[Rustydawg]

I've searched on this topic and it's confirmed my beliefs that there is sometimes a noticeable duration increase going from 1.5 to 1.6 ratio rockers - I'm curious to hear what resulted from anyone else experimenting with this.

I bought a set of unused Crower rockers off a friend, 8 are 1.5, 8 are 1.6 ratio. When I assembled my 355 SBC the 1.6 ratio went on the intakes.

The camshaft is a single pattern Comp 268H (218/218* @ 0.050") and Perf RPM heads, 10:1. The engine likes a surprising amount of initial advance - I'm running 14* and it would like more if I could hot crank it.

Just how much 'detuning' can one expect from swapping the 1.5 ratio rockers to the intake and the 1.6 to exhaust?

The exhaust on this car is squeezed down from headers to a single 2.5" exhaust to clear the chassis (exhaust-unfriendly '86 F-body) so the exhaust side probably deserves the 1.6 ratio more than the intake.

[curtis73]

I think you'll pick up some power and torque by swapping. Especially if you're running a single pattern cam, that exhaust side might really like the extra flow time and area.

[Rustydawg]

Yeah Curtis, I'm hoping you're right!

I'm also hoping someone has done a similar swap but for the most part a guy's going to have a whole set of 1.6 or 1.5, not half & half.

And a racer doing the same thing will be working with another 30 degrees of .050" duration too, and not likely exhaust restricted like my case.

[automods]

dual profile camshafts use longer duration and higher lifts on the exhaust side. im sure it will be better to swap to 1.6 on the exhast

[ScoTFrenzel]

Basicly swapping rockers is just compensating for a mis-speced cam. Or maybe I should say, fine tuning the cam specs.

Every situation will be different. There is no way of knowing what will help your car without trying them all on a chassis dyno. It might be that 1.5s on all will be the best. You could even try more or less cam advance.

Generally speaking, more lift makes more power because more fuel gets into the engine in the same amount of time.
Remember you are fighting a restricted exhaust.
More exhaust lift/duration @ .050 might actually increase back pressure into the cylinder and reduce power.

[F-BIRD'88]

You're wasting your time. Fix the exhaust system.

Changing the rocker ratio does not change the valve open/close event timing.
Not one degree. It only changes the valve motion. (rate of lift)

[curtis73]

Originally Posted by F-BIRD'88

You're wasting your time. Fix the exhaust system.

Changing the rocker ratio does not change the valve open/close event timing.
Not one degree. It only changes the valve motion. (rate of lift)

I agree and disagree at the same time. Rocker ratio doesn't change seat-to-seat valve timing, but it DOES change .050" lift numbers. It reaches those lifts faster and therefore simulates faster ramps and increases .050 duration.

Regardless of where the events are altered (at the lobe or the rocker) the net result is the same. The most important factor is how the valve moves. If you use higher ratio rockers to alter the valve movement, the cam events are changed in the eyes of the engine.

I also agree that rocker ratio changes are a fine-tuning thing. I would also suggest putting a proper exhaust on it and consider using a split-duration cam. SBCs typically can benefit from more exhaust duration since they are slightly hindered in that side of the head.

[F-BIRD'88]

Originally Posted by curtis73

I agree and disagree at the same time. Rocker ratio doesn't change seat-to-seat valve timing, but it DOES change .050" lift numbers. It reaches those lifts faster and therefore simulates faster ramps and increases .050 duration.

Regardless of where the events are altered (at the lobe or the rocker) the net result is the same. The most important factor is how the valve moves. If you use higher ratio rockers to alter the valve movement, the cam events are changed in the eyes of the engine.

I also agree that rocker ratio changes are a fine-tuning thing. I would also suggest putting a proper exhaust on it and consider using a split-duration cam. SBCs typically can benefit from more exhaust duration since they are slightly hindered in that side of the head.

The .050" lift duration does not change cause it is measured at the lifter. The rocker has nothing to do with it. The camshaft or its specs do not get bigger.
The valves open and close at exactly the same time reguardless of rocker ratio.
Anything else spun into it, is deceiving and wrong.
changing the exhaust rocker ratio will not change the exhaust valve timing at all.
Most small block chevs in fact respond to a change in intake rocker ratio more than exhaust rocker ratio.
The exhaust side responds to changes on valve event timing way more than rocker ratio changes. That requires a new camshaft.

Changing the camshaft or the rocker ratio will not fix the MAJOR issue of the exhaust pipe being way way way too small.
You can fit a much bigger main exhaust pipe and Y collector on a third gen F-Body. Try blowing air thru a straw.

[malc]

Power improvement was found using unequal rocker ratios by installing the higher ratios on the four corner cylinders, 1, 2, 7, 8.
This apparently makes up for the differences in intake manifold runner lengths.
Four horsepower increase was found @7600rpm using 1.7 rockers on the intakes of 1, 2, 7, 8 and again an increase when 1.65 rockers were installed on the exhausts.
A power drop was experienced when higher ratios were added to the middle cylinders.



Higher rocker arm ratios increase valve lift and effective duration.
Like stated above most of us have either 1.5 or 1.6 ratios all round.

[curtis73]

Originally Posted by F-BIRD'88

The .050" lift duration does not change cause it is measured at the lifter. The rocker has nothing to do with it. The camshaft or its specs do not get bigger.
The valves open and close at exactly the same time reguardless of rocker ratio.
Anything else spun into it, is deceiving and wrong.
changing the exhaust rocker ratio will not change the exhaust valve timing at all.

I'm not saying it changes the CAM timing, nor am I saying it alters seat-to-seat valve opening and closing events. I'm saying the lift profile at the valve changes, and that is what sets the operating parameters of the engine. Since a larger ratio rocker lifts farther and faster, it reaches .050 at the VALVE quicker. The actual lift values at the valve have been altered. Changing from 1.5 to 1.6 ratio rockers is the same as altering the lobe profile by 6.7% greater lift at all points of the duration. It has the same net effect at the valve as altering the cam lobe. You haven't changed the seat-to-seat duration, but you have altered the lift profile and .050" duration as it is seen at the valve. I know that .050" duration is measured at the lifter, but it has the same effect as if you had more aggressive ramps on a cam. The engine acts as if it has a cam with the same seat-to-seat duration, but more .050" duration when you switch to larger rocker ratios.

Of course it doesn't change cam timing, but if it didn't change valve events then it wouldn't change power attributes either.

[ap72]

Originally Posted by curtis73

I'm not saying it changes the CAM timing, nor am I saying it alters seat-to-seat valve opening and closing events. I'm saying the lift profile at the valve changes, and that is what sets the operating parameters of the engine. Since a larger ratio rocker lifts farther and faster, it reaches .050 at the VALVE quicker. The actual lift values at the valve have been altered. Changing from 1.5 to 1.6 ratio rockers is the same as altering the lobe profile by 6.7% greater lift at all points of the duration. It has the same net effect at the valve as altering the cam lobe. You haven't changed the seat-to-seat duration, but you have altered the lift profile and .050" duration as it is seen at the valve. I know that .050" duration is measured at the lifter, but it has the same effect as if you had more aggressive ramps on a cam. The engine acts as if it has a cam with the same seat-to-seat duration, but more .050" duration when you switch to larger rocker ratios.

Of course it doesn't change cam timing, but if it didn't change valve events then it wouldn't change power attributes either.

There's no point in trying to argue with Fbird about this one, he's adamant that the rocker arms do not play a role in seat to seat duration. We all know that it does, but as many times as I've seen this point argued on this board he refuses to conceed it. j
ust like I refuse to conceed that 250º duration on the intake lobe of a cam in a chevy 350 is not perfectly acceptable on the street-
well maybe its not the same thing exactly since mine is a matter of preference and a higher rocker ratio extends seat to seat time regardless of drive preference.

And it does actually change seat to seat timing, that is one area where you're off. Its just like tightening the lash- the cam stays the same, the valve's action changes.

[curtis73]

I see what he's saying... we're just arguing semantics. He's right when he says that cam events are fixed with the grind. Rocker arms don't change the cam. I'm just saying that rocker arms can change the valve events and he's just arguing the semantics of the definition of cam timing events.

Its all good, but you're right... rocker arm ratio changes will change valve timing events, and they would tend to lift the valve off the seat a little quicker, but since hydraulic lifters have no lash its effect would be immeasurable. The oil suspension in the lifter begins to lift the valve when its pressure overcomes the valve spring. Since you aren't changing that pressure it would theoretically still open at the same time... or at least within a fraction of a degree. Technically changed, but not noticable to the engine I wouldn't think. Solid lifters would be affected since the lash is set at the valve side of the rocker. It would take less lift at the lobe to take up the lash and therefore the seat to seat duration would increase slightly.

The main thrust of what I was trying to demonstrate is that by increasing ratios you are in effect simulating a taller lobe with more aggressive ramps. It creates more area under the curve, which (if your heads are up to the task) will create more flow and more power.

[ap72]

Originally Posted by curtis73

I see what he's saying... we're just arguing semantics. He's right when he says that cam events are fixed with the grind. Rocker arms don't change the cam. I'm just saying that rocker arms can change the valve events and he's just arguing the semantics of the definition of cam timing events.

Its all good, but you're right... rocker arm ratio changes will change valve timing events, and they would tend to lift the valve off the seat a little quicker, but since hydraulic lifters have no lash its effect would be immeasurable. The oil suspension in the lifter begins to lift the valve when its pressure overcomes the valve spring. Since you aren't changing that pressure it would theoretically still open at the same time... or at least within a fraction of a degree. Technically changed, but not noticable to the engine I wouldn't think.

The main thrust of what I was trying to demonstrate is that by increasing lift you are in effect making the lobe taller. It creates more area under the curve, which (if your heads are up to the task) will create more flow and more power.

hydraulic lifters have "preload" which is very comparative to lash, like lash it can be adjusted and like lash rocker arm ratio can cause it to "fill in" faster and in effect reduce the seat time a slight amount for a given cam profile.

[curtis73]

Originally Posted by ap72

hydraulic lifters have "preload" which is very comparative to lash, like lash it can be adjusted and like lash rocker arm ratio can cause it to "fill in" faster and in effect reduce the seat time a slight amount for a given cam profile.

Agreed, but in the perview of how it actually affects seat-to-seat valve timing events when comparing rocker ratio changes is minimal. The important value is the time spent at mid and higher lift values.

[ap72]

Originally Posted by curtis73

Agreed, but in the perview of how it actually affects seat-to-seat valve timing events when comparing rocker ratio changes is minimal. The important value is the time spent at mid and higher lift values.

Well, it can be, or it can make a great deal of difference. Sometimes it can affect dynamic compression enough to require adding a degree of timing or carb jetting requirments, only in extreeme cases like moving from 1.5 to 1.7, but it can make a difference nonetheless.