Difference in ET times with 4.10 gears?

[billy_tripper]

I have an 05 GT auto with 93 performance tune (SCTII), JLT2 Intake, Steeda UDP, 4.10 gears, and Loudmouth axlebacks. I also have 20" wheels with 255/35zr20 tires.
I went to the 1/8th mile track in February in sub 40 degree temperatures and ran a 8.74 at 81 without the gears. I went back last week in sub 60 degree temperatures and ran a 8.72 at 82 with the gears. Thing is, it felt like a lot of the weak spots in the first run were gone with the gears (taking off and 3rd gear). I just figured the gears would help my time more than that and was hoping to at least break 8.5.
My question is, does the air still make a big difference even though 60 isn't really "hot air" and do the 20's make that big of a difference. They are not a heavy 20" wheel but they are just a cast aluminum wheel, not forged or anything. They weight like 32 lbs each.
Just looking for some input. The 4.10's are great though! I love them!

 

[DarkFireGT]

4.10's should take more time off than that, most definitely. They're good for 3-5 10ths in the 1/4 mile I believe. We'd have to see the other stats on your slip to figure out what's going on. What RPM are you shifting at? Are you powershifting? What RPM are you launching at? What are your 60ft times? What are you trapping? These are all important.

Just to give you an idea, my '07 has intake, tune, CMDPs (about the same gains as your UDPs) and off road pipe, and I went out to the track for the first time last week and my 1/8th mile was 8.771 with stock 3.31 gears. You just need some practice, that's all. So do I, for that matter. You should be running around 8.1-8.3

 

[kooldawg6]

I will say the temps do make a difference for sure. The 20s are hurting you because you have litte or no sidewall flex to aid traction.

Here is the timeslip for my car(#42)back in April(temp was in low 70s). Not many mods, but fairly similar to yours. I am running OEM 18" tires.

Here is the timelip for my car just a couple weeks ago(temp again was in low 70s). No different performance mods, but the car was 195 lbs lighter than the April run. On the same tires, which with the weight reduction killed my traction, all but my 60' times improved as well as MPH.
Best ET

Best MPH

 

[stangdude2000]

Just curious, were you running the 3.31's or 3.55's before?

The 20 inch wheels will make a much bigger difference that you'd imagine, but I don't know how it would relate in terms of times at the drag strip. Beyond the added weight, the extra rotational inertia is killer. If you double the moment of inertia, you need to double the torque to accelerate at the same rate.

Here is a very simplified calculation. We will say you go from 18" to 20" wheels. We will estimate them as just hollow hoops that are infinitely thin with a diameter of the rim. This moves all the mass of the spokes out to the rim and all the mass of the tires into the rim. The equation for interia is Inertia = Mass * Radius^2. By increasing the radius by 1 inch (about 11%), you increase the moment of inertia by about 23%. Angular acceleration of the wheel is alpha = torque/moment of intertia. This means to increase the inertia by 23%, you'd need to increase the torque applied to the wheel by 23% to get it spinning at the same rate. This doesn't, however, mean your engine needs to make 23% more torque since the mass of the car slows down the process anyways. I don't know how much of a actual torque increase you would need to offset it.

If the weight of the wheel increases 10% (30 lbs -> 33 lbs), the wheel needs about 36% more torque to get spinning at the same rate.

*DISCLAIMER* Just incase you missed the previous less obvious disclaimer, the 23% and 36% does not mean the engine. This would be if the wheel were spinning freely with no friction, as if the car was on jack stands. Friction and the car's weight throws a monkey wrench into the equation and makes it REALLY difficult to calculate how much extra engine tq would be required to offset the increase in inertia.
/Doctoral Dissertation lol

 

[billy_tripper]

I was running 3.31 gears. I've tried launching at different rpms but I've found about 1500-1800 to be the best. After I got the gears it seems like I can't go over about 1800 and the tires try to break loose. It's an auto so I don't have to worry about powershifting, and shift rpm and I didn't get a timeslip either time. Thanks for the help guys and any more input is appreciated. I may try the factory 17's to get my time down a little, but I still am boggled by the small gain from the 4.10's when they feel so much stronger. The only thing I can figure out is the really cold temps from the first run helped me.
Also, without the gears I ran an 8.9 first, then an 8.8, then a 8.74, then an 8.8 something. With the gears I ran 7 runs and even when I got a horrible start I still ran a worst run of 8.76.

 

[kooldawg6]

The weather was your biggest obstacle this past time.

 

[kooldawg6]

A brief explaination of stangdude2000's post...F/I Problem solved

 

[stangdude2000]

A brief explaination of stangdude2000's post...F/I Problem solved

 

[billy_tripper]

A brief explaination of stangdude2000's post...F/I Problem solved

Yea I wish!

 

[slammed89swb]

I will say the temps do make a difference for sure. The 20s are hurting you because you have litte or no sidewall flex to aid traction.

Here is the timeslip for my car(#42)back in April(temp was in low 70s). Not many mods, but fairly similar to yours. I am running OEM 18" tires.

Here is the timelip for my car just a couple weeks ago(temp again was in low 70s). No different performance mods, but the car was 195 lbs lighter than the April run. On the same tires, which with the weight reduction killed my traction, all but my 60' times improved as well as MPH.
Best ET

Best MPH

Dude those are impressive numbers for a lighty modded car! What all do you have done to your car? I bet with some lite wheels and slicks you would with out a doubt have a 12 second car! Man my wife's 07 GT 5speed needs to run that good!

 

[DarkFireGT]

Actually, with one to two tenths off his 60ft's he'll be down in 12.9's. I have faith that he'll hit it next time

 

[anticubicle]

With 3.55's I was running 13.8's @ 102'ish

My best with 4.10's as of tonight is consistant 13.1's @ 104-105. I also tossed on Pulleys and an H-Pipe but the 4.10's made the huge difference.

 

[slammed89swb]

Actually, with one to two tenths off his 60ft's he'll be down in 12.9's. I have faith that he'll hit it next time

Yep!!! and with slicks that will be easy to do!!! I dont really see that 60 foot goin down that much on facotry rubber

 

[kooldawg6]

Yep!!! and with slicks that will be easy to do!!! I dont really see that 60 foot goin down that much on facotry rubber

Those runs were on the factory 235/50-18s running 33 psi. Next year I will have som MT ET Streets I took out the passenger seat, back seat, Shaker1000 sub box/amp rack and once to the track, the spare and jack. Read "Put my car on a diet" thread for detailed info.

 

[Casey4s]

Those runs were on the factory 235/50-18s running 33 psi. Next year I will have som MT ET Streets I took out the passenger seat, back seat, Shaker1000 sub box/amp rack and once to the track, the spare and jack. Read "Put my car on a diet" thread for detailed info.

What type and size wheels are you going to use with the MT's ?

 

[DarkFireGT]

Yep!!! and with slicks that will be easy to do!!! I dont really see that 60 foot goin down that much on facotry rubber

Actually, you can do that with 20psi in stock rubber. I've seen 1.8 60 ft's with stock rubber. Just takes low pressure and practice.

 

[maxima]

With 3.55's I was running 13.8's @ 102'ish

My best with 4.10's as of tonight is consistant 13.1's @ 104-105. I also tossed on Pulleys and an H-Pipe but the 4.10's made the huge difference.

what mods did u have before the pulleys and h pipe , and what did you run thanks

 

[anticubicle]

what mods did u have before the pulleys and h pipe , and what did you run thanks

In horrible weather @ one of the worst tracks in Florida...

Stock - 13.9+ @ 100
Pre 4.10s - CAI & Tune 13.8+ @ 102
Posst 4.10's - 13.1 @ 105

 

[maxima]

what do guys think will spin more stock 17 oem tires or rims or the 18s oem s ??????????with 4.10s

 

[kooldawg6]

What type and size wheels are you going to use with the MT's ?

Probably the 26x11.5-17 on factory Bullitts.