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Discussion Starter #1
I think I'm missing something because this is a very popular mod that gets many positive reviews.
I have a 2002 Z. My performance mods consist of AR LT headers with H/F Cats through stock TI's, Halltech CAI and a tune.
I'm trying to get an understanding of the actual performance advantage in swapping out my stock 3.42 to 3.90 gears.
From my view it seems as though it would render 1st gear quite useless on street tires. For me, properly launching the car with the stock 3.42 gears takes alot of skill and finesse. I know that changing to 3.90 gears would lower the shift points of all other gears and drop top end speed.
It seems it would be great for burnout contests.......something I'm not at all interested in. Would there be any performance advatage from say a 40 MPH roll?
Can someone explain what I'm missing in my logic here?
Thanks.
Silverbird :z:
 

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Ultimately the only thing gears will give you is a higher RPM at a given speed. So, with 3.90 gears from a 40 mph in second you may be at 4500 in 2nd, where someone else with 3.42 would be at 4000 rpm. so when both of you launch you will pull harder. BUT you will also shift sooner, at which point the other car will make up the time lost from the initial role. with this repeated a few times, by the time you all hit ~160 it will be a wash. IMHO gears do not provide any advantage when dealing with a long distance (>1/2 mile), but would be great from turn to turn since the exit speeds would be that much punchier, stop light to stop light, or at the strip (assuming shift points dont hinder performance).

The only reason I havent done them yet is because I have a bigger issue which is the large rpm drop between 3-4th and 4-5th gears with the m12. yes the 1st and 2nd are very close (but who uses those anyway? :lol:). this can only be fixed with a closer more consistent tranny...once i get a closer ratio tranny (possibly the mm6) i will use the final drive ratio to determine the max speed in each gear. IMO this will be allot more beneficial than just a ring & pinion change.
 

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I have 410 gears and a cam package. If you are doing a 40 mph roll with a stock geared z with the same power, to say 120mph You with 390's will get their first. You will shift sooner, but you both will still shift the same number of times, 2 times. The 390 or 410 geared car will be faster than a stock geared until you max out 5th gear. But you will be able to pull in 6th gear. My 5th tops out about 142mph, but it gets there a lot faster.
The gears also are good for a big cammed car, The big cams lose a lot down low in the rpm range. the gears will get you back into the power band faster.
 

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I have 410 gears and a cam package. If you are doing a 40 mph roll with a stock geared z with the same power, to say 120mph You with 390's will get their first. You will shift sooner, but you both will still shift the same number of times, 2 times. The 390 or 410 geared car will be faster than a stock geared until you max out 5th gear. But you will be able to pull in 6th gear. My 5th tops out about 142mph, but it gets there a lot faster.
The gears also are good for a big cammed car, The big cams lose a lot down low in the rpm range. the gears will get you back into the power band faster.
This is a great explanation! I've run 3.42's, 3.90's, and 3.73's. The car definitely accelerated faster with the 3.90's. 4.10's are even quicker. Gear choice really depends alot on what you're doing with the car and your power level. Street and strip H/C cars really work best with 3.90's or 4.10's.
Once you get into turbo and blower power, 3.42's and even 3.15's seem to work better. For road racing the 3.73's are a perfect fit, probably up to 500 hp at least. I just went back to 3.42.s anticipating a rather large increase in HP and TQ. Drove tha car today after putting it back together with the 3.42's and can really say I'm VERY pleased.
Good thread! Keep it coming! :yeadog:

Jimbo
 

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I have 410 gears and a cam package. If you are doing a 40 mph roll with a stock geared z with the same power, to say 120mph You with 390's will get their first. You will shift sooner, but you both will still shift the same number of times, 2 times. The 390 or 410 geared car will be faster than a stock geared until you max out 5th gear. But you will be able to pull in 6th gear. My 5th tops out about 142mph, but it gets there a lot faster.
The gears also are good for a big cammed car, The big cams lose a lot down low in the rpm range. the gears will get you back into the power band faster.
i couldn't agree more!
 

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Gear choice really depends alot on what you're doing with the car and your power level.
I totally agree.

For a street driven car, there is probably nothing negative (possibly slightly higher engine temps) from doing a ring and pinion swap. You get the extra punch in each gear and you pull higher gears which allows you to cruise in 6th at a relatively low speed.

On a road course, however, it takes allot of planning. For instance, you have a 3rd gear turn, that leads onto a straight away whose breaking zone begins at the top of 4th. Exiting that turn with 3.90/3.73 gears you may gain an edge since you will be higher in your powerband, but you would lose that edge on the following straight due to having to hit your rev limiter, or shift into 5th for 1 second only to have to brake and downshift again. Ultimately, almost every turn (entry and exit), must be considered when choosing the correct ratio for a given track :eek2:.

Same applies to the strip...I do not have any personal experience with that, but have read where many people lost a couple tenths because they geared the car to low and now had to use an extra gear they never used before.

I found this in another post, here is a top speed calculator for your reference:

Speed/RPM Calculator

Also, keep in mind that 5th & 6th gears are "overdrive" gears and your power output is not nearly as efficient in those gears (16% less HP for a m12 and 26% less for a mm6). So on a road course, or maybe anywhere, you want to spend as little time in those gears as possible (until someone releases a 5th or 6th gear 1:1 tranny ofcourse :)).
 

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Howdy Vette nuts. My C6 Z06 has kooks headers , head and cam , air and free flow muffler and quick shifter . It runs great Dyno at 570 @6200 rpm at 6200’ Ele. 505 torque. With bigger cam I personally think 3.90 would be perfect. About 200 rpm higher across the board in all gears . It would take pressure off the clutch and drivetrain and drop rpm in 6th which is good. I’m at 2000 @ 86 mph. When speed limit hits 60-65 I’m shifting to 5th. I plan on installing 3.90 when I do clutch also adding MSD with 102 intake. Because of cam and heads , should add 34-37 hp. Should push 600 hp point and crack 9.8 1/4 with an experienced driver ( it’s been done and documented) 600 hp rwhp is about 705 crank . And you will get longer life out of the clutch. But if you dump it you will smoke those 300$ apiece tires . It will do it now with 3.42 just easier with 3.90. Years ago late 70s could destroy the ole L70s but they where 65$ a pc
 
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