Our AWD questions

[jepherz]

Just a few simple questions on the AWD system on a STOCK galant vr4 that I'm a little confused about...

As far as I understand it, we have an open front and rear diff on our cars from the factory. What this means, is if one of your wheels starts spinning (let's say you're on ice) then the other wheel will get no power. Does this sound right?

Now I'm confused on the center diff. As far as I had always thought, it is either:

1. a 50/50 F/R split when traction is not an issue, and can transfer power from 100/0 to 0/100 when needed to the wheels that HAVE TRACTION. This would explain why all tires should be replaced at the same time and also why you should only tow the car on a flatbed. If this is the case, what are people gaining by buying a better center diff?

or 2, the center diff is open just like the front and rear and functions the same. I could then see why people upgrade the center diff, as it seems as soon as you lose traction in one corner, you are SOL.

Can someone explain how the system works?

 

Jeff, I don't know whether these are what you're after /ubbthreads/images/graemlins/dunno.gif:

 

[Armitage]

Quote:

As far as I understand it, we have an open front and rear diff on our cars from the factory. What this means, is if one of your wheels starts spinning (let's say you're on ice) then the other wheel will get no power. Does this sound right?



Correct. That's why for competition a lot of folks upgrade to the 92 DSM 4-bolt rear end to gain a limited slip differential in the back as well.

Quote:

1. a 50/50 F/R split when traction is not an issue, and can transfer power from 100/0 to 0/100 when needed to the wheels that HAVE TRACTION. This would explain why all tires should be replaced at the same time and also why you should only tow the car on a flatbed. If this is the case, what are people gaining by buying a better center diff?



This is the one. For drag racing and other hard launch applications people may need to upgrade (or weld) their center diff to prevent it from breaking. Also, our center diff (and the 4 bolt rears) are viscous coupled units, so they take a bit of time to transfer power. I don't know anything about upgrading our diffs but other types of mechanical diffs (torsen) the distribution of power to the wheels that grip are quicker.

I'm no expert on the subject but hope that helps a little.

 

[cheekychimp]

Cusco does a clutch type centre diff that's pretty good, locks up quick and is stronger giving a 50/50 rear split and is machined so as to eliminate the VC. They also do a sun/planetary centre diff that is supposed to give a rear bias split but it suffers from the same disadvantage as the open diff in that it directs power to the wheels slipping (very, very bad news if the back end starts to go ... think about it!)

I have nothing against Torsen type diffs as long as you are on decent tarmac, but they do give problems where one wheel lifts off the ground. I personally prefer to stick with clutch packs for that reason alone. But it is very much a case of picking the correct diff for your application and NOT a case of saying one diff is better than another. Many people have had fantastic results using a Torsen type front diff for track racing to pull the car through bends.

 

[jepherz]

That answers all of my questions. So basically our center diff, and the 3 bolt/4 bolt LSDs are viscous, meaning the oil has to flow in order to start locking them up.

Thanks for clearing that up!

 

[464/2K]

so our cars have a 50/50 ratio then?

 

They have a 50/50 front/rear split when all four tires are on the ground and have equal traction. But the viscous coupling allows that ratio to change front to rear as needed when the traction of one or more tires changes. For example, if you're drag racing and launching the car off the start line, the traction at each wheel is going to change and the viscous coupling is going to adjust the ratio. That is why some people change to a welded diff to permenantly lock the diff into a 50/50 split that does not change.

 

[skivittlerjimb]

The center VC, at least on my car, takes a noticeable amount of time to transfer power from the slipping front to the rear. It is a noticeable delay in at least two distinct scenarios:
1) Slowly churning up a rather steep and very slick hill in town in first or second gear on cold mornings when it has a layer of ice and slush. All four wheels are on an equally slippery surface, so the side to side open diff issue is somewhat negated (I have the stock open rear diff in 1432), but you can tell the VC is doing it's job to transfer torque to the end that has more weight and thus more traction, but it takes it's sweet time. Maybe a half to a full second of front wheel slip before the rear tires take up the slack?
2) On grippy asphalt coming out of a slow corner in 2nd gear at full boost at about 3700-4000 rpm. It sounds a bit like clutch slip, but it's not. (At least I don't think so- the clutch in 1432 has less than 10k on it and the tranny was fully rebuilt at the same time.) As the car transfers weight to the rear under accel., the front tires begin to slip. Again the VC takes it's time, but gets the job done and transfers power to the rear.

I've only begun to read up on how diffs. can lower lap times, but my hunch is a LSD front diff. would be the best upgrade diff upgrade to get the most lap time reduction, especially in tracks with slower corners. Not sure if this extra grip would be offset on a tight autox course by the extra steering effort/tugs, but I bet being able to get on the power a tad sooner would be a benefit there, too. LSD centers can make things even more interesting at the steering wheel, and in some cases, can increase power on understeer, but in skilled hands, I'm sure a better center diff. than the stock VC system would lower lap times further.

-Jim B.
1432/2000
168k

 

[cheekychimp]

I've heard the best combination for the track is a torsen front, clutchpack centre and either the open diff at the rear or a clutchpack!

 

[skivittlerjimb]

I had a ride in a 4g63t, AWD '93 Dodge Colt coupe at the track with clutchpack diffs. front, center, and rear. Talk about noisy at slow speeds! Wicked grip out of corners, but the car needs more suspension development. The owner had tested it on standing starts on gravel and said accel. was really not much slower than on asphalt. A little tough to live with as a daily driver, though.

-Jim B.
1432/2000
168k

 

I'm running a Quaiffe front diff., stock viscous center diff. & a DSM viscous LSD rear diff. & run lots of road tracks. I am putting down 340WHP & 330W ft-lbs. I have no operational or handling problems with this modest $ setup on my daily driver for the past four years.

On the road track, the slight delay of the viscous center diff. locking up doesn't seem to hurt me, even on a wet track. The Quaiffe front diff. ensures that both front tires are getting all their power. I believe the most important issue for fast laptimes on my street tires (even sticky Azenis,)is smooth and careful power application past the apex and managing the weight transfer of our front heavy cars (from entry under braking thru turn-in to power-on for the rest of the turn.)