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THIS DISCUSSION / SOURCE-DERIVED OVERVIEW
Discussion summary
YOUR RESEARCH2004 996 C4S Tires
Leading reported evidence
Use on subie wrx
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997.2 uses a completely different electromagnetic center diff which can be retrofitted into 996's with a standalone diff controller as commonly used on Subie WRX.
Leading evidence: 'Use on subie wrx' (no explicit attributable resolution; this is a reported action or suggestion). Across 1 matching discussions, The most frequently discussed action is 'Check reviews and tests at tire rack' (1 discussions). For that action, 0 discussions report a resolution and 0 report an unsuccessful attempt. No action has an explicit, attributable resolution report under the current rules. These are forum reports and suggestions, not a confirmed diagnosis.
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Suggestion
I would recommend a set of 4 General GMax tires in factory sizes - check the reviews and tests at Tire Rack.
Recommend set of 4 general gmax tires in factory sizes
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Suggestion
I would recommend a set of 4 General GMax tires in factory sizes - check the reviews and tests at Tire Rack.
Explicit resolution reports before identified causes, performed actions and suggestions. Within each level, matching component labels precede direct query mentions and inherited thread context; successful versus unsuccessful discussion counts, owner confirmations and independent discussion support determine order. This is not a probability of success.
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Suggestion
Are you guys running your 18 in 285 or 295 in the back?
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Suggestion
With the C4/C4S and Turbo you have to be within (I believe) 3% between front and rear diameters or you may run into differential problems.
Use completely different electromagnetic center diff
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Suggestion
997.2 uses a completely different electromagnetic center diff which can be retrofitted into 996's with a standalone diff controller as commonly used on Subie WRX.
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Performed · outcome unconfirmed
997.2 uses a completely different electromagnetic center diff which can be retrofitted into 996's with a standalone diff controller as commonly used on Subie WRX.
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17 eligible posts · 0 unverified posts · 1 discussions match the subject in their title or opening post.
Rennlist: 1 discussions
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I just picked up a 2004 C4S needs new rear tires, the manufacturer of the fronts only makes a 285/35 18 as opposed to a 295/30 18 on the car now will this make a difference? Thanks for the help.
That would be getting close to an inch larger in diameter over the stock. A 285/30 would be slightly smaller in diameter than stock, though.
Which brand/model tires are on the front?
Be very careful of tire diameters. With the C4/C4S and Turbo you have to be within (I believe) 3% between front and rear diameters or you may run into differential problems. At least that is what I have read
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...which means the fronts have to be slightly larger diameter than the rear. Look at revs/mile as a reference. (example Front 804 rev/mi, Rear 808 rev/mi) This difference is needed for the "4" to work correctly. (in C-4, C-4-S)
YMMV
I would recommend a set of 4 General GMax tires in factory sizes - check the reviews and tests at Tire Rack.
Don't take chances with mismatched tires on a 4S.
That has been said a lot, but I never found a technical document that confirms it.
Actually, the 997.1 S, that has the same AWD system, does not follow that rule in the 19" standard wheels sizes
The 996 and 997.1 AWD system has a viscous clutch between front and rear axle (not a mechanical differential as was hinted earlier). This viscous clutch is integrated into a single housing together with the front axle differential.
Having the rear wheels spin a little faster than the fronts by means of a larger front tyre radius than rear tyre radius, the viscous coupling will transmit some drive torque to the front wheels.
Equal tyre radii/diameters will result in no torque transmitted to the fronts until the rear tyres start turning faster than the fronts, as they will when loosing traction at the rear.
Having smaller diameter tyres at the front than rear, results in requiring more rear tyre slip/spin before the viscous coupling will start to transmit torque to the front.
The maximum allowed difference in tyre diameters is to ensure that the heat inside the viscous coupling, caused by the different turning speeds of its input and output shafts is not so great as that it reduces the lifetime of the viscous fluid and thereby of the viscous clutch itself.
Another, lesser factor is that having more torque transmitted to the front than designed and everything calibrated to, can result in unexpected cornering behaviour.
That is all correct.
However, here in the forum, a lot of times has been said that the rear wheels have to be smaller than the front ones, and that is not correct.
OEM 19" Sizes for 997.1 Carrera 4 and 4S are
Summer:
235/35 R19 Rear = 647mm diameter
305/30 R19 Rear = 666mm diameter
Winter
235/35 R19 = 647mm diameter
295/30 R19 = 660mm diameter
In both cases, rear wheels are larger than the front ones.
First, +1 for the General GMax tires in factory size which for me is 295 rears. The Generals have good grip, wear acceptably well, and importantly are MUCH quieter than the Michelins or Pirelli PZeros I've run.
I converted my 2003 C4S to a "C2S" - just removed front axles, front transaxle, etc. and have zero regrets. I find the car a little livelier and a little more tail-happy, which was the outcome I was looking for. I don't know if your needs align with that but it's a very easy and satisfying project.
Link to my version is here.
Forgot to add that among the reasons for converting to RWD only is that tire size doesn't matter anymore.
Good point. Is it different from the 997 rear diff ratio, though? Only then it would be a way to have the rear tyres turn faster than the smaller fronts turn.
But the final drive ratio does not impact on the behaviour of the viscous coupling of the differential between front and rear. That differential is the same for 996 and 997 and as we can see, yes, there is a limit on the difference of rotation between both axles, but it is not mandatory that the rear spin faster than the front.
Yes the diffs have different ratios in 997.1. 997.2 uses a completely different electromagnetic center diff which can be retrofitted into 996's with a standalone diff controller as commonly used on Subie WRX.
This is a source reference. Confirm exact model applicability in the original material.
