Why no tighter than 112 lsa in marine engine?

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DaveMcLain
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Re: Why no tighter than 112 lsa in marine engine?

Post by DaveMcLain »

MT Jeff wrote:
gmrocket wrote:There has been a lot of guys saying narrow LSA will cause reversion and moisture to enter the exhaust port/combustion chamber.

That's only true "if" the cam duration is too much for the application. Over camming in a marine app may cause that.. But that's not what I'm saying, or suggesting in the discussion. Why would anyone go to big? A cam that improves on plane time, has better mid range and top end speed is the point! And I've found that a narrower LSA will do the trick.

A blanket statement like "narrow LSA causes water reversion" is just plain false! That's totally out of context when so many other factors that are not even mentioned.

The wide LSA marine cam "spec" can go right into the BS garbage bag.

That kinda what I was thinking. Most would want too much duration to run less than 5000 rpm and then a tighter lsa could cause a problem.

Do you have the timing card on the can you used? I went to Comp and done was available. Shows as "special purpose" cam. Card specs not available as least online.
This is the cam I ended up using in the boat engine and it runs really nicely. stock 350 Chevy, marine exhaust manifolds, stock intake, Q Jet etc.

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wyrmrider
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Re: Why no tighter than 112 lsa in marine engine?

Post by wyrmrider »

shaft or inboard/ outboard I am guessing
should jump right out of the water and pull up lots of skiers at the same time and still idle slow enough to fish
DaveMcLain
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Re: Why no tighter than 112 lsa in marine engine?

Post by DaveMcLain »

Inboard outboard ski boat not a jet boat. He's really pleased with how it runs.
CharlieB53
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Re: Why no tighter than 112 lsa in marine engine?

Post by CharlieB53 »

There are very few of the old style exhaust riser/elbows remaining, most have rotted and replaced with a 'newer' style which has a small 'dam' across the bottom of the passage designed to hold back that puddle of water that is pulled backwards by the reversion pulses.

This one change in design has greatly reduced the rate of reversion problems and allows the use of better cams today.

Picking the right cam for the application is still not to be taken lightly.

Edit: I just read that pdf linked earlier. Perfect explanation with examples of the design changes I was attempting to explain. The author really did his homework.
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Re: Why no tighter than 112 lsa in marine engine?

Post by Tartilla »

I'm going through this myself. Old post date, but rather than fore up a new one, I figured I would add to this, and spike some conversation.

The 'rule' just seems to be parroted for the most part. Vs the complete understanding of the dynamics of the raw water injected marine exhaust, with various cam factors. Likely for the 'ease' of it, and for the pros to not have to worry about reversion being an issue with the substandard manifold elbows that were the norm for a long time.

LSA by itself, is not going to cause reversion. The int/exh lobes and total overlap, and how the engine behaves from that overlap, will either enable the potential for reversion, or not. The 2nd factor, is the injected raw water even able to get mack to the primary exhaust manifold, with the cam actively drawing back aerated water.

Larger CID engines, will likely tame the overlap. Well planned raw water injection locations, and effective dams to prevent water returning up the pipe, will go a long way to eliminate reversion issues. Running raw water cooled manifolds can also have condensation issues as well.

So planning out an cam to support the engine, should not start at the LSA, rather, it should be a factor that accounts for the need of the engine breathing, exhaust/intake draw through on overlap, intended RPM use etc. Being massaged to be yet another compromise for the marine engine.
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Re: Why no tighter than 112 lsa in marine engine?

Post by rp930 »

bill jones wrote: Sun Aug 21, 2016 5:41 pm -here's a story regarding Hal Wing’s 10 meter long (nearly 40feet) Reggie Fountain boat engines.

-Hal Wing was the owner of Little Giant ladders and the guy you see on the "little Giant ladder" commercials.

-I don't know if any of this is pertinent here but this big boat had wet exhaust manifolds
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-in 1984 I built two 454's 60 over for a 40 foot boat---and changed hydraulic flat tappet cams in that thing like 4 times over four years.

-This was NOT quite a true pump gas engine having to use whatever was available at the marina but deep pockets allowed the owner to buy and mix racing fuel.

-Cast iron rectangle port heads and 10.2cr---ran it at lake Powell where the altitude is 2000 or so elev.

-The idle factor had to be really right because the boat had to come into the dock without creating a wake which meant an idle speed around 500 rpm.
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-I used hydraulic flat tappet cams and I ended up changing the cams 4 times (every winter for 4 years) after running the 1st summer with the stock cams that came in the brand new engines from Reggie Fountain.

-I started out with Sig Erson jet boat cams called JB100's, something like 227/233 x .546" lift & 108 lobe spread----then went up to the JB 200's--233/241 x .550" lift & 108 lobe spread, and each time we'd get the engines tuned to idle at 500rpm even tho they sounded pretty lumpy.
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-Then we decided that we'd go for the biggest hydraulic cams we could buy at the time which happened to be a Crane's 252/262 x .609/.633.

-I bought a pair of them and they ran better and I could still get the single carbs to idle.

-So I got brave and bought one new cam that Crane had just added to their catalog which was a 262/270 x .600"/.615" with a 114 lobe spread.

-Then I had a local cam grinder guy get me a pair of cam blanks and had him grind the 270 exhaust lobes onto both the intake and the exhaust and put the lobe spread at 110.

-I really didn't want 114 lobe spread cams because I'd never had any joy with wide lobe spreads.
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-These 270/270 cams took nearly a week on my old Go-power dyno---working with a very light load at idle---to get the idle down to a "pock-a-tah----pock a-tah---pock-a-tah ----idle----where I could do anything with the throttle and never kill the engine.
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-Each engine's carburetor were #3418 855 vacuum secondary carbs that I had done a ton of work to and they flowed just under 1100cfm like 1091 with a special 4x14 stainless steel wire mesh spark arrestors I had fabricated and on top of a two inch tall 4 hole spacer that was flared outward from 1-3/4" to 2-1/4" on all four holes.

-The manifold flange was opened up to 2-1/4" in all four holes----then I slotted each side and actually left a full divider in there.
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-I guess the moral to the story is: lots of duration isn't bad if you can tune it to idle and accelerate like you want.
-AND--it wasn't just any one thing---it was everything that made this thing work.

-The manifolds were 180 GM marine that I had ported to the point I had to use epoxy in a number of areas to seal up where I had ported thru the walls.
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-There was NO PLACE this engine lacked for anything concerning response or right now power---but with the huge props we had on it we had pretty well limited the top rpm to about 5600 around102mph at 4200 elevation.
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-The main thing it took for the idle was getting the mechanical curve short and quick with lots of idle timing---like a minimum of 24 before the vacuum advance----and I used a vacuum advance that worked around the 5" of idle vacuum and that added another 12 degrees anytime there was vacuum above 5"---and a normal total of 38 to 42 degrees above about 2500 rpm.

-The carburetor needed to have extra idle air---part of which I got thru PCV systems---and thru adjustable idle air bypasses I built into the carburetors.

-Once I got the throttles adjusted down to within .020" of the bottom of the transfer slots that and the advance curves was the magic it took to make it idle.

-nothing trick was done to any of the fuel circuitry of the 3418's.
Hal was a great guy. Into all kinds of cars.
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