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  #16 (permalink)  
Old 11-03-2008, 11:58 AM
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Quote:
Originally Posted by 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.
1.6s don't add duration, the events of the cam you have will start and end in the same place they do with a 1.5 rocker. What the 1.6 does is increase the rate of valve opening per degree of event. They also have a higher net opening over the top of the lobe.

In the first instance of increasing lift per degree of rotation, this makes space for greater flow thru the valve both at the start of lift and at the end. For the intake it makes the engine behave as if it had a few more degrees of overlap on the opening side and a few degrees later on the closing side. But this is from greater flow potential related to lift not any change to duration. This will result in a bit of a choppier idle, less bottom end torque and higher top end power. In the second instance for the intake there will be more over the top of the lobe opening, at low RPMs this will reduce mixture velocity in the port's reducing bottom end torque and increasing top end horsepower.

Similar things happen to the exhaust, the closing valve will hold more lift later into the cycle but the rate of change is higher, the effect like the intake is to make the engine respond as if it had more cam. Blow down will be more effective sooner as the valve will open quicker, the valve will be open further over the top of the lobe and will hold more opening at the valve on the closing side. These affects reduce lower RPM torque by reducing exhaust gas velocity in the ports and headers but they add horsepower on the top end.

Unfortunately if your exhaust system isn't up to flowing the exhaust the engine can produce, putting the 1.6s on the exhaust valve will buy nothing. The reason is that when a port or a pipe connected to a port has reached its maximum rate of flow, when you come back to the cam and valve timing, there is nothing to gain by increasing lift or the rate of lift because the pipe can't flow any more so opening the valve more has no effect. The solution here is either a larger pipe that flows more in any given moment, or a cam that holds the valve open for more moments, in other words an increase in the duration time will make better use of the pipes abilities to flow. However, you're constraint is 8 ports into one pipe. You either need to approach this from the thought of duals or a larger single say at least 3 inches. That of course means that the catalytic converter if so equipped and the muffler must also have 3 inch inlets and outlets. Since most of this stuff is just is made on housings designed for 2 inch pipes but have 3 inch pipes attached for mating purposes, there is usually no gain by using very large single exhaust systems.

The spark lead an engine likes is largely related to dynamic compression ratio, mixture quality, and combustion chamber design. The dynamic compression ratio ties back to the static ratio and how that is usually reduced by cam timing, induction and exhaust design. Basically the weaker the mixture (i.e. low idle vacuum, high contamination with exhaust, excessively lean or rich carburation) make for a mixture that dosen't burn well, this takes more initial advance to get the time to get the time necessary to have the reaction go toward completeness so the engine will run at idle. The shape of the combustion chamber also plays on this where older open chambers for example burn less efficiently than tight chambered heads, thus they need more spark lead than more modern design chambers. This goes on and on into squish/quench clearance, spark plug location, valve size, port shapes and more, but I think you're getting the point without me writing a book.

I think your immediate efforts need to be aimed at getting a decent set of dual exhausts on the car. There are aftermatket crossmemebers for the F -body that will pass pipes to either side of the gear box.

Bogie

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  #17 (permalink)  
Old 11-03-2008, 09:19 PM
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I look upon swapping the rockers around as a fine-tuning tool as someone had described it.

The exhaust is a 2 1/2" mandrel-bent piece, I'm sure it would breathe better with a 3" job under there but I have no complaints with power output. I am surprised at the RPM potential of the combo considering a relatively conservative cam and single exhaust - credit that to good-flowing cyl heads and some compression.

If I swap the rockers around I'll dig this thread up and post what I found. Thanks for your responses.
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  #18 (permalink)  
Old 11-04-2008, 12:02 AM
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Quote:
Originally Posted by ap72
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 250 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.
You have this preconceived notion of maximum "acceptable" streetable valve timing duration (probabily because you can't (yet) tune a carburator or ignition system currectly) and a false preconcieved notion of how rocker ratio effects valve action in a running motor that are both false and ill conceived. Probabily both from a lack of experience or know how and poor understanding of basic applied physical mechanics and high performance engine tuning.

Simple lever mechanics proves that you cannot change the valve event timing by changing the rocker ratio. the valves will start to.... and finish moving at exactly the same time reguardless of the rocker (simple lever) ratio.
The rocker ratio only effects the rate of lift of the valve, within that set motion duration as set by the cam lobe.
This has a very moderate and quite different effect on a running motor than a real change in actual camshaft duration. (a differnt camshaft)
Valve lash and its effect is a totally different thing and not comparable.

I didn't create the rules of mechanics and simple levers and reguardless of how you spin it you won't change those simple rules.
You'll find a lot more success (and a lot less knuckle damage) in fixing , tuning and hot rodding cars if you get the rules of applied mechanics straight in your mind and work within them.

Don't take it personally. We are not born with this knowledge or experience.

Do a google search "simple machine" there are 6 of them that haven't changed in 1000's of years. These are the basics of all applied mechanics.
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  #19 (permalink)  
Old 11-04-2008, 02:26 AM
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Sorry guys, I have to agree with F-Bird on this for sure.

When using a 'degree wheel' and a 'dial indicator' gauge, to degree your camshaft. It does not make any difference on lift or duration at all, whether you take your readings at the camshaft, the lifter, the end of the push rod, or the push rod end of the rocker. The valve end of the rocker, will still show the same duration specifications, but will multiply the lift by the rocker ratio only.

Why do you think the Cam Card only comes with 'lift' spec's for the valve, and the camshaft @ .050? Valve lift is cam. lift times 1.5

Have you not noticed the 'cam' card does not show any other different "duration" spec's, per rocker ratio? Like .060, or .070.

This is because there is none.

I'm sorry to be so blunt.

Are we all still friends?
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  #20 (permalink)  
Old 11-04-2008, 06:14 AM
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Quote:
Originally Posted by F-BIRD'88
You have this preconceived notion of maximum "acceptable" streetable valve timing duration (probabily because you can't (yet) tune a carburator or ignition system currectly) and a false preconcieved notion of how rocker ratio effects valve action in a running motor that are both false and ill conceived. Probabily both from a lack of experience or know how and poor understanding of basic applied physical mechanics and high performance engine tuning.

Simple lever mechanics proves that you cannot change the valve event timing by changing the rocker ratio. the valves will start to.... and finish moving at exactly the same time reguardless of the rocker (simple lever) ratio.
The rocker ratio only effects the rate of lift of the valve, within that set motion duration as set by the cam lobe.
This has a very moderate and quite different effect on a running motor than a real change in actual camshaft duration. (a differnt camshaft)
Valve lash and its effect is a totally different thing and not comparable.

I didn't create the rules of mechanics and simple levers and reguardless of how you spin it you won't change those simple rules.
You'll find a lot more success (and a lot less knuckle damage) in fixing , tuning and hot rodding cars if you get the rules of applied mechanics straight in your mind and work within them.

Don't take it personally. We are not born with this knowledge or experience.

Do a google search "simple machine" there are 6 of them that haven't changed in 1000's of years. These are the basics of all applied mechanics.

Lash and preload are VERY simmilar when considering rocker arm changes. And I do know how a lever acts, I also know that the lash or preload of a lifter is filled in ever so slightly quicker with a higher ratio rocker arm. That's not even a simple machine, its just a simple action. And I don't hold your lack of knowledge against you. And I don't think that "expierence" means anything unless you fully understand what is happening. Without understanding no knowledge can be derived from expierence, just anecdotes.

I'm not saying there's a great deal of seat to seat duration change- a few degrees at most and only on lazy cams, nonetheless you cannot ignore the effects of preload or lash if you want to have a full grasp of the action of the valve.
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  #21 (permalink)  
Old 11-04-2008, 08:15 AM
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Rusty,
rocker ratio's changes are explained on page #3 of this David Vizard article (but you need to read the whole article):

http://www.popularhotrodding.com/tec...ics/index.html

and here's a Dynomax's pipes to cubes/HP sizing chart pic'

malc,
thanks for the dyno graph chuckle...
8000rpm HP peak street motor combo (in your car?)!!!!!
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  #22 (permalink)  
Old 11-04-2008, 01:02 PM
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Red,

Thanks for the link, the Wizard and I go way back - he mentioned that I had the rockers on the right side last time he stopped in at the garage for a beer and to see how the car was running...

I've got a handful of his books and he says essentially the same thing, but here's a line from your link:

As for the exhaust, we find that it is relatively insensitive to valve acceleration but is sensitive to duration. For this reason the rocker ratio used on the exhaust is best kept about 0.1 to 0.2 of a ratio lower than the intake ratio.

I started the post to see if anyone else had experimented with moving rockers around, something like what Malc posted (but somewhere less than 800 HP peaking at 8K?) Still very interesting though.

The exhaust will have to stay as is, even if someone dropped a 3" Dynomax exhaust system at my front door I can't say when I'm going to find time to swap it. Not 'til the kids get older then I can utilize that child labour!
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  #23 (permalink)  
Old 11-04-2008, 04:41 PM
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Ok... I got frustrated and made a drawing. Its remedial, and not to scale, but it demonstrates my point.

The black arc represents the lift at the VALVE with 1.5 rockers. The grey arc represents the valve lift with 1.6 rockers.

Although both arcs still intersect the bottom line at the same point (same seat-to-seat duration) the point at which the VALVE reaches higher lifts is faster with the 1.6 rockers. The reason I was saying that it changes the effective timing is because what the valve SEES with larger ratio rockers would be equivalent to changing the lobe profile to have faster ramps and a taller lobe lift.

Rockers DON'T change the cam duration specs, but they in effect add .050" duration at the VALVE as indicated by the blue and red lines.

Increasing rocker ratio doesn't add seat to seat duration, but it DOES simulate additional @.050" duration. I think we're all arguing the same point using different words, but this drawing should help us all get on the same page.

I won't get into the disucssion about lash and seat-to-seat timing. I'm simply discussing area under the curve and valve opening/closing speeds.
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Old 11-04-2008, 04:52 PM
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your drawing also has to have a line right below the red one that has an area for .010" travel of the rocker arm tip which is where the lash is taken up and the valve begins to move off of the seat. That's where my argument lies.
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Old 11-04-2008, 06:22 PM
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Quote:
Originally Posted by ap72
your drawing also has to have a line right below the red one that has an area for .010" travel of the rocker arm tip which is where the lash is taken up and the valve begins to move off of the seat. That's where my argument lies.
The lash value is taken up by lobe travel on hydraulic lifters, not rocker travel, so regardless of what is on the valve side of the rocker stud, the fact remains that on the short side, it will still require the same amount of lobe lift to begin activating the valve. THEN, once it opens it moves faster, but the lash is in the lifter. If it was .010" before the rocker swap, it will be .010" AFTER the rocker swap.

With solid lifters, i would agree since lash is physically determined at the valve. If you set lash to .010" at the valve before the swap AND after, then the lash will be taken up faster and add a fraction of a degree to the duration... but still, not enough to practically measure.
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Old 11-04-2008, 06:53 PM
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Quote:
Originally Posted by curtis73
The lash value is taken up by lobe travel on hydraulic lifters, not rocker travel, so regardless of what is on the valve side of the rocker stud, the fact remains that on the short side, it will still require the same amount of lobe lift to begin activating the valve. THEN, once it opens it moves faster, but the lash is in the lifter. If it was .010" before the rocker swap, it will be .010" AFTER the rocker swap.

With solid lifters, i would agree since lash is physically determined at the valve. If you set lash to .010" at the valve before the swap AND after, then the lash will be taken up faster and add a fraction of a degree to the duration... but still, not enough to practically measure.
Thank you Curtis.

AP72, I hope this helps, to better explain to you, what F-Bird, and myself, have tried to do.

Stephen
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Old 11-04-2008, 07:11 PM
How fast is fast enough?
 
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Quote:
Originally Posted by curtis73
With solid lifters, i would agree since lash is physically determined at the valve. If you set lash to .010" at the valve before the swap AND after, then the lash will be taken up faster and add a fraction of a degree to the duration... but still, not enough to practically measure.
Thankyou. and anything is measurable with the right tools.
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