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1.70 to 1.80 rockers

525 Views 15 Replies 5 Participants Last post by  BogiesAnnex1
I have a BBC with 1.70 rockers? Jesel Sportsman. How much reliability am I going to sacrifice going to 1.80 ratio. I am running a hyd roller with I/E 242/249 @ .635//647

The reason for change is I increased displacement from 496 to 557 and believe it needs more cam to compensate for the increase in displacement. Due to wet boat exhaust I can't really replace cam with more duration as it would likely increase overlap and engine screwing exhaust reversion, currently at only 13 deg.
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There in lies some of the rub of increasing lift via higher ratio rockers.

While they get the valve off its seat at a faster and larger amount per degree of crank rotation which can be helpful as the rev’s get high enough to utilize that which will be a function of mixture velocity of the intake tract which is RPM dependent, the next and larger issue is whether the port can support sufficient increase in flow volume at the higher lift. Here your going to have to hunt down the spec’s for the head in question to determine whether that offers a large enough flow gain to bother with.

Th the greatest extent going to higher ratio rockers provides the most gain when teamed with a cam that offers a slow rate of lift rate combined with a modest peak lift, that being cams more like those of the 1960’s. Modern short ramp, fast lift rate between .050 points and high peak lift tend not to be as enhanced by bigger ratio rockers that those more classic cams.

So whether the result is worth the pursuit takes some examination.

Bogie
 

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Really the seat pressure is more important and to an extent the open pressure is a function of that. The big headache is getting the valve to close without bouncing. The next is getting it to track the cam lobe on the backside, the third is not to loose track over the top of the lobe where unwanted lofting might occur from front of the lobe acceleration, that’s not to say that lofting isn’t desirable in some situations.

High lift rocker ratios add lift and acceleration to that lift so to some extent there can be power added to the engine by enhancing breathing sooner in the lift cycle. The other component is adding over the lobe top max lift. Here this is a function of whether the port gains enough added flow for the added lift to be worth while pursuing.

Increasing the lift over the top is harder on all the components so it there isn’t any benefit to the additional lift if the port is unable to substantially increase flow to feed that additional lift. Then your money needs to include improving the ports or replacing the heads for something that shows significant flow increases at the lift you’re chasing. Short of that the better choice is a cam with a more aggressive lobe rate of lift rather than adding more lift over the top of the lobe. So project number one is to know what your heads flow to valve lift profile looks like. The other is put your money down and experiment. But keep in minds that most heads show a great reduction in the rate of flow increase as the lift gets toward or beyond .5 inch, so I go back to your should have a lift to flow map for the heads you’re using before going further.

Bogie

Bogie
 
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