Hypoid damage to 915 Transmission?

[jcsjcs]

I just replaced the Transmission Fluid in my '84 911 to Swepco 201. The stuff I removed appears to have been hypoid based. My friend (a BMW guy) is concerned that hypoid can damage brass synchros. Any concern for my 911?

[Amail]

I thought hypoid was a helical gear form?

[Steve@Rennsport]

Swepco 201 is a GL-5 lubricant and works VERY well with the 915 in your car. These do not have brass syncronizers; they are steel, just like the spiders and operating sleeves.

The GL-5 rating is required to protect the ring & pinion gears.

[Geary]

There are no transmission oils sold today (that I am aware of) that will damage brass synchros. Some of the very first GL-5 synthetics had gross amounts of sulpher which may have been hard on brass synchros, but I've never seen evidence of this myself, nor do I know anyone personally who has seen evidence of this. At any rate, the old wive's tale (about GL-5 gear oils being hard on brass synchros) was born somewhere, and will likely be around for decades to come.

Hypoid R&P designs see somewhat greater stresses, and require "extreme pressure" gear oil types. These gear oils are sometimes called "hypoid" gear oils. Since hypoid gear oils offer better gear face protection, this was the conservative recommendation for early VWs & Porsches with their relatively small-sized R&Ps, even though they are not of hypoid design.

Swepco 201 will win the Extreme Pressure contest against ANY synthetic sold today, hands down. I've only ever heard of one other fossil gear oil that offers better R&P protection (and I'm still trying to track it down).

[porsche930dude]

I just changed mine over today from what looks like redline but not sure. Anyway im sure it has been a while because the drain plug was totally full of gunk and metal Hope it shift better thad be sweet. I ajusted the clutch too though so i wont know what made the difference

[dvkk]

Originally Posted by Amail

I thought hypoid was a helical gear form?

You are correct. A hypoid gear set is a R&P where the pinion shaft's centerline is offset from the ring gear's centerline. Doing this causes the stresses on the gears to be greater, but at the same time the whole gear set is stronger.

Because the stresses are greater, to protect them properly, you must use some kind of GL-5 gear oil. Do not use any GL-4, it has an inferior extreme pressure rating compared to GL-5.

I agree with Paul about the myth about the problems with brass syncros and GL-5. This myth is rampant in the VW world, and I have put the call out for anyone to post their pics of brass parts damaged by GL-5. So far, no takers.

[chris_seven]

I have been concerned about the best oil to use in 915 transaxles for some time now and this threadf does seem to raise some interesting issues.

I have alwasy believed that GL-5 isn't great for synchro gearboxes as it has excellent lubricity and the low friction increases the time to spin up gears to the correct speed. As the 915 is already quite slow I can imagine that GL-5 isn't ideal for gearshifting but does improve the life of the R & P.

I have always thought that GL-4 was recommended for medium duty hypoids and that GL-5 was for heavy duty applications and I guess this leads to a debate about the loading in the 915 hypiod. If my understanding is correct the hypoid angle in the 915 is quite steep and this does produce more loading than in a conventional axle.

I spent some time looking though some of my old Workshop Manuals (genuine Porsche) and up to about 1976 Porsche specified GL-4 for 915 transmissions. ( I may have dates wrong but this certainly covered the mag cased 915s).

After this date they changed to GL-5. Obviously there is no explanation just a change of detail.

The effect of GL-5 on 'yellow metals' is also well documented and early 'EP' transmission fluids has ZDDT additives to give them thier pressure capability. GL-5 had around twice the ZDDT as GL-4.

These Zinc based compounds are known to attack bronzes, but as I never used a GL-5 in a manual gearbox I nevef had any problems.

I also believe that modern GL-5s tend to use a Boron based additive which has removed this problem and as stated the use of steel parts in the 915 removes this issue completely.

The only real concern then becomes the synchro action and the suitability of GL-5.

I tried discussing this with a couple of guys I know who make oil additives and one of them suggested using a Redline Transmission Oil which has been designed specifically for Transaxles which combine hypoids and synchros.

The fluid is basically a GL-5 but contains a friction modifier to allow good synchro action.

We have used it for a couple of years now and have not had any issues or wear problems.

Shifting is good but we haven't really compared it to Swepco or other fluids.

[Geary]

If I lived in the UK, I would be using Castrol LSX90 over RedLine.