Stock Head questions

Simon@London · 03-25-2009, 09:17:29 AM

On my old heads the valve springs were only holding at 80 lbs pressure which sounds quite light. These are stock GM 71 year 920 heads. Also had one valve go bad with a worn stem plus new seals and some guide work. The Cam was a very mild Crane Blue racer which has been changed to a Energizer to match the low compression of this engine with some good lift. Should have a nasty idle now but my question is this.

Will weak springs be one of the culprets in valve float. I experienced this issue with this motor and we found numerous issues when torn down. Bad rebuild didn't help with some crappy workmanship.

So are valves, springs a major issue.

Don't bother mentioning other heads please, this is a motor for this year only as I stated before we are building more of a project engine once I find the right core. Even have almost $500 in change saved up turned into bills already in my Z28 engine project jar. ( keep adding to it )

Thanks.

joeEZ · 03-25-2009, 10:27:46 AM

Yes, springs are a major issue. I have seen people install springs only to come back with a problem. The heads were then properly set up and shimmed the springs. They picked up at least 2 tenths in the quarter.

Make sure they get set up at the proper install height, and match your springs to your cam. Although, some people like a little extra spring pressure.

hhott71 · 03-25-2009, 11:23:46 AM

weak springs aren't good. The generic "Z/28" springs are more than enough for your Energizer cam.

Simon@London · 03-25-2009, 12:46:40 PM

Thank you both.

What pounds per square inch pressure are those Z28 springs rated at. I am trying to compare to see how lazy these old springs were. I am also guessing when they rebuilt this last spring they didn't even touch the springs but did have new rods installed. I have the old set but since these were running good enough they just left them in it. My buddy also said someone attempted a head port job but failed badly and never matched the intake with it neither.

Damon · 03-25-2009, 01:26:45 PM

Having the right valve springs INSTALLED AT THE PROPER INSTALLED HEIGHT is very important. Having weak springs is like leaving a good party at 10pm- you miss half the fun!

80 lbs on the seat is actually quite good for stock. I've seen them as low as 65 lbs quite a few times.

A mild performance flat tappet usually does fine with about 100 lbs. on the seat. The RATE of the spring doesn't need to be very high- around 300-350 lbs. per inch is fine. I haven't measured any Z28 springs on my bench but a similar aftermarket spring would be the Comp 981 spring. It checks in around 105 lbs. on the seat with a 323 lb/in spring rate (stock diameter, so it's a drop-in replacement for stock, as is the Z28 spring).

Simon@London · 03-25-2009, 04:46:52 PM

Having the right valve springs INSTALLED AT THE PROPER INSTALLED HEIGHT is very important. Having weak springs is like leaving a good party at 10pm- you miss half the fun!

That's what I figured. Coupled with a bad valve and some leaking it probably wasn't pushing out the max torque or HP. Can't wait to hear it run now with a nice fresh top end.

tom3 · 03-25-2009, 11:04:25 PM

Many of the newer cams have pretty fast ramps and older springs will cost some power for sure with these. Not a big job or lot of money to replace those springs.