Progressive/Digressive rockers, flat tapppet..pros/cons?

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Progressive/Digressive rockers, flat tapppet..pros/cons?

Post by modok »

for a long time I've been aware a lot of rockers are not constant ratio.
Some are very progressive.
And I had assumed in the past.....that this was potentially an advantage. Or at least.... not always a bad thing?


Camking mentioned that a digressive ratio would be what you want......interesting
and for roller cam I can see that.....but is it also true for a flat tappet setup?
I know the lift rate is limited by lifter diameter, nose raduis can only be so small.... but I don't think I understand enough about cam profiles to fully understand.
The opening and closing ramps can be as gradual as you want, so, that seems to work... BUT.....is it an advantage? maybe no.
A rocker with a high ratio during mid lift, and lesser ratio closed and open would certainly be a good thing, but that's not easily possible.


I got some aftermarket rockers, mocked it up and measured EVERYTHING, and found the ratio is about 1.62 at low lift, and about 1.52 at high lift, because the pushrod pivot is very "low'.
I'd have to do some heavy modification to back the adjuster far out enough get to constant ratio, and that would hit the valve covers too. :roll:
is this a big problem....not really, but
Got me thinking about it again.

Is there any possible advantage to a progressive ratio rocker on a flat tappet cam.... if the cam profile WAS designed to suit it?
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Re: Progressive/Digressive rockers, flat tapppet..pros/cons?

Post by Baprace »

Try changing the pushrod length and you will see the net valve lift changes.
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Re: Progressive/Digressive rockers, flat tapppet..pros/cons?

Post by modok »

Yeah I know I can set it up all kinds of different ways, but, it's still going to be progressive ratio.

the only way to make this rocker constant ratio is going to be... welding.
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Re: Progressive/Digressive rockers, flat tapppet..pros/cons?

Post by gmrocket »

Are you mocking it up using your actual running springs or some soft test springs?

It’s not uncommon to lose some lift with high poundage running springs.

Just tested some 1.65 T&D’s with soft springs to check clearances.. lift at retainer was pretty close through out the lift cycle. When tested with the 800 lbs open springs, lift at max figured out to about 1.6
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Re: Progressive/Digressive rockers, flat tapppet..pros/cons?

Post by modok »

I checked it three ways.
First I measured all the parts well enough to make at least a two dimensional blueprint, from which the instantaneous ratio could be calculated at any position. I can measure just about anything.

Second I mocked it up with a checking spring and actually checked it with a dial indicator on the lifter and valve.

Third I measured the sideways displacement of the pushrod, and contact point of the roller on the stem to see where the the arc each was.

Give or take a a few thou, all three ways reach the same conclusion.
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Re: Progressive/Digressive rockers, flat tapppet..pros/cons?

Post by modok »

And I got it progressive and digressive backward in the first post....oops.
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Re: Progressive/Digressive rockers, flat tapppet..pros/cons?

Post by Rick! »

I think the antonym to progressive is regressive - which fits what you are posing to the forum.
Digressive, though what many posts could be considered, doesn't seem to be the word you are looking for to describe the reduction in rocker arm ratio as cam lift increases.
Back on topic, cam lift profiles seem to be "smooth" enough not to induce jerk once past the ramp so a progressive rocker ratio could be feasible. One could possibly build in ratio near the end of the lift to compensate for bending that occurs in the rockers. For a flat tappet cam, the "on the nose" spring forces are probably not great enough to bend aluminum roller rockers significantly, at least not like with a healthy solid roller IMO.
I guess the other side of the coin is: Does the change in rocker ratio during lift make a significant change in head flow and performance? I don't have that answer and after going thru all the Spintron info, making prettier lift curves doesn't necessarily make for measurable improvement.
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Re: Progressive/Digressive rockers, flat tapppet..pros/cons?

Post by CamKing »

If the rocker ratio is higher at the base circle, then it is at max lift, you are increasing the velocity and acceleration in the worst place possible, right where you are taking the valve off the seat, and compressing the valvetrain, and right where you are seating the valve.
Performance wise, you are increasing the area before TDC on the intake, and increasing overlap area. Neither of those, are a good idea.
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Re: Progressive/Digressive rockers, flat tapppet..pros/cons?

Post by hoffman900 »

CamKing wrote: Thu Apr 30, 2020 12:22 pm If the rocker ratio is higher at the base circle, then it is at max lift, you are increasing the velocity and acceleration in the worst place possible, right where you are taking the valve off the seat, and compressing the valvetrain, and right where you are seating the valve.
Performance wise, you are increasing the area before TDC on the intake, and increasing overlap area. Neither of those, are a good idea.
Breaking down any pressure trace, simulated mass flow, etc, I couldn't agree with you more. That also isn't included valvetrain dynamic reasons either.
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Re: Progressive/Digressive rockers, flat tapppet..pros/cons?

Post by modok »

Rick! wrote: Thu Apr 30, 2020 8:15 am I think the antonym to progressive is regressive - which fits what you are posing to the forum.
ha! maybe. Or maybe it's degressive (but spell check says that's not a word)
Used concerning shock absorbers and springs.
I googled "regressive shock valving", and there are a few hits, degressive gets a lot more, digressive gets the most, but they may be spelling it wrong due to spell check same as I did. i don't know, interesting trivia question
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Re: Progressive/Digressive rockers, flat tapppet..pros/cons?

Post by modok »

CamKing wrote: Thu Apr 30, 2020 12:22 pm If the rocker ratio is higher at the base circle, then it is at max lift, you are increasing the velocity and acceleration in the worst place possible, right where you are taking the valve off the seat, and compressing the valvetrain, and right where you are seating the valve.
Performance wise, you are increasing the area before TDC on the intake, and increasing overlap area. Neither of those, are a good idea.
Yes sir. using a degressive rocker on a "nromal" cam profile seems to me it would have no advantage.
but
If you had cam profiles designed to produce the same lift curve with both kinds of rocker.....how would they perform?
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Re: Progressive/Digressive rockers, flat tapppet..pros/cons?

Post by CamKing »

modok wrote: Thu Apr 30, 2020 12:59 pm
If you had cam profiles designed to produce the same lift curve with both kinds of rocker.....how would they perform?
Exactly the same. The engine doesn't care how the valve lift curve is created.
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Re: Progressive/Digressive rockers, flat tapppet..pros/cons?

Post by modok »

CamKing wrote: Thu Apr 30, 2020 1:10 pm
modok wrote: Thu Apr 30, 2020 12:59 pm
If you had cam profiles designed to produce the same lift curve with both kinds of rocker.....how would they perform?
Exactly the same. The engine doesn't care how the valve lift curve is created.
CamKing wrote: Thu Apr 30, 2020 12:22 pm Neither of those, are a good idea.
It's worse,
and yet can perform exactly the same. ok

Are you implying that I need to have you make the cam? :wink:
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Re: Progressive/Digressive rockers, flat tapppet..pros/cons?

Post by CamKing »

modok wrote: Thu Apr 30, 2020 8:16 pm
CamKing wrote: Thu Apr 30, 2020 1:10 pm
modok wrote: Thu Apr 30, 2020 12:59 pm
If you had cam profiles designed to produce the same lift curve with both kinds of rocker.....how would they perform?
Exactly the same. The engine doesn't care how the valve lift curve is created.
CamKing wrote: Thu Apr 30, 2020 12:22 pm Neither of those, are a good idea.
It's worse,
and yet can perform exactly the same. ok

Are you implying that I need to have you make the cam? :wink:
Those are two different things.
If the cam is designed, so the valve lift curve is the same, it will perform the same, power wise.
If you just pick a profile, designed for a constant rocker ratio, then
Performance wise, you are increasing the area before TDC on the intake, and increasing overlap area. Neither of those, are a good idea.
There are no performance benefits for running a rocker that's a higher ratio at the base circle, then it is at max lift. At best, you could spend an extra $1,000 to have profiles designed to make the rockers not cost you power.
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Re: Progressive/Digressive rockers, flat tapppet..pros/cons?

Post by MadBill »

Didn't Crane used to talk up a "hot off the seat" rocker design of theirs?
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