Determining a engine's redline speed

[Kevlar]

Ok... BMW makes engines and during the design process, they figure out that a car can redline at 8100rpm (let's use the E46 M3 as an example). In their search for ultimate power, they know what mechanical components are in the engine and how fast it can spin without blowing itself up.

Now, along come the aftermarket tuners and they come along and they bump it up another 500rpm. On what basis do they bump it up 500rpm, why not 1000rpm? Why not 250rpm?

Do they just take a guess and say, well, BMW is normally pretty conservative, we'll up to 500rpm and hope nothing breaks, or do they do their own testing and see why BMW limited it to 8100rpm and do they do analysis to say another 500rpm shouldn't be deterimental to engine life or running operations.

I'd like to hear your thoughts...

[mryakan]

I think it is probably "Let's hope it won't break and if it does we'll just blame BMW".
As an engineer, you always learn to be a bit conservative because your design will probably be used (or abused) in ways you probably did not anticipate. If you design it on the edge, you are asking for trouble. There is the often mis-quoted "5% Engineering margin of error", which really is a 5% minimum safety margin that engineers like to build in for such cases. Well what happens if you remove that safety margin. You may not necessarily break anything, but you are now just on the edge, all it takes is one unfactored variable to change and poof, blue smoke.

[ezbmr]

Well depending on what components the engine has, it can run much higher than perhaps specified, but what would be the advantage if the hp tops out at 6000 rpm? Formula one is at 18000 or so, motorcycles are around 13000 or so. I think the smaller the engine, the more advantage the engine might have to rev high, the bigger the engine, the heavier the internals are. There are ways to get more hp at red like, but most people like their hp at the lower and mid part of the rev range. If you could have all your hp at 3000, wouldn't you?

[DrewDude320i]

Well depending on what components the engine has, it can run much higher than perhaps specified, but what would be the advantage if the hp tops out at 6000 rpm? Formula one is at 18000 or so, motorcycles are around 13000 or so. I think the smaller the engine, the more advantage the engine might have to rev high, the bigger the engine, the heavier the internals are. There are ways to get more hp at red like, but most people like their hp at the lower and mid part of the rev range. If you could have all your hp at 3000, wouldn't you?

:confused:rolleyes:(

[xatlas0]

Aside from valve float, the biggest limitation to maximum RPM (in a gasser, anyway) is piston speed. This speed determines the maximum loading of the rods, crank, and various connectiong points, like the wrist pins and connecting rod bolts. These parts are loaded alternatingly in extreme compression and extreme tension, by virtue of the Otto cycle. A large part of the loads stem from the actual motion of the cylinder itself, not just the explosion part. Compressive failure (of the bottom end, anyway) is pretty rare, as the materials used have excellent compressive characteristics, and there is more material being compressed. Most of the design cases for engines I have read cite the redline limiting factor as either the connecting rod bolts or the wrist pin, as those provide the lower limit to the maximum tensile force/pressure allowed.

That is why, in most cases of over-rev, if valve float isn't an issue, you'll read about a piston+rod blowing out the top of an engine, as that is the point of maximum tensile force, which causes the connecting rod bolts to let go. If you know the bolt type, size, and piston + rod specifications (material, dimensions, manufacturing procedure) you can make a pretty good guess as to when the engine will go pop, if you disregard valve float.

[brandonmatthew]

I would go to guess bmw sets the redline lower than it could be mechanically for added reliablility and engine life. People who mod the engine usually don't care as much about reliability or they wouldn't up the redline anyway. I'm guessing if you sat at the raised redline for any significant time the engine would start to have more problems than normal, it just wouldn't blow up immediately.

[tmaher]

you said a bunch of stuff here about RPM max In most manufacturing processes there are roughly a 10% leeway - Tweaking up towards that 10% is probably a given that any Joe Schmo will attempt - There are other cases though, like the detuned Yamaha V6 in the SHO that was good for 9200 RPM, yet only reached an astonishing 7200 RPM - Warranties, warranties...

[Kevlar]

The question is more about ... how do tuners establish the new redline, do they do extensive homework to calculate the new redline or do they just bump it a few hundred and hope that BMW left them some room on the table.

It's one of those things of thinking why something is the way it is before changing it to something 'supposedly' better.

[xatlas0]

The question is more about ... how do tuners establish the new redline, do they do extensive homework to calculate the new redline or do they just bump it a few hundred and hope that BMW left them some room on the table.

It's one of those things of thinking why something is the way it is before changing it to something 'supposedly' better.

If you take apart the engine and properly identify certain material properties and design specifications (pretty easy and cheap to do) then you can fairly conclusively determine the maximum tensile strength, and therefore the maximum engine speed. If a tuner does this, I am not sure. However, it is very possible that they could.

[Codeman]

I think it's one of those 'hope doesn't break' things.

I don't understand why they raise the rev limiter, doesn't horsepower drop off substantially the closer you get to redline?

[brandonmatthew]

I think it's one of those 'hope doesn't break' things.

I don't understand why they raise the rev limiter, doesn't horsepower drop off substantially the closer you get to redline?

I'm pretty sure most change the peak power also. I have a dinan chip and while my power used to drop off pretty soon after 6k, it now pulls all the way to 6800+

[Lantz]

If you take apart the engine and properly identify certain material properties and design specifications (pretty easy and cheap to do) then you can fairly conclusively determine the maximum tensile strength, and therefore the maximum engine speed. If a tuner does this, I am not sure. However, it is very possible that they could.

I think they simply establish that there might be a few extra horses with the extra rpm and then over reving will put the engine back at a high rpm during a shift. Hoping there would be some dyno'ing as proof at least.

The way i look at most tunes these days is there is very little consideration taken into the engine design. They are looking for numbers and benefits to sell. The increased rpm to a laymen might seem like a benefit and hence another selling point. Not saying that it is not a benefit but engine reliability may not be considered.

[Kevlar]

I don't understand why they raise the rev limiter, doesn't horsepower drop off substantially the closer you get to redline?

Normally yes... some cars will continue to make power up there if you add cams to take advantage of the high RPM running. The majority benefit for stock cars though is to raise the redline so when you change to the next gear you are higher in the powerband (so I would imagine).

[NikosX]

I know some companies adjust redline for simpler reasons. A friend of mine works for a custom BMW tuning organization in AZ. Its called Auto Individual. He raised the redline on his E46 M3 by a couple hundred simply so that he could finish the quarter mile in 3rd gear, thus eliminating the time lost shifting into 4th at the very end.

[creeves328]

I'm pretty sure most change the peak power also. I have a dinan chip and while my power used to drop off pretty soon after 6k, it now pulls all the way to 6800+

Yes sir! The tuners software makes its power mainly from spark advance changes (combustion chamber pressure increases). In this case, the Dinan software will continue to advance spark timing further, whereas the stock software likely hits a ceiling at some point (limiting the explosion, ie power). Since flame propogation time never changes, whereas the piston speed does (as rpm increases), you dynamic compression will lower if spark timing doesn't continue to advance in relation. The tuner 'recommendation' of using high octane is a safety measure as this all generates more heat.

So... The tuner can change peak power, or extend its peak to a high RPM point (before falloff).

Kevlar --- Speaking the e46 M3 as an example (double vanos), the tuners can change cam timing for changing power peaks. Valvetronic changes as well.

[Serious]

Manufacturers take into account stress testing on every component, how much HP they want to make, and then find a RPM under that stress limit for a bit of safety margin. In the case of e46 m3 piston speed is a big limiter.

Aftermarket tuners for the most part just jack up the limiter until there is either no power to be made by additional RPM, or there is risk components failing (how much testing they do is most likely very little). So its likely they are just assuming the safety margin BMW put in.

BTW there is a rumor that the s65 (e9x m3 engine) was initially designed with 9000rpm redline in mind and near the end of development engineers turned it down to 8400.

[Phore]

I don't understand why they raise the rev limiter, doesn't horsepower drop off substantially the closer you get to redline?

Well my Dinan chip raises the rev limiter and I always assumed it was to take advantage of the supercharger they also sell. With superchargers you will always make more power with more revs so if they sell it with a chip they can claim a better peak HP.

[robmpulse]

I would assume it's through R&R and trail and error.....

but then again.... I know what you get when you assume...

[Rakshas]

I would imagine most tuners just bump it 500 and hope things don't break, assuming that BMW built in a decent safety area.

The larger companies (Dinan, Alpina, Schnitzer etc) probably do more testing.

[///AlpinePower]

Ok... BMW makes engines and during the design process, they figure out that a car can redline at 8100rpm (let's use the E46 M3 as an example). In their search for ultimate power, they know what mechanical components are in the engine and how fast it can spin without blowing itself up.

Now, along come the aftermarket tuners and they come along and they bump it up another 500rpm. On what basis do they bump it up 500rpm, why not 1000rpm? Why not 250rpm?

Do they just take a guess and say, well, BMW is normally pretty conservative, we'll up to 500rpm and hope nothing breaks, or do they do their own testing and see why BMW limited it to 8100rpm and do they do analysis to say another 500rpm shouldn't be deterimental to engine life or running operations.

I'd like to hear your thoughts...

Probably to the limits of the warranty and whatever internal mileage limit they deem fit. The aftermarket can only move it so much further. In the end, the aftermarket engine does not last as long.