Camshaft Lift or Rocker Arm Ratio??

[theMelvster6]

I'm sure this is an arguable topic and possibly a topic that has been argued before...I'm not having much luck finding a lot of info to satisfy my question....so here we go...

Red Shift 567 cams have a .567 lift on intake and exhaust using stock 1.625 ratio rocker arms

NOW stock XB cams have a .551 lift on intake and exhaust using Stock rockers, but if you go to a 1.675 ratio rocker arm...you've created .567 lift on intake and exhaust using OEM .551 lift cams

Same goes for Red Shift 585 cams...which have .583 lift on intake and exhaust using stock rocker arms
With the stock XB cams and 1.725 ratio rocker arms you get approx. .585 lift on intake and exhaust.

From JEGS.com

Rocker Arm Ratio Explained
You've heard of 1.5:1 ratio and 1.6:1 ratio rocker arms. What does that really mean? How can you make more horsepower by using the larger ratio rocker arms?

The ratio of a rocker arm refers to the amount of movement on the valve side of the rocker arm in comparison to the pushrod side. A 1.5:1 rocker arm will move a valve 1.5 times the lift of the cam, assuming all things are in proper working order. If your cam has a lift of .4 inches, multiply by 1.5 to get .6 inches of actual valve movement. With the same cam lift of .4 inches: If you do the math on a set of 1.6:1 rocker arms: Multiply 1.6 x .4 inches of cam lift. You get .64 inches of actual valve movement. Essentially, changing to a larger ratio rocker arm is the same as increasing the lift of your cam the same amount. Opening both valves further usually is a benefit in the power department as your engine is capable of moving more air and exhaust through it.

From Crane Cams

How does an increase in Rocker Arm Ratio improve the engine's performance?

The engine reacts to the movement of the valve. It doesn't know how the increased lift was generated. It responds the same way it would as if a slightly larger lift cam had been installed. In fact, since the speed of the valve is increased with the higher rocker arm ratio, the engine thinks it has also gained 2° to 4° of camshaft duration.

The end result is an easy and quick way to improve the performance of the existing cam without having to install a new one.

So is there a preferred way to achieve higher valve lift? Or does it even matter? either way you have to use adjustable push rods and match them to your valve train geometry...cost wise New cams are $400-$650 depending on using your cores or buying new and New Rocker Arms...depending on brand and quality...on the cheap end $150 for ULTIMA roller tip rocker arms, up to $450 for Crane Cams roller tip rocker arms...and you can find rocker arms within that wide range of prices.

I guess the curiosity bug hit me and now I'm curious what y'all think? LOL

 

[lawdog]

It is not a bad idea in theory(especially in car engines where you can adjust the valvetrain geometry easily), but here are the problems I see.

1. From my understanding the stock heads are pretty much maxed out in flow as it is. Increasing lift will not provide much benefit unless you modify the stock heads to increase flow.

2. Modifying the geometry of the valvetrain will have negative benefits. It is not a simple bolt-in affair to correct the geometry and odds are you may not even get much more lift with poor geometry. Not to mention the increased stress on the components.

If you want to change just the rocker arm ratio I would read up on valvetrain geometry and how you will need to adjust it, ie shorter pushrod, shimming the rocker box, valve stem height, etc.

If you want more power I would start with the basics, opened airbox, K&N, aftermarket muffler, and of course tuning for every mod. If you need more power look at a big bore kit, headwork, and a cam. From there things get very expensive! I would recommend staying away from altering the rocker arm ratio and instead use a cam to get the lift and duration you want.

 

[theMelvster6]

So larger Cams don't change the geometry? Just the length of the push rods need to he adjusted? I was mostly just curious about it all. There is actually a great instructional on the Rev Performance website all about valve train setup, geometry, etc. I'm planning on doing the 1050cc kit and possibly some head work to start...

 

[jetlee]

Since duration is measured from .005" lift and that is achieved earlier due to the higher ratio, it can increase duration by a couple degrees on each cam.