acao

This was also the case with BMW's S54 engine. That engine was installed in the E46 M3 and the M Coupe/Roadster. In the M3, it made 333 hp. In the M Coupe/Roadster, it made 315 hp. The difference was in the redline because the M Coupe/Roadster didn't have the same intake plumbing for free breathing.

acao

For my work, I hired an engine designer for a Big Three manufacturer for some consulting. He was the lead engineer for the modern version of a certain famously branded engine that rhymes with shemi. I asked him about engine redlines, and the e-mail thread with his response is below [read from the bottom up]:

[Thing 1],

There is no general rule that establishes the over-speed capabilities of an engine, or which components will fail first. In a perfect world all the components would have the same over-speed capability. Often, there are historical reasons why one component might be the “canary”. I’ve seen where the crankshaft and bearings are overdesigned and the valvetrain is the limit. I’ve also seen where the valvetrain is more capable than the bearings. 20 years ago the engine would start to lose power (Valvetrain going out of control) and it was obvious that things were being pushed too far. Today’s hardware is designed to a higher requirements and the power won’t drop off like it used to.

The rpm limits that are put into the engine calibration is generally 200 to 400 hp above the max “shift speed” of the transmission. The engines are designed to sustain that condition for many hours. I’m not aware of OEM’s testing much above those speeds since they are regulated by the engine controller, and therefore are not part of the durability schedules. Going above these speeds is uncharted territory; an engine might be able to run above the max rpm for a while, but then again, it might not! My recommendation to anyone attempting this is to make sure they have new, high viscosity (40 weight) oil in the engine, and keep it cool ( below 280F) it will go a long way towards protecting the bearings.

Regards,

[Thing 2]


From: [Thing 1]
To: [acao]
Subject: Re: Redline

It's complicated. It could be a function of the valvetrain or the cranktrain (i.e., crankshaft, con-rods, pistons). From a valvetrain perspective, the valve springs must have enough force to close the valves. The faster you spin the engine, the more spring force you need to overcome the inertia of the moving valve. If you spin too fast, you can get what is (or was) called valve float. Generally, it won't make the engine go "kaboom", but it's still not good. On the crankshaft side, there's a lot of mass accelerating and the bearings need to be sized to react it. I remember looking as the relative contribution of accelerating mass and cylinder pressure on main bearing loads a long time ago. It turned out the the loads from combustion pressure dominated at low engine speeds, while the loads accelerating the masses (pistons, con-rods and crank arms) dominated at higher engine speeds.

Of course, this is the long winded version of "I don't really know". I don't remember from my engine design days. I'll have to ask a friend still in the business...

[Thing 1]

[acao] wrote:

On engines for consumer automobiles, what's the typical safety margin for determining redline and rev limiter?
If a manufacturer sets redline at 7k, and limiter at 8k (or 7.5k if that's more realistic), what is the number that makes the engine go kaboom?

Adam@Autometrics

Ignoring tactical situations (which would be an endless discussion), the thrust chart like Bill Verburg shows above tells the tale. Upshift either at maximum rpm or when thrust in the higher gear equals (or exceeds) thrust in the current gear.

Generally the engine builder (dependent on components, limitations, and desired life - higher is universally better) determines maximum rpm (with a rev limiter), and shift lights are tuned for maximum acceleration. A car without shift lights (street car) will use a "redline" (that is necessarily below maximum rpm) as an indicator of a good shift point.

2BWise

Why the notes to not shift at redline (7k)? Looks to me that you'd want to run each gear to 7k, except 5th and 6th which look like you could argue it either way.

SeanR

IBKFUTT

In Before Kibort Fcks Up The Thread.

Bill Verburg

You want to maximize thrust in the next gear and you want to stay in the light blue as much as possible and you need to know the rpm drops which for this trans are 2561, 1680, 1418, 1069, 963 when shifted at 7K

Veloce Raptor

Most of the time, it makes sense to upshift at the point of peak HP. Sometimes, however, it makes more sense to focus more on torque peak (and how flat the torque curve is), and shift accordinglyy. And sometimes, it makes sense, for vehicle stability & the ability to use more throttle position for more track real estate, to upshift short of peak HP (what some might call "short shifting")...

Mahler9th

I think all of the technical stuff on shift optimization is interesting, but again I doubt that a lot of rigor goes into the process of "deciding" for most DE drivers and amateur racers. Even those with big budgets and ongoing success.

Even those of us with custom gears probably don't go through the further process of figuring out the best shift points. I'd further guess that even folks that have to dyno their cars for class rules probably do not do this as a general rule.

Is fully or partially optimizing the gears relatively "low hanging fruit," given allowance by rules and favorable budget? I think historically yes in many instances. I know this to be true for cars like mine. Thank heavens for Guard Transmission.

Is going further to use dyno or data acq. to optimize shift points low hanging fruit? I don't think so for many of us.

As to the technical information about what manufacturers do regarding a design decision on red line, that is quite interesting, but not surprising. I would surmise that it would be even more interesting to know some of the other factors that are considered, like various business aspects of these design choices. As an engineer by training, I can see the value of considering all of the technical variables. But as a businessman, I can also see the possible influence of other factors involving marketing and projected service and parts revenues. It may not be as complex at GM or Porsche AG as I think it is... should be easy to find out.

If I were going head-to-head with another manufacturer for customers, non-technical aspects of that competition would factor into the final decisions I'd make on red line to drive toward design sign-off. For example if I was the guy with final sign-off on the Porsche GT3 or the Ferrari (pick the model). I wonder if the industrial design guys get input...

GTgears

What's the Kibortian term? Horsepower seconds?

winders

As scary as it sounds, "you know who" isn't making the term up. Look here:

http://www.convertunits.com/from/ter...epower+seconds

and here:

http://www.w8ji.com/rotating_mass_acceleration.htm

Scott

Mahler9th

Matt:

Made me laugh out loud. It was great to see you at Sears Point.

It was also great to see Mr. Kibort, who wheeled in driving a nice 928 street car.

Anyway, regarding shift points. I think of this concept like the sweet spot on a tennis racquet.. The sweet spot is bigger than the tennis ball. Inside the sweet spot is the fat part of the curve... outside, not so much!

Mahler9th

Hitting the sweet spot dead center yields only theoretical benefits. These benefits are "in the noise."

This is probably true to a lesser extent even in F1.

2BWise

Why maximize the next gear? Why not maximize overall? It appears to me that you have more force in each gear all the way to 7k than short shifting it. Ex: 2nd to 3rd. 2nd gear is over 1250 lbs from 3500 and up. 3rd gear is noticeably below 1250, so you will maximize your accel capability if you wind out 2nd gear than short shifting 3rd.

I've found that plotting this against speed, like your first plot, is the best way to go (at least for me because it makes it easier to visiualize). RPM on the track is more of an abstract, but with speed you have a concrete idea of and can optimize force for the entire range you will expect.

I've got the benefit of changing gears whenever I want (about 1.5 hr job) with the FC, and I've done the full on gear selection route with all the fancy math and plots to optimize gear selection. The problem is you can get gears so close in ratio that the end result is intellectual masturbation. What works in theory or math doesn't necessarily shake out on track. If you've got one box and you can't easily swap gears then its probably a worthwhile exercise to get a really close baseline.

The problem is this:
Some times after making what looks like the theoretical optimum is thrown out the window because the range suddenly put me right inbetween gears at the apex of the corner. Now, the whole setup is compromised because I'm too high in 1st gear and too low in 2nd. This is where I've found that the on track subjective is worth as much as the math. I decide to swap in a taller 1st gear, which by the math is not quick, but the speed and rpm shift up actually gives me an improvement in accel at the apex of this one corner, which in turn matches 3 other corners on the track.

This leads to an interesting anecdote that I've yet to conclude. At my home track the FC track record was set by a guy that claims his setup was compromised by only running 3 gears instead of all 4 (was geared for another track and didn't decide to swap back). I've raced this track with what I mathematically found to be the optimum 4 gear setup. Run competitively with it and it is nearly bang on. I ran a test day once where my gearing was setup for another track and it left me using only 3 gears. What I found subjectively was the car was much easier to drive and still as fast. Following that I threw the 3 gears in my optimization script and found that it has the potential to be even quicker. In the spring I'll get the chance to test this theory.

The point: just drive it like you stole it. You can run thru lots of math to try and optimize gearing and shift points, but until you get the subjective and objective data from the track it doesn't mean anything. Until you have the hard data (ProCoach's suggestion with Ax data) from the track to validate your shift point as optimal then the theory really doesn't matter.

mark kibort

No, this is entirely incorrect. MOST of the time, it pays to shift at near redline, not near HP . this i can prove 9 times out of ten on the 10 engine dynos you can show me. by the way, its simple math that can show the answer. flat torque curves and peak torque curves mean realtively nothing as well.
you are correct, that sometime max acceleration is not required or needed at tactiical points on the track as it might upset the car or create more effort to make such a shift. But, if we are talking about straight line acceleration, it always pays to flog the engine well past max HP.

Better go after Jim Watt for the responsibility of that term or most any higher ed physics book! HP-seconds ------ def: a unit measure of Work.

Thanks Scott. Yes, exactly!

mark kibort

This is why after all these years, i still always say, just look at the HP curves!
they tell all you need to know. after all acceleration=power/(mass x velocity) . this means acceleration is proportional to power! (as long as the mass is the same, as in two same cars, and the speed you are comparing is also the same. The later would make no sense if you were not)

So, take your hp curve, find your RPM drop percentage after a shift and maximize your HP to maximize the HP-seconds. (area under the HP curve in otherwords).

Power tells more of the story, but you can do it the more complicated way and look at gears, tire diameter, efficiencies, torque curves, etc etc. you will get the exact same answser in the end, but it will take you a little longer to get it.

Power and acceleration are inextricably tied together, in the same way that power is tied with RPM and torque.