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ChrisBennet's Avatar
 
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Valve clearance and stiffer valve springs

Hi guys,

Stiffer valve springs made sense to me for stock pistons for the margin of safety they offered with the 964 cams and for overruns from missed shifts.

The valve pocket on some 98mm 10.5:1 JE's I have is so deep that I have a minimum of something like 4mm of clearance between the valve and the piston with 964 cams.

The next motor in the pipeline is a 3.2 short stroke with JE pistons and 964 cams. I don't want to spec stiffer springs unnecessarily.

I'm wondering how much clearance you need before stiffer valve springs don't make sense? Any thoughts?

-Chris
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Old 01-23-2005, 10:46 AM
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Oops, could someone move this to the Engine Rebuild forum for me?
-Chris
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Old 01-23-2005, 11:38 AM
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Chris,

You are correct; you don’t want any more valve spring than absolutely necessary. Strong valve springs add wear to the entire valve train. The crank gear to jack shaft gear sees more load, as to the chains, chain sprockets, cam and rockers. Even the tensioner and probably the valve guides also. Besides it uses horsepower and causes heat just where you don’t want it.

With low compression piston (T, E, CIS…) there is not much valve cut-out. If the valve contacts the piston it is at one side of the perimeter of the valve. This almost always bends the valve.

With high compression pistons (S, C6, RSR…) the valve cut-out matches the full circumference of the valve. In an over-rev situation the piston can just help close the valve by contact. Not desirable but not necessarily bending the valve.

Of course the type of cam has a great deal to do with this. With some cams, the piston chases the valves closed and the valves chase the piston down. There are situations where the closest valve-to-piston clearance is not when the valve is at maximum lift or the piston at TDC.

FWIW, I have never had or built a street or race engine with anything other than stock valve springs set to stock settings.

Best,
Grady
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Old 01-23-2005, 12:04 PM
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With performance cams the ramp acceleration is faster and the valve opening longer (time) coupled with increased total lift. You need a spring that can handle the additional stress. The spring keeps the valve train under control.

MO here is that a slightly higher spring rate ensures that the valves will be where they are supposed to be at high rpms. Stock springs are designed for stock lift, duration and rpms.

Anytime a performance cam profile is installed you know that the driver will rev it more because the engine will now make power at a higher rpm.

I'm using the Supercup grind in my 3.4 build and choose the 'race' spring set-up from John the camgrinder. This may be slight overkill but this engine should make power to @7K rpm. I don't want any valve float or valve to piston contact in case I accidently over-rev it.

His 'race' springs spec @15-20 lbs more closed and @30lbs more at lift than the stock springs. I would consider this increase modest with virtually no trade-off in frictional drag.
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Old 01-23-2005, 12:24 PM
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Asphaltgambler,

I agree with everything you posted. How much more radical are your cams than RSR Sprint Cams?

Porsche didn’t use stiffer valve springs for the racing 911 engines.
I approached the problem (as Porsche did) by using lighter weight components; Carrera 6 non-adjustable forged rocker arms with lash caps on the valve stem, Factory Titanium retainers, and more. This allowed the valve train to run 8500 RPM and more without ever a malfunction still using stock springs and settings. Yes, if it went to 9000 accidentally, I could see a shadow of the valve on the piston. If there was contact it was slight and full circle, never a bent valve.

Standard street 911 valve train will handle 7300 just fine every shift, occasionally 8000 and possibly some more. When someone “buzzes” their engine by putting it in the wrong gear that may be 9,000 or a lot more. With low compression pistons, they are sure to bend valves or worse.

I think the best investment is in proper stock or improved shift linkage to prevent any problem in the first place. Perhaps lots of track time and DE events. In almost 40 years, I have never put a Porsche in the wrong gear.

The race valve train is expensive but worth every penny if you intend to run much above 7500.

Best,
Grady
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Old 01-23-2005, 02:10 PM
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From my research the SuperCup grind is the most aggressive cam that you can use with stock injection. I bought the cam and spring set from John (the camgrinder). He was very helpful and gave me great combo pricing. The profile is just slightly hotter than the Webcam 20/21 and significantly hotter than the 964 grind.

Not sure of how the profile compares to the RSR but I think the overlap and duration on the RSR is better suited to carbs and power concentrated in the upper rpms only.

I guess I maybe somewhat old school as my experience leads me to be a little on the safe side. These engines have valve to piston clearance that is tight even in stock form. This car is to be a week-end runner and playtoy. I like the extra insurance because I've missed shifts at one time or another in everything I've beat on!!
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Old 01-23-2005, 02:37 PM
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A couple of observations;
1) Chris I believe is talking about a 3.2 in which we've documented that the valves are larger and heavier then in the earlier 2.2-2.7 family of engines. These larger valves have more inertia.

2) It's not really the "wildness" of the cam that requires the stiffer springs, but rather the accelerations combined with the mass (aka: inertia).

3) Your cam supplier should be able to share with you the valve accelerations for the different profiles. While a 906 cam is more radical then a GE60, the GE60 may have faster valve accelerations. If you were to rev the engine with the GE60 cams to 8500 RPM with stock springs you quite likely will have valve issues that would not have existed if you had used the 906 cams or stiffer springs. The more radical CIS friendly cams tend to have higher accelerations in order to fit the maximum airflow without having overlap or closing the intake valve too late. This is generally not a big issue since most CIS engine's don't rev beyond 6500 RPM. If you were to rev a 3.2 engine with one of these cams to 7800 RPM, you very well may have valve contact issues.

As a rule, if you're trying to do something "off the beaten path" of norm 911 modifications, you'll need to get beyond "hot" or "radical" and review the specific charactoristics such as accelerations and valve train mass.

4) A couple of halfway steps between stiffer springs and not are...

- As Grady described, reduce the mass of the valve train, specifically replace the retainers with Ti.
- Shim the stock springs so that they are under more pressure. Keep in mind that you'll need to confirm that you don't have spring bind if you go down this path.
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Old 01-24-2005, 03:05 AM
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John's 'race' spring set comes with Titanium retainers. From other non-P car performance engine builds I've found that valve springs weaken over time, especially in air-cooled apps because the thermal cycles are much quicker (in time) and the temperature extremes are wider.

In light of what an average Porsche engine build costs just in parts, again I believe it is cheap insurance compared to shimming a 20yr old stock spring. Even John's 'street' set is a a bargain @$195. offering @10lbs higher seat pressure and @20lbs additional open over stock.


There's another post here about a guy over-reving his 3.2 and the $$$ and time consequences. True that at some extreme rpm's the valves are going to hit the pistons regardless of what spring set-up is choosen, but maybe in his situation if he had a stronger set there would be no 'accident'
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Old 01-24-2005, 05:06 AM
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My goal is to spec a cost effective valve train with 964 cams that will allow a misshift to happen without consequences. While racer's like Grady may have the skill to never miss a shift, I think he's the exception. I can't in good conscience just send someone off with a motor and tell them "Just don't ever miss a shift and you'll be fine."

I guess I was hoping to hear from someone running stock springs that as long as you had Xmm of clearance on the valve with 964 cams the piston won't hit it unless you go over Yrpm.

-Chris
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Old 01-24-2005, 06:17 AM
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Chris, even then that would vary from car to car. Again, being cost effective, good replacement springs, even stock spec is cheap insurance that all is well.

Most builders only check the installed (seat) spring pressure. You should also check it @max lift. That's where any weakness will show itself. I'll bet that if you wanted to only lighten the valve train the Titanium retainers are $$ about what a new stock or above stock spring set is.
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Old 01-24-2005, 06:36 AM
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Can you or someone else get the cam data for the 964 cam and the stock cam? It's not that difficult to figure out "equivelant" inertia levels, so a 964 cam at X RPM has the same inertia as a stock cam at Y RPM.
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Old 01-24-2005, 08:05 AM
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To compare an RSR sprint cam with the Super cup cam,
The Rsr sprint intake has 280 degrees at 1mm and the Super cup has 242 degrees. Both have the same valve lift. The super cup intake cam accelerates the valve at a much higher rate than the Sprint.
Porsche engines have a very stable valvetrain compared to most engines. The biggest advantage to using an aftermarket valve springs is keeping the rocker arm in contact with the camshaft lobe. At higher rpms the lobe can cause a condition called "lofting" where the rocker arm leaves the cam lobe at full lift. This condition can cause more wear than the added valve spring pressure of a non stock valve spring.
Valves coming in contact with the piston is usually caused by valve seat bounce. Inadequate valve spring closed pressure at high rpm can cause the valve to bounce off of the seat when closing.

If you keep the 964 camshaft under 6800 rpms a new set of factory springs will be fine. Ti retainers would be a good investment. Hoping the valves wont hit the piston because you have added more room is not solving the problem. The valvetrain will be separating, the rocker arm not contacting the cam lobe or the rocker arm coming out of contact with the valve stem tip etc. When these parts come back into contact the force can be extremely high, causing broken rocker arms etc.
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Old 01-24-2005, 08:16 AM
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If I was a rich guy with some time on my hands I'd digitize the rocker and cams and model the valve train using Common Model...
-Chris
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Old 01-24-2005, 08:17 AM
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Quote:
Originally posted by camgrinder
To compare an RSR sprint cam with the Super cup cam,
The Rsr sprint intake has 280 degrees at 1mm and the Super cup has 242 degrees. Both have the same valve lift. The super cup intake cam accelerates the valve at a much higher rate than the Sprint.
Porsche engines have a very stable valvetrain compared to most engines. The biggest advantage to using an aftermarket valve springs is keeping the rocker arm in contact with the camshaft lobe. At higher rpms the lobe can cause a condition called "lofting" where the rocker arm leaves the cam lobe at full lift. This condition can cause more wear than the added valve spring pressure of a non stock valve spring.
Valves coming in contact with the piston is usually caused by valve seat bounce. Inadequate valve spring closed pressure at high rpm can cause the valve to bounce off of the seat when closing.

If you keep the 964 camshaft under 6800 rpms a new set of factory springs will be fine. Ti retainers would be a good investment. Hoping the valves wont hit the piston because you have added more room is not solving the problem. The valvetrain will be separating, the rocker arm not contacting the cam lobe or the rocker arm coming out of contact with the valve stem tip etc. When these parts come back into contact the force can be extremely high, causing broken rocker arms etc.
Thanks John. That is just the sort of thing I wanted to know. I'll be calling.
-Chris
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Old 01-24-2005, 10:54 AM
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