drcamshaft's performance valve spring
 

Hi,

I'm having my head rebuilt and John Dougherty reground my cams and recommended his performance valve springs with my stock retainers.

My mechanic has reservations about these springs as they are an interference/friction fit type and he's never used any of this type before. His concerns are that the rubbing between the dual springs will cause problems. He's also worried the heavier springs will overload the stock retainers.

We both spoke with John and he recognised my mechanic's concerns but assured us this was not a problem.

Has anyone else used these springs? Is the interference between the springs a problem?

Help...needed from someone who doesn' t know enough to know how to proceed.
Simon

I have them in my race engine. I have had no problems with them and rev my 3.0 engine to 7200 RPM. The shop I use is German Precision and they have installed several sets of these springs with no issues.

I didn't know they were an interference fit with each other but perhaps the coils are interlaced so that they never actually touch each other.

According to data I've found on the DC website and other sources the stock spring have about 227 lbs of pressure at max lift and the DC springs have 265 at max lift. This seems to be a safe limit with the stock retainers.

Having said all that why are you using these springs. If you are not going to rev above the normal redline then they are not needed. I use them because I have a race car with carbs and need to turn the engine to 7K to get the most out of it.

-Andy

I have s set as well but added the Ti retainers for good measure. The springs do not rub against each other. Fret not this is a great option for the price. They went into a short stroke 3.3 with decent cams from John as well. It revs beautifully all the way up to and beyond 7k. Mechanically things are good to about 8k but the rev limit is set a bit lower.

I would not hesitate to deal with John again as it was a very straightforward and painless experience. Helped me out quite a bit with cam selection and did not lead me astray.

Best of luck

The dual springs rub against each other causing noticable friction. When placed on the table some of them seem to tilt to one side- dont sit level.

Do you think I have a defective batch or are we overly concerned?

Simon

Andy,

Could you give me the details of German Precision? I"d like to give your mechanic a call to get his feedback.

Simon

Andy,

The reason we are using the performance springs is because we've reground the cams to DC20 profile and John recommended the upgrade in the spring rating. My mechanic concurs in this rationale.

Simon

I'm no expert so if Henry Schmidt or Steve Weiner weigh in, ignore my response and use theirs. My understanding is that an interference fit in some cases is by design.

1) Single springs by definition have a resonant frequency where it's possible to generate valve float. The higher the pressure, the higher frequency, so it is possible to design the resonant frequency so that it outside the engine's rev range -- generally by going to a stiffer spring.

2) Dual springs have two different spring rates, which when chosen and designed well will result in the resonant frequency of one of the springs being out of phase with the other. The result is a reduction in the tendancy to float valves. This is why most high RPM competition engines nowadays use dual springs. It's possible to reduce the overall pressure some compared to a single stiff spring, while also reducing the tendency to float valves.

3) If you are really clever, you can design the springs to have a slight interference fit which will cause the springs to dampen each other, as opposed to just being out of phase with each other. So this is a better solution still.

4) As I understand it, all coil springs have a tendency to fret some, at least at the ends where the coils are closed. So apparently there have been some good studies about increased spring life by using anti-friction coatings on springs. This also applies to interference fit springs.

That's what I've been able to learn from the fairly extensive reading that I've done on the subject.

"When placed on the table some of them seem to tilt to one side- dont sit level."

Unless spring technology has changed, springs should sit at 90º. Contact John and ask to confirm.

Sherwood

German Precision is in Sunnyvale California. Ted Robinson is the name of the owner/machinist. (408) 747-0728

-Andy

I too bought the performance springs from John and noticed there was a slight interference fit as well. I had my springs coated with an oil release treatment as well, the fit is still slightly snug. These are for my 84 Carrera project and I'm using them as added assurance against floating my valves and the upper end of the rpm range. It's been a while since I talked to John but he is pleasant and very helpful. Does anyone know how much more cam wear I may expect to see by using these?

Regards,
Alex

The springs are designed to be an interference fit. The thought is that both the harmonic and the resonant frequencies will be dampened by the contact of the two separate springs. This should improve functionality and longevity.

Thanks for your feedback Henry, I greatly appreciate it.

Regards,
Alex

Thanks for everyone's replies. I'll keep you updated as I sort things out!
Simon