Engine Issues at Talladega: Vapor Lock, Gas Cans and Oil Coolers

An usual number of teams “ran out of gas” or had engine troubles during the Talladega race.   The TV analysts had some ready answers for what might have caused these problems.  Their extemporaneous theories tend to elicit sighs from engine builders, who know that problems can rarely be diagnosed at the track – and even more rarely by someone who hasn’t looked at the car.

A wonderful aspect of blogging is that we’re not called to have answers on the spot like the television broadcasters and we have the leisure of time.  Let’s examine some of those theories.

The Gas Can

SPEED reported engine builders suggested that the teams weren’t getting a full tank of fuel into the car and that’s why they were running short.  This doesn’t really make sense given how fuel mileage is calculated.

Prior to putting the gas in the car, the gas can and gas are weighed.  After the fueling is complete, the gas can and any remaining gas are weighed.  The difference between those two weights is the weight of gas that actually got into the car.  This assumes that all of the gas missing from the can made it into the car.  If gas is spilled, it will affect the validity of the calculations.

Sunoco provides the teams with the density of the gasoline – how much one gallon of gas weighs.

You can calculate the volume of gas using this information.  For example, if the density of gas is 6.073 lbs/gallon and you find that you’ve put in 133 lbs worth of gas, the volume of gas that got into the tank.

(Yes, I know I’m using weight instead of mass, but as long as both the density is given in terms of weight/volume, the ‘g’s cancel and we’re OK.)

Teams know from this how much fuel they actually put into the car and they base their calculations (and what they tell the driver) on these numbers.  So even if they aren’t getting a ‘full tank’, the crew chief is well aware of it.  It doesn’t matter whether the lack of fuel is due to human error or a malfunction on the part of the gas can.  This is not a likely cause of the fuel problems we saw Sunday.

There are two places that this might be an issue:  First – if the engineer does the fuel mileage calculations incorrectly, you’re going to have the wrong prediction for the number of laps you can run.  The problem with this is that it is highly unlikely that multiple groups from different teams made the same mistake in the same direction.  The number and breadth of problems suggests something more systemic.

The second issue is that the density of fuel depends on temperature.  Fuel becomes less dense at higher temperatures, so putting in the same weight volume would mean less volume fewer molecules. (Thank you Barry!) It’s a little confusing because all of the calculations the team makes are done in terms of gallons, but that assumes a particular density. I’m checking to see whether teams take this into consideration.

Vapor Lock

Another theory proposed on the network broadcast (as a result of a crew chief comment, I believe) was “vapor lock”.

What is Vapor Lock?

Vapor lock happens when liquid fuel vaporizes (changes to a gas) prior to entering the combustion chamber.  The pumps in a fuel delivery system are designed to pump liquids, not gases.  The fuel pump cannot pump gas well, so the fuel pressure drops and fuel stops being delivered to the engine.  Since engines don’t run without fuel, the car ‘locks’.

How easily a fuel causes vapor lock depends on its vapor pressure:  the higher the vapor pressure, the more susceptible the fuel is to vapor lock.  (Although it’s not relevant to Talladega (elevation 596 feet), vapor pressure increases at high altitudes and this may also cause vapor lock at high altitudes, even when the car behaves fine at lower altitudes.)

Does EFI and/or Ethanol Cause Vapor Lock?

Vapor lock is LESS likely to happen with EFI than with carburetors.  The NASCAR carbureted system ran at low pressure and lacked a fuel pump in the fuel cell.  Those factors made it much easier for the engine to vapor lock.  The EFI system runs somewhere around 70 psi and has a fuel pump inside the fuel cell, which decreases the probability of vapor lock.

Vapor lock can happen within the engine (prior to the cylinder) or at the fuel pick-ups in the fuel cell.   The most likely place for vapor lock to be initiated would be at the fuel pick-ups because the fuel cell itself isn’t pressurized; however, the two engine builders I spoke to this morning both said that none of the data they have indicates that vapor lock was an issue in their cars.

Ethanol also makes it LESS likely that a car would experience vapor lock because ethanol has a lower vapor pressure than gasoline.  Ethanol-containing fuels are less likely to vapor lock than pure gasoline.

So What IS the Issue?

My sources suggest that high oil temperatures are causing the engine problems.  This problem is exacerbated by high outside temperatures and the reduced cooling inherent in the rules package that was implemented to prevent the two-car draft.

Two fluids help cool the engine:  water and oil.  Both are in turn cooled by the air coming in through the grille.  As the air flows in through the grille, it first encounters the radiator used for cooling the water circulating through the engine.  The air comes in at temperature Temp 1 and leaves at temperature Temp 2, where Temp2 is larger than Temp 1.  The air picks up some of the heat from the radiator and carries it away, which is why Temp 2 is larger than Temp 1.  (For more on this, see my blog on radiators)

Behind the water radiator is another cooler for the oil.  It also depends on cool air coming in through the grill.  The problem teams are having is that Temp 2 is so high that the air can’t cool the oil efficiently.  The problem is exacerbated because 1) the cooling air coming in (Temp 1) is hotter due to the outside temperature and 2) the air is warmer after passing through the water cooler because the engines are running hotter.

Thanks to the EFI data, teams can look at how the temperatures change in a much more detailed way than they could back when they relied on the driver relaying temperatures.  My engine guys report that they are seeing a difference of up to 50 °F between Temp 1 and Temp 2.  That difference is normally only 15-25 °F.  In addition, Temp 1 is higher to start with when the external temperature is high like it was at Talladega on Sunday.  (And like it no doubt will be in Daytona in July.)

Oil is a combination of different types of long-chain hydrocarbon molecules that unfortunately break down at high temperatures.  If you’ve ever heated oil on the stove above its smoke point, you’ve seen firsthand the decomposition of oil molecules due to high temperature.  The result is usually a gummy dark tar-like substance deposited on the pan surface.

The same thing happens with engine oil:  when it starts to decompose, it can’t lubricate the engine. An engine cannot run at peak power for very long without functional oil.

Yes, I did suggest that it would make sense to put the oil and water coolers in parallel instead of in series so that some of the cooler air would get to the oil cooler without having to pass through the water cooler first.  No dice – it’s been tried and deemed to be against NASCAR rules.



  1. I know you’re probably tired of comments like mine but I’m really appreciative of the analysis and giving us some specific numbers such as the temperature spread between the water and oil. It’s great to have someone on the “inside” with access to the engine builders. No idea why this info isn’t part of the TV broadcasts but even if it were, we’d still not get analysis like this. Great stuff Diandra!

  2. Hi Dennis: Nope, I never get tired of being told I’m doing something useful!

    I have a huge advantage over television in that I get to talk to the people who know after they’ve gotten to analyze their data. There’s no way of knowing these things in real time. The more subtle an effect is, the longer its going to take to understand it. Thanks!

  3. Diandra, you make a statement that I find confusing: “The second issue is that the density of fuel depends on temperature. Fuel becomes less dense at higher temperatures, so putting in the same weight would mean less volume. I’m checking to see whether teams take this into consideration.” You make it sound like the energy content of the fuel is a function of volume rather than mass. Can you clarify what you’re trying to say here?

    • Barry: Thanks for catching that. The key (as I’ve corrected it) is that the fundamental unit of energy in combustion is the number of molecules. Since that’s a hefty number (on the order of Avogadro’s number), you use more macroscopic variables. The next best thing to use would be mass because each molecule weighs the same regardless of whether it is part of a solid, liquid or gas. As soon as you go to volume, you’ve made an assumption about the density that may or may not be correct. Mass would be a more fundamental (and less confusing) quantity!

  4. I was puzzled during the race by the comment on TV about vapor lock. Thank yoiu for all of the information. I like knowing the “whys and where-fors”. I had been wondering if there was something basically wrong with either the new fuel or system.

  5. I wish that the TV “tech” commentators would spend their air time on things like this rather than telling me that a race tire is sticky and a shock absorber goes up and down.

  6. Enjoy your explanations. Have referred your site to every racer I know. After a couple of years you will have enough to do another book.
    PS: we’ve won the first two out of three.

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