Hello,
I'm wanting to change out the rearend of my 87 El Camino SS, to something with a posi unit. I don't know which to get, the 86-88 had 3:73 and the eairler ones had 3:43's, am I right? I don't know what mine has at the moment, but I'm tired of when I punch it I get the one wheel spin. When I swap what else will I need to do? Do I need to recalibrate the speedo? Anything else? The engine has a mild buildup (rv cam,intake,headers, dual intake,ZZ4 chip) any thoughts? Thanks for any help.


August

The 86-88 MCSS had 3.73 and 83-85 MCSS had 3.42. You'll need to change the driven gear to get the speedo to read correctly. Here are a few sites to get information on how to do it.

TCI Automotive Speedometer Information (http://www.tciauto.com/tech_info/speedo_gears.htm)

Performance Automotive and Transmission Center (http://www.transmissioncenter.net/2004rtip.htm)

:mrgreen:

Ernie,
Thanks for the reply, what I was also looking for was which gear ratio would suit me better. I know thats a loaded question, but fro example how much of an RPM rise would it be from stock to 3:43 and then how much more if you went to 3:73? At freeway speeds, would MPG go way down if I went with the 3:73 vs the 3:43, or are we only talking a couple of hundred rpm's? Thanks fro any info.


August

That change will make a few hundred RPMs difference in the 60 MPH speeds and it will lower the top speed accordingly. It will lower the gas miliage accodingly also. Would your engine act better with it? Well, that would possibly be yes but it depends on what your power band is and transmission.

Final drive ratio X Rear End Ratio X 168 X Forward Speed/Tire Radius

Fd * Rr *168 * Sp/Rt = RPM

So with a THM 350 or 400 Final drive being 1:1 , Tire diameter of 13" (26" overall) @60 Mph with a 3.43 RPM= ~2659

With the 3.73 gear RPM= ~2892 Larger tires lower RPM.

With a 700R4 your final cruise RPM @60MPH is ~ 1914 RPM
With a 200r4 your final cruise RPM @60Mph is ~ 1809 RPM
These were with the 3.43 you have now and you'd launch like you had about a 5.60:1 rear gear ratio. :o

I think the Camaros with the LS1s' have the 4L60E (700R4 Computer controlled) with a 3.73:1 rear end. The Buick Grand National had the 200R4 in them as did some of the Vettes at the time or so I understand.

...what I was also looking for was which gear ratio would suit me better...how much of an RPM rise would it be from stock to 3:43 and then how much more if you went to 3:73?
My 87 El Camino has a 2.41 rear and my 86 MCSS has a 3.73 rear. They both have 235/65/15 tires (27.02" height). At 65 MPH the Elky is at about 1300 RPM and the MCSS is at about 2000 RPM. The Elky gets around 22 MPG and the MCSS gets about 15 MPG. A 3.42 rear would be at about 1900 RPM.

Try this RPM calculator:
Interactive RPM and MPH Correlation Program By Bowling & Grippo (http://www.bgsoflex.com/rpmmph.html)

Or, check out my RPM Calculator:
Malibu Nirvana Tire Height and RPM Calculator (http://www.malibunirvana.com/technical/tire_diameter/index.html)

:mrgreen:

My 87 El Camino has a 2.41 rear and my 86 MCSS has a 3.73 rear. They both have 235/65/15 tires (27.02" height). At 65 MPH the Elky is at about 1300 RPM and the MCSS is at about 2000 RPM. The Elky gets around 22 MPG and the MCSS gets about 15 MPG. A 3.42 rear would be at about 1900 RPM.

Try this RPM calculator:
Interactive RPM and MPH Correlation Program By Bowling & Grippo (http://www.bgsoflex.com/rpmmph.html)

Or, check out my RPM Calculator:
Malibu Nirvana Tire Height and RPM Calculator (http://www.malibunirvana.com/technical/tire_diameter/index.html)

:mrgreen:

According to your calculator your a little bit low on your estimate.

Differential Gear = 3.42 to 1
Transmission Gear = 1.00 to 1
Tire Diameter (Inches) = 27.02
Vechicle MPH = 60
Computation Results:
Computed Engine RPM is 2553

Your Elky should be more like
Differential Gear = 2.41 to 1
Transmission Gear = 1.00 to 1
Tire Diameter (Inches) = 27.02
Vechicle MPH = 65
Computation Results:
Computed Engine RPM is 1949 @ 65

And the MCSS

Differential Gear = 3.73 to 1
Transmission Gear = 1.00 to 1
Tire Diameter (Inches) = 27.02
Vechicle MPH = 65
Computation Results:
Computed Engine RPM is 3016

That is a big difference from 2000 even at 55 your spinning better than 2500 RPM. Larger diameter tires changes the overall RPM but not a lot they do throw off the speedo. So a gear change should be followed up with a new Speedometer gear(s), could be drive or driven and...

Anyway I'm sorry man but taller gears result in RPM and at some point the ability to drive "Comfortably" at highway speeds. My limit is probably somewhere around 3000 RPM with my exhaust system. Sometime just after 3000 my exhaust really start to bark loudly. The only way to have your cake and eat it too is to move up to overdrive and then that final drive (Transmission Ratio) can be reduced. 200R4 is .68 the 700R4 is .71/72 (something like that) the T56 Viper transmission is .50 the T56 GM 6 speed is .68 (I think) both of those are standard six speeds.

Want a killer machine, Go with a T56 (Viper Version) and 5.88:1 rear, 1 through 4 is pretty much all for fun, 5th gets into the overdrive at .7 and then 6th is .5. Need a 12 bolt or a 9" for that. Put only 420 or so ponies up front and you own it. :D

According to your calculator your a little bit low on your estimate.
No Pete, my estimates are correct. Both of my vehicles came stock with the 200-4R transmission. The fourth gear is not 1:1 as you assumed. It is actually 0.674:1.

:mrgreen:

:oops: Alright I assumed you had the typical 350 thing going on. So now it all makes since.

Sorry I doubted you! :D I didn't know the Elky had ever came stock with an overdrive. But I do like the 200R4 over the 700R4 as far as gear ratios but the 700R4 is a little bit stronger internally.


I checked recently at a local transmission shop to see what a well built 200R4 was going to cost me and to my surprise "Installed" I was quoted $950.00 and a warranty came with it. I made it clear that I would beat it up when I got the feeling and they say "Not a problem!" So my next upgrade to the 76 is to add the 200R4 and a 4.10:1 rear end gear and redo all the bearings and seals in the rear. My 2.73:1 rear gears really are nice on the freeway but lacks severely in the "Stop Light Fun Factor." At 60 my elky is turning 2100 RPM which is good miliage RPMs.

Sorry I doubted you! :D

I didn't know the Elky had ever came stock with an overdrive.
That's OK, but don't let it ever happen again. :D

The 200-4R overdrive transmission (MX0) was an optional transmission for the 1985 El Camino. Starting in 1986 the the 200-4R became the standard transmission for the El Camino.

:mrgreen:

Guys,
Thanks very much for the info, it has been most helpful. The "fun factor at the stop lights" just isn't the same. My milage already sucks I get about 10-11mpg, still can't fighure that one out, will be putting in a new O2 sensor to see if that helps. So if I go with either 3:43 or 3:73 I should be ok. I mainly do some highway but mostly street driving, so light to light is important, I also like to keep the 14" rims as it keeps it looking stock. At highyway speeds Rpms for 60 are around 1800 to 2100 so it would not hurt to much to go up some. Again thaks for the info.

August

I installed 3.23s in my stock rear . I am pleased with the performance increase (I had 2.73s). Highway cruising isn't to bad.

Tommy,
How come you went with 3.23's instead of a higher ratio? I'm still trying to make up my mind as to which ratio to go to. Thanks for the info.


August

With my turbo 350 trans, I wanted a gear that was good off the line but still give me decent mileage on the highway. Most of my driving now is 65 mph or better. I did alot of research first to find the best compromise gear for me. It came down to 3.08s or 3.23s because mileage would suffer to much with 3.42s.

I put a 700R4 behind the big block in my 74. It has 373 rears. I replaced the TCI turbo 400 because it was killing me on the highway down here in FL. About 3000 at 60. With the 700R4, it drops the highway rpm a bunch but with the 308 1st gear in the tranny, the final drive is about 12 to 1 which is too low for me. The tranny is in 2nd gear in about 10 feet unless you really launch. I think 342 is the right gear if you have an od tranny because you'll still have about a 9 or 10 to 1 final drive in first, but you won't lug the motor at 60 or 65. If you have a 350 or 400, I think 323 is the way to go for the same reasons. You can find all of the gear ratio data and various calculations at the TCI website. Just remember that when you are doing calculations that include tire diameter, you need to use the loaded radius, not the unloaded diameter that the specs show. Calculate the loaded radius from the rev per mile spec for the tire. If you compare the two, you'll find that the loaded radius is less that 1/2 of the diameter, but the loaded radius is the actual measurement of the tire when it is rolling down the road which is what you need to calculate speedo gears and rpm.

Bgstk,
Thanks for the info, I think I have finally decided on the 3.42 gear ratio. Now I just need to find a rear end, I would like to just swap the whole rearend rather than mess with ring and pinion if I can. I'm in search of a rearend :oops: at the salvage yards. I'm sure I'll run across something soon enough.



August

August,
I just changed the entire rear in my 85 with a 1984 Monte 3.42. It was an easy afternoon project for me and two other guys, with air tools. Going from the stock 2.41 to this posi is a word of difference, especialy at the street lights. Good luck with your search.

Chris,
Lucky you, I'm still looking for a rearend out here :(. Anything I should look out for when doing the swap?


August

August,
Some things to do while the new rear is off are 1. replace fluid. it is easier to remove and install the filler plug when the rear is off. (I went right to a Chevy dealer and he set me up with posi additave and oil). 2. Spray the bleeders and brake line fittings w/ WD-40. This will help out once it is installed and you connect to the brake system. It's also a good time to evaluate the quality of your springs and u-joint. I replaced my shocks but used the original springs. In the future I might get a set with a little more height to rake the car a little. Since I used the original control arms I did not have to deal with the exhaust pipes being in the way with the uppers. I could not justify spending $600+ on adjustable control arms.
Good luck in the quest.

First August, what year Elky do you have, and what if anything do you plan on doing to the motor?

It's an 87 SS, has headers, cam, intake, chip, custom exhaust, will shortly be getting some head work done and a new cam, a Comp Cam HE268. I have to keep everything smog legal here in CA.

As many of us here are aware of, the stock GM 7.5" rearend in 5th Gen Elkys, definately leaves something to be desired! You can look for a GM 8.5" out of a Grand National, Hurst Olds, or a 442...good luck, if you do find one your probably going to pay $700-$800 if it's in half way decent condition. The end result is that you'll have a rearend that can probably handle a 400 HP motor, but much more than that and your pushing it. I have decided to go with a 9" Ford rearend (Yeah, yeah, I know what your thinking, "OMG NO! :mad: a Ford rear in a GM") Sounds horrible even when I say it to myself. But I have decided to build this brute strength rearend capable of withstanding any punishment I can give it, (454 w/Super T10) AND on a budget...half the price (or less)of Curries or Mosers! This limits me to no more than $1250. I am purchasing the housing this coming week...and it has already been modified to bolt into 78-87 GM G bodies. This rear will not get installed until it's time to swap the entire drivetrain. If any members are interested you can contact me, if your also thinking of this swap, and I can either point you in the right direction or answer questions or maybe answer mine if you've already done this swap.