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Old 11-24-2008, 10:26 PM   #1 (permalink)
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Cooling problems with undertrays?

I'm working on an undertray for my car and have come up with some cooling questions for anyone that knows about such things. Both my oil pan and transmission case are aluminum and extend down slightly into the airflow beneath the car. Could blocking with an undertray cause any overheating? The oil temp would be somewhat moderated by the fact that the engine itself is cooled via the radiator. The transmission has an ATF "warmer" that has coolant hoses running to it, but I don't know if it also helps cool the transmission. The "warmer" has two warmer and two bypass hoses running to it, and I've no idea how it functions.

I may be able to tap into the ATF temp sensor with a multimeter, but this will only give me a voltage reading. I wont' have a good idea of how that reading correlates with temperature.

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Old 11-24-2008, 11:54 PM   #2 (permalink)
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Yes, one does have to watch temperatures. I have gotten my oil so hot that it caused the rear main seal to leak. But that was a 95 degree day.
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Old 11-25-2008, 02:13 AM   #3 (permalink)
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On a warm day and a long trip, ATF gets hotter than engine coolant. This fact is derived from working in a service station back in the 1970's. eg. Car Owner: "Fill her up and check the oil." ... Me: "The oil iis a quart low, Sir. And it looks like it's due for a change." Car Owner: "Yeah, I should have changed it before I left, .... How long for an oil change." "We're almost done with the one in the first bay. Should be less than 1/2 hour." With the car on the hoist, the transmissions were always obviously physically hotter than the radiator.

FWIW. In the 1980 I operated a specialized 6x6 20 ton vibroseis truck with automatic transmission. On 100F air temp days, the transmission oil temp ran around 280F-290F. Engine oil temp ran about 220F. Engine coolant temp ran around 200F.
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Old 11-25-2008, 02:13 AM   #4 (permalink)
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although my undertray doesn't cover the oil pan and the exhaust it would mostely shield it from any direct airflow, because i didn't want to risk anything and because the exhaust would contact the undertray i made a cutout in the trailing edge of the tray to give them both some breathing space....haven't noticed any problems with overheating and such.

i might get away with better aero, but i wouldn't want to find out my mods cause trouble if i'm on holiday for example and my driving is a bit more demanding than on my dayly commute

o the other hand i've wondered if an aluminum undertray wouldn't actually function as an heatsink and improve cooling. if you do a full undertray you might also consider louvers below the parts you want to keep cool they will actually extract hot air at speed
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Old 11-25-2008, 02:19 AM   #5 (permalink)
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If your tranny and oil pans stick out into the air stream, sacrifice the extra bit of airflow, and clearance your undertray for them...

If you later decide you'd like to chance it awhile, you can always make some nice scoopy kinda things that cover them up, just make them removable until you're sure they won't have any effect.

IF either has vanes on it, it's designed to help cool, end of story. That doesn't mean its' actually necessary, but that's the intention. With Honda cars, it's pretty common that they have more cooling capacity than necessary, especially if you're only using it as a DD and not really working it out.

BTW, temp gauges are cheap... and trans temp gauges arent that expensive either... might be a good investment if you're worried about your trans.
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Old 11-25-2008, 05:40 PM   #6 (permalink)
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"Most ATF can withstand normal operating temperatures of around 200 degrees F for tens of thousands of miles. But if the temperature of the fluid rises above 220 degrees F the fluid starts to break down quickly. Above 300 degrees, fluid life is measured in hundreds, not thousands of miles. And above 400 degrees, the fluid can self-destruct in 20 to 30 minutes!"

Also above 175 degrees ATF life is cut in one half.

You may ask "why the radiator/ATF warmer?"

ATF (depending on individual Brookfield Viscosity rating) can become ineffective at extremely cold temperatures.

If in doubt, a transmission fluid temperature gauge sender can be TEE-ed off the transmission-to-radiator line.
Installing a Transmission Temperature Gauge
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Old 11-25-2008, 06:48 PM   #7 (permalink)
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I've never seen an ATF warmer... always coolers, where the lines are put into a secondary radiator that is held to or near the radiator, or a part of the radiator...

Those were always used to keep the ATF cooler though, since the engine's coolant doesn't usually go much over 200* (sarcasm) I believe the average thermostat is 190*, correct?
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Old 11-26-2008, 07:14 PM   #8 (permalink)
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Many Honda's use a warmer/cooler. The warmer/cooler uses engine coolant to warm the atf at low temps, once the engine & trans are at runnig temp the warmer then will act as a cooler.

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Old 05-18-2011, 03:10 PM   #9 (permalink)
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This is a great idea. I thought about doing this to my project.
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Old 08-22-2020, 06:44 PM   #10 (permalink)
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The undertray, if properly designed, should not impair engine / gearbox cooling, but help them greatly.


On my car, there's in front an undertray running from front splitter lip to just after front axle subframe, at the front end of the catalytic converter. It can't be flat, due to wrapping the engine oil sump and gearbox, what matters is that it's quite smooth, no bits and bobs dangling in the airflow.

Car is pretty low to the ground, minimum ground clearance is in the 80 mm (3.15 inches) range below the front axle subframe. The undertray acts like a funnel, dropping smoothly the ground clearance from the front lip to the lowest point below axle.

As Bernoulli's principle tells us, if the speed of the airflow increases, as the air is squeezed in a funnel, the pressure decreases.

This is good for 2 reasons:

1. Decrease in pressure exactly below driven front axle cancels part of the car's lift (does not "create downforce", this is not possible in a road car), which aids high-speed stability;

2. The air enters the engine bay in a high-pressure area (the pockets below the middle line of the front bumper) and it finds the path to exit into a low-pressure area, at the rear side of the front axle.

The drag of the cooling air flow is huge. According to SAE paper 2004-01-1307, 33.4% of all drag in a road car comes from the flow of air in the engine bay (including radiator, intercooler, oil cooler etc). The entire exterior of the car barely makes 31.7%. A car with no airflow through the engine bay may be shaped like a brick and still be more aerodynamic than most cars on the road. (This can't exist in the real world, even electric supercars need some cooling for the battery, motors and brakes.)

So, if we draw the air from a high-pressure area, exhaust it into a low-pressure area and add heat in the middle... we just have described a jet engine It increases greatly the airflow through the engine bay and therefore improve cooling.

Real world testing

Engine coolant temps in the 85-88C (185-190F) highway in cool weather, 90-93C (194-199F) hot weather, 93-96C (199-205F) city driving, occasional jumps in the ranges where fans go into full speed (over 102C/216F) in stop and go traffic. Intake post-intercooler air temps as read at the manifold 13-15C over atmospheric temps.

Incidentally, more than 80 years ago, Ettore Bugatti and Louis de Monge thought of the same problem for the 100P prototype aircraft: how to get rid of the drag generated by cooling system's ducts, funnels and scoops?

Solution: think outside of the box. Drop everything people knew about fans, ducts, funnels and scoops... use the pressure differences created by the act of flight itself.

Therefore, the 100P drew air for engine cooling at a high-pressure edge (the front edge of the tailfins), flew it back to front into the fuselage and left it to exit in a low-pressure area, at the wing trailing edge. This not just allowed a very clean shape of the fuselage itself, but it even added a bit of thrust in flight. They expected to hit 500 mph with only 900 hp, if some minor unpleasantness called WWII didin't happen.

US Patent no. 2268183, filed by Bugatti on March 16, 1939. Granted on December 30, 1941, when people were too concerned with some pesky Mitsubishi A6M Zeroes in the Pacific to pay too much atention.

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