Auto Transmission Question

[Coug]

When you change your transmission fluid you're only changing about 30% of it, right? When you put in the new clean fluid how long does that take to mix with the other 70% of old fluid? Is it instant?

 

[GTBATV]

It's a dilution calculation. So 30% is
x=(3/7)^n. First change is 3/7 = 42%, second change is 9/49 = 18%, etc. As far as how quickly the old mixes with the new, I don't know.

 

[butchna]

 

[Coug]

It's a dilution calculation. So 30% is
x=(3/7)^n. First change is 3/7 = 42%, second change is 9/49 = 18%, etc. As far as how quickly the old mixes with the new, I don't know.

Are you being serious or is this a joke? Assumption was if I change the fluid once it's not 30% but 42%? So if I changed it twice in a period of say a week would it then be 84% changed?

 

[GTBATV]

Are you being serious or is this a joke? Assumption was if I change the fluid once it's not 30% but 42%? So if I changed it twice in a period of say a week would it then be 84% changed?

This is depends on what the capacity is. I based this equation on my wife's van, it holds 7 total. When I drain it, I can get 4 out. So on my first change, I leave 42% {x=(3/7)^1} of the old fluid in there. Assuming it's all homogenized after driving it, the second change would leave 18% {x=(3/7)^2}, and so on. On my F150, it holds 13 quarts and I can drain out 7. So with that, I'm leaving 46% {x=(6/13)^1} old fluid with the first change.

Plug in your capacity for my 7 and what you couldn't drain out for my 3 and work it out. For easy math, divide the amount left in the transmission by the amount it can hold then multiply that number to the power of whatever number of changes you are on. Multiply that number by 100 and that's the percentage of dirty fluid still in there. But it's a law of diminishing return. After about the fourth change, you are only moving the needle maybe 1%. You will never get to 100%. And you are draining out some of the new fluid with each change.

Or you could just take it somewhere and have them flush it and get it all.

 

[Buzzbait]

and you consciously chose to ask this question on VolNation?

 

[Coug]

This is depends on what the capacity is. I based this equation on my wife's van, it holds 7 total. When I drain it, I can get 4 out. So on my first change, I leave 42% {x=(3/7)^1} of the old fluid in there. Assuming it's all homogenized after driving it, the second change would leave 18% {x=(3/7)^2}, and so on. On my F150, it holds 13 quarts and I can drain out 7. So with that, I'm leaving 46% {x=(6/13)^1} old fluid with the first change.

Plug in your capacity for my 7 and what you couldn't drain out for my 3 and work it out. For easy math, divide the amount left in the transmission by the amount it can hold then multiply that number to the power of whatever number of changes you are on. Multiply that number by 100 and that's the percentage of dirty fluid still in there. But it's a law of diminishing return. After about the fourth change, you are only moving the needle maybe 1%. You will never get to 100%. And you are draining out some of the new fluid with each change.

Or you could just take it somewhere and have them flush it and get it all.

Ahh! I see where you came from now. Thanks!

and you consciously chose to ask this question on VolNation?

Yes, and I got a fairly decent answer. Although my local mechanic answered the timing mixture question for me today. He said once you change the fluid it only takes about a half hour to allow together.

He also said flushing is definitely NOT recommended on a Honda Accord.

 

[Firebirdparts]

Are you being serious or is this a joke? Assumption was if I change the fluid once it's not 30% but 42%? So if I changed it twice in a period of say a week would it then be 84% changed?

You both bad at math. Calculate based on the old fluid and you’ll figure it out easily. It’s a first order decay (diminishing returns).

 

[GTBATV]

You both bad at math. Calculate based on the old fluid and you’ll figure it out easily. It’s a first order decay (diminishing returns).

First order decay is for chemistry, not fluid dynamics. And diminishing marginal returns is a financial term as far as I know. But if you have an easier way, I'm happy to learn.

 

[Firebirdparts]

Okay, so it's actually very easy. Please for the love of God, do you own. For illustrative purposes let's say 30% new, 70% old.

change it once, and the old oil is 70%
change it twice and the old oil is 70% of 70%
change it three times and the old oil is 70% of 70% of 70%.
If you use the old oil in an obvious calculation, it's very simple.

if you don't like the 70% do you own.

 

[GTBATV]

Okay, so it's actually very easy. Please for the love of God, do you own. For illustrative purposes let's say 30% new, 70% old.

change it once, and the old oil is 70%
change it twice and the old oil is 70% of 70%
change it three times and the old oil is 70% of 70% of 70%.
If you use the old oil in an obvious calculation, it's very simple.

if you don't like the 70% do you own.

I think my equation was a condensed version of this.

 

[GTBATV]

Okay, so it's actually very easy. Please for the love of God, do you own. For illustrative purposes let's say 30% new, 70% old.

change it once, and the old oil is 70%
change it twice and the old oil is 70% of 70%
change it three times and the old oil is 70% of 70% of 70%.
If you use the old oil in an obvious calculation, it's very simple.

if you don't like the 70% do you own.

And with your math, you're also not counting for the 70% change of the new fluid.

 

[GVF]

My math is easier. 100% old. Have never, ever, changed auto transmission fluid. Milan is on 246K miles of original and only fluid. S-10 is up to 370K and still drives by my house by the guy that bought it. CTS had 220K on trans oil when I traded it. Engine oil and other stuff, I change and/or check regularly. But, I don't mess with trans oil.

 

[Coug]

Math Wars