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My problem with the 19" setup is that there really isn't a readily available tire combo that properly biases the AWD viscous coupler in widths that make sense....
Ideally, the rear tires should be a slightly SHORTER diameter than the front, so it spins just slightly faster, and begins to pre-load the viscous coupler for torque transfer. When the rear starts to spin, it further engages the viscous coupler quickly and transfers torque to the front. When biased this way, the car feels very much like a RWD car unless and until the rear breaks traction.
When the rear is TALLER than the front, the viscous coupler is biased the wrong way... The front is spinning faster, and it's preloading the coupler the wrong way. This causes some drag on the front axle, and requires the rears to slip/spin even more to first overcome the incorrect bias, then actually start to transfer torque.
If you have a RWD conversion, this makes no difference. If you have a 997 Turbo, this also is not an issue, as they replaced the viscous coupler with a electronically controlled clutch pack. But on a 996 running AWD, I think it makes a notable difference in how the car feels.
Example of my preferred 18" tire/wheel setup:
Front: 245/40-18 (25.7" diameter)
Rear: 315/30-18 (25.4" diameter) Rear is slightly SHORTER, therefore turns slightly faster, and biases the viscous coupler CORRECTLY.
Example of typical 19" tire/wheel setup:
Front: 235/35-19 (25.5" diameter)
Rear: 305/30-19 (26.2" diameter) Rear is slightly TALLER, therefore turns slightly slower, and biases the viscous coupler INCORRECTLY.
There are theoretically other fornt/rear tire combos in 19" that would provide correct bias, but not readily available these days. For example, 235/35-19 (25.5 inches tall) + 315/25-19 (25.28 inches tall). This would be freaking perfect, BUT tire rack lists not a single tire available in 315/25-19 let alone a matched set with the fronts...
You could also run 225/40-19 (26.1") and 295/30- 19 (26"), but that gives up so much needed tire width (20mm front and rear vs. my preferred 18" setup), and it's kind of tall in the front causing potential fitment issues. But at least it biases the viscous coupler correctly.
This makes me happy I've removed the front diff - I am going to go 245/305, and I believe there will be 1/2" difference between them, with the rear being a bit taller, because I absolutely hated the taller front on an 18 in 245.
My problem with the 19" setup is that there really isn't a readily available tire combo that properly biases the AWD viscous coupler in widths that make sense....
Ideally, the rear tires should be a slightly SHORTER diameter than the front, so it spins just slightly faster, and begins to pre-load the viscous coupler for torque transfer. When the rear starts to spin, it further engages the viscous coupler quickly and transfers torque to the front. When biased this way, the car feels very much like a RWD car unless and until the rear breaks traction.
When the rear is TALLER than the front, the viscous coupler is biased the wrong way... The front is spinning faster, and it's preloading the coupler the wrong way. This causes some drag on the front axle, and requires the rears to slip/spin even more to first overcome the incorrect bias, then actually start to transfer torque.
If you have a RWD conversion, this makes no difference. If you have a 997 Turbo, this also is not an issue, as they replaced the viscous coupler with a electronically controlled clutch pack. But on a 996 running AWD, I think it makes a notable difference in how the car feels.
Example of my preferred 18" tire/wheel setup:
Front: 245/40-18 (25.7" diameter)
Rear: 315/30-18 (25.4" diameter) Rear is slightly SHORTER, therefore turns slightly faster, and biases the viscous coupler CORRECTLY.
Example of typical 19" tire/wheel setup:
Front: 235/35-19 (25.5" diameter)
Rear: 305/30-19 (26.2" diameter) Rear is slightly TALLER, therefore turns slightly slower, and biases the viscous coupler INCORRECTLY.
There are theoretically other fornt/rear tire combos in 19" that would provide correct bias, but not readily available these days. For example, 235/35-19 (25.5 inches tall) + 315/25-19 (25.28 inches tall). This would be freaking perfect, BUT tire rack lists not a single tire available in 315/25-19 let alone a matched set with the fronts...
You could also run 225/40-19 (26.1") and 295/30- 19 (26"), but that gives up so much needed tire width (20mm front and rear vs. my preferred 18" setup), and it's kind of tall in the front causing potential fitment issues. But at least it biases the viscous coupler correctly.
I don't know if I don't understand you or if you don't quite understand Porsche's design for the 996 awd cars.
If you look at the winter tire sizes in the manual it's 225-40-18 and 265-35-18. This means that the rears are taller and Rev slower per mile.
We are talking 10-30 revs difference per mile on a viscous coupler in both scenarios (factory winter or summer sized tires). So it's clear that the system can handle a faster front or faster rear.
Based on the winter tires size where the rear is taller it seems that the faster front will heat up the viscous coupler faster and be more AWD.
In summer mode it's definitely more RWD, I agree with you on one thing, the 19" size makes the bias more winter like and I don't know if that's a problem for the viscous coupler or detrimental to its health.
Personally I ,have converted to RWD so not an issue for me
For argument sake let's say the difference either way is within Porsche recommended sizes.
In winter trim the difference is 0.2-0.3" depending on the tires and the fronts are smaller
In summer trim the difference is fairly consistent at 0.1" and the rears are smaller
I believe that the viscous coupler will wear out faster if the fronts are turning faster because the front wheels warm up the coupler faster and keeps the AWD engaged.
Going to 19" in awd is not a good idea because of the taller rear from the way I understand the system, drawing from the recommended tire sizes of course.
Hello everybody finally aquired my dream car and excited for the road ahead, i also own a few vws but this car has me relearning alot lol and looks like this site is a wealth of knowledge so time to make like a sponge 😁
The following 6 users liked this post by Rdirtytoo:
awdmig (01-27-2024),Freier (01-27-2024),jpflip (01-26-2024),Scrh1 (01-31-2024),SSDVC (02-01-2024),vtec_ (01-27-2024) and 1 others liked this post. (Show less...)
Hello everybody finally aquired my dream car and excited for the road ahead, i also own a few vws but this car has me relearning alot lol and looks like this site is a wealth of knowledge so time to make like a sponge 😁
Hello everybody finally aquired my dream car and excited for the road ahead, i also own a few vws but this car has me relearning alot lol and looks like this site is a wealth of knowledge so time to make like a sponge 😁
Congrats on the new ride! A fellow Floridian here too! If not a member of the PCA I would join as there is a giant Porsche community in FL.
Thinking this would get some comments..after I put my selected design lens cover..I spliced in blinkers on the grill.
kind of an old school thing
Anyway...I kindah like it..but I like the stock look as well..
01-11-2024, 09:04 PM
