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Discussion Starter #1 (Edited)
This isn't exactly a BBF question but a topic thats been discussed before. I have been helping a friend run a 565 BBC in a 5.30 Index class. Car weighs about 3000 lbs and we spray it just a little bit to dial in in to the 5.30 index. It will run about a 5.40 on motor. This is a good, well built motor that has performed flawless up till now.

It seems after a basic freshen up of the engine we lost 6-8 numbers on E.T. We are having to spray longer to run the 5.30 index. We have looked over everything trying to figure out where we lost E.T. After looking back in his records he saw where he replaced the exhaust pushrods that were not the same wall thickness as the old ones. This is the only thing that has been changed in the engine from last year. He had concerns about his pushrod size anyway.

Has anyone seen a noticeable loss/gain from this type situation? We are searching everywhere for a problem.

I know it's not a BBF, but there are some great engine guys here, I appreciate any help and advice
 

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Off the scorpion site

What is pushrod flex and how does it affect power and durability?

The pushrod deflects or flexes upon valve opening and then again just prior to valve closing. As the cam tries to move the valvetrain to open the valve, it has the weight of the valvetrain against it and in the case of the exhaust valve, cylinder pressure also. Upon valve closing, the cam is trying to slow the valvetrain down. So on valve opening and valve closing the pushrod flexes and as rpm increases it gets worse. Here’s how it effects rpm, power and durability;

Rpm is reduced when the pushrod flexes, and then rebounds like a pole vault. It throws the rocker and valve out of control. As rpm increases, the more the pushrod flexes, the more out of control the valvetrain becomes and the more valve "float" or "bounce" to the point that the engine rpm no longer increases.

Power is naturally affected by the loss of valvetrain control at rpm, but something else happens. Remember the engine responds to valve timing not cam timing. Even if the valvetrain stays in control. When the pushrod flexes upon valve opening and valve closing, the valve opens later and closes earlier. The engine "sees" less valve open time (like a smaller duration cam) and as rpm increases it gets worse. Loosing 6 to 8 degrees of valve timing is not unheard of!

Durability is affected when the valvetrain parts are "thrown" out of control by the rebounding pushrod. When this happens the valvetrain parts are jerked apart from each other, and then slammed back against each other by the valve spring pressure. This "slam" can break parts anyplace in the valvetrain and at the very least break the lubricating oil film causing excessive wear. All of the above can happen without a pushrod failure!


Hope that helps
 

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I've been moving the p. rod package up as of late after seeing evidence of some valve float while the springs were good for the task. I think that p. rod flex is an issue here.
The exh. side opens against pressure so it's a bigger deal there....more so with the big pressure of spray.
 

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What type/size pushrod are you running? Guide plates or no guide plates? The biggest pushrod you can fit in the engine, preferabbly a taper design if your setup allows it (no guide plates) is the ideal setup. It's on the slow side of the valvtrain, so weight wont be an issue. Stability is what your after, and wall thickness is also very important. The thickest you can go without affecting oiling up top any. Hope it helps
 

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What type/size pushrod are you running? Guide plates or no guide plates? The biggest pushrod you can fit in the engine, preferabbly a taper design if your setup allows it (no guide plates) is the ideal setup. It's on the slow side of the valvtrain, so weight wont be an issue. Stability is what your after, and wall thickness is also very important. The thickest you can go without affecting oiling up top any. Hope it helps
The biggest, thickest wall pushrod is not always the IDEAL set-up especially if the pushrod gets rigid enough to make another part of the valve train flex instead.
To exclaim "ALWAYS" is not close to reality here; do some SPINTRON testing and you can see it for yourself.

Get that pushrod CORRECT for whatever application it is required to serve.
 

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Well I didnt at any point say "ALWAYS" But ideally yes. The pushrod transfers energy from the source (lifter) to the desired endpoint the rocker and valve. If you are getting deflection, your losing stability. A large dia. tube lessens harmonics and spreads the load out and lessens deflection. The heavy walls prevent compression of the tube. Im not saying Go grab some 1 3/4" tubing and make some pushrods, but size the pushrods to your cam/rpm/spring pressure. Hopefully I worded it better this time.
 

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An example would be...This 565" motor is running 200hp worth of nitrous, jesel shaft rockers, and lets say 850lbs open pressure. Now you've got high cyl. pressures on top of alot of spring pressure. The exhaust especially since it is opening under pressure. A 7/16dia .168 wall on the int and a 1/2 on the exhaust would be plenty of pushrod for this motor and could take you to 400 seat 1100 open if you ever needed.
 

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Do MMC - aluminum matrix - pushrods have a role here? Do the lifters have to be heavy steel slugs? It just seems like more heavy parts, moving further, faster meaning exponentially higher forces in the valevtrain.

Mebbe someone should cast a block with the cam moved up several inches, shortening the pushropes a bit. I s'pose the pushrods are less an issue than the valves, but they're long and look like frustrated springs.
 

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Discussion Starter #14
Thanks for the info guys. Pushrods have been changed and we plan to test this weekend. I'll post up results.
 

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Maybe I'm missing something but my C-headed 600 with over .900 lift my 3/8 x.080 pushrods seem to be fine and when cycled through lift they get within about .030 of touching the pushrod holes but there has never been any evidence of the flexing and touching
Frank
 

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Something better than a 3/8" X .080" :-}.
Perfect.....who knows. But, the 7/16" stuff gives a better gain.
All of this depends on spring pressure, p. rod lenght , cam design, etc.
I'd doubt most of us will ever get it "right"....it is a good thing to move in the right direction.
 

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ok, I got it now I wasn't sure what you were talking about his current set up or the idea of upgrading. To me a 3/8 080 is a little inadequate for the spring/cam etc that im assuming is controlling his setup, but thats also my opinion. I agree none of us will get it "right", but hopefully help in moving the right directon like you said. Just out of curiousity do you only use Trend pushrods, or do you use others as well?
 
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