Best of Deals Car Reviews Repair Shops Cars A-Z Radio Show

More horsepower after a rain storm?

My 1966 Ford Station wagon seemed to have more power right after a rain storm.
Did cylinder gasses cause moisture in the air to produce steam, creating more expansion and more horsepower?

If so, would it have made sense to aerosol distilled water into the intake?

Thank you.

No, it was simply greater air density that produced the extra power.
Robert, don’t tell me you’re considering these “water injector” add-ons that claim to make more power and increased mileage, are you? I assume you’ve been hanging around here, and I hope you’ve read too many of our threads on the subject, to actually consider this. IMHO you’re a good man, Robert, and I’d hate to see you mess up your Suburban with one of these scams.

Read all about water injection here:

Won’t do anything good for your car. As TSM says, don’t do it.

I know nothing of “water injection”.
Surprised that the barometric pressure increases enough to make a noticeable difference.
Distilled water would not have minerals which could deposit in the cylinders.

You live at elevation, Robert, iirc. Every time you drive you see how significantly air pressure affects engine power (except for turbo & supercharged engines, where the drop in performance is much less).

Climactic changes in air pressure are much less pronounced than those due to altitude, but probably enough to have a slight effect on power. The storm may have also cleared enough smog from the air that your car isn’t holding its nose. Oh, a 1966. It’s designed to inhale crud and like it.

I’ve read that station wagons of that era are a new area of collector interest, pure nostalgia for someone like me. Even though we only had Fords, I’d choose an Olds Vista Cruiser with the extra little windows for an improved view. I believe 1966 was the first year of the two-way doorgate tailgate that either folded down or swung to the side. We had just bought a 1965, but my father was so envious of those who had that feature. We had to wait until we got a '71.

Go for it Robert. Sounds like a great idea !

TSM is correct, the greater air density is allowing the engine to run better. The rain cools the air off, which is denser than before the rain. More air molecules to burn fuel gives more power. This is more noticeable after a rain because it is a quick change. Driving up and down a mountain takes a while and the effect of the air is not a dramatic change

Not only does the rain cool the air off, but if it was a large enough storm you get outflow, which is a large downdraft of air that comes from higher elevations where the air is cooler.

When tornado chasing I used to bring a jacket - the outflow from a supercell can be COLD, especially if you’ve been sweating in the muggy heat just before.

Strange that a self-professed enviromental carebear would have a smog bleaching 60’s wagon.

@SteveCBT is exactly correct though.

The old radial piston aircraft engines frequently used water injection at take-off power settings to suppress detonation…If the water-injection system failed, there was a noticeable loss of power…

There was only one car produced, in the sixties, that had water injection. I believ it was a GM intermediate, maybe OLds F-86 which used it to prevent knock. The F-86 became the Cutlass later.

Thank you, all.
The 66’ Ford Country Sedan SW was years ago when our relative humidity was lower than now.
(More lawns, trees, swimming pools, etc.)
The increased power diminished to usual level in about 20-30 minutes.
Barometric pressure usually is higher after a storm front has passed, but I did not think it was enough to notice.
I did an increase in power when the air was cooler at night.

@Docnick–you are right. The car was the 1962 Oldsmobile F-85 which had a special model called the Cutlass. The engine was either turbocharged or supercharged ( I am not sure which it was although I think it was turbocharged) and it had water injection (an alcohol mix was actually used) to prevent detonation.
@Robert_Gift I remember my mother thinking that the 1949 Dodge my parents owned had more power on cool, damp evenings. It probably was most noticeable on that Dodge because it was really underpowered with its flathead 6.

For what it’s worth, I’ve noticed that pretty much every car I’ve ever owned, whether carbureted or injected, have all seemed to have a “sweet spot” at around 55-65 degrees F where they ran the best, were most responsive, and made a little more power. Maybe this is what fuel system designers use as a base ambient temp range when designing a system, so the fuel delivery just works best there, I don’t know. But I have noticed it too.

Makes sense for carbs. But wondering if rain storms have the opposite effect w/modern fuel injected cars using a mass airflow sensor? Causing the engine to run sub-optimally I mean. The same mass of air during a rainstorm will have less O2 in it to burn, as water vapor is displacing part of what would on a dry day be O2 containing air. The net result would be the MAF indicating more O2 is getting into the engine than actually is, and so it would tend to run rich. This also might be a reason to have an emissions test done on a dry low humidity day, particularly if your car is a little high on the HC’s.

@GeorgeSanJose: You may be on to something, but the cars I’ve owned have all used MAP sensors instead of MAFs.

Same effect, George. Denser air means more molecules get drawn in (or pushed, for those with turbos or superchargers), and that’ll be sensed by either a mass airflow or manifold absolute pressure based system. The ECU will lengthen the injector pulsewidth accordingly, to keep the ratio of oxygen to fuel correct.

George, you are on to something, but in most areas of the US the amount of moisture in the air is negligible. As TSM says the denser air has more molecules, that way over comes any additional moisture. While a few more water molecules are also in the air, it is temperature dependent. Cooler air holds less moisture. You would need to be in New Orleans in August (100 + F ) and have 99% RH to have enough of an effect by the moisture in the air.