ALL things related to the Ford 351C
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PostPosted: Mon Apr 27, 2020 5:54 pm 

Joined: Mon Oct 29, 2012 1:52 pm
Posts: 733
Today I was working on a set of small block Chevy heads for a customer who bracket races. They are an old set of Dart aluminum heads that are in nice condition despite being raced a lot for many years. I gave them a fresh valve job and a new seat of seals and springs and here's where it gets interesting.

Due to availability of parts I ended up using a set of PBM 3850 springs on the intake and a set of 3600s on the exhaust. On the 3850's in installed them at 1.900 and with the cam's .630 lift and .025 lash they end up if I remember right on average about .150-.200 away from coil bind at max lift and give approximately 240lbs seat load and around 580lbs open. On the exhaust side the lower rate 3600 is stacked tighter at 1.840 installed hight and this gives loads that are on average nearly identical to the 3850 both on the seat and open. The 3600 ends up running between .055 and .080 away from coil bind.

With the same cam lobe working each spring thorugh the same rocker ratio which spring would give better valvetrain control if each one was placed on a valve that had the same amount of mass? Would the 3850 with its higher rate and undoubtably higher natural resonant frequency but taller installed height and shorter bind height give better control? Or would the lower rate and frequency 3600 stacked tighter tend to self dampen and have better control because it is running closer to coil bind?

It would be fun to have a spin rig to find out!

PostPosted: Tue Apr 28, 2020 9:48 am 

Joined: Mon Oct 29, 2012 9:05 am
Posts: 2881
I think that's the only way of knowing, throw it on a Spintron. The "internet" would tell you that the spring setup at .050" would last the longest, but I'm not too solidified on that theory. I usually aim to set springs up at .050-.060" from coil bind but I've also seen spring manufacturers recommend that their springs be setup at .100-.120" in some cases and I have done so without issue.

I do some really high end pulling truck engines, 8500-9000 rpm, with >1200 lbs spring pressure. I use PAC springs for one of them and their tech guy told me to aim for .100" or tighter. On that particular engine, there are two install heights, like you're looking at and I use the same spring for both. On one side it's setup at about .060" from coil bind, and about .100" on the other. There are never any valve spring issues with that one and when that engine comes back for freshen up, it's usually only lost about 15-18 lbs of seat pressure (with 380-400 lbs seat) after a season.

One thing for all the other guys reading: don't always assume that springs bind up where the manufacturer says they do. In most cases, it's always lower and I think the manufacturers do that to give guys who don't check some added cushion.

I just tested a full set of Comp 933 springs. The box says 1.200" coil bind. They all bound up anywhere from 1.110-1.140".

Always check them yourself.

Brent Lykins, Lykins Motorsports

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