All,

I just upgraded my stock 955 Porsche Cayenne Turbo spec wheels and tires (20x9 | 275/40/20 | Porsche Sport Design) to aftermarket wheels and tires (22x9.5 | 285/35/22 | Avant Garde Ruger Mesh M310s with Pirelli Scorpion Zeros).

I haven't put the new wheels on yet and am seriously considering doing some dyno runs on the stock wheels after a flash (I'm about to flash the car in a couple of weeks).

While flywheel HP output won't be any different with the new wheels, I will definitely loose some wheel HP due to the increased weight and increase in resistance on the dyno rollers. Has anyone done any similar dyno comparisons?

I have done some ratio calculations but I haven't found a single ratio calculation I would call accurate as such a formula doesn't account for resistance.

For example:

29.85 = 22" - Aftermarket
28.66 = 20" - stock
Divide = 0.96 ratio
0.96 x current HP estimate (500 HP crank ballpark) = 480.06
Approximate Loss =20 HP loss

I don't think this even comes close to telling the whole story. I'm betting it's worse than this due to weight and resistance.

FWIW, I'm not a fan of 22" wheels, but at the same time, I can't stand the massive gaping wheel wells the Cayenne's have.

 

Yep, you will lose acceleration due to the much larger diameter of the 22's adversely affecting the final drive gear ratios. Plus, as you note, the additional weight of the larger wheel/tire assemblies will feel like anchors at all four corners. Handling will become more sluggish while the engine is being asked to motivate the extra weight all of the time.


22's may offer some visual appeal, but they are significant detriments to improved performance. Sadly, there are always trade-offs when we pursue the upgrade path.

 

If you want to save weight, you would be better off going with a forged wheel (1pc or 3pc).

 

I disagree. Other than handling, I could not tell any performance differences in a straight line between my old 18" winter setup and my 22" summer setup. I think the difference here is the CTT has loads of torque throughout the powerband. Any acceleration performance deltas are negligible, IMO.

 

I have to agree. I have 21's and while tires are more expensive, I find its the best compromise of filling the wheel and having the perfect amount of sidewall.

 

100% agreed. To me the 21's just look perfect. I've seen a lot of nice 22" wheels very reasonably priced, but I think they just look a little too big.

 

If you are worried about the weight I would take the new wheels and weight them against the current set you have on the vehicle. Plus going with a bigger wheel and tire package will throw off your speedo as well as your mileage (odometer). Not to mention if the new wheels do weight more you will also see an increase in break wear.

 

The flaw with this assessment is the assumption that the factory designed the Cayennes performance ideal around a smaller/lighter wheel like a 17-18-19-20-21!? ALL of those sizes are offered from the factory stock and the highest "performance" variant according to the factory is the 21", with the 20" next in line. Everything else is not considered their performance wheels (17-18-19).

Seems the 22" for the cayenne application falls closer to (and technically above) the performance hierarchy than anything below a 20" wheel.

 

No flaws in the assumption. The OP was discussing moving from his 20" diameter wheels to a 22" wheel. Every point made regarding this type of swap is valid. It's a simple fact that the negative effect on gearing from going from a factory 20" diameter tire to the most common 22" tire (285/35/22) reduces the ability of the vehicle to accelerate as rapidly. Most would consider slowing a vehicle's acceleration as a result of a taller net gear ratio down a reduction in performance.

 

The tiny net change in in gearing is not even enough to be mentioned based on the size and hp/TQ of this vehicle. The difference from the largest factory tire/wheel to the 22" is less than 3/4" and barely over that from the 20 to 22". Porsche themselves allows for a much larger margin and variation across the whole range!

Having your mother-in-law in the car could make more of a net change in acceleration than the difference in these tire sizes.

The difference in outside temps from my morning drive to my afternoon drive has more of an effect on acceleration than the difference of these wheels!

There is a TON more to the calculation than just saying a minor gear ratio change equals such a notable hp change. Hp/TQ is not linear, so a change in gear ratio up/down doesn't directly up/down the ability to accelerate. Spending more time in the bulk of the power band is what is important. A reduction of gear ratio may allow certain Cayennes to spend more time in their power band, not less.

For example the Cayenne S has a much lower final drive than the Turbo. Porsche put taller gears in the turbo to make it faster, not slower! The lower gears of the S would make the Turbo burn through the power too quickly and not accelerate as well. (Similar to running too small of a tire)


Need to keep this in realistic perspective!

It's not like we are talking about putting 40" tires on a Jeep that came with 29" tires

 

The ability to accelerate is dependent upon the acceleration force the tire puts to the road, that is, the torque at the wheels.

All other factors remaining constant, and assuming a 2006 Cayenne Turbo S, increasing the diameter by 4.2% (29.85 diameter of 285/35/22 tire versus 28.66 diameter of stock 275/40/20 tire) reduces the acceleration force at the tire by 4% between 2,250 and 4,750 rpm (range of peak torque of 518 ft. lbs.), regardless of which gear the vehicle is in.

Simple math, put the 22's on and you WILL accelerate slower. That was my original point, and it's still valid. That said, when the PIG weighs in around 5,600 lbs., it doesn't really matter that much anyway. It's all good.

 

It's not that simple, but good try.

Why did the factory put taller ratios in the turbos if their goal was to make it faster? That flys directly in the face of your theory. So does the fact that the factory put 21" on their sport model(GTS) that they were making faster than the standard S.

Everything the factory does seems to exactly opposite of your "simple math" Maybe you should go show them how its done

I've built jeeps and race cars for 30years and sometimes to go faster, you need the taller (numerically lower) ratio, not the other way around. Just depends how that particular car and its needs put the power down. Not a simple linear equation.

 

I went from stock 17s to 22s and I actually think I noticed increased performance cornering and driving.

In terms of speed or 0-60, didn't feel a difference really or at least it wasn't noticable.

 

Sorry, but for the conditions I stated, it is that simple. Given a specified amount of torque at a given engine speed, the effect of an adverse gearing change as a result of a significantly larger tire diameter does, in fact, result in less torque being available at the wheels to accelerate the car.

There are indeed, as you correctly note, many variables to consider in selecting gears for a vehicle, but that doesn't change the relationship between effective gear ratio and torque available at the wheels to actually accelerate the car. All else being equal, a taller effective gear ratios reduces torque at the wheel at a given engine torque output.

Honestly, if someone want to run 22's, who cares? But it's fair to point out that the larger diameter tires adversely affect acceleration if no other factors are changed.

 

You are still missing the point.

In the example of the Turbo. Porsche raised the gear ratio(same as adding bigger wheels bit in a MUCH LARGER increment) to allow the car to spend more time within its power band and therefore accelerate faster than it would with much lower gearing where it would simply never get its full power down.

Simply saying that this is a constant and always equation is false! You will not "always" make a car slower with bigger tires.

Which again, Porsche themselves added bigger tires/wheels to their "fast" cars and smaller wheels to the "slow" cars. So they even think the magical gear ratio vs tire diameter is closer to 22" than 17"