Radiator Cap?

wstar

So looking around random parts sites, one of the "performance" mods I keep seeing is replacement higher-pressure radiator caps. Some of them even word it like you're going to get better cooling (and thus better performance) from installing such a cap.

My understanding is that the rating on a radiator cap is the pressure at which is will pop open and dump boiling coolant out the top of your radiator (which is a point you never want to reach, it's just a safety system to prevent blowing out some other part of the cooling system, like a hose). Assuming you're not already blowing your cap by running insanely hot, a higher-pressure cap isn't going to make a damn bit of difference to your cooling right? I'm not getting it.

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travisjb

newer high performance radiator caps use this:
Cold fusion - Wikipedia, the free encyclopedia

wstar

so it's just snake oil, as I suspected, at least on a relatively stock street car?

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travisjb

Honestly, I've never used them... but the claim is that by allowing for a higher pressure release point, the boiling point of the fluid is therefore also higher (remember from physics that pv = nrt... higher 'p' means higher 't')... that means the fluid can continue cooling the engine at higher temperatures before it boils... the rest of the cooling system needs to also accomodate these higher temps... i suspect nissan spec'd the cap appropriately to allow for sufficiently high psi... also, not hearing of any excessive water temps, mostly oil temps...

I'm interested to hear other opinions, but my gut tells me this is not needed

reference
PIAA High-Performance Radiator Cap :: Sigma Automotive

wstar

Yeah I think unless you're already having problem boiling your coolant (esp if running all water for racing), increased pressure rating at the radiator cap valve isn't going to change anything. Anyways, check this out:

I checked on my car, my stock radiator cap is labelled 137kPa, which is 19.87 psi. Our service manual gives the following values under "radiator" (units are kPa (psi)):

Cap relief pressure: Standard 122.3 - 151.7 (18 - 22)
Cap relief pressure: Limit 107 (16)
Leakage testing pressure: 157 (23)

I'm interpreting that to mean that the stock system was designed ideally for a cap that opens in the 18-22 psi range, and that any cap that opens at less than 16 psi should be replaced (faulty). Additionally, leak-testing the rest of the system (the radiator and hoses, etc) should be done at 23 psi, so we know it can handle that much pressure, but who knows how much beyond. The stock cap rating is right in the middle of this "Standard" cap relief pressure range.

Now over at: The Z Store, they're selling an increased performance Nismo radiator cap that they claim works on all kinds of older Z's as well as our new 370Zs, offering increased pressure, blah blah. Their description says:

So the "performance" radiator cap they're offering to us for the 370Z is actually a lower pressure rating than our stock caps (although still within the "standard" range). On the other hand, the PIAA one you linked is 186kPA, which is well above the leak test pressure for our stock cooling system. If something goes bad enough to cause those pressures in our stock system, I think I'd rather have the cap let loose than blow a hose (or worse).

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travisjb

Great research wstar... REP coming your way !