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Discussion Starter #1
I'm curious. I always thought that VTM-4 would send a maximum of 50% of torque to the rear wheels, but now I'm beginning to wonder.

VTM-4 has three engagement modes:

1) "Acceleration torque control (ATC) mode." When you are accelerating, it sends torque to the rear. Acura says "rear wheel torque rises smoothly from zero to the maximum setting in proportion to vehicle acceleration" but note they don't say what the setting is.

2) Torque is sent to the rear when slippage is detected. Acura says "torque is proportional to both slip rate and the rate at which the slip rate is increasing," but once again doesn't say how much torque is actually sent.

3) "VTM-4 Lock mode." When you hit VTM-4 lock, and are in 1st, 2nd, or reverse gears. You get max torque at low speeds, and at a certain point, it begins decreasing progressively until 18mph.
E.g. on the Acura Canada site, they say "the maximum amount of rear-drive torque is locked in until the vehicle gets moving and exceeds 10 km/h, at which time rear drive torque is gradually diminished. By 29 km/h, the lock mode is fully disengaged." But they don't say how much torque is actually sent.

I seem to remember "50% as a number, which is quite good, but now I am wondering if I was wrong about that. Or is it that 50% is only sent under certain modes.

For 2003, Acura claims that "for 2003, the VTM-4 system has been remapped to provide up to a 30 percent increase in rear torque for enhanced handling and increased steering feel, especially in slippery situations." That makes me really doubt if it was at 50% before. Once again, Acura cagily avoids saying what is the maximum amount of torque that can be sent to the rear.

Has anyone any literature that specifies the actual amount (percentage) of torque sent to the rear?
 

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Darn good observation wmquan.

Didn't the MDX brochure have a blurb on this? Sorry, I'm still at the office and don't keep a brochure here!

Beyond that, NAC/NAFC.
 

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According to 2002 MDX brochure

"up to 55% of power can be sent to the rear wheels". So based on this, plus what we are seeing about this "extra" 30% for this year- one could logically deduce that the VTM-4 can now send up to 85% of the torque to the rear wheels.
 

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Re: According to 2002 MDX brochure

GABulldawg said:
"up to 55% of power can be sent to the rear wheels". So based on this, plus what we are seeing about this "extra" 30% for this year- one could logically deduce that the VTM-4 can now send up to 85% of the torque to the rear wheels.
For my understanding. If it is 55% now, an extra 30% means 71.5%, instead of 85%.
 

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The marketing-types who write the release information are either math-challenged and/or think their readers are. It's impossible to tell from what they wrote whether it's 55% + 30% = 85% or 55% + (30% * 55%) = 71.5%.:(
 

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If the new figure is 55% of power going to the rear wheels, it's more likely that the 30% increase to arrive at 55% means that the previous amount sent to the rear was in the neighborhood of 40-45%. A 30% increase on 42.5% comes to 55.25%.

Marketing types really can make numbers say almost anything!
 

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Discussion Starter #7
Re: According to 2002 MDX brochure

GABulldawg said:
"up to 55% of power can be sent to the rear wheels". So based on this, plus what we are seeing about this "extra" 30% for this year- one could logically deduce that the VTM-4 can now send up to 85% of the torque to the rear wheels.
An extra 30% probably means 30% of 55%.

Now, here's the key question Acura does not answer: up to 55% in which VTM-4 mode? All modes? Or just two modes? Or just one mode?

Or, do they mean that if 55% is the max of one mode, that one or two of the other modes has been increased by 30%, possibly meaning that no one mode goes more than 55%?
 

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These are all very good questions!!!
I , too, wondered about this!!:confused:

If anyone can shed some light on this....PLEASE DO!!
 

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Hey all,

If i may inject a little comment. The extra 30% to rear wheels is probably not going to be useful, especially not "for enhanced handling and increased steering feel, especially in slippery situations..." VTM-4 is a on/off system. The benefit of this is only really felt when the system is on. When does it come on? WMQUAN answered that already. So, does this "enhanced" handling in slippery condition?? Not really, because you want FWD in slippery condition, don't you?? Understeer is better than oversteer in THIS condition.

Now, during NORMAL driving, VTM-4 is shut off (if no slippage). Thus, you don't see any benefit to this extra 30%. Your MDX will respond like any other FWD car/SUV.

BMW X5, new 4Runner, etc. have a rear-biased FULL-TIME 4wd system (on all the time). Now, this is useful during normal driving because you get the advantage of RWD-effect when apexing the corners.

Thanks.
 

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Sorry, tigmd99, but VTM-4 is NOT an on/off proposition. The current going to each individual VTM-4 clutch coil is controlled separately, and can vary anywhere between zero and maximum. I have seen charts in the Service Manual, but nothing to indicate the actual current or torque percentages.
 

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Frost,

So, is VTM-4 on/activated when you're driving on the highway and there is no slippage??? Are the rear wheels engage in this situation?? Isn't it "off" in this situation??

I understand the VTM-4 is always "monitoring" but the rear wheels do not get power until: (1) slippage; (2) accelerating from slow speed/stop; and (3) VTM-4 LOCK button is press.

Thanks.
 

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tigmd99 said:
I understand the VTM-4 is always "monitoring" but the rear wheels do not get power until: (1) slippage; (2) accelerating from slow speed/stop; and (3) VTM-4 LOCK button is press.

Thanks.
No, that's not entirely correct. 1 - slip detected, 2 - ANY acceleration, *OR* 3 - VTM4 lock.

But what frostyra said is that it's VARIABLE torque; not an all or nothing proposition, since the amount of torque to the rears is GRADUAL (either up or down) and not a "fixed percent or nothing" proposition. It might be shooting 5%, 12%, 35%, whatever, or nothing back there.
 

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Discussion Starter #13
Some FWD-biased systems always send some torque to the rear. With the MDX, it appears that no torque is sent during normal cruising conditions; pure FWD.

E.g. the Subaru 4-cylinder automatics always send at least 20% to the rears, all the time. The theory is that at least you're getting some power to the rear at all times. And as tigmd99 has pointed out, some systems send a lot, or most, of the torque to the rear at all times.

Acura has always been a bit elusive when it comes to when torque is sent (e.g. kicks in during acceleration; but how much acceleration)?

Unfortunately there are some limits that are imposed by VTM-4. The propellor shaft to the rear clutch packs (not a true differential) results in a decent amount of heat being built up. That's one reason why the design doesn't send torque as often to the rear. It's also why you have to change that VTM-4 fluid so often compared to most (all?) other vehicles.
 

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Hey all,

Yeah, you guys may be right. However, it does not really matter. Acura's claim of improving handling, especially in wet, is a bit wrong. Why in world would anyone want more power to the rear in wet conditions?! (Assuming that the front wheel still have traction.)

In wet, i want FWD or, at least, FWD-biased 4wd system! On dry land (no slippage), i probably want a more RWD-biased system to improve handling...thus, X5's main goal in it's (unreliable) 4wd system. Either way, i don't really care because SUVs are not sports car. All i want is stability control (VSA) at highway speed, and the '03 MDX has that...cool. Finally!

Thanks.
 

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tigmd99 said:
Hey all,

Yeah, you guys may be right. However, it does not really matter. Acura's claim of improving handling, especially in wet, is a bit wrong. Why in world would anyone want more power to the rear in wet conditions?! (Assuming that the front wheel still have traction.)


Can you please explain why it would be "wrong" as you claim?

In wet, i want FWD or, at least, FWD-biased 4wd system! On dry land (no slippage), i probably want a more RWD-biased system to improve handling...thus, X5's main goal in it's (unreliable) 4wd system.
Also, can you please explain why a RWD biased 4WD/AWD system would improve handling on dry pavement, and additionally, why this would not apply on wet surfaces if it is indeed true.

The X5's AWD system is pretty simplistic in design with only a main single stage transfer case splitting the torque 38% fore/62% aft. Haven't heard of any problems with it...no clutches to wear out/fluid to change, etc. Just gears basically.
 

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Drew,

In rain, when cornering, if you send more power to the rear, you're going to spin. Same in snow. Pulling is better than pushing when surface is slippery. Thus, FWD cars are preferred in these conditions.

On dry land, a RWD car can be induced into gradual oversteer around a corner, thus allowing to go around faster...while a FWD car will have a tendency to go straight (understeer). Thus, most REAL sports car have RWD.

Uhhh, you haven't heard of any problems with X5's system?? Check Consumer Reports next time. BMW has had much problem with the electronics on it's 4wd system (X5 and 330xi). X5's system is all electronic...relying on slippage to engage brakes on each wheel. It is not just a "simple" 38/62% split.

Thanks.
 

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tigmd99 said:
Drew,

In rain, when cornering, if you send more power to the rear, you're going to spin. Same in snow. Pulling is better than pushing when surface is slippery. Thus, FWD cars are preferred in these conditions.


That's not true actually. What you're forgetting is that we're not just pulling OR pushing. What we are doing is pulling and pushing at the same time with two pairs of driven wheels on two axles. Since more power gets transfered to the rear, it is less likely that the front tires will get overwhelmed past their tractive limit, resulting in understeer and directional control losses. This is why cars with 50/50 split permanent 4WD/AWD systems have very neutral handling. While transfering power is not as ideal as having a pro-active system since power transfers can affect other things as well, it is certain better than nothing.

On dry land, a RWD car can be induced into gradual oversteer around a corner, thus allowing to go around faster...while a FWD car will have a tendency to go straight (understeer). Thus, most REAL sports car have RWD.
Um, when was the last time you saw someone doing a power slide around a corner on city streets during daylight hours with traffic around? Anytime you oversteer, you lose speed and hence you get a slower time. Ask anyone who has done track driving. The trick is to apex at the proper speed so that your vehicle does not start sliding out, requiring corrections (more corrections results in slower lap times).

The reason why performance cars have RWD, or better yet AWD, is because the wheels that do the steering don't have to do the propelling as well. If you get one set of wheels to do both tasks, they get overwhelmed a lot quicker. Additionally, FWD cars tend to be quite weight biased towards the front, whereas RWD cars are better balanced. Weight transfer is important since on acceleration, the weight is transfered back, to where the pair of drive wheels are on a RWD car.

Uhhh, you haven't heard of any problems with X5's system?? Check Consumer Reports next time. BMW has had much problem with the electronics on it's 4wd system (X5 and 330xi). X5's system is all electronic...relying on slippage to engage brakes on each wheel. It is not just a "simple" 38/62% split.
Curious. You must have a special version of Consumer Reports. I have been a subscriber for about 8 years now and from what I've seen, they don't distinguish between electrical problems with the 4WD system, etc. It's all one big lump category. I have been following BMW message boards before the X5 made its debut and the one thing that I can say for certain is that the AWD system as well as the stability control + traction control system has been pretty much trouble-free not just with the X5, but with their 3-series as well. :confused:
 

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VTM-4

It had better be an on-off system or those clutches will not last very long!
The fluid is doing a few different jobs, that most 4WD systems dont require of there differential fluid.
It is gear oil, hydrolic system fluid, and wet clutch fluid all at the same time. This has more to do with why it needs changing than the torque the system transferes


The torque supplied to the rear wheels has more to do with how much traction is available than what the system is doing.

One more tidbit:
50% torque to the rear is what a good old fashion 4WD system gives you when all wheels have traction.
 

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Pilot VTM Fluid Change ?

Now that most of you have owned a X for awhile, I have a question for you-

The Pilot's manual suggests that during NORMAL conditions, change the fluid at 15k. During SEVERE conditions, change it at 7.5k.

What have most of you guys done with this???
 
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