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Discussion Starter #1
Hello everyone, the other day I decided to try and spin my new cam up to 6500 RPM as it's advertised to make power that high. When I got to about 6300 RPM it started missing and power flattened out, it sounded/felt like I hit a rev limiter. I thought it was odd so I let off and cruised it back home with normal driving, and it acted fine. I pulled the valve cover to check lash and I noticed about half the rockers on the passenger side cylinder head were loose as hell, then I pulled the pushrods out and found that about 5 or 6 were bent (1 being bent pretty badly). I'm just curious as to what could of caused this???


It's an Isky Z-25 solid flat tappet cam with Lunati solid lifters, and Comp 986-16 1.430" dual valve springs with damper (132 on the seat and 290 open) installed on Edelbrock performer RPM heads. I installed the valve springs myself, and made sure to check and shim each valve spring to the proper installed height of 1.750" +/- .020". I checked the seat pressures on the head and they fell right around 130lbs. Also the valve springs are seated in their pockets, it looks like they were machined for a 1.460" or 1.470" spring so there is a little bit of side clearance in the pockets.


The only thing I can think of it being is improper pushrod length. I have stock replacement Sealed Power 7.795" pushrods in there, but I overlooked changing them after I had some machine work done (heads decked .040" and block decked .010" for a total of .050").


Is it possible that pushrods being .050" too long caused this? But why only on one head and not the other? (I checked the other side and lash was fine and all the pushrods were straight).


Maybe stock replacement pushrods don't like RPM above 6000 or too much spring pressure for them???


Help me out here guys! I'm a bit lost.
 

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Hello everyone, the other day I decided to try and spin my new cam up to 6500 RPM as it's advertised to make power that high. When I got to about 6300 RPM it started missing and power flattened out, it sounded/felt like I hit a rev limiter. I thought it was odd so I let off and cruised it back home with normal driving, and it acted fine. I pulled the valve cover to check lash and I noticed about half the rockers on the passenger side cylinder head were loose as hell, then I pulled the pushrods out and found that about 5 or 6 were bent (1 being bent pretty badly). I'm just curious as to what could of caused this???


It's an Isky Z-25 solid flat tappet cam with Lunati solid lifters, and Comp 986-16 1.430" dual valve springs with damper (132 on the seat and 290 open) installed on Edelbrock performer RPM heads. I installed the valve springs myself, and made sure to check and shim each valve spring to the proper installed height of 1.750" +/- .020". I checked the seat pressures on the head and they fell right around 130lbs. Also the valve springs are seated in their pockets, it looks like they were machined for a 1.460" or 1.470" spring so there is a little bit of side clearance in the pockets.


The only thing I can think of it being is improper pushrod length. I have stock replacement Sealed Power 7.795" pushrods in there, but I overlooked changing them after I had some machine work done (heads decked .040" and block decked .010" for a total of .050").


Is it possible that pushrods being .050" too long caused this? But why only on one head and not the other? (I checked the other side and lash was fine and all the pushrods were straight).


Maybe stock replacement pushrods don't like RPM above 6000 or too much spring pressure for them???


Help me out here guys! I'm a bit lost.
Sounds like the Isky cam and the Comp springs aren't playing together. There's more to springs than just pressure, the spring has to stay out of harmonic frequencies generated by the cam. When your expecting to rev over 6000 its a good idea to pick a complete kit from the cam manufacturer as these have usually been tested to insure that the spring doesn't loose control.

The stock type push rod is in way over its head, you can only push 5/16ths 1010 steel so far.

Why all the disparit parts Isky cam, Lunati lifters, Comp springs, Sealed power push rods and what are the rockers?

Did you put spring locators between the valve springs and the soft aluminum of the head?

The rockers need to miss the springs and retainers.

The Edlebrock heads use valves .1 inch longer than stock GM.

You did a lot of milling on the heads and block, why?

Bogie
 

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If your rocker arm is/was centered on the valve stem upon assembly(Did you do the rocker arm geometry test with a sharpie?), then I would think your length is fine but perhaps you need a higher quality push rod, just a guess.
 

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Discussion Starter #6
Sounds like the Isky cam and the Comp springs aren't playing together. There's more to springs than just pressure, the spring has to stay out of harmonic frequencies generated by the cam. When your expecting to rev over 6000 its a good idea to pick a complete kit from the cam manufacturer as these have usually been tested to insure that the spring doesn't loose control.

The stock type push rod is in way over its head, you can only push 5/16ths 1010 steel so far.

Why all the disparit parts Isky cam, Lunati lifters, Comp springs, Sealed power push rods and what are the rockers?

Did you put spring locators between the valve springs and the soft aluminum of the head?

The rockers need to miss the springs and retainers.

The Edlebrock heads use valves .1 inch longer than stock GM.

You did a lot of milling on the heads and block, why?

Bogie


Well the reason I have these Comp Springs is because that's what my old 282s cam used, and The recommended spring from Isky for the Z-25 cam is only 10lbs less spring rate (310lbs as opposed to 322lbs from Comp). Lunati Lifters because I've heard good things about Lunati products and they were significantly less than the Isky lifters (I was trying to stay a little on the budget side when I swapped that cam out).


I have Comp 1.6 Roller Rockers, and they do clear the retainers.


I milled the block deck because the pistons I used (didn't realize it until after I purchased them) have a 1.548" wrist pin height instead of 1.560", so I had to have the deck milled to get the piston .025" in the hole (which ended up being .010" shaved off the block) so I could use a .015" gasket for a proper quench.


The heads I got from a friend, but were 70cc chambers, and I didn't want pop up pistons to get the CR I want, so I called Edelbrock and they said I could shave up to .060" from those heads. I had .040" removed and then CC'd them and it put the chambers at 65cc.


I never used the sharpie/length checking method, and now I know I need to go and do that.


I do think it's time to upgrade those pushrods..


I guess I just didn't expect to bend one, especially at a pretty modest spring rate.
 

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Discussion Starter #7
I would look at a spring nearer 150 seat and 400 open pressures and a 3/8 PR.


I would love to have a good chromoly 3/8 pushrod, but I won't do that until way down the road when I get better heads and a roller cam which is gonna use a heavier spring pressure anyway!


Besides, these springs are actually a tid bit fatter than Isky recommends.


Isky recommended a P/N 205-D
115lbs seat
268lbs open
310 lbs/in spring rate
1.250" single spring


I used Comp P/N 986-16
132lbs seat
290lbs open
322 lbs/in spring rate
1.430" dual spring with damper


But maybe bogie is right with the harmonics??
 

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Discussion Starter #8
. Are you using stock, unhardened pushrods with guideplates?


Lol well damn when you say it that way I think I already know where you're going with this!


Yeah I am, and from the sound of everybody's feedback this is probably a crutch/weak point for my valvetrain.
 

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I am (was) running the comp 5/16 .080 chromoly "hi-tech" push rods... Haven't bent them yet just crushing seals.


spring specs 135 lbs. @ 1.750, 325 lbs @ 1.200
 

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The instruction sheet with my RPM heads say to use a .100” longer pushrod than stock if I’m not mistaken. Also if I’m not mistaken, the recommended springs are for the stock ratio rockers, but not certain. I am certain that the 1.6:1 rockers will put a good bit of extra force on the pushrods, cam, and lifters.
Did you check valve to piston clearance? Worth doing IMHO especially if you shaved the head and block, and only a .015” gasket. I know its only .480” lift, but you don’t know unless you check. Might want to check out this youtube video as you asked about mid lift check on another thread.

https://www.youtube.com/watch?v=o5is9BsH5OU&feature=em-upload_owner

Also might want to look into some locators for the springs like Bogie mentioned. If the spring jumped up onto a part of the head that’s not machined for it I’m sure you can guess the results.
Ssmonty
PS I just looked at the Comp catalog and they don’t offer a spring cup for the 986 springs.??? At least in the catalog I have (106-07).pdf. Might want to check out this other video too?

https://www.youtube.com/watch?v=PXzO9u93-n8
 

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Discussion Starter #11
Thank you monty,


I did check piston-valve clearance after I had it all machined and it came out to somewhere in the ball park of .200" clearance which is plenty.


Yeah and if IIRC the Edelbrock instructions said .100" longer pushrods if using the stock stamped rockers.. I THINK. Either way I'm about to checked my rod length and definitely am going with the Comp Magnum chromoly 5/16" pushrods.


In regards to spring locators I do not have any, the only thing between the pocket and the spring is a shim. Each pocket needed a .030" steel shim to get to 1.750" installed height, a few pockets actually needed a .030" and a .015" to get to 1.750".


Thanks for the valvetrain geometry video, that's pretty cool how they checked it that way.. I'm gonna try and check it with the sharpie method and the stock length pushrods just to see how far off I may be with the current ones.
 

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You definatley need a hardened pushrod if you have guideplates. .100 longer valve means longer pushrod, and the geometry needs checking. .080 wall with 1.6 rockers, thats a lot of spring pressure on that pushrod.
 

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Discussion Starter #13
Yeah you're right Richie,


I just checked the rocker tip in relation to the valve stem with the sharpie method. I wasn't able to get it to leave a very good mark, but it looks really really close to center, maybe a tid bit high, so I'm thinking .050" longer (7.850" .080" wall chromoly) pushrods.


What do y'all think??
 

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. Also, no self-guiding rockers if using guideplates... yours don't look to be self-guiders...

. I've seen guideplates cut stock pushrods in half in about a half-hour of running...


. Those witness marks look fine on the valve tip...
.
 

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Well the reason I have these Comp Springs is because that's what my old 282s cam used, and The recommended spring from Isky for the Z-25 cam is only 10lbs less spring rate (310lbs as opposed to 322lbs from Comp). Lunati Lifters because I've heard good things about Lunati products and they were significantly less than the Isky lifters (I was trying to stay a little on the budget side when I swapped that cam out).


I have Comp 1.6 Roller Rockers, and they do clear the retainers.


I milled the block deck because the pistons I used (didn't realize it until after I purchased them) have a 1.548" wrist pin height instead of 1.560", so I had to have the deck milled to get the piston .025" in the hole (which ended up being .010" shaved off the block) so I could use a .015" gasket for a proper quench.


The heads I got from a friend, but were 70cc chambers, and I didn't want pop up pistons to get the CR I want, so I called Edelbrock and they said I could shave up to .060" from those heads. I had .040" removed and then CC'd them and it put the chambers at 65cc.


I never used the sharpie/length checking method, and now I know I need to go and do that.


I do think it's time to upgrade those pushrods..


I guess I just didn't expect to bend one, especially at a pretty modest spring rate.
It's not the spring pressure that bends push rods, it what happens when the spring loses control and either space opens up allowing the valve to freely accelerate till it hits the seat and bounces, sometimes into the piston or the seat has bounced and is closing again as the valve train is opening it again.

Keep in mind that at 6000 RPMs any valve is cycling 50 times a second so it gets pretty easy for things to get out of hand.

Bogie
 

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Discussion Starter #16
. Also, no self-guiding rockers if using guideplates... yours don't look to be self-guiders...

. I've seen guideplates cut stock pushrods in half in about a half-hour of running...


. Those witness marks look fine on the valve tip...
.
They're not self guiding, so I guess I'm okay with the stock length then, just need hardened/heat-treated pushrods.
 

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Discussion Starter #17
It's not the spring pressure that bends push rods, it what happens when the spring loses control and either space opens up allowing the valve to freely accelerate till it hits the seat and bounces, sometimes into the piston or the seat has bounced and is closing again as the valve train is opening it again.

Keep in mind that at 6000 RPMs any valve is cycling 50 times a second so it gets pretty easy for things to get out of hand.

Bogie


I see what you're saying Bogie.. I hope these springs don't give me more issues. I'm going to get the hardened chromoly pushrods and if I still have issues near peak RPM I'll probably have to look into different springs :mad:
 
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