Getting ready to swap over to 3.42 posi

Old January 18th, 2019, 11:41 AM
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Getting ready to swap over to 3.42 posi

Getting ready to swap from 2.76 open to 3.42 posi. Wondering about RPM's. Right now I turn about 2500 at 50mph, 3200 at 70mph, I see and read a lot about 5k + rpms, how different will my RPM's be after the swap? It will be higher, correct? With my build ( 380 or so olds stroker with full roller and aluminum heads, TH350 with a shift kit, 2400 stall, 15" 60's) will the additional RPM's "wear out the engine" faster? Should I be concerned with, or double check, anything due to the increased RPM's? What will it "feel" like on the freeway after the swap? How much of a dog will it be on the freeway? I am aware of swapping to a different tranny but not in the budget right now. Any other input on what to expect and or be aware of would be appreciated.
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Old January 18th, 2019, 11:57 AM
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Originally Posted by boese1978 View Post
Getting ready to swap from 2.76 open to 3.42 posi. Wondering about RPM's. Right now I turn about 2500 at 50mph, 3200 at 70mph,
Dang, that seems high. With short 245-60-14 tires and 3.23 rearend my engine turned 3200 RPM at 70 MPH. What size rear tires do you have?
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Old January 18th, 2019, 12:09 PM
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as noted, 60r15 BFG, My RPM numbers are not 100% to the nut accurate, just have a small tach......
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Old January 18th, 2019, 12:24 PM
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If those numbers are correct it looks as if you have around 600 RPM converter slippage. Seems kinda high as my Hughes Performance converter has around 250 RPM slippage at cruising speed. But maybe that's typical for the converter you have?

Swapping to 3.42 rear gears would result in 50 MPH = 2900 RPM and 70 MPH = 3800 RPM.
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Old January 18th, 2019, 01:19 PM
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my car has 295/50/15 tire, 3.42 rear and the engine spins about 3k rpm at 70mph.
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Old January 18th, 2019, 02:26 PM
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I upgraded from 2:56 gears to 3:42 posi. Huge difference in response. Huge difference in RPM's on the highway too, felt like I was stuck in 2nd. Lots of torque though. Woke the car up. I put an over drive tranny in, no issues now. I didn't go much over 65 mph. It felt like the rev's were getting up there at 70.

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Old January 18th, 2019, 04:28 PM
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We have 3:42 gear in our light Blue 76 cutlass, TH350 , TCI Saturday night special T/C . I forget the rear tire size, I'm thinking 265 60 15. On my tach , it hits about 2800 @ 50MPH . My speedo is still off due to the trans gear not being changed but my GPS gives me my MPH pretty accurately. The RPM's get up at 70MPH but I forget the number since I haven't drivin it much in the past couple years. I'm thinking 32-3300 but that's without taking my Ginko Biloma daily.

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Old January 18th, 2019, 07:00 PM
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RPMs are more of a comfort thing . I drive to the track with 3.90 gears 3500 stall and usually drive in the 70 to 75 mph range. What i found was the small block olds really sings at about 4k rpm. While cruising it moves almost effortlessly like it's pulling no load obviously with a numerically lower gear ratio with would be at a much higher speed at 4k rpm. A good exhaust should keep things quiet ..with a 3.42 it should still be in a "comfortable zone " . just fwiw. I was affraid to go to 3.90's now I want 4.33 's lol. I got used to it screaming down the road but I need a much quieter exhaust.
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Old January 20th, 2019, 12:33 PM
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I also have 2.78 gears and thanks to new cars spoil us for rpms we drive. I need to get the lock up working on my TH350C, it will put me at 2500 rpm at 70 mph with a 26.3" tall tire. The 60-15 tires mean nothing, we need the tire height. Expect around 3200 rpm at 70 mph. It sounds like you tach reads 500 rpm too high.

Last edited by olds 307 and 403; January 20th, 2019 at 12:38 PM.
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Old January 22nd, 2019, 04:25 AM
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Thanks for all the input and thoughts. Copper- funny you now want the 4.33 I am looking forward to the change. Need a more accurate tach as well...
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Old January 22nd, 2019, 06:34 PM
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The 3.42 will be a big upgrade. You will enjoy it.
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