What are the primary determinants of engine redline RPM?

Factual Questions
1

See subject line. Obviously there is an absolute upper limit determined by the mechanical strength of the parts involved: wrist pin saddle, wrist pin, con rod, crankshaft, rod bolts. Spin too fast, and one or more of these parts will break.

A brief internet search suggests that before this strength limit is reached, there is an RPM beyond which the engine wear happens at a rapid pace. this page is pretty old, but it shows that for production passenger cars in which the engine is expected to last for well over 100K miles before being considered “worn out,” mean piston speed seems to be an important factor.

I applied their MPS formula to my 2003 Maxima and came up with 3479 ft/s, which fits within the guidelines they prescribe. OTOH, I tried the formula for my G37 which has a longer stroke AND a higher redline; I came up with 4297 ft/s, which is way out in the “very short life” region.

So maybe mean piston speed isn’t the primary determinant for longevity? What else is there? Can anyone point to technical publications (e.g. SAE papers) that might shed some light on this?

2

Valve float is a limiting factor especially with pushrod type valve trains. The valves can’t close fast enough and the engine will lose power well before any mechanical damage to the crank, pistons, etc. It’s not quite as much of a problem with overhead cam engines.

3

The balance of the rotating assembly (crank, rods, pistons) is critical, as is the ability of the valve gear to cycle fast enough without float (as above). Beyond that, it’s the ability of the intake system to feed that much reciprocating volume. All other things being equal, it’s possible to build an engine that starves itself out of higher revs while having the mechanical ability to handle more. The reverse is usually true with mass-production engines; without limiters of some sort, they could easily rev into damage or destruction ranges.

4

“Losing power” is different from engine damage, the latter being the factor I always associate in my mind with redline RPM. Does valve float cause damage?

Sure, this sounds like engines with intake systems tuned to provide peak torque at very low RPM. But not breathing well at high RPM is different from engine damage. An engine may be gasping for breath at 8,000 RPM, but if it’s mechanically sound, why would the manufacturer deliberately keep you away from those kinds of speeds by setting the redline somewhere lower?

5

I didn’t mean that gasping for breath equated to damage, only that it will be a limiting factor on maximum RPMs if the engine is otherwise capable of them. I’m pretty sure some engines are redlined for power and drivability reasons in addition to mechanical limitations. The maker doesn’t want drivers to push the revs into a range where the engine drastically loses power.

And, of course, some cars are hard-limited on speed to match their tire capabilities… so the overall answer isn’t always simple.

6

that page is a bit over-simplified as it’s only talking about mean piston speed. The peak speed may be much higher and that is dependent on the ratio of the length of the connecting rod vs. the stroke. A short connecting rod relative to the stroke will drastically increase peak piston speed as well as piston/cylinder wear.

7

Valve float can weaken the valve springs.

8

It’s also possible, but not all that probable, for the valves to hit the piston tops on an interference engine if they float bad enough.

9

Again, if it’s not hurting anything, why would the mfr care?

Do you have a cite for that? Cheap tires are easily replaced with tires that have a higher speed rating, but then the owner would still be screwed because the ECU says “no, I’m still not letting you go any faster.” I am aware that some vehicles (e.g. the infamous Suzuki Hayabusa motorcycle) are top-speed governed, but it’s not because of the tires.

I’m familiar with the kinematics of a reciprocating-piston engine, and yes, a short conrod will do funny things to the piston motion. But given that peak piston speed only happens for a brief instant during each stroke, is it actually an important parameter? Peak speed doesn’t last long enough to dump appreciable amounts of friction-related heat into the cylinder wall.

10

Poor PR. Having Car and Driver choke out your sports sedan at 110 would look bad. No other reason. I suppose a serious power falloff could be a driving hazard if it startled the driver. Few engines are designed to allow such power falloff before mechanical RPM issues anyway. It was mostly a theoretical.

That’s exactly what happens. Read almost any recent review of a Euro sports car or sedan and there will be discussions of US vs. EU speed limiters and their relation to the speed rating of the tires the car is shipped with. Getting a US model Euro-chipped to raise the limit speed is not uncommon. (I mean, it’s horrid that your S model will only do 137 instead of 155, isn’t it?)

11

Talking about pre-ECU cars:

Valve-float determines the point at which adding more fuel won’t increase RPM, especially under load. So, it serves as a practical “red line”, but one that can’t really be crossed.

You can decrease valve float by replacing springs with heavier ones, giving what’s called a “higher friction” engine. However, if you do, you’ll eventually run into other limiting factors, which I’d have to guess about. In any case, you can raise the red line considerably on an engine that’s been “blueprinted”, meaning, all the parts have been machined or replaced to meet spec, rather than living with what happend to come off the assembly line. After that, the limits are based on design choices.

My guess is that a high friction engine also has shorter life just due to increasing stress on everything (especially the starter!), even if you don’t run at high RPMs.

With an ECU, they can limit RPMs wherever they want. I worked on Ford EEC III and IV back in the late 70’s early 80’s (mostly in Sci Lab, not for production). For an 8-cylinder engine running at 6K RPM, that’s a spark cycle every 1.25 msec. In that time, the ECU has to schedule coil on and coil off (the off causes the spark, and there are minimums for the amount of time it can stay on without overheating, and off to discharge, determining the timing profile for any given RPM). It also has to schedule events for fuel delivery, for direct-inject engines (but not for CFI, as those don’t need to be per-cylinder). And it has to do this in little enough time to let enough background processing do recalculations of operating parameters.

Well, back in those days, there really wasn’t enough time. IIRC, we didn’t run 8-cylinder engines faster than say 4500 RPM, because even if the engine could go faster, the computer couldn’t quite keep up. The EEC-III was 1M instructions per sec, IIRC, and that was pretty fast at the time, for a processor cheap enough to put in an engine.

But what gets damaged first, when you push it too far? I hope I don’t find out!