Oct 052012

Every return to a restrictor plate track brings suggestions about how we might eliminate the restrictor plate.  Restrictor plates serve the very necessary function of limiting car speeds at Daytona and Talladega so that the cars stay on the ground.  The negative is that they remove throttle response.   One suggestion from some readers that I hadn’t heard of before suggested that NASCAR could just change the rear-end gearing parameters to shift the power curve and reduce horsepower that way. Will that work?

The amount of horsepower an engine make depends on the rotation rate of the engine.  The faster the engine runs, the more combustion events and the more power generated.  This works up to a point, because if you rotate the engine really fast, you start having problems getting enough air into the engine and the power goes down.  The graph below is for a typical unrestricted engine that makes its maximum horsepower around 9300 rpm.

In order to cut the horsepower back to what you’d need to run safely on a plate track, you would need to restrict the engine to run at about 450 hp – which would mean the engine would have to rotate at about 4500 rpm.

Looking at the curve above, it’s evident that the engine is designed to run at its peak horsepower.  What dictates that curve?  Cylinder displacement, engine configuration, head configuration, etc.  But mostly  NASCAR determines the curve because of the rear end gear rules.

NASCAR engines run up to about 10,000 rpm (revolutions per minute).  Rpm is a unit that measures how fast something rotates. It’s like miles per hour, but miles per hour corresponds to a linear motion rather than a rotational motion. The minute hand on your clock, for example, makes one revolution every hour. The seconds hand makes one revolution per minute.

The circumference of a typical tires is around 88 inches.   Every time the tire rotates once, the car moves 88 inches, so 1 tire rpm = 88 inches per minute.  You can convert this into miles per hour.  Since I chose a nice round number like 88  inches for the tire circumference, it works out to a really simple equivalence:  1 tire rpm = 1/12th of a mile per hour.  This means that if you want to go 200 mph, the tires have to rotate at 2400 revolutions per minute.

The power curve above shows that the engine makes the most horsepower at 9300 rpm.  This produces a problem:  the engine is driving the car at 9300 rpm and the wheels are rotating at 2400 rpm.  That’s why you have a transmission and a rear-end gear, as illustrated at right.  The diagram shows the gear ratios for a Borg Warner MM6 manual transmission and a GU6 3.42 rear-end gear, as might be found in a Corvette.  Note that NASCAR cars are not allowed to use any gear that increase the rotation rate between the engine and the wheels.  No 5th or 6th gear, either.  1:1 is the best you are allowed — which means that the rotation rate coming out of the transmission is the same as the engine rotation rate.

At maximum speed, the transmission is using a 1:1 gear, so the only reduction occurs at the rear end gear.  A 4:1 gear means that one gear makes four rotations for every one rotation the other gear makes.  If the engine is rotating at 10,000 rpm, and it passes through a 4:1 rear-end gear, you have 2500 rpm at the tires (which is 208 mph).

The whole point of this discussion is to keep the cars at lower speeds so they stay on the ground.  Let’s say you want to limit the cars to 190 mph – that requires the wheel rotate at 2280 rpm.  We don’t want more than 450 hp, so the rear-end gear has to take the rotation rate from 4500 rpm to 2280 rpm, which means 4500/2280 = 1.97, so you need essentially a 2:1 rear-end gear.  (Just for comparison, a typical rear-end gear is 3.3-3.9, depending on the track.)

So it is possible to gear the car down so that it simply doesn’t produce as much horsepower.  It is a better solution than what we currently do?

Restrictor plates work by reducing the air coming into the engine, which means you can give the engine less gas and thus you produce less horsepower.  Gearing down would reducing the horsepower by making the engine run in a much-less efficient range.

One consequence of a lower rpm is that you would have really back problems with knocking.  Knocking happens when the air-fuel mixture auto-ignites (ignites before the spark plug fires).  Knocking is much more likely at low engine speeds because the the combustion happens so much more slowly than in a fast-running engine.

Another consequence is that my engine design friends tell me that they can probably tweak an engine, within the rules, to produce more horsepower at 4500 rpm such that NASCAR would have to further change the gear and run the engines at 3500-4000 rpm, which exacerbates the knocking problem.

I wondered whether taking the engine speed down might increase throttle response, but none of the experts I spoke with thought that it would.  The problem, they say, is that as long as cars are running at full out power, aerodynamics will dominate plate tracks.  You’d have to decrease downforce and increase drag to really make a difference.

Finally, there’s an aesthetics issue.  The sound of an engine changes with its frequency.  If you went to Indy and we blindfolded you and asked you tell us whether the car on the track was a NASCAR racecar or an Indy car, it would be easy:  Indy (and F1) cars sound like mosquitoes.  They run at about twice the speed of a NASCAR engine.  If you forced a NASCAR engine to race at 3500-4500 rpm, the car would sound like it was in pain – you’d get a low moan instead of the engine sound associated with 9000 rpm that we’ve all come to know and love.

If you were paying attention, you ought to be wondering why you couldn’t just run at very HIGH rpm – the power curve goes down on each side of the peak, so you could have the engine run at 13,000-14,000 rpm and output 450 horsepower.  The problem on that side is that NASCAR initiated the gear rule so that teams wouldn’t have to deal with the incredible stress on engine parts that have to run at those very high speeds.  High-speed engines would significantly increase the cost to teams – it would be cheaper in the end to just let them build a dedicated open (not restricted) plate track engine.

In conclusion, yes – gearing down would work in theory, but it would introduce its own unique problems that would offset the advantage.

Jun 092012

NASCAR engines like to run at about 8000-9500 rpm (revolutions per minute); however, the tires on the car rotate around 2400 rpm at 200 mph.  The gearing in the transmission and the rear end gear reduce the rotational engine speed, with different gears providing different reductions.  When you talk about the size of a gear, you’re actually talking about the relative sizes of a pair of gears.  The gear on the left in the diagram has 20 teeth, while the gear on the right has 10 teeth, so this gear would be a 2:1, meaning that the smaller gear rotates twice every time the larger gear rotates once.

If the engine is running at 9000 rpm and goes through a 3.0 gear, the result is a 3000 rpm revolution.  Pretty straightforward calculation.  A car is just a little more complicated because there are two sets of gears. All that means is that you multiply the two gears together to find the effective reduction in speed.  In the diagram above, if you were in first gear, 9000 rpm coming from the engine would be 9000/(2.66*3.80) = 890 rpm at the wheels.  Note that the fourth gear is the smallest it’s allowed to be in NASCAR:  there’s no change in rotational speed.  NASCAR doesn’t allow overdrive, which would be a number less than one.

There is actually no rule against shifting – but there are rules about which gears you can use in the transmission and rear end gear.  The third hear in the drawing that third gear is 1.14, which is the gear you use at Pocono.  At a place like Michigan, you’d use something more like a 1.30 gear.

It’s a seemingly small difference in numbers, but it makes a really big difference in how useful third gear is on the track.  Let’s say you’re running 8500 rpm in fourth gear.  That translates to 2237 rpm (186.4 mph) at the wheels.  If you have a 1.30 gear, running the same speed of 186.4 mph, the engine would have to turn at 11,050 rpm, which is well beyond the range in which engine builders start to get ulcers.  If you want to use third gear and keep the engine at 9000 rpm, then the maximum speed would be about 150 mph.

On the other hand, if you’re only running a 1.14 gear, running at 186.4 mph requires you to run at 9,690 rpm.  All of a sudden, third gear gets useful on the track.

Remember that horsepower and torque depend on the engine speed, as the graph at left shows.  There’s a sweet spot (a peak) where you get maximum horsepower or maximum torque.  (The position of that peak changes depending on how the engine is built and tuned.)  One way of changing the engine rpm is changing speed.  Another is changing gear.

Pocono features two very long (3000+ feet) straightaways and two very flat (6 and 8 degrees) corners.  This means that drivers pick up a huge amount of speed coming into turns 1 and 2.  Turn 1 (where drivers have been entering at 211+ mph) has 14 degrees of banking – but teams are going into the 8 degree banked turn 2 at 200 mph.  You can’t take the turn that fast, so there’s a lot of slowing down going on.  Instead of staying in fourth gear and letting the engine run slower, you can shift into third gear and keep the engine in its most favorable operating range.



Apr 262012

The defining characteristic of the Kansas race was the surprising number of engine problems.  Many of those problems can be attributed to the change in rear gear from a 3.89 to a 4.00.  At  190 mph at a track like Kansas, your wheels make 2270 revolutions per minute (rpm).  If you watch the telemetry on the television broadcast, you know that the engine is rotating around 9500-9900 rpm.  Since the engine is attached to the wheels, there has to be something to change the rotation rate between the engine and the gears.

Gearing Up

That something is the transmission and the rear gear.  As shown at right (with the values given for a Corvette ZR-1), the engine rotation passes through the transmission and then through the rear-end gear before reaching the wheels.  A 4.00 gear means that the ratio of rpm in to rpm out is 4.00:1.  It takes four revolutions of the input to produce one revolution on the output.  If you have something rotating at 8000 rpm and you add a 4.00 gear, then the rotation is reduced to 8,000 rpm/4.00 = 2,000 rpm.

Note that NASCAR does not allow 5th or 6th gears and does not allow overdrive (when the first number is smaller than the second).  The lowest gear you can have is 1:1 in NASCAR.

Let’s compare running at 190 mph with the two different gears.  Last year, a 3.89 gear was used. At 180 mph, you’d better be running in 4th gear (which means 1:1 and the speed coming into the rear end gear is the same as that coming from the engine.  The engine speed required to go 190 mph is this 3.89*2270 rpm = 8830 rpm.  This year, with a 4.00 gear, you’d need to be running at 9080.  If you’re running 200 mph, last year you needed 9293 rpm and this year it would be up around 9556 rpm.  You’re basically running 250 rpm (or so) higher this year than last year at the same speed.

Andy Randolph, Engine Technical Director at ECR Engines tells me that engines were running at 9800 rpm for sustained times.  Although the engine rotates that fast at some places, doing it continuously places huge stress on the mechanical parts – that’s why most of the failures were due to mechanical breakage.  (Because I know he’s too modest to mention it, I’ll point out that none of the engines that had problems at Kansas were from ECR.)

The Math

For those of you wondering about where my numbers come from, here’s a calculation I did for Las Vegas.  The only difference is the slight variation in tire circumference.  If you plug in the numbers to the formulas and don’t get what I got above, I probably screwed up on the calculator.

Left-side and right-side tires have difference circumferences.  The circumference of a left-side Vegas tire in 2008 was 87.4″, while the right-side tires had a circumference of 88.7″.

To calculate how many times the tires rotate each minute, I first convert the speed into inches per minutes.  I know to use those units because I’m trying to get an answer in revolutions per minute, so I need to convert hours to minutes. I also know that every time a right-side tire makes one complete rotation, it has traveled 88.7 inches, so I’m going to convert miles to inches because I know I will need that later. Convert 45 mph to inches:

45 mph corresponds to 47,520 inches per minute. Looking at the right-side tires (for no particular reason), the car travels 88.7 inches every time it makes one full rotation. The number of times the tires rotate each minute is 536 rpm, as shown below.




Jun 112011

The big news for Pocono is that drivers can shift…again.  Which brings up the obvious dual questions of: Why would you want to? and Why didn’t you before?

Compare how fast the wheels have to rotate with how fast the engine rotates.  Both are measured in revolutions (or rotations) per minute – rpms.  Assuming a tire circumference of 88.6 in, tires have to rotate from 417 rpm (at 35 mph), to 1490 rpm (125 mph) to 2146 rpm at 180 mph.  The graphic tachometer on television tells us that the engine runs between 7000 rpm and 9500 rpm most of the time.

Gearing for a Borg Warner MM6 manual transmission and a GU6 3.42 rear-end gear, as might be found in a Corvette.

You can’t connect the engine directly to the wheels because of the difference in rotation rates.  This is where the gears come in.  A car has two sets of gears:  The first I’ll talk about is the rear end gear, which I seem to remember is somewhere around 3.8 or 3.9 for Pocono.  The rear gear reduces the rotation rate coming from the driveshaft and sends that rotation to the wheels (as shown in the diagram).  A 4.0 gear would produce a rotation rate coming out of the gear that is 1/4th the rotation rate coming into the rear gear.  If the driveshaft is rotating at 5000 revolutions per minute (rpm), the wheels would be rotating at 1250 rpm. (A 4.0 gear would mean that for every four rotations coming in, one rotation goes out.)

With a 4.0 rear gear, your engine would have to change speed from 1600 rpm to about 8500 rpm going from 35 mph to 180 mph.  The problem is that an engine produces its maximum power over a narrow range of rpms.  (It also produces its maximum torque over a small range of rpms, although not the exact same range as the maximum power band.)  You’d like to have the engine operating in the target range all the time.

This is why you need a second set of gears, which are found in the transmission.  This series of gears (usually 4, 5, or 6 different gears) gives you different sizes so you can keep the engine running near its sweet spot — regardless of how fast you’re going.  Fourth gear on most transmissions is 1:1, meaning that there is no speed change through the transmission.  On a passenger car, like the one from the gearing figure, the higher gears (overdrive) reverse the ratio.  0.50:1 means that the rotational rate coming out is higher than the rotational rate going in.  NASCAR prohibits overdrive.

In trying to go faster and faster, teams were moving their engine’s target range to higher and higher rpms – which means higher and higher costs.  In 2005, NASCAR instituted a gear rule to keep engine speeds (and thus cost) down.  NASCAR gives you a limited choice of rear-end gears and dictates the transmission gears as well.  Those choices keep the maximum engine rotation rate below about 10,000 or 10,500 rpm without having to implement a difficult-to-enforce engine rule.

NASCAR changed the gear rule for Pocono this year.  First gear can be anything you want.  Second gear can be 1.70:1 or greater, and – this is the big change – the third gear limit changed from 1.28:1 to 1.14:1 or greater.  Fourth gear stays at 1.00:1.   (“or greater” means that the first number may be larger, but not smaller.)  NASCAR still doesn’t allow overdrive.  Normally, the rule book prohibits gears between 1.00:1 and 1.28:1 except for road course events.

Pocono - certainly one of the more unique tracks on the NASCAR circuit

Why Pocono?  Most oval tracks have four turns, with the frontstretch and backstretch close to the same length.  Pocono has three turns and three straightaways:  a frontstretch of 3740 feet, a backstretch (Long Pond) of 3,055 ft and a short straight of only 1,780 feet. You can imagine that the rpm the car reaches is very different coming down the two long straights (i.e. coming into turns 1 and 3) compared to coming down the shorter straight (i.e. into 2).  What you’d like is for the engine to be turning at about the same rpm into each turn.

It seems like NASCAR’s change is too small to be meangingful – from 1.28:1 to 1.14:1 is only 0.14, right?  Actually, it’s a factor of two.  What makes a difference is how much above 1.00 the gear is.  The important thing about moving from 1.14:1 to 1.28:1 is moving from 14 to 28.

For the sake of argument, let’s say the engine is ideally in 7200 rpm in fourth gear.  When you shift to third, a 1.28:1 gear (which used to be the lowest for third), requires the engine to run at 9216 rpm (=1.28*7200) to maintain the same speed.  That takes you far away from the best rpm range for your engine.  Changing from 1.28:1 to 1:14:1 means that third gear only requires your engine to run at 8208 rpm.  That may seem like it is still a big shift; however, given the way the power and torque curve vary with rpm, it’s small enough to mean that you’re close enough to your power band for it to work. It’s a shift of about 1000 rpm instead of 2000 rpm with the 1.28:1 gear.  That gives the engine shop – and the driver – some interesting options.

This type of a rules change is, in my opinion, exactly the direction NASCAR ought to be moving to open up areas for people to be innovative.  It’s a relatively minor change in terms of enforcement.  It keeps the teams from pushing into the higher rpm ranges (and thus steeply pushing up engine costs), but it allows the engineers and the drivers to pursue different strategies.  For example, most drivers will be shifting in turns 1 and 3, but others (like Denny Hamlin) plan to shift only in turn 1.  Another aspect is how shifting affects fuel mileage.  Overdrive gears are there because the more rotations an engine makes, the more friction it has to overcome.  And, as Carl Edwards points out, every time you shift, you run the chance of screwing up and damaging the transmission.  Most NASCAR drivers aren’t used to shifting this much during a race.  Do you try for what might be a small advantage and shift at the cost of possibly screwing up the transmission?  Do drivers like Marcos Ambrose, who have a lot more experience shifting, have an advantage?  Does the engine shop adapt different strategies for drivers who are comfortable shifting compared to those who are not?

Unfortunately, this rule really makes a difference only at Pocono due to it’s unique configuration.