I have a grille shell that is 21" tall and 16" wide. What is the best radiator for a mildly built Merc flathead? Is that going to be big enough to cool the engine?
You might as well be asking why the sky is green. Details Please. Pictures help. What is mildly built, hp expectations, transmission, AC, etc.? Do you have a hood, fully enclosed or open sides? No one can possibly answer your question with any degree of accuracy with the information provided.
They don’t cool very well with a stock radiator. How do you think it will cool if you make the radiator smaller?
It is for a speedster that I am imagining. It will have closed hood sides. The grill shell is a 28-29 A that will be narrowed to 16"s and shortened to 21"s tall. I don't have pics but that is what I have drawn. No AC. Manual transmission most likely a T5. Engine is a .060 over 51 Merc with a Max1 cam, dual Stromberg 97s and stock heads. I am hoping it will be 140-150 horsepower but that may be optomistic.
It will be a challenge, but I think you may be able to get it done with a 16" x 21" cooler. Maximize the airflow through the cooler and engine compartment (lots of louvers, one big ass fan), and maximize the heat rejection capacity of the cooler (lots of fins, big tubes, thick core, etc). Just for comparison's sake, 1958 to 1960 T-Birds came with FE's, automatic transmissions and a fairly tight engine compartment. The T-Bird radiator has an 18" x 24" core, and handled the cooling requirements of a bigger, more powerful (though at least slightly more thermally efficient) engine. Your rad has 78% of the core area as the T-Bird, so I think there may be a chance.
I’m using a rear mounted rad in my speedster build from a Dodge Dakota. Probably is slightly too big for what you need, but it is way bigger in terms of cooling capacity than what I’ll ever need for my mostly stk flathead. Core dims - 21 7/8” H x 17 11/16” wide https://www.amazon.ca/dp/B08FBJ3QQ5/?tag=atomicindus04-20
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