Rev limit and rod bolts on 3,2

[Thorsten S.]

Hello

I have got a question about the rev limit on the Carrera 3,2. In W. Dempseys book "How to rebuild...." and in some PCA Panoramas I read that the 3,2, 3,3 and 3,6 engines have problems with the rod bolts at high revs over 6300 rpm.
Now we know that the 3,3 turbos had their limit on ~ 7000rpm with the same parts.
And the 3,2 Clubsport CS had his limit at ~6800rpm and Porsche built in hollow intake valves for 300 extra rpms, but no other rod bolts.
So, in Porsches opinion the weight of the intake valves is the part that limits the rpms.
My own Carrera 3,2 had his maximum rev ever measured at 7300 rpm for short time (I got an other ECU with extrem storage where I could read this number). And on the autobahn I sometimes drive 10 miles at top speed where the gauge shows ~6800 to ~6900 rpms.
Did I only have luck? I dont think so, because I know no 3,2 here with a rod bolt problem and we can drive sometimes long distances at high speed here.
My question now: Did anyone of you have exactly this rod bolt problem and at which rpms? Was it during driving at high rpms or when you shifted down and missed a gear?

Thanks for help

Thorsten

[KobaltBlau]

This is a good question. Who knows of 3.3 Turbo or 3.2 rod bolt failures? I do know that most builders like to replace these bolts with ARP during rebuilds, but (jerry macguire voice)show me the failures!

[Randy Webb]

If you take Jerry Woods' & Bruce Anderson's engine class, Jerry will show you a nice set of failures.

[89911]

The rod bolts are undersized on the 3.2's compared to earlier years and are a weak point. Doesn't mean this will be the reason the engine fails at high rpms. But many that do fail find these as the culprit. Recently switched to ARP rod bolts in my 3.4 rebuild

[Thorsten S.]

89911: Did you change something on the valvetrain on your rebuild engine and what is your maximum rpm now?

Thorsten

[KobaltBlau]

Originally posted by Randy Webb
If you take Jerry Woods' & Bruce Anderson's engine class, Jerry will show you a nice set of failures.

Randy, can you tell us a little more about the circumstances of these failures, I won't be taking the class this week!

[Thomas Owen]

Yes, there was a slight reduction in the diameter of the 3.2 rod bolt. But, did that reduction lead to an increase in rod bolt failure? Or, was the diameter excessively large to begin with? There are many opinions on this, but for me, I am satisfied with the stock bolts. I am sure a mechanic that does many rebuilds can show you an example of almost any type of failure. Then there is the argument about how to lube the rod bolt threads during torque, and whether to use locktite or not....

[89911]

Originally posted by Thorsten S.
89911: Did you change something on the valvetrain on your rebuild engine and what is your maximum rpm now?

Thorsten

At high rpm, the bolts can stretch and break. Didn't really change my rev limit, I just feel more comfortable operating in that vicinity now. My rev limit is set just shy of 7000k.

[Quicksilver]

The dead give away of the problem for the 3.2 is rod bolts that are "stretch to torque". Read this as a disposable rod bolt. The bolts are torqued into their plastic deformation and any more distortion of them WILL cause more plastic deformation. This means that the bolts will permanently take an additional stretch which means your clamping load is reduced and the rod nut's preload torque is reduced.

The common failures will be a spun rod bearing or a thrown rod.

I can’t imagine why an engine that is expected to operate regularly near the limits of its rev range would be built this way. Stretch to torque bolts are used to reduce production costs by allowing assembly by torque angle (how far you turn it) instead of actually torquing each bolt.

If you don’t want to believe it then ask why nobody makes a stretch to torque high performance replacement bolt.

Oh, where are the failures? Lets start with the 3.2 on the stand in my garage...
...Repeat after me: “I will not use my rod bolts as a rev limiter!”

Wayne

[KobaltBlau]

Missed shift, Wayne? estimated RPM?

[RSupdate]

Well, I have been worried about the rod bolts in my 3.2 for a long time...... They didn't fail but my tranny did...so "while I was in there...."

I just replaced (among other things) both the rod bolts and head studs with ARP parts. whoa...talk about $$$...cha-ching !!!

At least I have piece of mind now......

[Thorsten S.]

Very interesting, Wayne!
I am sitting here with my dictionary to understand it : You say the main problem is not the diameter of the bolt- it is the kind of bolt ("stretch to torque") itself.
Can you tell me, if of the older cars (before 3,2) had an other kind of bolts?
Or is the problem caused by two facts: The smaller diameter AND the "wrong" kind of bolts?
(I hope you can understand my english)

Thorsten

[WERK I]

ALL bolts stretch when tightened. ALL bolts stretch when stress cycled(i.e. downstroke/upstroke) Measuring bolt "stretch" is the MOST accurate method of installing rod bolts. ARP prefers this method over the more common torque measurement. Elasticity makes the world go round! The problem is that when a material is over stressed, it becomes permanently deformed, losing some of its elastic properties, failure soon rears it ugly head. Larger bolts of the same material will have a higher tensile strength, higher quality(ARP/Raceware) have greater tensile strength still.

[KobaltBlau]

Thorsten, perhaps this will help. When these bolts are torqued down, they stretch. when a 3.0 is revved hard, they do not stretch any more. when a 3.2 is revved hard, maybe they stretch a little more. over time, this may allow a bearing to spin or even failure of the bolt.

I still want to see more information on the circumstances under which these 3.2 (and 3.6) bolts have failed.

Here is a hypothesis, no data at all:
- When a 3.0 is over-revved due to a missed shift, the valves maybe kiss the pistons, and a top-end rebuild to correct this is fairly straightforward, for a race engine (polish out the kiss-marks, check valves, do a normal top end)
- When a 3.2 is over-revved in exactly the same way, maybe some rod bolts fail, causing lots more damage.

I have no idea whether these scenarios actually happen this way.

[Thorsten S.]

Kobaltblau: O.K, now I know what can happen with the rod bolts and why it can happen.

But like you: "I still want to see more information on the circumstances under which these 3.2 (and 3.6 and turbos) bolts have failed "

and:

How often is this the reason for a big engine failure?
Most times I hear about about broken valves, and sometimes broken valvesprings.

Grüße

Thorsten

[KobaltBlau]

Thorsten, I suggest you post this question on the "911 Engine Rebuilding Forum" with a link to this thread. There is at least 1 person who hangs out over there who can contribute more.

[Thorsten S.]

Kobaltblau: I did it. :-)