massive understeer
Thread Starter
Track Day
Joined: Sep 2007
Posts: 23
Likes: 0
From: Seattle, WA
massive understeer
My 91 C4 Cab has severe understeer when pushed hard. Turns in fine but on power application the car just pushes until lift off. The car has Powerflex bushings and all of the front end is in good shape. Tires are contisport 215 on 7inch wheels and 255 on 8.5" on the rear. Struts are old but seem to be fine ( this winter for replacement). I am looking for some ideas as to where to look for this issue. Has anybody installed 225 pilot sports on a 7" wheel?
Rennlist Member
Joined: May 2007
Posts: 2,292
Likes: 35
From: Portland, Oregon
Have you read up on the philosophy of the C4 and the Anti-Blocking system in the car? IMHO, that is what the C4 is designed to do, unless you do some extensive suspension mods to dial that out. Take a look at Adrian Streather's C4 book (search here ) or do a search on the C4 differential systems.
My C4 does the same thing... Porsche's design was to provoke understeer as a way of stopping oversteer in critical maneuvers.
My C4 does the same thing... Porsche's design was to provoke understeer as a way of stopping oversteer in critical maneuvers.
Three Wheelin'
Joined: Apr 2004
Posts: 1,433
Likes: 1
From: bloomington, IN
How old are the tires? Whe I first got my c4, it pushed like a hog. Just replacing the ancient tires that were on it made a huge difference(I did put 225's on 7" cup 2's, seems to be fine) in how it turned in. Now, after doing suspension, an aggresive alingment, and some other small tweaks, it handles much better. Do a few searches here and pelican, lot's of good info.
Thread Starter
Track Day
Joined: Sep 2007
Posts: 23
Likes: 0
From: Seattle, WA
The tires are 3 years old and I am going to replace them this year. I know the C4 pushes but I don't see how you could get this car around a track without crashing. If this is the way it is them so be it. By the way Davisriley mine is a guards red 91 C4 Cab, love the car.
Rennlist Member
Joined: May 2007
Posts: 2,292
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From: Portland, Oregon
You might indeed have issues... but I wanted to calibrate what you are experiencing against the design elements of the C4 driveline... Many C4 buyers don't know what they are getting when it comes to "track" like performance. There are ways to dial this out with the right suspension components. You can check with Steve W of Rennsport Systems for his point of view. Many have opinions on this.
The only other big hazard has been fuel spillage on the front driver's tire in some rare track instances. The fuel filler neck can crack over time and let fuel dump out on hard left turns. This has caused some issues. This is rare, but something to check in an older car. The fuel filler neck is very easy to replace.
The only other big hazard has been fuel spillage on the front driver's tire in some rare track instances. The fuel filler neck can crack over time and let fuel dump out on hard left turns. This has caused some issues. This is rare, but something to check in an older car. The fuel filler neck is very easy to replace.
Three Wheelin'
Joined: Apr 2004
Posts: 1,433
Likes: 1
From: bloomington, IN
There are a lot of people(myself included) that do a bunch of track work with a c4. Worn out suspension, worn out control arm bushings, worn out sway bar links, low tire pressure, a poor alingment, or improper ride height are only a few things that can make it handle poorly. There are a couple of guys on this board that club race c4's, and seem to do quite well. Best to have someone who knows these cars and there specifics take it for a spin. What size wheels are on the car? Pretty sure 16's should be 29 front, 36 rear, and 17's seem to work best at 36 all around. Good luck.
Rennlist Member
Joined: Apr 2006
Posts: 1,777
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From: The 909, CA
Have a read of Excellence magazines article 'Wringing out the Carrera 4' at Johns 964 web site. Talks about the understeer, ways to reduce it and how to drive a C4.
http://www.porsche964.co.uk/rev_rep/excellence.htm
http://www.porsche964.co.uk/rev_rep/excellence.htm
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Racer
Joined: Aug 2002
Posts: 432
Likes: 1
From: Stockholm, Sweden
Replace the 18mm rear sway bar with a 21mm version, install a strut brace in the front in and go with 225/45 front tyres (assuming you are on 17" rims).
These three mods are very cost effective and will also help reduce understeer.
These three mods are very cost effective and will also help reduce understeer.
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Burning Brakes
Joined: Feb 2004
Posts: 898
Likes: 0
From: Derbyshire, England
I thought understeer on the C4 was mainly a result of the PDAS (?) system; that being dependent on all the other parts being set up correctly. And, that tapping the brake pedal dis-engages the PDAS. Not that I have tried this on the street!
Don't seem to have any issue with understeer when street driving mine at a good clip. It feels different at speed to a C2 and will push a little when going hard through tight corners, but nothing disconcerting. Mine has HDs/Greens and is lowered on 17" Cups ( 7 and 8s...are you sure you have 8.5 on the back...didn't think they existed? ) to between RS and RoW height; nothing else other than a good geometry set-up.
EDIT: infact, re-reading your post, it sounds like it is a PDAS issue and a question of learning to drive around/with it when needed.
Don't seem to have any issue with understeer when street driving mine at a good clip. It feels different at speed to a C2 and will push a little when going hard through tight corners, but nothing disconcerting. Mine has HDs/Greens and is lowered on 17" Cups ( 7 and 8s...are you sure you have 8.5 on the back...didn't think they existed? ) to between RS and RoW height; nothing else other than a good geometry set-up.
EDIT: infact, re-reading your post, it sounds like it is a PDAS issue and a question of learning to drive around/with it when needed.
Man of Way Too Many Avatars
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Joined: Jul 2006
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From: Auburn, WA
My 91 C4 Cab has severe understeer when pushed hard. Turns in fine but on power application the car just pushes until lift off. The car has Powerflex bushings and all of the front end is in good shape. Tires are contisport 215 on 7inch wheels and 255 on 8.5" on the rear. Struts are old but seem to be fine ( this winter for replacement). I am looking for some ideas as to where to look for this issue. Has anybody installed 225 pilot sports on a 7" wheel?
It takes a little bravery and you should learn it on a track or other safe area, but try turning in later and harder than you normally would and as the car starts to come around give it WOT. It's scary the first few times, but once you realize the car will grab and go its kind of fun.
Do not try when wet until you have a FIRM grasp of the car on dry or you will be very sorry when you turn your C4 into a jeep and meet a tree or other hard object offroad. Likely backwards.
Rennlist Member
Joined: Apr 2006
Posts: 1,777
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From: The 909, CA
It comes down to learning the car. I was a little concerned when I got my cab. Once I learned how to drive a C4 agressively I am no longer worried.
It takes a little bravery and you should learn it on a track or other safe area, but try turning in later and harder than you normally would and as the car starts to come around give it WOT. It's scary the first few times, but once you realize the car will grab and go its kind of fun.
Do not try when wet until you have a FIRM grasp of the car on dry or you will be very sorry when you turn your C4 into a jeep and meet a tree or other hard object offroad. Likely backwards.
It takes a little bravery and you should learn it on a track or other safe area, but try turning in later and harder than you normally would and as the car starts to come around give it WOT. It's scary the first few times, but once you realize the car will grab and go its kind of fun.
Do not try when wet until you have a FIRM grasp of the car on dry or you will be very sorry when you turn your C4 into a jeep and meet a tree or other hard object offroad. Likely backwards.
