Trans cooler lines too big?
 

Somehow I accidentally kinked one of me OEM transmission cooling lines on my 05 Civic. No idea how that happened, my best guess is it happened when I changed the trans. So far the kink is not causing any problems, everything is operating correctly and the trans is running nice and cool with my aftermarket cooler. However, the kink obviously can’t be good, so I am going to replace those lines.

I would rather use braided stainless AN hoses all the way to my cooler than those OEM metal lines and rubber hoses because IMO they look much better and may be more reliable. I was initially going to go with 8AN lines (1/2 inch) rather than the stock 3/8 inch lines since the price difference is negligible and I figure less restriction could never hurt. In my mind, even if flow doesn’t increase, the pump won’t have to work as hard to circulate the fluid through the larger lines, possibly extending its life, reducing fluid temps, and slightly improving efficiency by reducing the power required to run the pump.

However, I have read that using cooler lines that are too large can actually be a bad thing because they may flow too well and will cause lower line pressure in the trans making it shift softer, which I certainly don’t want since I did mods to raise the line pressure since I like firm shifts.

So my question if restriction in the cooler lines is actually desirable. I don’t see how restricting the flow could ever be a benefit, but I just want to be sure that I am not going to cause a problem by installing larger lines. Thank you very much in advance!

As long as it's designed like other auto transmissions I have worked on it won't.

You mean it won’t cause a problem?

I’d bet the main restriction is the cooler(s). 3/8 or 3/4 lines might see almost the same pressure and flow.
If you did flow more, I would think it would be maintained at the same pressure, and actually increase the load. You would also be warming up more fluid, and increasing vehicle weight. Probably going to cost you 0.00021mpg
I would think the factory setup would be the most reliable, unless they have to be modified to use.
It may be slightly easier to route and secure smaller lines.

The cooler wouldn't be a significant restriction, my cooler has 1/2 ports on it and is quite easy to blow through compared to a couple feet of 3/8 rubber hose. I don't have the stock cooler in the rad anymore because I installed an all aluminium Mishimoto racing rad years ago, so the only cooler the fluid flows through isn't restrictive.

I would think that the load on the pump would be a tiny bit lower when it is working against less pressure, but I may be overlooking something. I don't think the cooler line pressure is regulated.

Cooler flow is a controlled and calculated bleed to the system. The new Fords I work on have a thermostat in the cooler line system that only allows flow when the oil is up to temp. This makes my flush machine not work properly because the thermostat stops the flow. So it is back to drain and refill on the new stuff.
Use whatever lines you feel good about.

Thank you. What is the order of fluid flow? Pump to valve body to torque converter to cooler I think? Or is the cooler in parallel with the pump's output so more flow through the cooler means less flow through something else?

I think it is pump to converter then everything else. On a Ford 4r100 if the cooler plugs up the rear bushing doesn’t get lube and fails quickly.

Thanks. So what is the cooler supply line connected to? Is that the fluid directly out of the torque converter?

Thanks. Does the trans rely on restriction in the cooler lines to build enough pressure to operate correctly, or is less restriction in the cooler lines good? For what it's worth the return line dumps the fluid right back into the case of the transmission, it doesn't go to anything. Thanks