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Old 03-27-2008, 05:46 AM   #1
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Default Chevy 350 rocker ratio

Would raising the rocker ratio on a 1974 350 produce any horsepower gains?
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Old 03-27-2008, 09:32 AM   #2
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If it is stock it may help out a bit. I would say a rough estimate of 10-15hp. If you are completely stock on the intake and exhaust then it won't be worth the time, but if you have a aftermarket carb, intake, and headers then you would benifit from the upgrade.
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Old 03-27-2008, 02:52 PM   #3
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Stock rocker ratio on a */B is 1.5 to 1, move up to 1.6 or 1.65 to 1 and you will see a good jump in hp. This has been done for a long time, i'm talking 40+ years. If you get a new intake and headers you want to make sure that the rpm where the intake makes its power is the same as the cam. If the cam makes power down low(rpm) and the intake up high the motor wont make good power. You can retard the CAM(4* to 6*) to make better power up in the higher rpm range. Go to Jegs or Summit, the cost is dirt cheap.

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Old 03-27-2008, 04:32 PM   #4
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Thanks for the advice.
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Old 03-27-2008, 06:16 PM   #5
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Make sure that your piston and cam combo is such that you can run more lift. A totally stock 74 motor should be fine with 1.6 or even 1.65, but who knows if that motor really is totally stock.
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Old 03-27-2008, 09:08 PM   #6
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Default Re: Chevy 350 rocker ratio

Quote:
Originally Posted by bonny99
Would raising the rocker ratio on a 1974 350 produce any horsepower gains?
Probably a little....considering what the base power output of a low compression smog motor is, don't expect much. (gains are always a percentage of the motors current output...which in 74 wasn't much at all.)

All a larger ratio rocker does is extend the cam a little and open the valve a little deeper into the heads. If the intakes and heads can actually flow any more is a different story.

Instead of putting money into just rockers, consider a new camshaft and heads. Bumping the compression up a little, or a better pair of heads will go a lot further towards upping the power output instead of just a set of rockers.

Before you get too far into this, measure the cam you have now. (degree wheel, dial gauge, and paper & pencil) This can tell you the lift and duration of the cam in the motor, which you can use to translate what the rocker change would bring.

Example, let say you have a mild grind (like a 248 grind) that has lobe lift of 0.2667 for a duration of 192 degrees...your typical stock emmissions grind. With 1.5 rockers, that works out to a valve lift of 0.4000". Change to a 1.6 Rocker, that will change you valve lift to 0.4267" and maybe added 2 degrees of useful duration. *YAWN*...that'* maybe worth 2 hp. And you spent about $70 bucks to get it.

Do a cam swap to a mild performance cam for about $160 and get a brand name cam that produces 0.450" of lift with 1.5 rocker and a 210 degree duration. Now you might extract another 15 to 20 hp out of the motor...more if you heads and intakes are up to the task of flowing the extra air. See what I mean, the cost per hp is cheaper going with switching cams vs. playing games with rocker ratios.
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Old 03-28-2008, 03:32 PM   #7
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Well me and my buddy are trying to fix up his 1973 GMC sprint, hes replacing the motor with a 1974 350 with a rochester 4 barrel. We are most likely going to put a cam in it and maybe heads in the future.Thanks for the pointers.
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Old 03-28-2008, 10:44 PM   #8
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Quote:
Originally Posted by bonny99
Well me and my buddy are trying to fix up his 1973 GMC sprint, hes replacing the motor with a 1974 350 with a rochester 4 barrel. We are most likely going to put a cam in it and maybe heads in the future.Thanks for the pointers.
No sweat. The time to do it is while the motor is out of the chassis.

Doing the heads sooner rather than later would be a good plan. Consider this, a '74 350 probably has honkin huge 72cc iron heads that do not flow very well, and probably has shot valve seals. Loosing those will go a long way towards pumping up the volume on the happy-meter. Even if you stick with the stock dished pistons, you can swap to late model heads, like centerbolt iron heads from the late 80'*/early 90'*, and pickup a smaller chamber in the process...they'll flow better too. Aluminum looks cool and flows even better, but if your budget is tight, Iron heads are a good option.
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