Rocker Arms

Travis Miller · 05-07-2010, 06:32 AM

Quote:

Originally Posted by Pat Cook

So, if we have a cam that is suppose to have .400 lift, but the closest rocker we can find figures out to 1. 46 ratio. it would be ok to have a cam ground with a.273 lobe to make up for the rocker ratio, but if we have a rocker that actually has a ratio of 1.56 you can not have a cam ground that has a .256 lobe lift to get the lift to check legal?

Just curious.....

If you are talking about an engine that has a spec of 1.5 rocker arm ratio, the first senerio would be okay. The second one would mean you have used a rocker arm that is sold as a 1.6 ratio and that would be illegal. While the total lift at the valve retainer would check correct, the ratio would be incorrect if it were to be checked.

The difference is you have used a rocker that has altered the rate of lift as the valve is opening and closing from the original rocker arm ratio spec but hid it by meeting the total valve lift spec once the cam reaches max lift.

Travis

(Disclaimer: Opinions expressed by me on this forum are exactly that, my opinions.)

Tom keedle · 05-07-2010, 07:48 AM

Quote:

Originally Posted by Travis Miller

If you are talking about an engine that has a spec of 1.5 rocker arm ratio, the first senerio would be okay. The second one would mean you have used a rocker arm that is sold as a 1.6 ratio and that would be illegal. While the total lift at the valve retainer would check correct, the ratio would be incorrect if it were to be checked.

The difference is you have used a rocker that has altered the rate of lift as the valve is opening and closing from the original rocker arm ratio spec but hid it by meeting the total valve lift spec once the cam reaches max lift.

Travis

(Disclaimer: Opinions expressed by me on this forum are exactly that, my opinions.)

how would that be any different from a cam being ground different?

Alan Roehrich · 05-07-2010, 08:02 AM

With a given tappet diameter, especially a flat tappet, there is an absolute limit as to how fast you can accelerate that tappet. So, given the tappet diameter, and a factory rocker ratio, there is a design limit on how fast you can accelerate the valve. Increasing the rocker ratio allows you to accelerate the valve faster. So, if you reduced your maximum lobe lift in order to use a higher ratio rocker, you'd be disguising a method of exceeding the design limit of the original valvetrain with regards to valve acceleration.

When you have a limited amount of lift, the secret to power, within reason, is how fast you can get to that lift limit from the valve being on the seat, how long you can hold it there, and how quickly you can get it closed. It's called, in general terms, "area under the curve". Again, this is a generalization, not an exact rule. For a given combination, there is an ideal, that combination may or may not be able to achieve that ideal. And that ideal may not always be the maximum amount of "area under the curve".

It is really hard to explain this, especially using generalizations as opposed to actual examples and the math involved. That's about as clear as I can make it here.

Pat Cook · 05-07-2010, 08:42 AM

Quote:

Originally Posted by Travis Miller

If you are talking about an engine that has a spec of 1.5 rocker arm ratio, the first senerio would be okay. The second one would mean you have used a rocker arm that is sold as a 1.6 ratio and that would be illegal. While the total lift at the valve retainer would check correct, the ratio would be incorrect if it were to be checked.

The difference is you have used a rocker that has altered the rate of lift as the valve is opening and closing from the original rocker arm ratio spec but hid it by meeting the total valve lift spec once the cam reaches max lift.

Travis

(Disclaimer: Opinions expressed by me on this forum are exactly that, my opinions.)

I am no "engine master" by any means, but is there any real power by doing that? and isn't it the same thing as the way stocker cams are ground anyway? the dwell cam verse the rpm cam?

Is there any dyno proof? just curious.....

Dgal · 05-07-2010, 09:40 AM

I reserve the right to be wrong, but I believe the Nascar teams have experimented with larger ratio rocker arms in order to reduce the amount of spring pressure on a given lift/duration. Less spring pressure should equate to less parasitic hp loss within the valve train.

Greg Reimer 7376 · 05-07-2010, 10:00 AM

On another thread, I remarked about a 283 racer who went through about 100 stamped OEM rockers to get the 16 that he used. That was back in the day when OEM was all that was allowable, and the best arms were the ones with the little triangle on top of the tip above the valve stem tip.I don't know ,however, if he was attempting to achieve accurate lift, or whether he was striving for as accurate a ratio as possible.With all the latitude that various length pushrods can produce,there is an almost infinite degree of possibilities. Yes, it's interesting what an open forum can produce.Thanks to Travis for the expose' on how rocker arm ratios are calculated.

Dwight Southerland · 05-07-2010, 03:58 PM

The higher ratio rocker arm will increase any action ground into the camshaft profile by a mathematical amount. One of the limitations of how radical a camshaft profile can be ground is the lifter diameter. So, when the camshaft has been designed to maximum acceptible rate of lift change dictated by the lifter diameter, then there is no more that can be done to make the valve open any more at a given lobe lift. The higher rocker arm ratio allows a little bit more lift at a given lobe lift, hence "more area under the curve". Also, you can grind a faster ramp into a camshaft profile if the lobe lift is less, also providing for more area under the curve.

See what Alan said above. If I could draw pictures here, it would be easier to see.

Alan Roehrich · 05-07-2010, 06:52 PM

Quote:

Originally Posted by Dwight Southerland

If I could draw pictures here, it would be easier to see.

Ain't that the cryin' truth.

05-07-2010, 08:02 AM	#3
Alan Roehrich Veteran Member Join Date: May 2006 Location: Murfreesboro TN Posts: 5,125 Likes: 1,582 Liked 1,880 Times in 422 Posts	Re: Rocker Arms With a given tappet diameter, especially a flat tappet, there is an absolute limit as to how fast you can accelerate that tappet. So, given the tappet diameter, and a factory rocker ratio, there is a design limit on how fast you can accelerate the valve. Increasing the rocker ratio allows you to accelerate the valve faster. So, if you reduced your maximum lobe lift in order to use a higher ratio rocker, you'd be disguising a method of exceeding the design limit of the original valvetrain with regards to valve acceleration. When you have a limited amount of lift, the secret to power, within reason, is how fast you can get to that lift limit from the valve being on the seat, how long you can hold it there, and how quickly you can get it closed. It's called, in general terms, "area under the curve". Again, this is a generalization, not an exact rule. For a given combination, there is an ideal, that combination may or may not be able to achieve that ideal. And that ideal may not always be the maximum amount of "area under the curve". It is really hard to explain this, especially using generalizations as opposed to actual examples and the math involved. That's about as clear as I can make it here. __________________ Alan Roehrich 212A G/S

05-07-2010, 09:40 AM	#5
Dgal Member Join Date: Dec 2009 Posts: 169 Likes: 0 Liked 0 Times in 0 Posts	Re: Rocker Arms I reserve the right to be wrong, but I believe the Nascar teams have experimented with larger ratio rocker arms in order to reduce the amount of spring pressure on a given lift/duration. Less spring pressure should equate to less parasitic hp loss within the valve train.

05-07-2010, 10:00 AM	#6
Greg Reimer 7376 VIP Member Join Date: Nov 2008 Location: Glendora,Calif. Posts: 1,137 Likes: 172 Liked 719 Times in 220 Posts	Re: Rocker Arms On another thread, I remarked about a 283 racer who went through about 100 stamped OEM rockers to get the 16 that he used. That was back in the day when OEM was all that was allowable, and the best arms were the ones with the little triangle on top of the tip above the valve stem tip.I don't know ,however, if he was attempting to achieve accurate lift, or whether he was striving for as accurate a ratio as possible.With all the latitude that various length pushrods can produce,there is an almost infinite degree of possibilities. Yes, it's interesting what an open forum can produce.Thanks to Travis for the expose' on how rocker arm ratios are calculated.

05-07-2010, 03:58 PM	#7
Dwight Southerland VIP Member Join Date: Dec 2001 Location: Arkansas - In the middle of everything. Posts: 2,002 Likes: 64 Liked 783 Times in 195 Posts	Re: Rocker Arms The higher ratio rocker arm will increase any action ground into the camshaft profile by a mathematical amount. One of the limitations of how radical a camshaft profile can be ground is the lifter diameter. So, when the camshaft has been designed to maximum acceptible rate of lift change dictated by the lifter diameter, then there is no more that can be done to make the valve open any more at a given lobe lift. The higher rocker arm ratio allows a little bit more lift at a given lobe lift, hence "more area under the curve". Also, you can grind a faster ramp into a camshaft profile if the lobe lift is less, also providing for more area under the curve. See what Alan said above. If I could draw pictures here, it would be easier to see.