gonzilla

Interesting. Thanks Steve.

As an aside, in your opinion, how high can a stock NA engine safely rev? In other words, how high would you be comfortable raising the redline in your car?

Mike J

The redline???

You figure Porsche would engineer a redline that is in accordance with the engineering of the engine and a good view of longevity, so why would you exceed that, never-mind that the power/torque is dropping off anyways?

Cheers,

Mike

gonzilla

I asked that question just as a point of curiosity. I'm just wondering, regardless of powerband, how fast can the thing spin before rods, bolts, bearings become a problem? I know that the same bottom end has been used in other cars that rev much faster. For the record I don't plan on trying to raise the redline in my car.

I'm coming at this topic probably not knowing as much as you, Mike. That's why I'm posting the question here. I'm trying to learn, dude!

Mike J

Got it, sorry if I was coming across as a dick...

gonzilla

Nah, it won't keep me from stealing bandwidth off your site!

Now do YOU have any ideas of how much stress these bottom ends can take?

Quadcammer

Just me, but I wouldn't rev the car much past 7200rpm, not that there's any reason to do so anyway.

Perhaps the bottom end can take more, but the valvetrain is another important consideration.

No HTwo O

What's redline on a NA 993? IIRC, the Turbo is just over 6,?00. Do you mean 6,200 Oliver?

Quadcammer

I believe both the turbo and N/A cars rev to 6,750.

I meant that, with the stock internalled engine, I would not rev higher than 7200....i.e. thats the max I feel the internals can take.

I haven't looked at many dyno graphs, but power starts dropping after about 6400rpm or so pretty quickly.

Mike J

I have not see any numbers or statistics - other then some real example failures when I took an engine rebuilding course with Gerry Woods/Bruce Anderson years ago. Pretty messy examples, but those mostly attributed to missed shifts.

How much stress tolerance did the Porsche engineers build into the calculations? No idea, and finding out can be pretty expensive!

Cheers,

Mike

Steve Weiner-Rennsport Systems

In stock form, I would not take these engines past 6800 RPM unless the valve springs, spring retainers, and rod bolts have been replaced with better parts. I do not raise rev limiters in software unless these things have all been done.

I would not continuously run past 7K without better rods, bearings, and some oiling modifications.

Mike J

Ok, so let go with Steve's suggestion. That is a tolerance of 6800/6750 => .7% over, 7000/6750 = > 4% is considered risky.

Now, what is the accuracy of the gauge, and more importantly, your eyes when you are driving the car that close of redline on being able to distinguish 4%, never-mind 4%?

So, I would treat anything past the redline as a no-fly zone, and you can have both short and long term failures if you go beyond that.

Cheers,

Mike

gonzilla

Wow. I guess they weren't messing around!

camlob

I did a money shift recently, fortunately nothing happened. My internals, I must say are pretty good.

No HTwo O

Steve Weiner, have you ever seen over-rev damage on a 993 from a high speed spin on the track, whereby the engine was spun in reverse, and was caused damage because the rev-limiter does not protect in this situation?

I've heard this can be done, but wanted to hear from an expert. Thanks.

KaiB

What happens here is progressive failure. Rod bolts can get stretched over time with the occasional forced over-rev. For a while, no ill effects are noted.

Ultimately bearings can (and will) spin with catastropic results. The failure is often blamed on something else, but it's the cumulative effect of "micro stretches" which is actually the primary cause of failure.