Results on an AWD dyno vs. a 2WD dyno: closer to actual power on the road?

15296 Views 12 Replies 6 Participants Last post by desperado callado, Jan 31, 2009

desperado callado

Discussion Starter · Jan 29, 2009

So, I saw a quote in a post elsewhere from DynoDynamics which states that 1/2 or more of apparent powertrain loss on a dyno is actually friction loss from the tires, while loss from the actual powertrain components is only 1 or 2% per gearset. If true, it occurs to me that this is the major reason that an AWD car can conceivably dyno significantly lower than a 2WD cars and still accelerate as fast. Look at the dyno on Cobb's site for the MS6. On it's face, it's not even in the same ballpark as the MS3. (I know: different dynos, different days, different locations, the delta is all that matters, etc., etc.; but even allowing for that I think my point holds water.) Yet we know that even with the weight penalty, the MS6's acceleration is neck-and-neck with the MS3's.

Sure part of this is better traction, and not just at the dig, but at higher speeds as well. But the more significant part may be is that dynoing an AWD car takes into account 100% of the friction losses from the tires, while dynoing a 2WD car on a single roller may account for only 1/2 of those friction losses.

This may be another reason (in addition to possible interference from your traction/stability control) to dyno 2WD cars in AWD mode. Now, I'm not sure exactly how it works when a 2WD car is dynoed in AWD mode. On a Mustang, the rollers are linked so that the rear wheels spin at the same speed as the front, but rear wheel friction losses still may not be accounted for somehow. Maybe somebody who knows more about the subject can shed some light on this question.

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08crystalwhite_socal_ms3

· #2 · Jan 29, 2009

because the rear wheels are linked in no way to the front wheels, having the rear wheels spining while going through with a FWD dyno would not make a difference in dyno traction.

If that's what you're trying to get at?

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eurotuner

· #3 · Jan 29, 2009

why are people trying to reinvent the wheel?

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desperado callado

Discussion Starter · #4 · Jan 29, 2009

When you dyno a 2WD car in AWD mode (on a Mustang, anyway), they mechanically link the rear rollers to the front rollers so that they are spinning at the same rate as the front wheels. The question is whether the friction generated by the rear wheels against the rollers is somehow factored into the force/power measurements.

Since the front roller is powered by the front wheels, presumably the rear wheel friction factored in. If so, the readings would be lower but would more accurately reflect the total powertrain loss when you're on the road. Clear as mud?

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smakdown61

· #5 · Jan 30, 2009

If the front and rear rollers are linked into awd mode, then with a fwd car you are not only spinning the front rollers, but the back rollers AND back wheels as well.

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eurotuner

· #6 · Jan 30, 2009

edit

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08crystalwhite_socal_ms3

· #7 · Jan 30, 2009

The MS3 is not AWD, why would you want to try and duplicate a AWD dyno on a FWD car.

Powering 4 wheels is more complex and involves more components = higher % of power lost through drivetrain. The reason they accelerate off the line quicker is because they have more traction to propel them from a stop to speed. Put a FWD car and a AWD car in the snow and see which of the 2 has more traction.

*The point of a dyno is to gauge how much power your drive wheels are putting to the ground.*

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desperado callado

Discussion Starter · #8 · Jan 30, 2009

[quote author=08CrystalWhite_SoCal_MS3 link=topic=134751.msg2900105#msg2900105 date=1233307659]
The MS3 is not AWD, why would you want to try and duplicate a AWD dyno on a FWD car.

Powering 4 wheels is more complex and involves more components = higher % of power lost through drivetrain. The reason they accelerate off the line quicker is because they have more traction to propel them from a stop to speed. Put a FWD car and a AWD car in the snow and see which of the 2 has more traction.

*The point of a dyno is to gauge how much power your drive wheels are putting to the ground.*
[/quote]
The possible effect of DSC/TSC is one reason. When the front wheels are spinning and the rear wheels aren't, these functions may effect what the ECU is doing. As far as the point of a dyno, I thought the point was to gauge how much power the engine is making while you are rolling. The effect of the friction developed by all the wheels is a significant factor that AWD cars have to take into account. It makes sense to me that 2WD cars should take it into account, too. That is, if you want a measure of power which is as close to the real driving experience as possible.

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smakdown61

· #9 · Jan 30, 2009

[quote author=desperado callado link=topic=134751.msg2900149#msg2900149 date=1233319676]
[quote author=08CrystalWhite_SoCal_MS3 link=topic=134751.msg2900105#msg2900105 date=1233307659]
The MS3 is not AWD, why would you want to try and duplicate a AWD dyno on a FWD car.

Powering 4 wheels is more complex and involves more components = higher % of power lost through drivetrain. The reason they accelerate off the line quicker is because they have more traction to propel them from a stop to speed. Put a FWD car and a AWD car in the snow and see which of the 2 has more traction.

*The point of a dyno is to gauge how much power your drive wheels are putting to the ground.*
[/quote]
The possible effect of DSC/TSC is one reason. When the front wheels are spinning and the rear wheels aren't, these functions may effect what the ECU is doing. As far as the point of a dyno, I thought the point was to gauge how much power the engine is making while you are rolling. The effect of the friction developed by the all the wheels is a significant factor that AWD cars have to take into account. It makes sense to me that 2WD cars should take it into account, too. If you want a measure of power which is as close to the real driving experience as possible.
[/quote]

+1. When my car was on the dynojet the ABS light came on at some point. When it senses a very high difference in front/back wheel speed it throws a fit. I still dyno'd 240/250 but not sure what effect it had.

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mgmsIII

· #10 · Jan 30, 2009

The only reason to dyno a fwd car on a awd dyno that has both rollers linked is to get the most accurate numbers based on friction losses like the op said. However there is absolutely no need for this as it is not the industry standard. As for the tuning aspect, a dynojet with load control or a mustang dyno are preferred to mimic the load that would be present on the vehicle while actually driving but again that is only for tuning. While tuning a turbo tacoma on our dyno with MAPECU, we hit a completely different set of cells on the dyno then when tuning on the street because we do not have load control. Is it a huge deal? No not really but there is a difference. Dyno we use is a dynojet 224x btw.

BTW never dyno a car with trac control on as it will severely screw with your numbers and in the case of auto cars it will usually just stall.

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08crystalwhite_socal_ms3

· #11 · Jan 30, 2009

If the point you are trying to get at is load bearing? Than that clears things up for me.

mgmsIII: your post made perfect sense, and I think this is what he's trying to get at.

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Sinitron

· #12 · Jan 30, 2009

The short answer to all of the theorizing in the first post is: No.

If you hook up a 2wd vs. awd car (mazdaspeed and subaru for instance) to dynapacks which mount to the hubs, you get proportional differences vs. any other system.

If someone triggers an ABS light, it's most likely from letting off the throttle on the dyno instead of pushing in the clutch.

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desperado callado

Discussion Starter · #13 · Jan 31, 2009

[quote author=Sinitron link=topic=134751.msg2901293#msg2901293 date=1233350725]
The short answer to all of the theorizing in the first post is: No.

If you hook up a 2wd vs. awd car (mazdaspeed and subaru for instance) to dynapacks which mount to the hubs, you get proportional differences vs. any other system.

....

[/quote]Short answers followed by vague statements aren't particularly useful. What exactly are you disagreeing with? Here's the exact quote from Dyno Dynamics on which I based my "theorizing": The biggest source of loss in the entire transmission system of a car is in the tires - they account for half or more of the total losses between the flywheel and the rollers.

If you disagree with that statement, then please provide another more accurate figure for the losses attributable to the rolling resistance of the tires. If you accept this as roughly true but disagree with the extrapolation that doubling the number wheels on a dyno (and therefore the amount of rolling resistance) will yield significantly lower hp/torque measurements, then please explain why you disagree.

As to 2WD vs. AWD yielding proportional differences on a Dynapack vs. another system, I don't really follow. I assume that you are speaking from experience, so can you explain better what you mean and what the significance is?

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