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Thread: Why 6-Speed?

  1. #1

    Default Why 6-Speed?

    I've been doing some thinking about my manual transmission. It came to my attention in another thread that you can swap gears and change your ratios if you want (just never really thought about it).

    Quote Originally Posted by erratic50 View Post
    i had a Gforce T5 built for my 85 project car with the Astro bearing retainer and the mid brace. I went 0.59 on the overdrive because that puts the final drive ratio of my 3.55 rear gears with a 0.59 at the same place as a 3.08 with a 0.68. 1-4 close together with lots of acceleration, 5th spaced out a long ways for cruising. I won’t actually need 5th during aggressive pulls really. Building a motor with a 6500 redline. Good to go until around 150 then 5th is required.
    Didn't want to derail that thread too much, but it got me thinking. My understanding of it is, overdrive doesn't really do anything for you during heavy acceleration, like Erratic says. So, you get to 1 to 1 and you might as well leave it there, say if you're racing. All overdrive does for you is reduces RPM for highway cruising.

    I knew cars like my Fusion's 6-speed had 2 overdrives for even better mileage advantages, but I was looking at LMR's manual tech page, and the Cobra's 6-speed is the same way. Why? What does that do for you, performance-wise? I thought for sure in the Cobra it would have given you another gear before 1 to 1, but not so, apparently. Why does having a 6-speed even give you bragging rights, if all it does is something lame like giving you better MPGs on the highway?

    And for that matter, why bother with the additional step between 1 to 1 and the final ratio? It seems pointless to me. I can see in an automatic, you're going to want additional ratios for smoothness in shifting, but in a manual, does that even matter?

    I'm not planning on swapping to a 6-speed, but since i have 4.10 gears, if I can make highway cruising more pleasant by changing my 5th gear ratio, I might just have to do that! Erratic, did changing that ratio for you make your car less smooth while accelerating? I guess that's the question.
    Brad

    '79 Mercury Zephyr ES 5.0L GT40 EFI, T-5
    '17 Ford Focus ST
    '14 Ford Fusion SE Manual

  2. #2
    FEP Senior Member cb650's Avatar
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    If you have the HP to pull it better top end. But how many places are there to go that fast unless your actually doing track time.
    Believe it or not some of the new automatics are 8 speed.
    mostly its a macho thing.

  3. #3

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    There is a ten speed auto in the new Mustang and it is really good. I ran it on a racetrack in sport mode and basically it was fine with the chosen gears almost all the time.

  4. #4
    FEP Power Member 4-barrel Mike's Avatar
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    Brad, why don't you swap your 4.10 for something more streetable like 3.55? It would make the entire driving experience much more enjoyable and be much cheaper that modding the trans.

    Mike

  5. #5

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    Quote Originally Posted by 4-barrel Mike View Post
    Brad, why don't you swap your 4.10 for something more streetable like 3.55? It would make the entire driving experience much more enjoyable and be much cheaper that modding the trans.

    Mike
    Because I like acceleration But, I suppose you have a point. On the other hand, I consider myself VERY lucky I was able to set up my gears without destroying them in the first few miles. Any messing with them I do at this point, I consider pushing my luck. I'd rather tear open the tranny than mess with those stupid shims again.

    But more gears for top end, huh? That's another thing I really don't care about for the exact reasons you say, cb. When am I ever going to be going over 80 mph? Never.
    Brad

    '79 Mercury Zephyr ES 5.0L GT40 EFI, T-5
    '17 Ford Focus ST
    '14 Ford Fusion SE Manual

  6. #6

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    My plan was always to do a 363 stroker with a "viper" spec t56. I wanted the trans for its strength, but also for the decent first gear ratio when combined with gears and the double overdrive (.5:1 on second over drive).

    The t56 is still one of the strongest transmissions out there if you want to go manual and has almost identical gear ratios up to 5th gear compared to a t-5. With some 3.55 or lower gears, it can take you from a 2500+ rpm highway speed back down to a more stock 1500rpm or so.

    I always wanted to have my car look and drive stock, including relatively quiet exhaust. Something I could take on long freeway cruises getting decent gas mileage, hit the local grocery store, but still have plenty of power on tap to have a good time.

    But, I drive my cars. My 86 tbird is my daily, ive put about 70k miles on it on the last two years. Its not uncommon for me to do over 1000k a week. Every thing from runs down to the gas station, to a two hour round trip to get the kids from school.
    2 1986 cougars (both 4 eyed and 5.0)
    1 1987 cougar

  7. #7
    Moderator wraithracing's Avatar
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    More gears means less change between gears and generally less RPM drop between gears improves acceleration. That is why so many transmissions now are 6, 8, 10 gears. Obviously most of these are automatics now, but having all those gears spaced closely together improves the acceleration and improves fuel mileage as the computer can always find the right gear for almost any occasion.

    A double overdrive in a 6 speed allows you to still accelerate in 5th gear for better top end and still gives you that HUGE RPM drop in 6th gear to lower RPM on the highway. That generally will improve fuel economy and lessen wear/tear on the engine due to lower RPMs.

    There are options in regards to gear ratios in a WC T5 that will give you benefits of a steeper overdrive gear without the added expense and headaches of a transmission swap. Not saying building up a WC T5 with aftermarket parts is cheap, but generally will be less $$ than a T56 swap. Good Luck!
    ​Trey

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  8. #8
    FEP Super Member erratic50's Avatar
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    You have to look at effective ratios.

    Think of it this way — Motor RPM in each gear vs ground speed. Ideal gearing will bring the gears closer together so you can stay closer to the power band that you WANT to use.

    When you’re racing you want it to run a bit fat on fuel and near the point of pinging and sweep from near peak torque up into where you reach peak horsepower

    when you cruise down the road you want it to run lean and around the best torque output per volume of fuel consumed.

    etc

    Youve probably read some of my other posts. Remember when I suggested an off the wall 6-speed transmission borrowed from Nissan and known for high RPM shifting and power capacity??? How many said it would never work before even considering the ratio math?

    It comes with a 4.0:1 low and 0.80 OD. And nobody thought about the rear gear or tire sizes — those combined with the trans gears determine ground speed vs RPM in each gear don’t they!?

    so think about this - 4.0 paired with a 2.73 ..... many thought I was crazy to even suggest it. what is the ground speed of the car in a 4.0:1 1st with a 2.73 vs a 2.95:1 1st with a 3.90. Calculate the drive ratios by multiplication — those two will end up quite near one another.

    Trans gears vs ground speed at a give RPM is ALWAYS relative to both tire size and rear gear. The lower the rear gear the closer together the gears become when looking at ground speed changes across the entire RPM range of the engine in a given gear.


    The reason I am making sure I’m all done raising hell by the time I need to shift into 5th is because the RPM drop from 1:1 to 0.59:1 is 41%. No engine running 6500 is going to keep pulling near the same rate when all of a sudden the engine speed drops to 3,835! Also with the T5 design the Astro and Gforce kits even are much weaker in 5th vs 1-4. They can’t really tolerate that sudden hammering from an engine spinning 6500 suddenly running 3835. The clutch helps absorb it but it’s fairly brutal still. So my answer — stop beating on it before shifting into 5th.

    There are better transmissions out there for sure. I may go T56 in my 86GT when I do it’s build

    The big advantage of a viper T56 6 speed is you can run pretty crazy overdrives in 6th and quite low rear gears and still maintain a great stair step in every gear 1-6. You only need a 0.75:1 5th to split the difference between 1:1 and 0.5:1

    the driveshaft RPM at 6000 engine RPM is 12000 ..... that’s the only thing that’s not awesome about extremes on overdrives

  9. #9

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    Well, it's interesting to think about, anyway. I probably wouldn't change anything but the 5th gear ratio, but probably won't even do that. I need to log some more highway miles with this thing and actually try and figure out what kind of mileage I'm getting too. Once I quiet down the exhaust it might actually be pleasant on the highway.

    Sent from my SM-G960U using Tapatalk
    Brad

    '79 Mercury Zephyr ES 5.0L GT40 EFI, T-5
    '17 Ford Focus ST
    '14 Ford Fusion SE Manual

  10. #10

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    The 6r80 auto in my 2014 GT has a 4.17 first gear and 3.15 rear gears and that thing is a street holeshot king, even with a converter I consider too tight. It has two overdrives and will easily pull in 5th, because the engine has a lot of power. There is actually no 1:1, 4th is closest at 1.14:1 Over all ratios being matched to the engines power curve is critically important. Ford stuck us with weak rear gears in the 80s but made up a bit for it by making torque filled V8s with good throttle response and relatively steep first gears. That's what made these cars feel fast when they weren't. The deep OD got them past fuel economy regs at the time.

  11. #11
    FEP Super Member erratic50's Avatar
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    It’s not that a foxbody with over 400HP under the hood will have any trouble cranking the speedometer well past insane in 5th gear. A stock HO with a 2.73 or 3.08 rear gear and a 0.68:1 overdrive needs little encouragement to keep right on pulling after a driveway tune up optimizing the timing and clearing out the air inlet restrictions.

    my point was with a 3.55 rear gear and a 6500 redline on the motor being built you’re rolling 140 at the top of 4th. Will it go faster, sure. No problem. But why?

    will it go 238? Probably not, definitely not with me driving it. Will it blow past the 170 mph mark on the speedometer? Guaranteed... heck my 86GT will even do that and it has WAY less horsepower than my 85 build will have.
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    Last edited by erratic50; 12-16-2018 at 02:20 PM.

  12. #12

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    Do it
    Brad

    '79 Mercury Zephyr ES 5.0L GT40 EFI, T-5
    '17 Ford Focus ST
    '14 Ford Fusion SE Manual

  13. #13
    FEP Super Member erratic50's Avatar
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    Viper T56 is awesome. 0.5:1 OD plus 4.10’s will get you to basically the same final drive ratio as a 3.08 with a 0.68:1

    3.08x0.68 = 2.0944
    4.10 x 0.5 = 2.05

    You can’t go with things like a 3.55 or a 3.27 or a 3.08 if you want to use 6th anywhere other than the salt flats

    road manners wise .... my experience in two 86GT cars both running the same VM1 ECU and turned up to 13.5 degrees of base timing...... anything final gear ratios beyond a T5Z 0.63:1 with a 2.73 is not useful with anything except a stock HO cam or other low end torque oriented grinds. The engine has to make fairly big torque right off idle to deal with it. Stuff like a lumpy cam or huge intake or other type mods and you can’t pull it until 1800-2000 rpm which is 90-100 mph or beyond

    my typhoon EFI and SD friendly cam do not like overdrive these days on a windy day until 85. Also If you’re headed up into the mountains you WILL have to downshift.

    stock intake with the same cam .... the 0.68 and 2.73 .... I never had to downshift once on I80 between San Francisco and Omaha.

    Stock cam stock intake, etc, it’s fine at 65 and beyond and will go up any hill in front of it in OD

    I’ve been in a car with stock cam and E7 and later AFR heads with these gears. It didn’t live the OD. Heads that swirl more give a better low end torque result


    heres how those ratios play out

    2.73 x 0.63 = 1.7199 (too much unless high swirl heads with stock low end torque imo)
    2.73 x 0.68 = 1.8564 (works just fine even loaded down with stock low end — even in mountains)

    With a T56 and 0.50
    3.73 x 0.5 = 1.865

    You can easily justify more gear — a 3.90 rear gear or a 4.10 based upon 1st gear ratio results. Or if you like that kick in the butt a 4.30 or a 4.56.

    Your shaft RPM gets up there but it will go faster than any sane person would ever drive it with any of those combos

  14. #14

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    I'll report back here when I get the engine dynoed and you can tell me what to do.
    Brad

    '79 Mercury Zephyr ES 5.0L GT40 EFI, T-5
    '17 Ford Focus ST
    '14 Ford Fusion SE Manual

  15. #15
    FEP Super Member erratic50's Avatar
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    Quote Originally Posted by ZephyrEFI View Post
    I'll report back here when I get the engine dynoed and you can tell me what to do.
    Lol

    Add more gears.

  16. #16

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    I would... if "6-Speed" didn't carry about as much weight as "Cobra" on the used parts market.
    Brad

    '79 Mercury Zephyr ES 5.0L GT40 EFI, T-5
    '17 Ford Focus ST
    '14 Ford Fusion SE Manual

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