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Help with wheels can I run 17's in back 16's in front?

I'm painting the car and something that has always bothered me is the car doesn't have it's flares filled out. It's an sc but with turbo flares installed shortly after it's initial purchase by previous owner.

Currently running 16x7 front 16x8 rear. I'm thinking of getting a set of 17x11's sticking them in back and pushing the 16x8's up front.

Is this possible? Is it dumb? It will save me from having to buy a whole set. I wouldn't mind 16x11s but i guess tires are hard to find. Finding tires for 17x11 I hear is hard, but not terrible.

Anyone have experience with this? Should i have the clearance for this kind of project? Any brand recomendations?

Thanks ahead
Old 07-02-2019, 11:05 AM
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Additional thoughts.... I've heard more rubber will give more grip but i'll have more unsprung weight and car might feel a bit more sluggish. I have a 3.2 in there euro spec so it's decent on power, freshly rebuilt.
Old 07-02-2019, 11:12 AM
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I used 17x10 rears with 16x8 fronts on my '81 SC with Turbo flares. It was a wonderful combo!

https://1drv.ms/u/s!Anptue1ohMb7rQqf-Z7lZqimnx27
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Old 07-02-2019, 11:24 AM
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Quote:
Originally Posted by Peter Zimmermann View Post
I used 17x10 rears with 16x8 fronts on my '81 SC with Turbo flares. It was a wonderful combo!

https://1drv.ms/u/s!Anptue1ohMb7rQqf-Z7lZqimnx27
Nice!! Exactly what I was looking for.

Was there a reason to go with 10 rears specifically? Over 9 or 11 for example.

Any issues with tire rub? Did you need spacers? I like how it sits in your real wheel well would love to replicate that.

Sorry for all the questions.
Old 07-02-2019, 11:54 AM
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Quote:
Originally Posted by nkotselas View Post
Nice!! Exactly what I was looking for.

Was there a reason to go with 10 rears specifically? Over 9 or 11 for example.

I did the measurements and had HRE do the rears using alloy centers that I had at the shop. I felt that the 10s would be a nice balance in traction and also "fill" the wheel openings.

Any issues with tire rub? Did you need spacers? I like how it sits in your real wheel well would love to replicate that.

No rub (car was set at Euro ride height specs). I don't believe spacers were needed because HRE did a great job doing the offset that I requested.


Sorry for all the questions.

NO PROBLEM!
I will try to find a YouTube piece I did back in the day and post a link for you.
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Old 07-02-2019, 12:13 PM
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OK, try www.youtube.com

Enter "zimmaster62" in the search bar

Click on "Willow Springs Raceway - California"

You will notice that I switched the front wheels to 9 x 16s at that point.
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Last edited by Peter Zimmermann; 07-02-2019 at 12:38 PM..
Old 07-02-2019, 12:22 PM
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a double stagger would work, but why not just commit to 17s all around?
tire options galore plus more room for brakes and suspension sorcery.
You don't have to run steamrollers, 17x9/10 with appropriate offsets and tires would fill the arches very well and keep the weight down...relatively.
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Old 07-02-2019, 12:53 PM
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Quote:
Originally Posted by chrisbalich View Post
a double stagger would work, but why not just commit to 17s all around?
tire options galore plus more room for brakes and suspension sorcery.
You don't have to run steamrollers, 17x9/10 with appropriate offsets and tires would fill the arches very well and keep the weight down...relatively.
My main concern was both cost and more importantly weight.

So I know generally what offset means but not much in terms of what i should go with. I really love the deep Fuchs look so I would want a negative offset correct? But that would leave less room for future upgrades? Is there a sweet spot people have come to a consensus on?
Old 07-02-2019, 01:17 PM
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Quote:
Originally Posted by Peter Zimmermann View Post
OK, try www.youtube.com

Enter "zimmaster62" in the search bar

Click on "Willow Springs Raceway - California"

You will notice that I switched the front wheels to 9 x 16s at that point.
Wow that's some sweet footage. Could watch stuff like that all day.

Last questions i promise...

Did you notice a major falloff in acceleration after going to the 17 out back? I'm worried about the car feeling sluggish. What about benefits? Did the extra contact help you through the corners?

Thanks again
Old 07-02-2019, 02:05 PM
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Quote:
Originally Posted by nkotselas View Post
Wow that's some sweet footage. Could watch stuff like that all day.

Last questions i promise...

Did you notice a major falloff in acceleration after going to the 17 out back? I'm worried about the car feeling sluggish. What about benefits? Did the extra contact help you through the corners?

Thanks again
It's not so much the inertial cost but the gearing cost that affects performance, while on average a 17" tire is initially more expensive than a16 the difference isn't huge

What really hurts are the gearing cost from taller tires

Another issue is that car setup becomes more difficult the greater the difference in tire height front to rear

a third issue is the lack of short 17" rears, you can put a 275/40 x17 on an 11 but that tire @ ~25.7" tall will kill acceleration and braking. a 315/35 x17 is ~the same height & is very costly inertially too boot

The tallest non race tire I'd use is a 255/40 x17 @~25.2"

Currently the best wide rears are in 18, 285/30 x18 @ ~24.8" is great and a matching front would be 245/35 x18

wheels would be 9 & 10, Ruf used those 9ET17.7 & 10 ET5

Here's a typical 16 to 17 change w/ the Ruf wheels




and here is the 16 to 18 change w/ the Ruf wheels


*actual 17 or 18 wheel weights will likely be different by +/- a few ls and won't affect the results overly much

The thing to look at is the bottom left corner net torque delta
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Old 07-02-2019, 02:52 PM
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Quote:
Originally Posted by Peter Zimmermann View Post
OK, try www.youtube.com

Enter "zimmaster62" in the search bar

Click on "Willow Springs Raceway - California"

You will notice that I switched the front wheels to 9 x 16s at that point.
Wow that's some sweet footage. Could watch stuff like that all day.

Last questions i promise...

Did you notice a major falloff in acceleration after going to the 17 out back? I'm worried about the car feeling sluggish. What about benefits? Did the extra contact help you through the corners?

Thanks again
Old 07-02-2019, 03:16 PM
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Quote:
Originally Posted by Bill Verburg View Post
It's not so much the inertial cost but the gearing cost that affects performance, while on average a 17" tire is initially more expensive than a16 the difference isn't huge

What really hurts are the gearing cost from taller tires

Another issue is that car setup becomes more difficult the greater the difference in tire height front to rear

a third issue is the lack of short 17" rears, you can put a 275/40 x17 on an 11 but that tire @ ~25.7" tall will kill acceleration and braking. a 315/35 x17 is ~the same height & is very costly inertially too boot

The tallest non race tire I'd use is a 255/40 x17 @~25.2"

Currently the best wide rears are in 18, 285/30 x18 @ ~24.8" is great and a matching front would be 245/35 x18

wheels would be 9 & 10, Ruf used those 9ET17.7 & 10 ET5

Here's a typical 16 to 17 change w/ the Ruf wheels




and here is the 16 to 18 change w/ the Ruf wheels


*actual 17 or 18 wheel weights will likely be different by +/- a few ls and won't affect the results overly much

The thing to look at is the bottom left corner net torque delta
Hey there Bill. I remember you being around a lot back when i first "restored" my car, nice to see you are still active.

What do you recommend I do? The wheel change is entirely cosmetic and although it would look cool to everyone else i would hate getting in the car each time knowing i sacrificed performance.

I would be glad to get 16x10 or 16x11's but I hear tires are impossible to find and when found are only race slicks. Otherwise I think a 16x10 option would really be ideal. I just have such wide flares and wheels and tires that are comparatively thin looking. I've always wanted to address this and never did so I'm hoping to make the change now, just not wanting to make the wrong one!
Old 07-02-2019, 03:22 PM
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Quote:
Originally Posted by nkotselas View Post
Hey there Bill. I remember you being around a lot back when i first "restored" my car, nice to see you are still active.

What do you recommend I do? The wheel change is entirely cosmetic and although it would look cool to everyone else i would hate getting in the car each time knowing i sacrificed performance.

I would be glad to get 16x10 or 16x11's but I hear tires are impossible to find and when found are only race slicks. Otherwise I think a 16x10 option would really be ideal. I just have such wide flares and wheels and tires that are comparatively thin looking. I've always wanted to address this and never did so I'm hoping to make the change now, just not wanting to make the wrong one!
Tire selection in 16 is really bad too

The trick will be to find some wheels that you like and can afford

Unless you have a a really big motor or track the car you really don't need or want really big tires.

for stockish 3 liter 911s I usually recommend 8x17 & 9x17 but w/ those sizes you will need some expensive custom wheels w/ 215/45 & 245/40 x17 tires

If you already have the Fuchs the rear 8x16ET10.3 is a decent fit in front you could use 225/50 on that wheel then source some 9x16 ET15 Fuchs for the back 245/45 x 16 would be the tire to use but Pirelli P zero Rosso seems to be about the only street 245/45 x16 now available
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Old 07-02-2019, 03:35 PM
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Quote:
Originally Posted by nkotselas View Post
Wow that's some sweet footage. Could watch stuff like that all day.

Last questions i promise...

Did you notice a major falloff in acceleration after going to the 17 out back? I'm worried about the car feeling sluggish. What about benefits? Did the extra contact help you through the corners?

Thanks again
My biggest performance glitch came with the wide body changes. Aero had a dramatic effect on lap times until I worked out slightly different suspension settings, etc. I made the big mistake assuming that going to the wide body configuration would give me two seconds/lap at Willow Springs, but it actually cost me 1.5 seconds.

No, it's not a lot, and without the clock might not have been noticeable, but the car was a bit slower with the wide body flares. When it was all said and done, after some suspension tweaks, closing off some of the front oil cooler inlet, and adjusting the rear wing, the car was about one second/lap better, and easier to drive. I can't answer specifically about the 17s, but they could have played a part in the initial slow down.

I went from a narrow bodied car to a wide body car with no other changes other than the wheels/tires. Torsion bars, anti-roll bars, bushings, and custom valved Bilstein shocks all stayed the same, and the car felt good in the corners, but obviously lost something on the straights.
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Old 07-02-2019, 04:56 PM
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Cover your eyes Bill

I'm running 9x16 Fuchs on the front and 12.75x17 3pc custom Fuchs on the rear of my 930S. Purely for aesthetic reasons. I love the way it looks, it handles fine, and with turbo power the extra sprung weight is meaningless (to me). With a 3.2L Carrera engine you will notice the engine is not quite as powerful. If that is a concern then mount up a set of 4" wide space savers and have a ball! Seriously though, wide look cost a bit of power so choose what is important to you with consideration of how you use the vehicle. Mine will NEVER be on a track and is plenty powerful enough for the street.



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Old 07-03-2019, 07:01 AM
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Originally Posted by rarlyl8 View Post
cover your eyes bill

i'm running 9x16 fuchs on the front and 12.75x17 3pc custom fuchs on the rear of my 930s. Purely for aesthetic reasons. I love the way it looks, it handles fine, and with turbo power the extra sprung weight is meaningless (to me). With a 3.2l carrera engine you will notice the engine is not quite as powerful. If that is a concern then mount up a set of 4" wide space savers and have a ball! Seriously though, wide look cost a bit of power so choose what is important to you with consideration of how you use the vehicle. Mine will never be on a track and is plenty powerful enough for the street.



nice!
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Old 07-03-2019, 08:47 AM
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The #s just don't support a big difference due to aero when comparing a Carrera to turbo body. I don't have the aero #s for the '78 up 930 body but here's a comparison of a '74 Carrera 2.7 and a '74 3.0RS





aero difference is ~100# max, thrust from the drivetrain is in the 500# range+/- in the upper gears
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Old 07-03-2019, 09:36 AM
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Quote:
Originally Posted by Bill Verburg View Post
The #s just don't support a big difference due to aero when comparing a Carrera to turbo body. I don't have the aero #s for the '78 up 930 body but here's a comparison of a '74 Carrera 2.7 and a '74 3.0RS





aero difference is ~100# max, thrust from the drivetrain is in the 500# range+/- in the upper gears
Paraphrasing to make sure I understand...
The extra drag alone of the widebody is like adding 100# to the mass of car.
and
Taller wheels can have the same effect on acceleration in higher gears as adding 500#

So the ideal recipe is a wider car that's 100# lighter with wider tires and making sure the rolling diameter of those tires is as close as possible to their narrower counterparts?

ugh. the slippery slope lurks ever near.
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Old 07-03-2019, 10:24 AM
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Fullquack over on the turbo forum runs a 930 with 17s in back, and 16s in front. You can do a search to see how his car looks. He seems to be happy with the handling.
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Old 07-03-2019, 10:38 AM
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Quote:
Originally Posted by chrisbalich View Post
Paraphrasing to make sure I understand...
The extra drag alone of the widebody is like adding 100# to the mass of car.
and
Taller wheels can have the same effect on acceleration in higher gears as adding 500#

So the ideal recipe is a wider car that's 100# lighter with wider tires and making sure the rolling diameter of those tires is as close as possible to their narrower counterparts?

ugh. the slippery slope lurks ever near.
No, Lets pick a realistic speed since on a track, say 112mph, at that speed the 2.7RS has 232 lbs of drag,the 3.0RS ahas 278 lbs of drag. So at that speed the wide body has 46 lbs more drag.

That's a very different thing from having extra weight.

I have the #s for a '73 RS which will be very close to te '74RS
at 111 mph a stock '73RS is in 4th or 5th gear, in 4th at that speed it as ~587# of thrust pushing it forward, subtract the drag of 226# and you have a net thrust of 361# available for acceleration.

if the car and driver weigh 2600# the acceleration will be ~4.445 ft/s^2 if the car weighed 2700# the acceleration would be 4.28 ft/s^2

weight always acts to inhibit any change in speed, drag gets bigger the faster you go, w/ modern cars downforce also gets larger the faster you go
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Old 07-03-2019, 12:41 PM
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