Could worn valve guides significantly affect performance?
 

2 years ago I had a compression/leakdown test which showed good #s, no imminent problems leading to a rebuild. However, this year I've noticed my car smoking significantly on deceleration at the track, indicating worn valve guides. I've also been chasing down a horsepower problem for a few years now, whereby when I get the car hot at the track I lose probably 20-40% performance (feels like I'm running on 4 cylinders). If I let the car cool down it's back to normal.

Could worn valve guides, and whatever else might be causing the smoke on deceleration, be affecting my horsepower when hot? I've already rebuilt the carbs (Zeniths) and replaced all ignition components (MSD coil/CD, new plug wires, rotor, cap, points, plugs, etc and all timed by a pro).

If the engine needs rebuilding, I'll most likely just do a transplant for a 3.0 (I don't want to replace the tranny, hence no 3.2 or 3.6, at least not yet).

Thoughts?

Here's a pic of what I'm talking about:
http://i148.photobucket.com/albums/s...t/DSC_7161.jpg

Thanks!

well, i've had people comment that after i did a valve grind that it felt like they got an extra 30hp. loose guides cause the valve to wobble around and beat up the valve seat. a round valve trying to seal on an oval seat leaves some power on the table.

Would that get worse with more driving at higher rpms? I'm pretty sure it's the rpms that are causing the power drain, rather than heat, because I don't get the drain during regular highway driving, even on really hot days where my oil temps reach 120C - it's only at the track. (fuel delivery problems have also been suggested, which I haven't yet investigated)

Christian,

I suspect the difference you see between highway and track is thermal stress. The engine simply runs hotter. The result is thinner oil that gets past the worn valve guides. The higher rpm permits the valves to ‘flop around’ more, never finding a seat (as John notes a round valve in an ovel seat or vice versa). It is the complete and proper seating of the valve on the valve seat that allows heat transfer from the head of the valve to the cooler valve seat and allows the heat to be dissipated to the air via the head fins. Poorly seating valves allow combustion gasses to pass between the valve and seat, greatly exacerbating the heat in the valve, seat and head. This progressively increases the overheating of the engine.

Another important heat path is via the valve stem to the valve guide. Worn guides restrict this heat flow path and allow the valve to run hotter.


The problem is this can be a very dangerous situation for the engine. The head of an overheated valve can break off causing huge damage.

A current compression test and cylinder leak test will give you more information.

Best,
Grady

+1
"2 years ago I had a compression/leakdown test"

Grady, thanks for explaining that so in-depth. If I'm understanding what you're saying, these areas that are getting too hot, even dangerously so, won't be reflected in the temperature gauge on the dashboard, because that only measures oil temperature, right? That would explain why I've never considered temperature a problem, because I often see higher temps on the highway, cruising at 4000 rpm for an hour (in 4th gear, not 5th! it's a 4-speed 915) than I do on the track.

So I'm guessing a top-end rebuild is in the cards for this winter? Or a transplant...

Will a compression/leakdown show problem, though, if the engine needs to be driven hard in order to cause the problems? If I just get the car warm driving it to the shop, the engine will behave normally - i.e. no smoke on decel (or at least not significant) and no power drain. It needs to be run at high rpms for the problems to show themselves. Will putting the car on a lift and running it at 5000 rpm for a few minutes work? Or does the engine need the load of actually moving the car?

You really have to have current compression/leakdown tests to get a sense of what direction to head in.

I noticed no increase in HP after doing the top-end on my '88. I did notice a HUGE decrease in oil consumption. Especially on track days.

Rob, did you have any kind of HP drain before the rebuild?

Not sure what you mean about not using the same trans? My 3.6 was mated to my stock '72 trans. Just need a different flywheel is all.

Really? I've read (several times) that the torque of the 3.6 is far too much for a 1972 915 and would destroy the gears if not driven extremely gingerly. Obviously I'm not interested in tiptoeing around in this car :)

Christian,

Here is a related thread “valve guide question”
http://forums.pelicanparts.com/porsche-911-technical-forum/439478-valve-guide-question.html

Here is Pelican Flieger’s and my contributions to that thread.
http://forums.pelicanparts.com/newreply.php?do=newreply&p=4283051
http://forums.pelicanparts.com/newreply.php?do=newreply&p=4285905
This has a lot of application to your situation.


The heat is mostly generated as a byproduct of producing maximum power. You can’t do that free-revving and seldom do that on the highway. At 4000 rpm, your 915/12 4-speed with a 28:23 4th gear should be going about 82 mph. You are not at full throttle and there is a lot of air passing under the car to keep things cool.



If you go the transplant direction, carefully preserve and store your MFI 2.4 911T engine. You will want it again.

I recommend you go completely through your 2.4 and make it new(ish) again. A wonderful upgrade is the (expensive) Mahle Nikasil 90 mm P&Cs for the ’73 911 Carrera RS 2.7. They are 8.5:1 CR and suitable with 911T cams. Your MFI will handle the increase in displacement from 2341 cc to 2687 cc (~15%) just fine.

Long term you will benefit from a front fender oil cooler. Unless you can source the ’72 911S only parts, the best option is a kit from Elephant Racing. You should also convert your fan to the 1.82:1 ratio.

I also recommend you get a 915 5-speed and also make it work like new. This you will really enjoy. :D

All this you can DIY with Pelican Forum help. You can buy an interim transplant engine and a 5-speed and rebuild things at your leisure while still driving your 911. The only downtime is to swap components. With good planning that is a weekend job. :cool:

Best,
Grady

Just wondering what the before/after consumption figures were/are?

I have an '86 with over 200K and am anxiously watching my oil consumption

Of course the folks here so far have given you super advice. However, you may be experiencing a more simple problem once that heat builds and the RPM's go higher due to track use, I'd suggest a complete tune up if you haven't done so recently. Plug wires are important, are they old? MFI cars need to be spot on with respect to the timing and general condition of the plugs, points, rotor and cap. Then the fuel delivery system must be checked for performance to spec.

Bad piston rings can lead to smoking on decel. You may have a broken one or two. You might do another compression test and then leak down if you find a low performing cylinder.

The dramatic drop in power is the automotive equivalent of your engine screaming out in pain. Had this happen with my 2.7 last year, the valve guides (and valves) were just shot. Oddly, the engine didn't smoke. Once mine got good and hot, the engine would lose all of its power. Once I got inside, my guess at the cause was pistons binding in the cylinders - I had fresh scuffing in the jugs.

One big danger that hasn't been mentioned yet is valve dropping. As has been expertly explained, a good valve seal is required for heat transfer. If that is compromised, the valve head gets way too hot. Eventually all the wobbling (from poor sealing/seating) and heat can cause the valve head to drop off the stem and into the combustion chamber. Thought a rebuild of a mag-case engine was expensive? Try a grenaded mag-case engine...

Sorry, I know it sucks, but it sounds like you've got a winter project assigned....

No drain. As I said, there was no discernible difference after the top-end rebuild. ...In other words, the issue is not your worn guides.

I suggest you quantify the problem by doing a compression check (warm) and a leak down test. Post your findings.

2 more good thoughts there, but the heat would have to really hot to expand the pistons to the point of almost seizing. Especially old pistons. Not saying it can't happen, but the oil temps must get way out of sight when that is about to take place.

The heat could reduce the hardness of the piston alloy, making it wear more (even with in-spec clearences) due to the higher power demand.

Maybe the metal weakens quicker than it expands, causing the "fresh scuffing" of Aluminum residue on the hard Nickel-Silicon-Carbide cylinder wall coating.

...or the heat could amplify a broken ring problem.

That's my bet.