Technical Writer for Internet & Print Media
In reading this thread, I see there are a few misconceptions about rocker arm changes.
First, on an L83 engine, which has 200-205hp dependiing on model year, the typical SBV8 rocker arm ration change, 1.5-1.6 ratio, will result in only a modest gain in performance. The 3-5hp number listed above is possible but might be overstating the case on a 200hp engine.
The statement about roller tips significantly decreasing friction is incorrect. In fact, there is very little decrease in friction in using a roller in the tip of a rocker arm especially at the stress levels in a 200hp engine. The big decrease in friction comes when you use a roller bearing in the trunnion. This is why all current GM OE rocker arm designs on V8s have roller fulcrums but not roller tips. In reality the roller tip increases weight of the rocker but is used by aftermarket companies either 1) to mislead the consumer into thinking the rocker is better because of the fancy-looking roller tip or 2) in the case of aluminum rocker arms, to avoid the cost of adding a hardened steel insert on the tip.
We must be careful when we discuss the effects the ratio change has on duration. At lash point, there is no change in duration, however, once the rocker begins to translate the motion of the lifter and pushrod, duration begins to slightly increase above the duration which was present with the 1.5 rocker. The overal change in duration is slight with the chief advantage being extra lift.
As stated earlier, when changing the rocker ratio, it's best to check for coil bind and interferrance between the retainer and the valve guide, however, on an L83 getting only a 0.1 increase in ratio, it's unlikely there would be a problem.
Lastly, a more aggressive rocker arm ratio moves the engine speed at which valve float will occur lower in the rpm range. Again, with the L83 and only a 0.1 change in ratio, it's unlikely that the limiting rpm of the valve train would be reduced enough to matter, however, if the valve train has been in service a long time, the normal wear and fatigue that occurs may have the valve springs already weak enough to put a valve train with 1.6:1 ratio rockers into valve float in the upper rpm range you'd see in street driving.
Maybe I'm hijacking this thread but I don't think so. I am thinking of changing from a flat tappet cam to hyd roller. In order to get the proper seat pressure from my current springs Crane cams suggested I use .030" shims. The springs have a max lift of .575" now. The cam I am considering has an exh lift of .539". With the spacers, this only leaves .006" of compression. Can this be acceptable? The rocker ratio would be staying the same--1.5.
Technical Writer for Internet & Print Media
You're going to be real close to coil bind. When that happens, parts break.
I wouldn't do it.
I'd purchase the proper springs for the hyd. roller.
I got on the Crane cams website and the specs for the cam call for 112 lbs seat pressure @ 1.650
Originally Posted by Hib Halverson
open 318 lbs @ 1.140
The cam has a max lift of .539
The springs on the heads are
installed height 1.800
seat pressure 120 lbs
open 320 lbs @ .500 lift
The part # for the cam is Crane # 119701, the heads are Edelbrock Performer RPM 64cc
Does this look like it will work?
Whatever the max lift at spring is you MUST have an additional approx 0.060" to 0.075" it can compress to avoid coil bind. Does the .575 mean it can compress 575 and then 060 more until it becomes solid? or ... or does it mean .515 + .060 = .575??? Does it mean 1.800 spring is solid at 1.225" ?? ...OR ... on seat at 1.800" / solid at 1.150" ?? Springs specs often shown with both open AND solid heights ... check it & reply. VERY important.
Letsee your springs = 1.800" (usually means on seat) and 120# on seat AND max lift .575".
And your new cam requires only 112# on seat but lifts .539.
If your springs Truly have an additional approx .060 more travel beyond .575 'til solid ... and they're Truly 120# on seat ... then I'd stick the cam in just as it is. But ... what things are supposed to be and what they really are ain't always the same. So ... there ain't but one way to know ... that's measure stuff ... HIGHLY recommended!
as for rollertips ... don't waste your time on em ... and if you add ANY roller rocker arm to sbc/BBC ... you must check that roller is centered on valve tip & adjust guideplates as needed ... more often than not they're Not aligned w/vt... if roller runs off edge of valve tip it can easily drop a valve & destroy motor ... that's happened alot. IIRC, CAC member fine69 (ralph) had similar problems w/ misaligned rollers-VTs & guideplates w/rpm heads nearly 3 years ago ... fugly but not destroyed.
as for changing ratios ... Not mentioned so far is: clearancing head for pushrod. When you change from 1.5 to a higher#/ratio ... the lever arm between valve tip & trunnion always stays same as stock ... BUT the lever arm between pushrod cup & trunnion changes ... it Shortens ... and pushrod gets closer to head ... head usually needs grinding clearancing. Also, when you change increase ra ratios ... it usually adds a degree or 2 to the 0.050" duration.
By scotch1 in forum C4 Technical and Performance
Last Post: 10-22-06, 10:31 PM
By Runge_Kutta in forum C6 Technical and Performance
Last Post: 10-05-04, 05:29 PM
By Aaron D in forum LT4 Forum
Last Post: 07-27-04, 07:47 PM
By mntpi in forum C4 Technical and Performance
Last Post: 05-19-04, 03:13 PM
By 3 Deuce in forum C4 Technical and Performance
Last Post: 04-22-03, 08:07 PM
Owned and Operated by © 2000-2015 Corvette Action Center | |
© CORVETTE is a registered trademark of the General Motors Corporation & Chevrolet Motor Division. Neither Chevrolet Motor Division nor any subsidiaries of GM© shall bear any responsibility for CorvetteActionCenter.com content, comments, or advertising. CorvetteActionCenter.com is independent from GM© and is not affiliated with, sponsored or supported by GM©. Copyright/trademark/sales mark infringements are not intended, or implied. All Rights Reserved