stickypatch

I've never raced or tracked a car (but I certainly intend to soon) and I hope this is the right place to ask this question:

I understand that most racecar drivers change gear at red line - yet I also understand that the torque drops off after a certain rev point. Isn't there better acceleration with greater torque?

winders

Oh no........



Scott

acao

Nowadays, redline is a bit below the engine's mechanical limit. Instead, redline is often where the torque curve drops off. In other words, redline for modern engines is often set at a point where a car could conceivably spin faster, but there's no reason to because there's no additional torque. No additional torque but additional wear means that's a good place to paint the red line. This is simplified, because there are other considerations including keeping it alive during the warranty period, durability tables, fuel economy expectations, and bragging rights.

Linking back to your question, performance drivers go to redline before shifting because that's where peak power is. Torque indeed drops off after a certain point, and that point is usually after redline.

There are still yet other factors informing the shift at redline (or at another time), including the fact that any time spent in between gears is essentially time and distance coasting. If a driver can shift in 0.20 seconds, reducing shifts can mean real time when multiplied over multiple shifts and multiple laps.

But, road courses aren't drag strips, and a track design may indicate that short shifting is the fastest way through a segment if the normal upshift point falls at a sub-optimal location on the track.

Evan Fullerton

Simple answer is you want the highest wheel torque for any given vehicle speed for best acceleration. Even if torque is decreasing at the crank, it can still be higher at the rear wheel in a lower gear due to the gear ratio multiplication. Most stock cars make enough torque up near redline and have wide enough gear ratios that reving all the way out is generally faster but it doesn't always hold true. To find ideal shift points you would need a dyno sheet and gear ratios.

IcemanG17

Paging Mark Kibort...please start typing ASAP

IcemanG17

On a racetrack there are other factors in play vs just going for peak power shifts.....for example you might have a slow corner, followed immediately by other corners (S turns)...so lets say 2nd gear is optimal, but it won't last through all the S turns....so you must shift it short, or past redline to shift in a mostly straight part of the track....a better option is to choose 1 gear higher that can accelerate cleanly through the entire section....remember shifting DOES slow the car down...so you must have enough time in the lower gear to make it worthwhile....

Shift points are also extremely dependant on the cars power curve.....many older V8's might redline at 5500rpm, but make peak power at say 4800 and fall off a cliff power wise after that....vs others that pull hard all the way to redline....

another HUGE HUGE HUGE factor.....since you haven't started driving on track yet....seriously the LEAST important pedal for beginners is the throttle.....don't worry about peak power shifting for your 1st several track days....just learn the line and work on proper threshold braking is more than enough to keep you busy out there!!!! I have LOTS of laps around my home track in the past 5 years.....I like to think I know the line, and am pretty good at feeling what the car is doing.....but I still need to work on being better at "feeling" what the brakes are doing....

stickypatch

Thanks for the replies. I'm getting a better picture now.

I had the impression (somehow) that the redline was a number set by pure engineering mathematics. ie: that it represented the maximum feet-per-second the pistons could go given balance and bearing clearances etc, and that it's specifically tied to the stroke length.

Jake951

Maximizing wheel torque (not crankshaft torque), which maximizes acceleration, is EXACTLY the same as maximizing wheel horsepower. Maximum horsepower usually, but not always, occurs somewhere near redline. That's why you shift near or at redline.

aj986s

Shifting will depend a great deal on the course design, and where redline will be approached. As mentioned, some places are "safer" to shift than others. And you need to assess the benefit of a lower gear + an extra shift, versus smoothness & efficient pace in the higher gear. Also remember when you upshift, your RPM's drop. The goal is to keep the RPM high enough to maintain good power/torque. IIRC, HP & Torque graphs all cross around 5,250 RPM. My rule of thumb is try and keep at least 4,500 RPM on the engine. If a section requires a shifting compromise, and a shift point drops me below 4,500 or so, then I'll think more about maintaining the lower gear, and finding a later point to upshift.

This is also where gear ratios become important. Many street cars have gear ratios designed to optimize realtime driving and fuel economy. Sometimes spacing (often gears 2-3) results in a big drop in RPM, in order to achieve good highway mileage. Purpose built race cars have gears ratios spaced closer together, and often equally spaced, so that each gear change results in nearly the same RPM change. And some race cars/teams will even have different gear sets for different tracks, in order to optimize equal spacing of RPM changes with expected top speed at the particular track.

ProCoach

^^Some of us are lucky enough to do this^^

For objective measure, THIS is a worthwhile benefit of data acquisition, whether you have a stock or altered gearbox, a stock or altered engine.

There are simple ways to plot with engine RPM, speed and forward/aft g's to determine optimum shift points, which do vary, in relation to redline.

Chris Brown's book "Making Sense of Squiggly Lines" has the simplest and most succinct explanation on the easiest, data-supported way to optimize shift points.

2BWise

Shift when HP in gear A is equal to the HP in the next gear.

Bill Verburg

As mentioned as a general strategy you want to maximize wheel torque aka 'thrust', but there are tactiacal and situational reasons for altering the general strategy
This info can be derived from a dyno run or from data aquisition
here we see the thrust(wheel torque used to push the car forward) of a 993 Cup car in 3 different ways
thrust vs speed, red is engine blue is wheel torque


only looking at 2 gear sets


thrust vs rpm

Mahler9th

As may have been mentioned, the term "red line" may need to be first defined before the questions can be truly answered.

To me, the term often refers to an rpm level usually identified on a tach in a production car. Like a 911. Or a 997. Or a 991. The manufacturer chooses this rpm point based upon business decisions. Technical information informs the business decisions. But it may or may not drive the business decisions.

Now, for a car where instead the owner and/or builder has chosen a point for "red line," decisions may be based on a variety of variables. Race car or street car or in between, this point wasn't chosen by a production car manufacturer. Can be a professional race team, or a teenage Spec Miata driver. I'd call these decisions that are based on economics. Sure, technical information informs them as well.

In an extreme case, this latter type of decision may be based on extracting the ultimate performance for a given situation. In a situation where money is no object. Like...

An engine in the current Red Bull racing car driven by Vettel. Let us say we know it has 20 hours of unrestricted use left. The final race will use only 5 hours. So we want max performance... for this track, with this set of gears, with this diff, with this strategy... everything optimized. Maybe they use a supercomputer.

Now Vettel may not shift at this "red line" every time he has the opportunity. Because his "supercomputer" brain is adjusting to variables on the fly. He wants to win the race.

Now in more practical terms, many of us amateur racers have "red line" points chosen based upon fairly simple variables. Known engine configurations, gearing, and such. All with economics as the overarching concern.

In terms of "where to shift," which may really be the focus of these questions, as suggested by various previous responses there are a lot of variables. What I have learned in my 25 years of DE driving and racing is that the best place to begin is the beginning... at an actual event, with an actual qualified instructor. With an actual set of variables.

And in many places in the USA, if you have a Porsche, then a PCA event is your best place to start.

In terms of optimizing the "shift points" for a given car, of course data acq or dyno work can lead to answers. I seriously doubt whether a lot of amateur racers go down this path with any level of rigor. Why? Probably cost/benefit. This despite the fact that chassis dyno machines are popping up everywhere and data acq, is so much more reasonably priced. The cost is probably time/opportunity cost. Working out, dinner with the wife and kids, a few extra hours at the office, having a beer with the folks at the track may all be given higher priority in terms of time spent. Heck even checking out Rennlist forums.

Regarding the use of data acq. on this, I wonder when someone will come up with a simple way of accessing these tools toward this type of goal. I see Peter has mentioned at least one reference that may achieve this... but I am not too pysched about squiggly lines.

I would want it to be simple enough for me to discuss with an intermediate student with a street car. And I want her to be able to gain enough lap time benefit for the discussion to be worthwhile.

May be impossible. We will see.

analogmike

Not true sometimes, as in the '73 911 T E and S. All used the same crankshaft and rods so they should all be safe at the same RPM. But the cam and piston differences makes the T run out of breath much earlier than the S, and make better power down low, so they gave it a lower redline in order to maximize acceleration and prevent needless wear. Also the T could not reach 7000 RPM in top gear so it didn't need a redline that high.

Mahler9th

It might be interesting to hear more about how a manufacturer chooses a red line for a given car. For example the 991. Maybe someone can find out from Porsche AG. And it would be interesting to learn how the the variables and/or process has changed there, since say 1964 or so.

Similarly, it would be interesting to hear how a pro race team chooses redline and/or shift points. It is not impossible that the former is driven by mfg requirements. For example, I would not be surprised if the Flying Lizards had to stick with the RSR factory red line as a requirement in their support relationship with PMNA/PAG. I suppose I could just ask them. I think the discussion about shift points would be far more interesting. Perhaps over a beer or glass of wine at Sears Point some time.