Radiator Sizing

I've been told that the formula for radiator sizing is 1 square inch per HP. That brings up several questions:

-Is that for single row or double row?

-What's the ratio of conductivity of aluminum versus steel cores?

-How much heat rejection can I anticipate from piping"? For instance if you had a rear-engined car and were transplanting the radiator from the rear to the nose, and as a result had 15 feet or so of rigid 2 inch tubing as well as some additional rubber, how much heat might that tubing radiate?

Thanks,

Berk

That's a pretty rough rule of thumb. Very rough.

Take a look at the fin density, that can have quite an effect on efficiency - and the more efficient the radiator, the smaller it can be. Stacking cores decreases the efficiency of every new core, as the air going through the core heats up and heat transfer slows down.

For your steel tube, it'll depend on the amount of airflow over the tube and the diameter. There's probably a calculation per square inch of surface area that you can use, assuming a constant supply of ambient air.

according to that, my Fiesta is overcooled.

when ice racing, the heater does most of the cooling.

Racing cars, Nascar, Indy, F1 would have Huge radiators.

Funny, A 1980 Fiesta radiator is the only one that I can find that fits my nose well. I think that we may have spoken about this earlier. It's going to be on a 250 HP motor, so is probably inadequate, but there is all that bare piping as well. Alternately they make an Aluminum race version of the Fiesta radiator. It's just a bit more money than I'd like to pay.

There has to be other radiators that will fit? I raced a 1980 Fiesta in IT years ago and did get a larger radiator to replace the OEM. The OEM was old and not as efficient as a new one would be. The VW radiators seem like they may work? My current race car is a 1988 Scriocco 16V and I have no issues with cooling, and the radiators are common and cheap. Are you using an air dam? Forcing air into the grill helps and an air dam does that.

There are three aspects to cooling an engine:

Transfer heat from the engine to the coolant
Store the heat in the coolant
Transfer the heat from the coolant to the air

You can't do a whole lot about the first without some fairly fundamental modifications, but some engines do much better than others. A 500 hp LS3 is easier to cool than a 250 hp turbo Miata engine for this reason.

Storing the heat in the coolant is about having a high boiling point, a high specific heat capacity and magic juju. Basically, make sure your cap is good and you're not running 50% antifreeze.

The last is the one everyone focuses on, because it's big and shiny. You need an efficient radiator that maximizes heat transfer and you need lots of air moving across it. Don't just concentrate on getting air to the radiator, make sure you can get air out of the engine bay - or whatever's behind the rad. If you have high pressure in the engine bay, you'll have a hard time forcing air through the rad. This is why Exocets don't have any trouble with cooling despite the same hardware that's on a suffering turbo Miata, because the open sides of that engine bay ensure there's no pressure buildup.

This may sound stupid or like cheating but what if you looked up the size of radiators for various 250-300hp cars. I dont like doing math and seems plenty of manufacturers have already done the math and testing.

And every thing keith said is golden, im just here in the cheap seats shouting ideas.

I seem to recall that the locost guys like the cheap, ebay, Honda Civic "racing" radiators (the thick core, all aluminum, "half width" kind), should be fine for 250 hp, especially with the underbody piping.

Problem is that I've got a wide narrow slot for the radiator, and precious few fit well. This one shows the most promise:

http://www.ebay.com/itm/40MM-ALUMINUM-ALLOY-RADIATOR-FORD-FIESTA-I-MK1-M-T-1976-1983-1982-1980-/161271739189

Xj Cherokee is the next step up from the fiesta one if you don't have more height but can go wider.

What dimensions do you have?

Summit has universal radiators that you can scope out by size. Keith, Say I picked up one of those chevy 6.0 LS truck engines and threw a small turbo on it and was making around 600HP. How big of a radiator do you think I'll need? The biggest I have ever been able to stuff in the nose of a TR8 was 26X19 with a 3" core. The distance between the frame rails is just a tad over 26" and leaning the rad at an angle, I can get a 19" tall rad in there. I'm thinking I might need a little more than that, and I'm not oppose to cutting away the frame rails at the front to make more room. The car is basically tube framed inside the shell anyway, and the original 70 pound front bumper has been replace with a fiberglass replica, so there is no need to carry the frame rails that far forward any more.

I too have heard of that rule and maybe guilty of spreading it here.

As mentioned, there are a ton of variables that will influence how it will work. Ducting/fans and how well hot air can escape the engine compartment are two big ones.

Hence I have modified the rule of thumb to say that if you can fit 1 square inch of rad per HP then you should be able to design something that works.

I've been doing some research on this topic lately, and like Keith said, the transfer of heat from the radiator to the air is only one part of the equation. Unfortunately I'm starting to accept that there is no rule of thumb that's adequate since the specifics of each car are SO important to how well a given radiator works. Things like ducting and flow management are hugely important.

If anyone figures out a reliable way to calculate this, I'd love to know about it.

I wouldn't discount all that tubing. MR2 radiators are right up against the frunk -- there's probably ~ 3" of space behind the radiator (with electric fans taking most of that) and then it hits a wall. But then there's ~20ft of aluminum tubing under the car.

Stock MR2 cooling systems perform just fine with BIG hp.

This is probably one of those things that you have to guesstimate and monitor. Log temps in and out, and engine temps and go pound on it.

Some numbers: the rad we use on our V8 conversions (and is installed in both my MG and my V8 Miata) is 25.375 x 15.25 overall. Core surface area is 345 square inches. The core is 32mm (1.25") thick. Cools a rampaging LS3 nicely, needs more help with a turbo Miata with around half that area. Some of that is pre-rad airflow (intercooler and AC condenser vs no other heat exchangers and a big airdam/splitter), most of that is the efficiency of the engine. Now, this is a rad that was designed to be particularly efficient, with a crossflow design (dual pass for the V8 version, mostly for packaging) and a very high fin density.

TR8Todd, I don't know how it would cool a turbo LS motor. But I'd be wary of most generic radiators, they're generally competing on price more than anything else. Ours, with good ducting (especially with a V-mount giving separate airflow to the IC) would be a pretty good bet. Here's some interesting data we generated - the dual pass was a monster with a 3" core from AFCO.

https://www.flyinmiata.com/tech/rads.php

BTU's

It would be nice if radiators had their effective BTUs listed, could make doing swaps easier.

Kreb, if you can't fit one big radiator, it may be possible to have two smaller radiators. One may have to be located elsewhere in the car, which may cause more issues than it fixes.

Radiator sizing always, always boils down to: How much will fit in the nose?

Theory this and theory that, it all just tells you how much you should have if you could actually fit it. Which you won't.

BTU/hr doesn't tell you anything other than looking at horsepower increases relative to stock.

Heat exchangers for cars aren't rated for a certain heat dissipation rate since they're so highly dependent on airflow. Maybe someone that works with heat exchangers professionally can chime in, but I believe BTU/h ratings are usually based on clear airflow.

well.. wide skinny opening.. you need to think outside that "box" My father's old 300ZX had a radiator that was laid down on it's side, with ducting to pull air up from under the bumper.

Do not be afraid to tilt the radiator to make it fit the space. You can always remove the standard cap (if your radiator has one) and put a pressurized reservoir in the engine bay and just a simple bleed screw at the highest point at the radiator.. or vice versa

Knurled wrote: Radiator sizing always, always boils down to: How much will fit in the nose? Theory this and theory that, it all just tells you how much you should have if you could actually fit it. Which you won't.

This. Just jam in the biggest radiator you can reasonably fit. Bigger is better for cooling. Your car is already designed and built with an opening for the radiator, don't leave any of that space on the table.

Also more passes are better, and the next best thing is more rows. The 3-row 3-pass is the holy grail of radiator designs.

(Theoretically more width & height is even better than either more passes or rows, but practically, they can only be so big).