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As a result of developing the supercharger kit, I have learned an awful lot about the 968 engine and what it can and cannot do or tolerate. Developing it allowed me the "opportunity" to try a number of things and see what the results and effects were. Doing over 140 dyno pulls, and over 20 hours of on the road tuning showed a LOT.



What I wasn't suspecting though was to find any serious weak links in the engine. I expected certain limitations, but not truly weak components.



We knew about some oiling issues, but as this was a street kit, that seemed to be a non-issue.



We knew about overheating issues, but again, given the recommendations for improvements given with the kit, that seemed to be fine as well.



We knew about the head gasket issues, and knew what to do about that.



We knew about the valve guides and such, and knew what to do about that.



Over the years we had been told that the bottom end of this engine was pretty bullet-proof. I took that as gospel, and didn't worry about it. Sure, the rod bearing thing was a potential concern, but that was also manageable. While the crank itself is truly stellar at resisting wear, the bottom end is by no means bullet-proof.



Here's where it gets messy. As a part of development, I wanted to see what kind of boost the engine could take. I wanted to push things up just a bit. I was able to handle the mixture and heat issues. That was no problem. I expected more trouble with that, but it was actually pretty easy. What I did not expect was an OEM major component failure.



We knew that the early rods were found to be weak and fail at the connection of the upper and lower half of the rod. Porsche beefed that up and issued an updated rod. That was not the problem. The problem is the material used in making the rods, and the design of them.



Every engine has a designed in "weak link". This is to help reduce the chance of catastrophic failure and grenading. Porsche decided to make the rod the weak link. Many engine designers do this. They essentially have to choose between the rod and the bearing/crank. The rod is usually less expensive and less damaging. So, to how I figured this out.



In trying to resolve an inexplicable oil consumption issue, I changed out quite a few things with no success. I changed out the head, the Air/Oil Separator, and tried multiple cleanings of the engine internals with products well known at succeeding at curing problems due to dirty rings and such. No luck. It became obvious that something else was going on, and it was likely worn oil rings. so, out came the engine.



Upon inspection at disassembly the problem became obvious. Rods had bent, causing the piston to go cock-eyed through the cylinder as it went through each stroke. They basically collapse under the load. That meant that the oil rings were hitting the side wall wrong and wore out, which resulted in the oil passing by the rings.



The head gasket was perfect, as were all parts of the combustion chamber and pistons. We never over-revved the engine, as the redline was always kept stock. There were absolutely no signs of detonation. We knew that already though, as there were monitors on the knock sensors to tell us if we had that problem, and they never did.



The damage was purely as a direct result of boost creating a final compression ratio beyond the limits of the rods to withstand it.



This will be the case with both early and late rods. It has nothing to do with the connection point. The failure occurs well up the rod nearer the wrist pin. This will happen to any engine that pushes boost up too far. It may not happen right away, but it will happen. You probably won't know it either for quite a while. My engine continued to run just fine, even though it had bent rods. There was no sign whatsoever that there was a problem. No noise, no pressure loss, not even a discernible power loss. There was no clue whatsoever that this kind of problem existed, but for the oil consumption.



In looking back at what was happening, we know exactly when this happened, and at what boost level. We know exactly the point at which it will quickly happen again, and can take a good guess at which point it would gradually begin to happen.



These engines are capable of handling boost, and are still excellent candidates for low boost kits. They do extremely well under the right conditions. They have their very real limits though, and exceeding those is problematic.



So, the bottom line is that if you want to play around and try to run higher boost than about 6 psi, and you want to keep the 11:1 ratio, you have to change the rods to Carrillo or something like that which can withstand the pressure. Porsche rods will not do. It has nothing to do with heat, fueling, detonation, or anything like that. It is the design of the rod itself. Changing to the stiffer aftermarket rod though also means that the rod bearings now become the weak link, as they will be taking the entire load of the constant pounding, and will need more regular changing.



As with pretty much everything else, there is no free lunch.



So, basically another $10k is going into this project. I could have had a V8 - LOL.



I'll post pics next week when I get everything all cleaned up and have good ones to post.

Flash

So the bottom line is that a stock DR1 super charger system, installed as designed, with well maintianed top end , shouldn't have a problem?

correct. there are no indications whatsoever that there is any issue at all with the kit being used as designed.



all of this just points to confirmation that my initial instincts and evaluations of the engine and the supercharger kit were correct, and the current levels of boost are about as far as you can go safely on an OEM engine.

#4 cylinder? Ive had stock 951 rods do that when running too much boost. Upgrading the rods are a must with higher boost levels. Curious what PSI you got to before it happened.

bummed to hear about it, but glad you finally figured out what's been plaguing you for the last year!



are you going to upgraded rods and bearings now?

carrillo rods. oem bearings, but we are coating them.

What kind of boost, hp and torque do you think caused the problem. What was the highest that you ran it?

again, the problem is final compression ratio, not hp or torque.



adding much over 6psi results in too high of a final compression number (pretty much anything over 15.5:1) and the rods begin to collapse.



i really don't want to get into what number we saw, because that is a very misleading bit of information, and we never really got to tune that, since we found out right away that the injectors were too small to support that level of boost.

What is the size of the stock injectors?

small



321ml/min @ 300kPa

I think thats the same as 321cc/min. and I agree, that is too small to produce a much additional hp with safe A/F ratios. My boosted Mazda 2.65l KLDE runs 6 -370cc/min, and they are maxed out (over 85% duty cycle) at the max torque rpm at 9psi, making about 275whp (~310 crank hp + hp to drive the SC).

The tested flow of the 968 injectors is 296cc/min at 3 bar (though I have seen some anecdotal evidence they can be up to 340 cc/min) at 3 bar). At the 3.8bar FPR the 968 runs, the theoretical flow is 333 cc/min. I'm running the Ford version (brown tops) of the 951 injectors. The 951 injectors provide 416 cc/min at 3.8 bar, and the ones I am using a little less, around 390-400 cc/min @ 3.8 bar and they are maxed out with 242 rwhp. I always thought it was pretty amazing the 968 injectors could provide enough fuel for 240+ rwhp.

[quote name='Eric_Oz_S2' timestamp='1330757685' post='122677']

The tested flow of the 968 injectors is 296cc/min at 3 bar (though I have seen some anecdotal evidence they can be up to 340 cc/min) at 3 bar). At the 3.8bar FPR the 968 runs, the theoretical flow is 333 cc/min. I'm running the Ford version (brown tops) of the 951 injectors. The 951 injectors provide 416 cc/min at 3.8 bar, and the ones I am using a little less, around 390-400 cc/min @ 3.8 bar and they are maxed out with 242 rwhp. I always thought it was pretty amazing the 968 injectors could provide enough fuel for 240+ rwhp.

[/quote]



Are you running a 951? If not how are you fooling the DME so you get the right on-time for the injectors?

Running an S2. Not sure about the 968 code, but the S2 code has an injector constant tied to the Fuel Quality switch (don't think the 968 had this?). I simply changed the "0" setting to -20% in the code to scale all the pulse widths back for the larger injectors. This changed all fuel maps except the idle maps which are not affected by the FQS. The latency of the injectors is very similar, in the end there was almost no change to the dead time maps to get the 951 injectors to work correctly.

nope - we don't have that

back to the issue at hand.



i'll be over at the shop later this morning, and will take pics of the rods. it isn't pretty. what bothers me is how quickly it happened when we super-revved at higher boost. let me paint a picture.



on the track, rev limit bumped up a bit, light flywheel. go into a tight turn, blip the throttle, downshift to second, go through the turn, hit the exit apex, hit the throttle and go back on boost revving up to redline................bang!



this is not all that different from what we did on the dyno. there was no warning. we know exactly when it happened, and we all stood there trying to figure out what had happened. the disturbing part was that the engine continued to run seemingly fine, though all outputs were lower from that point on, and we were consuming oil from that point on. we thought it was valve seals, but obviously that was not it, as i replaced the head with no change. i also replaced the AOS with no change. this pointed us to rings, which was essentially correct, but the assumption that it was limited to those was the error.



this does not paint a pretty picture for anybody contemplating anything much over 6lbs on 11:1 pistons with OEM rods. i'm even guessing in saying that 6lbs is ok. i haven't seen or heard of any failures yet at that level, and they have all been at about 7lbs, but i also haven't seen the internals of the engines to see if they are only slightly bent like mine, and yet still running. there just isn't enough of a sample yet to know exactly where things start to go bad. given the 2 years of the D1R kit running without issue, i think we have enough of a sample to know that 5lbs is fine, especially on the street, but we just don't know about more than that.



pretty scary stuff.

If the stock rods have failed in compression, then it is probably not specifically boost that is an issue but more what torque the engine makes - and more specifically the Brake Mean Effective Pressure on the crown. In my case if I have 7psi, but lower output, then in reality the stress on the rod should be lower. The other variable is how the pressure wave generates down the cylinder bore, if you have too much ignition advance it can peak earlier resulting in detonation and increasing the peak pressure momentarily (knocking). This may be enough to bend the rods, but won't result in an increased engine torque due to the transient nature of knocking.

we don't have any knocking. we looked closely at that, and had to make some timing corrections for that.



remember that boost is relative. 7psi on one engine is not the same as the other. it's not just a factor of pulleys and such. you can have 2 identical motors, and they can both have 7psi boost, and if the intake and exhaust are different, or the combustion chambers are different, you can have very different outputs. as an example, all you have to do to increase boost numbers is choke down the exhaust. it won't make more power, but the boost numbers go up. a similar thing happens with an intercooler. boost numbers go down due to the restriction, but power goes up due to the cooler mixture. same pulley. same revs. same engine. just some changes in how much compression there is.



so, i think we are chasing the same thing here though. on a 968 engine, with stock rods, 7psi is trouble. the cylinder pressures are too high, resulting in too much force (torque) being applied to the rod.



on an S2 engine with stock rods, since the intake and exhaust is smaller, the boost numbers will be inflated relative to the same amount of volume. your torque numbers are consequently a lot lower.



i'd have to do the math on cylinder pressures and pressure ratios on that engine to know if it is too much or not, but i can't think that it's too far off, based on the power numbers you have with the setup you have. remember that the rich mixture you have costs some power, so in reality, you'd be quite a bit higher. the pressures won't change due to mixture, so if that is a problem, it will still show itself. if you are on the edge on timing though, that could change the pressures.



frankly i think you might have the same power, and no detonation issues, if you removed the intercooler and increased the pulley size. that would lower the cylinder pressures, and alleviate the worry. i went through the same thing. i actually found that i lost power by adding the intercooler in this boost neighborhood. the 951 intercooler cost us about 1 psi of boost and only dropped the temps about 25 degrees F. i would have had to overspin the rotrex to make up the power if i didn't want to lose the bottom end torque. it wasn't a good trade. if i were to let the boost climb though, an intercooler might help things, but it would have to be really small so as not to increase resistance too far. i was out of fuel though, and didn't want to change injectors, so we never got that far.



something to remember is that i am limiting only the top end boost. it boosts up normally all the way to 5.5k. if i uncorked this, i would make 7psi, but then i would have more bent parts in more engines.



tough stuff to figure out. let's hope you don't figure out the limits the hard way like i did.

Flash,



I thought long and hard on whether to upgrade the rods when I had the engine apart - now I am kicking myself. Hopefully the combination of intercooler, lower initial compression ratio and upgraded rods will help, plus if we don't dial in too much advance. If not - there will be more tears from Australia (and a possible divorce). Hard lesson to have to learn and I am not sure I want to repeat it. I might run the car for 12 months and then strip it back down and replace rods.



I would not have thought that the rod would have been the weak link - I would have expected rings to go first or potentially valve guides/seats or head gasket.

yeah - many engine designers choose the rod to be the weak link. i didn't expect it to go that early though. that's pretty soft. but then, the final pressures were more than double stock before they went, so i guess that's still a pretty good margin.



as for yours, depending on where you started in compression, you may be on this side of the danger line. you could always install a boost limiter too.