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-   -   Big performance increase 911SC (http://forums.pelicanparts.com/porsche-911-technical-forum/1015408-big-performance-increase-911sc.html)

Bill Douglas 12-11-2018 08:26 PM

Big performance increase 911SC
 
I swapped out the heavy 17" Carrera Cup fakes for the factory fitted 16" wheels.

Wow, the car feels so much faster, responsive, snappier, the list goes on. I loved the look of the 17" wheels with big rubber, but all of that extra rotating mass was not good for the car.

911SC Sport with 930/10, cams and SSIs.

RSBob 12-11-2018 09:02 PM

That is so cool that you got that kind of gain.

Makes me wonder about my Linea Corsa 17” 7s and 9s.

Bill did you reduce the tire size as well or just the wheels?

antares 12-11-2018 09:13 PM

… having to install spacers to kick out those wide 17 wheels that the Carrera and 930 suspension design did not need to do to such a large extent I suppose that makes sense. Boy if you can feel the difference on your 930 motor then a non aspirated 3.0 SC stock with 17's /wide rubber must really be a slow boat in comparison. What particular stability and control issues has this move corrected ? also in what range does the 'much faster' factor come into play …. taking off or acceleration ?

aircooled pegasus 12-11-2018 09:44 PM

The SC/Carrera looks so proper with 16s.

Those fake Carrera Cups are quite heavy also.

madcorgi 12-11-2018 10:31 PM

I'm a fan of running stock-sized tires on my car (1985 Carrera--205 and 225s), though I have swapped out the original 6&7 x 16s for aftermarket Fuchs 7&8x16s with the stock rubber. Just because I like the looks . . .

Bill Douglas 12-11-2018 10:57 PM

Quote:

Originally Posted by RSBob (Post 10281274)
Bill did you reduce the tire size as well or just the wheels?

The circumference is the same but the back ones went from being 245s down to the stock 225 in width. I just think the fake Carrera Cup wheels were SO heavy.

Quote:

Originally Posted by antares (Post 10281278)
what range does the 'much faster' factor come into play …. taking off or acceleration ?

It might be placebo effect, but the car feels more lively, more responsive. More acceleration I would say. Haha, easier to park too instead of muscling the steering wheel around.

Mike964_61 12-12-2018 03:47 AM

You should have weighed the wheels so you would have an actual number. I would have like to known the difference.

2.7RS 12-12-2018 03:59 AM

Scrub Radius. The answer. No black magic, butt dyno or witchery

T77911S 12-12-2018 04:07 AM

my 930 has 18's. I sometimes wish it had the 16's for performance but the 18's look awesome

Bill Verburg 12-12-2018 04:32 AM

Just changing from 17 to 16 is no guarantee that you will experience and improvements

It's all about tire OD and the weight of the rotating assembly, tire OD is potentially the most significant and tire weight is more significant than wheel weight which is more significant than rotor weight which is more significant than hub weight.

Judicious choices can make a big difference

here's on example using only 1 tie line, Bridgestone RE71, 16s vs 17s. No change in scrub radius

Here the 17s are going to be ~1.7lb-ft better, for acceleration ~.6 lb-ft worse for braking and also provide better grip in the corners
http://forums.pelicanparts.com/uploa...1544621407.gif

GH85Carrera 12-12-2018 06:03 AM

It is strange you can feel in in the seat of your pants.

My 85 came with 15 inch Fuchs, 7s and 8s. It is pretty much impossible to find good performance summer tires that can be used on the highway for long road trips in the 15 inch size. I finally gave up and bought some Euromiester wheels. Pilot Sport tires have orders of magnitude more grip for cornering, and work great in the rain. I did not notice any set of the pants difference in acceleration.

For sure the 15s are lighter that the 17s I have now, but I sure like the driving feel of the 17s.

Bill Verburg 12-12-2018 06:31 AM

Quote:

Originally Posted by GH85Carrera (Post 10281508)
It is strange you can feel in in the seat of your pants.

My 85 came with 15 inch Fuchs, 7s and 8s. It is pretty much impossible to find good performance summer tires that can be used on the highway for long road trips in the 15 inch size. I finally gave up and bought some Euromiester wheels. Pilot Sport tires have orders of magnitude more grip for cornering, and work great in the rain. I did not notice any set of the pants difference in acceleration.

For sure the 15s are lighter that the 17s I have now, but I sure like the driving feel of the 17s.

W/o knowing details of you specific case I can only guess, but here goes
stock 7 & 8 x15 Fuchs w/ 185/70 & 215/60 tires changed to Michelin PS2(no data on PS) on Euromeister 7 & 8 x17

There would be only a small ~4.6 lb-ft difference, every one is different but that's likely to be not very noticable. But on others changes i've seen far larger differences up to ~45lb-ft


http://forums.pelicanparts.com/uploa...1544628244.gif

1979-930 12-12-2018 07:38 AM

Based on the title I was sure you added a turbo. :D

Steve@Rennsport 12-12-2018 07:41 AM

Quote:

Originally Posted by 2.7RS (Post 10281391)
Scrub Radius. The answer. No black magic, butt dyno or witchery

Nossir.

Moment of inertia (and weight)! :) :)

This is what Bill felt when he switched from his heavy 17" wheels to the original 16" ones.

QueWhy 12-12-2018 08:02 AM

Quote:

Originally Posted by antares (Post 10281278)
having to install spacers to kick out those wide 17 wheels that the Carrera and 930 suspension design did not need to do to such a large extent I suppose that makes sense. Boy if you can feel the difference on your 930 motor then a non aspirated 3.0 SC stock with 17's /wide rubber must really be a slow boat in comparison. What particular stability and control issues has this move corrected ? also in what range does the 'much faster' factor come into play . taking off or acceleration ?

He has a non aspirated stock 3.0. I think the engine code is confusing you 930/10 simply denotes a ROW 911SC engine.

jac1976 12-12-2018 08:31 AM

Quote:

Originally Posted by QueWhy (Post 10281681)
He has a non aspirated stock 3.0. I think the engine code is confusing you 930/10 simply denotes a ROW 911SC engine.

This

john walker's workshop 12-12-2018 08:45 AM

My77 930 felt more nimble when I lost the Ruf 17X9/10s and put the fuchs 16X7/8s back on. Lot less railroading too.

GaryR 12-12-2018 08:54 AM

Read the OP and then scanned down to see (I KNEW he would respond!) Bill V's input on this. He was of great help when many of us switched our SC race cars from 18" (very light) CCW wheels with 275/35 Hoosiers to 15" Jongbloeds with 275/35 15's.. We did the same with our E class 3.2 cars, the gearing is changed across the board, similar to running a 7:31 R&P. Acceleration out of corners was improved and it changed our cars from virtual 4 speeds (on all tracks I frequented) into 5 speeds..There was a substantial running diameter difference in these two tires, doubt if going 17" to 16" on a similar tire would have the same or even similar effect?

T77911S 12-12-2018 08:57 AM

my brothers 930 still have 16's, we have not raced much but I think he is quicker.

also, my car sits much higher than his, especially in the rear.
I have 285-35-18 in the rear.

Bill Verburg 12-12-2018 09:11 AM

Quote:

Originally Posted by T77911S (Post 10281757)
my brothers 930 still have 16's, we have not raced much but I think he is quicker.

also, my car sits much higher than his, especially in the rear.
I have 285-35-18 in the rear.

Hopefully that is a 285/40 x18, @24.8" OD that is a sweat rear tire only ~.1" taller than a 245/45 x16 and w/ roughly equal inertial cost, hopefully w/ 245/35 or 255/35 fronts for matching balance.


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