Swapping to 7.31 R&P
 

I am considering changing the 8.31 R&P on my 915 tranny to either a 7.31 or an 8.35.
With regards to using a 7.31, is it a straightforward process of swapping one with an 8.31?
Can I use any good, used 7.31 R&P from earlier 915's, and put it into my later-915 tranny?
I know the 8.35 from Guard Trans is available, and is a more robust solution.
Just wanted to know the pros and cons of of both options...before I jump into it.

I'd call Matt at Guard, he will give you the correct advice.

To look at the finished installation of each, the 8:35 is a far more confidence inspiring solution.

Pete ... Whats are the pros/cons for a 8:35 RP vs the others. I dont think Ive ever seen/heard of one .. Im curious. Thanks!

Lot's of info here:

http://forums.pelicanparts.com/porsche-911-technical-forum/722760-has-anyone-used-8-35-ring-pinion.html?highlight=pinion+bearing

...and here:

http://forums.pelicanparts.com/porsche-911-technical-forum/720070-broken-915-pinion-bearing.html?highlight=pinion+bearing

Thanks Pete. Good info. Im guessing a modified 3.6 will tune up one of these in a hurry? Slower then a 7.31 but faster then a 8.31?

And an 8:35 is much stronger than a 7:31!

Thanks for chiming in Pete...and I've come across that link you provided, and read it multiple times to familiarize myself with the nuances of a 7.31 vs. 8.35 r&p's.
From a purely "$" perspective, it's a substantial diff... $500-600 for a good, used 7.31 vs. $2895 for the 8.35.
And there are many 915's happily running around, both cruisers and track cars, that have the 7.31 r&p...with no apparent negative consequence.
No question, the 8.35 is intrinsically stronger...but definitely a more expensive proposition.

I was hoping that I could be part of the "7.31 gang"...and enjoy the benefits of a 4.42 final drive ratio on the track...:D

What does this entail?

I would be interested to know why the 8:35 is stronger than the 7:31 as the cost difference is significant (at least for the Tandler parts).

Is a tooth loading or a scuffing issue?

I thought the rule of thumb was to have no fewer than 9 teeth on a gear to reduce the speed variation (maybe it was 11 to have a prime number). Isn't it also undercut?

More teeth = higher Lewis form factor. Not sure how much difference there is between 7 and 8 teeth, offhand. I recall Porsche once ran a gear with 6 teeth.

The issue with the number of teeth between the two gears is to make sure that you don't have the an individual tooth on one gear making contact with the same teeth on the other gear every revolution. As an example, an 8:32 pair would not be ideal.

JR

That is a good question.

Porsche used a stronger differential side plate on the Carreras. I wonder how a stronger side plate enters in the equation.

I think that when I wrote that an 8:35 was stronger than a 7:31, I was suggesting that based on how Porsche engineers parts. For instance, as soon as the 930 Turbo went into production, Porsche switched to a 9-tooth pinion. It's big and heavy, and if Porsche didn't think it was necessary to cope with the Turbo's increase in power, they would have continued to use the earlier part, or something similar.

Porsche used the same reinforcing strategy on the Sportomatic in 1975. When they switched to a 3-speed unit, they changed the R&P from a 7:27 to an 8:27. Not sure how long they would build Sportos, they probably had an eye to the future and made the change based on higher future hp & torque numbers.

When Porsche switched from a 7:31 to an 8:31, I'm sure they did that with an eye toward the future; bigger displacement engines producing more hp and torque. I feel that an 8:35, for those reasons, is a better choice. I know that they're new, and expensive, but under normal conditions should last the life of the car. A used 7:31 might not be able to deliver that.

The load limit for a gear set is proportional to 2N2/(N1 + N2) for the 7:31 the proportionality constant is .36897 for the 8:31 it is .41025 a strength increase of ~12%


The potential issue w/ swapping cwp besides strength is that it doesn't change the rpm drops which is what most really want to address.

Changing the final drive ratio will change the absolute rpm drop between gears but not the percentage change in rpm.

Nope...

The Z man knows the deal
 

The Z man has a very valid point. Look up the sheared gear teeth threads on overloaded 7:31 gear boxes here on Pelican. I went with a 915/72 for my Euro 3.0. The 7:31 would work nicely on a 2.7 high RPM motor (call Henry at Supertech for one) or a pleasure use 1980s US stock SC 3.0. However, javadog hit the nail on the head about gear design calculations. The 7:31 went away after 911 motors went up in displacement and started making lots more torque(not just peak HP @ RPM). Basic Torque/HP equation. The 7:31 behind a built 3.6 will have a very short life span and a high maintenance/replacement frequency. Torque needs more "teeth in contact" and/or "bigger gear teeth" to keep from killing a gear box too quickly. IMHO Save the 7:31 R&Ps for the long hood restorations and use a R&P that will consistently hold a 3.6.

Nope

Here's an example of a g50/30 that was configured w/ 9:31 for use in Cup cars and 8:32 in RSR's
http://forums.pelicanparts.com/uploa...1386132605.gif

Bill according to 'your' equation the 8:35 has a proportionality of 0.372 which is a strength improvement of 0.8% from a 7:31.

Hardly seems worthwhile bearing the cost involved?

I am also not sure the teeth in contact argument works either as a spiral bevel generally has two teeth in contact - sometimes 3 depending on the accuracy of the pitch - I am not sure changing from a 7 to 8 tooth pinion changes this factor.

7:31 r&p...
 

I was faced with the same decision a few years ago. If you're not in a hurry and have a good 8:31 R&P already, I'd suggest swapping the gear sets. Stock 1st is low enough, and based on feedback I thought a 7:31 would make 1st a bit too low.

I picked up a good used GT set and found another new in box GT set for a great price. I went with new Albins for the other two and negotiated a fair price on the set.

I went with:

2nd - 15/30 (2)
3rd - 21/31 (1.476)
4th - 24/27 (1.125)
5th - 28/24 (.857)

5th gear was tough for me because it was only a slight drop at a big expense, but I didn't want to spend all this coin on the box (I also have a Wavetrac LSD, one piece bearing retainer plate, billet side cover, Wevo internal gateshift and I plan on plumbing it for a cooler - $$$...) and have a "compromise" flat 5th shift.

My 3.0 build (Max Moritz P&C, early big port heads & CIS, 964 cams, SSI's, M&K, etc..) will hopefully compliment it. I plan on a conservative 6500 prm redline.

I guess my main point is that if you are patient and have some time to wait there are usually some nice gear sets that pop up every few months. You could also post a WTB, it seems that some folks on the forum are very helpful and reasonable in their pricing.

If it's a pure "budget" build, it's hard to beat the 7:31 R&P swap though...

I think there is one question from the original post that remains unanswered. Installing a new ring and pinion is never a straightforward direct replacement. It is a complicated installation and without the proper tools to set up depth and backlash, as well as measuring bearing preload, it would be very difficult to get right.

Also, contrary to Mike B's comments in one of the linked threads, our sets don't require that one guess at set up. The depth is marked right on the unit, just in a slightly different manner than his Porsche marks it.

As for the strength question? Let us not forget that there have been huge advances in metallurgy between 1973 and today. It's not just tooth count that dictates that a brand new just made cwp set is going to be stronger.

Regards,

Matt Monson
Guard Transmission llc

Matt,

I am not sure that I can agree with this statement at a basic level.

Although there have been many new materials developed in the last 50 years most of the steel compositions we use in basic engineering have been around for the best part of 70 years.

The old traditional gear steel in the UK is EN36 and this has always had a specified tensile strength of around 1100MPa.

It is still in common use today and still manufactured in significant quantities.

http://www.tatasteelnz.com/downloads/CaseHard_BS970-1955EN36A.pdf

There is no fundamental difference today than when the designation was initially introduced in around 1940 (EN Series - Emergency Numbers created at the start of WW2)

The majority of the EN Steels are still used EN24 for example is virtually identical to SAE4340, EN19 is similar to SAE4140 etc. all have been around for a considerable time.

It is certainly true that some of the 'smart' varieties of these steels offer significant improvements over the standard production forms of the similar materials.

300M for example is an enhanced version of 4340 and by virtue of modern processing (typically Vacuum Arc Re-melting) has much better properties but at a significant price premium.

The area in which commercially produced steels - to differentiate from speciality steels- have improved is in the area of cleanliness. The introduction of steel making practices such as QBOP and LD converters have had a significant impact.

It also fair to say that grain refinement techniques have also improved and tended to make steel more consistent.

The practical significance is that most commercial steels have significantly less 'prior' defects than was the case in the Sixties (when I began my career as a Metallurgist) and Seventies but the 'strength' of materials that are correctly manufactured has not changed much during that time period.

It would be true to say that less defective parts are produced and that Weibull B10 lives are better but I think this is a different argument to saying that a given part is stronger.

Again the one area where this statement is inaccurate concerns fatigue strength and I would certainly agree that cleaner and more consistent materials will improve this area of performance.

^^^Wow.

OK, now that that was taken care of.

Maybe I missed it but, it seems to me that the OPs car, motor, and intended use should not be overlooked. IMO in most cases on SCs and Carreras close ratios with the 8:31 would be the best option for a track car. Autocrossing, well that could be debated, I suppose.

According to what I have read the added strengh of the 8:31 comes largely from the fact that the pinion gear is larger in diameter, resulting in less forces on the teeth due to the longer torque arm. I hope that makes sense, not sure if that is the correct way to explain it.

I also think a major consideration when Porsche went to the 8:31 was the fact that the newer cars with the larger motors benefitted from the higher overall gearing.

Everytime I drive on the freeway I want a sixth gear. But on the track with my 2.5 it is perfect with the 7:31. I may change my mind when I finally get my 3.0 in there. We'll see.

I guess it's just magic that my Cup Car ring and pinions and gearsets last on average twice as long as the factory Holinger bits. Wouldn't have anything to do with our choice of alloys or heat treat( heat treating IMO falls under the broad umbrella of metallurgy).

So you are free to challenge my assertion but I can tell you that we aren't using common grade gear steel and get our steel from the same mills and of the same grades as are used by the top Formula One teams.

Very interesting...and spirited discussion...and I'm getting a lot of useful info that'll be helpful when I finally decide to pull the trigger on which path to take:

a. Swap to a 7:31 R&P
b. Swap to an 8.35 R&P
c. Stay with the current 8.31...change to a close-ratio 2-5 gear set.

Right now, I'm using a newly rebuilt 915 with stock ratios, 8.31 R&P, Wavetrac diff, Hargett shifter, Wevo gateshift, with an OEM Porsche oil cooler.
So I've spent a pretty penny on this tranny at this point.
I'm very pleased with the tranny action...no sloppiness, no grinding, no issues.
Great for cruisin'...but leaves a lot to be desired on the track.
It is mated to a 2.8 MFI...with the usual "bells and whistles" that make it a "giddyup" motor...:D

This car is bound to grow old with me...and I am enjoying what the journey is all about...;)

Really all that matters is the tooth length, as the gear won't fail in the "disk" center, and the tooth length is a function of pitch rather than diameter. If the 7 and 8 have the same pitch, then they are equal from that perspective.

If it was my car, I would replace 3,4&5. Better rpm drops, more cost effective and the correct route to a faster track car.

DocDan - the 8/31 to 7/31 swap, keeping the original 8/31 gear sets, is a great and relatively low cost (as hot rod options go) way to pep up an older car. When I ran one in my stock 3.0, it noticeably pepped the car up. My wife asked what I had done with the motor.

I'd be a bit hesitant to do this with a motor much more powerful than a stock or mildly pepped up 3.2 motor, though. I put the 8/31 I had kept from when I built the 7/31 box into my race tranny where I was getting 250 RWHP. I had two non-standard gear sets (low 3d, stock 3d for a 4th, 1:1 5th) for the box to get the ratios I wanted, and they did the pepping up on the track.

Of course heat treatment techniques are metallurgy but case hardening, austempering, martempering and many other 'advanced' techniques have been around a long time. The more advanced treatments just weren't commercially viable.

Are you are saying that you are using the recently developed VAR gear steels (2008/2009) with high Molybdenum content and core strengths of around 2000MPa that have been developed by Corus?

If this is the case then I accept that there will be an improvement in strength but the cost increase in using this material generally results in extremely high prices.

For example an XTrac LSD for a GT3R is currently around $12000. I am sure that they are excellent but certainly beyond my limited means.

The more general point, however, that all steel is stronger than it used to be isn't accurate and to assume that recently made gears are stronger as a consequence is the statement that I was challenging.

To equate life directly to strength is also misleading as surface hardness, oil film retention and many other properties affect the result.

I guess that I just like to see comments of this type backed up with some data.

Here's a graphic showing a comparison of a 915/63('78-79 SC) w/ the 3 different cwp choices discussed here also added are 2 properly regeared 915s, both were used in SC/RS, one is stock w/ 8"31 the second is a rally version w/ gear and cwp change, lower lines will feel and be quicker acceleration, also added are the rpm drops, all the 63 are the same and I only included the SC/RS rally version drops, all use the same rear tire and same rpm limit even though the SC/RS will rev higher and thus extend the lines up and to the right to higher speeds
http://forums.pelicanparts.com/uploa...1386260082.gif

Notice that the difference between a 7:31 and 8:35 is almost nil, you might as well use the stronger 8:35

also note that a higher top speed in first even though acceleration will be marginally decreased is generally a good thing as long as the motor is reasonably torquey, I'd much rather have a 39mph 1 vs a 35mph w/ a 3liter

^^^ Hear ye, hear ye. Two years ago I went from a 7:31 to a 8:31 and I never looked back. I love being able to squeeze a little more mph out of first gear. The 731 was/is fun, but first gear is useless just like in my saab viggen. Granted, I'm talking about a street car with a 2.7 and not Bill's cars, if I was still auto-xing then the 7:31 would go back in.

As a metallurgist, certainly you understand that heat treatment is an evolving science, therefore a continually improving process. I'm aware of at least two small changes in the “basic” heat treatment process of Matt's gears during the 2004-2009 time period that have made a noticeable difference in strength (although I would attribute the longevity of Matt's gears more to tooth design than to material and heat treatment). If I'm aware of these two improvements, there have likely been many others.

I'll presume to speak for Matt .. yes.

Could you expand on this? You are implying that you know the material costs of an Xtrac LSD, and how this translates to the ridiculously high price tag. I challenge this assertion, so I would appreciate your back-up data.

Yes, you seem to challenge and twist any of Matt's contributions. Matt did NOT say that “all steel is stronger than it used to be”. This is you distorting what he said. What Matt said was, “There have been huge advances in metallurgy between 1973 and today.” and he refers to “our choice of alloys”, obviously not common EN36.

You seem to be debating yourself here, as you simultaneously challenge and yet support Matt's simple statement, “There have been huge advances in metallurgy between 1973 and today”. Why should Matt have to qualify a simple factual statement with a five-paragraph explanation?

Thank you. I've been badgered by Chris enough to know when to walk away. It gets old being followed around the board being challenged at ever turn.

I really don't want to badger Matt, the only comments that I ever make are to try to get some facts beyond the statements being offered.

For example if you know of two simple changes to heat treatment methods that add significant strength why not share them.

What are the heat treatments and what strength improvements were produced?

As I am sure you know XTrac have a range of steels that are produced on their behalf by Corus (now TATA), I don't believe that these materials are commercially available.

In a previous life I commissioned small batch production of steel for the manufacture of Military Vehicle Drive Shafts (30 tonne quantities) which we manufactured on behalf of the MOD in the UK and the cost difference between this and a commercially available alloy was around 7 times greater than a similar steel produced in large volume.

I would agree that there is more to the price than material cost but the differences may be significant.

We buy S155 in small quantities and this is also a VAR steel but produced in reasonably commercial quantities and the cost is around 3-4 times a conventional Air melt steel with similar chemistry.

This seems to say that if it is new it must be stronger - I am not sure how I misinterpreted this information.

The simple point I was trying to make was that there have been many advances in metallurgy but not all of them are employed on a day to day basis.

EN36 is still a common choice for the manufacture gears with SAE8620 also still being used in large quantities. I think even F1 gears were commonly made from EN36up to around 2001 - at this time I was developing the first of several gear fatigue test rigs for the FI industry as well as Military Helicopter Gear and Gearbox test systems.

When I trained as a Scientist I was taught that factual statements should be backed up with data and this is always sadly lacking.

It is clear to me that you guys just don't like being questioned and as I am an 'unbeliever' you never seem to respond with anything other than rhetoric.

I would be more than happy to know how much stronger the 8:35 is when compared to the 7:31 but no-one seems to have the answer :(

Chris,
You say you don't mean to badger me but you bumped a thread that had not seen a reply in a month to challenge a statement I made regarding forging being a type of casting and that what really mattered was the choice of materials as much as the actual process of forming the metal.

You responded with one of you multi-paragraph rants talking about how casting and forging are not all similar and completely ignored the point of my post, which is that the Japanese automotive aftermarket has a long history of throwing out "forged" as a marketing buzzword meaning to imply their stuff is better made, even if their choice of alloys is crap. That is typical of how you interact with me and target my posts.

Matt,

I didn't 'bump' a thread after a month it was just the first time I saw your comments and I don't agree that forging is a type of casting and never will.

If you believe this that's fine but I think it is acceptable to disagree.

It is interesting that you view comments that disagree with you as a 'rant'- I see it as trying to provide the correct information and to explain why I disagree.

I believe that the comparison that you made was a fundamental error an of more significance to the general understanding of metal processing and that it needed to be challenged.

If you are confident that forging is a type of casting please explain why and we can then have an informed debate and I would be happy for you to prove me wrong.

I don't target your posts but do believe we make significant statements that deserve to be challenged.

I would prefer you to consider it as an exercise in trying to improve understanding and not aimed in your specific direction.

the 7:31 vs 8:31 debate is typical. I asked Tandler the same question several years ago and never gained an answer.

We can argue forever but without explanation it is just a matter of belief.

I presented a paper on the influence of Shot Peening on the fatigue Life of Case Hardened Gears at the European Conference on Fatigue Prevention and Design in 1986.

The presentation, even though backed up by hard data caused a huge debate with some of the participants suggesting the concept had no merit. The debate meant that more work was done and the preparation of gears using this technique is now well established and commonly used.

I see it as a means of making progress but I believe you disagree and regretfully I will no longer respond to your posts.

I'd like to badger Chris and Matt for fun, maybe another day on a different post.

No real science here, just my personal experience - I've been using a 7:31 with a high rpm / high hp (400hp / 300+ ft. lbs. 3.4l) in a 915 in the same gearbox for 13 years, I make no attempt to baby it, 3 to 4 and 4 to 5 clutchless up shifts. The case has all of the Wevo Race Case bearing mods, shot peened, lightened gears, cooler, spray bar, standard RSR flywheel, pressure plate, clutch, hydraulic VW clutch release, and it has a 935 spool instead of a LSD. I've never had any gearbox / ring or pinion failures, oil temp never an issue. Mobile 1 gear oil.

Aluminum or magnesium case?

JR

65rsr if you are making clutch less upshifts you know how to drive. A good driver gets a lot more out of these cars than the least common denominator.

I've got clients who go through 3rd gears annually. Other guys who race the same series can run 4 or 5 years on the same gear.

Aluminum case. The 3.0 case with the big round plate under the sump.

I guess I'm easier on it than I let on. I have pedals that mount from the top and make it easy toe and heel, (actually toe and side foot) a very percise early wevo external gate shifter, and an extremely well built / blueprinted gearbox. Also, the way my gearing is, I don't shift alot, and I don't use my transmission to scrub off speed.