changing rocker arms effect vacuum?

i have a 270-HR cam in my car w/ 1.52 roller tip Comp rockers. they say it has 15" of vacuum at idle. if i put 1.6 roller rockers on it,i know it will make the lift bigger but will that in turn take away some vacuum?

thanks

nope your vaccum should be fine.

It will effectively add some duration and could impact your engines vacuum. I think it will be slight. By opening the valve farther, it also opens it faster, this adds a slight increase to the duration at .050 and advertised duration (@.006) because, it gets to those lift levels sooner. So, it can, but I doubt it will be much of a difference.

My thoughts.

I'm with transxtreme, I don't believe it will effect your vacuum, your only modifying the amount of lift @ the valve, the cam is the same and the lobe seperation angle (LSA) is the same, thus keeping the centerline the same which in turn is responsible for the amount of vacuum produced in the motor.

just like 1sickS10 sad, he just elaborated on my answer, a different cam is the only thing that will change your vaccum on the valvetrain

thanks guys. im not changing rockers right now...but if i want to in the future ill know then b/c i honestly wasnt sure if it would affect vacuum or not.

Ok. Cam duration is measured at a specific lift (either .006 or .050). If we increase the ratio of the rocker arm, we will reach that lift more quickly (valve opens sooner, and takes longer to close). This effectively adds a slight amount (usually less than a few degrees) to the cam. While I agree that a few degrees may not make a world of difference, any changes to cam duration can impact vacuum. If it didn't change any of the characteristics of the motor, it wouldn't produce any more power either. So, while you are making a change that impacts lift more severly than duration, it will still have an impact on duration (which can impact vacuum). Some of the statements above would indicate that I can run any cam I like as long as the intake centerline and lobe seperation is the same.

However, by adding lift, you will also add a little duration. Making the cam lobes appear larger. More duration on the same LSA will decrease vacuum because you will gain a slight amount of valve overlap. Again, I'm not saying it will "change the world" as far as the characteristics of your cam, but it will have an impact on overlap and therefore vacuum. Simply discounting this is change is to oversimplify camshaft operation or demonstrate a lack of understanding in that area.

If a cam is rated at 270 degrees of duration, it is measured at a valve lift of .006". This requires .004" of "lobe lift" and a 1.5 rocker arm to generate the .006" that is our spec. By switching to a 1.6 ratio rocker, we only need .00375" of lobe lift to get our same measurement point of .006". By reaching the .006" spec with less lobe lift, we have effectively added duration to that cam (as a cam rotates, it reaches .00375 before .004 in it's rotation).

At .050 (the more standardized spec) we need a lobe lift with a 1.5 rocker of .03333333", with a 1.6 rocker, we need .03125 inches. So, the valve will be open the required .050 with less lobe lift. If we need less lobe lift to reach the specified duration, and we know that lobe lift increases as a cam rotates, then we know that the valve reaches the specified amount of lift sooner and therefore camshaft duration will increase.

So, as I said, it can affect engine vacuum.

Have fun!

sorry dude, but i disagree. Your not changing the duration, duration is the amount of time (in degrees) that the valve is open, your not physically changing the grind of the cam, sure it opens more, and the profile of the lift might be different, but it still opens the same amount of time. The valve doesn't open until the lobe smacks the lifter and the force is thrusted up the pushrod into the rocker arm. So it doesn't open any sooner, or close any later. It simple generates more force, and opens the valve more.

What you are missing is that duration is the amount of time a valve is open .006" (or .050), since it opens farther for the same amount of cam lobe lift, it will reach the "measuring point" sooner, therefore adding a slight amount of duration. If 1.6 rockers had no impact on engine operation at all, they would not make any additional power. There is no free lunch.

You are partially correct. You are not changing the duration ground into the lobe of the cam. That remains constant. You are however changing the way the valve follows that cam lobe by lifting it faster (sooner) and further (higher).

You said:
Your not changing the duration, duration is the amount of time (in degrees) that the valve is open , your not physically changing the grind of the cam, sure it opens more, and the profile of the lift might be different , but it still opens the same amount of time.

You are so close to getting this right.

duration is the amount of time (in degrees) that the valve is open

Yes, and if we lift the valve faster (higher ratio rocker) we will reach the specified lift point more quickly (.006" or .050"). Less "lobe lift" will equal more "valve lift" and cams are measured at the valve.

your not physically changing the grind of the cam

This is correct. You are not changing the grind of the cam.

sure it opens more, and the profile of the lift might be different

Right! It is that change in "profile" that adds the duration to the "valve lift" (the cam grind didn't change, but the "lift curve" or "profile" of the lift/duration curve did.

but it still opens the same amount of time

The change in "profile" (you said it yourself) results in a slight change in duration at the valve, this will also have the effect of increasing the period of overlap (where both valves are open, this can impact vacuum production).

So, in order to lift a valve higher, we use a rocker arm with an increased ratio. This "ratio" is the ratio of "lobe lift" to "valve lift". By altering that ratio, we will begin to lift the valve sooner in order to get it to lift higher. This is how the profile changes. Since valve lift is a ratio of cam lift and we increased that ratio, we now lift the valve farther for the same amount of cam lift. The lift gain is not just at peak lift, it is everywhere along the profile of the lobe.

Think of it this way. If you multiply everything by a ratio.

Take a dollar (one dollar is a "small amount", think of a small amount of lift just as the valve begins to move). Add a 1.5 multiplier (rocker ratio), that dollar becomes 1.50 (a nice increase), now, with a 1.6 ratio, it becomes $1.60 (we gained 10 cents, so what?).

Now, take $100, it will be $150, or, at 1.6, $160. Now we have big gains, no longer did we gain "10 cents", we gained "$10". This is the same way the increased ratio of the rocker works. The gains are minimal at first (but there) and become much greater as things increase.

Better?

I started wondering if we were going to go back and forth on this forever... I opened up my Lingenfelter book and came across this line here..

"Increasing rocker ratio does change the camshaft's duration slightly but the main gain is the added lift through the valve curve."

With this, I apologize if I seemed at all ignorant and thank you for opening my eyes to this factor. I can sleep good tonight knowing, I learned something new today :D

oh ya, your $ thing just confused the hell out of me, as good as I am with $, it didn't do the trick ;)

Sorry about the bad example. Was trying to show that the nature of a ratio is "the more you have, the more you gain". That gains in the lift curve are small and get progressively larger as the lifter rides up the lobe.

Sure buddy, whatever you say, I agree with track bird all the way:D :cool: You must be one smart SOB when it comes to mathematics... wish I had your brains:bow:

~Jon

Jon,

I've just been at this for a very long time (I worked Tech support for one of the big mail order parts places for a while and had to explain this to many people). I did it for fun, then, I got paid for it. Now I just like to help out.

Kevin