Valve spring mistery!!!

[sam_james30]

Hi

I've overhauling the cylinder heads of my 964 C2 with 150k on the clock. Basically I bought a force gauge and measured the force my old one were exerting when compressed to different lengths.

When compressed to a length of 30mm I was getting between 80-100lbs of force from the spring. When I look up the value quoted in Dempsey's engine rebuild book and Anderson's performance hand book they quote 80kp or 176.4lbs of force...

So I order a whole new set (OEM) from Pelican parts and had them shipped over to the UK as they were a lot cheaper than any I can get over here.

They arrived and I deiced to test them too, and guess what they are giving out the same 80-100lbs of force!!!

So is 80-100lbs of force correct? Have I misunderstood the specifications given in the book? Or are these OEM springs underrated? Should I have got genuine Porsche springs.

Also if 80-100lbs of force is correct which should I fit? here Dempsey and Anderson's books seem to disagree. Anderson says the spring are top quality and will last the life of the engine, Dempsey says replace as a matter of course.

Thanks

Sam

p.s. before you ask I'm check the force meter on a number of weighing scales and it seems accurate, certainly not having a error of nearly 2x....

[manbridge 74]

Misprint in book?

I like to double check all engine specs since I don't rebuild 911 engines 24/7.

[chris_seven]

How did you test them?

You do need to fit them into the spring base and use the valve spring retainer

Apply the load with these parts in place.

As the springs are progressive in nature any reduction in displacement will have a significant influence in the force applied and may result in the error you have observed.




I would expect that providing you measure displacement carefully forces should be within 5%.

[sam_james30]

Ok it might not have been a mystery after all! I just assumed the specification was on the spring on it's own.

With the addition of the seats the spring is almost bound when compressed to the 30mm or so specified and the progressive nature kicks in sending the reading sky rocketing.

Now the new spring seem to read with in 5% of the correct reading, and the old springs are way out.

I'm not sure where you got the picture from, but non of the books I read mentioned this fact. Although looking at the photos closer they do have the seats installed.

Thanks very much!

A slightly dumb feeling...

Sam

[cgarr]

check them with the lower and top retainer, you cant just measure the outside dimension: note you will have to calculate or add back in the thickness of both retainers to compress to the proper installed height, as I remember it was something like 205 thousands I had to add back in?

[chris_seven]

Originally Posted by sam_james30

I'm not sure where you got the picture from, but non of the books I read mentioned this fact. Although looking at the photos closer they do have the seats installed.
Sam

Drawing was from the 911 Workshop Manual - Volume 1

[sam_james30]

Hi

I'm assuming that when the book says Compressed length "30.5-31.0mm" it is meaning the length from valve seat to valve retainer as shown in the diagram chris_seven posted above?

Then I get 80kp or 176Lbs of force.

Thanks

Sam