350 MAG modifications

[ROBIII]

I'm running a 1990 Celebrity Andretti (26') with twin 350 Mags. Just bought it and am still tweaking and tuning. Right now, it'll only do about 58MPH on GPS. Props are 23" which I think should be 21".
What mods will make a significant difference on the HP of these engines?
K&N flame arrestors? MSD box? Performer intake? Etc.? I don't want to turn them much past 5000RPM but would love to spin a 23" prop to this RPM.
Also, how much difference would adding a stroker kit to these blocks make if I don't change the heads? What is HP/torque difference in 5.7" and 6" rods when it comes to stroker kits?
Thanks,
ROBIII

[Bondo]

you don't mention what you're turning now?? do you have thru hulls?? the K&N will get you little,looks good..the merc. thunderbolt ing. is plenty of spark, up to 5500rpm or so..the manifolds,+ carb work will get you a bit more,if you're running thru hull exh..if you go with the stroker kit,i'd be interested in your forged cranks you're taking out..the 6" rods require a LOT more block grinding,+ the net gain is limited...the increase in stroke,+ a strong cam will show the Most gain...

[ROBIII]

Thanks for the feedback.
Yes, I'm running thru-hull exhaust. It runs through an S-pipe and then exits about 10" below what is typical as other components don't allow the traditional straight-thru mounting.
Thanks for the info on the 6" rods. I didn't realize that it took so much clearancing to run the 6" vs. 5.7" rods. Sounds like a good project for next winter. Question: if a cast crank is good enough for Mercruiser/GM, why wouldn't a cast stroker crank (Summit) be acceptable? I like forged stuff better, too. I'm just asking.
Do you not think that MSD's would make a difference? As for carbs and intakes, can I keep my stock carbs but change to an Edelbrock Performer manifold and show any gains?
What cams are you talking about that won't cause reversion? What about 1.6 ratio rockers vs. the stock 1.5?
P.S> I'm currently turning about 4400RPM at WOT.
ROBIII

[Havasu Hangin']

I'll throw my 2 cents in...
Originally posted by ROBIII:
Question: if a cast crank is good enough for Mercruiser/GM, why wouldn't a cast stroker crank (Summit) be acceptable?
I was told that the cast stuff is good up to 500HP. Over that, you are turning some revs, and will need the forged...
Originally posted by ROBIII:
Do you not think that MSD's would make a difference? As for carbs and intakes, can I keep my stock carbs but change to an Edelbrock Performer manifold and show any gains?
I agree with Bondo that on a relitively stock engine, I don't think the MSD will make a big difference. However, once you upgrade your other parts, the MSD setup is the way to go, as you can dial in your curve, and have a longer spark.
The manifolds will work together with the heads, cam and exhaust to help the engines breathe. Alone, you will probably only see small gains.
Originally posted by ROBIII:
What cams are you talking about that won't cause reversion? What about 1.6 ratio rockers vs. the stock 1.5?
Tight Lobe Separation Angles allow for more HP, but are not good in boats. If you stick with a cam that has a LSA of 110+, you'll be fine. Most marine camshafts don't mess with anything lower anyway, as the rough idle will be hard on you drives when shifting.
IMHO, if you are trying to get those engines to breathe better, head work is in order. The higher ratio rockers will help with a few ponies, but I don't think they will raise your powerband into the 5,000 RPM range.
I agree with Bondo, a good set of cams (with the other changes) will wake those engines up. If you go stroker, pick a cam that will work with the extra cubes.
http://www.planetsmilies.com/smilies/happy/happy25.gif

[Bondo]

Question: if a cast crank is good enough for Mercruiser/GM, why wouldn't a cast stroker crank (Summit) be acceptable? I like forged stuff better, too. I'm just asking.
in 1996 chevy(merc.) switched to nodular iron, still cast,But much stronger(probably what sumit has),but you've already got steel cranks..the only problem i've seen with the cast nodular stroker was an elgin crank that snapped the balancer off the front, got blamed on the blower load on the pulley...
as for induction, the carbs you have will flow more air than the 350 can pump, atleast up to 5000+...i'd tend to go with the torquer manifold..(remember torque moves a boat,horsepower keeps it there)
ditto on what HH says about the cam,your Much Better off if you under cam it,+ advance it a bit, than over camming,+ trying to retard it...110-112LSA will make excellent power, without the reverion problems..the 1.6 rockers will net you very little at these RPMs, for the money spent...cleaning up the heads will get you MUCH more Bang for the bucks..
while you're there,Balancing will give you piece of mind,+ longevity to your new motors...money well spent...
good luck on your way to 5000Rpm........

[ROBIII]

You guys are great! I know 4 wheelin and jeepin and suspensions and stuff but don't know much about making HP for a boat.
On the head work, are you referring to a bowl blend or much more? Also, is the Torquer manifold really better than the Performer for boats?
What about exhaust...Will the Mercruiser manifolds flow for decent torque/HP for the 350? Would different risers make any diff or just stay stock? The only that sucks about twins is that you automatically double your expense when you go to upgrade!
Thanks for the info on the carbs. They'll stay stock.
Any particular brand of cams better than another? Model #? It has a roller cam from the factory.
Thanks for all the input! Keep it coming!
ROBIII