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Thread: GEARS: SYNCHRO vs DOG/ Straight cut vs helical cut

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    2.5T kgj996's Avatar
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    Default GEARS: SYNCHRO vs DOG/ Straight cut vs helical cut

    So your looking for a gear set for your ride and can't decide what gears best fit your application. Between dog boxes and synchro gears, to straight cut and helical cut gears, there are lots of choices and lots of differences. Here is a quick down and dirty:

    Synchro: These type of transmissions are found in 99.9% of manufactured cars on the road today. They use "synchros" or synchromesh rings to speed up or slow down the gear that you are shifting to, synchronizing the speeds so you can select the next gear smoothly. You can get gear sets in either straight cut or heical cut gears or a combination of the two.

    Some of the pros to using synchro gears:

    • They are very drivable and easy to live with in a daily driver. With smooth quiet shifts, they make the best choice for a daily driver especially for a city commute.


    • They are less expensive because of the availability of parts. They have been making synchro transmissions for the past 50+ years so lots of companies make parts for these.
    • They are relitively maintenance free, besides the occational gear lube change and a magnetic brain plug is always a good idea.

    Cons of a synchro gear set:

    • They are weaker than dog engagement gears. Synchros take up more room than dog rings, so there is less room in the box for thicker gears. Thicker gears mean more torque handling and less breaking.
    • Synchros often are made of bronze alloys and they wear out after time and can corrode if the wrong type of gear lube is used. On late model Subarus like 02-05 WRXs, 1st gear syncro will wear out and not allow you to shift into first unless you are near a complete stop. Also 4th gear syncro is prone to premature wear.
    • Requires the clutch to be engaged to shift gears.


    Here is a pic of a helical gear synchro box:


    Dog Gears: Dog boxes were frist developed for the race track and used to only be found in competion race cars. Like a lot of other race developed technologies, the dog box eventually found it's way to the street and the prices became withing reach of the weekend racer and hobbiest. Because of there extreme power handling, dog boxes have become more and more popular with high power street applications, regardless of there poor everyday drivability.

    Dog gears or a "dog box" use dog rings to select gears instead of synchros. These are very stout rings made of steel alloys similar to the gears, that directly interlock with the gear when the selector fork moves it. These dogs instantly select the gear. There is no extra time to wait like with synchros, because the next gear doesn't have to speed up or slow down to mach the speed to be selected like in a synchro box. This greatly cuts down on time between shifts and can make a big difference in competition like dragracing of auto-x. Dog rings are also a great deal thinner than synchros, making room for thicker gears and allowing the box to handle more power. Because of the sudden gear engagement, dog boxes can be tricky to drive and get used to. They require fast and positive shifts to work properly. If you shift them slow and timidly, you can damage or wear out the dog rings. Kind of counterintuitive, but they need to be slammed into gear to work properly.

    Because of the dog ring engagement, the clutch doesn't need to be engaged to shift, on or off power! This can not be done with a synchro box unless you rev match and are not under power. Even if you do the above, you still are wearing out the synchros.

    Pros of a "dog box:

    • Massive power handling! With the room to fit wider gears, a well built dog box will handle 1000whp+!
    • Can handle extreme shock load from drag launches and hard full power shifts.
    • Less parts in the transmission to wear out if driven properly.
    • Fast shifts
    • Clutchless shifts

    Cons:

    • Expensive. They can cost up to $10K for a 6 speed and 7K for a 5 speed.


    • They are hard to drive. If you have a daily driver, these are probably not for you.
    • Loud! the shifts are very clunky and very noisy.
    • If not driven properly, they can need to have the dog rings replaced.


    Here are a pic of a straight cut 5 speed PPG dog Box.


    After you have figured out what type of box you want, now you need to choose the cut of gears you want. There are two cuts, straight and helical.

    Helical cut gears are the most common found in most late model cars on the road today. They are relatively quiet due to there curved profile, with only part of the gear tooth contacting the other at any given time. Due to the limited tooth contact area, they can be more seceptable to breaking due to shock load. With only part of the tooth contacting the other, only a fraction of the tooth is taking all of the stress from driving, of launching, or racing.

    Straight cut gears are as they sound, straight cut. The teeth contact straight on to each other and the whole tooth width is used during contact. These gears are considerably louder due to the increase of contact surface, and make a loud whine, like a super charger of a gear driven timing set.This whine is quite loud and can get old on a daily driver, but with the increase in tooth contact area, the gears can handle shock a lot better. A lot of people who do drive there cars daily compromise and will get straight cut gears 1st and 2nd, were the most shock load occurs, and get helical cut gears for 3rd through 5th. Either way you go, consider your application first and decide what best fits your ride. If you are planning on running a 16g with a conservative tune, a dog box might not be what you need. But if your goal is to put down 600whp and race every weekend then a dog box might be right for you.



    If you own any of the above, chime in and post your set up and experience with it. I'm no transmission expert, so correct me if I have anything wrong.
    Last edited by kgj996; 03-04-2009 at 08:10 PM.

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    $25 Donation wagonmafia's Avatar
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    very nice write up man.

    so why nothing on the ever so cool "fagmatic boxes"

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    2.5T kgj996's Avatar
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    I was saving that thread for the expert (yourself). Plus my pain pills just kicked in from my wisdom tooth extraction.

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    Nice thread! must read again

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    $25 Donation wagonmafia's Avatar
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    Quote Originally Posted by kgj996 View Post
    I was saving that thread for the expert (yourself). Plus my pain pills just kicked in from my wisdom tooth extraction.

    wow that sucks man, hopefully they gave you the goodstuff.

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    3.0L morbidgorilla's Avatar
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    So its possable to get staight cut syncro gears?
    The summation of BCT was eloquently inscribed on a port-a-potty wall "I stood in line, I did something, I stood in line again"

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    2.5T kgj996's Avatar
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    Yes, or a combination of straight and helical.

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    Gaddis, The Seraphinwolf's Avatar
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    Nice writeup Kevin. For now I'm getting a V6 RA DCCD for my GF but will likely in time go to Dog Box. You know when it's less DD and I can get used ot a Dog engagement. Gotta get that power to the floor!
    -Gaddis
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    The Busballah
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    I still believe the case flex theory out there. These gears are just a super expensive fix.

    Some rally guys out there a while back made a case reinforcement where they thought it flexed and the transmissions started lasting 10 rallys instead of the 2 rallys without the case brace.
    rubber baby buggy bumpers!

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    SCS Original Kevin's Avatar
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    I loved the ratio's of teds 6-speed dogbox.

    thats what I would get if it didn't cost so much lol

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    Post Whore Robk0000's Avatar
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    how hard is it to install a gear set into a 5speed?

    Fromt he pics of the tranny it doesnt look to bad, maybe time consuming but not difficult?
    09 Wrx Hatch Stg II - Electrical guru

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    The Busballah
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    The process is supposed to be very complicated and you follow the service procedures which include lots of measuring but as I have seen many technicians do transmission work they don't ever take the time to measure.

    So really what I am saying here is do the best you can to follow procedure and just go for it. I want to rebuild a transmission one day.
    rubber baby buggy bumpers!

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    2.5T kgj996's Avatar
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    Quote Originally Posted by Nate8409 View Post
    I still believe the case flex theory out there. These gears are just a super expensive fix.

    Some rally guys out there a while back made a case reinforcement where they thought it flexed and the transmissions started lasting 10 rallys instead of the 2 rallys without the case brace.
    There is a guy on Nasioc who has developed a bracket to install on the 5 speed case and it has shown to cut case flex in half. And it is very reasonable at $199 for the whole kit! here is a link.

  14. #14
    2.5T kgj996's Avatar
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    I know some one here must have some experience with aftermarket gears. Any first hand experience with any of the above?

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    Founding Father/SCS Original
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    Great write up!

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