Limited Slip

[DougE]

I'm installing a limited-slip differential in my coupe. I want to add a friction modifier to the oil. I have heard both Red Line 80301 Limited Slip Friction Modifier, and
Ford XL-3 Friction Modifier is a good product. Does anyone have a preferred brand of additive?
Thanks, Douglas

 

[ElGrecko]

I had a bad experience with the Redline product. I've had LSD's in various cars for over 40 years and I've used them brutally in autocross and worn out a good number of them and modified them to increase lockup. Once a Sailsbury type LSD like these starts to slip things go to hell in very short order. You want them to slip enough for normal driving but you want them to tighten up when you want them to work or they will wear very quickly. You want a fluid that properly lubes the gearset but allows the clutches to lock up when pressure is applied and those are opposite attributes. If you get too much boundary layer lubrication (like a hypoid gear lube) the LSD won't tighten up and it will actually wear a lot faster.

My experience with the Red Line product was not good. I had a C4 Corvette and after about 4 years of autocrossing it the clutches were worn out. I had an excellent shop that rebuilt it and we he understood that it should be built tightly to maximize life and my friend happened to be there as the work was being done and they built it to the tight side of blueprint specs. They filled it with Redline diff fluid and their LSD modifier and in about 3 weeks of street driving and a couple of autocrosses and it was loose again. The shop rebuilt it again to the same specs and used the proper GM rear end lube and friction modifier and it lasted for another 4 years of abuse and was nice and tight when I sold the car.

You can buy differential fluids with the modifier already in it. Castrol Syntrax is what I am using now in my C5 Z06. I had the LSD rebuilt two years ago and it's working nicely. It was what the race shop that rebuilt the diff put in after the rebuild and it has a friction modifier in already and they don't use an additive anymore. It's been in there for 2 years of street driving (about 6000 street miles) and probably a dozen autocrosses and I checked it recently and it still had the proper stock break away torque.

The side loader LSD's don't have a high lockup percentage and thus they are going to tend to wear more quickly. They only have 25% lockup. There are a couple of shops that will mod your diff for a higher percentage of lockup and that's probably a good idea just from a wear standpoint. I think Ireland is one of those shops and if you go to around 40% it works a lot better. If you use a friction modifier at all use the factor modifier from GM. You can get that at any GM dealer and that's the best additive that I've used when I was using one.