Just getting around to fabricating a water expansion tank. What pressure radiator cap should I use. ETK says the cap is 2-bar (29 psi). TIS says to pressure test to 1.5 bar (21psi) So I suppose at least a 25 to 29 psi cap?

I run a 28psi cap with my zionsville aluminum radiator and aluminum expansion tank.

Just getting around to fabricating a water expansion tank. What pressure radiator cap should I use. ETK says the cap is 2-bar (29 psi). TIS says to pressure test to 1.5 bar (21psi) So I suppose at least a 25 to 29 psi cap?

Why would you run higher than stock? Temps should be lower if anything right?

I suppose you want the cap to blow in a controlled way prior to any of the expensive parts or catastrophic failure of a hose.

I suppose with lower temps, you could run a lower PSI cap for a bit more margin in the system.

Just wondering what people run with non-stock radiators and tanks.

I suppose you want the cap to blow in a controlled way prior to any of the expensive parts or catastrophic failure of a hose.

I suppose with lower temps, you could run a lower PSI cap for a bit more margin in the system.

Just wondering what people run with non-stock radiators and tanks.

Gotcha, I was just wondering what was behind running a higher pressure relief cap. Overall system pressure should be slightly reduced if the temps are reduced and if they're higher than should be it could hide a potential problem until it's too late. At least that was my thinking. I have an aluminum radiator but the stock plastic reservoir so I always run the stock cap.

Cheers.

Can I get lower pressure radiator cap anywhere for a e36 1995 325i ?

There are 140kPa caps that fit. Stock E36 325 and M3 cap is 200kPa. Late-model E30 325i (1990-91) take the plastic screw-on 140kPa cap. It's either embossed on the top of the cap or inside the bottom of the cap.

But the stock E36 can handle 200kPa / 29psi. You should never see more than a few psi on the track. If coolant pressure increases over 10-15psi, look for a blown headgasket.

There are 140kPa caps that fit. Stock E36 325 and M3 cap is 200kPa. Late-model E30 325i (1990-91) take the plastic screw-on 140kPa cap. It's either embossed on the top of the cap or inside the bottom of the cap.

But the stock E36 can handle 200kPa / 29psi. You should never see more than a few psi on the track. If coolant pressure increases over 10-15psi, look for a blown headgasket.

James, this doesn't even sound close to me. Granted, I'm a little new to the BMW engine but from my limited experience, the system IS designed to carry high psi to squelch bubble size from the cooling passages(nucleate boiling). I'm thinking something as high as 20psi is normal at about 195degF. Of course, that pressure is material/temperature dependant.

But you are saying race cars are coming off the track with "a few psi" cooling system pressure? You guys undoubtly have more experience than I do but this info just doesn't jive? I know several low pressure domestic systems(like our SBC sprint cars)run at low coolant pressure(about 10-15psi).

Thanks,
Don

When the coolant system temperature rises to a stable 190-200F from a track session (typical M50-based temps from most stock-style cooling setups), the hoses are not rock hard. Even though the cap is designed to hold 29psi or 21psi, the system should never see those pressures if the radiator is able to stabilize the temp. The high pressure caps are more for hot climates with AC on and engine at idle, where the coolant flow is slow and airflow through the radiator is limited. The radiator requires a much higher temp to stabilize the coolant, and that's when you need the higher pressure cap. Running hard on a dyno with modest, even inadequate airflow, the coolant pressure doesn't rise more than 5-6psi. I recently tested this on a 265RWHP IP car with factory water pump and Euro E36 OEM radiator.

Not surprising you're unlikely to reach those pressures. 29 psig = 44 psia. Saturated steam at 44 psia is 273 degrees F. I hope your coolant doesn't get that hot! (I'm neglecting temperature gradients in the cooling system of course.)

Have fun,

Chris

^^ Formerly a thermal hydraulic analyst, now a vehicle dynamics controls engineer