can somebody explain to me rocker ratios? what is a high/low ratio? the benefits of each? thanks.

 

Long story. Could take several pages on a basic intro. The short version is that stock SBCs came with 1.50:1 rocker arms. So a .300" lift at the cam should be a .450" lift at the valve. If you swap to 1.60:1 rockers, that .300" becomes .480". Plus there is a small effect on duration. If your cam was ground to make 212 duration at .050" and .450" with 1.50:1 rockers, and you switch to 1.60:1 rockers, the valve will now "see" the at .050" duration closer to 214 degrees. The other considerations woth changing rockers are adequate clearances, forces on the lifters, and rate-of-lift. The rate of lift, or acceleration of the valve, is increased with higher ratios, such as 1.6:1. This is generally better for intake than exhaust, and too fast of a rate can limit your rpm range. Clearances to check include the pushrod to it's slot in the head, the rocker to retainer, and the retainer to seal. Higher ratio rockers nearly always include a longer slot, so clearance to the rocker stud is seldom an issue. And lastly, your valvespring pressure is multiplied by the rocker ratio, So, if your springs make 100 pounds on the seat and 240 pounds open, the lifter sees 150 psi and 360 psi pressing it against the cam. This isn't an issue at this point with roller lifters, but with hydraulic lifters, too much pressure through the pushrod can lead to lifter collapse, which is why rev kits were invented.

 

but what does higher ratios mean? if you have a cam with a .450 valve lift then with 1.60:1 rockers will give you .522 valve lift?

 

if you are set up for 1.5 which is standard and your lift is .450 [you did not say exhaust or intake] and you move up to 1.6 then - and atilla help me if i have this wrong - you will get a 10% increase in lift - so 450 + 45 will get you to about 495. that is a short way to get some info. but im sure that there is a real formula for that. and if you went to 1.7 you should be at about 450 + 90 or 540.
but you have to know what the max lift of your heads are and maybe you have to do a little machining to make sure your rockers dont hit at the bottom and then you need springs to handle that lift and compression, then you need to screw in the studs, and you may have to shim the springs and offset the locks an equal amount.
i was not a math major but it is very confusing.
and some cams are designed to give you that lift using a 1.6 rocker - i think the LT4 hotcam stats are based on a 1.6 rocker.
it will also change duration so a zz4 with 208/221 moves up to about 212 and 224 with 1.6 rockers. or something like that - i dont remember the exact figures. everything changes with each modification.
but you should do a search and see what the real pros say about it. i check with other people all the time.

 

here is a good site with a chart to show you the difference http://www.comeracing.com/products/c...streetperf.pdf
take a look at it. i may download it just for reference.

and it sounds like you have an lt4 cam so here is something i have that might help.

LT4 1.6 ratio .476/.480 203/210 115 alum LT4 head 10.8:1 195cc 6300rpm redline in LT4
LT4 1.5 ratio .450/.450 201/208 115 alum LT4 head 10.8:1 1.5's reduced a little duration

looks like my initial numbere were all wrong for that cam. but they were just estimates this was from a site where they were doing the math the correct way.

and here is another chart and it is slightly different too. http://www.reedcams.com/hyd_flat_profiles.htm

in the morning there will be a pop quiz - no true and false answers. lol

I just learned something also, whenever you increase rocker ratio by 1 point, like from 1.5:1 up to 1.6:1 that increases duaration by 2 degrees.

 

oo i think i kinda get it. I'll look around thanks.

 

is it just easier to buy a whole kit ( cam, lifters, rods, rockers, valves, timing and all that good stuff) just to eliminate the guess work

 

complete kits are better for newbs. If you have a cam atvertised as .450" lift, and you switch to 1.6:1 rockers, you get .480" valve lift. Again, this is because .450" valve lift divided by the OE rocker ratio of 1.5:1 means the lobes have .300" lift. .300" times 1.5:1 is the .450" advertised. Except for COMP CAMS' XFI series, and the GM "hot" cam, all SBC cams are sold with the advertised lift being valve lift with a 1.50:1 rocker ratio.

 

finally found what i was looking for http://www.chevyhiperformance.com/te...nfo/index.html and here is the correct formula - pretty simple
To determine maximum valve lift, simply multiply the lobe lift by the rocker ratio. In the case of a 0.340-inch lobe lift with a 1.6:1 rocker ratio, this would produce a max theoretical lift 0.544 inch (0.340 x 1.6 = 0.544). We say theoretical because valvetrain deflection and tolerance stack-up can prevent the engine from generating maximum valve lift.
im keeping that site

here is an example using my zz4 cam - took me a minute to figure out. if the 474/510 is based on the use of a 1.5 rocker - and using the .510 - the formula is (.510/1.5) * 1.6 = .544
the .511/1.5 = 340

now lets do yours

you are 450/450 and it looks like an lt4 cam, which i think is based on a 1.5 rocker.
so .450/1.5= .30 and .30 * 1.6 = .480
your other numbers will increase by 2 degrees also


WHY DID I BOTHER I SHOULD HAVE KNOWN THAT ATILLA HAD THE FORMULA IN HIS HEAD.

good job atilla - as usual.

 

so actually with a cam card thats just showing how much lift the cam will contribute. to figure out your actual valve lift you need to incorporate the rockers into that. And by the way atilla, the xfi is the cam i most likely plan on getting

 

No. for the third time, a .450 cam with 1.6 rockers gets you .480" at the valve.

 

so if a cam says .480 lift then thats the valve lift when using 1.6:1 rockers correct

 

Cam companies always give lift numbers for cams with stock rocker ratio (1.5, as stated above).

 

usually it is based on 1.5 bus some cams - like the lt4 hot cam - show the stats with a 1.6 because it comes with a 1.6.
so you really have to check to see what they come with and if not installed you have to check the factory. 1.5 is like a standard for most - like using .50 to get the stats as a standard.

 

ok i got it thanks