So I'm pretty much halfway into re-gearing my differential to a 4.083 FD on my 2006 5AT Coupe and I know running at higher speeds in top gear the RPM difference compared to the normal final drive is higher.

Now, about once a month I do a 140 mile, 2-and-a-half-hour commute and use the interstate and I can easily find myself doing 80 to 90 mph (I know, I speed, I'm sorry, its an addiction) for a lengthy portion of the trip which means I'm definitely going to be increasing the RPM's the engine sees constantly for those 2 and a 1/2 hours.

So my question to you guys is, do I need to get any sort of additional cooling mods (automatic transmission cooler, bigger radiator, both) to prevent damage, or do you think the stock cooling system will suffice for this kind of use? That commute is pretty much the longest trip the car will see, but like I said, I do it pretty much every month.

Thanks for your help!

 

youll be fine,only chance of overheating is coming from smashing the pedal down constantly to feel the new gears in action...but your golden my friend happy motoring

 

If you really wanted to you could install the nismo finned differential cover but I don't think you'll need it. Some people install an external cooler, again I don't think you'll need it.

I'm thinking about installing a fan on my transmission cooler, this summer is going to be miserable.

 

I had it all last summer in Phoenix with no additional engine cooling and had no issues.

I am adding a larger oil pan, the Pathfinder cooling mod and an oil cooler during my stroker build.

 

Awesome, thanks for the answers guys, I appreciate your expertise.

Kind of an off topic question, but where exactly is the stock tranny cooler? I want to say the the lines go into the bottom of the radiator IIRC. Is there a dedicated portion of the radiator that strictly cools the tranny fluid?

 

AT has a cooler at the bottom of the radiator. MT none.

 

4.08 gears on a 5at!?.. get a trans cooler. the rpms will be too high for too long at 90mph. i've put transmission into limp mode so i can tell you will will need it. the radiator only has a tube that runs in the lower tank of the radiator. i'm not sure about 2005 ecu's but the 2003 ecu does not have support for 3.7 gears and greater. 2003 only supports 3.3 and 3.5, at 3.7 the tcs kicks off too much depending stagger versus square tire setups. 3.7 on a 2003 the speedometer stays about 5mph off at those speeds. please chime in with the 4.08 gears and let us all know if the 05 ecu is able to read the 4.08 gears and maintain accurate speedo and if the traction slip light starts blinking. supposedly cruise control is lost too. i'm curious since i don't know anyone with 5at that went to that great of a diff gearing. after 3.7 the 5at with DE engines gets slow because it doesn't rev up like the HR motors do. when i had the 3.7 it was great in city, but got doggy at 90mph+ pulls against other z's. i only had the advantage pulling from under 60mph. after that the longer gears from other z's would pull on me.

chime in after the 4.08 install!

 

Like cabrio said, factory cooling is in the radiator. Most aftermarket radiators are for mt only. I mounted my cooler under the radiator.

 

With any AT car, I think a nice external cooler is always worth having. If you are mechanically inclined at all, you can install one for less than $100. Heat kills ATs, so anything you can do to reduce heat will increase the service life. The extra RPMs shouldn't hurt due to the fact that your fluid is still circulating/being cooled. But yes, a cooler would help reduce temps.

 

I have a 2006 G35 5AT, I don't know if I made that clear or not. From what I read about others installing 4.083's they had no speedometer errors, apparently the ECU reads speeds from a source (I'm not sure where) that is independent of the gearing so the speedometer should not be off due to swapping gears? I do know that Cruise Control is lost, no biggie though, I don't use it anyway. I completely understand the top speed loss, I'm in it for improved 0-60 times anyway, not top speeds and do mostly city driving so I'm glad you say it was great in the city. Thanks for the input man, I'll be sure to keep you posted, but could you clarify what you mean by the ECU not supporting shorter gears?

 

OK, so lets say I want to play it safe. Can I get an aftermarket radiator for a manual transmission car (without the lower auto tranny cooler section) and run the auto tranny cooler lines to and from a dedicated aftermarket auto tranny cooler? Would this work? Would this be a good idea? Or is there more fluids that get cooled by the stock radiator that I need to take into consideration? Or any other problems with this concept?

 

It should work with zero problems. It would definitely increase the cooling capacity if your transmission. As far as I know on autos, antifreeze and at fluid are the only things that run threw the radiator.

If your radiator works fine then why replace it? I would say kink the tranny lines so you don't loose a lot of fluid, install a at cooler, connect the lines and check the fluid levels. You could plug the transmission lines in the radiator and no longer use them.

I'm looking at a summit catalog and I'm thinking of running a cooler with a fan and wiring it to my idle fan.

 

YES! You are reading my mind man, I was just looking at some stand-alone coolers and noticed a few with those electric fans mounted right on them, call me easily impressed, but I think that is a pretty cool (no pun intended) feature for an independent cooler.

I guess you're right man, no use in replacing a perfectly good radiator, I guess I just had it in my mind that not using the built-in tranny cooler on it made that portion of it dead-weight and useless (space that could be deleted for weight savings or used to cool the coolant more), but from what I gather from you guys its just a tube running through a bath of coolant on the underside of the radiator anyway?

 

do not bypass the radiator cooling. there is a minimal operating temperature that the fluid needs to reach for everything to run efficiently. depending where you live, you route the fluid to the radiator first then to the cooler and back. that is ideal for non freezing climates. reverse for freezingt climates.

though it is true that the engine heat itself will warm the whole transmission, the radiator will assist on warming up the fluid and maintaining a close to efficient operating temperature.

speed readings come from the rotation speed of the output shafts from the differential with some kind of calculated input from the engine rpms and/or the drive axle rotating speed. 2003 the ecu registers the same speed for a 3.5 and 3.7 output shafts. what that means is, 3.7 in reality turns slower, but the speed on the speedometer shows that it does not, so it says you are doing 80mph, but you're really doing only 75 when using the 3.7 gears.

i do not know how the 2005 ecu is, but i would like to believe it will respond similar to mine with 4.08 gearing and show speedometer reading inaccurately. you will be able to confirm this for us.

my cruise control on 2003 worked fine with 3.5 and 3.7 tested up to 100mph no problems. tested with 3rd, 4th and 5th. since later models suffered the CC loss, it leads me to believe there will be incorrect speed readings with 4.08 gears. i think the 5at ecu was never really suppose to see gearing that was too short.

 

You have a valid point sir, however I think the OP will be fine since he lives in Florida. Maybe the fan on the cooler could be overkill for him, but for those of us who have VB upgrades and a external cooler is a must. I always let my car idle to operating temperature before I pull out of the driveway. I usually run slow for a few minutes before I hit boost, and even after I let the car idle down before I kill it.

I think running the fan to mooch off the idle fan wouldnt hurt, since it wont be constantly running. I live in Arkansas and the weather gets hot, but our winters arent as bad as up north.

 

Alright, so it seems that 4.083's are too extreme a choice for a 5AT from what you guys are telling me, luckily that was just my plan and I have not yet purchased a set yet, however I do have an offer for a set of 3.9's for dirt cheap with practically no miles. I'll probably end up taking those to be perfectly honest.

Then, I guess the best option for me is to run that tranny cooler in series with the radiator like power2rice said and leave it at that?

Also, power2rice, do you or anyone else know if it is possible to have the speedometer re-calibrated to read accurately with the new gearing then?

Thanks for all the help so far guys!

 

Yeah Arkansas weather sux!

Compared to sunny southern california!!! Lol

I lived in Rogers for 2yrs worst two years of my life
What can I say, I love Cali lol

Keep is updated op, you got me subscribed to this thread already
When are you looking to do the swap? And have you considered a VB upgrade?

 

Im thinking of pulling the trigger on this part:
http://www.summitracing.com/parts/HDA-526/
^^^The overall size is what Im looking for. Since I have an oil cooler that Im looking to install as well. (I could always install it behind the front bumper)The price is fair, the outlets match the diameter in the hoses that I have now, so I would'nt end up purchasing fitting, hoses and clamps. Not to mention it performs better than B&M products according to the rate of cfm.

35Alive, lmk If you decide you want to upgrade your valvebody, I have a tranny FS w/a stillen vb upgrade. I might even decide to throw in a free trans. cooler

 

Well currently I'm waiting on the rest of my bearings and seals from courtesy parts, they had all the bolts in stock so they shipped those out first, but I can't even use those until the bears and seals show up, as well as the ring and pinion which I am waiting to hear a reply from the seller. So right now I have an empty 6MT VLSD differential housing and a pile of bolts lol. I'm thinking about thoroughly cleaning it (get any old shavings out), but I don't want to ruin the VLSD mechanism, anyone know a good cleaner or method of cleaning the inside of a diff. that won't mess with the VLSD?

I have definitely thought about a VB upgrade, they sound great from what I've seen in the reviews, but I have neither the the cash to buy a prebuilt one nor the skill to modify one so I find myself kinda in a rut.

Looks good man, I'm glad you found a better performing one, and it looks like for a lower price too. Thanks for the offer on the tranny man, but I'm going to have to pass, between this build and the new body kit I really don't have the cash to spare. How is the Stillen VB upgrade treating you though? I've actually heard bad things about it compared to the Transgo kit or whatever else they use, but haven't experienced either.

 

from what i have researched online, i do not know of a solution for that calibration. the system reads pulses from the same abs ring on the axle shafts from the differential but also obtain other readings thats why a 3.5 and a 3.7 diff shows the same info... so i will assume 3.9 will show the same readings even though they are wrong in the real speed world.