safe to rev to 8500 ?

Below red line is below red line. Cold starts are much worse.

 

I used to over rev Kawasaki motorcycles all the time, when I was racing.

Never hurt anything, but it did make minor slapping noise. I dont think I would want that to happen on one of my cars, just to expensive to find out...

 

Redline is where you take a chance of catastrophic failure, so unless you know why the redline is what it is and you have altered it to correct the issue, it’s best to stick to redline.

Engine wear rate is a whole different question. The higher you rev, the shorter the life will be and it's an exponential relationship. With simple math, driving in say 3rd gear at 6000 rpm vs 6th gear at 3000 rpm, the engine turns twice as much and therefore has twice the wear…..but acceleration load loads such as opening valves and the piston changing direction are proportional to rpm squared, so driving at 6000 vs 3000 you are actually doing 4 times the wear.

As a practical example, I used to rave an engine with a 9000 rpm redline. If I shifted at 9000 it would last 2 race weekends before needing to be rebuilt. It I shifted at 7500 it would last the entire season.

 

I don't claim to be an expert, but here is some "theory" behind rev limits and why you might want to stay under them. Engines are designed and developed to do certain things within certain limitations. One of the design parameters is the length of the stroke the piston makes from TDC to BDC. The longer the stroke, the higher the piston "speed" will be at a given RPM, as it must accelerate from a stop to a speed and back to a stop many times each second. This piston speed needs to stay within certain safe limits to avoid excessive inertial loads which are passed through the rod bearings to the crankshaft. If the piston speed, and thus the inertial loads, are too high, engine damage usually results. The damage frequently manifests itself as severely damaged rod bearings, which overheat the crank journals, and can destroy the rod, crank, and even the entire engine if a rod decides to go sight-seeing out the side! Race engines were often designed so that piston speeds stayed below around 4500 feet per minute to avoid such damage; this would equate to maybe 8,500-9,000 RPM for a 3" stroke; without doing the math...there are other factors involved of course, including cylinder bore, piston weight, rod weight, valve train design, materials for all of the above, etc. The engine is also designed to make maximum horsepower/torque at certain RPM's, and revving very far beyond the maximum HP/torque numbers is just creating more noise! The bottom line is how many cubic dollars do you have to gamble with???

 

I have missed a shift and seen 10,500RPM without the engine under load..

But the carbon flakes in my rear view mirror looked like sparks off a welder's torch....

 

Just to add fuel and confusion to the fire, I would mirror a lot of whats been said above. Its hard to find technical details on racing engines, sometimes even on engines 40 years old. But I came across some details on the 166 Dino F2 recently in Cavallino. This particular motor began life as a three valves per cylinder 1500cc V-6, with a 86mm bore, and a 45.8 mm stroke making 200 HP at 10,000 rpm. A later 4 valves version had a reduced bore to 79.5 mm, but increased stroke to 53.5 mm, for a total capacity of 1593cc, with power 225 HP at 10,600. In its final form it became a 2404cc engine, producing 285 HP at 8,900 rpm. Notice that although its small, with short strokes, it does not rev all that high.

What does any of that mean? A 45 mm stroke is barely 1 3/4 inch, very very short. yet even with short strokes, Ferrari kept rpm's fairly low. In any racing there are constant compromises between performance and longevity. When you push materials far enough, you will learn thier limits. For example, take the wing of an airplane, say a Cessna 150. Initially it was designed to handle 3.8 g's in standard conditions. But there is a built in safety load that exceeds that figure by roughly 1.5 times. But thats when its new. If the airplane keeps getting exposed to 3.8 g's long enough, while simultaneously aging, oxidising around rivets etc., its original design strength will be compromised. Eventually it wont handle 3.8 g's anymore either.

There have been a few people who claim the 308 has a harmonic imbalance at around 8,500 rpm. If its true, thats not a good area to run the engine. Get above it or stay below it, but dont hold it in that range. But if you keep passing back and forth through it changing gears thats not good either. I guess the point is, as others have stated, its an area of diminished returns. If the world championship were on the line, or your pride, it might be worth revving it beyond its safe limits. But just to drive around hearing it sing way up there will eventually take its toll. Pistons can shatter, valves can let go, anything. And at high engine speed the resulting damage if something does let go can be almost beyond reason.

One last area that should be of concern. I think it was Mark (correct me if Im wrong) who once seen a chunk of flywheel imbedded in the dashboard of a 308. No one was in the car when it happened, but consider the result if someone had. Not only does the engine have a redline, but every rotating component. The engine may very well be safe revving to 10 or 11K, but that does not mean the clutch or gearbox internals are. Take a good look at its location and think about it. These cars are aging, and shops that do our work for us are increasingly cutting corners. If your flywheel or clutch assembly is out of balance by even the slightest amount, extra rpm will not be good to it.

 

A lot, that is a big no no redlining an engine for your car especially high compression. turbo & Supercharged engines and worse if its a cold start hard rev. no need for you to be bouncing off the rev limiter as your not making any more power your just shorting the engine life.

http://www.youtube.com/watch?v=wyoDpMVwxFw

that should explain it.

 

Remember that all the points ignition cars have no rev limiter. I am not fully sure that even the 2V injected or if even the QV cars have one. You need to watch your tach especially in the upper ranges in lower gears as these engines will rev to over 11,000 rpm or until somthing fails if you dont stop them. If you cant watch the tach and drive at the same time, you shouldnt be revving near or past redline. There is a reason decent sports cars have tachs directly in front of you, and why race cars have large ones front and center, so you can see it. Some racing engines will litterally explode if you go past redline even a tiny fraction, and postively if you miss a shift. You have to pay full attention. If your going to make plus 8,000 rpm shifts, be aware that if you accidently miss a shift into a lower gear your engine may exceed 11,000 rpm. You might not get to do that to many times. And you may not get away with it the first time. Think of over revving your engine like playing russian roulette with five in the chamber and youll probably be okay.

 

It's a lot more fun to shift at high rpm, but the more time you spend at high rpm the shorter the engine's life will be.

 

Yes , so true and if you see the video I posted in the reply to the thread starter its very easy to do that, with all that is possibile for missing a shift and granading the engine or even the transmission/transaxle at lower speeds, some dont realize to have this failure at 100mph+ IS LIFE CHAGING HORRIBILE.

 

I would honestly say that if you dont exceed the redline, if you maintain the car with good fresh oil, keep good clean coolant in it, and keep it continually in a good state of tune, etc., it will run for many many years without any major trouble.

The problems arise when you deviate, and to what degree. Safe to rev to 8500? Flip a coin. If I were truly planning to rev it there on any regular basis, I would want to tear it down and see the bottom end with my own two eyes. Taking a 20 year old car thats never been apart up to those rpm's, and I dont care if you have a stack of reciepts a foot thick, your asking for a whole lot of hurt.

 

Without seeing Ferrari's technical data we'll never know the truth. But generally speaking on most production cars the fuel cut (redline) is set based on valvetrain dynamics - both when the valve spring stresses surge excessively or when in extreme cases the valve starts floating. The US and Japanese major automakers test for hundreds of hours on dyno at the worst case rpm (which is not necessarily redline). So if this were a 'vette or Viper I'd say no problem hitting redline often. I have no idea what Ferrari uses for acceptance criteria on their engine tests, but I think they would expect many cars to see redline often (moreso than a Chrysler minivan!). In summary, yes, engines will wear faster and potentially fail sooner at high rpm. But I think Ferrari did their homework and sold engines that are fairly stable up to redline. FWIW, I take my 355 to 8500 whenever I have an open stretch of road and don't think twice about breaking parts!

 

I think it depends on what engine were discussing. The 308 is redlined at 7700. The 355 is at 9000 IIRC.

 

355 redline is 8500. If I had a 308 I'd take it right to 7700 if the roads permitted. The thing to keep in mind is there isn't much time actually at redline if you shift there. Heck, in my STi I ride the fuel cut for some time in autocross because there's no time to catch 3rd before the next gate...

 

gee..my lowly six year old honda redlines at and easily revs to 9000 rpm.

 

And when it explodes the engine will be cheap to replace. When some poor sap grenades a Ferrari, even a dumb ol 2V could set you back over $15K by the time the ink dries.

 

Since we're also talking 308s and engine revs, how accurate is the tach in any given 308? I notice that my temp guage varies with lights on (dash lights as someone pointed out), I guess due to bad grounding (also as someone pointed out). And others have mentioned that oil guages on 308s can read low (sender units).

Given the electrical issues that the older cars have, would you still feel comfortable going to the redline? Or would you recommend stepping it down just a bit (maybe max at 7K) to account for errors in the tach and other electrical issues?

James in Denver

 

Now your thinking Actually its pretty easy to test. If you check your speedo by mile markers to check its accuracy to speed, and if you have roughly the correct circumference tires, you can crosscheck the tach by speeds in gears according to the manual.

 

Just a minor point, but the original setup in my Lotus was points and had a rev limiter. The rotor itself had a mechanical inertia thing that limited the RPMs to 6800.

Ken

 

Never thought of that, thanks!

There is actually one of those radar "you are going this speed" signs just down from me, so maybe I can check my speed going by one of those (at legal speeds of course).

Now as far as the tire circumference, I believe that I have the correct sizes (I'll double-check with my tires and the manual), however, would different brands of tires vary enough in the circumference that it would matter doing this type of calcuation/check for the tach?

James in Denver

 

I agree with all the tech. points mentioned.
On a tech. and enjoyment level I think it is very wastful and point less to bounce an engine off the redline, but I think it is as also wastful to not run an engine into its power band for fear of wearing it out, Why bother having a high RPM engine at all? why miss the power and sound? if you have a healthy engine and stay a couple of houndred revs. below the red line on shifts no problom that is what you brought the car for. Have fun!

 

At what point do the other parts fail, Alternators and AC pumps can't like high revs much, and V belts do have a limit too

 

Its all a tradeoff. Never run it and it will never wear out. But it will rust and rot. Run it and it will wear, but it wont rust and rot. Drive it like a granny and it will run a slight ways farther, but not always. Drive it like an insane person, and it will live a very short life. Strive for that middle ground. Dont ever over rev it, but dont be afraid to hit redline once in a while. Dont drive it like a granny either, it certainly wasnt built for that.