Why no tighter than 112 lsa in marine engine?

[MT Jeff]

I have a Comp Extreme Marine XM270HR the cam card is here. http://www.compcams.com/Company/CC/cam- ... d=202&sb=2
Why wouldn't the next size smaller cam on a 110* lsa be better? http://www.compcams.com/Company/CC/cam- ... d=186&sb=2

I know reversion is an issue on marine engines but the 110* cam has 2* less overlap at the .006 numbers for timing. And if installed at the recommended icl closes the intake valve 7* sooner. I realize the installed center line can be changed.

This is for a future project 383 for my boat. I have a sound 355 now so its not a rush job this time. I already have the first cam but if a tighter lsa would be better I could sell it and get something else made. This is a cruiser boat inboard outboard. I know hot boat or jet would be different.

What is an allowable overlap on a marine engine? Everybody says no tighter than 112* but that is pretty general statement. Looking at timing cards for Comp, Lunati, and Crane I realize direct comparison is difficult to say the least. But comparing Comps cams I would think the 110* lsa cam would make more torque in the under 5000 rpm range it would operate with 1500-3500 being the most important to get on plane.

[Keith Morganstein]

Primary reason is if you have wet exhaust. You don't want water getting back into the engine. Secondary is having a smooth low idle so you can dog it into fwd and rev without issues.

[turbo2256b]

http://www.michel-christen.com/ExhaustElbows.pdf

[MT Jeff]

Primary reason is if you have wet exhaust. You don't want water getting back into the engine. Secondary is having a smooth low idle so you can dog it into fwd and rev without issues.

Wouldn't the two cams listed have similar idle and reversion with the overlap almost the same? I don't know. The 110* cam has less duration.

[MT Jeff]

http://www.michel-christen.com/ExhaustElbows.pdf

Thanks for that link. I have a Volvo. I'll go look at my old risers/elbows and see how they are made.

[Walter R. Malik]

I don't think that is really the case in most circumstances.
Though, It must be remembered that most marine transmissions will not even shift out of neutral above 1,000 RPM so, it has to idle below that.

[bill jones]

-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.

[MT Jeff]

Thanks Bill, although I'm not in that league that was very informative. The point of my question was, and maybe I didn't ask correctly, is why do people automatically say no tighter than 112* LSA?
The two cams I pointed out are within 2* overlap of each other with the tighter LSA cam having less duration. I would think and I might be wrong that the tighter LSA cam would idle similar and make more torque down low so to speak in the rpm range I would operate. I understand idle and reversion issues but I always here "No tighter than 112*" no matter which cam it is.
I believe Dennis Moore some time ago said up to 224/230 @ .050 on 110 lsa will work although I think that is beyond my operating range and comfort level for a cruiser boat. I realize Dennis Moore wrote those books some time ago but degree's haven't changed as for as I know.

Thanks Jeff

[gmrocket]

It's an old wives tale in most circumstances.
I've done many marine builds, mild.. Hot and wild... Jet drive, and the typical stern drive.


Just finished a mild 350 Chevy in a 20 family cruiser/ski boat.. Pretty heavy, big pitch prop and not much gear multiplication in the out drive. Not much compression either, 1995 stock mercruiser bottom end.

Went with a comp on a 108 in a a couple advanced... It was night and day compared to the old marine cam. Gained on out of the hole.. Mid range acceleration and top end speed.

Idle was 750, a little choppy, yet went into and out of gear no issues.

[MT Jeff]

Thanks gmrocket that's what I thought/know. I'm not a pro but I do know a narrower lsa can pick up the mid-range. I think know it's like most other cams if you call for a recommendation the customer thinks "I can go up one or two and get more" . It's kind of frustrating if you want to optimize for a certain result.

[user-23911]

Marine engines normally want peak torque lower in the rev range than you'd want in a car, you need low down torque to get it up on the plane.
Closing the LSA moves the peak torque up in the rev range. Opening it moves the torque down.

[oldjohnno]

Closing the LSA moves the peak torque up in the rev range. Opening it moves the torque down.

Say what???

[F-BIRD'88]

You got to remember that most marine V8 engine applications are running thru Marine cast iron exhaust manifolds (and many also go thu the drive exhaust)

Thus the exhaust breathing and exhaust scavedging action at overlap is NOT THE SAME as with long tube racing/dyno headers.
Therefore the BesT exhaust valve timing events ( duration and overlap, LSA)
is not going t be the same as typical is best with long tube tuned exhaust headers.

Most people NEVER these the marine engine on the dyno with the MARINE exhaust manifolds. It is going to want a different cam than with tuned headers for best performance. Especially when working against the back pressure of a thru the drive marine exhaust system. Further the cam that gives the most WOT power and torque
@ WOT will not eh be the same cams that works the best at part throttle running in the boat.. it will tend to over scavedge at part throttle and waste fuel.
That total day running fuel consumption adds up fast in a boat that runs on love.
going 2-3 MPH faster at WOT in the boat soon looses its luster when the boat eats gas like its going out of style.

If you are building a boat motor for RACING and are using Tuned long tube racing marine headers and open exhaust knock your self out.
If you are building for a mix of top speed and pleasure cruising and are using Marine cast Exhaust manifolds be sure to verify the engine power and torque curve on the dyno USing the manifolds you will run in the boat.
ya ot matters. Often you can get a old set of marine exhaust manifolds that are maybe cracked or corroded water jackets and were retired and run then "DRY" on the engine dyno for dyno testing.

This does not mean you cannot use a 110LSA cam. But it is up to you to verify that it actually makes power thru your marine exhaust manifolds.

Sucking back water thru the marine exhaust into the engine on shut down or a engine stall is another matter. There are ways to deal with this on a high overlap cam with water jacketed ( with injection) exhaust, but most won't be bothered dealing wit this, then get all pissy if when the racy motor gulps water.

We built 2 540+ HP marine race 350 SBC's with 106 LSA big solid lifter race cams
and ran them thru merc magnum marine exhaust. But these were purely race engine stuff..... for boats (12.5:1cr Methanol fuel) The boat did go fast thou.
Ran WOT at 6700 RPM... (sounded like 2 alcohol funny cars running side by side @WOT, from a good distance.

Cannot say it was good on gas or very practical....

Again do not assume that the cam that makes best dyno power with tube headers is going to be the best thru your marine manifolds.

and if running your boat at part throttle Cruise (with any kind of fuel efficiency)
is a priority, (as opposed to WOT top speed running) you may well need to adjust your cam choice including the LSA, for this.

[F-BIRD'88]

So a mild moderate 110LSA cam may not be best thru the manifolds
and the typical 4 deg advance 110LSA 106/114 phasing may not be best cam position when running thru the marine exhaust system. If you want to get more than a warm fuzzy feeling from your cam choice in your boat you need to verify it is actually working for you running thru the exhaust you will be suing in the boat.
Testing on a dyno thru dyno headers or making your choice based on that experience will not get your the right camshaft for the boat.

Also most guy scanning and making a catalog cam choice for their boat will want to use the existing Marine ignition system which has a built in ignition advance curve
(usually 24deg) that cannot be modified to allow correct (increased) timing at idle that a tighter LSA cam tends to want to idle nice at low rpm.
The typical pleasure cruiser I/O boat guy won't change the $$$ ignition system $$. And will get all pissy when the cammed up motor won't idle in gear etc.

[Walter R. Malik]

Closing the LSA moves the peak torque up in the rev range. Opening it moves the torque down.

Say what???

Spreading the separation generally widens the power band with more at the bottom AND the top at the expense of the mid range peaks. There are, of course, some exceptions especially when the overlap gets to be quite a lot.