What is the difference between the valve springs used for a solid cam and a hydraulic cam? Are they interchangeable or will the use on the wrong application cause damage?

I was looking at a set of heads that were rebuilt with springs for a solid cam, and need to know if I will have to replace the springs because I am running a hydraulic cam.

 

I think the spring rate is higher for the mechanical cam.

 

So would a solid spring have a greater resistance and therefore damage the cam?

 

Your can go to the Comp Cams web site and look at the spring section and it might be able to expand your knowledge base.

Spring basics: Spring have three ratings. Closed, open, and max lift before coil bind. At a certian installed height you have what is called closed valve seat pressure. Or (closed) Generally you will see H-flat springs anywhere from 90 pounds closed to @ 120# closed. When I ran H-flat cams I found that 125 -130# was about the upper limit.

It's not that it causes cam damage per say. It's more of the design of the oil pump up h-lifter. Your oil pressure and oil vescosity play a role pushing the internal piston up against the base of the pushrod. If you had too much spring pressure the internal piston would just depress instead of lifting the valve off the seat.

The open pressure is determined by the spring rate times the valve lift. If spring has big strong coils it might have a rating of 500 pounds to compress it one inch. Then if your cam and rocker combination lift your valve .500 inches your open valve spring pressure is 250#s plus the #'s on the closed seat. If the "Spring Rate" is too high the H-flat internal piston will compress instead of lifting the valve.

The higher rpm you rev. a motor the greater the spring pressure has to be. I found that 7200 rpm was about the upper limit of H-flat lifters. Engine builders install solid or solid rollers to exceed 7000 rpm because they can install very high pressure springs and not have to worry about compressing the internal piston plunger of a H-flat lifter.

So to answer your question. Without testing the spring you can't tell if it would work or not.

 

Thanks gkull. The springs are rated at 135# seat pressure. So it looks like I will be replacing the springs if I get the heads.

 

I didn't want to explain to much and over fill your head all at once!

135#s at the installed height is the next spring subject.

Lets use the example of 1.800 inches installed height. Which with shim, retainer, 7 or 10 degree lock combination causes 135#s of seat pressure. Then lets say that you bought a set of 16 +.050 chromemoly retainers and locks. Your installed height is now 1.850 inches and the seat pressure is only 120#s because the spring is less compressed because of the additional installed height.

bonus number two in doing this is: lets say that this spring gets coil bind at 1.200 inches. 1.800 - 1.200 = .600 this spring is rated for a max valve lift of .600 inches. Add .050 to this with your new retainers and your max lift is now .650 inches and the the spring will run cooler and last longer. never run springs at or near max rated lift. Your just asking for failure. Just spend the extra money and get the correct springs.

The only other problem with those 135# springs is the spring rate might also be too high for your H-flat lifters. So even with setting the installed height to get the seat pressure right, the spring rate did not change.

 

Sounds like I would just be much better off with a set of Z28 springs rather than risk using the other ones.

Without knowing exactly what spring it is, that could cause trouble in the long run.

Thanks again.

 

If you go to the cam card for your cam it will give recommended spring. Then call up the manufacturer and ask for a the next stronger spring and one with more max lift. Any head repair shop could help you out on spring selection.

I'm also pretty sure that z28 the 302 ci screamer engine was a solid cam.

 

Interesting tidbit - Factory Z/28 springs were just the same part as the springs for the run of the mill base hydraulic cam small block engine. No screw-in studs and no guideplates either just 100% stock base engine parts there.

There was a HP service part "Z/28" spring that was available from the parts department. Might have been identified with a brown stripe but I would have to double check my CPM to be sure. These were an upgrade and might have been the ones specified for the "Off-Road Special" -140 cam but all the Z/28 engines were sold new with bone stock springs that were the same as the ones used in your gradfather's Caprice.

-Mark.

 

The -140 cam had a spring package that went with it-- 3927142 if I recall.
I used that cam and spring setup in a Camaro many moons ago. Z-28 car, came from the factory with screw in studs, guideplates, cast rocker covers, 2.02/1.60 valves, and 76cc chambers for low compression. I added that 3927140 cam, headers, a good intake and a 750 Holley dp. Used to surprise the 455 Trans-Am guys a bunch.

 

I didn't look like things added up or I am not thinking correctly. Open pressure is the sum of the closed pressure plus the additional pressure created by the lift of the cam.
For example using gkull numbers. The closed pressure is 100lbs. The springs are rated at 500 lbs/inch. The cam is .500" lift so it will add 250 lbs of pressure. The open pressure is 350 lbs. Springs such as these in this example would probably not be recommended for a hydraulic flat tappet cam.
After looking at different cam cards, it seems the cam manufactures recommend about 300 lbs or less for open pressures. The general rule is the higher the lift the cam has, the lower the spring rate. A spring that is suitable for .400" lift cam may also work for a .450". A cam with .500" lift may be pushing things too far and a weaker springs are needed to keep the open pressure from being too high.

 

According to my cam card, the spring requirements are as follows:

Closed - 114 lbs @ 1.700
Open - 318 lbs @ 1.240

Are these minimums or maximums? How would a 1.6 ratio rocker affect the spring selection, if at all?

 

Changing from 1.5 to 1.6 ratio rockers yields about 6.67% more lift. That would add about 14 lbs more open pressure.

The specs I looked at for valve springs are neither minimum or maximum, they are what the manufacture recommends.

Will you have to change the springs? Unless you know who made them and what the item number is, it would be a guessing game to know if they are compatable. You could take them to an engine shop and have them checked to see what rate(s) they are.

Are you using pressed in studs or screw in studs? There is an acceptable limit of how much pressed in studs will safely handle, roughly about 280 lbs from Crane's website. As to how much above that value is possible, I don't know.

 

You are correct! It's seat plus the lift spring rate. I just wrote this in a hurry and didn't proof read! thanks for catching it.

 

those are good low lift spring pressures for an h-flat. As posted the valve lift is only .460 with 1.5 ratio rockers. 1.6 rockers would only increase the lift to .490. Marginal gain at best, but every little bit helps.

 

Yes, those are the ones (-142 springs). Do you remember if they had the brown paint stripe?

With 76cc heads and a -140 Off-Road cam it must have been pretty soft at low RPM. That cam needs about 12:1 c/r in order to work right. Even then it is a very high RPM grind.

You are right about the '70 and up (350 cube) Z/28 engines having those guideplates etc. The first generation 67-69 302's did not. Amazing what they accomplished back then with those very basic production parts.

-Mark.