jasontmcv

I've been looking high and low to find the installed valve height spec for 996 gt2 heads. Not in the service manual or anywhere I can find. Thanks in advance.

Slalom996T

It might be better to simply check what the factual seat spring pressure is, i.e. taking in consideration the shape of the springs.

Sometimes it also seems to be forgotten that increased boost pressure need a corresponding seat spring pressure increase?

Slalom996T

Macster

Not sure I agree with the statement an increase in boost pressure requires an increase in spring seat pressure, unless of course the increase in boost pressure is extreme. I seem to recall the seat pressure is around 70psi so it would take some boost to lift a closed valve off its seat. With my 03 Turbo -- stock -- I've seen 0.9bar boost (at 8K+ feet above sea level) and 0.9bar equals 13psi.

Generally a higher spring pressure is required if the red line is increased or if the cam lobe is more aggressive (higher lift) or bigger valves are used. (Generally a turbo/supercharged engine doesn't require an extremely high lift or huge valves as the cylinder filling is accomplished by the presence of boost. In NA engines the saying is while Man empties the cylinder only God fills the cylinder. In boosted engines Man both fills the cylinder and empties the cylinder.)

I'd be a little leery going overboard with higher spring pressures with these engines. The valve lifter bucket face/cam lobe interface is splash lubed and a higher spring pressure may result in an increase in lubrication that may not be met with the existing splash lubrication.

Also, I have to note the intake valves have a dual lift feature. I have not done any measurements but I believe the surface area is reduced so the actual pressure of the lobe against the bucket could be higher due to the smaller contact patch the smaller surface area offers. IIRC these engines have a rather low spring seat pressure and this is I think partially in recognition of the reduced surface area. It also can be partially due to the lower weight of the valve hardware and the use of progressive (and two) valve springs.

For the OP's question about installed spring height I have no numbers to supply. Porsche has been pretty (make that very) tightlipped regarding any engine details like this.

All I can offer is the installed spring height wants to be such that when the valve is fully open the spring doesn't experience coil bind. But how much margin to allow for I don't know. There may also be a fully open spring pressure callout that of course is selected to ensure there is no valve float at maximum RPMs.

One might check to see how much margin there is with a stock engine and that the open valve spring pressure is and duplicate those two readings.

Slalom996T

Macster

Out of theory and in general I propose:

Since the inlet valve area in the inlet channel is exposed to boost there is a inlet valve lifting situation at hand when the valve is closed. And this is obviously included in the oem settings like peak rpm, valvetrain weight, oem cam lobe design, etc.

With increased boost there is a need for increased seat spring settings vs oem settings.

Larger inlet valve/s also increase the need for increased seat spring settings.

Overall lighter valvetrain reduces the seat spring pressure need on the identical lobe OR allowing a higher peak rpm with oem seat spring setting OR increased boost with oem peak rpm with oem seat spring setting.

Since the unseated inlet valve "always" restricts/interupts the intake flow (i.e. the resulting VE and unregarding boost/non-boost engines), should the intake valve simply be as far away as possible from the valve seat, i.e. high lift.

Measuring the spring force is simply the only way to know the level of force since using the installed height procedure is more of a "should be" situation, unregarding new or used springs.

On a "normal" 4v DOHC valvetrain with direct acting buckets and boost the valve seat spring pressure is often around 30kg +/- 5kg i.e. 66lbs +/- 11 lbs.

Slalom996T

Macster

When the turbo is making boost the intake valve when closed is under pressure from the combustion chamber side during the compression, power, and even part of the exhaust stroke.

Slalom996T

Macster

What you say is obviously true.

But since the intake valve lifting action remains within the 4 stroke process it technically needs to be taken into account when leaving oem city more than "marginally".

Tell tale signs of the dilemma might be a drop in the torque curve and/or exhaust gases and/or lambda values or even intriguing initial sounds followed by evil technical gremlins.

Slalom996T

m42racer

I always thought that spring pressure was all about controlling the valve train at RPM. I have also thought that when more seat pressure was used in Turbo engines it along with valve face widths was to remove the extra heat from the valve when using higher boost. Never was it about boost pressure.

Slalom996T

m42racer

Correct, on a naturally aspirated engine is the purpose of the valve spring (pressure) to keep the valve train components in contact with each other up to the cam design red line.

But, as per above, in a oem boost engine there is a boost lifting situation acting behind the intake valve that will lower some of the valve seat spring pressure. And this is typically taken into account in the oem design of the cam shaft.

The purpose of the valve seat width and design is at least three folded.

* Enabling a seal vs the combustion pressure

* Heat transfer from the valve face (and stem) into the cylinder head and this in particular regarding the exhaust valve (some times with the help of sodium filled stems)

* Enabling best flow transition and value, especially in the low lift phase.

Slalom996T

m42racer

Good discussion but we do need to answer the first question. We have kinda of got off subject.

I looked on some of my old build sheets I was given. I see that in one case the seat pressure was 68 lbs with an installed height of 1.369" Not sure how much was taken off the seats. In another engine build it shows the seat pressure down around 58 lbs. But that was with a custom cam.

Hope this helps the first request. PM me for more info. I will see if I can email you a copy of my build sheet.

As for the comments about pressure and RPM.

Question, how does an engine know its an NA or Turbo, with regards to spring pressure. Valve train mass and RPM could be the same.

I have an Turbo engine that runs above 8000 and has 58 lbs of seat pressure on the Intake according to the build sheet I was given. I run normally 1,4 boost and sometimes when I feel like it a little above 1.6. No issues with anything.

I always thought that narrow seats were better for air flow? That is what I have always been told.

I guess its a different thinking. Not all builders follow the same convention. Probably why some have more success than others.

Slalom996T

m42racer:

Since I'm not aware of the oem valve spring seat pressure, nor do I have any new ones to verify the proposed oem spring seat pressure @ installed height I can't elaborate on the spring issue, sorry.

[Simplified: Since the oem intake valves are 37mm the valve lifting @ 1 Bar may be:
Pi x 18.5 x 18.5 (neglecting the valve stem dia being neutral) = 10.7kg/23lbs on the "full" valve diameter.]

Since the boost pressure only "sees" the valve inside of the valve seat width and assuming that

* The valve seat is located on the outer edge on the intake valve

* 1.5mm valve seat (never mind the angle...) we might have an effective diameter of roughly 37-2x1.5 =34mm

=> Pi x 17 x 17 = 9 kg/20lbs

I.e. should 1Bar above oem boost need 9kg/20lbs valve seat spring pressure, theoretically, neglecting the valve stem dia.

I'm not aware how much margin Porsche engineering might have installed in the oem valve train settings.

The "detected difference" NA vs boost engine with the same valvetrain is simply as per the reply in the #4 posting:

" But, as per above, in a oem boost engine there is a boost lifting situation acting behind the intake valve that will lower some of the valve seat spring pressure. And this is typically taken into account in the oem design of the cam shaft."

When you run @ 1.6Bar you're roughly 1Bar above oem boost settings and this means, theoretically, a need for ca 7-8kg added seat pressure ASSUMING the oem valve train settings vs added boost have no margins.

You maybe have lighter intake valves and/or higher valve seat spring pressure vs oem and/or might the possible margin in the oem settings take care of it?

Often does a 1mm valveseat with a inner radii correction favour the intake flow.

Slalom996T

Kevin

Two quick facts.. The 996TT stock valve springs are stiffer than 996GT3 Stock springs.. The installed seat pressure in the 996TT is higher than the STOCK 996GT3... From memory it 28 to 30% greater..

We also need to remember that the 996TT and 997TT has the Variocam plus low lift and high lift tappets> impacts valve spring specs.

m42racer

Jason,

PM sent with contact name and number to call for correct info.

m42racer

Jason,
You have no contact info. Call Performance Developments 949 646 7461. They will know for sure.

Slalom996T

Kevin

Is the GT3 oem intake bucket/tappet and/or intake valve, etc, lighter than the Turbo counterpart? Is the width of the "cam walk line" on the tappet/bucket equal?

Since the Gt3 oem valve train has no boost lifting action on the intake valve (although the allowed peak rpm is a little higher; roughly 1k rpm on the early versions) I guess this is one of the reasons?

(The Variocam lift on the low lift phase is quite low (3.6mm vs 11mm) as well as operates in the lower rpm's, hence I guess the valve spring pressure demand is lowish.)

Slalom996T

m42racer

Good discussion as I'm always interested in the tech stuff.

Don't understand anything you are saying.

If I try to understand, you are saying that there is some lifting going on, on the backside of the intake valves when on boost? If the seat force is 50 lbs/sq in to lift the valve you would have to see over that as a boost value?? No????
Not to mention as the valve opens the spring rate increases. Also, the pressure at the back side of the valve will be a lot less that you think as some of the valves in the other cylinders will be open.