Please help settle a debate!

[jimz61]

This has been a debate among friends for two days now. Will someone who knows please help settle it?!?

Here's the debate:

When installing a set of valve springs, there are a number of factors to be considered. And they all relate to each other. Among them are minimum and maximum spring height and seat pressure.

Accepted assumptions:
1. seat pressures vary slightly from spring to spring
2. install heights vary slightly from head to head

Debate:
Should the installer adjust each head/spring for consistent/optimum spring height thereby accepting a variance in seat pressure from valve to valve?

or

Should the installer adjust each head/spring for consistant/optimum seat pressure thereby accepting a variance in spring height from valve to valve?

Thanks!

[rick conrath]

Since the distance between the rocker arm valve stem tip is set mechanically by the valvetrain, I would argue the the spring pressure should be consistent to minimize valve float. That's really the purpose of the springs....to push the valve back in place. A spring with a higher seat pressure isn't going to be any longer when constrained by the keepers/valvetrain geometry. I believe shimming is the answer to the question.
Rick
'78 930

[Wil Ferch]

Rick:
The counter-argument might be that setting pressures equal ( and allowing a slight variation on the height)....might cause one of the springs to "bind".....obviously not a good thing.

I'll check the Porsche factory manuals which I believe answers this question...but I think ( for this reason), the height might be the "control" parameter.....

Wil

[Zeke]

Race engine builders buy lots of springs and use matched sets. I don't know if this solves your problem as I don't completely understand the question. But I do know there is a difference in springs from the same box. You have to sort thru them and send the ones you don't use back.

[andrewmr]

Originally posted by Zeke
Race engine builders buy lots of springs and use matched sets. I don't know if this solves your problem as I don't completely understand the question. But I do know there is a difference in springs from the same box. You have to sort thru them and send the ones you don't use back.

Milt,


Here's an attempt to explain it further.

And I'll use specific numbers in this example. Let's assume that the install height is 40mm and the required seat pressure is 90 pounds. And I'm assuming all of the springs are usable.

In the first scenario it's said that you should test all of the springs to determine that they have 90 pounds of seat pressure at 40mm. This would require that you accept a variance in that seat pressure (as you said if you had batches of springs to work with you could pick the best set). And then do the head work to set all of the install heights at 40mm. The end result would be that you had all of the springs at the required height and the seat pressures would vary within the range accepted.

In the second scenario you would compress the spring to the required seat pressure and then set the install height at whatever height the spring reached that pressure. If you reached the required pressure at 40.20 then you would shim that valve/spring accordingly. The end result would be that all of the seat pressures would be 90 pounds but the install height would vary.


Michael

[jimz61]

Now everyone knows who is debating. LOL!

-Jim

[Bill Verburg]

The shop manuals only specify installed spring height. The asssumption is that all of the seat pressures will be within spec, i.e. not too high not too low