$1,000,000 Question: Rear End Gear Ratio (specifically for a 2.1L KB blown 2V)

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  1. #1

    Icon5 $1,000,000 Question: Rear End Gear Ratio (specifically for a 2.1L KB blown 2V)

    Hey guys, new to this site; been out of the Modular Mustang scene for a LONG time (military deployments and all). Getting back into the grind of things I recently decided to do a complete rear-end rebuild (after discovering fractured spider gears in the stock differential during its last fluid swap); its time to do this the way Ford should have if cost wasn't a factor in 1999. We are welding the axle tubes to the housing, wedling on end-flanges, welding a dual-plane cross brace kit from Wild Rides, Energy Suspension's upper housing bushings, dropping in a True-trac & Moser 35 spline axles, and now I just need to set the final drive gear ratio. I'm not a novice when it comes to how all this works (its just now its my time to undergo this actual upgrade), so am looking for some real bona-fide input from the experienced guys on this forum. I'm personally swaying on the Motive Performance 3:31 (so I can entertain doing a 1/2 mile airstrip attack some day too... yes I realize 1/4 mile & 1/2 mile are quite different). I want to maximize my time in the power band while in working gears (1st-4th), and as it is/was before my diff broke apart, 3:73 was just too steep. So here are some prime critical questions (SN-95 specific PLEASE):

    * What final drive gear ratio would you recommend for a 2.1L KB blown setup?
    * What are your experiences when it comes to gear ratios with Positive Displacement (PD) blower setups?
    * What tire sizes are you running with your PD setups?
    * What experiences do you have with Motive vs Motive Performance ring & pinions?

    Factors:
    * This build is NOT meant to be a daily driver; 100% street/strip BEAST. Comfort & MPG is NOT a factor.
    * 473 rwhp / 447 rwtq (w/ current 3:73 gears); plans to build the bottom end and do the pulley swap are coming.
    * Stock 5-speed (until this one explodes).
    * FRPP Aluminum Driveshaft
    * BBK Solid LCA & UCA
    * Stock/Gabriel Quadri-shocks (yes I am still running them)
    * QA1 12-Point Adjustable Shocks
    * Eiback Sportline Springs
    * Wild Rides Upper & Lower Battle Box Upgrades (installing this week)
    * Nitto 555DRs our back (315/35-17); honestly thinking of switching to the M&H 325/35-17s (28" tire)...

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  3. #2

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    Quote Originally Posted by AZ Cherry Bomb View Post
    ... time to do this the way Ford should have if cost wasn't a factor in 1999. We are welding the axle tubes to the housing, wedling on end-flanges, welding a dual-plane cross brace kit from Wild Rides, Energy Suspension's upper housing bushings, dropping in a True-trac & Moser 35 spline axles, and now I just need to set the final drive gear ratio.
    OK.

    TruTrac is nice but is relatively weak when it comes to torque biasing differentials. You are operating with only six pinion gears inside a TruTrac. Here is an exploded pic of the unit;

    Name:  Eaton TruTrac.jpg
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    Those little pinions are important because they transmit the entire torque load to the driving wheels, that the engine is capable of producing. As a daily driver six is not a bad compromise between cost of goods and product life. In a race application, a six pinion configuration is not your friend. Eaton helps you understand that issue and just how much it is not your friend by not warrantying the TruTrac if you subject it to race track operation.

    The preferred alternative to the Eaton TruTrac differential is the Wavetrac. The Wavetrac is made entirely out of heat treated billet and forged components using very high strength and shock load resistant steels in all stressed components — like pinions, side gears etc. Additionally Wavetrac is a 12 pinion design rather than a six pinion design. Here is a disassembled view of the unit supported in Lexan for viewing relative component positioning. See the pic below,

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    The twelve 9310 nickel steel pinions and corresponding side gears along with their patented central pressure application mechanism, that proportionally applies torque biasing force to the pinions when one wheel experiences a reduced tractive force, are easily visible. The physical appearance of the Wavetrac gives immediate visual clues to the strength of the unit. The clincher is the warranty. Wavetrac does not care where or how you use the differential! They provide a lifetime warranty to the original purchaser, irrespective of whether you use the unit on the street or on the race track — the warranty difference alone speaks about the profound difference in strength between between the Eaton product and the Wavetrac product.

    If you have Adobe Flash enabled on your computer here is a link to an animation of the Wavetrac differential operation, click here => Wavetrac Animation


    Quote Originally Posted by AZ Cherry Bomb View Post
    ... I'm personally swaying on the Motive Performance 3:31 (so I can entertain doing a 1/2 mile airstrip attack some day too... yes I realize 1/4 mile & 1/2 mile are quite different). I want to maximize my time in the power band while in working gears (1st-4th), and as it is/was before my diff broke apart, 3:73 was just too steep.
    Why 3.31 instead of 3.55 or 3.27 or 3.08 or ?? ...


    Quote Originally Posted by AZ Cherry Bomb View Post
    So here are some prime critical questions (SN-95 specific PLEASE):

    * What final drive gear ratio would you recommend for a 2.1L KB blown setup?
    What is the car's target top speed, preferred tire choice and engine power curve (dyno test)?


    Quote Originally Posted by AZ Cherry Bomb View Post
    What are your experiences when it comes to gear ratios with Positive Displacement (PD) blower setups?
    Lower gear ratios (numerically higher number) cause the engine to operate at higher engine speeds. Higher (sometimes called taller) gear ratios (numerically lower number) cause the engine to operate at lower engine speeds. The engine induction model whether it be n/a, s/c. or centrifugal has no impact on the rpm the engine sees. The only variables that affect the engine speed are vehicle speed, tire size and gear ratio.


    Quote Originally Posted by AZ Cherry Bomb View Post
    What tire sizes are you running with your PD setups?
    That depends on engine torque, vehicle weight, weight bias from front to back and intended use, street or drag race or top speed event like the Texas Mile.


    Quote Originally Posted by AZ Cherry Bomb View Post
    What experiences do you have with Motive vs Motive Performance ring & pinions?
    By far the best ring and pinion offerings are from Ford through FRPP. They typically have the least run out, the most consistent backlash through a full 360˚of ring gear rotation and the least predisposition to whine when installed at the proper Master Housing Depth dimension for the ring and pinion being used.


    Quote Originally Posted by AZ Cherry Bomb View Post
    ...
    * This build is NOT meant to be a daily driver; 100% street/strip BEAST. Comfort & MPG is NOT a factor.
    OK


    Quote Originally Posted by AZ Cherry Bomb View Post
    * 473 rwhp / 447 rwtq (w/ current 3:73 gears); plans to build the bottom end and do the pulley swap are coming.
    Not a relevant factor in the selection proces


    Quote Originally Posted by AZ Cherry Bomb View Post
    * Stock 5-speed (until this one explodes).
    Not a relevant factor in the selection process.


    Quote Originally Posted by AZ Cherry Bomb View Post
    * FRPP Aluminum Driveshaft
    Not a relevant factor in the selection process.


    Quote Originally Posted by AZ Cherry Bomb View Post
    * BBK Solid LCA & UCA
    Not a relevant factor in the selection process.


    Quote Originally Posted by AZ Cherry Bomb View Post
    * Stock/Gabriel Quadri-shocks (yes I am still running them)
    Not a relevant factor in the selection process.


    Quote Originally Posted by AZ Cherry Bomb View Post
    * QA1 12-Point Adjustable Shocks
    Not a relevant factor in the selection process.


    Quote Originally Posted by AZ Cherry Bomb View Post
    * Eiback Sportline Springs
    Not a relevant factor in the selection process.


    Quote Originally Posted by AZ Cherry Bomb View Post
    * Wild Rides Upper & Lower Battle Box Upgrades (installing this week)
    Not a relevant factor in the selection process.


    Quote Originally Posted by AZ Cherry Bomb View Post
    * Nitto 555DRs our back (315/35-17); honestly thinking of switching to the M&H 325/35-17s (28" tire)...
    This could be significant but again the choice is dependent now on maximum target vehicle speed and whether or not the competition is drag racing or top speed specific event.



    Ed

  4. #3

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    Ed, thanks for your input on this. I do see the benefit of having something like the Wave-Trac; interesting product, will need to consider it. Doing the math using available online calculators for the car (3,250# stock curb weight - 155# weight reduction + 205# driver weight + 110# 2.1L KB kit = 3,410# approximate race weight) and my dynoed HP, the math solves an 11.25 @ 120.10 mph. The recommended gear ratio for that specific speed with a 28" tire = 3.82:1. But I don't want to necessarily constrain my gear selection for 1/4 mile... I want to be able to run on the street from a roll, and hit the airfield attacks when available too. Trying to find a good compromise.

    But drag racing isn't simple math, and while physics & math are the root of the problem at hand, we have suspension, driveline, environmental, and driver factors at play here. Referring to my dyno sheet, my KB is making almost instant HP from 2,500 rpm through 5,500 rpm. The torque curve is a smooth scale all the way to 5,500. Since I am makingaround 350 rwhp at such a low RPM, my interest I think lies in "dropping" the gear to take advantage of that power and have a higher trap speed at 1/4 mile. A 3.90:1 gear doesn't seem to me like it would afford me more time in gear, but rather less time in gear, forcing me into overdrive. That's why I am asking for advice. I do believe PD blowers matter in this discussion. A centrifugal builds boost, giving more power as the power band grows, and guys have always used "higher" gears to make up for a lack of power on the 1/4. My feeling is that I don't need to compromise gear if I have the power? You don't know what you don't know, hence my questions on the matter. As far as my details that have been labeled "irrelevant", I put that in there so anyone who knows anything about our stock SN-95 chassis knows that there is some suspension, drivetrain, and chassis work in the car, for hooking up. Its never going to solve a lack of skill in a driver, but it is an important factor when talking about higher power levels and the forces at play on a rear end in this equation. I think I see your point (which I believe I addressed first and foremost as being a math problem), but I do need some real world input as it stands with others' experiences with PD blown cars, the gear selection, and what actually works in practice, not on paper and in theory, which is where I am stuck at. I need to decide on a gear so I can move forward with finishing this rear end, getting it back in the car, and getting it to test & tune. You feel me?

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  6. #4

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    It's late, and my eyes arn't what they used to be... I had the blue lines mistaken, HP grows, the TQ was immediate. So its making huge torque already at low RPM, building HP over the band.

  7. #5

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    For a drag car all you need to do is decide what point on your power curve you want to go through the quarter mile traps at. Look at the RPM, pick a drag tire that fits into the rear wheel well and work the math backwards to find the gear ratio you need to hit that RPM and your target MPH. All the other information you are cluttering the calculation with is not a relevant factor in the gear ratio selection / calculation problem. This is the real world solution to your question.

    If you would prefer to use a software tool instead of doing the math yourself, try this one, click here =>Second Strike Gearing Calculator

    Select your transmission, pick a starting rear gear ratio, select the tire size you want to run and watch the fourth gear RPM and MPH. Change the rear gear until you see the numbers you are looking for. When you fnd them you are done, go buy a ring and pinion.

    In the FWIW bucket this is not a physics problem it is just a simple mechanics problem with a four function math solution that does not use all four functions.


    Ed


    p.s. Your transmission will be most closely represented by either the 3550 or TKO transmission choices. Either on will work fine. Even with slight differences in the gear ratios below fourth gear, the important gear ratio for the calculation is fourth (1:1).
    Last edited by eschaider; 07-22-2019 at 12:57 AM. Reason: Added Postscript

  8. #6

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    That is an absolutely amazing tool! Thank you. Using this, I'm even thinking of stepping down to a 3.08:1 in an effort to find a trap speed around 117 mph for an 11.49. That's in a perfect world of course. Does this sound crazy to step down to a 3.08 from 3:73s???

  9. #7

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    Quote Originally Posted by AZ Cherry Bomb View Post
    That is an absolutely amazing tool! Thank you. Using this, I'm even thinking of stepping down to a 3.08:1 in an effort to find a trap speed around 117 mph for an 11.49. That's in a perfect world of course. Does this sound crazy to step down to a 3.08 from 3:73s???
    In supercharged form our engines make power by increasing rpm. PD blown engines have notoriously flat torque curves that can seem to last forever. Because of that, increasing RPM is increasing HP, almost linearly.

    One of the first comments someone who has never previously driven a car with a PD blown engine makes is that it never seems to stop pulling — that is because of the flat torque curve. By limiting your engines rpm in the traps you limit the horsepower you allow the engine to apply to accelerating the car. As long as you don't mind, it don't matter.


    Ed

  10. #8

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    Man, so even working all of this math, at the end of the day I will need to test the setup and see if I am even trapping the car at the top of 4th. If I have gear left in 4th, then I will need to switch gears again and gear 'up' (numerically higher) to get as much out of the gear as possible in that distance, which also means that there is no real compromise between a 1/4 mile build & a 1/2 mile build. They are completely different animals.

    So if I want to try to attain a theoretical 11.41 @ 118.47 mph, then I would need to run either a 3:31 (using 315s out back), or a 3:73 (using 325s out back). Accounting for some 'room for growth' after fortifying the bottom end, I may want to select the gear ratio that will attain the mph trap I am seeking later on, which also generally fits my current needs, based on a faster ET & trap for that final build, or a 3:31 to put it shortly.

    Good looking out, that really helped me work that out.

  11. #9

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    Trap speed is exclusively a matter of HP. A good cheat sheet approach is to look at what other cars in your class are trapping for MPH. Then look at your dyno sheet and see where your max power is (usually at peak rpm). Use that rpm and your chosen class typical trap speeds to back into your final drive ratio with the calculator. If you find the car does not MPH like the other cars in the class when you take it to the track then, that is telling you you are down on power compared to the competition.


    Ed

  12. #10
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    Ed, What about the old tried and true Detroit Locker?

  13. #11

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    Quote Originally Posted by Trojan Horse View Post
    Ed, What about the old tried and true Detroit Locker?
    The Detroit Locker is not a torque biasing differential. In fact it is at the polar opposite of the torque biasing diff pendulum swing. Using a series of locking ratchets the locker will as its name implies lock both axles together in a turn as if it were a spool. As the turn progresses the locker will release the current ratcheted lock and grab the next. The gradation is very coarse and produces loud clunking and jerking in the automobile as it progresses through the turn.

    In a straight line the locker is effectively a spool but not as strong as a spool. In fairness it is stronger than virtually all the limited slip differentials but a far cry from a spool. The technology is a half century old (actually older) and has been eclipsed but other alternatives that are better suited to any of the application environments you might want to use a locker in. Lockers are great display pieces in museums.


    Ed

  14. #12

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    I originally wanted to run a spool, but couldn’t see any real tangible way of “bypassing” the ABS. If I had gone with a spool, my brakes wouldn’t work right.

  15. #13

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    Quote Originally Posted by AZ Cherry Bomb View Post
    I originally wanted to run a spool, but couldn’t see any real tangible way of “bypassing” the ABS. If I had gone with a spool, my brakes wouldn’t work right.
    The Wavetrac is by far the Premium solution to the, 'which differential should I use?' question. It modulates the torque applied to each wheel in direct proportion to the available traction or lack of traction depending on your perspective. It is the strongest design of all the differential designs available today, by a wide margin. Additionally it is the only one that comes with a lifetime warranty irrespective of where or how you use the product — that is about as good as it can get!

    In your original post you said,

    Quote Originally Posted by AZ Cherry Bomb View Post
    ... its time to do this the way Ford should have if cost wasn't a factor in 1999.
    This is your opportunity to do just that. All the components are already designed, manufactured, tested (in service), ready for you to use and warrantied for life, no matter how or where you use it. All you have to do is buy one and

    Quote Originally Posted by AZ Cherry Bomb View Post
    ,,, do this the way Ford should have if cost wasn't a factor in 1999.


    Ed

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    Quote Originally Posted by eschaider View Post
    The Detroit Locker is not a torque biasing differential. In fact it is at the polar opposite of the torque biasing diff pendulum swing. Using a series of locking ratchets the locker will as its name implies lock both axles together in a turn as if it were a spool. As the turn progresses the locker will release the current ratcheted lock and grab the next. The gradation is very coarse and produces loud clunking and jerking in the automobile as it progresses through the turn.

    In a straight line the locker is effectively a spool but not as strong as a spool. In fairness it is stronger than virtually all the limited slip differentials but a far cry from a spool. The technology is a half century old (actually older) and has been eclipsed but other alternatives that are better suited to any of the application environments you might want to use a locker in. Lockers are great display pieces in museums.


    Ed
    My Detroit Locker has never behaved as you described.
    It will lock the axles together in a turn if you are under power, but if you ease off the gas it releases one wheel.
    It does not produce a loud clunking or jerking either. It merely clicks as you're are turning and I've never noticed any jerking.
    Of course, mine is pretty old, I bought it in the late 70's early 80's IIRC.
    Maybe they have changed them since then?
    All I know for sure is that it has held up to hundreds of launches and a lot of street miles since it was new.
    Thank you for your reply.

  17. #15

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    Quote Originally Posted by Trojan Horse View Post
    My Detroit Locker has never behaved as you described.
    It will lock the axles together in a turn if you are under power, but if you ease off the gas it releases one wheel.
    It does not produce a loud clunking or jerking either. It merely clicks as you're are turning and I've never noticed any jerking.
    Of course, mine is pretty old, I bought it in the late 70's early 80's IIRC.
    Maybe they have changed them since then?
    All I know for sure is that it has held up to hundreds of launches and a lot of street miles since it was new.
    Thank you for your reply.

    The Detroit locker design is a very strong design, its just that in the fifty plus years since its introduction there have been substantial improvements made in commercially available differential technology. If you use a pendulum swing type of analogy the lockers are at one end of the swing and the torque biasing diffs at the other. If your locker does not misbehave as you negotiate turns you should consider yourself fortunate. Most do not handle turns very well at all, especially under power.


    Ed

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