Popping Off: Breaking the Two-Car Draft by Heating up the Engines

Any closed vessel that is subjected to high temperature will experience increasing pressure.  When that pressure gets high enough, we change from calling it a pressure vessel to a bomb because if(when) it explodes, the vessel itself becomes a collection of high-speed projectiles.  For safety, we don’t heat closed containers if there’s a chance they will reach high enough pressure for them to explode.  A pressure cooker, for example, has a relief valve that at one time was as simple as a rubber stopper tightly fitted into the lid.  The rubber stopper fit in the hole securely enough to handle up to some cutoff pressure, then popped out when that pressure was exceeded.  (This is not an ideal safety mechanism because the flying stopper can injure someone, as can the blast of steam that dislodged the stopper.)

A more practical version is a valve that automatically opens when the pressure exceeds some cut-off value.  The open valve allows excess steam (and sometimes water) to escape.  As soon as the pressure is below the cut off, the valve closes again.  In addition to being safer, it eliminate the time-wasting step of looking for the stopper.

The cooling system on a car is a prime example of a closed system that is heated to high temperature.  Water is pumped through holes in the engine block, where it collects heat.  The now-hot water moves out of the engine and into the radiator, where the heat is transferred from the water to air surrounding the radiator.  The cooled water returns to the engine to pick up more heat.  A Sisyphusian task, indeed.

A radiator is a twist of metal tubing onto which is fastened thousands and thousands of fins that help cool the water that circulates through it.   A typical stock car radiator (like the one at left) might have 20 fins per inch (compared to 10 fins per inch on a typical car radiator).  The more fins per inch, the more surface area available for exchanging heat between the radiator water and the outside air; however, air has to pass through the radiator, so if there are too many fpi, the air flow is decreased and that lessens the cooling.

The water can only carry away so much heat on each trip, so the water temperature gets hotter and hotter as long as the engine keeps producing heat.  The water increases in pressure as the temperature increases.  (See Equation, Clapyron for more on that.)   Water, of course, boils at 212 degrees Fahrenheit, and that would seem to set a limit on how hot you can run an engine; however, there’s a caveat.  Water boils at 212 F only at atmospheric pressure.  As the graph below shows, the higher the pressure, the hotter the water can get before it boils.  Atmospheric pressure is right about 14.7 psi, and that’s where the 212 degree Fahrenheit number applies.  But if you can get the pressure of the system up to about 45-48 psi, the water won’t boil until 275-280 F.  If you can maintain a high pressure in your radiator, you can prevent the water from turning into steam.  Water is much better at carrying away heat than steam is.  Water also flows much better.  Most radiators have a pop-off valve that blows when the pressure gets too high.  A typical radiator cap on a car would be about 15 psi, which actually means 15 psi above atmosphere.  Atmosphere is 14.7 psi, so you’re looking at about 29.7 psi in absolute terms.  This is why your radiator cap has all of those warnings about not removing it while the car is hot:  when the system is vented (opened to atmosphere), the super-hot water will turn into super-hot steam and gush from the opening.

A pop-off valve serves as a ‘weak link’: it has to blow before anything else in the system blows.  Most radiator caps on passenger cars are spring loaded:  When the pressure gets too high, the cap lifts off its seat, opening the system and allowing the hot water to escape into a reservoir.  As soon as the pressure is back down, the radiator cap goes back to being closed.

In a NASCAR car, the pop-off valves open and route the escaping steam and/or water through a tube that passes up near the right-hand side of the car’s windshield.  When you see a car “pushing water”, the maximum pressure has been exceeded and the pop-off valve opened.

For the last couple of years, most of the top NASCAR engine shops have focused on strengthening radiators.  It’s not difficult to get a pop-off valve set to 100 psi.  The problem is that if the pop-off valve isn’t the weak leak in the system, something else breaks.  It’s much more expensive to replace a radiator than a valve – so the size of the pop-off valve is really limited by the strength of the radiator.   A stronger radiator allows a higher pressure to be maintained.  Tim Brewer said that teams were pressurizing their systems to 80 psi (which would be 94.7 psi on my graph were it to extend to the right.)

Two-car drafting produces very high speeds, and that makes NASCAR nervous.  Cutting down the restrictor plate (which they did today) slows down the cars, but NASCAR doesn’t want to change the plate more that 1/64th of an inch or two because the change in plate size significantly affects how air enters the engine.  Teams have been designing engines around a particular plate size, although you would think that by now, they’d know to test not only the announced size, but plates one or two sizes up and down.

The limiting factor on how long two cars can stay in a draft is temperature.  The air intake of the trailing car is blocked when it is drafting, and the water temperature increases.  Two cars could go twenty laps or more before they had to separate.  NASCAR’s plan to limit the two-car draft started with a mandated pop-off valve.  NASCAR requires all teams to use a 33-psi pop-off valve, which corresponds to (33+14.7=47.7 psi) in my graph above.  All the work teams did to manage an 80 psi pressurized system is now out the window.  They also decreased the size of the opening through which air enters the car to cool the engine.  Less air reaching the radiator means less heat transferred from the water and a warmer engine.

Now if someone only could come up with a pop-off valve for drivers…


EXTRA:  Wondering about the different between a tapered spacer and a restrictor plate?  Check out this video, which illustrates very visually how a fluid flows differently through an orifice (the plate) and a nozzle (the spacer).  They’re using both on the Nationwide cars now.  The way the air enters the engine really makes a difference in the combustion dynamics.  Making a smaller spacer would have created too big a perturbation.  The holes in the new space are actually larger, but the plate will help decrease the overall flow.


  1. I’m curious as to why the rear cars are offsetting to the right, when offsetting to the left would let the rear driver see what is happening ahead of them and keep the radiator in cooler air, since the exhaust on these cars is on the right. I know that all those drivers and crew chiefs are smarter than I am, so I must be missing something.

  2. I’m kind of curious, the reason for two car drafts was mostly stated as being a result of the smoother surface at Daytona. Now we’ve had a race at Talledaga as well and the two car draft happened there as well. Talledaga was not repaved last year so does that mean the change to two car draft is due to something else?

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