Radiator cap question

[Brd.Prey]

My Mishimoto radiator says not to use the cap it came with on the radiator because it is a two way cap and put it on the expansion tank. If it is on the radiator you could overheat. Can someone please explain what is going on here? I put a 2.0 bar cap on the radiator and 1.3 bar on the expansion tank both are Mishimoto. Is this a problem?

[aquillen]

I put a 2.0 bar cap on the radiator and 1.3 bar on the expansion tank

Good.

[Brd.Prey]

But they are both Mishimoto caps?

[aquillen]

I don't have that setup, but others have discussed this.

The lower pressure cap should certainly be an overflow style, and should vent off from the expansion tank to an open collection reservoir tank, i.e. non-pressurized tank, as Mishimoto says. At warmup the 1.3 then allows expanding fluid to go to the open reservoir for temporary storage. At engine cool down the 1.3 cap will allow the system to draw coolant back into the system from that reservoir tank.

Up front you essentially want to cap off the radiator so you don't have two places allowing expansion coolant to leave the system during warm up. Hence using the higher pressure cap in the front. BUT should the 1.3 bar cap ever fail to relieve pressure during an overheat problem (cap fails), that 2.0 up front would be a backup. So... some guys put another reservoir up there, which in your case go with another open to atmosphere tank. It should never really see expansion coolant go into that tank unless the 1.3 cap does not vent when it should.

One caveat which I don't have experience regarding your caps - it may (probably will) be that both caps will "draw" fluid from the reservoir at the same "vacuum" (actually just a lower than atmospheric pressure) during cool downs. I would bet that both caps will allow the system to draw back fluid at the same pressure which I think have nothing to do with the outgoing pressure settings of 2.0 vs 1.3 bar. If so then it would be a toss up which reservoir tank was drawn from first. If it were the front tank then you have a fluid transfer problem where every heat up pushes fluid to the rear tank and every cool down pulls fluid from the front tank. Some guys have reported that. You can see where that will go after a few heat cycles, empty front tank & overfilled back tank. In that case and if that happens, I think the best solution (what I'm doing actually) is to get the front system capped off so it can't vent period. I've been fiddling with where to install a sealed tank up front to catch any air that shows up there, but no vent or pressure cap - just a sealed up system in the front. --- or --- find a one way cap rated 2.0 bar so you have that backup pressure relief but no draw back through the front cap.

Someone like to simplify my discussion and just say what to do?

[Brd.Prey]

Thanks, yeah, that's what I thought but didn't know the front could refill at less than 2.0 bar. I looking for what the deal is on the Mishimoto caps.

[fletch]

Having trouble finding a decent link, but wouldn't "Wayne's mod" work for this venting problem? IIRC, the front radiator cap isn't a pressure relief device, but has a small hole that flows coolant continually to the degas tank. Any air in the radiator gets pushed to the degas tank by the constant, low volume fluid flow from the radiator to the degas tank.

[Brd.Prey]

I have Waynes mod.

[fletch]

I have Waynes mod.

Sorry for the misunderstanding. Looks like I better find that "Wayne's mod" reference and do some more reading...

[killerbmotorsport]

If so then it would be a toss up which reservoir tank was drawn from first.

The Subaru system caps are not only different in pressure, but function as well. The fill cap (commonly called the Upper Turbo Reservoir) has lower relief pressure and allows reverse flow into the system as it cools and comes under vacuum. The cap on the radiator is higher pressure, but ONLY a one way valve. It does not allow flow back into the system as it cools (vacuum).

If you guys are ever looking for a smaller version that retains proper factory function and utilizes the factory mounting points, we've been making them for many years. It was designed specifically for the guys looking to run rotated kits.