ROCKER ARM RATIO:
Don't have the facility/ability/budget to do a cam swap. How much can I effectively increase my cam lift by installing higher ratio rocker arms? Does this require other mods (springs/lifters/pushrods etc.), and would the results be worth the effort?

 

Changing rockers will only have an effect at higher rpms, where the engine is breathing the heaviest. It won't help your low-end torque.

I thought about it, too. And decided to save my money for something else, like a "Windjammer"! :yesnod:

 

Worth a few ponies but you prolly won't feel it. Would not do it unless you know your springs can handle it, you know your heads flow better at the higher lift and you know they will fit under you current valve covers.

 

First, it's not a bolt-on modification. You have to enlarge the pushrod slot in the heads since the ratio change moves the pushrod closer to the rocker fulcrum.

Second, it will increase lift. By how much only requires a bit of math. The theoretical ratio of the factory arms is 1.50 which means that you multiply the lobe lift by the theoretical rocker ratio by 1.50 to get your lift at the valve. So now you multiply the lobe lift by the new ratio (1.60) to arrive at the new theoretical valve lift. You can subtract the old ratio from the new ratio to arrive at your lift increase. There is also a slight duration increase, again theoretical, since your .050 duration now has a higher multiplier.

The question remains as to what will it do for you. Most stamped rockers come in toward the middle of 1.4x true ratio. Some of the aftermarket stampings (1.5 or 1.6) aren't much better. The precision rockers, whether they be investment castings, aluminum extrusions or forgings, tend to be at or near their advertised ratios. So if you start on the low side of a factory stamping and get a true 1.6 arm you stand to see a measureable increase in lift and duration. That increase may or may not be something you can feel. The better the heads, intake, and exhaust flow, the more sensitive and responsive it will be to these tuning modifications. If everything is pretty much factory stock, they are chicken soup mods -they don't help but they don't hurt either but they make you feel a bit better.

So if you have a bit of money and you feel compelled to do something, anything, go ahead. But you'd do better to just put the money in a cookie jar for the modifications that have a measurable return on your investment.

 

Spend the money on headers and duals if the engine is stock. You'll gain 30-40hp there.The rockers are worth 10hp and most likely you won't even feel the difference.

 

For $50 u add 0.030" lift to ur stock valves. Reports are of 10-15 hp gain. That about as cheap as it gets.
But yes u may have to enlarge the pushrod slots and this will require removing the intake. They call it a Lewis tool that uses a drill bit to open up the slots - Pro Form now sells a cheap copy.
Most stock springs can handle this 0.030" lift but u should still research this more.
I don't think headers are worth the effort with a short (very) period cam as I just installed a pair and let me list the headaces here:
exhaust pipe offsets needed from collector to exh pipe at trans member
-no one sells these and each side is a little different
A/C brackets need modification as stock pieces won't fit with headers
spark plug wires need heat shielding protection as stock heat shields won't work
should heat shield the starter too as you'll find out later - the hard way
And those stock corvette ram exh manifolds will produce 75% of the gain a header even with a big cam. For stock/small cams it's not worth the major effort and i wish i had left mine on.

BTW since the '81 has F.I. your cross-fire intake won't support high power, big cams etc. Ur time would be better spent on optimizing induction sys. For help Turbo City here in California has done a lot of research with the cross-fire system. :seeya


[Modified by cardo0, 10:51 PM 3/31/2004]

 

What's wrong with increases @ high rpm?

 

Got headers, got duals. Going to have to replace carb and figured I'd do rockers at that point, if worth it.

 

My biggest prob w/headers is reduced ground clearance. I would agree, not worth the effort. BTW, '81s have a computer controlled Q-Jet (my ccm is disabled). '82s are the only C-3s w/crossfire.

 

Doesn't sound worth the effort. Especially since I have to have the work done by a mechanic.

 

Well if u need to elongate those pushrod slots install a good Performer or Wieand intake too. Normally an '81 has the backfire injection but ur signature or post didn't say a thing about a carb sys.
Next work on optimizing advance and fuel mixture.
Oh yes, my bad. I'm thinking '82.
Increases at high rpm? I was saying that backfire intake is rpm/flow limited.
Also the stock 929 cam is kind'a rpm limited too as I expect it stops breathing before 5,000 rpm.
What i'm trying to say is get the stock system optimized before moving up to the hi power parts so u know how to correct your timing and carburetion.
I wanted to leave in the stock cam on my Goodwrench LM1 and try the hi ratio rockers. But other circumstances had me removing the radiator and support anyways. And that cam change was a ton of dirty work . But I found a wiped lobe and glad i did. :yesnod:

 

Hi Runner81,
I fitted 1.52:1 roller tipped rockers on my '81. Pushrod holes were fine for clearance & the job itself was easy. I followed the sequence for statically setting the rocker adjustments, but instead of adjusting the rocker I took it out & replaced with the roller tipped one, set it, & then moved on to do the next one in the sequence. I was told that the stock springs would be OK with the minor amount of extra lift, but the way I see it is that springs are crucial & fairly cheap, so it could be a false economy not to change them (& the seals) while you're at it (ever seen the result of a piston hitting a dropped valve? Ouch :( ).
The stock rockers are supposedly about a ratio of 1.47:1 so there should be an improvement in fitting 1.52:1. I didn't notice any because I changed the cam at the same time. I didn't really fit them for performance gains (but hope they do give some). I like the fact that the roller tip reduces side loads on the valve (increasing valve stem, guide & seal life) & the fact that they are all the same ratio, giving identical lift/duration for the valves on all the cylinders (well, that's the theory....). Hopefully this will give a smoother running engine, which is a good thing.
:cheers:

 

I didn't word my original post very well. I should have said nominal increases at high rpm -- others are suggesting 10-12 hp (although I think that's optimistic). It wouldn't be anything that's going to give you seat-of-the-pants feeling of increased horsepower. Put your money in the piggy bank and save up for a ZZ4. :yesnod: