9 replies to this topic

[Tomf]

Well ill be changing the rockers on my 1380, running a 286 cam and im looking at 1.5 roller rockers at the moment and i have been told this will give the eqivelant of a 296 cam...

What would the effect be using 1.7 roller rockers?

[Sleepy Stu]

As far as im aware 1,7 rockers are only really for race engines

Usefull article here as well

Clicky

[Jammy]

The type of engine spec that would necessitate 1.7 rockers would generally not be the type of engine you'd want to use on the road, or at least use on the road and expect to get a decent amount of life from.

But essentially, increased ratio rockers just increase the lift of the valve, but don't alter the duration that the valve is open for.

[Guess-Works.com]

don't alter the physical duration, correct, but do alter the air flow massively, as the valve opens quicker meaning the 'effective' duration is increased..

[Jammy]

Surely that shouldn't be confused with duration though? Duration is measured in degrees, not cfm....

[Guess-Works.com]

correct duration is the degrees of rotation the valve is open, but at the start of the valve opening... it's only open a little ( in direct ratio according to the rocker ratio )...

But if the valve opens quicker, there's more time in the total duration where the valve is in a position where it does not effect the airflow so much... it's like openeing a door slowly and quickly

[Jammy]

But it technically it doesn't open any quicker, the 'door' just opens up more, at the same speed as whatever it would open at if you were using 1.5s?

But, yes, it would achieve the same amount of lift that 1.5s would give you quicker/sooner.

[Jupitus]

It won't open sooner, but it will open quicker...

[Tomf]

Cheers for the replys

Yea i dont think ill be buying 1.7 roller rockers then

The only reason i asked was there was a couple on ebay and i couldnt find any 1.5 roller rockers...

[Ethel]

Guessworks is right,

The valve will be open by almost half as much again at any point after it comes off its seat, so if the valve needs to be open by say 2mm before it has any noticeable effect on air flow it will get there after the cam has rotated less with higher ratio rockers. The extra full lift will probably be more than the engine can make use of: especially on the asthmatic A series. The real benefit is that the valves will spend longer at or above optimum lift.

Someone ought to develop a cam with shallower ramp angles and a broader peak to make best use of high lift rockers.

Edited by Ethel, 14 July 2008 - 04:40 PM.