question on valves

[bell]

i know of the importance of properly adjusting valves, i have not heard of what happens if it does not get done however and am curious.
i know that on "normal" cars if you don't adjust them you will eventially destroy the valve seats and possibly break the valve, will something more destructive happen with the 911? i'm sure the tolerances on these engines are much closer than on your average car and although i always hear how imprortant this job is but i never heard what the consequences can be if it is not done.
thanks (oh....and my valves were adjusted when i bought my car, i'm not being neglectfull )

bell
85'911 targa

[RoninLB]

bell...Great topic. Not a pro. but, IMHO, an out of spec. valve adj. changes the the timing of when the cam makes contact with the rocker. A change of .001 can make about 3 deg. difference in cam timing [not positive on actual number]. Although .004 in. can be involved in valve train compressing when hot and an exhaust valve that does not seat usually burns. Duration may become an issue with improper adjustment. Duration is when the valve is off its seat. this is the time when gas is passed in and out of the cylinder. I believe that maintaining the same technique thru out the adjustment is important so that all valves are equal. An adjustment on the loose side of spec. shortens the duration which may add low end torque because the cyl. builds more cyl. pressure on the longer compression stroke. A correct adj. ensures that the camshaft is phased to the crankshaft correctly so valve opening and closing is correct to piston position. I think I got most of this correct.

[bell]

good description ronin, i think you've pointed me in a good direction
i've always understood the mechanicals of engines very well but i think i may learn something new here. i'm now doing some research on the changes in cam timing to the valves, i've never looked into that, we'll see what i come up with on this and i'll share what i learn

bell
85'911 targa

[Superman]

When valves are properly adjusted, there is very little play when the engine is warm. That .004" becomes near nothing at operating temp. If a valve gets too tight, like if the seat wears a little, the valve may never close. If the valve head does not contact the seat, it will have not place to shed its heat. So, exhaust valve clearances that are too tight are real trouble.

Disintegrating valves can destroy the engine. when valve heads pop off, they make a real mess.

[ChrisBennet]

Originally posted by Superman
When valves are properly adjusted, there is very little play when the engine is warm. That .004" becomes near nothing at operating temp.

Superman,
Have you actually measured the clearance on a hot engine? I could have sworn someone posted that they had measured the clearance and [to my surprise] found that it got bigger instead of decreasing. I can't seem to find it on Rennlist but I know it was posted there. Here it is:

Tue, 9 Feb 1999 15:13:7

Author: "Scott Crawford"

Subject: Valve adjustment particulars Part 2

Body: Hello gang,

I sent out a plea for help on the finer points of valve adjustment about
two weeks ago. I got a terrific response, I guess confusion, frustration
and misery love company. It's really not all that bad, I guess.

Anyway, I was stuck on finding out a couple of things. For instance, how
tight is too tight? At what point is a gap of less than .004 dangerous to
the engine? How can you tell it's too tight BEFORE you munch your engine?
Toward understanding the ramifications of this setting, I also was trying
to find out what happens to the gap as the engine gets hot? If the gap
gets bigger with temp, it seems to me the .004 is gravy, margin for error.
If the gap gets smaller with temp, then it becomes insanely critical. So I
set about finding out conclusively.

I measured #4 exhaust overnight cold. The .004" guage went in tight, the
.003" went easy, I could not get the .005" gauge in. The .003 and .005 are
"conventional" feeler gauges, about 2" long, 3/8" wide, difficult to get in
there. Of course, I made sure that piston was TDC.

I put it back together, drove around for about an hour to get the temp up
over the first hash mark, pulled the passenger side exhaust valve cover off
to measure the same valve at TDC.

The results: .005" went easy, .006" went easy, didn't have .007", .008"
went tight. All these sizes were a "conventional" gauge, therefore a bit
difficult to work with, but they went in. Thus, the valve gap gets bigger
with heat, and the cold gap is something of a safety factor.

All that said, there is no substitue for a correct setting. It's
comforting to know that there is a margin for error there. Hope this is
helpful for someone.

The best news is, the burns aren't permanently disfiguring

[dickster]

just another thought

between adjustments the clearances can become tighter, so clearances have to allow for service intervals?

i just had the clearances done on my daytona m/bike and to my surprise some had gone tight. this is due to the seat/valve head wear.

richard

[jluetjen]

A. Graham Bell has a good descussion of this in his book "Four-Stroke Performance Tuning" (Excellent Book!).

RoninLB seems to have summed it up pretty well. I just want to add two thoughts.

1) Valves will expand as they heat up which often takes up the clearance. In addition to the earlier comments about burnt exhaust valves, keep in mind that an engine where the valves don't close completely will have sh***y compression when warm.

2) On some cars, a "cheaters" trick when racing in a limited prep class is to tighten up the valve clearences and gain duration. This does increase the likelyhood of toasting the motor though. Given Porsche's reputation for tight specs, I doubt that this trick will get you anything on a 911.

[Roland Kunz]

Hello

To less play will let the engine run crocky and burn the valves or even can lead to piston/valve contacts.

To much play will make noise add waer due hammering from the parts and the performance will not be like it could.

Grüsse