Under hood temperature while moving

There is always the talk to improve engine performance by lowering the intake air temperature. Cold air intakes apparently being the most effective way to achieve that.
I do get the concept as the temperature difference between under hood and ambient air can be quite significant but how big is the difference while driving - especially on a 68-70 B-body? Since the engine compartment isn't sealed there should be constant air exchange. Most of the incoming air has to pass through the radiator and will be heated up but by how much?

any real world experiences? Is it even worth it to make up my mind about a cold air intake for my 70 Charger?

One advantage might be the location of the cold air intake opening - if placed in a high pressure zone.

Cold air intake and underhood airtemp are actually to be seen a bit as different things.
You don't change underhood temps by installing a cold air system. Your engine (performance) wants cool air as its more dense and this is what can make more power.

During driving, underhood temps will usually drop or stay low, but it's only the 'air' that is being replaced with cooler air.
When underhood parts have heated up, they don't cool down as quick.

This is when underhood airflow control comes into play.
The trick is to properly 'vent' hot air from the engine bay, and not try to cram in more outside air.

This is why the front of older cars start to lift and steering gets scary light when driving at high speeds. Air is just 'stacking up' in the engine bay and can't flow out quick enough. The car is almost riding on a cushion of air between the front wheels.

If you look at modernday cars, most all of them have heatshields around the exhaustmanifolds.
Just look at the 3rd gen 6.xL Hemi's, which have 'shorty' headertubes, enclosed in a sheetmetal covering to retain heat inside. Part of this is maybe for emissions as well as hotter exhaust going into the cat makes it more efficient.

Another thing that's very helpful (and just about every car has these days) is some sort of front airdam.
This creates a low pressure area behind it which helps 'sucking' out hot air from the engine bay (and lessen air turbulence under the car).

I've created a cold air intake on my daily '73 Dart, as well as an front airdam and my engine temps are pretty always about 10-20°F below the thermostat's temp rating.
Only when I'm driving/sitting in traffic is when the engine temps gets around or little over the thermostat temp.

My cold air entry is just next to the radiator facing forward to the grille.

Another thing I noticed with the cold air intake, is the engine doesn't tend to feel 'a bit lazy' anymore after everything is warmed up.

Thanks for the reply! Maybe I wasn't clear enough in my first post. I'm strictly talking the intake air temperature. I was comparing a cold air intake (e.g. drawing air from the front or base of the windshield) vs. drawing air from the engine compartment. Obviously the temperature difference decreases with driving speed as the heated air is replaced faster (if the engine compartment is properly vented).
Let's say you are driving @60mph - what's the temperature difference between engine bay and outside air?
Actually it all boils down to the pressure difference (colder air being denser) - so drawing cold air from a high pressure zone would be the best but is worth the hassle to make all the ducting etc.?
I might buy a differential pressure gauge and do some investigating..

Because I have a MegaSquirt ECU in my car, I was able to simply install a temperature sensor in the intake and log its readings.

On my car, the intake manifold is usually 40°F cooler than the engine coolant during highway driving.
Intake - 95-100°
Coolant - 140°

I've also had this tempsensor in the aiffilter housing before, and there temps would get close to the same as the outside ambient temperatures during highway driving.


But whenever you get into stop&go traffic, intake/air temps will climb quickly to engine/bay temps again.

Good to know! That confirms my suspicion that cold air intakes don't make any difference in the air temperature the engine sees at highway speeds. So the only performance increase is due to the placement of the air inlet (I assume the air pressure in the engine compartment is lower due to air moving around compared to high pressure areas such as the front of the vehicle or the base of the windshield).

When idling the intake air temperature might be much higher but that's when I could care less about ultimate performance

Hot intake air, when idling (for a trafficlight) is bad too, because when the light turns green you want to have 'performance' again. It doesn't help if your engine then "pings its brains out" because everything is hot and heatsoaked again...

I've been thinking about insulating the cold air ducting to the engine, as enginebay heat also warms up the tubing.
A double wall tubing should help in keeping the incoming air cooler longer I would think. Also applying a heatreflecting layer on the tubing should be helpful I guess.

how much of an air dam is needed to create the low psi zone? Wondering if my turbo tubing that hangs below the rad support on my Fury might be giving me some of that benefit?

An airdam would need to block or 'catch' and try to push away the air it goes through to the side.
Round tubing won't do much for blocking as it pushes the air down and 'rolls' right over it.

makes sense, I wasn't sure if the resistance of having to go over the pipe would be enough to create any pocket.

I am running MS controlled MPEFI on a 480ci Hemi '69 B-body.
Edelbrock intake with 2x1000 cfm throttle bodies, with the IAT sensor originally in the air cleaner base between the throttle bodies.

I had the Ramcharger ductwork off the hood for a while. The IAT ran 60 degrees or so above ambient.

With the ductwork on and a good seal to the air cleaner, on the road IAT went to about 10 degrees above ambient.

Air temps 11 degrees cooler offer a potential HP gain of about 1% -assuming the mixture is corrected for the temp change.

As a sidenote, getting meaningful IAT is not easy. I dealt with sensor heatsoak issues on hot restarts. Moving the IAT sensor into the ductwork and modifying the sensor transfer function table cleared it up.

Cool air helps. I remember the article on a 69' HEMI Road Runner with the "Air Grabber" cold air system in Super Stock and Drag illustrated in Dec 68'. 80 plus degree day. The car was run with and without the cold air. The et and mph changed enough that they ran the rest of the tests with the cold air.

I disagree with your conclusion.
My thinking, cold air does make a difference, even though it may vary, and the pressure differential advantage at our highway legal speeds would likely be negligible, if even measurable.

Best way to figure this stuff out is to switch over to EFI and then data log the MAT (manifold temp)

Once I started driving my car with the Sniper installed I could see what was happening. I have an opening in the hood and my MAT stays fairly close to ambient temp. So that tells me that the throttle body is pulling outside air into the engine.

My car is a different car with cold air intake,sitting in traffic or running down the freeway runs better, had to retune the carb.

Here's a screenshot from MegalogViewer of my car running a short trip on the highway.
(Right-click the image to show its larger/original size).

It's showing engine RPM (White trace) and MAP (=Engine load or 'reversed vacuum') as the Red trace in the top row.

The lower part shows Engine Coolant temp in Green and Intake air temp in Yellow.

The car had been driven before this day, but you can see the engine temp rising from the start, while the intake air temp rising very slowly until after the highway part (center section) and the car encounters slower stop&go traffic.

Do you know what the ambient air temperature was when you logged the data? Coolant temperature is really low which seems odd since you are using a thermostat ( = no coolant flow through radiator).

I wonder how the chart would look like without the cold air intake.

Coolant temp is low because of 3-row stock radiator and shroud.
The moment the thermostat barely opens the occuring coolantflow is enough to drop the engine temps below thermostat rating.
Tried different 'stats. Some had a wider opening temprange and ran the engine temp different, but none could keep it at its designed rating.
But I rather have an engine running too cool then too hot, especially when running on propane.

I know in wintertime the intake air could pretty much reach ambient temps, but that was when I had the temp-sensor located in the airfilter housing.

Coincidentally, my '60 NewYorker (w/496" rollercammed stroker engine) is has currently developed hot running issues during the highway trips, while staying fairly cool with slow driving.

Underhood temps are also very much an issue with this car, especially when both carburators are hanging over the exhaust headers, sucking in well-heated air.

I need to add more vacuum timing in this engine, but it's rather cumbersome getting it right. The engine is temperamental and the underhood heat isn't helping.

Might want to go digital on the ignition and do some airdam fabbing as well.
Also checked the lower radiator hose and it appears to have no spring inside, which is mostlikely the main issue for the higher highway temps.

General rule of thumb:
Heat issues in slow traffic = airflow issues
Heat issues at highway speeds = coolant flow issues.

Turned out the radiator in my '60NY was a bit clogged on top with softish rust/scale debris.
Took it out, laid it flat on its back and filled it with citric acid to dissolve any scaling and rust over the next few days.

Chrysler (and the others) got it right 50 years ago.
The air cleaner snout has a diaphragm (vacuum-operated) flap with 2 positions:
1. is open to a duct leading to the radiator support, and allows ambient only.
2. is below the snout, selected by the flap closing, and admits pre-heated air passing over/through a shroud over the driver's side exhaust manifold.
3. what temperature the can's interior sees is determined by a thermal vacuum switch inside the can. When the engine and underhood temperature is low (cold day, cold start) it admits only pre-heated air until the can interior temperature is 150 degrees (?). Above this, it blends cold air to keep the temperature nearly constant for best mileage and accurate mixture.
4. in addition, low engine vacuum (WOT) opens the flap and admits only cold air for max power regardless of can temperature.

Other than a larger snout, this idea still works perfectly with very few components.

My experience is roughly the same... ambient + 50-60 degrees F in summer with open element air cleaner and EFI measuring intake temps. Colder denser intake air helps power.