valve guide question
 

OK, if one has a Carrera, and I expect it is much the same with an SC, valve guide issues will likely rear their ugly head around 75-125K miles. From what I have read, this seems to be the average.

This appears to be one of the few weak links in the air cooled engine. What is the root cause for this? Is it a design flaw? Part and/or material defects? After the top end gets redone, is this a problem that will keep cropping up every 100 thousand miles or so, or after the fix, it is fixed and no longer is an issue? Kind of like with the dual mass flywheel problem on the 964s - once replaced, problem is no more.

-kevin

Horizontally-opposed, air-cooled piston engines have a hard time cooling the cylinder heads because the valves and cams are in the way of cooling fins and air.

Exhaust valves are on the bottom of the Porsche engine, further away from cool air, suffering from reduced flow and pre-warming of the air from the top of the engine.

Exhaust valves on a horizontally-opposed Porsche engine have a harder time with oil leaks also because the oil is pulled by gravity towards them.

Porsche experimented with metallurgy for the valve guides. As they ran engines hotter for emissions reasons (or more power creating more heat), the metal started to show more wear. They went to harder manganese bronze (from phosphor-bronze) and this may be a reason why SCs have valve guide issues but Carreras are better.

Furthermore, the valve stems seals can either be Viton or Teflon. Some say Teflon seals too well, not allowing enough oil and librication to the valve stem (but the seals don't leak ;-)) The seal can complicate the valve guide metal performance due to lubrication dynamics.

Give the exhaust valve guides a break, they have a tough life. A little oil now and then is all they ask...:-)

Flieger, great explanation! Thanks!

I'm confused by this. I've been reading and hearing for years that it's the Carreras that are prone to premature valve guide wear, not SCs.

Yes - that's what I too have read over the years. Carreras more prone to valve guides, SC's more prone to broken studs.

My understanding is that if a top end has been done properly these engines can easily last 100k miles - sometimes multiple of 100's if they are treated properly with consistent oil changes, etc.

Well, may be mistaken.

I know Porsche experimented with the alloy, though, leading to differences over time in valve guide wear.

I understand that the phosphor-Bronze performs best according to modern engine tech experts

Yes, this is backwards, the Carrera has a worse valve guide wear record than the SC. No one seems to have a good explanation for this as the material spec did not go down.

The original questions implies that 75-125 K is a "weak link" for air cooled engines. If you look at other OHC, two valve, hemi head engines designed in the same era, BMW M10, M20, Jaguar XK, they all have valve guide wear at 75K. I've been working on these engines for 30 years and this is not unusual at all. It has much more to do with the mass of the valves, and the speed of the engines. Four valve engines with CAFE mileage gearing will always have less wear. The main issue with valve guides is to avoid pattern guides made in someone's basement, which can only be done with "trust".

Wouldn't that be true for the intake valves (which would be "helped" further by manifold vacuum during part throttle operation), not the exhaust valves?

The exhaust valves point upward, so they should not get oil running up the valve stem unless someone has done some very indiscriminate driving and gotten the shiny side down ;).

Seems like the exhaust side would have a hard time getting lubed at all, which I think is what you intended to say.

I presume all valve guides, or at least all intake or all exhaust ones, in the same engine would wear at the same rate, hence all six spark plugs would look alike. So if only some plugs are oily and fouled, it's likely a problem of piston/ring/cylinder, but then the latter is not a common problem for our well-cared engine, especially if with low milage.

Now, are there other common causes for combined fouled plugs and oil consumption?

The problem is that not all the cylinder heads run at the same temperature like in a water cooled engine.

Don't know if relevant but one of the reasons I use Mobil 1 15w-50 is that it has a higher temperature level before it cokes than dino oils. It's also higher than other grades of M1

I have read a few times that the upper side and top of the piston gets very hot due to being air cooled and there is a noticeable difference between synthetic oil & dino. The upper ring land gets beat up. The synthetic remains intact where the dino doesn't.

don't know if synthetic would help in valve guide to exhaust valve stem interface.

If I had oil burning, good leakdown, over 50,000 miles, and tight valve guide wear I'd probably suspect the intake valve guide seals, not the exhaust guide seals.

when I rebuilt my engine in 1999 I dealt with one of the most respected engine builders in the US. I brought up the issue of valve guides and wanted the best. The most expensive were the silicone bronze[I think].

I was told that the SC valve guide was all I needed.

So far so good. 80k miles on rebuild and almost 3k miles to a quart traveling cross country many times.

Thanks, Craig, that's what I've suspected as cylinders 3 and 6 seem to have fouled plugs more often than do others but I don't have any proof for this speculation except that No 1 and 4 plugs look perfect in my engine.

So, do we have to replace the valve guides of those cylinders with fair-looking plugs?
And have any efforts ever been made to correct this uneven cooling problem, such as a horizontal fan like that of the 917?

The 3.0s as far back as the original 930/02 in the C3 and the 3.2 all have the same valve guide issues, they all use pretty much the same guides w/ a high Cu content Thermohedul FS for '76 -81 and FS15 for '82 -89, they are no match for US made Phosphor-Bronze guides.

any of them can have broken head studs, I've seen them broken right up through 964, the 3.0s are older and there was a lot more experimentation going on back then.

other than ignition, induction, and tendioners the 3.0 and 3.2LS aren't that different. The 3.2LS just has the turbo 74.4mm stroke crank instead of the SC 70.4mm crank, the purposely, made the 3.2 smaller by using the same 95mm bore p/c instead of the turbo 97mm

Question for Bill V.
 

Bill,
My valve guides on my 1987 went bad at 74K. As far as I know the previous owner took good care of the car. I recently had a top end job. Ollies's did the machine work on the motor. Can I assume they used the superior US made valve guides? Can I expect the valve guides to last longer than 75K, say 100K? maybe?
Thanks Mark

In my opinion, the material in the guides did not change between the SC's and 3.2, however, the fuel did.

This happened as the nation was transitioning from leaded fuel to unleaded. The guide material was just not up to spec for unleaded US gasoline.

Replacement guides in the Phos-Bronz are a higher quality and don't need the "lead" for lubrication. The OEM guides from Porsche were behind the times for fuel quality and suffered premature wear.

I built an engine, top end, in 1982. A 2.4 with a burned valve on #4. Engine had about 20, 000 miles on it. Replaced the guides on #4 with the Phos-Broz. The other five heads were fine.

In 1999 I did a rebuild on the same engine at 140,000 miles. When I disassembled the top end, #4 valve guides were perfect. The OEM guides in the other five heads were completely shot with the valves rattling around in the guides.

Since then I use MMO mixed with the fuel in the older cars. With the use of ethanol, our Porsche valve guides need all the help they can get.

+1 on a dash of MMO for the top end

What's MMO pardon my ignorance?

Scott

while on gas topic


I've been using RedLine gas treatment for many years.

anyone have info on this besides what RedLine says ?

Marvel Mystery Oil