rear axle change 331 to 391

Old January 27th, 2019, 08:41 AM
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rear axle change 331 to 391

Hi, this is a post i had done on aug 27 2016 there has been a change and i need some info on the 1/4 mile rpm ??

Hello, thinking of doing a upgrade to our 67 442 car, right now we have a chevy 331 12 bolt posi in it, were thinking of going to 355 or even 373 the car seems to need a little better gear, we are in the country in only gets out like 4 times a year and we like to bring it to rock falls and run the car on the track we don't care if she tacks up cuz like i said we don't really go anywhere with it, ill try here to list as much info as i can to see if any body has some suggestions on a good gear for it, we have a 455 +40 in our 67 442 right around 1000 miles on it, jm 18-20 cam 260/268 488/496 Ka iron heads coan converter 2800 stall turbo 400 trans with shift kit, L69 3x2 we have 65 jets in the center and 70 jets on the ends, w/z manifolds, with cut outs, best run closed all the way to the back 13.64 at 99 mph, we have not opened it up yet... any suggestions would be much appreciated, thanks john.....

jan 27 2019 the only thing here that has changed is we uncorked the car and ran 13.34 at 100.22 i know i was supposed to get back to you all with the upgrade, well it s finally going to happen this year, but with one change, i picked up a 1969 442 391 12 bolt for it, this is what were going to put in it this spring, we were going to put 373 in the chevy 12 bolt but are going this way, iam just a little concerned about getting to the end with it, i checked with this website
cwawlpedia.com and used there calculator with my 26x10x15 slicks, and 391 gears, i figure the car for what it is will only hit 102 mph there calculator shows it will cross at 5154 rpm, i read that a stall converter 2500 to 3000 will have about 200 to 250 slippage, so that puts me at 5404 that's about it for the car about 5400 is when i shift it now with the 331 if there are any opinions out there on all of this i sure like to here them, thanks john..

Last edited by john mann; February 23rd, 2019 at 09:37 AM.
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Old January 27th, 2019, 09:05 AM
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I think your ET will be better. 5400 is close but really think 5200. Jmo
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Old January 27th, 2019, 09:15 AM
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Jmo, I don't think it'll make much difference. What Muncie is in it M20 or M21? 3:31 is a good street/strip gear.
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Old January 27th, 2019, 09:28 AM
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Rock Falls, Wisconsin ? Those "calculators" are just a guide, the resulting numbers aren't written in stone. 3.91 shouldn't be to bad, except for convertor stall. Traction might be your biggest problem. Are you running air bags ? What rim width on the rear wheels ? What tire pressure do you run ?
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Old January 27th, 2019, 09:59 AM
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i have run 4.10s with a th400/ 2800 converter behind a 455 but with 28.5 tall slicks.rpms (about 5200) were not a problem at the far end. i think you will need a taller tire to launch tho.
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Old January 27th, 2019, 10:10 AM
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Kennybill the car has a turbo 400 in it with a 2600-2800 coan stall converter, not a 4 spd. OLDSter Ralph Yes that would be the place were we go, rockfalls in Wisconsin, the car hooks right up no spinning, it runs 1.85 to 1.90 60 ft times, no air bags, just air shocks let down to nothing, 9" wide aluminum rims, 16 lbs in the slicks,
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Old January 27th, 2019, 10:16 AM
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jcdynamic88 i do have a brand new set of mickey Thompson 255-60-15 ET drag radials there 27" tall ! i do really like the slicks, just cant find any that are 27" tall !
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Old January 27th, 2019, 10:58 AM
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Originally Posted by john mann View Post
OLDSter Ralph Yes that would be the place were we go, rockfalls in Wisconsin, the car hooks right up no spinning, it runs 1.85 to 1.90 60 ft times, no air bags, just air shocks let down to nothing, 9" wide aluminum rims, 16 lbs in the slicks,
John, I haven't been to Rock Falls for many years. I would like to get over there this summer. You won't spin much with 3.31 and slicks.

I am not impressed with air shocks, all I ever saw from them was "wheel hop" on other cars. You would also need to vary air bags from left to right, I am thinking 10 in the right side, 5 in the left as a starting point.

You have pretty wide rims on the rear. I would experiment with tire pressure. Do a burn out and check uniform blackness across the tire marks. You want it uniform. If tire marks are lighter colored in the middle, add air.


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Old January 27th, 2019, 11:11 AM
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i have a brand new set of lakewood drag shocks i will put them in it !!
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Old January 27th, 2019, 11:23 AM
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"the car hooks right up,no spinning" rest assured leaving the line with 3.91s will be different than with 3.31.
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Old January 27th, 2019, 01:10 PM
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iam sure !! i will post what all happens, i will start with the slicks, see what happens, then next round we go there i will run the et drag radials the 255-60 15 27" tall ..
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Old January 28th, 2019, 08:33 AM
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Are you running aftermarket axles?
If you are not its a very good investment with a heavy car and slicks.
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Old January 28th, 2019, 08:54 AM
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Originally Posted by Bernhard View Post
Are you running aftermarket axles?
If you are not its a very good investment with a heavy car and slicks.
the 69 442 rear with 3.91s would/should be 31 spline axles from the factory
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Old January 28th, 2019, 09:39 AM
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Originally Posted by jcdynamic88 View Post
the 69 442 rear with 3.91s would/should be 31 spline axles from the factory

A 1969 axle should have 28 spline axles. pull the cover. The casting number on the carrier will tell you for certain if 28 or 31 axles

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Old January 28th, 2019, 10:44 AM
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from what i remember 3.42 and up were 31 spline axles from 67-69.maybe only 67-68 ?
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Old January 28th, 2019, 11:08 AM
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69 should only be 28 spines,sorry for interjecting incorrect info.only 67/68 can have 31.
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Old January 28th, 2019, 11:28 AM
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Originally Posted by jcdynamic88 View Post
69 should only be 28 spines,sorry for interjecting incorrect info.only 67/68 can have 31.

But this many years later parts were swapped. Rear could have been "removed" from a 69 car and have 31 spline axles
The last 31 spline 391 axle I had was removed from a 1970 he wanted 323ratio

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Old January 28th, 2019, 11:54 AM
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Hi all, its all stock even the axles, should have 28 spline axles, will be opening it up this weekend or next weekend, i will post everything on here !!
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Old January 28th, 2019, 12:02 PM
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the last 3.91 31 spline rear i got came out of a 72 monte carlo....special order i guess
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Old January 28th, 2019, 12:33 PM
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Originally Posted by john mann View Post
Hi all, its all stock even the axles, should have 28 spline axles, will be opening it up this weekend or next weekend, i will post everything on here !!
Good luck.. I Hope that axle is something you can utilize
cheers
b
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Old January 29th, 2019, 08:39 PM
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The numerically higher gear ratio will greatly help your 60 ft. any drop in the 60ft. will greatly reflect your 1/4 mile time. If you wanna go faster I would highly consider headers . Suspension helps a lot. ditch the sway bar and put 90/10 shocks in the front , replace the control arm bolts that allow you to double nut the bolts so you can just run them snug. free movement of the front suspension helps drop 60 ft. times (wont make street driving sloppy) . Any unsprung weight helps too so light front wheels will help, In the rear boxed lowers and long bolts sugged and double nutted helps with 50/50 shocks and an airbag in the right rear spring with 10lbs should help it shake out right. Are you manually shifting the TH400 ? There is gains to be had there too. launch method helps too. when I had a 2800 stall it liked to leave at about 1300 rpm. when I went to 3500 stall I could launch at 2200 off the brakes. Lots of little tricks to drop E/T. I think your car can easily run high 12's with your gear change and some simple yet effective tweaks.

I was running open headers years ago and my small block olds powered 72 cutlass wouldn't get out of the 14.0 range. I put an x pipe on it and ran a 13.86 through the mufflers vs 14.3 with the mufflers and regular pipes. I eventually added collector rings to my exhaust so I could remove the mufflers entirely off of the car and the open x pipe ran 1 tenth faster than through the mufflers and x pipe . Power steering eats up power alternator eats up power water pump eats up power. I run manual steering and only run the alternator and I have it on a cut off switch so once I hit the beams I shut it off so there is no load on the engine. The water pump gets turned by a simple yet effective moroso water pump drive.

My last engine combination (355 olds) started life in my car running 13.3's . It ended up running 12.20's with a bunch of tiny changes. Going from a vacuum secondary carb to a double pumper literally dropped 1 tenth. dropping the sway bar although its sounds laughable dropped .15 . Going from .040 to .015 lifter preload really helped my last time out testing the 3500 stall . although I was spinning the tires with the new 3500 stall ( now need suspension work) I still managed to to run a 12.20 with a horrible 60 ft. so I think there was a 12.0 left in it but I sold the engine.

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Old January 30th, 2019, 11:51 AM
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coppercutlass thanks for all your input, iam thinking the car should run like 13.0 to 13.10 at like around 102 just my thought, i was just hoping to cover the 1/4 mile with the 391 with out it nosing over !! wr1970 i think hes spot on at 5200 so i think ill be ok !!
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Old January 30th, 2019, 12:25 PM
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John, I think you'll be ok for RPM's. You may even exceed your ET and MPH expectations. I'd like to be there to see it.
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Old January 30th, 2019, 01:39 PM
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i think you are looking more like 12.8 @ 105
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Old January 30th, 2019, 02:47 PM
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OLDSter Ralph did you get my email ralph from 2 days ago john ?? jcdynamic88 i dont think the car has the HP to hit 105 maybe just maybe 102.5 i would be real happy with 13.05 at 102
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Old January 30th, 2019, 03:06 PM
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John, I only got the email telling me you would email later. I will check email in a few minutes.
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Old January 30th, 2019, 03:18 PM
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I think your combination can do high 12's. It just depends what you are willing to do. For me it was constant little changes it took 3 seasons to get a 12.2x when everyone said i needed more HP. All those little things add up.
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Old January 30th, 2019, 05:10 PM
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coppercutlass ya know i thing she just may have it in her, like you said a little bit here a little bit there !! iam a believer !!
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Old January 30th, 2019, 05:12 PM
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OLDSter Ralph iam going to let you know when we go to the rock this year ok !!! i sent you the email !!
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Old January 30th, 2019, 05:57 PM
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I doubt you make your way far enough south to go to great lakes dragway. But if you ever do let me know.
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Old January 30th, 2019, 06:39 PM
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Coppercutlass Fore sure ! thanks for all your input !! john...
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Old January 30th, 2019, 08:56 PM
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Only Olds and Pontiac had 31 spline axles in 1968 only period. look under all 68 H/Os.....
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Old January 31st, 2019, 04:17 AM
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Originally Posted by Gerald Nickels View Post
Only Olds and Pontiac had 31 spline axles in 1968 only period. look under all 68 H/Os.....
And to add I've broke Olds 12 bolt axels at the track on street tires.
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Old January 31st, 2019, 04:43 AM
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Originally Posted by VORTECPRO View Post
And to add I've broke Olds 12 bolt axels at the track on street tires.
were they 31 spline? also gerald 67/68 3.42 and higher ratios got 31 spline
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Old January 31st, 2019, 04:12 PM
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well seeing this and that i got the rear end on the bench, any suggestions on where i can get a pair of these 28 spline axles made in the good old USA !! Thanks john...
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Old January 31st, 2019, 05:47 PM
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Originally Posted by jcdynamic88 View Post
were they 31 spline? also gerald 67/68 3.42 and higher ratios got 31 spline
3.23 Olds 12 bolt factory stock with posi. I've broke 12 bolt Chevy axels as well.
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Old January 31st, 2019, 06:26 PM
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I would just stick an 8.5 in there. They can usually live into the mid 11 sec range on stock axles. The 12 bolt in stock form still takes money to make strong and its heavier. I know you already have it but on the rear end subject that's where I stand lol.
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Old February 2nd, 2019, 05:43 AM
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Originally Posted by coppercutlass View Post
I would just stick an 8.5 in there. They can usually live into the mid 11 sec range on stock axles. The 12 bolt in stock form still takes money to make strong and its heavier. I know you already have it but on the rear end subject that's where I stand lol.
plus,that 12 bolt is probably worth good money to someone restoring a real 442.no offense to the OP but the combo he has probably isn't going to break either one.
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Old February 2nd, 2019, 10:07 AM
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jcdynamic88 i concur !! its only a 13 flat car hopefully !! i think ill be safe, thanks so much for all your info !! everybody to that has replied !! john..
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Old February 23rd, 2019, 09:30 AM
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Hi, update !! ok the rear has been opened up now, here are some pics !! looks like a 1970 casing 40227 with 1967-68-69 axles in it with 28 splines ! the differential only had 1 quart of fluid in it, but it was super clean, the one axle has a break in the outside wheel bearing, that's where i think the differential fluid went out,







Last edited by john mann; February 24th, 2019 at 07:49 AM.
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