What would happen if..

[francesca]

I asked someone the idle question - what would happen if I put my left foot on the brake, buried the throttle then took my foot off the brake. He reckons my gearbox would probably promptly become a widespread heap of bits...

Is he right, has anybody tried it? Is that what you mean by power braking?

[Ian C]

That's power braking alright, and you can get away with a couple of seconds of it.  Good for launches.  

 

What you don't do is rev it up in neutral and slam it into drive, that's when you get the exploded diagram of an autobox.

[Rich J]

If your talking AUTO the rear brake calipers will loose grip on the discs and will start to spin brfore the gearbox blows up beleive me, launch  tech. on AUTO is to keep your foot stomped hard on the brake with left foot, rev the engine upto about 2500rpm, then lift the brake off quickly and stomp hard on the accelerator pedal. Obviously keeping the brake on and revving to 2500 for too long is going to heat up the Trans. fluid so don't do it too often for too long.

[Guest Ash]

 

Yes, that is commonly called a power-brake launch. It will not damage the gearbox mechanically. Well, not directly. What it does to is to cause a RAPID rise in gearbox oil temperature.

 

All that energy, held back on the brakes, has to go somewhere. Most of it gets converted to heat. If the oil gets too hot then its molecular structure begins to break down, which can eventually lead to mechanical failure.

 

According to Toyota, the condition should not be held longer than 5 seconds. Which is plenty of time for a quick getaway at the lights.  :biggrin:

 

Yours,

J

[Branners]

Also, if you plan on doing it quite often then get the gearbox oil changed on a fairly regular basis. The costs of oils in miniscule compare to the damage that can be caused by oil breakdown.

 

I use the powerbraking at Pod for a good launch, I have only used it twice when not at Pod, once was in a race against Terry and Leon at Bentwaters and the other way in a race in Brighton against Justin. Mostly I just stomp the accelerator unless Im going up against something I know if seriously quick.

 

JB

Shop!!

[Guest Ash]

 

 

Yes, I agree on regular gearbox oil changes. Problem is, you can only drain a small quantity of oil from the box. Which means, any breakdown of your existing fluid will simply contaminate the new fluid.  

 

To do a full oil change you have to remove the oil-cooler pipes at the bottom of the radiator, extend them, then put the outlet pipe to a drain bucket and dip the inlet pipe into another bucket containing about 3 gallons of fluid.

 

Start the engine and let it tick over. This will flush new fluid through the box *and* most importantly the torque convertor.  

 

Yours,

J

[Phil]

I was thinking of doiny mu auto fluid but the job seemed a bit of a pain, and from what you are saying Ash it doesn't even work too well.

 

I take it Toyota wouldn't employ your method so I guess I'll have to have a go myself

 

 

[Guest Ash]

 

 

All Toyota will do is drain off a small quantity of fluid from the oil-pan at the bottom of the box, then top-up. Reason being, that's what it says in the service manual.  

 

The only way to do a proper full change, with the gearbox attached to the car, is how I describe above. It's easy to do. All you need are a couple of lengths of three-eigths bore pipe, two push-on straight connectors, two big buckets and 3 gallons of fluid.

 

Yours,

J

[Phil]

Any particular recommendations on which fluid to use?

[Guest Ash]

 

I use Castrol ATF III but I suppose any decent ATF III fluid will be okay.

 

Yours,

J

[Phil]

I best get some clean buckets, fluid and hose then

 

Will it matter that the front of the car is on axle stands, or should I put all four corners up to keep it level?

[Guest Ash]

 

 

Probably best to keep it level.

 

Yours,

J

[Phil]

Will do.

 

Thanks Ash.

 

Phil

[francesca]

Not such a daft idea then - thanks guys! :biggrin:

[Steve Gee]

Is that really the way it's done?

 

If I took my car to Toyota, or any garage, and asked them to change my auto-box oil, I would expect them to change it. ALL of it!!

 

Would all garages do the same??

 

Steve

 

[Phil Wall]

 

If I fit RLTC I will have to use power baking to get any benefit from the launch system.  So my oil will get cooked after 'some time'.

 

OK the auto box oil change doesn't sound too much of a PITA.

 

Any one suggest what a reasonable ' some time’ might be.?

 

If I can fit a temperature sensor to the auto box oil cooler what would be the maximum acceptable temperature before the oil would start to break down?

 

 

[Guest Ash]

 

There is an "issue" that surrounds the Auto gearbox-oil change. In the sense that if you ask a garage to change your gearbox oil, all they will do 99.9% of the time is to drain off the fluid at the bottom of the box and top-up with fresh fluid.

 

Problem being, around three quarters of the old fluid will remain. But for a stock auto car, under normal circumstances, this situation is acceptable. After all, the oil-change regime has been set by Toyota and calculated to give a long service-life under the conditions imposed by the normal operation of the vehicle.  

 

However, once you digress from those normal operating conditions, other considerations come into play.

 

Doing repeated powerbrake starts is a perfectly acceptable way of getting a speedy launch. But it is not something Toyota would have figured into their definition of normal operating conditions. Likewise, the repeated use of manual mode and/or turning up the boost. All of which place increased thermal stress on the gearbox oil.

 

The reason why the oil gets such a hard time is not so much due to providing lubrication for the gearbox... it is the action of the torque convertor that screws-up the oil.

 

In an auto box, the oil has a triple role to play:

 

1) It provides lubrication for the whole mechanical assembly.

 

2) An oil pump contained within the gearbox generates a certain amount of pressure. This pressurised oil is switched along various channels, at precisely timed moments, in order to change the gears.

 

3) It is this third role that is the most difficult: which is, transmitting all the power that the engine generates into the transmission chain.

 

With a manually shifted gearbox, there is a direct mechanical connection between engine and transmission. But with an auto, no such connection exists.

 

The actual connection between engine and transmission in an auto is, in fact, made by the oil.

 

Apart from times when in top gear, at speed, where a condition called high-speed lockup is engaged, every horsepower the engine generates is actually transmitted first into the gearbox oil. Following which the energy is released by the oil into the transmission.

 

The whole torque-converter action is based on Newton's 3rd Law of Motion: which states that for every force there is an equal and opposite force. Another way of looking at it is to say that for every action there is an equal and opposite reaction.

 

Within the torque converter body, there is an oil pump that is driven by (and turns at the same speed as) the engine. Basically, this pump forces oil along particular channels which cause the oil to quickly change direction. As the oil changes direction, there is generated an equal and opposing force. And it is this very opposing force that drives the transmission.

 

So with a power-brake start: say the engine is generating 100HP at the stall-speed of the converter, which is about 2400 rpm. 100HP is 74.6 KW of power. In other words the heat energy of around 75 1-bar electric heaters is being transmitted into the oil. Normally, the car would be moving and virtually all this energy would be immediately be converted into motion. But in this case the car is being held stationary by the brakes. All that enery *has* to go somewhere, and it is simply converted into heat which will very quickly cook the oil.    

 

Having an oil to air transmission cooler doesn't help much as this type of cooler requires the car to be moving in order to generate sufficient air-flow through the vanes of the radiator. There is an oil to water cooler, built into the bottom of the water radiator, but it's too small to dissipate the heat energy of repeated power-brake launches.  

 

One idea I had was to have an oil to air cooler with an electric fan that came on under certain conditions, say, with the brakes on and throttle position at maximum.

 

The upshot of this whole thing is 1) being aware of the far greater thermal stress placed on the gearbox oil once you begin tuning the car, and 2) realising that the normal definition of changing the gearbox oil doesn't actually change the oil at all.

 

The total auto gearbox oil capacity is almost 9 litres. What Toyota call a drain and refill (i.e. what happens when you have the car serviced) reqires only 1.9 litres of oil.

 

Yours,

J

 

 

[Phil Wall]

Another excellent post Ash.

 

I remember Nowton’s laws from school days.

 

I knew the energy had to go into the oil, but I had no idea that the amount of energy transferred was so high.

 

75kw even for a few seconds is a hell of  a lot of heat.

 

The majority of us run auto boxes and most of those are dealing with condition beyond those assumed to be within normal range by Toyota.

 

I can also see that a temperature sensor activated system of cooling the oil would be of little use as the heat transfer is so rapid. The system would not kick in fast enough . 

 

The car needs a system to be active before the condition that lead to heat rise occur. Perhaps be cooing the oil down before the intend conditions are applied

 

Given that such a system does not exist, unless you have one under development

 

A- Advanced Transmission Cooling System?  

 

Are there any ways of examining the oil to ascertain whether it has passed its best?

 

Should we be looking at a fixed time scale related to hours of use and just assume that for a percentage of that time the oil had been used beyond its normal operating range and therefore will have changed  its characteristics?

 

 

 

 

(Edited by Phil Wall at 9:02 am on July 15, 2001)

[Guest Ash]

 

 

It is difficult to answer your points with any real degree of accuracy. Problem being, it all depends on the car's state of tune, how it's driven, etc., etc.

 

What I do is a complete oil flush every 3000 miles.

 

75KW is a *lot* of heat. You are dead right there. And you are spot on when you say about having a system to be active *before* the oil begins to rise in temperature. That's why I think switching on an electric fan, mounted on an oil to air cooler, under conditions of brakes on and full throttle, would be a better idea than detecting temeprature rise.

 

I don't know how relevant the following calculation is, but I thought if my 2KW electric kettle causes a temperature rise of about 80 degC to 2 litres of water in 10 mins. Then if those same figures were applied to 9 litres of oil and 75KW you are looking at an 80 degC rise in 16 seconds.  

 

Also, in the back of my mind, I remember reading a report about gearbox failures by some company that re-built them.

 

I can't recall right now who they were but no-doubt their name will pop in mind when I'm not thinking about it. They reckoned virtually all the failed boxes they came across, failed due to some lubrication problem or other that could be traced to excess heating of the oil.

 

It will be interesting to see the results of the tests I'll be doing to mine once I've got the engine side of things sorted.

 

Yours,

J  

[Phil Wall]

Obviously there are many factors that are variable to peculiar to individual cars and drivers.

 

Your example of an 80 degree rise assumes all 9l rise together.

I suspect that there will be a smaller volume of oil that will rise much higher than this, depending on flow rate.  I believe it would take some time for the heat to be evenly distributed round the system.

 

As your rule of thumb method suggested 3000 mile, my oil has to come out very soon.

 

Certainly in the next couple of weeks.

 

It certainly makes sense to include a full flush in a regular service routine.

 

Thanks for highlighting this for us.

 

 

 

[Flipfinger]

So the moral of the story is:

 

If you're parched, and need a cuppa pronto, put water in your transmission fluid reservoir and then do a power start!

 

16 seconds later, you're ready for the Pyramid bag!