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My last cam was set up for 1.75 so that is what I have.However my new cam is set for 1.8 on intake & 1.72 on exhaust.Motor sounds great should I just spend the money on 8 new 1.8? cause I already have a new set of 1.72.Duration stays the same but the total valve lift is effected right?
Motor is a 568 dart block 345 merlin heads blown gas motor cam is 320 at .050 & gross valve lift is a 780 ? solid roller.thanks
 

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Just another Wannabe
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Bob, why is it that I know what you are going to post, but sit in suspense for it anyway? I love it.
Yes, it does change the duration. Let's over simplify. With a larger ratio rocker, the @ .050 is earlier now, isn't it. Basically ramping quicker both up and down. But let's talk performance.
The amount you will see in the boat changing over what you have to new rockers will not be worth the price spent. Run whatcha got! (At least that is my opinion.)
Then if you absolutely need to spend that money, take the money you would have spent and pay someone to get the tune-up right on the money. That will give you more 'in the boat' performance than the rocker change.
 

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The ratio of a rocker arm will never affect duration at the valve. However, it will affect your "effective" duration to a small degree.
 

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The ratio of a rocker arm will never affect duration at the valve. However, it will affect your "effective" duration to a small degree.
See Master Sargent, thats where you are 100% wrong.It will in fact change the duration at the valve. It will nevert change the duration at the lifter. The lifter has no idea what the ration of the rocker is. The valve does.

Bob, why is it that I know what you are going to post, but sit in suspense for it anyway? I love it.

The amount you will see in the boat changing over what you have to new rockers will not be worth the price spent. Run whatcha got! (At least that is my opinion.)
I was hoping for a bigger fire storm before I said anything. Biggest misconception in valve timing. Wish i had a dollar for every person who said it has no effect on duration only lift.
IMO, your right about the rocker ratio. Thats why I put the picture of the hair being split. If he was buying new rockers anyways, fine. But i would buy new rockers just to gain .08 on the intake and .03 on the exhaust. You would need a damn good dyno to find that. The cam grind itself will have much more effect than the ratio difference will. Granted the valve timing is going to change, but I would have to ask how perfectly this cam is matched to the rest of the package to get to worried about a rocker ratio difference that small.



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I guess we will have to agree to disagree. Rocker arm ratio will not change the actual duration at the valve. Only the camshaft can determine duration. You could have a 10 foot rocker arm if you wanted and the valve would still begin to open and close at the same time. Lift yes, duration no. :)
 

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I guess we will have to agree to disagree. Rocker arm ratio will not change the actual duration at the valve. Only the camshaft can determine duration. You could have a 10 foot rocker arm if you wanted and the valve would still begin to open and close at the same time. Lift yes, duration no. :)
If it doesn't change duration at the valve - WHERE does it?? The valves only know what the rockers tells them. The valve is the END RESULT of the geometry. So ANY change in lobe or rocker WILL directly result in altered valve movement. However slight.

Also, how does the CAM care what ratio rocker is on it??? A cam just has lobe lift assigned to it. We get to add the rocker ratio that fits our setup.
 

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The EXACT moment it moves remains the same. So ADVERTISED duration remains the same. Nobody, absolutely nobody uses advertised duration when judging or comparing or picking a camshaft. Nobody. Its fool hearty. Now, the very first crankshaft degree after the lifter moves, this all flies out the window. The very instant the lifter moves. Now! Ask your self, on a solid lifter cam shaft, how far has the valve opened when the lifter has risen .0001. Let us not forget the lash!

Now, once the lifter has actually raised enough to in fact move the valve, which by the way, is going to happen sooner in crankshaft degrees, every .001 of valve is going to happen sooner, and end later with higher ratio rockers. The valve will in fact, remain open for more crankshaft degrees from .001 open to .001 from closed with higher ratio rockers. The amount of crankshaft degrees a valve is open is the definition of EFFECTIVE duration.

Also, the rockers will close the lash with less camshaft rotation than with the lower ratio. Something to think about when changing ratios on a agreesive camshaft with a quick lash ramp. You could be opening the valve while still on the cams lash take up ramp. This may not seem so bad on the opening side of the lobe. But on the closing side, it could slam the valve closed harder due to the valve hitting the seat while the cam is still on the decel ramp. Higher ratio rockers could require a larger lash than lower ratio rockers, depending on the cams grind. Probably not so important on a flat tappet, but could potentially break stuff on a very agressive roller. I would also consult the grinder before changing the ratio on a cam with a lifter high velocity.



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www.highflowdynamics.com
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When the rocker arm ratio is changed, the seat-to-seat duration stays the same; however, area under the curve changes.

The change of area under the curve can be altered due to a change from this ratio-to-that ratio, or even from one rocker arm brand/design to another design (because the different brand rockers have differing design geometry), and even valve train geometry (aka installed geometry) can significantly affect area under the curve.


Whether or not a rocker arm change from, say, 1.7 to 1.8, will show up as a significant hp increase on the dyno might depend how for off the combo's camshaft profile is to being optimized to the build in the first place....and how much hp gain one considers to be significant based on the application of the engine (sometimes ~6 hp is huge; sometimes it is not).

LO
 

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Couldn't have said it better. Thanks LO. and yes, like I said, unless this cam is some custom grind for this package, there is now way to know the effect it will have on the motor's performance. If the cam is more than the motor wants, the lower ratio could in fact help.

Boy, you really opened it with the "installed geometry". "Design geometry" was bad enough. You did it, you can finish it when it comes back. And it will.

IN COMING!



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I will give you this much: With a solid lifter cam when it is cold and has a lash the duration would be changed ever so slightly. Good point.
 

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Boy, you really opened it with the "installed geometry". "Design geometry" was bad enough. You did it, you can finish it when it comes back. And it will.
HA! Oh man (shaking head)...
 

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I will give you this much: With a solid lifter cam when it is cold and has a lash the duration would be changed ever so slightly. Good point.
I am not talking about cold lash VS hot lash. I am talking about a hi ratio rocker possibly requiring a different lash than a lower ratio rocker.


And I will state this again in case you didn't understand it. A hi ratio rocker does not change the ADVERTISED duration (seat to seat) but it does in fact change the EFFECTIVE lift duration at the valve. If you were to measure the crankshaft degrees that the valve is opens .100 and the point that it gets back to .100 on the closing side, the number of degrees is greater with hi ratio rockers.

Let me try and explain it this way. If you want the valves open the same distance for the same amount of time and your using lower ratio rockers, your going to need a cam with more duration @ .050 ground into it.


If the lifter rises .050 with a 1.50 rocker, the valve side of the rocker moves .075. But if you use a 1.80 rocker the lifter only has to move less than .041

If you have a lash of .020, you will hit that at .013 lifter rise. But you will hit it with .011 lifter rise with a 1.80 rocker. And the difference only get bigger as the cam rotates thru it's cycle.

Make any more sense?



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I am not talking about cold lash VS hot lash. I am talking about a hi ratio rocker possibly requiring a different lash than a lower ratio rocker.
It does.

If staying at manufacturer lash specs, you would change the lash for a rocker arm ratio change.

Most go by this:
Say you have .22" recomended lash with 1.7 rocker.
.22"/1.7 = .129 (lifter lash basically ...lol)
And you want to use 1.8 rockers
.129 X 1.8 = .233" lash

So....not huge difference, but that's where you'd end up.

Remember, it's really the lash between the cam lobe and lifter we are concerned about....even though we measure from rocker tip to valve tip.

If I messed up the math, excuse me, taking a 2 second break from work.
 

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Like you said, its not huge, but its there. May mean nothing on a slow velocity cam, or a cam that isn't some custom grind for your deal. But you wil definitely be opening the valve sooner, using a slower part of the lobe if you don't change the lash.

But lash is really only there to cover for seat wear, and pushrod and valve stem thermal expansion etc. So if you leave the lash as is, your only opening the valve sooner, and using a slow portion of the ramp to do it. But in a custom cam, that could have some effect.



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"And I will state this again in case you didn't understand it. A hi ratio rocker does not change the ADVERTISED duration (seat to seat) but it does in fact change the EFFECTIVE lift duration at the valve."

pn7, it is funny that you would say this as this is exactally what I said on the 4th post to this thread to which you responded on the 5th post that I was 100% wrong?
 

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The ratio of a rocker arm will never affect duration at the valve. However, it will affect your "effective" duration to a small degree.
This is where you are wrong. It does effect the valve open duration. The .100 open to close, the .200 open to close, .300 open to close etc, all get longer. Only the very seat to seat timing remains the same. The instant the valve moves, any give lift off the seat occurs sooner and ends later with a hi ratio. The entire time, duration, crank degrees, what ever you like to call it, that the valve is off the seat is extended. The amount of crankshaft degrees a valve is a off its seat a given amount is called DURATION!

Thats why NOBODY cares about ADVERTISED seat to seat duration. It how far is the valve open for a given period of time. And ALL points were the valve is open a given amount are longer with a hi ratio rocker.



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It does.

If staying at manufacturer lash specs, you would change the lash for a rocker arm ratio change.

Most go by this:
Say you have .22" recomended lash with 1.7 rocker.
.22"/1.7 = .129 (lifter lash basically ...lol)
And you want to use 1.8 rockers
.129 X 1.8 = .233" lash

So....not huge difference, but that's where you'd end up.

Remember, it's really the lash between the cam lobe and lifter we are concerned about....even though we measure from rocker tip to valve tip.

If I messed up the math, excuse me, taking a 2 second break from work.
So over a 1/4" lash at the lifter? Really??? So my .400 lobe only has an effective lift of .150"???
 

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This is where you are wrong. It does effect the valve open duration. The .100 open to close, the .200 open to close, .300 open to close etc, all get longer. Only the very seat to seat timing remains the same. The instant the valve moves, any give lift off the seat occurs sooner and ends later with a hi ratio. The entire time, duration, crank degrees, what ever you like to call it, that the valve is off the seat is extended. The amount of crankshaft degrees a valve is a off its seat a given amount is called DURATION!

Thats why NOBODY cares about ADVERTISED seat to seat duration. It how far is the valve open for a given period of time. And ALL points were the valve is open a given amount are longer with a hi ratio rocker.
OK, now we are really "splitting hairs" When I said "never affect duration at the valve" I meant "seat to seat" which is what I would consider the definition of the word "duration".
 

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The seat to seat, and the crank angle that max lift occurs, and the and the LSA are the ONLY things that don't change. Everything else does.

You are aware that you can have two cams with identical seat to seat duration, and same lift, and LSA, and be compeletly different cams.

So just because those remain the same with hi ratio rockers, every aspect of the cam including it's VALVE TIMING changes. As far as the valve opening and closing events are concerned, its a different cam shaft. All the valve lifts measurements occur at different crank angles.

Next your going to tell me the duration doesn't change when the lifter diameter is increased on the same identical cam.;)



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