Re-visiting Lobe Seperation Angle?

[Boomyal]

I tried to do a search for LSA and all it came up with were two posts that had the word puLSAting in it.

If someone could repost the LSA range suitable to avert reversion, I would appreciate it.

 

[calwldlif]

Re: Re-visiting Lobe Seperation Angle?

Are you talking about "overlap"?

 

[ron7000]

Re: Re-visiting Lobe Seperation Angle?

I remember reading threads here about cams and cam specs, but I'm terrible at remembering cam numbers. However, I can tell you that there's more to it than just LSA - lobe separation angle. You basically have to understand cam specs entirely.

My understanding of 'reversion' is exhaust pulses creating a backpressure or reversing flow causing it to back up into the cylinder and then into the intake when the exhaust and intake valves overlap.

valve overlap is not caused just by LSA. Duration also plays a big part along with centerline values (valve timing).
You can have a small cam with say a duration of 217/213 with a LSA of 112 and have X amount of valve overlap. But take a cam with the same LSA and increase duration and valve overlap will increase because the lobes are fatter, as a result they overlap more. So for a larger duration you'd need a larger LSA to maintain the same amount of overlap. But that's only if you keep the same centerline values. Change the centerline values (what crank angle the lobes are set at) and then you have to readjust either LSA or duration to maintain X valve overlap. So there's really 3 variables, 5 if you count intake and exhaust separate: Int duration, exh duration, ICL, ECL, LSA.

As to what prevents reversion is a matter of the engine as a whole, but all the cam specs I've seen for marine have been 112 LSA with < ~220 durations at 0.050". If you look up marine cam vendors, you'll see as they start increasing duration they'll first say suitable for dry exhaust only then not list it as applicable for marine. Generally, lower duration and less overlap results in increased vacuum and increased cylinder pressure at low rpm, resulting in better idle quality and torque at lower rpms- which you need for typical boat motors. When you start increasing duration and valve overlap the motor will breath better at higher rpm which is great for top end power, but it shifts the motor's power range up in rpm which then causes low vacuum at lower rpm, poor idle quality, and lost torque- which is not good for the typical boat motor which is expected to idle well at 600 rpm to shift into gear and have rather good torque from 1000-3000 rpm to get the boat on plane.

 

[Boomyal]

Re: Re-visiting Lobe Seperation Angle?

Here are my cam specs.

LSA 112.5
Valve overlap -15.3 crank deg

Int duration 206.5-max lift .3011
exh duration 210.8-max lift .30913

This is a factory 5.0 roller cam motor with a reground cam to the above specs.

Cam mfgr says that it is well withing the parameters to avert reversion, but....

...I backed the exhaust manifolds (round log style) off about 1/2" to get to my lower valve cover bolts. I each of the front 3 exhaust port openings of the manifold sat a thumb size puddle of water. None in the rear two ports.

One manifold is newer, one is older. It is unlikely that I would have internal leaks in the exact same ports. I also know it is not coming from the motor because it runs on coolant.

I did pull the risor on one of manifolds and could see no water crossover evidence at that point. I am off to pressure test the older manifold now, but I doubt it will yeild any thing. We'll see.

 

[Bondo]

Re: Re-visiting Lobe Seperation Angle?

The LSA Range for a "Marine Cam" is from 109* to 112*............

Your Bump Stick seems to be on the Ragged Edge,..........
And,.... Though the Duration Sounds Low,..... It's Measured at an Odd Points,........ Most Posted Durations are at .050..........

'Course,..... I'm usually messing with Chevies,+ I seem to remember you've got a Ford,..???...??????

 

[calwldlif]

Re: Re-visiting Lobe Seperation Angle?

Very interesting.8)

 

[Boomyal]

Re: Re-visiting Lobe Seperation Angle?

The LSA Range for a "Marine Cam" is from 109* to 112*............

Your Bump Stick seems to be on the Ragged Edge,..........
And,.... Though the Duration Sounds Low,..... It's Measured at an Odd Points,........ Most Posted Durations are at .050..........

'Course,..... I'm usually messing with Chevies,+ I seem to remember you've got a Ford,..???...??????

Not sure what you mean by 'bump stick' Bondo? As far as I know my duration numbers are at .050. That is what the cam tech asked for when I spoke with him today. This is Delta Cams from Seattle WA. He said that all of the numbers that I read him back from their spec sheet were well within the parameters of 'marine-non reversion cams'

That all being said, I pressure tested my oldest manifold this afternoon. Not a sign of of any leakage. Of course to pressure test I had to eliminate any leak participation from the risor/elbow gasket as that is where I had to block everything off to pressurize the mani.

Interesting to note, the puddles and rust spots were only in the manifold opening. No rust was evident in the opening of the head exhaust ports. I have a 1/4" stainless steel spacer with two gaskets between the head and the manifolds

Also, the dried rust puddles had a funny crystaline appearance. So I took a piece of rusty steel plate, locked it in the vice, poured a cap full of 50/50 antifreeze on it and applied a propane torch underneath until the coolant boiled. I did this several times in hopes of comparing the residue from dried coolant on steel as opposed to the residue left in the openings of the exhaust manifold.

Interesting to note when coolant evaporates it leaves an oily kind of residue with no sign of rust. Also interesting after a certain point of the coolant boiling, it began to burn with a soft blue flame until all the liquid was gone. I left the hot steel plate wet so that it would dry on it's own just to see the appearance of naturally evaporated coolant on steel. All this was done to eliminate any leaks from the motor side of the equation.

I discovered this issue after prolonged idling for engine analysis purposes. I am wondering if the small amount of residual moisture was sucked back in at idle or somehow dribbled back into the manifold, with hose water running, after the motor was shut off. It always takes a few minutes to jump down and shut the hose off you know.

 

[Olds Eddie]

Re: Re-visiting Lobe Seperation Angle?

Boomyal, A bump stick is a gear head term for camshaft.

 

[tommays]

Re: Re-visiting Lobe Seperation Angle?

On a lot of new merc motors its nessary to run them at a high idle after a long peroid of low speed to be sure any water is pushed out before shuting down

I think there is a service bulletin about it


Tommays

 

[Boomyal]

Re: Re-visiting Lobe Seperation Angle?

On a lot of new merc motors its nessary to run them at a high idle after a long peroid of low speed to be sure any water is pushed out before shuting down

I think there is a service bulletin about it


Tommays

Thanks Tommays, I think I'll just assume that is exactly the issue. I had been idling at 630-650 after soldering closed the pre-drilled holes in the primary butterflies. Doing that evened out my idle considerably. This cam has a mild but distinct lope at idle compared to a stock cam. I never ran up the rpm before shutting down which I will just make sure that I do in the future.

That seems to be the only explanation.