Hello all, For some time I've got a few questions regarding engine REDLINE. I understand engine basics, but what specifically happens when you enter the red area on the rev-counter is somewhat of a mystery to me; - How is the maximum rpm engineerd? Valve bouncing? Material strength? Temperature? Is there a margin build in to it: how far can you cross the line till it breaks? - I know modern cars have an electronic rev-limiter, where the ECU cuts the RPM. But how does it do that? Reduce the fuel-flow? Change the ignition timing? - Have the classic engines similar (mechanical?) limiters? - What are the dangers to the engine when you over-RPM? Not that I have any intention to cross the lines, but still... Best regards, Michel
When you fly a plane outside its design parameters it might stall it might fall apart or it might be just fine Same with an engine For example the 308 carb engine is visually limited to under 8000 rpm but without any protection - many have strayed far beyond without damage even though there is no real performance gain Modern rev limiters cut the engine at a safe point for the majority of users so allegedly Michael Schumacher's 355 had a raised limit as do some performance remaps The most normal effect of over-revving is valve bounce ie the valve gear cannot recover as fast as it is being asked
Indeed Jimmie, I never intend to cross the boundaries on a plane. Stalling is one (bad) thing but when you'd overspeed it...a bit...fortunately it doesn't fall apart! I reckon the same with engines. They must have build in a margin to make it safe, reliable and durable. But how big this margin is...? Regarding the valves: is the biggest danger that when you overdoo it, the cylinderheads may touch the still extended valves? Could you theoretically achieve higher RPM with stronger valve-springs? And how on earth are they able to achieve the insanely high revs in a Formula1 engine?
It is usually set by valvetrain control and stresses on the valve spring. For qualification of engines they are run for hundreds of hours either at redline or just below redline at peak torque rpm (which could be more harsh on the valvetrain). Some time at redline is ok and that alone won't cause a failure. But if you're on track and spend alot of time there your chances of a failure do go up.
You probably mean peak power because peak torque is usually several thousands rpm below the red line and it cannot be more harsh for the valvetrain.
Valves do not rely on springs to close them once opened. Read somewhere it could be highly pressured nitrogen that closes the valves. Mass is the big problem when you have to get something to suddenly move fast. Stronger spings might help but returns diminish due to the fact the spring has to be larger so more of its potential energy gets used up just moving the extra mass that was used to make it stiffer.
They use the pneumatic valve springs to avoid the harmonics that set up in a steel spring around 16,000 rpm. At least that's what I read. Rev limits are usually set by the valvetrain like everyone has said.
There are several systems in an engine that are subject to stress. When one of these systems is overstressed it may fail and "bad things happen". In addition, there may be an RPM band where the stress does not cause catastrophic consequences (breakage) but does so accelerate the wear that the factory does not you to use that part of the RPM band. The first culpret is the main bearings, Overstress these, and the crankshaft will wear out very fast. This kind of failure was found in the 1930-50 as hot rodders pushed the edges. Most Ferrari engines have enough journal surface area and oiling system that other systems go first. Certainly this is true today. The next clupret is the connection rods. At high RPMs there are tons of force on these rods--not pushing them down on the power cycle, but pulling them apart at TDC on the transition between exhaust and intake cycles. Light forged AL pistons and Ti conrods aleviate most of this worry. Here on FC, we have seen TR motors spinning at 10K RPMs, and RifleDriver has stated that the F355 motor was designed for 10K RPMs and then the life of the belts limited its ultimate RPM to 8.5K. The next clupret is the valve train. At high RPMs the valve must remain following the cam or all sorts of harmonic forces are imparted to the springs. The springs can overheat in a few milliseconds if the valve fails to follow the cam. This overheating will remove tension from the springs and simply make the problem worse. Pushrod cars still have these problems, and engineers generally choose the strength of the pushrods so that they fail first. Titanium valves, retainers,... aleviate up to 40% of the weight and allow higher RPM potential should one wnat to go there. The next culpret is the ignition system. There is a certain maximum RPM where the ignitioin system can properly fire the spark plugs. All sorts of bad things can happen if the plugs do not end up generating a smooth flow of power into the crankshaft. The crank may break and take the engine with it. A final culpret are the ancilaries. The alternator, A/C pump, water pump, power steering pump all have maximum RPMs, and the puleys are desinged to leave them inside their operating window. If you fix an engine so it has another 10-20% RPMs safely, you are going to need to slow down the ancilaries to allow them to live. Any failure of the above can allow an engine to self destruct. However, today, outside of racing, RedLine are typically set by where wear starts to take off and leads to failures within the factory warantee period. Thus, the factory has a wear limited RedLine, while a race motor may have a mechanical failure RedLine. Many of the failure modes are quadradic some even cubic. That is, an engine may be perfectly happy to spin at 8.5K for 100K miles, but it taken to 9K may fail (or wear out) in 10K miles. The added 0.5K RPMs (7%) increases some of the stress levels by 14%-20%.
My 911 Turbo would cut fuel, to the extent where all power would come to a halt and throw you forward in your seat (as if you were braking). I have not had a Ferrari at 8500 or higher to find out.
Some older cars (pre-electronic ignition) do indeed have mechanical rev limiters - one way is a centrifugal switch on the rotor whereby contacts disconnect at a set rpm. Once the rpm drops due to no spark then the contacts come together and ignition is restored. It's crude but it works and no doubt has saved an engine or two along the way!
Michel- To give you a specific example, the 575M has a 7500 rpm red line and the Motronic ECU cuts fuel at 7600 rpm. It is a fairly soft rev limiter and I have bounced off it a few times and it is not scary at all. Taz Terry Phillips
I had this on an old VW I had. This a very bad thing for catalyst equipped cars as it sends unburned fuel to the cat, and they went away.
as a sideline to your direct question: load varies with velocity squared redline is part of an overall engine design - the induction harmonics and resulting HP output are no longer in their sweet range above redline. Summary: dramatically increasing loads and HP falling off... no gain - just pain. Rgds, Vincenzo
Interesting thread. So has anyone used something Pertronix's rev limiter on their F car? http://www.pertronix.com/prod/ig/drl/default.aspx Never mind, I was just told it would fry the digiplexes.
If you are talking about extended time at high rpm on a Ferrari (really any overhead cam engine) it will almost certainly be the connecting rods or connecting rod bearings that let go first. I would guess the pistons are not far behind. If you are talking about a short burst to very high rpm (a couple thousand rpm over redline) probably valves hitting the pistons or maybe the rod bearings. As Mitch pointed out a small change in rpm can make a huge difference in engine life. I had a race engine redlined at 9000 (piston speed was the limiting factor) and shifting at 9000 I got 2 weekends out of the engine or about 3-4 hours of track time and the engine was shot (pistons, bearings, trans gears usually). Shifting at 7500-8000 I got a full season out of the engine with no problem, re-ring in the winter and it was ready to go for spring. Factory redlines are normally more about durability than anything else.
Also, alot of time this red line is determined by the power curve and transmissions gearing matched to the engine. Example, if your engine makes 300hp from 4000-7500 and then makes 350hp from 7500 to 8500 and then drops back to 300 from 8500 to 8800 then to 250 from 8800 to 9500. If your engine can turn 9500rpms, you wouldn't want to use that unless the course absolutely demands it (straight into a hard breaking where shift wouldn't make timing sense). if you shifted at 8500 back to 4000, you get to use 350 or 300hp throughout the entire range. The rev limiter is a protection for not paying attention/mistake (losing rear grip for example, is a very easy way to over-rev an engine). They are generally made to not cause any harm, though I wouldn't want to just drive at them. And generally red lines on indicated on the tach isn't the engines "terminal" speed, which is varible due to metal stress, etc, but is determined by several factors. Bascially the weakest link determines it. Shorter answer, that would be the highest I would purposefully rev an engine unless its being raced.
unfunnyclown is right. In racing engines, you generally rev as high as you make power or maybe a little past to fall back into the strong part of the powerband if it's such. Really you can keep revving higher if you replace components with lighter materials. The discussion of rev limiters is also somewhat moot in cars with manual transmissions. No rev limiter is going save you if you miss 4th and find 2nd, downshifting on track.
