pistons with high ring lands.

Good or decent pistons/rods/crank there is no worry about rpm'ng them.

In my case, the cool fuel unit allowed minerals to get shot down the fuel rail causing a lean condition that blew a piston.

Concur.

I have talked to a engine builder who mainly built these GM 6.2 L for some race prep engine for race cars as this was the only engine they could use in the class. He was the so called expert on the 6.2 's. He said it was a combination of things - weak pistons and the longer stroke but said some things on top of that is mentioned here about rods and cranks. Also balancing cranks helps as well for the internals.

He gave me his card to call him about the whole jist of it but I never called for the complete info. Also Dart is a firm believer the longer the stroke the harder it is on the pistons. I will try to find that info on youtube from them as I sit in most of their seminars when I can. They film most of their stuff for youtube when they put on seminars. FYI if anyone is interested as to what info they put out.

I totally agree about the pistons as well being weak. The 350 - 5.7 does not have problems like this with pistons and believe me I have seen a lot of lean running - poorly cooled, severely overheated, old gas ran thru the Merc & Volvo and OMC marine 350 - 5.7 that never ever dropped a piston. Now dropped a valve or cracked a valve seat or had a valve seat fall out - that's a different story, seen plenty of that going on.

I would agree but it's my understanding that some of that can be offset by changing rod length. For instance going from a 5.7 length rod to a 6.0" rod for the SBC example. This decreases piston side load, peak piston speed, and increases dwell time at TDC and BDC, which with appropriate cam timing events can optimize power/torque.

No kidding....I never even fathomed a valve seat coming out until my old motor showed me it could be done. :eekdrop:

Exactly.

No reason to worry about a 383 pistons side load. Tune it right and Rev the ***** and keep going.

As an fyi: I don't believe in the cheap parts...so thus part of why I said above.