I've changed from 18" to 19" on my M3 and and Supra and it's never been a "seat of the pants" difference in braking, turn-in or acceleration. I can tell a bigger difference by changing tire pressure.

Around a roadcourse, the time will show, but it's nothing you should notice on the street.

Later, Steve

 

Hey Steve!
Yeah but on that car you likely had better tire options so that you could match the over-all diameter properly. 1 inch is a lot and the options are limited. Plus an 11 inch wheel is pretty big adding to the increase when bumping up. Suspension is slower too. look at all the efforts and expense manufacturers go through to have aluminum suspension components for example.

 

No worries. Didnt take anything personally which is why I clarified.

I think the change in tire diameter has a much larger impact than any slight change in weight.

Porsche Hollow Spoke:
F: 22 lbs
R: 26 lbs

Michelin PS2
225/40/18 - 21 lbs
295/30/18 - 27 lbs
295/30/19 - 28 lbs

Toyo Proxes
315/25/19 - 29 lbs

Bridgestone Potenza SO4
305/30/19 - 33 lbs

Forgestar F14s are a popular choice for those not wanting to spend a lot on wheels. They clock in at 23.8 for a 19x8.5 and 26 for a 19x11

So, if you went with a 19x11 rear with the Proxes, you are looking at a net add of 2 lbs in rotational mass going from an 18 to 19 setup and keeping roughly the same diameter. If you went with the SO4, that would add 6lbs which is much more and you would have to have a very light wheel to compensate for.

 

Here is a good page for wheel weights, it´s in german but anyway

http://www.944racing.de/felgengewichte.php

 

The difference in tire weight should be mentioned too. They are rotating masses too. 0,5kg for example is a hugh difference. If you want a performance car, stay with 18. If you want show and shine go with 19+.

 

Actually on the Supra I did eventually add 1" to the diameter and ran multiple different options. Interestingly the rear sizing was very similar to a lot of what is typically ran on the our 911's.

Stock
Front: 235/45R17 on a 17x8" wheel 25.3" dia.
Rear: 255/40R17 oa a 17x9.5" wheel 25.0" dia.

18" Setup
Front: 285/30R18 on a 18x9.5" wheel 24.7" dia.
Rear: 295/30R18 on a 18x11" wheel 25.0" dia.

19" Setup
Front: 275/30R19 on a 19x10" wheel 25.5" dia.
Rear: 305/30R19 on a 19x11.5" wheel 26.2" dia.
Also ran a 315/25R19 25.2" dia. (Couldn't tell much difference versus the 305/30R19, except the taller 305/30 had better straight line traction)

I also went from a 225/50R16 24.9" dia. on my RX7 to a 275/35R18 25.6" dia. with very little seat of the pants difference.

Later, Steve

 

I agree with most that on the track, the 18" setup is superior. Of course 17" would be better than 18" if they would clear the brakes, but that's a different discussion.

For a 911 turbo, the change to a 19" wheel should make very little difference in the driving experience. Of course I would pay attention to wheel and tire weights. Keep in mind that the tires weigh more than the wheels and are further from the rotational center.

If the OP feels the seat of the pant performance is significantly different, then he should check for other things such as a boost leak or his wheels/tires are really heavy.

Seriously, the brand/model of tires has a bigger seat of a pants impact than changing from 18" to 19".

Later, Steve

 

+1 I agree.

Overall diameter stock is 25" (approx.) OP along with some others running 19's is at about 26" (approx.). But there are also guys running 18" wheels with/or closer to 26" overall diameter as well.

If we put two equal cars OR the same car but the only difference would be that Car A was on 19" wheels with 26" overall diameter and Car B had 18" wheels with 26" overall diameter (approximately same overall weight wheels + tires on both cars)...and you were to take out both cars for a typical street type spin.. a couple straight line pulls and a few somewhat aggressive turns I seriously doubt anyone would know the difference.

Assuming that OP's problem is in fact related to his wheels/tires. The main issues would be overall diameter & weight. For it to make a noticeable difference those wheels would have to be pretty heavy. Which is actually not as hard as you would think.

At Tire Rack there is a "twist" looking wheel from Sport Edition that are stated to weigh 26 lbs each front and 32 lbs each rear. Porsche Hollow spokes are 22lbs front & 26lbs rear sooo That comes to about 20 lbs more of total additional rotational weight not counting possible additional weight from the tires. I would guess that there are quite a few replica &/or lower end wheels (perhaps even some of the "boutique" wheels) that are also that heavy and might even weigh more.

Just for reference also at Tire Rack they have O.Z. Racing Atelier Forged Superforgiata in 19" they are 16lbs front and 19lbs rear!!

If you were very familiar with the car and went from an 18" wheel/tire combo to a MUCH heavier wheel tire combo with a larger overall diameter whether it be on 18" wheels or 19" wheels you certainly could feel the difference. BUT if you switched to light weight (perhaps even lighter) 19" wheels and carefully selected tires that would be a different story.

I weighed my 19's (I took pictures of them on the the scale) my fronts were 22 lbs and not only are they 19" but they are also 8.5" wide rather than 8" like stock. My rears weighed in at 25 lbs at 19x11 while stock hollows are 26lbs.

Couple different ways to skin a cat. All that said, I do agree that if you are setting up a 996TT for track or just shopping for dedicated track wheels I would go 18" on this car. But..With some of the very competitively priced, light weight 19's out there I don't see an issue at all for a well thought out street setup.

 

As an example, my 19x8.5" front wheel weighs 19 pounds and my 19x11" rear wheel weighs 21 pounds.

Original equipment 18" wheels/tires weigh a total of 218 pounds (non-hollow)
Original equipment 18" wheels/tires weigh a total of 200 pounds (hollow)
My 19" setup weighs 182 pounds

Of course a forged wheel in an 18" size with light weight tires would still be lighter than my setup.

Later, Steve

 

This is what I read once on a racing forum and remembered it. Guys did put some math behind it, so it read plausible and credible:

"In terms of wheel WEIGHT due to rotational mass, its said that every 1kg of wheel weight, is equatable to 10kg of car weight."

If this holds as long as you know your wheel weights you can easily work out bhp per tone and get your answers.

For an example if we presume an added weight of 1kg per corner due to wheel and tyre combo the resulting 40kgs of increased car weight can definitely be felt in your arso-o-meter.

 

It goes a little farther than that. Un-sprung weight effects wheel control (handling) at a 10:1 ratio, but braking and acceleration were closer to a 2:1 ratio in the tests I saw.

Later, Steve

 

I will definitely weight the wheels during my next visit to a garage in a couple of weeks. If the total weight of my 19" wheels is not significantly higher than the OEM's then it really means a potential leaking from a hose somewhere.