Stupid Newb question but, do i need two radiator caps? I have an sti one in the lower right of the picture near the front of the car and then there's another cap that looks like the radiator cap in the upper left of the picture near where the turbo heat shield is? Which is the radiator cap? Do i have it in the right place?
http://i115.photobucket.com/albums/n320/Ray101381/wrx/DSC00525.jpg

mods: i didn't know where this thread would go so move if necessary.

here's a pic of a forester and it looks like both caps were switched to STi rad caps.
http://i64.photobucket.com/albums/h197/GottMogen/SEMADAY1023.jpg

It's not really a good idea to use the STi cap with a stock radiator. The STi cap runs at a higher pressure which has been known to cause leaks. To be safe and still have your bling you can transfer the sticker to your OEM cap.

hey sorry to go off topic but what all did you need to buy to have the STI top mount fit... and how difficult was it to put on

throttle body coupler, 06+ sti splitter and bellows, 90 degree elbow. the elbow and the coupler will have to be trimmed to the appropriate length to fit. back to the rad cap question. What is that cap in the upper left of the picture? Is that also a rad cap? Install was easy except i had to remove my strut bar to get the intercooler to fit into place then put it back on. The strut bar is a really tight fit so it was no fun getting it off then getting it back on, the good ol hammer was needed. also the bpv gasket is different on the 07 wrx than it is on the 07 sti, so you'll need a new gasket or cut your own(i cut my own).

bump, anybody know what that cap in the first pic is in the top left to right of the airbox?

That is the coolant expansion tank, the reason that you don't need a turbo timer.

Ray,
I don't have a perfect answer for you, this is something I've actually contacted Subaru about, however Mulder is quite incorrect in his oversimplifying the situation.
First, that is not the coolant expansion tank. The coolant expansion tank is located to the right of the radiator if you are looking head on at the car. The cap in the top left of your picture is, in fact, according to Subaru the actual radiator cap on a WRX or STI. This does seem odd considering you would assume the radiator cap on the radiator to be the "radiator cap"
That is part of the cooling system and it is used to provide the turbo with coolant. As you can tell from the hoses it is connected to the radiator which means that both caps are in some way linked in the same system
The advantage of higher pressure radiator caps is that it makes the coolant more efficient so to speak by raising it's boiling point and allowing more heat to be transfered to it before vaporizing. Something that is very beneficial in say a race car. If there are two caps in a system it would stand to reason that if one cap is of a lower pressure rating than it is the "weak link" in the system and will always vent pressure at that pressure. However, if you look at your stock caps you will notice that they are not of the same pressure rating. So, again you have to assume the subaru engineers put the higher pressure cap on for a reason and that it will preventy coolant from vaporizing and escaping up to that pressure, so in theory increasing the pressure of the higher capacity cap should help if it works independantly enough from the other cap to warrant different pressure levels on the caps.
The real question is how changing the pressure of the turbo tanks cap will affect the system, which is the answer i can't give you with 100% certainty, it seems from the pressure ratings that it sees less overal system pressure, but who knows.
The only "right" answer we're ever going to get on this is from whoever designed the coolant system, otherwise it's just speculation. As I said, I've contacted subaru about this, and once I hear back from them I'll let you know, however I've never seen an STi radiator cap cause a coolant leak, and if anything it would have more to do with stock hoses than the radiator. If you're not racing the car I wouldn't worry about the safety of having it installed. But you're right, it is a great question, is it even affecting anything if you don't replace both caps?

Ray,
Just got off the phone with an engineer at Subaru, we've been discussing a car we're building with them. Lol, he gave me a set of cliff's notes for replacing the radiator caps on stock cars;

Cliff notes:

Functionality: replace reservoir cap, not radiator cap

Rice: replace radiator cap, not reservoir cap

Bling: replace both caps


Which makes sense, I was making the mistake of assuming that the cap on the radiator was functional since it is rated to 1.37Bar, but according to Subie, it is not a functional radiator cap, it simply aids in flushing the coolant system, the only truly functional cap is the one located on the turbo reservoir, so changing that cap is the only way to effectively increase the system operating pressure.

later,
Andrew

Which makes sense, I was making the mistake of assuming that the cap on the radiator was functional since it is rated to 1.37Bar, but according to Subie, it is not a functional radiator cap, it simply aids in flushing the coolant system, the only truly functional cap is the one located on the turbo reservoir, so changing that cap is the only way to effectively increase the system operating pressure.


yep, a quick glance under the hood of an '02 would have told you that. (they only have one, on the small tank)

Ok so I was mistaken in referring to the tank by that name, I was in a hurry when I made that post. Subaru calls it the "fill tank". It is in that location so that the heat rising from the turbo after hot shutdown will promote coolant circulation by means of convection.
There have been reports here of higher pressure caps causing leaks on the OEM plastic tank radiators, this is most likely on the older types that did not have the cap as they were weak to begin with and tended to leak at stock pressure. Maybe it's safe to use the STi cap on the updated/cap style radiator but unless the car is modded or an aftermarket radiator is installed it's probably best to leave the original cap(s) in place. The factory cooling system is more than adequate for stock to mildly modded HP levels.