Dave M vin 03572
http://dm-eng.weebly.com/
Location: CLE/PHX
Posts: 2,588
My VIN: 5646,5080, 5880, 10234, 3639, 2518, 10586, 1538
This is a loaded question... Short answer is extensive testing shows the car can not function without the fans. The placement of fans/condensor coupled with air inlet makes it very easy for this to form an unwanted diffuser element. This is why it can get really hot on top of our fuel tanks....
www.deloreanindustries.com Every Detail Matters
Location: Northern NJ
Posts: 8,569
My VIN: 10757 1st place Concourse 1998
The exact speed at which the fans are no longer required to move enough air through the radiator is about 30 MPH. Of course it depends on road conditions and ambient temperature, what grade of fuel, how well tuned up the motor is, a LOT of variables. As we all know if you turn on the A/C it will force the fans to run whenever the clutch is engaged on the A/C compressor so the above 30 MPH is with the A/C off. If you look up the specs on the Spall fans they claim more CFM then the OEM specs. Obviously the ductwork and radiator will affect the total CFM of any installation. The specs are in "free air" so you can't expect that much when installed. The bottom line is the newer fans can move more air with less electric. I believe a lot has to do with the shape of the fan blades. The newer style are probably more efficient. IMHO nothing wrong with the OEM fans that can't be overcome with a Fanzilla or comparable device. All of this conversation presupposes you are comparing good working fans. Trying to compare 30 year old fans that aren't working well vs something new is not a fair comparison.
David Teitelbaum
Location: West Sayville, N.Y.
Posts: 1,350
My VIN: 005058 000927
Club(s): (AZ-D) (DMA) (DOA) (DCUK)
I wasn't attacking your fan setup Josh. I think yours is better than most actually. I was simply giving a IMHO about the newer fans in general and you replied with a LOL but no further explanation. This prompted me to defend my position. I was giving my opinion based on the facts as I see them but without empirical testing. If you have such testing you wish to put forward I have no problem with that. What makes you think I was zeroing in on your fans with an "attack"? Rest assured that was not my intention.
Rob
Does CFM truly matter as opposed to actually keeping the car cool? It wouldnt be that complicated to test. You could set up an electric heater and water pump to a hose and hook it up to the radiator and fans attached to a front fascia. You could test a stock setup vs a new radiator and fan setup. It wouldnt represent the car perfectly but it would give fair working comparison of the 2. The stock setup has a brass radiator with lower thermal conductivity than aluminum radiator. So even if the new lower current fans have less CFM than stock, the higher thermal conductivity of the aluminum radiator might compensate.
Location: Northern NJ
Posts: 8,569
My VIN: 10757 1st place Concourse 1998
It gets a whole lot more complicated than your over-simplification. One problem is the thermostat. If the radiator was to "overcool" then the thermostat would just shut to keep the motor at operating temperature and the otterstadt switch would shut the fans off. The use of the CFM rating of the fans is to try to roughly compare the efficiency of one part of a system. Proper testing can get quite complex trying to account for all of the variables in the cooling system. One thing to consider is if your old fans are bad and you are considering replacing them, the newer style could be the better choice for a lot of reasons and not all have to do with thermodynamics. Economics (prices) also affect your decision. BTW one of the reasons the fuel can get so hot is because the pipes full of coolant run right alongside the bottom of the tank. Between that and the hot air coming out of the radiator, it is no wonder the fuel gets so hot. Some owners have installed a big flap of rubber to deflect some of that hot air downward away from the fuel tank. Others have wrapped the fuel line around the accumulator on the A/C to try to cool the fuel. When the fuel gets really hot the pick-up hose on the OEM style fuel set-up can collapse under the vacuum making the fuel pump noisy and shortening it's life. The fuel pump is cooled and lubricated by moving fuel through it. Running the fuel pump with no fuel because the pick-up hose has collapsed is very bad for it.
David Teitelbaum
Good thinking. Maybe the OEM fans were overpowered anyway. I think the fact they had to recall the fan fail suggests the OEM fans did draw more power after they aged. The OEM radiator efficiency is probably less than the new vendor replacements.
It all boils down to engine not overheating and AC cooling enough. I don't think anyone has complained that the low power fans don't work good enough. But there is an EU vendor selling 3.5 amp fans now and I am waiting to hear how those are working out.
Dave M vin 03572
http://dm-eng.weebly.com/
Location: West Sayville, N.Y.
Posts: 1,350
My VIN: 005058 000927
Club(s): (AZ-D) (DMA) (DOA) (DCUK)
CFM of moving air is what cools the radiator. The NOS radiator was actually a single row aluminum unit with plastic tanks that cooled the car well despite being fragile when encountering impacts of any sort. Changing from one aluminum radiator to another will only help if you increase surface area and/or coolant capacity.
Rob
Location: Northern NJ
Posts: 8,569
My VIN: 10757 1st place Concourse 1998
More CFM can increase cooling capacity (BTU transfer) but only up to a point. The moving air must maintain contact for a long enough period of time to allow transfer of heat, it is not an instantaneous process. You can get bogged down in static pressures vs positive pressures and other aerodynamic effects. As Rob said, if the cr-py OEM system was adequate, (at least when new and in good working condition) anything you do to improve it can't be a bad thing. It is NOT all about CFM. CFM is just one of many variables that must be considered.
David Teitelbaum
Oh, I read that it was brass on the DMC knowledge base site. http://support.delorean.com/kb/a44/radiator.aspx
How does the thermostat affect the efficiency the cooling? I didn't get much time to elaborate on my testing condition, but it would simulate sitting at idle in 70F ambient temp. As stated, CFM is difficult to calculate, so it would be easier to see how long it would take a stock setup to cool the running coolant from 220F to 150F vs a new setup in identical conditions. That way you eliminate the need to calculate CFM. If it takes both setups the same amount of time to cool the system, then you know even if the new fans have less CFM you have the same cooling efficiency as the stock setup with less current draw. I don't see why you would focus on one part of the system when you are changing at least 2.
Anyways, my fans don't seem to turn on when the temp is high enough, so Derek you mind helping me change out my fans??