No Additive
We began our testing with a 50/50 mix of Honda antifreeze and distilled water. Under stock conditions, cooling system temps skyrocketed to 215 F and stayed there. Before the Prelude engine swap, this Civic rarely saw anything over 185 F on a hot day. This happened despite our pleasant 72 F Southern California temperatures outdoors. We also realized cylinder head temps in the neighborhood of 210 F and idle temperatures that never seemed to want to stop climbing. With the stock mixture, our radiator showed a minimal decrease in cooling system temperature. The car was drivable, but not something we wanted to get stuck in extended traffic with.

DEI Radiator Relief
Our first test additive, DEI's Radiator Relief mixed with distilled water, brought us down to a cool and much more manageable 183 F during low-rpm street driving. Under heavy load conditions we watched temperatures rise to about 205 F, but they stabilized - unlike our initial stock setup, which didn't seem to want to stop climbing. We also saw cylinder head temperatures drop here to 191 F - a good thing since our H22A4 has a knock sensor that was probably pulling timing out from knock due to excessive combustion chamber heat with the stock cooling mixture. We suspect a slight power increase here too. Radiator Relief performed best when mixed with water alone. Once antifreeze was added to the mix, temperatures rose by a factor of about two percent, but only when subjected to extreme load and rpm. So, unless you're interested in a race-only application, there's little to lose here from adding antifreeze to the mix.

Red Line Water Wetter
Red Line's Water Wetter is one of the first cooling agents we came across years ago, and have always had good results with. We've just never tested it against other cooling agents, or against itself when mixed with straight water as opposed to antifreeze, for that matter. The Water Wetter produced similar results to DEI's Radiator Relief and was only slightly bettered by it when mixed with water alone. But when the products were mixed with Honda antifreeze, Red Line took the lead - over everyone in fact. We realized temperatures only 3 F higher than DEI's straight water mix in most cases even when combined with our 30/70 antifreeze/water mixture. This was quite impressive to see an antifreeze cooling agent mixture come this close to a pure water-based cooling system, which typically runs at least 10 F cooler. And the good thing about Red Line's Watter Wetter is that it passed the ASTM D2570 Simulation Service Corrosion testing. What the hell does that mean? It means this product passed the same tests the big coolant companies passed and it won't corrode your engine when mixed with water alone.

Hy-Per Lube Super Coolant
Hy-Per Lube's Super Coolant wasn't far off from Red Line's Water Wetter, also producing positive results when mixed with antifreeze and water. When subjected to our pure water test, DEI's Radiator Relief still took the lead in comparison. But the Hy-Per Lube impressed us, especially under extended idling conditions where it posted the lowest number for any of the agents when combined with antifreeze - 204 F. Even when subjected to heavy load conditions and when mixed with antifreeze, the Hy-Per Lube mix wouldn't let us go any higher than 207 F.

Justice Brothers Radiator Cooler
The Justice Brothers Radiator Cooler, which brought us down to 177 F at several points during our test drive and usually hovered in the low 180 F zone, caught us off guard. We weren't expecting results that low after testing some of the other products. These readings were all taken when using pure distilled water mixed with the cooling agent alone. When subjected to an antifreeze mix, as expected, the results weren't as impressive but still reduced temperatures a bit. We saw a similar temperature increase just like we did with DEI's Radiator Relief. The water-based Justice Brothers solution not only produced the lowest cruising temperatures but also the lowest temperatures at freeway speeds and high-load hill climbs. As for cylinder head temperatures, Justice Brothers' Radiator Cooler came in second place at 193 F.

And The Winner Is...
So who's the overall winner? Well, that depends on what's most important to you, since each product performed best in different ways when subjected to different conditions. For the best in overall cooling when mixed with water alone, Justice Brothers' Radiator Cooler takes the lead. Its cruising and freeway temperatures were the lowest, which is where we spent most of our test time. But DEI's Radiator Relief wasn't far off. And to be fair, Radiator Relief was subjected to 5 F hotter temperatures outside during its test period. This could explain some of the discrepancy between the two products when mixed with pure water. But despite the hotter outside temperature, Radiator Relief still outshone the competition with the lowest cylinder head temperature and by far the lowest idling temperature. As for Red Line's Water Wetter and Hy-Per Lube's Super Coolant, these two proved to be the best of all products when subjected to an antifreeze mixture. Temperatures were naturally hotter than when tested with pure water, but negligibly. As far as our test results are concerned, we see no reason not to run an antifreeze mixture when it comes to these two.

DEI's Radiator Relief gave our Civic the lowest cylinder head temperatures and lowest idling temperatures out of the bunch when mixed with pure distilled water. It also came in second place as far as average temperatures go when mixed with straight water.

Red Line's Water Wetter proved to be the best all around cooling agent when mixed with a 30/70 antifreeze/water mix. We saw readings as low as 197 F at the cylinder head and the most efficient heat transfer through the radiator at just over eight percent.

Hy-Per Lube's Super Coolant was almost identical to Red Line's Water Wetter as far as results go when mixed with antifreeze, but it did better the Water Wetter when it came to heavy load and lengthy idling - not by much though.

Justice Brothers' Radiator Cooler gave us the lowest temperature during our testing - 177 F - and provided the lowest average temperatures overall when mixed with water alone.


Mixture 50/50 Water 30/70 Water 30/70
Outside Temp. 72F 70F 62F 65F 65F
Cruising Temp. 196F 183F 187F 181F 188F
4000 rpm / 75 mph 198F 190F 186F 183F 187F
5000 rpm Hill Climb 215F + 205F 208F 203F 207F
Extended Idling 215F + 200F 205F 205F 204
Cylinder Head Temp. 208F 191F 197F 193F 198F
Radiator Temp. Drop 5% 7% 8% 8% 7%