Do Cam Designers Expect a Consistent Rocker Ratio across Lift Curve?

[CamKing]

If it's fast off the seat, it's fast coming back on the seat too. From a dynamics standpoint, why would I want that?

You don't.
It's the exact opposite of what you want. Having a rocker with a higher ratio off the seat, increases overlap, without adding much area under the curve. It also increases acceleration off the seat, and back on to the seat, decreasing valvetrain stability, and increasing pushrod flex.

What would be ideal, is a rocker with a slightly lower ratio off the seat.

[hoffman900]

If it's fast off the seat, it's fast coming back on the seat too. From a dynamics standpoint, why would I want that?

You don't.
It's the exact opposite of what you want. Having a rocker with a higher ratio off the seat, increases overlap, without adding much area under the curve. It also increases acceleration off the seat, and back on to the seat, decreasing valvetrain stability, and increasing pushrod flex.

What would be ideal, is a rocker with a slightly lower ratio off the seat.

Thanks, Mike. That's exactly what my intuition was telling me.

[BradH]

David Vizard has a rocker arm 'chart' in his latest BBC book that shows some large variations in initial ratio & overall ratio for various brands/styles of rocker arms. How does 0.614" valve lift compared to 0.680" valve for the same lobe lift grab ya! Both rockers were supposed to be 1.8 ratio, overall ratios were 1.744 & 1.932

I bought a copy of that book recently, but haven't done a "deep dive" into it to have found that info.

OK, had some time to read the latest DV BBC book in more detail, including his info on rockers. Where I THINK he's coming from on the advantage of higher off-the-seat ratios applies to short-duration & RPM-limited builds typical of his hot street engines with good driveabilty. But for something more wide open as far as RPM, then I'd like to hear if he agrees with the drawbacks of that approach.

[SchmidtMotorWorks]

David Vizard has a rocker arm 'chart' in his latest BBC book that shows some large variations in initial ratio & overall ratio for various brands/styles of rocker arms. How does 0.614" valve lift compared to 0.680" valve for the same lobe lift grab ya! Both rockers were supposed to be 1.8 ratio, overall ratios were 1.744 & 1.932

I bought a copy of that book recently, but haven't done a "deep dive" into it to have found that info.

OK, had some time to read the latest DV BBC book in more detail, including his info on rockers. Where I THINK he's coming from on the advantage of higher off-the-seat ratios applies to short-duration & RPM-limited builds typical of his hot street engines with good driveabilty. But for something more wide open as far as RPM, then I'd like to hear if he agrees with the drawbacks of that approach.

For a cam design that is intended for common builds, he must use the nominal ratio to design around unless there is a particular geometry that is specific and reliable for that engine.

Assuming the cam is well designed, using a geometry that changes the motion defeats the design intent of the cam.

[David Vizard]

I bought a copy of that book recently, but haven't done a "deep dive" into it to have found that info.

OK, had some time to read the latest DV BBC book in more detail, including his info on rockers. Where I THINK he's coming from on the advantage of higher off-the-seat ratios applies to short-duration & RPM-limited builds typical of his hot street engines with good driveabilty. But for something more wide open as far as RPM, then I'd like to hear if he agrees with the drawbacks of that approach.

For a cam design that is intended for common builds, he must use the nominal ratio to design around unless there is a particular geometry that is specific and reliable for that engine.

Assuming the cam is well designed, using a geometry that changes the motion defeats the design intent of the cam.

John,
The intent is to get the greatest output from the engine.If it works reliably then the goal is achieved. AS I have said many times all the stuff I put in my books is the result of hundreds of hours of dyno testing. That is not even close to an opinion. If you had taken the time to read my BBC book you would have realized I pointed out both the pros and cons of the situation and more to the point, told how to make the best of the situation concerning any negatives.

I did a test of two 1.8 rockers and the one with the characteristics I advise to use made 18 hp and about 10 lbs-ft more.

DV

[David Vizard]

If it's fast off the seat, it's fast coming back on the seat too. From a dynamics standpoint, why would I want that?

Some experiments I did while working on Harvey cranes stuff involved different closure rates. After a ton of tests i found the the rate of reduction of breathing area on the closing side affects torque - especially in the lower rpm range. Here is the crux of the matter. You may not like the fast closure dynamics but if you are a racer you will appreciate the stronger output.

My 'A' Series Mini cooper cams had virtually no opening ramp and almost no closing ramp. The dynamics could have been made better but output would have suffered. The old saying is that proof of the pudding is in the eating. The race version of that is the BS stops when the flag drops.
Even ten years after I did the work on these cams with my partner in crime David Anton of APT the deal was if you wanted to win in an 'A' series powered car then it was virtually mandatory to use a DV/DA developed cam.

Remember at the end of the day you should be engineering to win. If dynamics have to be compromised for power then so be it.

[GARY C]

OK, had some time to read the latest DV BBC book in more detail, including his info on rockers. Where I THINK he's coming from on the advantage of higher off-the-seat ratios applies to short-duration & RPM-limited builds typical of his hot street engines with good driveabilty. But for something more wide open as far as RPM, then I'd like to hear if he agrees with the drawbacks of that approach.

For a cam design that is intended for common builds, he must use the nominal ratio to design around unless there is a particular geometry that is specific and reliable for that engine.

Assuming the cam is well designed, using a geometry that changes the motion defeats the design intent of the cam.

John,
The intent is to get the greatest output from the engine.If it works reliably then the goal is achieved. AS I have said many times all the stuff I put in my books is the result of hundreds of hours of dyno testing. That is not even close to an opinion. If you had taken the time to read my BBC book you would have realized I pointed out both the pros and cons of the situation and more to the point, told how to make the best of the situation concerning any negatives.

I did a test of two 1.8 rockers and the one with the characteristics I advise to use made 18 hp and about 10 lbs-ft more.

DV

John, How well can one design a cam if they don't know what the rocker is doing with the motion provided by the lobe? Seems like they should be looked at as one.

[SchmidtMotorWorks]

For a cam design that is intended for common builds, he must use the nominal ratio to design around unless there is a particular geometry that is specific and reliable for that engine.

Assuming the cam is well designed, using a geometry that changes the motion defeats the design intent of the cam.

John,
The intent is to get the greatest output from the engine.If it works reliably then the goal is achieved. AS I have said many times all the stuff I put in my books is the result of hundreds of hours of dyno testing. That is not even close to an opinion. If you had taken the time to read my BBC book you would have realized I pointed out both the pros and cons of the situation and more to the point, told how to make the best of the situation concerning any negatives.

I did a test of two 1.8 rockers and the one with the characteristics I advise to use made 18 hp and about 10 lbs-ft more.

DV

John, How well can one design a cam if they don't know what the rocker is doing with the motion provided by the lobe? Seems like they should be looked at as one.

Right, in Utopia, the geometry is known, but in reality (as we often see here), people can't agree even on what the desired geometry is or determine what it actually is.

I examined a wide selection of rocker arms for a head I designed at Edelebrock to optimize the dimension for rocker pivot point.
The biggest variation I found between all of the rockers was on the pushrod side.
Let's say that if everyone agreed to use mid-lift for pivot point, they would still have a wide variation caused by the differences in the position of the ball/socket on the pushrod side. One of my colleagues there pointed out how difficult of a problem this is for a hobbyist that just wants to buy some parts and assemble them, so he started making a table to help sort it out but it became very complex before all of the issues were resolved.
The other problem was consistency and access to data, he had a vast collection of heads to take measurements from but there was no upside to publishing the data (which was inconsistent anyhow, QC in the performance aftermarket is very loose).

I think the best you can do is to try to find a pivot point that makes the lifter to valve motion ratio as consistent as possible, then choose a cam that is designed for lifter motion. That way, if you choose another cam, you don't need to reconsider the other variables.

[3V Performance]

I'm curious whether cam designers have a working assumption that the rocker ratios being used in their models have consistent ratios across the lift curve. I would think the fact that some rocker arm brands & systems can have regressive ratio curves (higher ratio off the seat, then the ratio drops off as the lift increases) would have an impact on the overlap triangle and area under the curve calculations, as well as modeling flow at different points in the induction & exhaust cycles. Thanks - Brad

Exactly right Brad! Even some experienced engine builders are surprised at how much difference can be made by utilizing rockers with a faster opening rate althoug it is not as cut and dried as just swapping one for another.

DV

Looking at spintron valve traces this can be good and can be bad. MOST systems fight opening deflection issues and when adding a more aggressive ratio early can make matters worse. ( even more deflection ). A lazy lobe and light spring package can benefit from a more aggressive valve motion so some times it a win.

[RevTheory]

I'd love to see spintron data on Comp's QXI lobes and what's needed to make them stay halfway settled down.

[SchmidtMotorWorks]

I'd love to see spintron data on Comp's QXI lobes and what's needed to make them stay halfway settled down.

Do you have a graph of the acceleration curve?
Without that, you are throwing darts in the dark.

[RevTheory]

I'd love to see spintron data on Comp's QXI lobes and what's needed to make them stay halfway settled down.

Do you have a graph of the acceleration curve?
Without that, you are throwing darts in the dark.

Throwing darts in the dark for wanting to see data that someone already compiled? I don't expect to see it as it's expensive and likely private but I'd still like to see what it looked like.

That's all I'm saying.

[SchmidtMotorWorks]

I'd love to see spintron data on Comp's QXI lobes and what's needed to make them stay halfway settled down.

Do you have a graph of the acceleration curve?
Without that, you are throwing darts in the dark.

Throwing darts in the dark for wanting to see data that someone already compiled? I don't expect to see it as it's expensive and likely private but I'd still like to see what it looked like.

That's all I'm saying.

Just measure one.

The results will vary by the mass and stiffness of every component.

The acceleration curve tells you how difficult the challenge is and what parts are most likely to be excited by the design.