This may be a simple/stupid question but here it goes: I need a radiator for my rotary swapped 1976 fiat 124. Im looking at ones for the fb rx7 because it has the outlet/inlet where i want them to be (both on the passenger side) and it will fit in my small car. Now here is the dumb question part; Looking at fb rx7 radiators, some of them have the tanks on the sides like this:
now the rotary I have is the 13b rensesis from the RX8 and their radiators look like this:
so my question is, do these function the same? tanks on the side and on the top?
Top pic is a double crossflow unit. Hot fluid goes in the top, across the top half of the core, down through the other end tank, then back through the core to the outlet. Second pic is a traditional on the top, out the bottom unit.
All else being equal, either should work.
The top one is a double-pass and the bottom one is a single-pass, but otherwise they work the same. The difference (for a fixed water pump and cooling area) is the speed/time spent inside the tubes. Think of it this way, the double pass has the water flowing twice as fast, but also has twice as far to go, so the net cooling time in the radiator is the same between the two styles.
There are some very small effects that can lead to using one over the other, but if the cores were designed for a production car of basically the same type/power, the end result is that they will both work fine. If you are upping the power significantly, you may need to look at other options since the radiator needs to be sized appropriately with the power the engine makes (roughly the same amount of power that gets to the wheels, goes out thru the cooling system).
In reply to stafford1500 :
That’s a really great explanation of the difference, except it doesn’t always mean water flows twice as fast.
Jaguar slowed water flow down because the big fat front bumper blocked air to the radiator. By slowing water flow down they exposed it to longer cooler exposure. An infrared thermometer will show higher heat in the center ( right behind the bumper ) where the coolant turns around
edit
I misstated the center was hotter. I should have said the gradation in temps is disrupted in the center. Top being hotter, bottom being cooler, except in the middle behind the bumper.
In reply to frenchyd :
I did state that for the same water pump, the water has half as much core to flow thru, so the water speed does indeed increase (not exactly twice due to friction losses) roughly to twice the speed thru the tubes. Slowing pumps down with different pulley sizes will help high engine speed cooling sometimes, but hurts at low engine speeds (generally).
frenchyd said:
In reply to stafford1500 :
Jaguar slowed water flow down because the big fat front bumper blocked air to the radiator. By slowing water flow down they exposed it to longer cooler exposure. An infrared thermometer will show higher heat in the center ( right behind the bumper ) where the coolant turns around
I'd like to some real data on that, along with some thermal images. Often, unexpected warm areas on radiators spotted with IR thermometers or imagers turn out to be hot engine parts peeking through the radiator tubes and fins, or sometimes reflections from hot components.
In reply to slantvaliant :
You’re the best judge of what I said. Grab your infrared thermometer. If you don’t have one they are really cheap I bought mine 30 years ago and paid $25 for it but they are probably a lot cheaper now.
Pop your hood and aim it at the radiator towards the front of your car so you don’t get the things you are worried about
You can use it for tune ups, brake checking, tire temps, overheating issues, and don’t get me started about how handy it is around the house. You’ll find your own uses too, I’m sure. But if you have any pretentious of being a mechanic and working on cars you need one
stafford1500 said:
In reply to frenchyd :
I did state that for the same water pump, the water has half as much core to flow thru, so the water speed does indeed increase (not exactly twice due to friction losses) roughly to twice the speed thru the tubes. Slowing pumps down with different pulley sizes will help high engine speed cooling sometimes, but hurts at low engine speeds (generally).
You know we are probably saying the same thing just in a different way. Jaguar’s double pass radiator goes across one direction and then turns and goes back across the other. By slowing the flow down they were able to extract more BTU’s
Soooo, I have a rx8 motor in the car and the core on the rx8 radiators is 25" x 14" and is 2 inches thick. Then this rx7 dual pass core size is: 18"x 15" and is 2.75 inches thick. You think this will provide adequate cooling? I have very limited space and this rx7 fb one is the widest I can fit
slantvaliant said:
frenchyd said:
In reply to stafford1500 :
Jaguar slowed water flow down because the big fat front bumper blocked air to the radiator. By slowing water flow down they exposed it to longer cooler exposure. An infrared thermometer will show higher heat in the center ( right behind the bumper ) where the coolant turns around
I'd like to some real data on that, along with some thermal images. Often, unexpected warm areas on radiators spotted with IR thermometers or imagers turn out to be hot engine parts peeking through the radiator tubes and fins, or sometimes reflections from hot components.
It is mostly a debunked myth. Slowing down the flow does give more time for the radiator to shed heat, but it also gives more time for the coolant in the engine to absorb heat. It doesn't really matter how fast or slow it moves, the bottom line is heat generated/absorbed - heat shed in the radiator = net heat. The speed of the coolant simply changes the DeltaH between the hot side and cool side. It's like filling a hole with a big wheelbarrow or a small one. You can transport the same amount of dirt in three slow trips or 6 fast ones.
Manladypig... they will both operate similarly, but the crossflow radiator is a bit more efficient than the vertical flow. Not by much, but a wee bit. The more important comparison is the ability to shed heat which has to do with volume, surface area, and airflow.
In reply to Curtis :
So how can I find out how much cooling I need/how big of a radiator?
manladypig said:
In reply to Curtis :
So how can I find out how much cooling I need/how big of a radiator?
The quick and dirty way, without needing to math a whole lot, is figure out how much power your car makes, and make sure the radiator you're using was designed for a car that makes about that much power. Heat is pretty agnostic. It doesn't care if you're making 100 HP in a Civic or a Volvo, the same radiator will work in either case, as long as it fits and the air flow is there.
Air flow being very key. I have learned the hard way to make sure the radiator is sealed very well so that the air that you expect to be going through it is not, in fact, going around it. Because air does NOT like to be forced into tiny passages between tightly-packed fins. It much prefers taking a nice easy route above, below, or to the side of said fins. 
I don't know that much about radiator differences, but remember that the FB was rated at 100-120HP where the FE is 200-225HP. An aftermarket aluminum FB radiator is probably comparable to a stock FE radiator (don't know for sure, just speculation).
Don't forget about oil cooling as well. Rotaries put more heat into the oil than piston engines. I think I remember reading once that 40-50% of the engine cooling is through the oil. I might be remembering the numbers wrong, but I know they reject a lot of heat through the oil.
If you want to jam a stupid amount of cooling power into a small area, get a 3-row 3-pass radiator. The fluid flows down the back row, then up the middle row, then down the front row. They're far from cheap though.
Sonic
UltraDork
9/6/19 12:14 p.m.
If you dig into the summit racing catalog online you can search all radiators by dimensions, then look through them for inlet/outlet locations in the right places and sizes, just had to do this in our 89 Civic as we did a K24 swap and needed more cooling than the standard use scirocco radiator that is good enough on the street, ended up with a Griffin circle track radiator that works great, barely fits, and was reasonably priced.
In reply to Curtis :
Your formula works. Sort of. The flaw is volume. ( which is the V12’s strength and weakness) it has over 5 & 1/2 gallons. For a 326 cu in engine. Most similar sized Similar horsepower engines have about 2 gallons.
The reason for the extra? It’s intended use. It’s prime buyer was in Southern California older wealthy man who tended to drive slow ( more than 50% of Jaguars production was sold there. ) as large as the car was the engine bay was narrow. A lot of stuff crammed into a tight space with little area for heat to escape from.
The extra coolant allowed a much longer time before overheating.
That was also its weakness, because it took longer to come up to temperature it’s coolness kept the engine in an overly rich mode for too long. It’s also why the so called HE was created
In reply to volvoclearinghouse :
Amen!!! My Black Jack Spl with a little 3.8 liter six cylinder engine would overheat the big Corvette L88 radiator I bought for it until I put shrouds in that forced all the air through the core instead of allowed it around the core.
After that it was a matter of how much tape did I need to cover the opening to bring it up to temperature. Most races I stayed under 200 degrees
manladypig said:
Soooo, I have a rx8 motor in the car and the core on the rx8 radiators is 25" x 14" and is 2 inches thick. Then this rx7 dual pass core size is: 18"x 15" and is 2.75 inches thick. You think this will provide adequate cooling? I have very limited space and this rx7 fb one is the widest I can fit
Yes, it will be fine. I was running a Scirroco radiator in my 13B swapped RX8 when it was N/A. Small 2 pass 12x20? core and only 1.25" thick. I'm now running 16.75x18x2.25 core 2 pass similar to what you have shown. I'm also running a turbo and oil to water cooler so all my cooling is handled by the radiator, no air/oil cooling. Temps are holding fine even with just a little 12" spal fan in stop and go traffic. The stock RX8 radiator end tanks might be 2" but the core is only 1" thick. The thickness helps with having more physical fin area for the same area core size providing you have the airflow to push through the thicker core. Duct it well. The big core also gives you a lot of added thermal capacitance so slower rise (and fall) and lower spikes in temperature.
