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Torque converter slip?

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Old Feb 1, 2025 | 05:47 PM
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Default Torque converter slip?
My 78 has probably the original TH350 torque converter behind an L48 with 98k miles. At 55mph with 3.08 diff and 245r/60 tires i show 2750 on the tach. Wallace Racing calculator shows more like 2100 which suggests 30% slippage. I checked the diff on jacks and it is accurate. MPH is accurate to gps. Tach checks to another meter. What do the experts think? Thx.
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Old Feb 1, 2025 | 06:39 PM
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I think your calculator is flawed. Your RPM's sound perfectly normal.
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Old Feb 1, 2025 | 06:58 PM
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4vettes—thanks but my manual calculation shows 70 mph at 2750. Alternately 2200 makes 56 mph. Other guys out there-what are you seeing with a setup similar to mine?
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Old Feb 1, 2025 | 07:02 PM
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Yea , 2750 RPMs is a bit much so something ain't right. My math is: (tire revs per mile) times (axle ratio) divide (RPMs@60 MPH) times (actual MPH) divide (converter efficiency) equals (RPMs)
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Old Feb 1, 2025 | 08:34 PM
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Originally Posted by Radovette
My 78 has probably the original TH350 torque converter behind an L48 with 98k miles. At 55mph with 3.08 diff and 245r/60 tires i show 2750 on the tach. Wallace Racing calculator shows more like 2100 which suggests 30% slippage. I checked the diff on jacks and it is accurate. MPH is accurate to gps. Tach checks to another meter. What do the experts think? Thx.
How did you determine you have a 3.08 rear gears?
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Old Feb 1, 2025 | 08:37 PM
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Melwff—i checked the code on the bottom of the diff case then confirmed by rotating the driveshaft until the tire completed 1 revolution.
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Old Feb 1, 2025 | 09:11 PM
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Last edited by QIK59; Feb 3, 2025 at 11:38 PM.
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Old Feb 1, 2025 | 09:14 PM
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Originally Posted by Radovette
Melwff—i checked the code on the bottom of the diff case then confirmed by rotating the driveshaft until the tire completed 1 revolution.
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Last edited by QIK59; Feb 3, 2025 at 11:38 PM.
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Old Feb 1, 2025 | 09:14 PM
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Qlk59–yes you are correct. Mph is exactly twice rpms at all speeds in 3rd gear. So ehat is your experience?
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Old Feb 1, 2025 | 09:19 PM
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To all—when i checked the ratio i only monitored the passenger wheel. I suppose i need to recheck and watch both wheels!
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Old Feb 1, 2025 | 09:23 PM
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Originally Posted by Radovette
To all—when i checked the ratio i only monitored the passenger wheel. I suppose i need to recheck and watch both wheels!
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Last edited by QIK59; Feb 3, 2025 at 11:39 PM.
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Old Feb 1, 2025 | 09:25 PM
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Originally Posted by Radovette
Qlk59–yes you are correct. Mph is exactly twice rpms at all speeds in 3rd gear. So ehat is your experience?
---
Last edited by QIK59; Feb 3, 2025 at 11:39 PM.
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Old Feb 1, 2025 | 09:27 PM
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Sorry, i meant in terms of rpm- mph correlation
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Old Feb 1, 2025 | 09:43 PM
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Originally Posted by Radovette
Sorry, i meant in terms of rpm- mph correlation
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Last edited by QIK59; Feb 3, 2025 at 11:39 PM.
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Old Feb 1, 2025 | 10:27 PM
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I would never trust a tach in one of these cars without comparing it to an Innova timing light, or similar.

2nd gear is 1.52:1. Are you sure you are seeing 3rd gear?

The only way your rear wheels turn at different speeds when on jackstands is if your limited slip differential (positraction) is shot.
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Old Feb 1, 2025 | 10:41 PM
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Bikespace—definitely 3rd as i count each shift. So your take is that it is not worth rechecking the diff ratio? The code on the diff case is:
OK 3.57W7E1 which is 3.08:1ratio. Everything else on the car is numbers matching so i believe this diff is unmolested and original.
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Old Feb 1, 2025 | 10:52 PM
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Originally Posted by Radovette
Bikespace—definitely 3rd as i count each shift. So your take is that it is not worth rechecking the diff ratio? The code on the diff case is:
OK 3.57W7E1 which is 3.08:1ratio. Everything else on the car is numbers matching so i believe this diff is unmolested and original.
If it's easy to do, go ahead and check. A lot could have happened in 46 years. But you'll likely find that both wheels turn together, and your initial measurement was correct.

Do you, or a buddy, have a digital timing light with a tach?
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Old Feb 2, 2025 | 12:03 AM
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I'm not sure how your calculating any of this. And frankly don't care. Those RPM's at that speed sound perfectly normal for a C3 Corvette.
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Old Feb 2, 2025 | 06:18 AM
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What does the trans fluid look like and does it smell burnt?
Assuming 26" tire height and 3rd gear trans ratio of 1.00, got just under 2200rpms.

60

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Old Feb 2, 2025 | 08:59 AM
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Check your speedo for accuracy?
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