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low gear for turbo

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Old 11-05-2006, 02:21 PM   #1
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Default low gear for turbo

i done a search and it seems like 3.42 are ideal for turbo, but i was wondering wouldnt 3.27 or even 3.08 or something in that nature would be better , i mean it would spool up better and u would stay in the turbo's sweetspot longer like automatics do, i have a moser 12 bolt ,6spd, i would get a 2step to help on the launch what do u guys think?
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Old 11-05-2006, 03:09 PM   #2
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It likely would, it (the launch really) would just be HARD on the drivetrain, especially the clutch and parts down-line.
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Old 11-05-2006, 03:16 PM   #3
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yea but if it was a highway car it would have some killer top end
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Old 11-05-2006, 11:02 PM   #4
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its all depends on what you expect to trap in the quater mile, or how fast you want to go on the street.

www.f-body.com/gears will let you put in those ratios with the size tire you have and rpm range to give you an idea.
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Old 11-06-2006, 08:18 AM   #5
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Old 11-06-2006, 09:02 AM   #6
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try this spread sheet
Attached Files
File Type: zip Th400.zip (13.7 KB, 302 views)
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Old 11-06-2006, 10:08 AM   #7
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Thanks for those spread sheets smokinHawk that really helps in my decision to go with a TH400!

Personally I plan on running a TH400 with a 3.08 gear and a 28" slick at the track, and then switching to a 32" tire for the street. With a tight TC with a stall around 2500, I think building boost on the line wouldn't be that much of an issue, especially with a 2-step, the high first gear of the TH400 and a 3.08 rear gear. It'd be a nice streetable combo with a lot of potential at the tack.
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Old 11-06-2006, 10:35 AM   #8
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Yeah, I'm doing the same
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Old 11-06-2006, 11:46 AM   #9
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Quote:
Originally Posted by vanillagorilla
Thanks for those spread sheets smokinHawk that really helps in my decision to go with a TH400!

Personally I plan on running a TH400 with a 3.08 gear and a 28" slick at the track, and then switching to a 32" tire for the street. With a tight TC with a stall around 2500, I think building boost on the line wouldn't be that much of an issue, especially with a 2-step, the high first gear of the TH400 and a 3.08 rear gear. It'd be a nice streetable combo with a lot of potential at the tack.
what 32" tire?

ive been looking at tires to run on the street and found this dulop suv tire thats 255/65-16. its about 10" wide and 29.05" high and rated up to 130mph, goes on my stock 16"rims.

what size 32" tire have you found and what rim does it go on? and are you planning on mni tubbing to get that 32" tire to fit?
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Old 11-06-2006, 11:49 AM   #10
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Quote:
Originally Posted by smokinHawk
what 32" tire?

ive been looking at tires to run on the street and found this dulop suv tire thats 255/65-16. its about 10" wide and 29.05" high and rated up to 130mph, goes on my stock 16"rims.

what size 32" tire have you found and what rim does it go on? and are you planning on mni tubbing to get that 32" tire to fit?
It's for a truck (see sig.) 275/55/20. Sorry.
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Old 11-06-2006, 12:20 PM   #11
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I just switched from a 2.73 to a 3.00 gear (9") and think I went too far. With a 26" tire, 3.00, TH350, and 146 mph, I'm going through the traps at 6400 and my peak hp is at 5700. I wish I had gone with 2.91's instead.

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Old 11-06-2006, 02:15 PM   #12
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but does the higher gear make up for it in the 60'?
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Old 11-06-2006, 02:42 PM   #13
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You could just borrow or buy a set of 27" tires to test that theory.

As far as the 60ft goes, I think it's a trade off as with everthing in life. What you lose in gear you'd make up in the ability to build boost and stall the converter a little more. I bet it'd be a wash if both scenarios were optimized.
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Old 11-06-2006, 04:42 PM   #14
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With the 2.73's, I never got a good 60' due to rich blubbering on the line. With the 3.00's, I also got my tuning straight, changed torque converters, added cid, and changed cam timing, so you really can't compare anyway.

Quote:
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What you lose in gear you'd make up in the ability to build boost and stall the converter a little more.
Never thought about that but I'm sure it's true. Similarly, some turbo converter companies deliberately go with a lower STR to allow you to load the motor more against the foot-brake to get more boost.

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Old 11-06-2006, 04:52 PM   #15
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Im going from 3.73's t 3.54's........ should help me 60' better, be easier on the street, and ill be able to turn the boost up now with out running out of gear like before....
I spin to 6800-7K.

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Old 11-06-2006, 05:10 PM   #16
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i believe longrange4u is running a 2.95 or something in that range and ran faster than 3.42 gear. im going to a 3.23 and ill let you know how it turns out i currently have a 3.73 and i feel it runs the turbo out to fast doest stay in sweet spot.
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Old 11-06-2006, 06:19 PM   #17
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I run 2.75 gears and cut high 1.5 60's. You just have to make up the difference in tq multiplication by leaving with more boost. My friend with 3.0 gears can go 1.5x with 8psi vs my 10 psi launches.
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Old 11-06-2006, 06:34 PM   #18
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Quote:
Originally Posted by Zombie
I run 2.75 gears and cut high 1.5 60's. You just have to make up the difference in tq multiplication by leaving with more boost. My friend with 3.0 gears can go 1.5x with 8psi vs my 10 psi launches.

Quick.. tell them what suspension mods ya have...
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Old 11-06-2006, 06:56 PM   #19
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I am going to try a 2.75 ratio on a 28" tire with a th400 with a lower gear planetary. We'll see how it works out.
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Old 11-06-2006, 07:01 PM   #20
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Quote:
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Quick.. tell them what suspension mods ya have...
LOL, umm, 80k mile OEM parts Ok, it does have lower control arms and subframes. Does a bent adjustable panhard count as a susp mod?

I should probably put an aftermarket tq arm on it soon.

I think my 60's are pretty decent, I don't even run skinnies at the track, stock SS wheels in the front.
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Old 11-06-2006, 07:01 PM
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