Rocker arm ratio
Thread Starter
On The Tree
Joined: Dec 2022
Posts: 172
Likes: 25
Rocker arm ratio As the title says rocker arm ratio.
the factory rocker arms arm 1.7 and cams are ground to suit 1.7 ratio rockers and all figger quoted on cam cards are for 1.7 ratio
But let's say you already have a cam but want to go slightly bigger and you are already looking at new rockers anyway.
The cheap option is to get a rocker arm with a different ratio.
Let's look at 1.7 to 1.75 ratio rockers.
Lets say your cam is 230 238@50 and 614 621 lift.
Simply divide the figgers by 1.7 then multiply by 1.75 to get you new duration and lift fingers.
Also note that inlet valve open and close along with exhaust valve open close events will happen earlier and later now because of the change in rocker arm ratio.
this is a old school trick to gain slightly larger cam in restricted racing classes for a few extra hp and rpm.
the factory rocker arms arm 1.7 and cams are ground to suit 1.7 ratio rockers and all figger quoted on cam cards are for 1.7 ratio
But let's say you already have a cam but want to go slightly bigger and you are already looking at new rockers anyway.
The cheap option is to get a rocker arm with a different ratio.
Let's look at 1.7 to 1.75 ratio rockers.
Lets say your cam is 230 238@50 and 614 621 lift.
Simply divide the figgers by 1.7 then multiply by 1.75 to get you new duration and lift fingers.
Also note that inlet valve open and close along with exhaust valve open close events will happen earlier and later now because of the change in rocker arm ratio.
this is a old school trick to gain slightly larger cam in restricted racing classes for a few extra hp and rpm.
Joined: Apr 2006
Posts: 3,889
Likes: 1,137
From: Grand Rapids, Michigan
Lift will change, but duration will stay the same. However, the area under the curve will get larger. This is because duration is measured at the lifter/cam interface, not at the rocker tip. Valve accelerations will increase, and depending on valvetrain setup, the engine may enter valve float sooner. The overlap triangle will increase, making the engine behave like you installed a cam with a tighter LSA. All of which is just to add to what you're saying, that yes, an increase in rocker ratio can mimic the effect of installing a larger cam.
Your duration will not change. However, for lack of a better term, I like to say your effective duration will. To elaborate, you now have some duration from .615" lift to .632", and back down to .615". In other words, you now have duration in an area you didn't have any before, because lift stopped at .614". Hope this makes some sense, and this is the only way I know to describe it. But, as posted, your actual duration from .050" lift opening, to .050" lift closing, will not change.
This has been discussed here on Tech a few times in the past, and it’s always a relevant discussion no matter when it comes up. Seat to seat duration, which is the total duration, doesn’t change by changing rocker ratio. Now an increase in ratio, from 1.5 to 1.6 will add about 2ish degrees due to an increase in speed, throughout the lift cycle. So, while the only way you can add duration to a lift cycle is by adding metal to a cam lobe, the under the curve duration, or effective duration, or seen duration, can be altered by rocker ratio, due to the increase in ratio adding speed to the valve throughout the cycle. An increase in rocker ratio will also be harder on valve springs do to obviously adding lift, but also by increasing the speed of the valves opening and closing rates.
TECH Enthusiast
Joined: May 2008
Posts: 641
Likes: 572
From: Carnation, WA
Trending Topics
LS1 Tech Stories
The Best V8 Stories One Small Block at Time
Retro Modern Bandit Pontiac Trans AM Comes With Burt Reynolds' Autograph
Verdad Gallardo
Top 10 Greatest Cadillac V Series Performance Models Ever, Ranked
Pouria Savadkouei
Top 10 Most Powerful Chevy Trucks Ever Made!
Hennessey's New Supercharged Silverado ZR2 Has 700 HP
Verdad Gallardo
Coachbuilt N2A Anteros Is an LS2-Powered C6 Corvette In Italian Clothes
Verdad Gallardo
Awesome K5 Blazer Restomod Comes With C7 Corvette Power
Verdad Gallardo
10 Camaros You Should Never Buy
10 LS Engine Myths That Refuse to Die
Verdad Gallardo