I would say that rev-related damage on a normally aspirated car is usually found in the bottom end (crank, bearings, rods) or perhaps in some scuffing of the pistons. Revs alone don't cause events that melt pistons. 

A hole in the piston, like others have opined, is associated with detonation and pinging. It's more likely high revs would cause a hole in the *block* (from throwing a rod through the block) than the piston. Of course, if this is a ERA Turbo model...

I know better than to jump into this thread. OK, I can't help myself got to do it. I ask for a picture of the side of that piston after it's removed. A better picture of #1 would help also. I'm thinking RPM didn't have anything to do with your damage. I rebuilt a 1275 Sprite engine for a buddy who could not afford to do the tranny at the time. To the point, while on vacation in Miami he lost 3rd and 4th. He called me and ask if he could drive 6000 rpm in second on the shoulder of 95 and come home to VA. I ask he richen the carbs a couple of flats and watch OP and water temp. He brought it home or should I say it brought him. I rebuilt the tranny changed oil and filter on engine and went on. Steve (CTR) 

I would suggest that your piston failed because there was something fundamentally wrong with the engine and you failed to notice/do anything about it.
Pinking / pinging / knocking is bleedin' obvious, and the damage didn't happen in the blink of an eye.

Elfs have a 3.76 final drive, and all of the ones I've had have cruised at 70-75mph quite happily, which is 4500-5000 rpm.
Tigger with his 3.2 fd will sit at 80mph all the way to Italy and back (twice) with no trouble.
If the engine is either standard or properly tuned, then the rpm is no problem at all.

I understood the question just fine.   Your piston did not begin to melt simply because you were cruising at high RPMs, I promise you that.   If there had been a direct correlation, all of your pistons would have shown similar damage, and they didn't.  

Under lighter load (driver only) 80+ mph and 4500 - 5000 rpm are easily sustainable on a properly tuned engine. Add the stresses of heavy loads and get lots of knocking/pinking (detonation) and any engine will be harmed.

Benny:

Let me reread the question for you since you didn't understand it.

1. The question was safe cruising speed, RPM and harm to the engine.  

2. My piston failed from prolonged high MP and high RPM.  

3. Understand the connection there?

It's irrelevant whether he can find a place to carry those speeds.  The point of Alex's post was whether the engine could sustain high RPMs and the answer is likely 'yes'. 

I also agree that the ugly pictures of the failed piston had NOTHING to do with RPM, and should have never been entered into this discussion.  They amount to nothing more than scaremongering in this context.  

You guys just added a couple hours to my drive from Miami to Virginia.  The past events have seen my Mini pushed pretty hard

Nothing wrong with 4500 all day long.  Tune on the road and get the fuel and timing set right.  

One of the choices when building the Mini and trailer combination with 1380 power unit was the final drive...I opted for the 2.95:1.

This sees steady 75 mph at 3,800 rpm (GPS speed).  Speedo says faster but even though it was recalibrated for the FD, wheels, tires...its out.

Just over 40 mpg fuel consumption...would be a little higher if the Mini was not pushed too hard.

Ian, from my experience an spi with the most used 3.1 diff will but the MPI's with the 2.76 diff take a little longer to squeeze that speed out of them. FWIW i think the 850's and 998's are smoother more rev happy running engines compared to the 1275's.

Way back when (I was somewhat younger and less susceptible to pain) I would drive my Mini home betwqeen college terms.  This meant a bit over 1600 miles in 3 days.  On the flat bits the 850 would sit happily around 65mph - which equates to about 4500 rpm - for hours on end.  In those days most of us did not waste money on tachos or temperature gauges etc. 

I would suggest that an SPI will not be able to pull enough revs in top to reach 92mph....  but with modern oils, 4500 will be no big deal.

Cheers, Ian

I believe that if the engine and subsystems are healthy, these small engines can rev @4500-5000 for long periods of time without harm. That's the benefit of a motor that's physically small.

When I was crossing Wyoming on I-80 in my Mini (with 2.76 final drive) it had not a prayer of pulling 4th gear at freeway speed (8,000 ft. above sea level.) This was much worse in the western part of the state.

I had to hold the Mini in 3rd and sometimes 2nd gear just to keep it moving - my Mini was so slow I was pulling into the left lane to let semis pass me on the right. (admittedly, with their turbocharged diesels, they feel the effect of the altitude far less.)

Anyway, I had that Mini pegged at 4500+ RPM for at least 4+ hours just to cross the western part of the state from Rawlins to the Utah border. I didn't quite have to go to that extreme between Cheyenne and Rawlins, but I had to downshift a lot. And I also had it revving high for hours climbing up from Amarillo, TX, through Denver as well (mainly after passing the Colorago border - though in the Texas highlands, I actually did see my engine temperature rise fairly close to red zone for the only time in my journey. I was probably running at 4500+ RPM for more than 12 hours in June weather on that leg.

I pulled the head a year ago and there was no damage whatsoever - just coking from rocker cover gaskets leaking oil into the intake manifold. I think that leakage is responsible for pinging, because when I 1st tuned up the Mini (and replaced the gasket) there was no pinging for a couple years. (tuneup was just plugs and valve adjustment, no timing.) And my oil dripping problem vanished.

Then the pinging returned. It wasn't till I pulled the head (and saw the oil in the inlet manifold) that it struck home why the pinging had returned. (along with oil dripping.) It was leakage from the rocker cover gasket.

Yeah malsal, maybe saying 5000 was a little high, I'll change my story and say 80mph with a 3.44...whatever that comes out to.

Tony and Don at MM are doing the necropsy today and there were other things going on too, it was the only remaining area of the car I had not yet gone into.

Alex with all due respect 5000 rpm roughly equates to 92 mph using the posters figures. Where do you think he can do this kind of speed in this country on a regular basis ? I still stand by my post he is on the road to destruction at those revs for sustained periods in any A series.

Wow, thats bad....

Actually I am quit happy with 4000rpm already, just wanna know if even 4000rpm is acceptable or its already too much.

The engine damage pictured has little to do with the rpm and speed involved, and more to do with other setup issues.  It must have been pinking like a sod for a long time before the piston let go.

A standard or well tuned (read timing and fuelling correct) Mini engine with decent oil will turn 4500-5000 rpm all day quite happily.  It's when you move away from standard and don't spend the time on the rolling road to get everything right that reliability issues appear.

Image Gallery

Kenji here is the result of my pushing 80 loaded with 3 people just north of the golden gate a couple weeks ago.  91 fuel, Kent 266, ported 12G938 big-valve.

But fortunately had a great trip yesterday taking the engine up to Nevada City to meet Don and Jemal, so it wasn't all for nought.

I would stay around the 4k area, short bursts up to 4.5k may be alright but 120km = 74 mph how fast do you need to go ? Any sustained speeds above 4k and up to 5k are asking for trouble imo buy something more highway friendly Mini's were not built for sustained highway speeds they did not even have a Motorway (M1) in the UK until the year the Mini was introduced 1959.

94 SPI 1275