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#1 |
Registered User
Join Date: Sep 2008
Location: Dallas,TX
Posts: 328
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Anybody running a big block with th400 and 3.55 gears?
I'm about to do some upgrades to my rearend in my 68'. Right now I have an open 3.73 setup. I'm cruising on the highway with normal flow of traffic at 3500 rpms. This is really not a problem besides drinking up the gas lol...but if I'm going to change to a limited slip set up I might as well change gears if I'm going to do it while I'm in there. I've been thinking about running 3.55 limited slip or 3.42's what you guys think and list you mods if you could and what rpms you are seeing.
Here's what I have... 1968 SWB 502 H.O. Crate Engine Minor head Work still has 2.19/1.88 valves Bumped the Compression up a bit over 9.5:1 Howard's Hyd Roller cam-Duration:235/241(ADV 288/294)Lift:618/618 LSA:112 Air Gap Intake with Holley 950HP DUI ignitionHooker Comp Headers 2" primaries 3.5 collector3" Exhaust Race Built Stage 3 TH400 2800 stall 3.73 gears open carrier Loving the way my truck is performing but the open rear is killing me! |
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#2 |
Senior Member
Join Date: Nov 2004
Location: Vacaville, CA
Posts: 2,272
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Re: Anybody running a big block with th400 and 3.55 gears?
I would run a 3.08 or even a 2.73 to let that big block live a long and healthy life. The 3.55 will not help much to lower the RPM's. I have a 402 in my 56 Chevy truck with a 400 trans and a 2.73 rear end from a 79 Seville and it works great. You could also have a 700R4 built up for the big block and it would work the best. I have a 4L80 in my 68 crew cab with a 454 and 3.73 gears which works great but also have 35" tires on it. I am running about 2000 RPM at 70 miles an hour and getting about 12 miles to the gallon with a lifted truck.
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#3 |
Registered User
![]() Join Date: Jan 2011
Location: Chattanooga TN
Posts: 6,208
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Re: Anybody running a big block with th400 and 3.55 gears?
DIA of your rear tires?
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Chad 1967 C10 SWB - Project Savannah - 6.0/4L80 *Currently underway* 1968 C10 SWB - TOTY 2018, 50th Anniversary Tribute Project * Sold * Pride and Joy 1986 Silverado Short Fleet - Scarlet *Sold* 1985 Silverado Short Fleet *Sold* 2022 Jeep Gladiator Mojave 2001 Jeep Cherokee XJ 4x4 - Lifted, Built 1992 Jeep Cherokee XJ 4x4 - Lifted 2013 Honda Accord EX-L v6 Coupe 6spd (wife's ride) |
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#4 |
Registered User
Join Date: Sep 2005
Location: Canton North carolina
Posts: 3,079
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Re: Anybody running a big block with th400 and 3.55 gears?
I swapped the 3.08 gears in my 71 to 3.55 and not impressed, wish i had gone with 3.73
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67 blue step side- bb 68 gold and white swb cst anniversary 68 custom swb hot rod Ls swap 69 swb cst 6 cylinder/ overdrive 69 black/black cst bb swb 70 orange and white cst Canada built truck swb bb 71 black and white super swb bb 71 red and white super swb 72 green and white super swb 4x4 62 nova wagon 400 series Ls swapped 68 Camaro 327/4 speed- 1 owner, original paint 66 Volkswagen bettle 69 auto stick Volkswagen Beetle |
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#5 | |
Senior Member
![]() Join Date: Feb 2001
Location: New Madison, Ohio
Posts: 21,365
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Re: Anybody running a big block with th400 and 3.55 gears?
Quote:
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A husband can be right...or...A husband can be happy. ![]() 67-72 Chevy and GMC Trucks...The Classic Truck for the Classic Folk. ![]() 1970 CST Two tone green, 402BB, 400 Automatic, Tach, Buckets, AC, AM-FM, Tilt, GM CB, GM 8 Tract, LWB, etc ![]() ![]() JOHN 17:3...The better side of "LIFE" Remember: Everyday is a good day...Some are just gooder! |
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#6 |
Registered User
Join Date: Aug 2010
Location: Edwards, CA
Posts: 7,503
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Re: Anybody running a big block with th400 and 3.55 gears?
3.07 was stock for BB powered C/10 trucks. My suburban would get 17mpg freeway @65 on L78 tires. That was a pretty much stock but tuned engine with a good air cleaner. With a 502 I'd imagine you are doing a lot better than the 402/210hp. So if you want mileage step up to a 2.73 or consider an o/d trans. swap or a Gear Vendors O/D.
__________________
Thanks, Tim * VIN/Model Decoders * Power Team Charts (engine/transmission/gear ratio) * Post Your Pickup SPID ** Blazer SPID ** Suburban/Panel SPID * RTFM ... Read The Factory Manuals... download 'em here Highlanders ** Do you have a 1972 Plaid Pickup? ** Plaid Blazer ** Plaid Suburban |
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#7 |
Account Suspended
Join Date: Jan 2007
Location: Tucson
Posts: 2,183
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Re: Anybody running a big block with th400 and 3.55 gears?
[QUOTE=Racie;5324138]I'm about to do some upgrades to my rearend in my 68'. Right now I have an open 3.73 setup. I'm cruising on the highway with normal flow of traffic at 3500 rpms.QUOTE]
I had the same problem but went a different way as I really wanted to keep the high speed starts but still get better mileage and less noise on the freeway. My Truck is running a 572 BB with a TH 400 trans giving the following ratios 1st 2.48 2nd 1.48 3rd 1.0 The diff is a 4.10:1 posi coupled to a gear vendors 0.7:1 or 1:1 overdrive The tires I use are 285/60R17's with a 30.46 diameter which gives final drive ratios of 5.94 in OD or 8.94 @ 1:1. that's 2340 rpm @70 mph in OD or 3340 RPM @ 1:1 Much quieter in OD but i can still burn the rear tires all the way through first and second. |
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#8 | |
Registered User
Join Date: Aug 2010
Location: Edwards, CA
Posts: 7,503
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Re: Anybody running a big block with th400 and 3.55 gears?
Quote:
__________________
Thanks, Tim * VIN/Model Decoders * Power Team Charts (engine/transmission/gear ratio) * Post Your Pickup SPID ** Blazer SPID ** Suburban/Panel SPID * RTFM ... Read The Factory Manuals... download 'em here Highlanders ** Do you have a 1972 Plaid Pickup? ** Plaid Blazer ** Plaid Suburban |
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#9 | |
Registered User
Join Date: Sep 2008
Location: Dallas,TX
Posts: 328
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Re: Anybody running a big block with th400 and 3.55 gears?
[quote=Fitz;5325768]
Quote:
Posted via Mobile Device Last edited by Racie; 04-22-2012 at 06:52 PM. Reason: Spelling |
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