Rear Diff Options - Power Oversteer Found Here
Aug 23, 2010, 07:21 AM
#154
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If you get any lockup or shaking in low speed turns, it's probably time to change your rear diff fluid. My stock rear diff started acting up around 20K or so, and I've been changing rear diff fluid religiously ever since - usually every 5K miles or so. Haven't had an issue since.
l8r)
l8r)
Aug 23, 2010, 11:35 AM
#155
My shep diff has been in for about 5 days. I've done about 400 street miles on it and 2 autocrosses. The original unit had 70k miles and was slipping badly (lots of inside rear wheelspin) -- more on that later.
Installation:
Easy enough, assuming you know, or can figure out, how to remove the rear pumpkin. The unit was returned to me in a brand new box, power coated. I opted for a 12 plate upgrade on the recommendation of trevor. I didn't really spend much time considering different options. I just knew I wanted more lockup in the rear. Total cost of rebuild, bearing replacement, and shipping was around $450.
Noise:
As mentioned in an earlier post. I filled the unit was OE diaqueen. I heard three clunks as I rolled out of the garage and nothing since. Seriously, zero noise. I still use the original rubber bushings and mounting hardware. All things considered, my car isn't very quiet. I have an open exhaust, intake, and no sound deadening. I can hear my walbro fuel pump, but not the diff. *shrug* In my case it wouldn't matter. I don't take the evo to the store.
Street Performance:
It's hard to know what to say here. It's not possible to _really_ push this car on the street. Throttle application result in significantly more rotation than before. The way my car is setup it will step out in any gear. It's not dangerous though. It won't snap. If you can drive a z06 without spinning at every right turn, then you can drive this. I made a quick road trip last weekend for an autocross. On 400 treadwear tires, I could still make the car push -- especially at turn-in. It's not something you'd experience in on-ramp driving.
Autocross performance:
My setup is not stock and I'm not a secretive guy. I'm running Zzyzx EM sport coilovers. They rock, but are completely unobtainable at this point. It sucks to be you. Springrates are 700F/1000R, with a larger front bar and stock rear bar. Those aren't typos. Camber is around -3.1 F / -1.2R. Zero toe. On crappy street tires (above) the car is an oversprung mess. All you can feel is the sidewalls flopping around. On race tires, the car works -- o brother does it work. I'm running cams, E85, and a stock 9.8cm turbo. It makes 300 and something horsepower. 350, 375. I dunno. It's not a dyno queen. My 5400' dyno chart would be useless to most people.
Now this whole process began because of a problem after midcorner. I'll call it 2/3 corner. Any appreciable throttle application at 2/3 corner would result in significant inside rear wheelspin. It took a while (30-40 runs) to diagnose and confirm that it was that wheel and repeatable. The rear wheel was unweighted enough -- and the diff was slipping enough -- that the available traction barely exceeded the scrub of the last 1/3 of the corner. There was a very noticeable point when the wheel loaded back up and the car took off. By that time you were near the exit though.
With the rebuilt diff, this problem is gone. On power, the rear end locks up and power goes straight to the pavement. Excess throttle will still spin the tires, but the rear end will step out rather than just make smoke. Consequently, corner exit understeer has been eliminated in my case. I wasn't really aware of this fact at the time, but I did 10 runs last weekend. There wasn't a single time that I had to get out of the throttle to tuck the nose back end. So cool.
The rear question is if it is faster. I'd like to scream an emphatic "yes", but I can't. Last weekend's event was an Rocky Mountain SCCA divisional. Attendance was somewhat low, but the quality of drivers was very high. I was running about 95% PAX times of several national champs. However, that's what I've been running all season. In class, I've run against the same driver 9 times this season. We are usually within about a tenth of each other. I won, but only by 67/1000s. The data isn't there.
There is no doubt that the car is looser. On the first day I nearly spun out of the start gate because I was used to using full throttle on cold tires. I can't anymore. Although, I never actually looped it so *shrug*. The second day felt much better, but I had probably adapted my driving.
For cars with chronic exit understeer, I would say this a "must" mod. For racecars, where time trumps all, it's more complicated. It will depend on specific setups and driver preference. I have never liked a very loose car. That data has not proven that it is faster for me. More fun: yes. Faster: maybe.
I haven't tried the new setup on the road course yet. Maybe this weekend.
donour
UPDATE: This week, I put the car on the alignment rack. I found out that the rear was not at 0 degrees toe like I had set it. The rear was toe'd out about 0.3 degrees total. For those of you doing mental math, that's a lot: around 3/16. That explains much of the loosey goosey behavior. Oops.
I set the toe to 1/16" in and the rear camber to -1.4 degrees. I took it out to the road course for some laps this morning. I don't know how many I did -- a tank of E85 worth -- more than a handful. I was on crappy street tires so grip levels were low, low, low. As marshall predicted, midcorner push returned. However, exit inside wheelspin was still gone. Hooray, it works. The toe in made catching entry oversteer easy-peasy. The only way to really move on those terrible tires was to throw->catch->accelerate. For a little while, the ferrari club was on track with me. They stayed FAR away.
Tomorrow I'm going back out, but this time I'm bringing kumho 710s and timing equipment.
UPDATE2: Ok. As promised, I was back out today. Same track as yesterday, but this time with the PCA. They (we) ran a high-speed autocross/DE hybrid course. Luckily the configuration was almost exactly the same as an event I ran last month.
On race tires the car is much, much better. Like my original impression, the difference is phenomenal. Small change in the rear alignment (-1.4 camber, 1/16" toe in) settled the rear down tremendously. Again, I'll say it's 100% more fun. Corner exit is more controlled and exit speed is higher. I'd even go so far as to now say the car _is_ faster. The delta from a well established local field is 4-5 tenths out of 85 seconds. Depending on how competitive you are, that could be tiny or small. For me that's huge and a no-brainer for the price. I can point to the exact places where I picked up time and another, very experienced, driver backed it up with almost identical times.
Final Verdict. It's not magic, but it does help. If you can't identify why you need it done, then you probably can't take advantage of the extra lockup.
Installation:
Easy enough, assuming you know, or can figure out, how to remove the rear pumpkin. The unit was returned to me in a brand new box, power coated. I opted for a 12 plate upgrade on the recommendation of trevor. I didn't really spend much time considering different options. I just knew I wanted more lockup in the rear. Total cost of rebuild, bearing replacement, and shipping was around $450.
Noise:
As mentioned in an earlier post. I filled the unit was OE diaqueen. I heard three clunks as I rolled out of the garage and nothing since. Seriously, zero noise. I still use the original rubber bushings and mounting hardware. All things considered, my car isn't very quiet. I have an open exhaust, intake, and no sound deadening. I can hear my walbro fuel pump, but not the diff. *shrug* In my case it wouldn't matter. I don't take the evo to the store.
Street Performance:
It's hard to know what to say here. It's not possible to _really_ push this car on the street. Throttle application result in significantly more rotation than before. The way my car is setup it will step out in any gear. It's not dangerous though. It won't snap. If you can drive a z06 without spinning at every right turn, then you can drive this. I made a quick road trip last weekend for an autocross. On 400 treadwear tires, I could still make the car push -- especially at turn-in. It's not something you'd experience in on-ramp driving.
Autocross performance:
My setup is not stock and I'm not a secretive guy. I'm running Zzyzx EM sport coilovers. They rock, but are completely unobtainable at this point. It sucks to be you. Springrates are 700F/1000R, with a larger front bar and stock rear bar. Those aren't typos. Camber is around -3.1 F / -1.2R. Zero toe. On crappy street tires (above) the car is an oversprung mess. All you can feel is the sidewalls flopping around. On race tires, the car works -- o brother does it work. I'm running cams, E85, and a stock 9.8cm turbo. It makes 300 and something horsepower. 350, 375. I dunno. It's not a dyno queen. My 5400' dyno chart would be useless to most people.
Now this whole process began because of a problem after midcorner. I'll call it 2/3 corner. Any appreciable throttle application at 2/3 corner would result in significant inside rear wheelspin. It took a while (30-40 runs) to diagnose and confirm that it was that wheel and repeatable. The rear wheel was unweighted enough -- and the diff was slipping enough -- that the available traction barely exceeded the scrub of the last 1/3 of the corner. There was a very noticeable point when the wheel loaded back up and the car took off. By that time you were near the exit though.
With the rebuilt diff, this problem is gone. On power, the rear end locks up and power goes straight to the pavement. Excess throttle will still spin the tires, but the rear end will step out rather than just make smoke. Consequently, corner exit understeer has been eliminated in my case. I wasn't really aware of this fact at the time, but I did 10 runs last weekend. There wasn't a single time that I had to get out of the throttle to tuck the nose back end. So cool.
The rear question is if it is faster. I'd like to scream an emphatic "yes", but I can't. Last weekend's event was an Rocky Mountain SCCA divisional. Attendance was somewhat low, but the quality of drivers was very high. I was running about 95% PAX times of several national champs. However, that's what I've been running all season. In class, I've run against the same driver 9 times this season. We are usually within about a tenth of each other. I won, but only by 67/1000s. The data isn't there.
There is no doubt that the car is looser. On the first day I nearly spun out of the start gate because I was used to using full throttle on cold tires. I can't anymore. Although, I never actually looped it so *shrug*. The second day felt much better, but I had probably adapted my driving.
For cars with chronic exit understeer, I would say this a "must" mod. For racecars, where time trumps all, it's more complicated. It will depend on specific setups and driver preference. I have never liked a very loose car. That data has not proven that it is faster for me. More fun: yes. Faster: maybe.
I haven't tried the new setup on the road course yet. Maybe this weekend.
donour
UPDATE: This week, I put the car on the alignment rack. I found out that the rear was not at 0 degrees toe like I had set it. The rear was toe'd out about 0.3 degrees total. For those of you doing mental math, that's a lot: around 3/16. That explains much of the loosey goosey behavior. Oops.
I set the toe to 1/16" in and the rear camber to -1.4 degrees. I took it out to the road course for some laps this morning. I don't know how many I did -- a tank of E85 worth -- more than a handful. I was on crappy street tires so grip levels were low, low, low. As marshall predicted, midcorner push returned. However, exit inside wheelspin was still gone. Hooray, it works. The toe in made catching entry oversteer easy-peasy. The only way to really move on those terrible tires was to throw->catch->accelerate. For a little while, the ferrari club was on track with me. They stayed FAR away.
Tomorrow I'm going back out, but this time I'm bringing kumho 710s and timing equipment.
UPDATE2: Ok. As promised, I was back out today. Same track as yesterday, but this time with the PCA. They (we) ran a high-speed autocross/DE hybrid course. Luckily the configuration was almost exactly the same as an event I ran last month.
On race tires the car is much, much better. Like my original impression, the difference is phenomenal. Small change in the rear alignment (-1.4 camber, 1/16" toe in) settled the rear down tremendously. Again, I'll say it's 100% more fun. Corner exit is more controlled and exit speed is higher. I'd even go so far as to now say the car _is_ faster. The delta from a well established local field is 4-5 tenths out of 85 seconds. Depending on how competitive you are, that could be tiny or small. For me that's huge and a no-brainer for the price. I can point to the exact places where I picked up time and another, very experienced, driver backed it up with almost identical times.
Final Verdict. It's not magic, but it does help. If you can't identify why you need it done, then you probably can't take advantage of the extra lockup.
Last edited by donour; Aug 29, 2010 at 07:39 PM.
Aug 23, 2010, 09:26 PM
#157
My shep diff has been in for about 5 days. I've done about 400 street miles on it and 2 autocrosses. The original unit had 70k miles and was slipping badly (lots of inside rear wheelspin) -- more on that later.
Installation:
Easy enough, assuming you know, or can figure out, how to remove the rear pumpkin. The unit was returned to me in a brand new box, power coated. I opted for a 12 plate upgrade on the recommendation of trevor. I didn't really spend much time considering different options. I just knew I wanted more lockup in the rear. Total cost of rebuild, bearing replacement, and shipping was around $450.
Noise:
As mentioned in an earlier post. I filled the unit was OE diaqueen. I heard three clunks as I rolled out of the garage and nothing since. Seriously, zero noise. I still use the original rubber bushings and mounting hardware. All things considered, my car isn't very quiet. I have an open exhaust, intake, and no sound deadening. I can hear my walbro fuel pump, but not the diff. *shrug* In my case it wouldn't matter. I don't take the evo to the store.
Street Performance:
It's hard to know what to say here. It's not possible to _really_ push this car on the street. Throttle application result in significantly more rotation than before. The way my car is setup it will step out in any gear. It's not dangerous though. It won't snap. If you can drive a z06 without spinning at every right turn, then you can drive this. I made a quick road trip last weekend for an autocross. On 400 treadwear tires, I could still make the car push -- especially at turn-in. It's not something you'd experience in on-ramp driving.
Autocross performance:
My setup is not stock and I'm not a secretive guy. I'm running Zzyzx EM sport coilovers. They rock, but are completely unobtainable at this point. It sucks to be you. Springrates are 700F/1000R, with a larger front bar and stock rear bar. Those aren't typos. Camber is around -3.1 F / -1.2R. Zero toe. On crappy street tires (above) the car is an oversprung mess. All you can feel is the sidewalls flopping around. On race tires, the car works -- o brother does it work. I'm running cams, E85, and a stock 9.8cm turbo. It makes 300 and something horsepower. 350, 375. I dunno. It's not a dyno queen. My 5400' dyno chart would be useless to most people.
Now this whole process began because of a problem after midcorner. I'll call it 2/3 corner. Any appreciable throttle application at 2/3 corner would result in significant inside rear wheelspin. It took a while (30-40 runs) to diagnose and confirm that it was that wheel and repeatable. The rear wheel was unweighted enough -- and the diff was slipping enough -- that the available traction barely exceeded the scrub of the last 1/3 of the corner. There was a very noticeable point when the wheel loaded back up and the car took off. By that time you were near the exit though.
With the rebuilt diff, this problem is gone. On power, the rear end locks up and power goes straight to the pavement. Excess throttle will still spin the tires, but the rear end will step out rather than just make smoke. Consequently, corner exit understeer has been eliminated in my case. I wasn't really aware of this fact at the time, but I did 10 runs last weekend. There wasn't a single time that I had to get out of the throttle to tuck the nose back end. So cool.
The rear question is if it is faster. I'd like to scream an emphatic "yes", but I can't. Last weekend's event was an Rocky Mountain SCCA divisional. Attendance was somewhat low, but the quality of drivers was very high. I was running about 95% PAX times of several national champs. However, that's what I've been running all season. In class, I've run against the same driver 9 times this season. We are usually within about a tenth of each other. I won, but only by 67/1000s. The data isn't there.
There is no doubt that the car is looser. On the first day I nearly spun out of the start gate because I was used to using full throttle on cold tires. I can't anymore. Although, I never actually looped it so *shrug*. The second day felt much better, but I had probably adapted my driving.
For cars with chronic exit understeer, I would say this a "must" mod. For racecars, where time trumps all, it's more complicated. It will depend on specific setups and driver preference. I have never liked a very loose car. That data has not proven that it is faster for me. More fun: yes. Faster: maybe.
I haven't tried the new setup on the road course yet. Maybe this weekend.
donour
Installation:
Easy enough, assuming you know, or can figure out, how to remove the rear pumpkin. The unit was returned to me in a brand new box, power coated. I opted for a 12 plate upgrade on the recommendation of trevor. I didn't really spend much time considering different options. I just knew I wanted more lockup in the rear. Total cost of rebuild, bearing replacement, and shipping was around $450.
Noise:
As mentioned in an earlier post. I filled the unit was OE diaqueen. I heard three clunks as I rolled out of the garage and nothing since. Seriously, zero noise. I still use the original rubber bushings and mounting hardware. All things considered, my car isn't very quiet. I have an open exhaust, intake, and no sound deadening. I can hear my walbro fuel pump, but not the diff. *shrug* In my case it wouldn't matter. I don't take the evo to the store.
Street Performance:
It's hard to know what to say here. It's not possible to _really_ push this car on the street. Throttle application result in significantly more rotation than before. The way my car is setup it will step out in any gear. It's not dangerous though. It won't snap. If you can drive a z06 without spinning at every right turn, then you can drive this. I made a quick road trip last weekend for an autocross. On 400 treadwear tires, I could still make the car push -- especially at turn-in. It's not something you'd experience in on-ramp driving.
Autocross performance:
My setup is not stock and I'm not a secretive guy. I'm running Zzyzx EM sport coilovers. They rock, but are completely unobtainable at this point. It sucks to be you. Springrates are 700F/1000R, with a larger front bar and stock rear bar. Those aren't typos. Camber is around -3.1 F / -1.2R. Zero toe. On crappy street tires (above) the car is an oversprung mess. All you can feel is the sidewalls flopping around. On race tires, the car works -- o brother does it work. I'm running cams, E85, and a stock 9.8cm turbo. It makes 300 and something horsepower. 350, 375. I dunno. It's not a dyno queen. My 5400' dyno chart would be useless to most people.
Now this whole process began because of a problem after midcorner. I'll call it 2/3 corner. Any appreciable throttle application at 2/3 corner would result in significant inside rear wheelspin. It took a while (30-40 runs) to diagnose and confirm that it was that wheel and repeatable. The rear wheel was unweighted enough -- and the diff was slipping enough -- that the available traction barely exceeded the scrub of the last 1/3 of the corner. There was a very noticeable point when the wheel loaded back up and the car took off. By that time you were near the exit though.
With the rebuilt diff, this problem is gone. On power, the rear end locks up and power goes straight to the pavement. Excess throttle will still spin the tires, but the rear end will step out rather than just make smoke. Consequently, corner exit understeer has been eliminated in my case. I wasn't really aware of this fact at the time, but I did 10 runs last weekend. There wasn't a single time that I had to get out of the throttle to tuck the nose back end. So cool.
The rear question is if it is faster. I'd like to scream an emphatic "yes", but I can't. Last weekend's event was an Rocky Mountain SCCA divisional. Attendance was somewhat low, but the quality of drivers was very high. I was running about 95% PAX times of several national champs. However, that's what I've been running all season. In class, I've run against the same driver 9 times this season. We are usually within about a tenth of each other. I won, but only by 67/1000s. The data isn't there.
There is no doubt that the car is looser. On the first day I nearly spun out of the start gate because I was used to using full throttle on cold tires. I can't anymore. Although, I never actually looped it so *shrug*. The second day felt much better, but I had probably adapted my driving.
For cars with chronic exit understeer, I would say this a "must" mod. For racecars, where time trumps all, it's more complicated. It will depend on specific setups and driver preference. I have never liked a very loose car. That data has not proven that it is faster for me. More fun: yes. Faster: maybe.
I haven't tried the new setup on the road course yet. Maybe this weekend.
donour
did the 04 evo rs come with acd? the acd reflash from gruppe s may be a good next step
Last edited by kyoo; Aug 23, 2010 at 09:34 PM.
Aug 24, 2010, 07:18 AM
#160
donour - That is a great review!!! It seems that with this diff, it may be advantageous to set the car up closer to what alot of the STis do, to get the rear to rotate some, but still plant and go. Coming from my autocross experience always in FWD, it definitely takes a different approach to make the evo fast (as judged by the clock), and mine also had been the on-throttle understeer issue, with that said if the car WILL actually rotate with throttle input, then the line changes a bit to be quick. I would imagine as you adapt to it, you will cut your times even more. my .02
Aug 24, 2010, 07:27 AM
#161
Donour great write up. To me, this was what applied most:
"More fun: yes. Faster: maybe."
I'm building my car to how I like without much regard to being the fastest. Fun factor wise, it is a great mod. I took a friend out for some street fun with the R-compounds on. I had him giggling like a school girl by taking a 15mph corner at like 45mph with the car sideways. He drives an 800 HP Celica so my 300 HP isn't too impressive to him, but the corner speed and car attitude sure was.
With regards to the diff rachet noise, it is gone with the 235s on the car. I'm thinking it just takes a decent amount of tire to get the rachet noise. Not enough tire and you just get tire slip because the diff is locked up enough and the tire can't over power it.
"More fun: yes. Faster: maybe."
I'm building my car to how I like without much regard to being the fastest. Fun factor wise, it is a great mod. I took a friend out for some street fun with the R-compounds on. I had him giggling like a school girl by taking a 15mph corner at like 45mph with the car sideways. He drives an 800 HP Celica so my 300 HP isn't too impressive to him, but the corner speed and car attitude sure was.
With regards to the diff rachet noise, it is gone with the 235s on the car. I'm thinking it just takes a decent amount of tire to get the rachet noise. Not enough tire and you just get tire slip because the diff is locked up enough and the tire can't over power it.
Aug 24, 2010, 10:13 AM
#162
did the 04 evo rs come with acd? the acd reflash from gruppe s may be a good next step
Originally Posted by Bassicfun
It seems that with this diff, it may be advantageous to set the car up closer to what alot of the STis do, to get the rear to rotate some, but still plant and go.
As I write this, I'm tempted to put the 800lb springs on and go to the track this weekend to see how the car behaves at 100+ mph
d
Last edited by donour; Dec 16, 2011 at 10:50 AM.
Aug 24, 2010, 10:26 AM
#164
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Good thread with some great info from different perspectives.
I believe the consensus is that an upgraded rear diff is a must...... no matter who you should choose.
Glad this has not turned into the typical evom pissing match.
I believe the consensus is that an upgraded rear diff is a must...... no matter who you should choose.
Glad this has not turned into the typical evom pissing match.
Aug 24, 2010, 10:34 AM
#165
Oh! I just remembered one important fact that I forgot to mention. About a month ago I drove Hsun Chen's Evo IX at a large autocross in Colorado. It's classed the same as mine and has a very, very similiar setup. He has a little more power and more tire. His rear diff has been rebuilt by TRE rather than shep.
Having driven both in competitive environments >> against the same people << I can't really tell the difference. His made a bit of noise, but the car had no rear seats and solid mounts. I found no basis to think that one is better than the other.
d
Having driven both in competitive environments >> against the same people << I can't really tell the difference. His made a bit of noise, but the car had no rear seats and solid mounts. I found no basis to think that one is better than the other.
d