patsmx5

iTrader: (16)

True, but for my application I'm thinking about cutting up the rear of the trunk and putting the a/c condenser and a couple oil coolers back there. Seen Porsche do this (a/c condenser in rear) to get clean air to intercoolers and radiator up front.

I have no idea for inlet/outlets, drain and fill plugs would be convenient, but I don't remember where they are. No way I'm doing a banjo on the drain, that would be too restrictive I would think, unless it's a custom high flow banjo I don't know of... I'd prefer a large hose that goes straight to the pump inlet, no restriction at all.

Leafy

iTrader: (1)

It shouldnt be that bad with the banjo. That pump as long as its the same level as the bottom of the tranny is going to have less head loss before the pump than your oil pump would so it shouldnt be a big deal. The inlet is the annoying one with it on the exhaust side.

codrus

I've pondered a tranny cooler, and was intending to use one of the Tilton pumps that Summit sells for that purpose. Dunno if it's a better choice than that ebay one, though:

http://www.summitracing.com/search/p...Cooler%20Pumps

Putting the AC condenser in the trunk seems like an odd choice. I wouldn't do it on a street car (because I'd want the trunk to carry stuff) and if it's a race car then why does it have AC?

Fittings and hoses should be fairly straightforward, also a thermostatic controller or perhaps just a manual switch in the cabin along with a temp gauge.

--Ian

patsmx5

iTrader: (16)

I've never used a Tilton pump, or the ebay one for that matter. The sticker on the pump in your link, the 40-527 pump shows 8 Amps, 1.9 GPM, 50 PSI Max. Vs the ebay pump has no pressure rating but "up to 10' lift pressure" (not much) and rated at 3.2 GPM.

Without testing either pump, my guess is if there is little restriction, the ebay pump will move more oil, if there's a big pressure drop, the Tilton will be better. So then the question is, which will move more oil if it has to push 200*F+ gear oil through an oil cooler?

Leafy

iTrader: (1)

Tilton sounds like the safer bet with the whole, being advertised for this function and not being all that much more than the ebay one.

patsmx5

iTrader: (16)

Finally found some info, this is specific to rear end cooling

Article: Keeping You're rear Gears Cool - Circle Track Magazine

Quote from very bottom of article:

That's for a diff, and they're saying 20 GPM is ideal flow, that's 10x more than the electric pumps we're looking at..... I would think a transmission would generate more heat than a diff, but I could certainly be wrong. I really have no idea.

If they are right, an electric pump wont' cut it at 1-2 GPM. I can say that from what I've read, NASCAR does use pumps to cool their transmissions and diffs, and they are mechanical pumps, not electric.

glade

iTrader: (2)

See. This is why I love this forum. Pertinent information being shared. I now rescind my earlier reservations!


One question though. If the 20gpm is ideal for differential oil, is it because it holds, and or transfers heat different than that of the trans?

patsmx5

iTrader: (16)

I think it's simply that to remove enough heat to keep the diff cool, you have to pump that much cooler oil into per minute to remove all the heat.

Rough numbers, but check this out:

This site post efficiency for diff gears Comparison of Gear Efficiencies - Spur, Helical, Bevel, Worm, Hypoid, Cycloid

From there,

I believe our diff is spiral bevel gear. So if we split the range and say we have 97% efficiency, then 3% of the power the diff sees will turn into heat.

So on a 300whp car, rough numbers, 3% of 300 = 9 HP worth of heat generated. If we use the worst case and say 95% efficiency, then 5% of 300 = 15 HP worth of heat generated.

I'm not gonna do the flow rate/temp calcs right now, but if the above is correct, our cooler needs to pump out 9-15 HP worth of heat on a mere 300whp car. Anyone please feel free to confirm/deny/laugh at the above!

codrus

20 gpm for the diff sounds crazy high. It's only got a quart of oil in it, so that's pumping the entire thing through the cooler in less than a second.

Let's try some back-of-the-envelope calculations. Poking around a bit, mineral oil has a specific heat of 1.6 kJ/kg-K. 15 hp is 11.2kJ/second, and with a 90K temperature different (250F vs say 100F) that suggests that the fluid could carry that much heat with a flow of only .077 kg/second. Gear oil is pretty close to the same density as water, so a 1L capacity is giving you around 1 kg total mass.

So if I did the math and unit conversion right, and assuming a heat exchanger big enough to output 100F gear oil, 20 gpm is about 10-12x higher than it needs to be. That'd suggest the 2 gpm of the Tilton is just right.

--Ian

Dustin1824

iTrader: (2)

+1, and I would say this is conservative. Also keep in mind though, you are assuming that all of the heat needs to be evacuated through the heat exchanger, and it doesn't. The gearbox radiates a large amount of heat.

It's also conservative because the heat exchanger does not need to output 100F fluid, I believe that is too low. I would guess you would want the tranny oil to stay in the 180-230F range.

According to AGMA(American Gear Manufacturers Association) gear design regulations, they do not apply a temperature safety factor to gears until they are over 240F, I believe. This is from memory, so the temps may be +-20F, but everyone here can get the point.

On a side note, a 20 GPM pump seems WAY too big for me. Even a 2 GPM pump would empty the gearbox off all oil in less than 8 seconds if a hose popped or got damaged in some way.

sixshooter

iTrader: (12)

I don't pretend to be as smart as some of you guys but I always thought I would use the power steering pump I removed from my car to circulate the oil from my transmission through a cooler. It is rated for oil and for high temperature. I thought it would likely be mounted to a bracket off of the PPF and use a belt over the driveshaft to spin it. When the car moves, oil flows. It wouldn't make any pressure except for the little bit of flow resistance from going through the cooler. It is a simple little gear pump that should last forever in that application. The belt wouldn't need to be very tight because there wouldn't be much pressure on the pump. A little spring loaded tensioner pulley like the one used under a lawn mower deck on a Snapper would work just fine. Keep it simple.

It would be good because it would involve no heavy additional electric motors or additional electrical power drain. In my car there is already a deficient alternator output when the radiator fans, headlights, or power windows are on. I need that power for spark plugs and Megasquirtseses.

patsmx5

iTrader: (16)

This is all good post guys.

I will say though, if his numbers rely on a 150*F temp drop, and the new system only drops the temp say 50, then to remove the same amount of heat we will need more flow.

BUT, as has been mentioned, we don't need to remove ALL the heat, just some.

I don't have the data, but if someone with say, 150whp, or 200whp miata could tell use, "hey, I have 150whp and my transmission oil is 220*F after a 30 minute track session", then we could basically guess that the cooler needs to be sized to remove the difference between YOUR whp and the "limit" of what the stock trans can handle without getting too hot all by itself.

I have no idea if anyone has this data though! But does this seem like a reasonable way to estimate the system requirements?

And +1 to Sixshooter's idea, i like that, simple and cheap!

sixshooter

iTrader: (12)

The cooler doesn't actually need to reduce the temperature of the fluid by the entire desired amount in a single pass. It's a small volume of oil and it is all going to make several trips through the cooler each lap. Less than 2 gallons of oil in the trans, 2GPM pump, 2.5 minute lap time = a pretty serious amount of cooling even at 15*F drop per pass.

codrus

The pump size gives you the maximum amount of heat you can transport from the diff to the heat exchanger. If the fluid coming out of the heat exchanger is too hot, then the problem is that the heat exchanger isn't big enough (or efficient enough, or have enough airflow, or whatever), not that the pump isn't transferring enough fluid.

And yes, the diff cools by conducting heat to the air passing over the fins as well, so the cooler doesn't have to handle the full load. My intention wasn't to do a full analysis, just a quick back-of-the-envelope calculation to look at the question of 20 gpm vs 2 gpm.

I don't think a PS pump is the right choice. For one, it's intended to make pressure, not volume. For another, you don't want the pump to run all the time, that would overcool the oil. I suspect the Tilton is lighter than a stock PS pump, as well.

--Ian

patsmx5

iTrader: (16)

That is true. But you can calculate the amount of cooling, it's m*CP*dT, mass flow rate time heat capacity times change in temp. So that takes into account how many times it passes through.

The tilton is rated at 1.9 GPM.

This 75W 90 oil Mobil 1™ Syn Gear Lube LS 75W-90 is listed at having a density of 0.859 g/cc. 1.9 gpm is 7,192 cc/minute, so that's .859g * 7,192 = 6,178 grams/minute, or 6.178 kg/minute.

Cp of oil is 1.67 kJ/kg*K I think.... Not 100% on that. Can anyone confirm?

So if our GOAL is to keep the oil at say, 230*F max, then the oil exiting the trans will be 230*F, and we'll cool it a bit and then return it to the trans.

230*F = 110*C
So if we cool the oil say 50*F, which I would think would be pretty reasonable/good, that's 230 - 50 = 180*F = 82*C exiting the heat exchanger heading back to the trans. So temp drop is 110-82 = 28*C I didn't put numbers in Kelvin but the delta T will be the same K or C.

heat removed would be m*cp*delta T, so 6.178 kg/minute * 1.67 Kj/kg *C * 28C = 288.88 KJ/minute, which is 4.81 kJ/sec, which is basically 4,810 watts, or 6.46hp worth of heat.

I think.... That's actually a decent amount of cooling, more than I thought it would be.

patsmx5

iTrader: (16)

Agreed! See my math, let me know if you agree or I messed something up....

FWIW, the article I linked that claimed 20 gpm was a racecar with a 9", not a miata making 300whp. No idea if there application needed more flow, just numbers I found on the internets!

sixshooter

iTrader: (12)

NASCAR belt driven pump that runs off of the driveshaft:







Hell, you can mount all kinds of interesting things back there on a race car:

Leafy

iTrader: (1)

That nascar pump, cooler, and lines all look way too small to be able to do 20gpm. I'm thinking the link that was quoted had a typo?

patsmx5

iTrader: (16)

That is possible. They said it ran at 5 PSI, which sure sounds more like 2 GPM vs 20.

If so, then everything looks ok for 2 GPM.

And from the pic above, that oil cooler isn't that big at all. But I'm guessing if it's from a NASCAR car, it probably gets 170mph air, so it doesn't need to be huge to be effective.

rex1223

Just a thought, why not call a nascar team and ask what they think! I suspect that they would stop laughing long enough to give a tip or two!!!