Laust Pedersen

There seem to be a few recent broken timing belts with the usual costly consequences. Danno has on a previous post mentioned that we are working on a solution for the 951 engine by machining valve pockets into the standard pistons.
This appears to be relatively straightforward and a picture of the pockets being applied to nicked pistons can be found here . The two pictures shown are SolidWorks models, giving an impression of how far away the pockets are from the internal surfaces.

Here are some concerns that could be raised:
1) Will it lower the compression ratio?
Yes it will, but only to 7.9, since the two pockets are about 1.5 cc.
2) Will removing the Fe-coating in the pocket area have an effect?
No, the coating is intended to make the piston compatible with the cylinders and furthermore the pockets will quickly be covered with carbon as a regular piston does.
3) Will it weaken the piston?
Yes it will, but the real question is if the piston still will have adequate strength to handle the combustion and inertia forces. In lieu of a thorough analysis and testing, this is a judgment call, which I will say yes to, partially based on the fact, that I have not seen a piston failure as being the root cause of an internal problem.
As seen in the 2nd and 3rd picture the deepest portion of the pockets are in the solid area of the piston above the piston pin, so the crown thickness is not compromised.
4) Will the modification be costly?
No, although the cost has not been calculated yet, but it will certainly be a very small fraction of the financial consequences (roughly speaking ~$500 parts & ~$1500 labor) of a broken T-belt in the current interference engine.

Personally I would do the modification on my engine only if the pistons were out (or almost out) for other reasons.

To go further with this we would like to gauge the interest in having this modification done. So please let us know if this modification is something you would seriously consider now or in the future.

Danno & Laust

sweanders

It would be nice to know if it works but I wouldn't be interested in buying an untested product.

adrial

Any concerns with hot spots causing detonation?

jns

I'd be very interested in it. I am a little concerned about the loss of material on top of the piston,though. Would it be possible to take less off the top of the piston and make up for the gap by a thicker head gasket. Seems like a good compromise if it's possible. It would be nice to be able to drive until the belt breaks or the water pump fails and then call AAA to tow her in. I'm not implying neglect in maintenance and would keep the belt tensioned but would save time and money by running the water pump until it's service life is over rather than changing it while in there to be on the safe side. Gotta keep an eye on that temp gauge,though.

Porsche-O-Phile

I'd have the same concern as adrial (hotspots) - the other would be cost. These things would be ungodly expensive to make and then have coated to be compatible with the alusil blocks. As an alternative that would possibly preserve compression would be to dome or "flat top" the piston with the cut-outs for the valves. One would partially offset the other.

z3bra

I think provided a sufficient radius exists on the new edges hot spots wouldn't be a problem.

The concern I have from looking at the pistons in the picture on gururacing.net is that there is a very thin spot on the edges of the piston on the sides of the pocket very close to the ring land. It's probably fine but it seems like having the material that thin even properly radiused it still has the propensity to get hot and stay hot as well as providing a good opportunity for a piece of piston to more easily break off should detonation occur. This is admittedly a worst case scenario.

Could you compare the amount of material there after the valve reliefs are machined vs an original? It just seems like a pretty thin piece of material on the crown of a piston for a turbocharged engine in the area of the top ring land. Given the pistons are forgings it's probably ok but I would think it at least merits some investigation. This might be a better modification only if you've fitted oversize pistons. That thin spot just makes me a little bit hesitant on stock size pistons. On the 104mm pistons it would be much less of a concern to me.

Overall I like the idea, I'm just not sure I'd want to put a set in my car without some real world testing first.

mroberts

My mechanic swears blind the turbo heads are non-interference. I noticed in one of he spec sheets that the US engines are 9.5:1, vs 10.5:1 in Europe - is this enough difference to turn a European "interference" engine into a non-interference one ?

hosrom_951

Laust & Danno: Wouldn't engines making 350+hp also be a concern since you brought up the issue of weakness?

Jeremy Himsel

I guess my concern would be the overall strength of the piston under high boost/load as well as the additional detonation sources. Also, what will the new valve to piston clearance be in a worst case scenerio and will that factor in minimum deck heights for shaved stock and performance rebuild heads? 8.3 to 1 seems to be a popular shave job. Very few people will pull a piston without at least cleaning up the head.

84black944

It seems like an interesting idea. Though how would it perform if you did what Porshe-o-phile said and get flat tops pistons and cut out areas just for the valves? B/c if you lower the compression you are losing low-end power correct?

Jfrahm

Wouldn't it be easier just to change the timing belt every few years? Also I'd be worried that the piston tops would be too thin at the cutouts. There are probably better ways to spend your time and money, and driving around in a 951 until the tbelt breaks is silly. If you happen to be spinning some decent RPMs the valves will hit anyway, and I hate having cars towed, and it might get vandalized on the side of the road, and and and... um, it's better to maintain it to prevent that sort of breakdown.

-Joel.

eclou

No way. That would be too inexpensive and responsible.

Evan70

Even on a non-interference style engine, you can still have piston to valve contact if you are running enough rpms. I had a timing chain break on a 95 Formula while on a roadcourse (at about 6K rpm) and had valves contact two pistons. Fortunately, I ended up just bending two valves instead of seriously damaging the pistons. My opinion, even is these cars were non-interference style with a heavy duty timing chain, I would still change it (belt/chain) at the recommend interval.

Jeff F

Anything done to combustion chamber design will have effects beyond the obvious compression ratio and reliability issues mentioned.

Flame travel, combustion speed, and percentage of complete combustion will all be affected. How much is the question. There is always unburned fuel/air after any combustion cycle. With these changes, there is likey more. Lost power. Someone else also mentioned hot spots. Good salient point. We don't know how much we affect these variables given the modifications. It is likely negative in any regard.

Given the factory combustion chamber is an old design, it was designed that way for a reason. I don't think we can change it that significantly without extensive testing (long term for reliability as well).

I plan to change my belts.

Jeff
'86 951 2.8. Lots of extra stuff.
'87 928 S4. Some extra stuff (SC install underway).
'92 968 Cab. Mostly stock, and staying that way.

Ahmet

Those specs are not for a 944 turbo engine. Where's your mechanic located? I'll bring in a 944 turbo running, we'll cut the belt, and spin the bottom end. I will make a bet with odds of 100/1 and wager $100. Ask him if he'll take it. Even properly timed 944 turbo engines sometimes make valve contact due to valve float if over revved, they are definately interference.
Ahmet