Effect of Soft Snow Tires on Dyno Measurement Results

We get this question quite often about the effect on dyno numbers with winter tires, namely:

Does the soft compound, thick tread blocks, and high number of deep sipes used in a snow/ice tire affect how much torque/power a roller chassis dyno would measure?

The answer is, Yes.  Here’s a great writeup, courtesy of Airboy Tuning, that was done on another platform but still very relevant showing why, if you’re after PEAK dyno numbers, you shouldn’t dyno on winter tires.

Now, dyno tuning can still be done on any kind of a tire (as long as its not slipping on the rollers) as tuners are after deltas (before/after gains) but for PEAK number measurements stay away from winter tires ;)

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Winter tires lower the power output as measured by the dyno.  The key being power being measured by the dyno.  The engine, in both cases, is generating the same power.

The tires used in this comparison are from the opposite ends of the spectrum; one is designed for ice and snow while the other is for Tarmac.  From the pictures below, one could imagine the difference in tread deformation when the tire is stressed.  In relation to the test results, I would expect winter tires that uses shorter blocks and fewer sipes to experience less loss than the WS-70 and All-Season tires will likely have more loss than the RE-01R.

The Tires:

Some information test setup:

• Chassis dyno used in the test is a Dyno Dynamics 450DS AWD unit.
• Vehicle used is a 2006 Subaru WRX with catted Turbo Back Exhaust (TBE)
• Tires used are Bridgestone Blizzak WS-70 (New, 215/45/17) and RE-01R (~50%, 225/45/17)
• Tires are mounted on stock 06 WRX 17″ alloys inflated to 45psi

Brief description of test sequence:

• Wheels with RE-01R tires are installed onto test vehicle and loaded onto dyno
• Engine is warmed up and car is driven on the rollers to warm up tires and drive train components
• Engine RPM to achieve 40mph (dyno roller speed) in 3rd gear is recorded
• 3rd gear pulls are done starting from ~2000rpm up to >6000rpm
• Car is unloaded from dyno and wheels with Blizzak WS-70 tires are installed
• Car is reloaded onto dyno, repeating the warm up procedure above
• Engine RPM to achieve 40mph (dyno roller speed) in 3rd gear is recorded
• 3rd gear pulls are repeated

Results:
Power and Torque

Boost and AFR

It should be mentioned that after the test, I did not find any deposits of black rubber powder on the dyno, which would indicate tire slippage.  What I did notice in the data is that the “road speed” for the Blizzak WS-70, as determined by the dyno rollers, is lower than one would expect based on the “RPM at 40MPH” measurements made with light loading on the tires.

The dashed lines in the graph below indicates the theoretical Road Speed vs engine RPM.  For the RE-01R, there is a slight deviation near peak boost/torque.  For The WS-70, the recorded road speed deviates from the theoretical value right at the beginning of the pull and the difference is significant.  The deviation diminishes as RPM approach 6500, where boost and torque is much lower than the peak values near 3000RPM.

March 28th, 2010 update

Tested another car with snow tires, this time it is the Aurora W403.  The picture below shows the tread pattern (tire tested did not have studs).  It has fewer sipes and the tire compound feels harder.

The difference in dyno output is not as much as the WS-70 used in the earlier test, even though this car has a larger turbo and is generating more power at the high rpm range.