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Old 11-13-2023, 09:50 PM
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Default Gear change effect on convertor?

Racing season is done here in the north east, I am flirting with the idea of going from a 4.10 to a 3.90. Right now I am running a 4.10 with a 30" radial and crossing the line at 6600 at 133 with 7% convertor slip. The car is cutting 1.31 60's. Right now I have an 850 Holley and 340lbs to try and slow it up for 10.0 index. Saturday I ran it all out and ran 9.94. I want to gear it for 142 or so at 6600rpms.
What effect will I gear change have on the convertor and launch? I am worried it will make the convertor less efficient. Will my 60's suffer?
Want to take the 340lbs out and put the 1050 holley back on it to see how close I can get to 9.5's.
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Old 11-13-2023, 11:01 PM
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The converter will act a little lower. In my experience there won't be much of an et difference.

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Old 11-14-2023, 10:22 AM
bluegoat65 bluegoat65 is offline
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Wow. Looking to gain nearly 10 MPH at the same Trap RPM. Would you not need the converter to slip at 0 to 1% also with the gear change? With that would your SLR then need to be changed to make up for the loss in converter at launch to keep it crisp? Is the 3 and a half hundred pounds you want to shed equals a tenth per hundred really an exact science or rule of thumb? Wallace calculators?

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Old 11-14-2023, 10:31 AM
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I want to take the 340lbs out of the car, that should knock .340 of the et on paper. Put the 1050 back on should knock of another .100. My problem is i have to much gear and would cross the line with way to much rpm thus the gear change.

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Old 11-14-2023, 10:44 AM
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Tuff question!
Maybe break it down into effects on the converter.

Less vehicle weigh = less load on the converter and that would bring down the rpm the converter flashes to.
Depending on the motors current power band this could be good or bad I would have to guess.

Reinstalling the bigger carb equals more torque and Hp.
I think you have found what effects this brings to the table if I recall right?

The gear change will load the converter harder and in turn using up more or all its stall speed.

All and all I don’t see you getting all of the mph increase your looking for, especially since no added rotating component resistance ( tires ) , Nor added wind resistance factor has been added as needed into to the equation.

My proven trusty Moroso slide calculator is telling me that to hit 146 mph you need 700 Hp and a 2900 lb car regardless of gearing.

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Last edited by steve25; 11-14-2023 at 10:55 AM.
  #6  
Old 11-14-2023, 12:00 PM
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Quote:
Originally Posted by Mike S View Post
Racing season is done here in the north east, I am flirting with the idea of going from a 4.10 to a 3.90. Right now I am running a 4.10 with a 30" radial and crossing the line at 6600 at 133 with 7% convertor slip. The car is cutting 1.31 60's. Right now I have an 850 Holley and 340lbs to try and slow it up for 10.0 index. Saturday I ran it all out and ran 9.94. I want to gear it for 142 or so at 6600rpms.
What effect will I gear change have on the convertor and launch? I am worried it will make the convertor less efficient. Will my 60's suffer?
Want to take the 340lbs out and put the 1050 holley back on it to see how close I can get to 9.5's.
Changing the rear gear should increase the convertor slippage. A 3.73:1 with 10% slippage should get you 142 MPH @ 6600 engine RPM.

Stan

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  #7  
Old 11-15-2023, 01:14 PM
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I'm very curious in this also as I run 4.11 gear and would like to drop to 3.90 gear to lower the rpm through the traps. I run a 29" tire and I would like to go back to the 28" tire. Dropping down tire size and gear would probably net same results? Thoughts

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Old 11-15-2023, 02:07 PM
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Quote:
Originally Posted by chuckies76ta View Post
I'm very curious in this also as I run 4.11 gear and would like to drop to 3.90 gear to lower the rpm through the traps. I run a 29" tire and I would like to go back to the 28" tire. Dropping down tire size and gear would probably net same results? Thoughts
If we ignore convertor slippage.

4.1:1 - 29" - 6500 RPM - 136.77 MPH

3.9:1 - 28" - 6500 RPM - 138.83 MPH

3.9:1 - 28" - 6405 RPM - 136.78 MPH

3.9:1 - 27.58" - 6500 RPM - 136.75 MPH

Stan

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Old 11-15-2023, 02:41 PM
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Quote:
Originally Posted by chuckies76ta View Post
I'm very curious in this also as I run 4.11 gear and would like to drop to 3.90 gear to lower the rpm through the traps. I run a 29" tire and I would like to go back to the 28" tire. Dropping down tire size and gear would probably net same results? Thoughts
I run a 3.90 gear and changing from a 29" tire to a 28" tire had no effect on ET. I can't remember if there was any difference in rpm going across the line, but there was no change in converter slip..

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Old 11-17-2023, 08:57 AM
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Thank you Stan and AG. I appreciate the info.

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68 Firebird. IA2 block, 505 cu in, E-head, Solid roller 3650 weight. Reid TH400 4:11 gear. 29" slick.
Best so far 10.12@133 mph. 1.43 60 ft.
76 Trans am, TKX .81 o/d, 3.73 Moser rearend,
468 with KRE D-ports, Doug headers, 3" Exh.
  #11  
Old 11-22-2023, 09:53 AM
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Decided to leave the 4.10 gears in. The calculator's say with current comb and slip I should cross the line at 138 at 7000 rpms. That is my limiter setting. I think 138 should be good enough for 9.6's. I don't want to give up the 60' times.
So in the spring I will remove 350lbs, put the 1050 back on, remove the exhaust and run it all out and see where I am at rpm wise.

  #12  
Old 11-22-2023, 11:34 AM
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Many years ago I did a computer simulation of a Pro Stock motorcycle run. Everything match up perfect. But to me something was off. The owner/crew chief/engine builder posted on a forum that I also posted on and had helped me for some information about the motorcycle. I said the only way I got everything to agree is to run out of RPM before the finish line. He laughed and said me too. He was willing to give a little MPH up for a better et.

Stan

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  #13  
Old 11-22-2023, 08:54 PM
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Tire dia growth, with all that tire rpm ought change a 28" to 29" after 100' or so.

  #14  
Old 11-23-2023, 09:22 PM
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I always ran a 3:73:1 gear for my best results.
I tried 4:56:1 then 4:10:1 and got my best ET and MPH
with the 3:73:1 this was a long time ago and not as fast
as the car is now.

GT

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