[357cutlass]

Okay I have a 350 +.040 sbc, 4relief flat top pistons, recently put on a set of AFR's 180 Street Heads, 1.6 Pro Magnum rocker arms, +.100 comp cam Chromoly pushrods, RPM Air-Gap intake. I'm all confused on the theory of valve lift and duration lost and gained from valve lash and 1.6 rockers. My cam is the XS274S comp cam, and the cam card shows .501intake, .510exhaust lift, 230intake, 236exhaust durtaion, set to .016" valve lash. I once read every .002" valve lash is 1degree duration. I'm wondering if the duration on the cam card is the duration when set at .016", or simply the duration on the cam. My gross cam lift is .536intake, .545ehaust lift with the 1.6 rockers istalled. But again i once read 1.6 rockers add about 3degrees duration. Is this true?? Because the difference on the intake side from .501" (1.5's) to .536" (1.6's) is .035". and if every .002" is 1degree duration, this doesn't add up but saying 1.6 rockers only add 3degrees duration more than 1.5's. My question is what is the duration and lift on my XS274S comp cam, with .016" valve lash, and 1.6's rockers??

Dyno2000 shows the motor at 460HP @ 6000-6500rpm, 448FT/LBS @ 4500rpm

[automotive breath]

Camshaft specifications are normally provided measured at the cam lobe. Duration
is provided at a given lifter rise like duration at 0.020" or 0.050". Lift is
calculated using lobe lift times the rocker ratio with out regard to valve lash.

[techinspector1]

Duration is ground into the lobe at the time of manufacture and no amount of different rocker ratio will change it. The motor might like it because the lift is increased and it gets there a little quicker with a higher ratio because the ramp looks a little more agressive to the motor, but the duration remains the same regardless of the ratio.

[cobalt327]

The total duration is the same. Duration at 0.050" lift, or any other amount of lift after the valve begins to move, will show an increase of duration with an increased rocker ratio, IMHO.

[techinspector1]

Originally Posted by cobalt327

The total duration is the same. Duration at 0.050" lift, or any other amount of lift after the valve begins to move, will show an increase of duration with an increased rocker ratio, IMHO.

Physically impossible. The motor will see it as an increase in the RATE of lift and in the total amount of lift and the potential is there to make more power because of those things, but duration cannot be changed with rocker arm ratio.

[curtis73]

I agree that cam duration can't be changed, but the lift at the valve (which is what determines engine operation) will change... but its just semantics. Like you said, its just a difference in rate at which the valve opens. Duration and lift is ground into the cam and can't be changed with rocker ratio, but the reason the engine changes its operation with higher ratio rockers is because the movement at the valve has changed.

I think what I'm saying is... changing the rocker's ratio would have the same effect as making the lobe taller with faster ramps. The seat-seat duration won't change, nor will the @50 duration change at the lifter or the lobe... but the @50 duration WILL change at the valve. It will reach those lifts faster, and therefore ACT as if you added a bit of @50 duration.

Kinda like arguing about "effective gear ratio" when you put on larger tires. The gear ratio doesn't change, but it "acts" like a different ratio when it comes to RPMs and speed. The net result is that you decrease highway RPMs which has the same effect as changing the gear ratio.

The same can be said for lash. Decreasing lash will not change the cam's timing specs, but it will pull the valve off the seat earlier and put it back on the seat later. The cam doesn't change, but the lift curve profile at the valve does change and that is why it affects power.

I'm wondering if the duration on the cam card is the duration when set at .016", or simply the duration on the cam

The numbers on the cam card are at the lobe, not the lifter.

.

[cobalt327]

If you measure the duration in degrees at the crank w/a degree wheel @ 0.050" lift with a 1.5 rocker ratio and change the ratio to 1.6, the 0.050" duration will increase.

Or any other "open" duration.

[curtis73]

Originally Posted by cobalt327

If you measure the duration in degrees at the crank w/a degree wheel @ 0.050" lift with a 1.5 rocker ratio and change the ratio to 1.6, the 0.050" duration will increase.

Yes... if you measure it at the valve, not the pushrod.

[cobalt327]

Originally Posted by curtis73

if you measure it at the valve, not the pushrod.

Agreed.
--------

[357cutlass]

so if I take the numbers off the cam card, .536intake, .545exhaust lift (1.6rockers), I minus the .016" valve lash from total lift. And the duration doesn't change from the .016" valve lash?? or it changes 8degrees, .002" being 1degree duration.

[357cutlass]

is there a given number of duration 1.6 rockers give over 1.5's, at the valve. I'm trying to plug all this info into a dyno program, but notsure of the numbers for the cam. thanx for all the help

[carsavvycook]

Here is a link to read for yourself. The results are dyno tested.

http://www.vetteweb.com/tech/0204vet...tio/index.html

[F-BIRD'88]

The .050" duration does not change. It is measured at the cam.

The duration you want to know to enter into a dyno simulation program is effective seat to seat running duration. or camshaft lash point duration.


If you have 1.6 rockers and set then at .016" valve lash then your net "lash point" on the cam is .010" lifter rise. .016" divided by 1.6 = .010". Therefor the number you are looking for is the duration @.010" lifter rise on the cam. You can measure that on a cam measuring machine "Cam Doctor" Or measure it with the cam in the motor using a piston stop, degree wheel and dial indicator reading off the top of the lifter body edge. or off the top of the pushrod.
This is not the same the "advertized duration".

Most modern cams have non symetrical valve lash ramps that are different on the opening side and on the closing side. may be entirly different on the intake and exhaust lobes as well. So you have to actually measure it (in this case @ .010" lifter rise) unless you're the guy who designed the camshaft lobes. In that case you would not be asking. You'd be telling.

You'll find desk top dyno much more accurate if you enter this measured net seat to seat "lash point" duration. and also enter the expected actual WOT engine airflow consumption @ full power (about 550cfm for this motor) instead of the rated carburator cfm.

I get 422hp using the .050" cam spec numbers and letting the program guestimate the seat to seat. And 426hp by entering my guestimation of the net lashpoint duration with the 1.6rockers and .016" valve lash. 10:1 compression ratio

The program predicts 417hp with a 1.5 rocker and .016" valve lash.

[F-BIRD'88]

Now if you want to see where "the rubber actually meets the road"

Take you tools and your 1.5's and 1.6's to the drag strip. Make 3 psses with one rocker set and then switch to the other and make 3 more passes. Then change back to where you started, all at .016" valve lash.

Then put the 1.6's just on the intake valves and drag test.

[curtis73]

Originally Posted by F-BIRD'88

The .050" duration does not change. It is measured at the cam..

That's correct, rocker ratio doesn't add cam duration, but we've talked about this before. It will alter the duration @50 as far as the valve "sees." It doesn't change the duration of the measured cam lobe, but it "acts" like you've added duration.

Just like when you say you have 3.73 gears, and then switch to larger tires, those gears "act" like 3.55s. You haven't changed the gear ratio, but the net result of highway RPMs has changed. It is the same effect as changing gears as far as the net result is concerned.

I apologize for the psycho-long article, but maybe I'll post it on a wiki later if we can finally come to an agreement, because we're all arguing the same thing.

Cam duration and lift is cam duration and lift. It can't be changed unless you regrind the cam or change cams. Period. What you do with that lobe profile after the cam can be changed. How those things will change depends partly on the type of lifter used. How those changes will affect the engine are entirely up to the rest of the operating parameters of the engine

First, lets look at some hypotheticals. Let's start with a solid cam with 275 degrees of seat-to-seat duration, 270 advertised duration, 230 degrees of @.050" duration, and a lobe lift of .325". Let's say that you set up this solid cam with .010" lash and 1.5:1 ratio rockers. Total valve lift will be a theoretical .3875" but actually less. Seat-to-seat duration at the valve will not be identical to the seat-to-seat duration at the valve because of the lash. Because of the lash, the cam has to rotate farther before it starts generating lift at the valve. Now, increase that lash to .015". You have now effectively reduced seat to seat duration at the valve because the lobe has to travel even further before it generates the .015" lift required to start moving the valve off the seat. You haven't changed cam duration, but you have changed the amount of duration seen by the valve.

Now let's remove lash adjustment from the equation and focus on rocker ratio. Let's take that above cam and install it hypothetically with zero lash and 1.5 rockers. Seat to seat duration will be the same at the lobe and the valve. Measure and plot valvelifts at equal points in the duration, and it might look something like this:


Now install 1.6 rockers and it might look something like this:


Notice how they both have the same seat-to-seat duration, but the valve lift on the 1.6 reaches .050" lift sooner. This effectively increased the .050" duration as seen at the valve. The duration of the cam hasn't changed.

Now... its very important to make a distinction. This is all semantics and word/definition games and here's why: If you wanted the valve lift profile as seen by the blue curve, you could install the cam with 1.5 rockers. If you wanted to then change to the pink profile you could do it two ways; either switch to 1.6 rockers, or use 1.5 rockers and install a cam with a lobe profile that would give you the pink curve. That new lobe would have the same seat-to-seat duration, but more .050" duration and more lift/faster ramps. Another reason why this is all just a word game is because of what the seat-to-seat and .050" duration numbers mean. When we are choosing a cam, the seat-to-seat, advertised, and .050" numbers are simply a relative measure of ramp speed. Keeping the same seat-to-seat numbers but increasing .050" duration (which is the same as installing the 1.6 rockers in this example) simply increases the "area under the curve" which is allowing a greater time and flow area for the engine to inhale air. At its most base level, increasing rocker ratio (or increasing ramp speed at the cam) simply gets it lifted higher, sooner. This allows more effective flow through the valve. Its also important to outline that (since lift is affected more than duration), the main benefit is being able to increase torque (and therefore power) within the original RPM range. The increase to .050" duration at the valve is very insignificant and does little or nothing to raising the powerband as set by the duration of the cam lobe. If, however the increased area under the curve allows for greater flow and cylinder filling at peak RPMs, the increase in flow may allow higher RPMs. If the increased flow mass effectively extends the HP curve after the peak, you can shift later and increase the average HP that you send out to the drivetrain over the course of the race... but that's another article.

To summarize; changing rocker arm ratio does NOT change cam duration. It will however change what the engine thinks the .050" duration is. This same property holds true for hydraulic cams because they are set to zero lash. Changing the lash on a solid cam will also change the duration as it is seen by the valve... but the cam duration stays the same.

Class dismissed because I ran out of brain.