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Story and Photos by Carl Heideman
In TV shows and web series, engi…
I've gotta say I'm impressed with these bite-sized posts dealing with engine swap issues. They're excellent ways to break down the process step-by-step, and provide detailed information on what to consider doing, and why.
I'm curious here, though, about what you're going to do about a radiator fan?
Psst! Check out steps 8 and 9.
I understand the urge to reuse off-the-shelf parts as much as possible - but I would not have used pusher fans. It's not like 40 year old MGs are renowned for their awesome cooling systems anyhow. I would have put a big puller on the backside with a shroud. There's loads of space and it would be a lot more effective.
Those hammer formed mounts are beautiful.
You can always count on brackets that are much prettier than is strictly necessary from Carl & co.
How did you determine the height for the expansion tank to ensure a complete fill such that the water jackets in the head have no air pockets?
I'm slightly disappointed that you didn't round off the corners on the bracket in step #17.
Thanks for the kind words.
Regarding the fans, I agree about puller fans 99% of the time. However, the stock 77-80 pusher fans really blow a lot more air than almost every aftermarket fan we see. We regularly remove puller fans from 77-80 MGBs and put them back to stock to solve cooling problems.
Regarding the expansion tank, we just want its inlet/outlet up high enough to bleed air from the radiator and honestly just go with the MG factory locations.
77-80 MGB radiator setups were carryovers from the MGB GT V8s, so they're very robust and we have found them to work well in a lot of engine swaps. I've had this setup on my Miata-powered MGB since the mid 1990s.
Of course, this swap is our first one with AC and a supercharger, so we may be testing the limits...if so, there will be a 2nd installment.
p.s. Since the story was written, the corners from step 17 were rounded off--good catch. I try to radius every corner because I'm not a fan of unnecessary bleeding...
I'll be interested to see how it does for cooling. Supercharging and AC are a double whammy that's going to work that system harder. Those MG fans do look like they have big, high-torque motors which is better than most of the "slim line" units on the market. If you do go to a puller, look for the biggest motor you can find. We have some good Spal part numbers if you're looking.
What about the water nipple on the radiator that is pointing the wrong direction from the expansion tank?
And thank for this type of write up! It is most helpful for a newbie like myself to see all the little things that make for a great swap.
That radiator picture is actually a mockup from before we sent it to the radiator shop. We had the nipple turned the correct direction while they had the radiator apart.
Regarding how this will hold up, it's a restomod street car that will get some aggressive driving in short bursts, so I'm fairly confident it will be okay. This setup will probably struggle on a track with extended WOT/high horsepower periods of use.
For me, radiators are very frustrating because they tend to be very trial and error. If there was better math published and understood, sizing would be so much easier. For example, I think they should be rated with a horsepower/duty cycle with some sort of multiplier for air flow. Start with the Otto cylce rule of thumb: 1/3 of the engine's power (heat) goes to the flywheel, 1/3 goes out the exhaust, and 1/3 goes into the radiator. At idle, an engine is making maybe 2-5HP no matter how big it is. All it's doing is spinning the flywheel and accessories and it doesn't take much power to do that. At WOT and high RPM, the engine is making its peak power. What really matters to the radiator at WOT is how long the engine is at that peak range of power. On the street, it's in bursts of a few seconds at a time. On a track, it's much longer periods of time, almost continuous, for maybe 20 minutes. Whenever it's making that peak power, the radiator has to absorb it. That's where the duty cycle comes in. What percentage of a period of time is the engine at peak power? Then you bring air flow in: At idle, the engine isn't making much power so the radiator isn't working very hard, but it's got to have air going through it. Most idle cooling problems are air flow, most peak power cooling problems are radiator sizing (sometimes with air flow as a factor).
Anyway, that's what's going on, but there isn't much useful information/math to help us with sizing. Frustrating.
By the way, some members of Hope College's FSAE team are working on just this math for their car--should be very interesting.
Yes, airflow through the engine compartment at idle. I have wondered if there were holes or vents at the bottom of the front fenders where the fenders turn under the car if that would give enough airflow without cutting up the lines of the car. (My sentence structure is horrible, not enough coffee...)
It’s not just sizing. Fin density has an effect, and doubling the thickness of the radiator does not double the heat transfer as the efficiency drops. The best radiator will have a high fin density and a thin core with a large surface area. We did testing on this years ago and found that the big fat “race” radiators didn’t work as well as the skinny stock units that they were replacing.
Fans are often rated at 0 pressure as well, and some fans will drop off dramatically in flow as you stack up the heat exchangers. The best rule of thumb there is to look at draw - the more power the motor pulls, the more torque it has to jam air through your rad. And intercooler. And condenser. And oil cooler. And power steering cooler.
In reply to Keith Tanner :
I agree totally. I should clarify that by "sizing" I'm thinking more in terms of "capacity to cool size" than physical size and I wish there was more real math and data available. I also agree on the fan rule of thumb. You can't trust the CFM numbers they publish so generally the size of the motor (amp draw) is much more trustworthy.
Timely article, I will be doing this to my 75 MGB for a V6 swap. I will be using a V8 radiator, which is the same except the upper and lower hose connections are opposite the radiator you have (upper on left, lower on right). You can still get the factory radiator panels as well, but yours look better.
Keith Tanner said:It’s not just sizing. Fin density has an effect, and doubling the thickness of the radiator does not double the heat transfer as the efficiency drops. The best radiator will have a high fin density and a thin core with a large surface area. We did testing on this years ago and found that the big fat “race” radiators didn’t work as well as the skinny stock units that they were replacing.
Fans are often rated at 0 pressure as well, and some fans will drop off dramatically in flow as you stack up the heat exchangers. The best rule of thumb there is to look at draw - the more power the motor pulls, the more torque it has to jam air through your rad. And intercooler. And condenser. And oil cooler. And power steering cooler.
However, if you're already maxed out on radiator size, going thicker can help provided you can get enough air through it. In the case of my Jeep, going from a 1.25" thick single row core to a 2.2 inch thick double row core (2 rows of 1 inch tubes vs 1 row of 1.25 inch tubes) made a huge difference in cooling ability. There's no space to make the radiator bigger and at highway speeds, pressure buildup in the engine bay limits airflow even with vents in the hood. So the thicker rad doesn't really reduce airflow significantly and is able to dissapate more heat into the air going through it (radiator exit air temperature is higher).
That isn't always true. On the Miata, we changed to a crossflow design and went with a thin, high fin density core and improved our heat transfer. The new radiator didn't take any more space but it cooled better than the typical dual core "race" rad that we also tested. Testing
As the air heats up passing through the core, the rate of heat transfer slows down. That's why you lose efficiency if you simply start stacking cores. Sometimes it's all you can do, but it should be the last choice. That's assuming you get to specify every aspect of your radiator, of course.
One advantage of increasing the number of cores is that you increase the heat capacity of the system. This gives it a bigger buffer to deal with surges in heat output.
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