Temperature effect on valve adjustments? - Page 4

Walt Fricke · 01-11-2010, 10:17 PM

John D: put another way, with the 993 you maybe ought not to convert to solid rockers without changing out a cam designed for hydraulic rockers?

Walt

jcge · 01-12-2010, 01:25 PM

Alternatively for the 993 conversion, the "Cat's whiskers" (hot hot hot valve) takes up the clearance when the engine's actually running, and contact actually remains on the 993's clearance ramp.....just a thought.

jcge · 01:39 PM

I assume this is a factory label, but may be applied at port of entry ???

From an 87 Carrera - bottom LH corner of the image states both clearance and tolerance

0.004" +/- 0.002" for Intake and Exhaust

In this instance, the reference to engine temp of at least 90°C may or may not apply to the lash setting - I'll leave that to your individual interpretation of the label (and I'll assume the Tech Spec book would carry some precedence - cold)

jcge · 01-12-2010, 02:35 PM

John D - camgrinder

re 906 profile

Perhaps its range of applications from the factory is a testimony to the those long clearance ramps and the range of impact to duration via simple lash adjustment.
906
911R
908/917 (I assume it underwent some geometric rearrangement for bucket followers)
and others no doubt

Any thoughts or further info on this profile - maybe worthy of a thread in it's own right.

John

Walt Fricke · 01-12-2010, 08:26 PM

Wow - factory thought you could get by with 0.002 lash? I'm not going to run out and readjust, though. Should provide some comfort to those who fret that they aren't getting super accurate lash settings, though.

Walt

psalt · 01-13-2010, 02:44 AM

Walt,

That looks like an emissions compliance sticker. Kind of like the sticker on a bottle of beer that says "Don't get pregnant while drinking and driving or operating heavy machinery". Everything on it is usually there for a reason, to meet the EPA emissions test for new cars. For example, the timing spec 3 BDTC +/- 3 means that the car does not meet the test (probably HC) past 6 BTDC. Unfortunately, since many (most) state tests are different than the EPA's new car test, you fail if you are not within the tolerances, even if you pass the state's tailpipe test. A common failure is idle speed. It must be that below 0.002" the overlap spoils one of the emission criteria, but I have never heard of a smog nazi opening up the engine to check the valve clearances in order to meet the failure quota set by politicians. It states that if anything is out of spec, refer to the workshop manual. That does not have a valve clearance range and states that if the adjustment tool (0.004") does not slide in, the clearance is too small. It would be a mistake to interpret the emission sticker as the valve adjustment procedure.

The most amusing story about emission stickers involves Cadillac. The emission sticker on one of their powerful V-8 cars also said "A/C Off". The smog nazi's followed the instructions and everything tested OK. However, when you switched the automatic climate control to On, the ECU used a different fuel and spark table that fattened up the mixture and improved the idle and drivability. 99.9% of Cadillac owner bought the car with the automatic climate control On and never shut if Off. The EPA was tipped off by an ex employee, sued Cadillac, and I think they settled for a big fine without admitting it was done "on purpose".

Eagledriver · 01-15-2010, 04:49 PM

So the engine tune-up specs on the emission sticker published by Porsche aren't really tune-up specs? Sometimes it's ok to be wrong.

-Andy

psalt · 01-16-2010, 04:48 AM

Yes, setting the valves to 0.002" is wrong. The purpose of the valve adjustment procedure is to allow for the wear between services, and 0.002" is too tight.

camgrinder · 01-16-2010, 12:05 PM

Quote:

Originally Posted by jcge

John D - camgrinder

re 906 profile

Perhaps its range of applications from the factory is a testimony to the those long clearance ramps and the range of impact to duration via simple lash adjustment.
906
911R
908/917 (I assume it underwent some geometric rearrangement for bucket followers)
and others no doubt

Any thoughts or further info on this profile - maybe worthy of a thread in it's own right.

John

you could go tighter than .004" cold, but not looser. If you have more than .012" hot on most profiles, the valve seating velocity gets too high and you can bounce the valve off the seat. The exception is that 315-300 Walt has in his engine. Those clearance ramps are higher, and need more lash. .006"-.008" cold.

I think its more to do with available valve springs at the time they designed the 906 profile. They wanted reliability at high rpm vs. HP from the looks of the profile. Very conservative profile.

jcge · 01-17-2010, 01:23 PM

Thanks John

fredmeister · fredmeister's Garage

So is there a consensus?
Is lash tighter or looser in a hot engine?

psalt · 01-20-2010, 11:44 AM

I believe everyone that has actually measured clearances on a hot 911 engine agree they are wider. Aluminum block and head, significantly larger (0.008-0.012'), iron block, aluminum head, larger (0.006-0.008"), iron block, iron head, closer to zero change (+/-0.002"). The major expansion is in your head, I am still looking for the cat.

rcaradimos · 12:45 PM

I copied this off the web...
Not sure if its true, whats the opinion?

************************************************** **************
The Problem with Using a Feeler Gage
In reality, the rocker smashes against the valve stem like a hammer on an anvil. It does so 25 times per second at 3,000 rpm. Although both are hardened steel, eventually the elephant foot and the valve stem wear, necessitating adjustment by means of the adjusting screw. Unfortunately, the rocker arm, which to account for tolerance build up is larger than the valve stem, tends to suffer concave wear to the center its active surface.
To adjust a valve clearance, a feeler Gage is usually placed between the valve stem and the rocker.
When the rocker tappet has worn, the measured gap, which is the thickness of the feeler Gage, is less than the actual gap, the feeler Gage failing to measure the concave wear point. This results in the valve opening less than it should, (adversely affecting performance), slightly off valve timing and above all, very noisy tappets.

************************************************** **************
Could adjustments made by using thread pitch and degree work better on worn (seated-in) parts?

I did a valve adjustment this summer, I did the intake first (cold) ran the engine up to temp on long spirited drive came home dumped the oil to get ready to do the exhaust next morning. I set one valve hot to see if temperature made a difference in gap, the next morning the valve I set hot had no rocker movement at all from the slight play (free float) to a bind. Say from .004 to .000, So setting @ .004 cold most likely goes to .008 hot.

rcaradimos · 01-23-2010, 12:54 PM

un-measured gap! My best rendition...

rcaradimos · 01-23-2010, 01:36 PM

This is off a motorcycle engine, excessive ware on both valve stem tip and swivel foot. Clearly a feeler gage would not measure the valve lash well. The poster of the pics wrote that a .004 was more like .008. Adjustments that are to loose hammer valve stem ends, swivel foot and valve seat.

Walt Fricke · 01-23-2010, 04:38 PM

Bob

Nice pictures. Wish I coult get my modest camera to focus through a magnifying glass! If I could, I'd supply shots of some random valve stem tips. Not a scientific sample, just what is sitting on my desk. None look convex. From memory, none of my elephant feet have been concave, either.

Others, and some certainly have a lot more experience than I do, may have seen cupping of the elephant foot, or of the tip of the valve stem. Your motorcycle engine being one example. But I never have with my 911 motors. I've had an elephant foot ball tip break off, and had a couple bend at the waist, all due to overrevs. Otherwise, anything that will screw out OK I have not hesitated to reuse, and have not gotten myself in trouble with this cavalier attitude. The valve tips I looked at look fine, despite fairly high mileage on some. Only ones which look beat up came from engines which blew up, as evidenced by dents on the valve head faces. And those don't even look bad - nothing you couldn't true with a valve grinder.

Maybe on some engines this is an issue. Not on my 911s not reved over 8,000 rpm, anyway.

There are lots of things one can worry about in life. Porsche 911 valve lash should not be one of them. Feeler gauge works just fine. Factory says to use it, which should say something.

But for those who are inclined to fret, there is the back of the cam method. And one can just set up a dial indicator and do it that way. Both would take care of any concavity issues.

Walt Fricke