Roller Rockers

[oldtuckunder]

Ok, I'll agree that with a higher ratio rocker you get to a specific opening lift point quicker and a specific closing lift point slower, so the duration between those to lift points is fractionally longer, however the total duration from when the valve first starts lifting to the point it closes isn't longer. Its also a bit imprecise to calculate as the ratio isn't constant as both the lift end and pushing end are going through arc's so the actual ratio between cam lift and valve depression varies during the duration, and a 1.5 and 1.6 ratio rocker will have different lift % profiles due to the different arc's the ends of each have to transcribe. It is actually possible to have a 1.5 ratio rocker lifting the valve quicker at specific points than a 1.6 ratio one, so as they say bigger isn't always better

 

Sorry Marcus but the link you posted opens a google search with hundreds of images so I'm not sure which one you were trying to show.

 

Alan

 

PS. did you solve your #1.,#12 clearance cause?

[Darth V8R]

 

Whilst they can increase lift, they can't increase total duration, the physical cam profile sets duration, a higher lift rocker doesn't start the valve opening any sooner (quicker maybe) and it will shut at exactly the same point. Like wise it can't increase valve overlap duration, all they can do is have the valves slightly more open during the period of overlap.

 

Depends how you define "duration". If you define it by a certain amount of lift at the cam lobe, then you are correct. If you define it as the amount of time the valve is a specific distance off the valve seat, then duration and overlap do in fact increase. Since engine behavior depends on what the valve is doing, not the lobe, this is the preferred definition for purposes of this discussion.

 

Hypothetically, lets say that with a specific cam the intake valve is open for 270 degrees, with "open" being defined as 0.020" of valve lift. Shift the rockers from 1.45 to 1.6rr, and at the 270 degree duration, the valve is now open 0.022" because the valve is opening faster. To get back to the 0.020" lift as being the open period, the cam duration is now something like 280 degrees AT THE VALVE. So with the same cam lobe, the effective duration has increased by something like 10 degrees. (Actual increase depends on the shape of the cam lobe).

 

The same phenomenon affects the overlap period.

 

So the higher ratio rockers do affect power, economy, and emissions. Add to that the decreased frictional loads for a roller tip, and you can add noticeable power by going to a higher ratio, subject of course to all the previously mentioned tradeoffs.

 

Vance