Understanding rockers - Pontiac GTO Forum
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post #1 of 17 (permalink) Old 04-09-2019, 09:22 PM Thread Starter
 
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Understanding rockers

Hey guys- trying to understand the impact of switching from 1.5 stamped steel rockers to 1.65 rollers. Besides the friction benefit, I thought I understood it that the bigger rocker had the effect of mimicking the lift of a bigger cam due to some geometry that I donít understand. I found this page- Wallace Racing - Pontiac Factory Cam Specs I have the 540233 cam from a 1962 389 SD. This shows the change in lift by moving to 1.65. Is it that simple? Just drop them on? Any issues with valve to piston clearance? Appreciate the insights. Thanks...
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post #2 of 17 (permalink) Old 04-09-2019, 10:18 PM
 
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The difference in valve lift is just simple geometry. If your cam lobe lift is .300, then 1.5 rockers will produce a valve lift of 1.5 x .300 = .450. With 1.65 rockers, it would be 1.65 x .300 = .495 valve lift.

1.65 rockers puts the pushrods closer to the rocker studs. So, most heads will need some grinding to give the pushrods enuff clearance. Because of the shorter fulcrum, the 1.65 rockers will put a little more stress on the pushrod side of the valve train, since more force will be required to open the valves. Just like using a shorter wrench will require more force from your hand to tighten a bolt than if you used a wrench with a longer handle. Again, just geometry.

As for the valve-to-piston clearance, there are too many variables. You'll just have to measure how much you have now & do the math.
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post #3 of 17 (permalink) Old 04-10-2019, 06:48 AM Thread Starter
 
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Thanks big d. That helps a lot. Wrench/leverage analogy is good. So assuming I confirm clearances for push rods and pistons, do I need to do anything to mitigate the extra stress on the push rods? Any danger there? This is a street car that will be pushed hard from time to time...

And just to say out loud, I presumed that taking the lift up from 405 to 447 by making this rocker switch would have some noticeable performance improvements? I am not planning on doing any head work to optimize flow for large lift cam so felt like this was a nice compromise without having to change cam. Thoughts?
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post #4 of 17 (permalink) Old 04-10-2019, 07:21 PM
 
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You'll also be changing other things. Pushrods for correct length, rocker arm studs, valve springs.
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post #5 of 17 (permalink) Old 04-10-2019, 07:40 PM
 
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Thanks big d. That helps a lot. Wrench/leverage analogy is good. So assuming I confirm clearances for push rods and pistons, do I need to do anything to mitigate the extra stress on the push rods? Any danger there? This is a street car that will be pushed hard from time to time...

And just to say out loud, I presumed that taking the lift up from 405 to 447 by making this rocker switch would have some noticeable performance improvements? I am not planning on doing any head work to optimize flow for large lift cam so felt like this was a nice compromise without having to change cam. Thoughts?
Performance improvements? Yes and no depending on what other matching engine parts you are using. Going to a higher lift may increase HP and TQ somewhat due in part to the faster lift (creating a little more overall duration) the valve will be opened by the 1.65 vs the 1.5's. Keep in mind that the faster lift of the valve with the 1.65's also means the faster the valve slams shut - and this could be a problem at higher RPM's and cause the valve to literally bounce off the valve seat, which is not a good thing.

.447" is a fine lift and it is said that about .450"-.460" lift is about where the factory heads stop flowing their best. So, IF you go with the 1.65's, you may also want to upgrade your valve springs.

The 1.65's as bigD noted, may require additional side clearances where the pushrods pass through the hole in the head. The 1.65's move the pushrod cup closer in towards the rocker stud to get that 1.65 ratio and moves the pushrod closer to the side of the hole. This move of the pushrod cup puts more pressure on the rocker arm stud and can in some instances with high lifts and high spring pressures snap off the factory "bottle neck" rocker. This is why the BB Chevy ARP 7/16" rocker arm studs are used to replace the factory "bottle neck" studs. Changing to the BB studs means that you will now have to use poly-locks to secure the rocker arms.

Duration is where you want to get your performance from, this includes valve overlap. If you look at the Pontiac cams, the lift stays essentially the same, but duration increases as does the overlap. More duration also moves your power band up the RPM scale.

Depending on your compression, the Lobe Separation Angle (LSA) can be used to build additional cylinder pressure for those low compression engines or bleed off some cylinder pressure on those high compression engines.

So switching to 1.65's may increase HP & TQ a little, but may not be that big of an improvement by itself. Start adding a better exhaust system, dial in the carb, use an open element air cleaner, dial in the ignition curve and total ignition timing, and now you will see noticeable improvements.

If an automatic transmission, add a shift improver kit and to give additional performance, go with 3.55 rear gears unless you do regular highway cruising.

Just my opinion.
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"...do I need to do anything to mitigate the extra stress on the push rods?..."


The Pontiac Ram Air IV engines had 1.65 rockers. For those engines, Pontiac used 7/16 rocker studs & larger diameter pushrods. So, it might be a good idea to buy some stronger pushrods.

Considering all the necessary parts & changes needed, I think that if I wanted more performance, I'd prefer to stay with 1.5 rockers & go with a slightly larger cam, new lifters, & matching springs.

I'm just curious. Is there some particular reason you wanna use that cam ? It appears to be a solid lifter, low lift, high duration cam. There are HFT cam/lifter combos that will make more power, without the hassle of solid lifters.
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Last edited by bigD; 04-10-2019 at 09:24 PM.
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post #7 of 17 (permalink) Old 04-11-2019, 04:56 PM Thread Starter
 
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Well I might be mistaken on the cam
Then. Solid lifters require poly locks, I think? These heads don’t have poly locks. Let me look at the markings on that cam more when I get back from trip. I can only see markings from top of motor, and I thought it had 540 233 stacked. I am
Just getting into hobby again and my confidence on changing a cam and dialing it in is low. Feel like I would screw up something. Replacing head gaskets is totally doable for me at this point. Not the best answer but it is reality,
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post #8 of 17 (permalink) Old 04-11-2019, 05:49 PM
 
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Well I might be mistaken on the cam
Then. Solid lifters require poly locks, I think? These heads donít have poly locks. Let me look at the markings on that cam more when I get back from trip. I can only see markings from top of motor, and I thought it had 540 233 stacked. I am
Just getting into hobby again and my confidence on changing a cam and dialing it in is low. Feel like I would screw up something. Replacing head gaskets is totally doable for me at this point. Not the best answer but it is reality,
No one has asked and you did not say, what size/year is the engine? This makes a big difference in what is being recommended.

Yes, solid lifters will require poly-locks because you have a specific rocker arm to valve clearance and you have to gap them.

If you swapped to the stronger 7/16" screw-in rocker arm stud, providing you have a high-performance 1967 and up 400CI with screw-in studs, then you would need poly-locks. If your engine is 1966 and earlier, then you have press-in studs.

You could confirm lift by installing a lifter, then place a dial indicator secured by a magnetic base (cheapo Harbor Freight will work) on the edge of the lifter and rotate to see what total lobe lift is. Then multiply by 1.5 to get total lift at the valve. This should get you pretty close to what you have on lift. To get the duration, you would need to hang a cam degree wheel on the front of the cam, and this would mean pulling off the timing cover - so I would just try to figure lift at this point.

The 540 233 could mean a 389SD cam or it could also be a manufacturer's number.
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post #9 of 17 (permalink) Old 04-11-2019, 06:06 PM Thread Starter
 
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Happy to opine. It’s a 1964 421 with 1965 77 heads and 1966 tripower. My thought was to put in the screw in studs, valve job and 1.65 rockers. But taking this advice into account of course. Sounds like I would need to buy new push rods and maybe springs too. I don’t know how to determine what length push rod to buy, and or the spring strength...
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post #10 of 17 (permalink) Old 04-11-2019, 06:27 PM
 
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Happy to opine. It’s a 1964 421 with 1965 77 heads and 1966 tripower. My thought was to put in the screw in studs, valve job and 1.65 rockers. But taking this advice into account of course. Sounds like I would need to buy new push rods and maybe springs too. I don’t know how to determine what length push rod to buy, and or the spring strength...
OK, that changes a few things. To install the screw-in studs will require head removal and a machine shop - if you did not already know this. If you go with 1.65's, then is is also the time to have those pushrod holes elongated.

Just added this as I was thinking about it - does the '64 421 use oiling through the rocker arm studs to supply oil to the rocker arms? I know the '64 GTO 389 was more like the '65 and up engines where the oiling to the rockers was changed, and they oiled through the pushrods. It is my understanding that the standard 389 for '64 still oiled through the rocker arm studs. Not 100% sure on the '64 421 engine.

Springs can be recommended by any of the several Pontiac engine builders or any cam manufacturer once you know the cam specs and if solid or hydraulic.

You can purchase a "push rod checking" tool which is simply an adjustable pushrod that you will screw-in or out to get the correct wear pattern on the top of the valve guide with rocker arm installed. Then that gets measured and you order a set closest to that length. This has been covered here before. You may not need the larger diameter pushrods as bigD pointed out, but you may want to get a thicker wall which is offered, and will provide greater stiffness. Most of the big name Pontiac builders offer these.

Last edited by PontiacJim; 04-11-2019 at 09:32 PM.
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