Thread: Gear Upgrade
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Old Jun 18, 2008 | 02:05 PM
  #25 (permalink)  
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parawxman
Joined: Aug 2006
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Default Re: Gear Upgrade
Not sure if the math all added up with your hypothetical, as it would require a 4.30 gear in order to top out at 108mph in 4th gear. Using this 4.30 gear, 5th gear would actually begin at 4900 for 108mph, which is similar to what we have already at the top end of the track and would allow us to continue to accelerate until 6K rpm and over 125mph, (HP permitting), or in other terms a 4.30 gear would give us an overall gear of 3.56 in 5th. Actually might not be a bad gear except for significant handicap of 1st gear. Gears like this would be better suited for an engine that runs up over 7K rpm. And better not to use the OD 5th gear as part of the ¼ mile equation

The problem I’ve seen with the SRT is we run out of gear / rpm before the end of the ¼ mile. Currently most basically stock SRT’s are shifting out of 3rd gear at around the 103mph mark, (assuming a 6Krpm shift point), leaving us only a 3.08 gear to accelerate the remainder of the ¼ mile. Even using the NA 3.27 gear this would have us shifting out of 3rd at about 98mph and dropping into 4th gear at about 4100rpm. This will give us extra gear multiplication and make a noticeable improvement in both speed and ET. But this is not enough of a change for me to make a swap worthwhile. However, as AMX notes, a gear more like a 3.55, would be my choice for drag racing this car, as it is decent compromise for our deep first gear, (about 35mph at 6K rpm, about 13:1 in first gear) and still leave us around 5500rpm at 120mph at the end of the track, of course assuming the engine is capable of the HP to run this rpm and speed.

Regarding traction, I’ve found this car to be fairly decent for a non posi/lsd and small street tires. The high tech approach to controlling wheel spin without the above has netted me 1.9 short times with the 255’s and with minimal wheel spin, other SRT6’s are even faster. On the street not going to WOT until it is rolling has proven to be an effective launch. Moving to a larger (285), high traction radial or slick would make a huge difference in my car. Once you are too the point of changing to a 3.55, high traction tires would be a requirement for best times, but all of this is a mute point, as recently discovered we currently can’t get gears beyond the NA’s 3.27 due to computer incompatibility issues. But we can always hope.


Originally Posted by LookieLou
I'll put a hypothetical example forward, For this example, let's say you currently trap at 106 mph, and your 4th gear tops out at 120 mph. You decide to change your gear ratio so that your 4th gear tops out at 108 mph (a little wiggle room, since the gears will hopefully let you trap at higher speed along with better ET). Now, you've dropped your ET and raised your trap speed to 108 mph. Congratulations! However, if you add some power (pulley,intake, whatever you like) which hypothetically allows you to trap at 112 mph, now you're going to waste precious time making a shft from 4th to 5th, minimizing your gains. Now, you have to re-gear to top out at higher than 112 mph to get any use out of that extra power.

In addition, IMO, our cars are already traction challenged right now. If you keep an open diff and street tires, a gear change could simply mean an extra half-second smoking the tires instead of grabbing traction sooner with less gearing. Changing gear ratios means a LOT more fun driving the car on the street (because it FEELS faster), but it doesn't necessarily translate to actually GOING faster.
 
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