Friction modifier, starting point?

[Mick]

I recently put new to me (used) diff in my car and added Redline 75W90 (with friction modifiers). It performed flawlessly all day during 30 minute sessions at NHMS. Later in the day during open track, about 50 minutes in, I started to get a howl from the diff but only under braking.

I assume that I need a *little more friction modifier. But how much is a little? Can anyone give me a starting point?

The label on the 4oz bottle of friction modifier says that you want ~4% concentration, but I can’t find any information on what percentage is already in Redline 75W90.

I don’t want to put in too much, but I also don’t want to have to put the car on jack stands 10 times, putting a teaspoon full at a time until I get the perfect amount.

Anyone have any experience with this stuff?

[offcamber09]

Mick, I can't answer your question from a Redline standpoint, but we usually add 4oz to the average capacity diff. for street use. I assume you've done some research regarding using the synthetic diff fluid for your app. For the Mustangs- we do not use it. The synthetic is inherently more slippery- not what you are looking for in a "limited" slip. For the Torsen T2R diffs- synthetic is not recommended by Reider Racing- and they know their gears, diffs, etc. The Torsen is not a clutch-type diff. so you would think that would be a good application for synthetic.

Not sure I've done much but confuse you here.

-Scott

[DanB]

I would try 2 oz. If it quits howling, you're done. If not, 2 more oz.

Damn, I am a limited slip diff setup genius...

[Mick]

Not sure I've done much but confuse you here.

Haha, you did.

Would you know if the the howl under braking could be caused by too much friction in the clutches? I hope that's all it is. It happened at the end of day 2, so I never got a chance to let it cool down then go back out and see if it chilled out.

A guy on another forum said "Growling under braking is usually due to excessive pinion to ring gap." ...what ever the f- that means. Diff's confuse me.

[Mick]

I would try 2 oz. If it quits howling, you're done. If not, 2 more oz.

Damn, I am a limited slip diff setup genius...

I'm probably over thinking it. I'm just paranoid that I'll put in too much and cause excess slip. This diff was $$, I want all the lock it has to offer

[jlwhorf]

If the rear is howling under straight line braking, I would not think it is the clutches, because they are not slipping under this condition. If it makes noise under trail braking, then maybe it the clutches making noise. If it makes noise under straight braking, you may have the ring and pinion set incorrectly, or hurt a bearing. When I have run a clutch type diff, I used a 4 oz bottle of GM additive in the fluid.

Jonathan

[Mark Swinehart]

What's a "diff"?

[offcamber09]

http://www.reiderracing.com/

These guys will know and are willing to tell you. If Mark calls up and asks them what a diff is- they may hang up.

Mark- how many times did you get calls asking if you sold tires when you were in the business? That used to crack me up.

[Mark Swinehart]

Scott - thanks for mentioning tires. It reminded me that I switch over my winter air (used for storage) to summer air for the Glen. Would you guys be able to do a set for me tomorrow if I dropped off the tires/rims?

Mark

[jadams]

If it is howling under braking, I don't think it has anything to do with friction modifier at all. It has to do with the ring gear pattern. Did you check the pattern when you re-installed the diff? How about the preload on the carrier bearings? Backlash set to .006-.010"? Not to make you worry, but I would suggest pulling the rear cover off and take a look inside there to see what is going on. It is easier to do at home that at the track (I know from personal experience!) I just replaced my diff as well, I broke my diff housing in half at the last event last year....no wonder I couldnt put any power down anywhere!

[TroyV]

After thinking about this a little bit, I think I agree with Joel in a way. Since you bought the diff used, there is no way to determine the condition of the bearings. Back when I did the rear gears in my 99 Mustang, in an effort to save cash, I just had the tech put it back together with the original bearings. If you recall the car howled like nothing I ever heard before, but it did so more under deceleration, than acceleration.

It might be possible that the box needs bearings as the slack is causing your pinion angle to vary or move inward/outward in subtle ways....thereby affecting the wear pattern and causing howl.

Just thinking out loud..

[Mick]

Dam. That's what I was afraid of.

[eastcoastbumps]

I'm probably over thinking it. I'm just paranoid that I'll put in too much and cause excess slip. This diff was $$, I want all the lock it has to offer

Have you considered welding the diff?

[offcamber09]

Scott - thanks for mentioning tires. It reminded me that I switch over my winter air (used for storage) to summer air for the Glen. Would you guys be able to do a set for me tomorrow if I dropped off the tires/rims?

Mark

Mark sorry for the slow response- I've been sick as a dog and didn't leave the shop until 9:00 last night.

Let me know if I can help tomorrow or Sat.

-Scott