2nd Radiator (real fix for your cooling problems for $750.00)

Kinetic Analysis

New Member
Hey so I don't normally post and I know this could/should go in the technical forum, but I think this one is important. Many of you have notice by now that the RC390 is not, actually, Ready-To-Race. In fact it's not even ready for particularly hot weather. There are three problems that I had to solve on this bike that had nothing to do with performance and have not yet been addressed by the factory:


  1. The head gasket has issues, particularly on the early models
  2. The fan is a grenade
  3. The radiator is too small. Period.

I can't be sure but I suspect that #3 contributes to a bunch of other engine problems. The good news is that for $750.00 there is now a fix. Assuming you don't ride much in very cold temps you can add one of these:

https://www.hsbkracing.com/parts-and-apparel/ktm/rc-390-390-duke/radiator/febur-ktm-rc-390-secondary-auxiliary-radiator-2015-to-2018/

If you have done the Spal puller fan modification you can retain the fan. I recommend keeping it even if you are racing since the fan is still going to come on as you enter the pits on a really hot day. Yesterday it was about 100F at Thunderhill. Data from the ODB says intake temp was about 113F while the coolant remained at 190F on the front straight at a GPS measured 105MPH top speed. That's about where it would be on my 1299 in the same conditions. In terms that most of you will understand: The dash now shows the temperature at the knee.

The modification is pretty straight forward. It's a bit of a pain in the ass to get everything fit, but the chance of screwing it up is low and you don't need special tools. It does eliminate the thermostat, so your bike will take longer to heat up and you might have some issues if you ride in really really cold weather. Also I have an aftermarket lower fairing so I wasn't able to make the attachment at the bottom of the radiator, but it still fit with no issues.
IMG_20190616_154950.jpg

More pics...
https://photos.app.goo.gl/c28AZacJxJy5YmRK8
 

Formula390

Supporting Vendor
Vendor
The WSS300 kit has an auxiliary radiator as well. The Febur second radiator is a known modification, tho there's some debate on how much this is NEEDED for racebikes. For street riding, the SPAL fan is more than enough to keep the bike in decent temp levels. For most, all they need to do is swap their coolant over to distilled water (man is this a hot topic for the day or what?!) but here in Central Texas, I'm regularly in 100+ degree days and I've never had a problem on the street, track, or screaming it's head off on the Dyno. I did swap over to the SPAL fan, and swapped to distilled water only for my coolant, and have had no problems at all. This is on a 15 bike. I've never popped the head gasket even {knocks on wood} and the test mule here spends the vast bulk of it's time at or near redline. Is the Febur radiator necessary. I don't think so. Would it help, certainly. For me, personally, it's not worth the $750 price of the secondary radiator... everyone needs to make their own decisions there tho.
 

Andy

Member
Every bit helps but I’m not sure $750 is it, far better to put that money elsewhere. But your money your bike, do as you want!

Must admit those rads are quality pieces of kit!


Sent from my iPhone using Tapatalk
 

Lima6

Member
Will the secondary radiator put more strain on the water pump? (also, a regular point of failure for the rc390)
 

Kinetic Analysis

New Member
@KodakMoment @othebe Ok, so after I finally tore down the engine I can now tell you what caused that. At first I thought it was a broken impeller fan, but no. Big end bearing failure. That caused the classic thrown rod.

I've decided to part ways with my RC390, so if you are interested in the extended radiator you can send me an email: [email protected].
 

Andy

Member
Personally I've found that everything is ok with the fan mods, high pressure cap, removal of rubber shroud/inner fairing bits, Samco hoses (thermostat retained) and a good quality coolant (Motul).
 
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