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Old 04-19-2006, 12:26 PM   #1
RalliSpec
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Default Gearbox technical discussion

We seem to recieve a tremendous number of questions about WRX gearbox issues, gear oil recommendations, etc. I thought I would post an overview of some of these questions and provide answers based on our experience and research. I am sure that some of this info may be floating around in various places but perhaps it will help to have it compacted into a single thread.

"Why did Subaru put a 5-speed gearbox into the WRX that is not strong enough?" The answer is that it is strong enough for what they considered to be normal useage of the car. At standard power & torque output, with standard clutch, and grip levels provided by the standard tires the WRX 5MT is more than sufficient to hold up to even aggressive driving styles. What Subaru can be accused of, however, is providing a gearbox with insuficient safety margin to handle the degree of modifications a significant portion of the owners are making to their cars.

Why is the WRX 5MT gearbox weak? This origin of this gearbox design dates back more than 20 years. The original engineering criteria were probably a lot different than they would have been now or even 5 or 10 years ago. The biggest problem is the gear centerline spacing. The 5MT uses a 75mm gear centerline spacing. This is the primary limitation for producing gears with teeth large enough to handle big torque levels yet quiet enough for the general public and not wear out within the normal life expectancy of the car. Combine this with splash lubrication, a longitudinally split case, inexpensive material choices, etc. and it is clear that it is not going to support power levels not typically seen even on many so-called exotic sports cars.

What is the exact cause of failure in these gearboxes? We've done some testing in this area. While many people have promoted case distortion as the cause of the failure (we were probably guilty of promoting this theory very early on)....strain guage testing of the case showed only a small level of distortion even at very high torque levels. While even a small amount of distortion is bad because it misaligns bearings and gears increasing friction and heat generation, it is not the main cause of failure. The main cause of failure is excessive gear tooth separation due to shaft flex. We found that at only 275lb-ft of input torque the gear tooth separation exceeded the recommended engineering limit for a helical gear profile. This separation occurrs because the mechanical leverage provided by the gears also creates a reaction force that wants to push the gears away from each other. The more torque you apply to the gears the more force there is trying to separate them. The input shaft is particularly thin so it can flex quite a bit. When the teeth separate the bending loads on the root of the tooth go up quite a bit. Ultimately the weakest tooth fails and creates a domino effect by wedging between the next set of teeth tearing them free and so on. It is very common to see every tooth on the gear torn clean off.

What can be done to strengthen the gears? The best option is to increase the root width of the teeth on the highly stressed lower gears. This is done by changing the gear tooth pitch. Tooth pitch is basically the number of teeth per inch of gear circumference. A comparison of RA and standard WRX gears will show a large difference on 1st and 2nd gears in this regard and a lesser increase on 3rd gear. Also, early WRX gearboxes (mid-2003 and earlier) had narrow (face to face width) 1st, 2nd, and 3rd gears. After mid-2003 the width was increased by 1mm on these gears and therefore matches the width of the RA and STi gears we sell. There are space limitations preventing further widening of the gears. The other thing is to alter material specification and treatments to improve either the ultimate strength or the fatigue life.

What about dog gears, aren't they a lot stronger? Most dog gears use a straight cut (spur) gear design instead of the helical gear profile used by Subaru. Spur gears are actually weaker than helical gears for the same width and tooth pitch. This is because there are fewer teeth meshed at the same time and the curve of the helical profile creates more surface area. However, replacing the synchronizers with dogs allows the manufacturer to significantly widen the gear widths so this loss in strength is reclaimed and then some. We've seen that some of the cheap aftermarket gearsets (dog or synchro) can be actually more prone to failure than an RA set. This is usually a result of insuficient heat treatment or excessively hardening the gears (creating a brittle gear not capable to absorbing high shockloads).

What is close ratio gearing? Gear ratios represent a ratio of the number of teeth on the driven gear to the number of teeth on the drive gear. A short gear (higher numeric ratio) creates a higher torque multiplication but at the same time requires the drive gear (driven by the engine) to spin very fast. So therefore a short gear ratio will provide a higher torque to drive the wheels and accelerate the car quickly but will require the engine to spin very fast (and it will reach redline very quickly). When you shift up the next gear is taller (lower numerically) so the torque multiplication is less but the engine speed decreases compared to road speed. A close ratio set means the numeric difference between the ratios (1st to 2nd, 2nd to 3rd, etc.) is reduced. Therefore when you shift the engine speed does not drop as much. The effect is to allow the driver to keep the engine operating in the optimum potion of the power band between peak torque rpm and peak hp rpm. Ideally for maximum acceleration you want to be able to shift a few hundred rpm after peak power and have the revs drop right to peak torque. However, if the engine has a wide torque band (as may be the case with a 2.5L w/ fast spooling turbo vs. a 2.0L with a big laggy turbo) then the need for a close ratio is not as important.

Why is the choice of gear oils so difficult for the 5MT?Two reasons. Number one is the fact that the front final drive gear is packaged inside the gearbox and shares the gear oil with the main portion of the box. The final drive gear is a hypoid gear design (tapered helical profile) and requires special additives to keep wear to a minimum (there are high loads combined with heavy sliding friction). On the other hand the synchronizers work best with a minimal level of these additives. Thus there are conflicting requirements. Number two is that when this gearbox is highly loaded it generates a lot of heat which is difficult to dissipate with just splash lubrication alone. This heat generation lowers the viscosity of the oil which also creates problems with the synchros (because to achieve a fast shift speed the synchro relies on oil drag between baulk ring and synchro cone to help match gear speed).

What gear oil should be used? Some people are recommending the use of GL-4 gear oil. GL-4 has a low level of additives mentioned above and therefore improves synchro performance. However, this is going to be at the expense of final drive gear wear. GL-5 has more EP additives which is better for the final drive but worse for synchro performance. Synthetic GL-5 oils tpically make this issue worse because of the properties of the base stock. Any synthetic GL-5 designed for "use with limited slip diffs" is going to be the absolute worst. To combat the heat issue, anyone running extended periods at the track or just putting long duration severe loads on the gearbox might consider running a 75W140 gear oil instead of 75W90. The gear oil should also be changed more frequently because the heat breaks down the oil and changes the properties.

What about GM Synchromesh gear oil?In theory this sounds like it should work very well because it was engineered for just this type of situation (synchros w/ hypoid gear). However, it was engineered specifically for GM synchro components which are not exactly the same as Subaru. Worth experimenting with though if you are having difficulty finding an oil that works well for you. It is important to note that GM synchromesh has a base stock similar to engine oil. It probably will thin out too much with high heat levels so for track day or competitive use its not ideal.

Do synchros wear out and are there upgrades available?Yes, the baulk ring portion (sometimes called synchro ring) of the synchronizer assembly is the wear item. Only a small amount of wear is enough to create issues. Forcing a very quick shift or forcing a shift when there is a large speed difference between road speed and shaft speed (like downshifting into the wrong gear) will accelerate the wear very quickly. In competition use it should be expected to replace baulk rings once or twice a season. If you drive your car every day like its a race car then expect to be replacing synchro parts. Unfortunately there are no upgraded synchros available for the 5MT that I am aware of. For the 6-speed STi gearbox a carbon-reinforced baulk ring is now being used on 4th, 5th, and 6th gears (which we can offer to '04-'05 STi owners who are having problems with synchros). It would be nice if a carbon-reinforced baulk rings were available to 5MT owners....perhaps there is a company out there that can produce a low volume run of them.

What about shot peening, cryo treating, microfinishing of gears?Shot peening helps with the fatigue strength by creating a compressive layer on the surface. Gear teeth are subjected to continuous load variations as the teeth engage and disengage from each other. Shot peening can improve life of the gears if the load levels are high but obviously will do nothing if you are exceeding the tensile strenght limit. Unfortunately, shot peening can distort the component in the process so it should only be done just prior to finish machining of the component. If done to a finished part then it may affect the fitment.

Cryo treating, although a controversial subject, has been shown by reliable sources to provide benefits to components such as gears. One effect of the cryo treatment is stress relieving. The gears are heat treated and then stress relieved by the factory but very often the level of stress relief is incomplete if only allowed to cool to room temperature. By taking the part down to cryogenic temperatures a more complete stress relief is achieved. Also, cryogenic treatment can compact and tighten the crystalline structure of the metal which improves the toughness and wear resistance of the metal. Cryo treating can also help extend the life of the gears when heavily loaded similar to shot peening by through a different approach.

Microfinishing is essentially a polishing of the gear surfaces. The process we subject our RA and STi gearsets to is a chemical-mechanical process that leaves the gear surfaces nearly mirror smooth without no measurable removal of material. This eliminates the need to break in the gears. It also reduces frictional losses between the gears and reduces the heat generated by the transmission. It can also improve fatigue life by eliminating micro-welding and pitting of the gear surfaces which can be a starting point for surface cracks.

--Dave
Rallispec, Ltd.
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Last edited by RalliSpec; 04-19-2006 at 12:57 PM.
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Old 04-19-2006, 12:53 PM   #2
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^^

Very nice write up.

So, are you saying the tooth pitch from RA's to WRX gears are different?
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Old 04-19-2006, 01:04 PM   #3
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Yes, noticeably different although not necessarily to the point that some of the aftermarket sets take things.

Both RA & STi gearsets have larger teeth than the standard WRX gears. That may have changed for '06. I have not had a chance to check. But since the WRX is now coming with the 2.5L turbo its probably got the same gears as the Legacy 2.5GT model which are similar to the STi gearset.

The tooth pitch unfortunately is limited by the diameter of the gears and the ratio. If you look at a 6MT you will find they have increased the gear shaft centerline distance from 75mm to 85mm. This allowed them to increase the gear diameters and run a stronger tooth pitch.

--Dave
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Old 04-19-2006, 01:12 PM   #4
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One last technical point:

Clutch choice has a huge influence on the reliability of the gears. This is due to the shockloading that occurrs when the clutch is engaged suddenly. A normal organic disc with sprung center hub and marcel springs will engage smoothly and gradually and the sprung hub will also help absorb some of the shock that would otherwise be transmitted directly to the gears. When you get into racing clutch designs most have been designed for fast and instantaneous lockup for performance and to keep heat generation to a minimum. This increases the shockloads transmitted to the gearbox pretty dramatically.

On the other end of things....tire grip also plays a part because tires that can slip and spin have the same effect in reducing the shock levels as a less grippy clutch would. With AWD systems you have essentially twice as much grip to begin with....when you add sticky tires and a harsh clutch you are just asking for trouble.

--Dave
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Old 04-19-2006, 02:22 PM   #5
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You rock Dave! Tons upon tons of good info there man!

I know that was alot of work to do this and I can say I appreciate it.

So, the RA gears are tougher than the 04'+ gears for more than the peening or cryo treating. Nice to know for sure

I bought my RA gearset from you almost three years ago and I thank you for all the questions you answered. They have treated me great for this whole time and looked still new the last time I had the trans open about 7K ago to install new synchos and front LSD.

I ran them with Neo RHD for the first 27K or so which I think may have sped up the synchro wear, but preserved the gears perfectly. I am now running Redline lightweight.

Thanks again Dave,
Kevin
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Old 04-19-2006, 03:31 PM   #6
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Yes thanks for taking the time to put that together Dave - a very informative read

Can you offer any words of wisdom on the various final drive ratios that Subaru has used, and how they might enter into the (failure) equation?
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Old 04-20-2006, 04:02 AM   #7
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Dave,

Where does the Forester XT 5 speed fall in the WRX, STI, RA, Legacy spectrum for tooth size and gear strength? Is the low 4.44 gearing in the Forester XT really helpful for reliability?

Thanks,
Justin Wade
2004 FXTi
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Old 04-20-2006, 04:20 PM   #8
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The Forester XT has been using the same gears and main shaft as the later WRX. So small tooth size but the wider face to face dimensions on the lower gears.

I don't think the lower final drive gearing really has any effect on the reliability of the individual speed gears themselves. If the 4.44 was used in conjunction with taller gears then it would be a different story. That brings up another good point...a taller gear is going to be less stressed than a shorter gear. Therefore the RA gear will be less stressed in 1st gear than a WRX.

As for final drive ratios from Subaru.... only 3.90, 4.11, and 4.44 have been used in the current Subaru models with 5-speed manual transmission. In the 6-speed only 3.90 and 4.44 have been used. However, the rear differentials are Nissan units so its very likely that alternative rear ratios exist....its just that there probably is not a corresponding front ratio for the gearbox. I do not think, however, that is possible to produce a reliable final drive gear shorter than 4.44 for the transmission.

--Dave
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Old 04-22-2006, 03:57 AM   #9
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Gears

Last edited by PacmanGSX; 10-20-2006 at 09:20 AM. Reason: Edit
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Old 04-24-2006, 01:21 PM   #10
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Quote:
Originally Posted by RalliSpec
However, the rear differentials are Nissan units so its very likely that alternative rear ratios exist....
--Dave
Typo... I think you mean Hitachi.
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Old 04-24-2006, 02:53 PM   #11
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No typo....the R160 and R180 are originally Nissan units. Sure they may be manufactured by Hitachi but that is not the key point. The fact is that Hitachi is not going to manufacture a specific ratio unless its in mass quantities. So by crossreferencing the models that the diff. was used on in a Nissan application it may be possible to find a ratio that is not currently being utilized by Subaru.

If you buy a rear ring & pinion from Subaru you will find that quite often the "Subaru" markings and labels are simply printed over top the Nissan ones. Peel back the labels and find the original Nissan part number!

--Dave
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Old 04-24-2006, 03:14 PM   #12
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Well, Hitachi makes the units that Nissan uses too... so I think it's a chicken and the egg problem. I am sure it is much easier to get replacement ring and pinion sets in various ratios from Nissan as you said. Pragmatism for the win.
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Old 04-24-2006, 03:31 PM   #13
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[quote=PacmanGSX]So if you see a pile of dog poop and you spray it with air freshner does that make it any better? No, it's still dog poop. RA gears suck, even if you spray them, polish them and freeze them guess what, they're still poop. PPG Rocks! QUOTE]

Thanks for your truly enlightening comment!

Obviously your small brain is not capable of comprehending even the most basic points being made. For one, I am not here comparing RA gears with PPG gears. The references to the RA gears are intended for comparison to the stock WRX gears. The majority of the points made are to show why there are gear failures with the WRX in the first place.

With regards to the cryo, microfinishing, and shot peening processes....these things are relevant to any gearset. Its basic metallurgy. These processes are being used and evaluated by many gear manufacturers and many top level race teams from Nascar to Formula 1.

PPG makes a good product and we are actually evaluating the possibility of carrying their products. However, there is a legitimate place for the RA gearset. If you had a bad experience with the RA gearset (assuming for a moment you are even speaking from any real world experience) its probably because of one of the following reasons:

1) You are expecting to "spin wheels" at every stop light to show off your "driving skills"

2) You are using an inappropriate clutch for the gears.

3) You are at least doubling the stock torque or power output of the engine and expecting the gears to do some amazing tricks to hold themselves together.

The bottom line is that if you have $4000+ to spend on a gearset and need it to support a lot of power then by all means buy the PPG gearset. If you want to spend just a fraction of that amount to get something considerably better than stock then the RA gearset is a legitimate option. The fact that we strive to improve it by putting it through additional treatments might make the decision not to spend $4000+ a little easier for some people. Regardless of what gearset you buy....you should know the limitiations of what you are buying. And that is the bottom line as far as what information I am trying to put out there. I could be like most other dealers out there and say "Buy my insert gearset name here because its bullet proof." but not give any supporting data or even an explanation of why its bullet proof (when there is no such thing).

But if you prefer that I keep my posts to myself, believe me I am more than happy to do so.

--Dave
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Old 04-24-2006, 03:37 PM   #14
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Quote:
Originally Posted by RalliSpec


What is the exact cause of failure in these gearboxes? We've done some testing in this area. While many people have promoted case distortion as the cause of the failure (we were probably guilty of promoting this theory very early on)....strain guage testing of the case showed only a small level of distortion even at very high torque levels. While even a small amount of distortion is bad because it misaligns bearings and gears increasing friction and heat generation, it is not the main cause of failure. The main cause of failure is excessive gear tooth separation due to shaft flex. We found that at only 275lb-ft of input torque the gear tooth separation exceeded the recommended engineering limit for a helical gear profile. This separation occurrs because the mechanical leverage provided by the gears also creates a reaction force that wants to push the gears away from each other. The more torque you apply to the gears the more force there is trying to separate them. The input shaft is particularly thin so it can flex quite a bit. When the teeth separate the bending loads on the root of the tooth go up quite a bit. Ultimately the weakest tooth fails and creates a domino effect by wedging between the next set of teeth tearing them free and so on. It is very common to see every tooth on the gear torn clean off.

...

What about dog gears, aren't they a lot stronger? Most dog gears use a straight cut (spur) gear design instead of the helical gear profile used by Subaru. Spur gears are actually weaker than helical gears for the same width and tooth pitch. This is because there are fewer teeth meshed at the same time and the curve of the helical profile creates more surface area. However, replacing the synchronizers with dogs allows the manufacturer to significantly widen the gear widths so this loss in strength is reclaimed and then some. We've seen that some of the cheap aftermarket gearsets (dog or synchro) can be actually more prone to failure than an RA set. This is usually a result of insuficient heat treatment or excessively hardening the gears (creating a brittle gear not capable to absorbing high shockloads).

--Dave
Rallispec, Ltd.
Quick question regarding these two paragraphs.

I broke an aftermarket gearset. Since it's no longer manufactured, I had to have a custom replacement made. The original was helical, but I asked for a straight cut second on the new shaft. I was under the impression that spur gears are stronger, ince they don't produce the axial loads that a helical gear design does. Is a spur gear design less likely to produce shaft flex?

If the spur design is not going to be as strong...I may have to call and change my order.
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Old 04-24-2006, 03:38 PM   #15
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Thanks Dave! Didn't want to go off on that dude and ruin your thread, but we all can't afford PPG gears. Plus, back when I did mine, they didn't even exist? At least not in the US market and I never heard of them. Either way I couldn't have afforded to do them.

Back when I installed your RA gears a 6MT was going for $6-7K used!
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Old 04-26-2006, 12:56 PM   #16
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Quote:
Originally Posted by PacmanGSX
So if you see a pile of dog poop and you spray it with air freshner does that make it any better? No, it's still dog poop. RA gears suck, even if you spray them, polish them and freeze them guess what, they're still poop. PPG Rocks!
Who said anything about PPG's? Rallispec is doing everybody a favor by explaining the differences between WRX and JDM 5mt STI gears, since the width is the same. This information has been MUCH needed for a while.
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Old 04-26-2006, 01:01 PM   #17
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Quote:
Originally Posted by fliz
Quick question regarding these two paragraphs.

I broke an aftermarket gearset. Since it's no longer manufactured, I had to have a custom replacement made. The original was helical, but I asked for a straight cut second on the new shaft. I was under the impression that spur gears are stronger, ince they don't produce the axial loads that a helical gear design does. Is a spur gear design less likely to produce shaft flex?

If the spur design is not going to be as strong...I may have to call and change my order.
Yes, a spur gear produces much less axial load than a helical gear. The axial thrust loads must be absorbed by the bearings. The high thrust loads create increased frictional losses which is why most racing gear sets opt for a spur gear (more power delivered to the wheels basically). But in terms of the actual strength of the gear....a helical gear is stronger than a spur gear all other things being equal.

A spur gear is actually going to create more shaft flex because nearly all the reaction forces are going to be in the direction that would bend the shaft. A helical gear, as mentioned, has a portion of those forces going along the length of the shaft in the axial direction. However, I believe that a spur gear can operate with less critical gear mesh so the gears need to separate more for it to become a real issue.

If you have this gear made I would suggest consulting an engineer to make sure that the gear being produced is as strong or stronger than the gear it is replacing.

--Dave
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Old 04-26-2006, 11:43 PM   #18
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Thanks Dave. I really appreciate the time and effort you took to explain the differences between gear sets and their impact.

Hopefully I'll never need a set, but if I do I'm definetly coming back to you.
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Old 05-02-2006, 12:51 AM   #19
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Thanks for the info Dave, it is very helpful. I wonder if my 3-puck cerametallic clutch is the "wrong" clutch for the RA box (that you rebuilt for me).

I am curious as to what you recommend for people who want the 5-speed (for me, it's the weight savings) with additional strength over what the RA's can handle. When are you expecting to evaluate the PPG set? I am interested in hearing your opinion.
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Old 05-15-2006, 03:50 AM   #20
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tell me dave is it possible to find a 5mt that will fit a rear diff. from an automatic transmission. thanks for advice.
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Old 05-16-2006, 01:24 AM   #21
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does anyone know how much, and what is required for a my02 jdm sti 6speed swap?
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Old 06-08-2006, 02:52 PM   #22
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Dave

Glad you got the "case flex" myth sorted out once and for all. Regarding spur gears vs. helical gears: There is some misunderstanding about overlap ratio and how significant this really is to overall gear strength. Not to knock you, because I also used to believe that the higher overlap ratio of a helical gear (2.0+) meant that it was stronger than a spur gear (1.35+), however other more important factors are involved.

If you look at how the helical gear engages, you will see that it starts engagement from one side, and ends on the other side of the individual gear tooth. Even in the most ideal conditions (totally fixed gears), one can visualize the load exerted on one side of the individual tooth, progressing to the other side as gear mesh continues to the next pair of teeth. This motion is how an individual tooth is torn off at the root, despite the fact that two gear pairs (2.0+ overlap ratio) are theoretically engaged. Add to this equation the fact that the idler gear needs to freewheel on a needle bearing whenever another gear is selected, and the fact that required needle bearing clearance = slop = deviation from ideal gear engagement, and you see how the problem is increased.

Now visualize the individual gear tooth in a spur gear pair. As engagement occurs, load is distributed across the entire width of the gear tooth, rather than load starting at one side of the tooth and working toward the other side of the tooth. If you can visualize this, you now see why "beam strength" of the spur gear tooth is greater than that of the helical gear. Beam strength becomes a much more important factor in overall gear strength than overlap ratio in most race applications.

Now consider one more issue: Shaft flex (which you have correctly pinpointed as being FAR more of a factor in Subie trans weakness than case flex). When the mainshaft &/or pinion shaft flex, and the meshing gear pair spreads apart, the ideal design strength of the helical gear pair is further compromised (loss of ideal mesh), while the spur gear's mesh is barely affected. Straight-cut gears that move apart from one another still remain in near-full engagement.

The times when helical gears are "better" than spur gears are:
1) Street applications in which strength is not as much an issue as quiet-running.
2) Road race applications in which gear strength is not borderline (not the case with the puny Subie tranny).
3) Dragrace starts in which some of the shockloading is absorbed by end thrust with a helical 1st gear, vs. the spur gear (in which minimal end thrust is available to help deflect these damaging forces). The question "which is better -- helical or spur 1st gear in drag race applications" can only be answered by practical testing, not theory. And it seems that with the Subie transmission, high quality spur gears do just fine in 1st gear position. So that really leaves us with #1 above as possibly the only time that a helical gear is "better" than the spur gear.

This all pertains only to the main gear teeth themselves, since spur gears can be synchronized (although almost all spur gear manufacturers also have opted to adapt the "dog box" design, eliminating synchros alltogether), and helical gears can be "dogged".

Hope this info helps ---

Last edited by RipVanW; 06-10-2006 at 02:07 PM.
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Old 06-09-2006, 02:56 PM   #23
C4NiNjA
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"Also, early WRX gearboxes (mid-2003 and earlier) had narrow (face to face width) 1st, 2nd, and 3rd gears. After mid-2003 the width was increased by 1mm on these gears and therefore matches the width of the RA and STi gears we sell."

I have a 2003 WRX, how would I know which side I fall on?
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Old 06-12-2006, 05:22 PM   #24
D Money
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Quote:
Originally Posted by C4NiNjA
"Also, early WRX gearboxes (mid-2003 and earlier) had narrow (face to face width) 1st, 2nd, and 3rd gears. After mid-2003 the width was increased by 1mm on these gears and therefore matches the width of the RA and STi gears we sell."

I have a 2003 WRX, how would I know which side I fall on?
You would need to know if you had an early 03 or late 03. I have an 03 and broke my gears two days off the dyno, most of the reason was due to the fact that I was double the whp I was stock and I put in an act HD clutch. Whatever, it happens and I just bought a full 6MT swap since I didn't want the wine of the PPG and the uncertainty of the RA gears holding up.
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Old 06-16-2006, 02:45 PM   #25
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^ Can you tell by vin or something? I have an 03wrx and I don't know what tranny I might have. I think I bought it around september of 02'. This may sound early but usually the cars are made well before that closer to april I believe and I may have a border line MY that may have the better gears. I've been wanting to find this out for a while. I have vf30 setup that I am installing this month and would like to have a little more piece of mind. I don't drive it crazy but it's always nice to have a little insurance.
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