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Old 04-12-2009, 10:32 PM
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Solid Camshaft lift and duration changes...fusterated

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

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Old 04-12-2009, 10:56 PM
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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.
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Old 04-12-2009, 11:14 PM
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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.
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Old 04-12-2009, 11:39 PM
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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.
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Old 04-12-2009, 11:41 PM
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Quote:
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.

Last edited by techinspector1; 04-12-2009 at 11:49 PM.
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Old 04-12-2009, 11:56 PM
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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.
Quote:
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.

.

Last edited by curtis73; 04-13-2009 at 12:11 AM.
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Old 04-13-2009, 12:12 AM
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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.
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Old 04-13-2009, 12:15 AM
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Quote:
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.
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Old 04-13-2009, 12:17 AM
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Quote:
Originally Posted by curtis73
if you measure it at the valve, not the pushrod.
Agreed.
--------
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Old 04-13-2009, 12:24 AM
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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.
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Old 04-13-2009, 12:31 AM
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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
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Old 04-13-2009, 01:26 AM
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Here is a link to read for yourself. The results are dyno tested.

http://www.vetteweb.com/tech/0204vet...tio/index.html
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Old 04-13-2009, 11:06 AM
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Quote:
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.
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Old 04-13-2009, 11:16 AM
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Ok, I lied. I just had a smoke and I have enough brain power for one last graph to demonstrate the lash/duration thing.

I used the original chart and I added lines to demonstrate how increased lash will reduce duration. I used completely unrealistic lash numbers, but I exaggerated to show the point more clearly. Take a look at this chart. It shows lines that represent lash settings of (a very exaggerated) .100" and .200". Notice how the more lash you add, the less duration and lift you'll be using of the cam's lobe profile. The cam's duration hasn't changed, but the effective duration as seen at the valve does.



This was my original reason for using the "effective gear ratio" as an analogy. Changing tire size doesn't affect the physical number of teeth on the ring gear, but the net effect is the same. If you want to decrease your highway RPMs you can do it two ways in this demonstration; you can increase tire size, or you can increase gear ratio. If you choose increased tire size (much like choosing more rocker ratio) you aren't changing the physical gears (like you're not changing the actual cam lobe), but you are changing the highway RPMs. (the .050" duration)

Saying that reducing lash adds effective duration is the same as saying adding tire diameter will change the effective gear ratio. In both cases, you haven't changed the parameters of the physical part, you have simply effectively exacted a change that simulates having changed the physical part.

Last edited by curtis73; 04-13-2009 at 11:22 AM.
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Old 04-13-2009, 11:33 AM
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My original comment was an observation as to what the engine "sees", not any attempt to rewrite the laws of physics and was offered in the context of the OP's first post- i.e. what effect that rocker ratio (and other parameters) had on his engine. In that context, my statement is valid, although easily misconstrued.

Duration- as seen on a cam card for instance- is tappet lift. Always unchanged- ground into the cam.

But as shown in detail by curtis73, the engine sees things differently when ratios are changed.

Perhaps it would have been more accurate to say,

“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.”

FIRST, instead of the non-qualified statement,

“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.”

My apologies if this was a concern.
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