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Intake Air Temp

I know there has been discussions all over the net on lower intake air temps, for a “denser charge” maybe/maybe not, making more power. But what are the thoughts on raising the temp. for more economy? Possible? Probable? Or just not worth the effort? Thanks for your thoughts. - Bob

It won’t raise the economy. The engine’s computer will read the exhaust oxygen levels and adjust the fuel to match the oxygen available. It will force a decrease in added fuel since the air is not as dense in oxygen but that gives a decrease in power generated forcing you to press the gas pedal harder. No net gain anywhere. Cold air intakes, or CAI’s, provide denser air, in theory, to raise the peak HP you can get from the engine. The CAI is usually a lower restriction air inlet which reduces the work required to suck in the air and that will increase the economy a bit. Combine that with a cooler air charge allows you to advance the spark a bit by reprogramming the computer. THAT will give you a bit more economy as long as you don’t go too far and create a need for premium gas. We are talking about 1 mpg or less here. All is connected and very hard to improve mileage… or the car manufacturers would have done it. You can improve your mileage much more by how you drive.

Well, what it will do is promote vaporization of the gasoline, leading to more complete combustion, and lower emissions. That’s the thinking behind the “heat riser valve” on malaise-era vehicles that came equipped with them.

Taking it one step further, burning any HC to soot only releases a portion of the contained energy (you’re only “part-way burning” it.) Burning it to CO releases more energy, and burning to CO2 releases all of it. So, in theory, it promotes better economy by lowering the amount of unburnt gases headed to the cat. (This also means the cat should run a bit cooler.)

In the real world, does it make a meaningful difference? I wouldn’t hold my breath if I were you.

Modern fuel injection does a great job of vaporizing the gas. no added heat needed.

I used to drag race many years ago and the engine performance in the afternoon and early evening were always better at those times. In the heat of the day…performance suffers so I would not add any heat at all. There was not a whole lot of difference but there was enough so that everyone would notice it though.

The logic behind messing with the intake air temp is to gain more timing advance which gives better power and economy but raises HC and NOX emissions and can cause pinging.

If the ECU thinks the air is colder it advances the timing.

Going the opposite way would retard timing and have no benefit.

Oh well, just another idea that goes nowhere. But thanks for the thoughts and comments.

The heat is already added. All the intakes I’ve ever seen get hot air from the stove created by an intake around the outside of the exhaust manifold. Many engines would stall if the pipe or hose were missing.

Most modern engines have cross-flow heads, the intake is on the opposite side of the head from the exhaust, and no intake stove.
Every 25 deg F change in temperature changes air density by ~5%.
Colder intake air does reduce the tendency of spark knock and detonation, so more timing advance would be an additional gain.
Colder, more dense air results in a smaller throttle opening for the same power and rpm, which increases pumping losses, which could reduce fuel efficiency slightly.
The best of both worlds would be hot intake air at light loads and cold air at high loads.

“Oh well, just another idea that goes nowhere. But thanks for the thoughts and comments.”

All the world’s great ideas derive from countless ideas that go nowhere. Never hesitate to keep trying. Leonardo DiVinci drew large numbers of crazy ideas that went nowhere… until centuries later when technology finally caught up with his genius. A new “cache” of his papers was recently discovered, AND some of his known papers are finally being evaluated by engineers instead of just artists, and it may be that we’re still catching up with DivVinci. Dr. Debakey, a legend in heart surgery technology, commented on DiVinci’s design of a heart valve. It was exactly like the artificial valve we currently use. It runs out that DiVinci had figured out the fluid dynamics that make a heart valve function. The comparison by Dr. Debakey was on NOVA.

Great comment “the same mountainbike”.

I have learned far more from my failures than I ever learned from my successes.


One thing we can agree on is that DiVinci was a man misplaced in time. I won’t list everything he invented or figured out as a web search can quickly do so, but some include: Helicopter, self-propelled car, parachute, armoured tank, several accurate clocks, scuba gear…

Back to the OP’s question: Those cold air intakes that are sold do add a bit of horsepower, but not really more efficiency. Most modern vehicles have the air intake as far away from the hot parts of the engine as can be–A lot take air from under the car instead of under hood. It really doesn’t matter too much though as long as the car’s computer can accurately measure the incoming charge temperature and either the air density or relative barometric pressure.

The “need” for heat stoves went away when the transition away from carburetors to fuel injection eliminated their purpose, which was to promote cleaner combustion when the engine was cold by introducing air warmed by the exhaust pipe. Fuel injection compensates based on various factors including engine temperature.

Nothing scares me more than initial success. It usually means you’ve overlooked something that’s going to bite you in the behind.

Many companies are little r, big D. They are often risk averse and train their employees to operate in that fashion. I’ve had to spend quite a bit of time re-educating people coming out of that environment; that failure is not something to be avoided at all costs. You will have many more strikeouts than homeruns. Learn from those failures and apply it going forward. One aw-sh!t does not wipe the board clean. Just the opposite is true. One homerun wipes the board clean and then some!!