Royal Purple – not a fan...
 

I’ve had gear issues. In fact I’m on my 4rd set of 4.10s. I nearly sold the car at one point it was so frustrating. My car was in the shop over 8 weeks total over a four month period trying to get my issues straightened out. I finally had success and a perfectly quiet set of 4.10s, until yesterday.

How did they get quiet finally? Well Ford did a warranty replacement on the entire axle housing, and in went a brand new set of 4.10s with races, bearings, cups, etc. To make a long story short I have 1000 miles on the rear and I’ve been breaking in this whole time, not leaving anything to chance. Just for that last piece of mind I decided to go ahead and switch out the factory “break in” fluid (regular Motorcraft stuff) with Royal Purple. Lord knows I don’t need any more issues with the rear of this car.

So I pull out on the highway and immediately notice a faint whirr. My stock gears did that, no biggie I could barely hear it but definitely not there before the fluid change. As I’m driving home I notice whine on deceleration! There was none before the fluid change, zip, zero zilch! I had the quietest set of gears I had ever heard including stock, until the Royal Purple went in. Nothing else was mucked with during the change; the mechanic took his time, cleaned it up real well and filled it up good. I used the 75w 140 stuff and no friction modifier (and I have no clutch pack noise).

So Aside from feeling like I wasted good money I’m now left wondering if I should leave this stuff in there or waste more good money going back. I know some will say maybe they were not right in the first place, and maybe that’s true, but they are right enough that they are silent with stock fluid. It’s a faint whine in a narrow range so I’ll probably just leave it be. It just chaps my arse!

Anyone else have a similar experience, and what’s your opinion on leaving this stuff in?

Tim

If that is straight rp without friction modifier that's your problem. It's a fallacy that it's already in the gear oil

I know lack of friction modifier will cause clutch pack chatter (binding), but you sure it can lead to gear whine and whirring sounds too?

Tim

Well it stands to reason that that was the only difference other than brand of gear lube. I am an Amsoil dealer and kinda have a passion for oil. Royal purple although very over-priced is a good product. This hobby of mine has taught me alot about applications of products and whatnot and friction modifier is needed. Gear oil doesn't come with it already in because some manufacturers don't require it. From my experience ford does. I suggest buying whatever for recommends and put in I believe 4 ounces. Now that they have begun to whine I dunno if it will stop but at this point it can't hurt,can it

I believe 75w90 is the correct weight for the 8.8"

According to a dealer, 75w140 was the proper fluid.

I know clutches NEED the friction modifier to prevent chatter/inside tire wrap up when cornering, but wonder if the extreme pressure aspects of that also protect the sliding gear contact faces...these cars put a heck of a lot of force thru there, especially in first gear, probably near 1000 ft/lb or more...if noisy, if any metal transfer has already occurred, that would be a bad thing- hope not...
did the noise start BEFORE the first time it was romped on? if so, I'd be inclined just to go back to std oil(and additive)... it worked before, right :)

Yea. Ford back specd that change last year I think

I just had a gear change and they used Royal Purple and the gears are quiet.

I've been running them for a couple weeks now and not even a slight bit of whine.

The gears are Ford gears though... maybe it was the gear or previous installer?

You guys got me thinking… 1st I have no idea what was in it to start however it was a dealership install so I assumed the Motorcraft stuff. They may have very well used something else to get them completely quiet like heavier viscosity oil. 2nd I did wait to romp on it until I did the fluid change, so perhaps it was the romping and not the RP itself that led to the whine.

I picked up a 2011 fusion for a DD a couple weeks ago, so not like I'll wear out that rear end anytime soon in the Stang. The whine is minimal especially compared to what I had before. It's nothing I'll even notice cruising around with the radio on, it just miffed me that after all that trouble I had a little noise come back and I was kicking myself for messing with it again.

Thanks for all the ideas fellas but I'm going to just leave it alone at this stage. I'm so done messing with the rear end and if the Royal Purple is truly good stuff then I should have nothing to worry about. If for some reason I do start getting clutch pack noise I'll add some modifier at that point.

Tim

I have had Royal Purple 75w140 in the rear end of my 07 GT for the last 2 years. I didn't add any extra LS friction modifier and everything works 100% and is dead silent. Usually gear noise is caused because the ring and pinion weren't set up properly when they were installed. The noise is usually caused by an incorrect wear pattern on the R&P because of improper backlash.

This is an absolutely correct statement. As long as the gears were installed properly any lubricant that meets the spec will do. I have motive 3.90 in my rear and heard horror stories about whine. Well 20k later I have not heard so much as a peep from my gears. Install is the critical factor and 0.010 of an inch does matter