Comp Ultra-gold rocker arm failure

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08-31-2015, 01:42 PM

#21

Exidous

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Care to elaborate? I'm pretty sure boost pushes on the back of an intake valve....

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08-31-2015, 01:55 PM

#22

Darth_V8r

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Quote:

Originally Posted by Exidous

15lb of boost on an intake valve @ 2.1" reduces open pressure avail by 52psi roughly. So at 15lb of boost you are essentially running a 330lb spring.

What I'm wondering is - what about the increased pressure on the exhaust valve? Would it not increase the force required to open the exhaust valve due to the boosted cylinder pressures? I missed in the thread if the rocker was on intake or exhaust.

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08-31-2015, 02:11 PM

#23

Exidous

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It was on the exhaust and I would think the added cylinder pressure would make opening the exhaust valve harder.

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08-31-2015, 03:08 PM

#24

Kip Fabre

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Quote:

Originally Posted by Exidous

It was on the exhaust and I would think the added cylinder pressure would make opening the exhaust valve harder.

Here is how it goes for a N/A verses a Turbo or a Supercharged. If the EX of a N/A engine has 300 LBS in the cylinder when the cam tries to open the valve with a 1.6 diameter valve with one of our HYD lobes it takes 603 lbs of force to open the valve. If its boosted and has 600 lbs in the cylinder it takes 1206 lbs to open the valve. So its a big difference when the presser goes up

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08-31-2015, 03:37 PM

#25

Exidous

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When you start talking about numbers like that it's easy to see when folks recommend stout pushrods.

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08-31-2015, 03:43 PM

#26

Kip Fabre

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Quote:

Originally Posted by nmccann

I suspect you are right. Springs are installed at 1.78" to get 155 on the seat, coil bind is at 1.13", tons of PV clearance .150"+. Chatting to my engine guy as well and he seems to be of the same opinion, told me he wouldn't sweat this much and to just change the rocker. I do appreciate everyone's replies on the matter as well. I am going to see about a warranty rocker but rather than wait a few weeks and let the summer pass I went out and bought another set to get the car mobile again. Based on the revised design of the new ones I suspect I'm not the only person that's had trouble with these... Those of out there with YT rockers, how similar is this to what you have?

Also, I had to grind the hell out of my valve covers to get enough clearance for these. Everything else remained the same, wipe pattern is good, no shimming req'd and the same pushrod length still works.

The difference between the two rockers are the top one is made from a extrusion
the lower one come from a casting. The aluminum grain structure is much better on the extrusion than the casting.

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08-31-2015, 04:33 PM

#27

TurboBuick6

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Quote:

Originally Posted by Exidous

Care to elaborate? I'm pretty sure boost pushes on the back of an intake valve....

It does, just not to the extent everyone thinks it does. How much cranking compression do you have? Now how much boost pressure do you have? Bet your cranking pressure is 10X that of boost.

For VE you close the intake valve while the piston is on the up stroke, correct? So for one very split second from the time it takes boost to recover in the intake tract from the velocity it was seeing your piston is probably half way up now. I bet that cylinder pressure is in excess of 300lbs by now.