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Discussion Starter · #1 ·
I have a 305 H code in my 84 gmc I installed a cam and now i want to upgrade to some 1.6 roller rockers (if the motor will allow the extra lift). The problem i am having is i cant find what the max lift is for these heads. Anyone have an idea or if you have a camed out 305 whats your lift?
 

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Running different motors through my DynoSim software, I've been able to find only 3 to 12 horsepower on a 400-500 hp motor using 1.6 rockers instead of 1.5's.

Considering the additional strain on the valvetrain with the increased leverage, I wouldn't do it. Unless you thoroughly check valve to piston clearance, retainer to valve guide clearance, rocker slot to stud clearance and check the valve springs at full lift for coil bind AND are running roller tappets, I just wouldn't do the 1.6's.

Flat tappet cams have a tough enough time surviving with stock rockers, much less asking them to do additional duty.
 

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I don't know what you've done to your Jimmy besides the cam, but headers will really help if you don't have them yet. Along w/a 2-1/4" to 2-1/2" dual exhaust w/an "H" or "X" connection between the sides, installed after the collector, before the muffs.

A cold air box (of your own making would be fine) will help as well.

As for the 1.6 rockers, besides the things already mentioned, also check the push rod slots in the head that may have to be opened up if there's interference between the end of the slot nearest the stud and the p-rod.

Sometimes the springs themselves will need changed, if they were borderline w/the 1.5 ratio rockers, or are stock.
 

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Discussion Starter · #5 ·
305

i have long lube headers with 2 1/2 true dual, no cat, edelbrock dual plain aluminum intake manifold, holley 4160 carb. im not sure what the lift is, my guess is about .450 or so maybe less its a mild cam with mid range power, i bout the cam used and i lost the part number.
 

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billy_bob said:
so who knows how much it is or the highest they've gone
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With the correct springs, you can run .500" lift, maybe more. All clearances must be verified before using a higher than stock lift cam or rocker ratio- there are many variables besides gross lift to consider.

The head quits flowing before .450" anyway, so lifting the valves higher won't get you any additional airflow- so there's no need to know how much lift the heads can take. It's a matter of how much lift the heads can USE.

It almost sounds like there's some kind of bench racing argument going on here- one guy says "I ran a .600 lift cam w/MY stock heads!" kind of deal. :rolleyes:

But the salient point- again- is the heads do not flow any more at .450" or so than they will at .800".
 
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