Does running hotter than stock improve MPG?
 

I know that engines run most efficiently when they are fully warmed up. So my question is can running hotter than stock improve efficiency? I don't think that you would gain much from running hotter, but not sure. So, for example, if the stock coolant temp is 190, would running at 210 improve efficiency? If so, how much? Thanks.

Yes and no. They are designed to regulate their temperature so going too hot will cause the cooling fans to come on and lower your efficiency. Example: my car runs at 200f, if it rises to around 208 the fans will come on.

Also, if the fuel you are using starts to pre-ignite (pinging) your ecu will pull timing to reduce peak pressures thus reducing efficiency.

Yes, higher operating temps increase efficiency but you're unlikely to see gains because the vehicles today are already tuned well and operate warmer than with past generations.

Depending on the vehicle.
If you don't have anything working against you, then it should work.

I would say yes. I got an MPG boost on my Metro by swapping in a higher temp thermostat. I live in a cool climate, so there's nothing but gains to be had out of running warmer.

You could also try messing with your temp sensors...on older cars, at least, it can make them run leaner/more efficient by tricking them in to thinking they're running hotter. Probably not good for emissions, though.

Yes, running the engine warmer (all other things constant), will allow you to loose less heat into the coolant. This means more is going to go into pushing the piston.

That assumes the ignition timing is staying constant, and heating things up more in the cylinder tends to cause pre-ignition earlier than in a cooler engine. So, it really depends on the engine design and its ability to avoid pre-igntion, and how conservative/aggressive the ECU programming is.

Running hotter might lead to better results, but sure it all depends on how the engine would handle that overheating. Oil and fuel quality might also affect the results. Well, oil coking in the cylinder head is sure not worth the economy of running hotter.


NOx emissions do increase while running hotter. That's been the issue with FCA EcoDiesel engines during the DPF regen procedure, since the injection got leaner during that procedure in order to build-up enough heat to burn down the soot trapped into the DPF.

In the Honda Insight, they did pretty much every little thing they could to eke out every last bit of efficiency.

Every other Honda I've seen has an 85C thermostat stock. The Insight's is 91C.

I forget the numbers, but the Civic VX has a hotter thermostat than the other Civics. I have one in my car.

Considering that hotter thermostats are usually found in flexfuel and dedicated-ethanol vehicles, and that ethanol has a higher octane-equivalent rating than regular gasoline, maybe running hotter could lead to a more noticeable improvement in the efficiency with higher-grade gasoline in order to either prevent knocks or resort to an enrichment of the mixture to overcome them.

A hotter thermostat seems to have helped on my diesel Golf. I had struggled to consistently get 55mph on the highway and now I regularly see 60 mpg average at 60 mph. The Stant Superstat keeps 207 F when the stock thermostat held 195 F. You don't have to worry about preignition in a diesel. Ignition timing is controlled at the injectors.