1/4 mile, Gear ratios, and Tire diameters

Discussion in 'SRT Hellcat Wheels and Tires' started by Tire God 2.0, May 1, 2016.

  1. Tire God 2.0

    Tire God 2.0 Senior Hellcat Member

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    For those of you crossing the finish line at an engine speed higher or lower than the desired RPM your engine likes you can change rear end gear ratios or you might consider changing tire diameters. The formula is as follows.
    Rear Axle Ratio x MPH x Transmission output ratio x 336 / tire diameter.
    We will use a stock 8 speed auto Hellcat as an easy example.
    Rear axle ratio= 2.62:1
    MPH (trap speed) 125mph
    Transmission output ratio ( last gear gatio used during race) I saw they ended in 5th gear. 5th gear ratio is 1.29:1
    Mulitiply by 336
    Divide by stock tire diameter (28.7)
    It looks like this.
    (2.62x125x1.29x336)/28.7 =4,946 RPM. This is the RPM in which the person finished the race. I used this and compared it with the Dyno sheets I could find for stock hellcats and looked to see at what RPM the power dropped off. The torque curve started declining right at 5000 RPM. SRT thought it through. So this post won't help stock guys. But for thoes of you who have put down a ton more power, this might be useful. I saw a trap speed of 140 mph out of this white HC 900hp beast. Dyno showed same torque curve (only alot more). But If you plug in the 140 trap speed into the equation using the rest of the same numbers, the RPM comes out to 5,539. (The drag tires i couldnt see what size).This is well past where the torque curve drops off. But if you change the tire diameter to 30 or 31 inches, it brings the RPM back down to right at 5k to keep it in its power band. In general you want to put the tallest, widest tire you can on the car for maximum traction. But sometimes, depending on gear ratios, changing the tire diameter will help you tremendously depending on the setup.
    I hope this was helpful. If need be I can go into depth of gear ratios and what they mean exactly ect.
     
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  2. 1 Fast Kat

    1 Fast Kat Rookie

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    Lot's of good info. Thanks!
     
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  3. Top Cat

    Top Cat Gold Member Hellcat Car Club Gold Supporting Member Fastest Cat/Record Holder HCC Charter Member

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    Good information others should take a look at,recently we switched to 3.25 gears and that change pushed the car into 6th gear about 100 ft before the traps, for us that was bad, so either a taller tire if it will fit or change back down to 3.00 rear gears, gears are less expensive, for those that think going past the 3.09 on the A8's it's not a good change.
    TC
     
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  4. DGatzby

    DGatzby Gold Member Hellcat Car Club HCC Charter Member Midwest Regional Pres.

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    Good deal, those 305/35's make 20 rpm difference.
     
  5. DGatzby

    DGatzby Gold Member Hellcat Car Club HCC Charter Member Midwest Regional Pres.

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    THAT is good info^^^^
     
  6. tmb828hellcat

    tmb828hellcat Rookie

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    Does any one have a preference on a 17" or 18" forged wheel and tire combo went to the track yesterday did a 11.5 pass with a trap speed of 129mph. I am currently running nitto 555 drag radials need something with more meat on them with that trap speed should easily be in the 10's
     
  7. sgoyal

    sgoyal Gold Member

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    I ran 11.15 with a trap speed of 125.31 using 17" Hoosier DR (26")... I had trans: sport, traction: sport and suspension: street.

    I'm still a rookie in drag racing (6 run ever). If I can fix my 0-60 (remove traction) then I'm sure I'll be in 10
     
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  8. BODUKE

    BODUKE Gold Member

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    Good you got to experience the quartermile dragstrip. It takes practice and the right conditions to get to tens.
     
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  9. Tire God 2.0

    Tire God 2.0 Senior Hellcat Member

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    hellcat ratios.JPG
     
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  10. baby girl

    baby girl Rookie

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  11. Tire God 2.0

    Tire God 2.0 Senior Hellcat Member

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    Overall top gear
     
  12. sgoyal

    sgoyal Gold Member

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    Top God, Do you think putting 26" Hoosier Tire would an issue on these cars (just for drag racing) ? Just trying to get your opinion.
     
  13. Wayne

    Wayne Gold Member

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    Didn't Stella use smaller diameter tires (I don't remember if they were drag radials, or slicks) to get to 10.6, or 10.7? Seems like it's a method similar to a higher numerical gear ratio. Not really sure though.
     
  14. BD2DBONE

    BD2DBONE Silver Member

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    SG - surprised you didn't use the track setting for transmission. Also given you had Hoosiers I would think you'd be better off with no traction as in a complete turn off by holding the button.

    I've never done the track but based on what I'm reading here. Might want to check with Stella.
     
  15. sgoyal

    sgoyal Gold Member

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    I spoke to StellaHC and going to remove traction. But the track had 3 incidents and they had to shut down for 20-30 Minutes. I didn't want to take a chance since I was the second next guy to run.

    Funny part, when I had transmission Track, I ran little slow not so sure. But I'll try next outing again.

    DA was around 2700 so that might have played role. So far, I'm really happy with these Hoosier.
     
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