will minor increases in rocker ratio help, your combo?

grumpyvette

Administrator
Staff member
HEY GRUMPY?
"I need a 1.7/1.6 shaft rocker setup up for my new engine build that needs to suit my AFR 227 heads.

1.65/1.55.???

Thanks
"


OK IVE just GOT TO ASK?
its a great question, but the source of the question is can I benefit from a higher rocker ratio? you need to ask yourself,
whats the slight increase in valve lift the higher rocker ratio going to do to benefit your combo?
every change results in a compromise at some point?
jumping up the ratio from the stock 1.5:1-to 1.6:1 is easy to do and has some benefits and a few potential problems, but once the ratios increased past about 1.65:1 the rocker stud needs to be re-located if you want the ideal geometry, and yes you can get 1.7:1 ratio rockers that will bolt on and work but you really should convert to shaft rockers if you want that ratio, for better results

heres your cam, and If your running your reasonable max piston speed, and valve control, both limit your engine with hydraulic roller lifters to about 6500rpm, even with a decent rev kit installed in most cases, before valve control issues become the limiting factor, and your port cross sectional area is about 2.15 sq inches so the port, with that duration cam will be limited to under 6500rpm, and having built lots of these engines over the years ID point out that theres a point of diminishing returns on increasing the rocker ratio on a SBC especially one limited with hydraulic roller lifters now if this was a race car and you were running a solid roller lifter valve train and not expecting to drive on the street, then, yes it might be a valid choice, but thats not the case .
BECAUSE the location of both the valve and the rocker stud are preset changing the rocker ratio in most cases forced the push rod to angle up closer to the rocker stud, this greatly increases the resistance the lift has on the cam lobe, resulting in increased potential for rapid wear, but has little effect on the valves rpm potential other than tending to lower its peak rpm potential because of the increase in cam lobe acceleration rates.
your average street hydraulic roller cam has springs that have about a 130lb seat and 330 lb open load rate
since the stock rocker ratio is 1.5:1 on a sbc that a load rat at the lifter of about 195lb/495 lbs
swapping to a 1.7:1 ratio boost the lifter lobe contact loads to about 221lbs/561lbs, and thats at static loads once you start adding the inertial loads at high rpms it gets significantly higher load rates
rollerrockfeed.JPG

rockergeometrychecker.jpg

auto0075kn.jpg

http://www.crower.com/misc/cam_spec/cam ... 1&x=22&y=2
stampedcvrockersa.jpg

rockeroilfeed.jpg

longslot.jpg

stock 1.5/1.5 ratio rockers- int .555/exh.559

with 1.65 and 1.55 rockers- int .610/exh.577

1.7/1.6 rockers- int .629/exh.596

do you really think an extra .020 lift, is going to be a meaningful improvement, especially if you were to factor in the more rapid effective ramp speeds and valve train stress, that will without a doubt knock a bit off the upper edge of your power band as youll tend to lower your valve float limit slightly?

your looking at about a 2% increase in peak lift, over the more common 1.55/1.65 ratio, and a similar reduction in rpm band, or giving up about 100 rpm to get the extra .020 peak lift??
keep in mind that some roller lifter cam lobes tend to have rather aggressive ramps to open the valves faster and hold them open longer
rollervsflat.png

read these
viewtopic.php?f=52&t=2203

http://www.wallaceracing.com/runnertorquecalc.php

http://www.airflowresearch.com/index.php?cPath=75

viewtopic.php?f=52&t=333

http://www.carcraft.com/techarticles/11 ... index.html

http://www.circletrack.com/techarticles ... index.html

http://www.circletrack.com/techarticles ... index.html

http://www.circletrack.com/techarticles ... index.html

http://www.vetteweb.com/tech/vemp_0204_ ... index.html

http://www.pontiacstreetperformance.com ... rArms.html

viewtopic.php?f=52&t=181&p=215&hilit=+girdle#p215


btw this might help

http://www.vetteweb.com/tech/vemp_0703_ ... index.html

http://www.vetteweb.com/tech/0204vet_sm ... index.html

viewtopic.php?f=52&t=196

http://www.enginebuildermag.com/Article ... itch_.aspx

viewtopic.php?f=52&t=126&p=1193&hilit=louis#p1193


viewtopic.php?f=44&t=2839&p=7344&hilit=adjustable+guide#p7344

viewtopic.php?f=52&t=126&hilit=louis+rocker
adjustablesbc.jpg


and you may need too use the correct adjustable guide plates when you find the push-rod alignment is in need of minor tweaking to get the clearance and geometry correct

STUD-12.jpg

louis1.jpg

louis.jpg


using a louis tool, this tool is a GUIDE /tool for use with a high quality DRILL,its made of HARDENED STEEL that FORCES the DRILL BIT to drill thru the head to correctly lengthen the pushrod slot for increased clearance, they usually come WITH INSTRUCTIONS AND THE NECESSARY DRILL

pushrodbind.jpg

its rather common to have push rods rub or bind when swapping to the higher ratio 1.6:1 rockers so its mandatory you carefully check EVERY one through its full arc for proper clearance and use a louis tool to extend the push rod guide holes is thats required
 
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