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rpm change with gear swap
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Picture of Fabman
posted
It's been a long time since I've set my car up specifically for 1/8 mile.

Sorting out a new car and the rear gear and converter need attention. Before I get into adjusting or replacing the converter, I feel the car needs a gear swap.

Car currently goes through the 1/8th at 7200 rpm. Would like to cross at 7600-7700 rpm, no more than 7800.

Car currently has a 4.11 gear, radial tire with zero/minimal growth.

Per the data logger, the 1/8 mile slippage is 14% and 1/4 mile slippage is 2.5%.

So what I'd like to find out is if I changed to a 4.29, 4.57 or even 4.71, what rpm increase should I expect to see assuming nothing changes on the converter. Now granted we all know the converter will tighten up with the less load of the deeper gear which in turn should make the slippage percentage less. But for this discussion, lets just say worst case how much rpm increase for each gear step up from 4.11.

again, this will be for 1/8 mile usage only.

thanks for any input
Todd
 
Posts: 277 | Location: USA | Registered: August 27, 2022Reply With QuoteReport This Post
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Consider an adjustment for bad air /good air. I’ve competed in 4000+’ DA all the way down to -500 DA. The swing in my car is 250+ rpm crossing at 7200 bad and 7500 mineshaft. My car is strictly 1/8 mile, has a loose convertor with 18% slip.
 
Posts: 2500 | Location: 53056 | Registered: December 30, 2009Reply With QuoteReport This Post
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Check out this page for calculators. Maybe something here will help.
http://www.wallaceracing.com/Calculators.htm


https://postimg.cc/gallery/np3zpruo/
"Dunning-Kruger Effect"
-a type of Cognitive bias where people with little expertise or ability assume they have superior expertise or ability. This overestimation occurs as a result of the fact that they do not have enough knowledge to know they don't have enough knowledge.

Before you argue with someone ask yourself, "Is this person mentally mature enough to grasp the concept of a different perspective?" If not there is no point to argue.

4X NE2 CHAMPION. 2020 TDRA NE2 Champion
 
Posts: 4061 | Location: United States of Texas | Registered: April 02, 2011Reply With QuoteReport This Post
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4.57


honesty is the best policy,insanity is a better deffense
1.036, 6.16@ 224

 
Posts: 1421 | Location: texas | Registered: February 17, 2006Reply With QuoteReport This Post
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quote:
4.57


I went from a 4.40 to a 5.14 and increased 700 rpm. Based on this I agree going from 4.11 to 4.57 should be 400+ rpm increase.
 
Posts: 2500 | Location: 53056 | Registered: December 30, 2009Reply With QuoteReport This Post
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quote:
Originally posted by markemark:
quote:
4.57


I went from a 4.40 to a 5.14 and increased 700 rpm. Based on this I agree going from 4.11 to 4.57 should be 400+ rpm increase.


Thirded. 4.11 to 4.29 was about +200 for me, so I was thinking 4.57 would get the OP closest to his goal.


Tony Leonard
 
Posts: 3172 | Location: Inver Grove Heights, MN | Registered: March 18, 2004Reply With QuoteReport This Post
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Depending on the whole combo, you may not gain anything in finish line rpm. I have made a change from 4.10 to 4.56 and saw absolutely no rpm change. But it did make the converter much more efficient (less slip). I then had to have the converter loosened up to get the rpm I was looking for.


Mike Boehner
 
Posts: 93 | Location: Lake Havasu City, AZ | Registered: January 19, 2011Reply With QuoteReport This Post



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^^^^^^ This, Agreed. Flash stall will change least but finish line slip (less) could change more. Don’t rule out a converter adjustment after gearing change. Depending on how tight the converter is now, going 5.29 and loosening the converter could be considered.

Prior to going 4.40 to 5.14 the converter company had missed the flash stall on a new build and missed again on the next week re-stall that I lived with for over 1.5 yrs with 4.40 gears (was 1/4 mile racing). Going 5.14 (strictly 1/8 mile) didn’t reduce the flash stall much (less than 100) but it did reduce the 1/8 slip dramatically ( 27% to 14%). The converter company agreed to fix the flash stall (free) to what was first requested, and it came out perfect. The 1/8 finish line slip is 18 – 19.5% depending on the installed tire RO. My biggest want was the higher flash stall and the gearing change put me close to the desired finish line rpm.

As I wrote prior, my flash stall falls into a 250+ rpm window depending on DA. I attribute this to how loose it is.
 
Posts: 2500 | Location: 53056 | Registered: December 30, 2009Reply With QuoteReport This Post
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Thanks for the input guys.

I'm gonna try the 4.57's. My intention is to get them installed and get some data before the season ends. Once I have some data I'll send the converter back to be adjusted and converted to a bolt together.

I spoke with Lane at PTC today and we determined the converter is actually pretty close to what they would want. Now granted it's gonna tighten up after the gear swap, but we'll handle that after we have some data.
 
Posts: 277 | Location: USA | Registered: August 27, 2022Reply With QuoteReport This Post
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Hey Fabman, we just did this with my wife's dragster, by accident (long story - it broke a pinion tooth and couldn't get gears, a friend that has a shop said he had them, I gave him the case and he set them up, turns out they were 4.57s not 4.11s so now I pick on him about not counting the teeth before setting them up).

Combination: 540 alcohol injected, 1.80 low, 4.11 rear, goes 4.80 with her in it, 1.09-1.10 60' 3.09-3.10 330.

We swapped from 4.11 to 4.57 and my friend drove the car in a 1/8 mile race since she has no interest in gayth mile racing. I'm going to guess that he's about 70 pounds heavier than her. With the 4.57 it went low 1.08 to 60', 3.08 to 330' and 4.79-4.80 to the 1/8. I was surprised it didn't pick up a little more than it did.

On the data/RPM side, the flash stall was the same. We shift on time at 2 seconds. With the 4.10 it's about 7300 weather depending, with the 4.57 it was high 7700- tickled 78 on one run. It really revved up quickly after the flash stall. With the 4.11 it slips 20% at the 1/8 and with the 4.57 it slipped 12%. At the 1/8 finish line, 4.10s are about 7150, 4.57s were about 7450, so 300 RPM higher.

Here's what the graph looked like:

 
Posts: 727 | Location: Upstate NY | Registered: July 02, 2013Reply With QuoteReport This Post
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quote:
Originally posted by Fabman:
Thanks for the input guys.

I'm gonna try the 4.57's. My intention is to get them installed and get some data before the season ends. Once I have some data I'll send the converter back to be adjusted and converted to a bolt together.

I spoke with Lane at PTC today and we determined the converter is actually pretty close to what they would want. Now granted it's gonna tighten up after the gear swap, but we'll handle that after we have some data.


I would agree with the 4.57 gears, that's about 800 rpm's based on the math but the lower gear will reduce slip and therefore rpm also. Looks like your converter is locking up pretty good now up top. How much rpm you pick up will depend on a number of factors but I'd say you will be in the 400 to 600 rpm gain at the stripe. More of less than that would indicate the motor loves or hates the added rpm's.
 
Posts: 2161 | Location: Tewksbury, MA,USA | Registered: November 03, 2000Reply With QuoteReport This Post
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quote:
Originally posted by 183N:
Hey Fabman, we just did this with my wife's dragster, by accident (long story - it broke a pinion tooth and couldn't get gears, a friend that has a shop said he had them, I gave him the case and he set them up, turns out they were 4.57s not 4.11s so now I pick on him about not counting the teeth before setting them up).

Combination: 540 alcohol injected, 1.80 low, 4.11 rear, goes 4.80 with her in it, 1.09-1.10 60' 3.09-3.10 330.

We swapped from 4.11 to 4.57 and my friend drove the car in a 1/8 mile race since she has no interest in gayth mile racing. I'm going to guess that he's about 70 pounds heavier than her. With the 4.57 it went low 1.08 to 60', 3.08 to 330' and 4.79-4.80 to the 1/8. I was surprised it didn't pick up a little more than it did.

On the data/RPM side, the flash stall was the same. We shift on time at 2 seconds. With the 4.10 it's about 7300 weather depending, with the 4.57 it was high 7700- tickled 78 on one run. It really revved up quickly after the flash stall. With the 4.11 it slips 20% at the 1/8 and with the 4.57 it slipped 12%. At the 1/8 finish line, 4.10s are about 7150, 4.57s were about 7450, so 300 RPM higher.

Here's what the graph looked like:



I did not think it would drop from 20% slip all the way down to 12%. That is more than I would have thought. Makes me consider another gear change on one of my cars.


https://postimg.cc/gallery/np3zpruo/
"Dunning-Kruger Effect"
-a type of Cognitive bias where people with little expertise or ability assume they have superior expertise or ability. This overestimation occurs as a result of the fact that they do not have enough knowledge to know they don't have enough knowledge.

Before you argue with someone ask yourself, "Is this person mentally mature enough to grasp the concept of a different perspective?" If not there is no point to argue.

4X NE2 CHAMPION. 2020 TDRA NE2 Champion
 
Posts: 4061 | Location: United States of Texas | Registered: April 02, 2011Reply With QuoteReport This Post
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quote:
Originally posted by TOP38:
quote:
Originally posted by Fabman:
Thanks for the input guys.

I'm gonna try the 4.57's. My intention is to get them installed and get some data before the season ends. Once I have some data I'll send the converter back to be adjusted and converted to a bolt together.

I spoke with Lane at PTC today and we determined the converter is actually pretty close to what they would want. Now granted it's gonna tighten up after the gear swap, but we'll handle that after we have some data.


I would agree with the 4.57 gears, that's about 800 rpm's based on the math but the lower gear will reduce slip and therefore rpm also. Looks like your converter is locking up pretty good now up top. How much rpm you pick up will depend on a number of factors but I'd say you will be in the 400 to 600 rpm gain at the stripe. More of less than that would indicate the motor loves or hates the added rpm's.


This engine is a 594 ci conventional headed bbc (4.650" x 4.375"). It makes peak power at 7400 rpm (1144hp) and peak torque is at 6100 (892 ft/lb).

The power holds on well to 8000 rpm.
 
Posts: 277 | Location: USA | Registered: August 27, 2022Reply With QuoteReport This Post
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