External Transmission Cooler
 

I know this has been debated several times….

I currently have my external transmission cooler connected in parallel with the radiator cooler…. However, after buying a cooler from NAPA (to put on my Astro Van), reading the documentation and talking to a mechanic, I think I’m going to change it to series.

Rationale:
1. Connected in parallel, the “majority” of the fluid will take the path of least resistance, thus eliminating the cooling efficiencies of the other cooler.
2. Connected in series, with the radiator first and the external cooler second, you get the cooling effect of the radiator and then the extra cooling from the external cooler.
3. I asked the mechanic about blockage in the radiator - he said it is very rare for a transmission cooling line to get blocked – (1) it’s usually a 3/8th. inch line, (2) it’s filtered (3) there’s no water in the system so there’s no rust, (4) the fluid is extremely hot, and (5) he said, "if you’re worried about it getting blocked… it could get blocked anywhere!"

Questions… does this make sense to anyone else and, how does the fluid flow, i.e. from the transmission to the right side of the radiator and return to the transmission from the left?

Thanks,
Bill

Re: External Transmission Cooler
 

Yes - to the fluid flow path.

Maybe to your mechanics opinion. In the early years (and if you have the OEM radiator), there is a fine mesh in the cooler part of the radiator. It's this mesh that caught and held clutch material (as they wore) and partially blocked fluid flow - thus heating up the ATF and in turn destructing the transmission.

Two solutions to the clogging problem: use the external add-on filter that Subaru recommended/installed under a TSB, or use a newer model radiator without the mesh.

BTW, I have a newer radiator and run mine in series for the same reasoning you use above - but that's never been tested by anyone that I know of - with a temp gauge to see if it's valid. The whole series/parallel argument has proven to be very popular - use the search button and you'll see what I mean. Might want to consider a back flush of the radiator cooler as recommended in the TSB for the filter install since you'll have the lines off anyway for the cooler change.

I would agree with Lee. I run mine in series for the same reasons. :)

Thanks Lee,

The one I’m changing this weekend is a 1994 L (FWD), I’m going to change the 1996 LSi later…Oh.. the ’96 has a radiator from a ’92…

Where is the wire mesh in the radiator??? Is there any way to remove it? I’d rather install the external filter.

Thanks,

Bill

the mesh is stuffed inside the cooler at the bottom of the radiator. I have no idea how to get it out, or if it can be gotten out.

the external filter was for the early models only, so I don't think Subaru would sell you one. I have read on the forum that a Legacy model (don't know year) uses the same filter, but an easier solution would be to go to your local auto parts place and get an inline filter. Also not sure how much this is needed as the later model transmissions had several changes made to materials used in the unit.

I have some pictures in my locker showing an installation of a PermaCool filter that works great. :)

I also have some pictures of my Permacool filter in my locker. Pretty much the same as Earl's but I put mine horizontal.

Now you ask - Which is better? Horizontal or vertical? Well, I don't think there is any difference in filtering, but when you change the filter, I think one is better than the other. I would say that I would rather have it horizontal because when you change the oil, it comes out of the thread end of the filter and down into the oil catch pan instead of all over the oil filter and down onto your hands.

Then why do I have mine vertical you ask! I installed it vertical without thinking about it. I may change that at a later time. :rolleyes: That info and $1.50 will get you a cup of coffee. ;)

fluid flow is fluid flow, and follows the same basic laws (okay, lets not get into laminar vs non laminar flow)and will follow the path of least resistance. Thus, a cooler hooked up in parallel will handle the majority of the flow unless it is more restrictive than the radiator-based cooler.
From all of the threads I've read, there is no way to remove the mesh in the '92 radiator. There is a TSB on flushing it out. I plan to follow the TSB before I install my cooler in series. I like the idea of having the "warming" aspect of the radiator during the winter.
-Bill

I have just got my hands on a B&M cooler and permacool filter.

I have a reasonable understanding of fluid dynamics, and all things being perfect, series is the way to go.

My concern is that I have a '92. If the original cooler mesh becomes clogged, there will be no flow through the B&M either.

If I install in parallel, I have 2 chances of cooling my tranny fluid, but will the B&M be man enough to give sufficient cooling if it offers the path of least resistance?

Also, if the fluid is travelling through both coolers, as it must in a perfect situation, (ie if the B&M has half the resistance of the stock cooler, it will carry fluid at a ratio of 2:1 with the stock cooler) then the velocity of the fluid through the cooler will be reduced, extending its contact time in the cooler, and therefore increasing the operation efficiency of the cooler.

Does it therefore follow that the benefits of series-vs-parallel depend on the cooling capacity/resistance of the two coolers being used?

Also is the fluid pump, pressure or flow rated? (ie does it try to maintain a constant pressure or a constant flow rate?)

If the pump is 'flow rated' then series would be best as the pressure would be increased to compensate for the increased resistance of the two coolers, thereby maintaining the flow through them.

If the pump is 'pressure rated' then parallel would be best as the flow rate would increase to compensate for the lower resistance.

If the pump is 'constant rated' could it then follow that parallel must be best, as in series the flow rate will be reduced by the increased resistance.


(Series RT = R1+R2 )

(Parallel 1/RT =1/R1+1/R2 )

(Velocity = Pressure/Resistance)



Sorry, brain smoking, overload imminent, time to bail out :confused: :confused: :confused: :confused:

Your concern is if the filter mesh becomes clogged? That is why we install a filter so that won't happen. :) If it isn't clogged now, the filter will keep that from happening, and if you think that it is a problem now, a back flush may be the answer. Good luck :)

I'm thinking about doing this this weekend. SO is there a way to remove the 'mesh' or not?

What benefits does backflushing the radiator do?

- Rob

I don't believe there is any way to remove the mesh, short of hacking the radiator apart (then you'd have bigger problems). There is a TSB on flushing the cooler out - basically involves a couple pieces of trans hose, a quart (?) of fresh trans fluid, and shop air.

I think the best way to flush the trans cooler
in the radiator is to flush it out with a parts cleaner solvent machine, blow it out with compressed air, and then flush it with clean trans fluid. I know not everyone has a parts cleaner but I work at a dealer so it's convenient. This is sure to remove any foreign particles.

As far as series vs parallel.....seems to me if you install a temp guage you will immediately know if any blockage occurs, right.

I have not seen any posts reguarding temperatures for a series or parallel system. And that is the point of the system.

By the way can anyone recommend a good trans cooler?

The filter kit from Subaru is currently on backorder. I've been waiting two weeks for it. I'll keep you posted.

Lee, Earl & others..

Thanks for all the help and suggestions… I didn’t have time to work on it this weekend… My daughter (claimed) she had a blowout and hit a curb in our Toyota Camry???? It bent the rim, the strut and the control arm – even cracked the windshield ??#$?!, so I’ve been working on that. Have you ever tried to compress a strut spring with a piece of crap compression tool!!! (and yes, it’s the same daughter (now 17) who totaled my 96 SVX into a tree) I have to fix it myself because I’ve afraid of my insurance man!

I’ll try to blow out the screen this weekend and let you know if it comes out… BTW, do you use just a regular oil filter in the Permacool?

Bill