LSA and exhaust
 

I will be running a 355 merc. with 490 lift, 230 dur. LSA of 108, with stock manifolds, 1.6 rockers on exh. 1.5 int. Am I looking for trouble? Guy at machine shop says probably not! As long as I blow out water before shutting down!

Yes, you have 14* of overlap. With stock Merc exhaust I wouldn't go over 7.

DAVE

I looked around , didn't see much in the way of smallblock manifolds! I guess I need diff manifolds and need to dump the water further downstream?

I'm at 110 LSA and Stainless Marine exhaust dumps 10 inches from the transom no problems.

Are you saying, go for it? I did find manifolds,but they weren't in the budget. I have 24' chapp. and can hardly find a place to go thru the hull!

Platinum, how far from the riser?

Miken- DaveF is right...it is overlap you should be concerned with. Wider LSA's will reduce overlap, but increased duration increases overlap...so, it's like dave says, its the degrees of overlap total. Now, is 7 deg. the magic number with stock merc manifolds? I don't know, but I'll bet it's close, maybe even a little less.
The best way to run your exhaust on an extended transom boat[like a Chaparral] is to run it out the sides...this gives you plenty of room to simply modify a switchable exhaust tip to run inline or you can run a 4" Borla muffler or similar...just make sure the muffler case isn't much longer than 12". Good luck!

Dave F so what is my total overlap with 224-234 110 LSA Thanks.

Sorry bout that Floridianson, I didn't scroll down far enough to see your name, my first time on the board!

Thanks, I'm in the process with corsa, but the question is, can I bolt on an elbow to the stock manifold to discharge the water further away,I was installing silent choice type, but it 90's right off the back, like 8-9'' from the end of stock riser? I know it would be easier to just buy a bigger, faster boat, but then I would have to make the garage bigger!

:D No problem I am still new to the high performance end of all this.I got Dennis Moore's book on the stuff and found it to be a real eye opener.In the book it talked about LSA and it stated that if the LSA is kept to within 109-112 an acceptable idle but one with a slight lope will be maintained and only a slight if any amount of water will be sucked back into the combustion chamber at idle.Mercruiser has suggested on there high per. engines that after extended periods of idleing the engine should be revved up a few times before it is shut off to clear any water.It did not say what exhaust was being used,I went with the stainless marine exhaust even before I had gotten the book and made my cam choise and with it dumping so far down the line I feel I should not have any problems .I do rev it up on long idles and before shut down.

FloridaSon, I think these guys are calculating based on 0.050", so if your numbers are also 0.050", you should be at 9* of overlap.

FYI, the cam recommended by Comp for my 350 with stock exhaust was an XE-256-12. This has 212/218 at 112, which gives no overlap at 0.050" (-9*). Out of morbid curosity, I looked at Comps Extreme Marine Rollers: XM264HR=-9*; XM270HR=-3*; and XM276HR=3*. The XM270HR is the largest recommended for an I/O.

Interesting thread.

220BR Thanks:cool:

I purchased Dennis Moore's book prior to rebuild,I figured 108 would work? I'll have to check and see if duration is rated at .004, .006. or .050 Iknow comp cams rates at .006 , I'm using melling.

Floridianson, are you using stock manifolds? Cpperformance makes an 's' pipe, which dumps further downstream, I wonder if it will bolt up to stck manifold?

Useing the Stainless Marine:cool:

220BR or anyone help me here I wish to learn.I called Crane and he said to look at the cam card.Now in my case I did not get a cam card.I went with the GM cam12366543 the ZZ502 from the local dealer and was real disappointed that it had no card and no numbers on it that could be traced back to a dam thing.(like a box of cracker jacks you never know what you will get)I do have an old cam card from my old motor so INT.opens at 34BTCD closes at 66ABDC and EXT.opens 78BBDC closes 30ATDC so what do we add or subtract to find the over lap.Also I had Crane look at the MER.500hp cam with 222-230 LSA110 and he said it only had 6* of over lap.Thanks :cool:

There's at least two ways to calculate overlap, but both require the LSA. I looked up the GMPP cam on the web and found that its duration is 224/234 at 0.050" but the LSA was not listed.

Using durations, just add the two together (224+234=458), then divide by 2 (=229). Dividing by two gives you half the duration on either side of the centerline. Now, double the LSA (because its in cam degrees not crank degrees) and subtract. So, for LSA of 108, overlap =13*; LSA=110, overlap=9*; LSA=112, overlap=5*

Good Luck

220BR, I checked with corsa, cpperformance, and melling. All of them were concerned with duration only, they have two trains of thought, of course there are no definite numbers! They say anything over 230 degrees dur., or an engine that will not idle, is cause for concern!

There are guys on this forum with a lot more experience and knowledge than me on this, but if your spin a conventional outdrive (Alpha, Bravo, etc...) a lumpy idle makes docking manuevers a pain. That seems like a very aggressive cam for a 355 I/O.

IMHO, for a boat, your better off making power thru displacement than an agressive cam. Why not turn that 355 into a 383?

Good luck

Your right! I was going that route, but no one could tell me which flywheel to run! Internal balance vs. external, and on and on. The 355 has been redone throughout, flattop hypers. w/ raised rings, 2.02's, besides, it's the challenge, anybody can go fast with a big block, or stroker! If and when the smblock peters out, I'll go that way. Thanks for the info. I'm new to the marine aspect of hi-perf.

That's a damn shame. I am currently building a 383 out of my '92 Merc 5.7L. Scat crank, TRW forged Flat tops, 2.02/1.6's, port matching, 9.7 SCR, 0.044 quench. My info is that all one-piece seal flywheels are externally balanced, so all you have to buy is an externally balanced damper. If your block was a 2-peice seal, then you have to spring for an externally balanced flywheel as well.

I'm building this engine over two seasons. This season I will run with the rebuilt stock heads and the cam from the 5.7L (Comp Cam XE-256-12) . Pretty weak, I know, but I still expect about 360 hp. Nest season, I'll upgrade to Dart Iron Eagle heads (72cc) and a roller cam (Comp Cam XM270HR). My biggest problem is the Alpha one outdrive.

Ya know that alpha won't do behind the final version 383, that's the main reason I didn't go that route, bravo and gimble housing are $$$$$. I'm still running the alpha, I'll just take it easy for a while, until I hit something underwater!!

220BR, I forgot to add this, do you use the 5.7 flywheel? If so, how do you account for the extra stroke on the bigger crank throw?

The answer is yes, I used the original 5.7L flywheel because it was orignally externally balanced. All one-pice seal GM 350s use an externally balanced flywheel. Evidently, the loss of material from the crank when going to the one-peice seal made GM use externally balanced flywheels.

It seems to be the luck of the draw on those Alphas. I have a buddy who has run a strong 383 thru a Model I (pre-Alpha) for three seasons. He even upped the gears to 1.89, which increases the stress on the lower unit. I bought a drive shower to help cool the Alpha. I just won't do WOT hole shots.

That does piss me off!, you didn't get any vibration with the 383? Enlighten me on the " drive shower"?

220BR, What boat, prop, mph, and rpm are you doing?

Whoa, there. Haven't put the engine in yet. USA Racing here in Houston is building the long block for me. They routinely balance stroker assemblies using the one piece seal flywheel with a new, externally balanced damper. I expect to get the engine back before Memorial day and may even be wet that weekend.

Drive shower is just a length of stainless tubing that is shaped to bolt to the drive so that the ends are at the cavitiation plate and the shower holes are up near the top plate. Under way, water is forced up and out to cool the exhaust passage in the drive. Really only useful if you are thru-prop instead of thru-hull.

Keep us posted, I'm curios to see what it does?