Do Some Manufacturers Have An Engine Advantage at Plate Tracks?

Elliott Sadler, in response to the question Dodge seems to be struggling a little more than the other makes this year. Is there anything that NASCAR should do, or can do, to help the Dodge teams out? said:

Just help us a little bit on restrictor plate races. The gears we have to run in restrictor plate races is not really for the Dodges; it’s really just for the Toyotas and the Chevrolets, so I really wish they would help us a little bit [with that]. Dodge is doing everything they can do; we’ve got a new engine coming [soon], so we’re just trying to do all we can.”

Jeff Burton also thinks that the playing field isn’t level at plate races. Not surprisingly, he disagrees with Elliott about exactly which manufacturer is at a disadvantage.

We’re closer today than we have ever been…I still believe, at restrictor-plate races, that Chevy and Ford are not playing on the same level (as Dodge and Toyota), but I believe NASCAR has done a tremendous amount to make it fair.”

When I sent my engine guru the Elliott Sadler quote and asked if he’d mind answering a couple of questions, Earnhardt Childress Racing Engine Technical Director Dr. Andy Randolph emailed back in a flash:

“Remind me who won Daytona again?”

True, a Dodge did win the Daytona 500; however, you can’t make a claim based on one data point. I plotted the race results for the Daytona 500 by manufacturer. A vertical bar indicates that the driver finishing in that position drove for that manufacturer. My analysis seems to confirm Jeff Burton’s comments that Chevy and Ford performed significantly worse than Dodge and Toyota.

Not only did a Dodge win the Daytona 500, the order of finish from 2^nd to 10^th was Dodge, Toyota, Toyota, Dodge, Dodge, Dodge, Dodge, Chevy, Ford. (And the sixth place Dodge was none other than Elliott Sadler.) So Elliott’s suggestion that the gear is designed for Chevy and Toyota isn’t 100% correct if you look at this year’s Daytona race results.

Note added 7/5/08: Elliott reminded me that a drivers’ ability to draft plays a much larger role in the results than the engine. He feels other cars would rather draft with a Toyota or a Chevy than with a Dodge because the Dodges are slower. So the conclusion, based on the finishes (and this is me, not Elliott concluding this) is that perhaps the Dodge drivers are good enough drafters that they overcome the engine issues. What I heard in Elliott’s voice was ‘Sixth is good, but if I had a stronger plate engine, imagine where I could finish.’

Let’s look at Elliott’s gear argument. A couple years ago, engine speeds were on the rise. You need very sturdy engine parts to stand up to high revolution rates. Sturdy generally means heavy, so teams would like to use metallic alloys that combine high strength and light weight. Those alloys (many of which were developed for the aerospace industry) are typically very expensive, which runs counter to NASCAR’s cost-limiting goals. NASCAR could have made an engine speed rule, but that would be complicated and tedious to enforce. Instead, they made a rear-end gear rule.

The wheels of a car going 180 mph rotate at about 2180 rpm while the engine is rotating at 9500 rpm. Two sets of gears are involved in stepping down the rotation rate between the engine and the wheels: the transmission and the rear-end gear. Gears always come in pairs, with the smaller gear having fewer teeth than the larger gear. The smaller gear thus has to make more rotations to keep up with a larger gear. If the ratio of teeth on the gear is, say two-to-one, then the smaller gear makes two rotations for each one rotation of the larger gear, which means that the larger gear rotates slower than the smaller gear. The transmission gears are limited and NASCAR mandates two and sometimes three rear-end gear values for each race. The rear-end gear thus determines the rpm range where the peaks in your torque and horsepower curves should be. That’s how NASCAR limited engine speed.

The challenge for the teams is to optimize their engines for the rpm range dictated by the rear-end gear NASCAR mandates. Every team is subject to the same gear rule, so why would the gear be “not really for the Dodges”?

In my last post, I talked about the importance of the shapes of the horsepower and torque curves. The engine department’s job is tweaking the engine to change the shapes of those curves. A longer camshaft, for example, will emphasize power and a shorter camshaft will emphasize torque. The factor that I found most interesting was the effect of the intake manifold, which is shown below.

The carburetor (and the restrictor plate) sit on the square inlet. Each one of the eight legs (called runners) goes to one of the cylinders. The intake manifold is responsible for getting the air to the cylinder where it combusts with the gasoline and produces energy. The more air that gets into the cylinder, the more gasoline it can be combined with to make energy. Not only is the amount of air important, but the way the air flows into the cylinder is also critical. Teams spend a lot of time modeling the airflow, as details such as the speed and degree of turbulence entering the cylinder affects combustion efficiency.

Air doesn’t flow continuously into the engine. The intake valve for the cylinder opens and closes, so air actually comes into the cylinder in pulses. You can model the runner and intake valve (with apologies to Andy) by a paper-towel tube and your hand. Place your palm over the end of the tube and aim a hair dryer down the tube to simulate the airflow. (Don’t leave the hot hair dryer in contact with the paper towel roll for too long.) If you place your palm over the end of the tube to mimic a closed intake valve, you’ll feel air build up at your palm.

When air hits the closed intake valve, it doesn’t just sit there waiting for the valve to open again. It reflects back toward the plenum (the region of the manifold where the eight runners come together). The air bounces around the plenum and then back down the runner. This process sets up a standing pressure wave, two examples of which I’ve shown below.

Regions of high pressure are indicated by the darkest color, and regions of low pressure by the lightest. The pressure wave ranges from about 10 psi at its minimum to 19 psi at its maximum. I’ve modeled the runner as a straight tube for simplicity, so that you can see two different possibilities. In the top situation, the highest pressure occurs at the intake valve, which means you get maximum air into the engine when the intake valve opens so that it is synchronized with these regions of high pressure. The bottom picture shows a less desirable situation, in which there is a low-pressure region at the intake valve. This allows less air into the cylinder and consequently generates less horsepower.

Where the pressure maximum occurs depends on the length of the runner, just as notes coming out of a wind instrument depend on the length of the tubing. Long, thin runners tend to give you low-end torque, while short, fat runners give you more power at high rpm. The timing of the intake valve opening changes with engine speed; however, the runner length is fixed. The pressure maximum will coincide exactly with the opening of the intake valve at only one engine speed. Andy says that you can get about a significant increase in airflow just by getting the geometry right so that the pressure maximum is coincident with the intake valve opening.

Getting the geometry right is trickier than it seems. I’ve modeled the runner as a straight tube. If you grab a physics book and look up “tube with both ends closed” in the waves section, you’ll find an equation that tells you how long the tube has to be to get the pressure maxima at the ends of the tube. If you look at the picture of the real manifold, you’ll see that the runners are nowhere near straight tubes and I haven’t even considered the effect of the plenum volume (which cannot be assumed to be spherical!). You’ll also notice that, since the cylinders are arranged in two lines of four, the runners are different lengths, which means that not all the runners are optimized at the same engine speed.

Each team can choose from four different manifolds: Two are supplied by the manufacturer and two by Edelbrock. All four must be approved by NASCAR. Teams are allowed to make some modifications – you might, for example, want to change the geometry of some of the runners to try to even out differences due to the differing lengths. When I first interviewed Andy for the book, he was working at Bill Davis Racing. The day I visited, they were cutting manifolds in half, modifying their insides, and then welding them back together for testing. Andy hasn’t worked with Dodge engines, but he says it is entirely possible that the Dodge manifolds used for plate tracks don’t allow them the flexibility to tune in the engine rotation range that is necessitated by the mandated rear-end gear.

So the gear choice to which Elliott referred can have differential impact on different engines; however, given Dodge’s performance in the Daytona 500, it’s a little hard to make the case that there’s an inherent design issue. Out of curiosity, I looked up the results for Talladega. Toyota, Dodge, Toyota, Ford, Toyota, Ford, Chevy, Dodge, Chevy, Chevy. Andy tells me that the big difference between Daytona and Talladega is that there is a handling component at Daytona that is absent at Talladega, so Talladega is much more dependent on a driver’s ability to draft. Andy suggested that qualifying might be a better test of the engine per se because there isn’t any drafting; however, since Daytona is an impound race, you would have to take into account that the cars locked into the race might not make as many compromises to qualify well. We’ll see if this week’s race produces similar results to the February race in terms of where the different manufacturers finish.