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My 84 SC has a 305 with the TH350 and no posi, so I'm I correct assuming I'm running 3.42 gears?

It's a bit of a loaded question since I'm wanting to pull the trigger on a SP383 Chevy crate engine - 435 hp (at 5,600 rpm) and 445 lb-ft (at 4,600 rpm) at the crank. I plan to replace the stock intake with a dual-plane intake given the smallish cam specs (222/230-degree duration, 0.509/0.528-inch lift, and a 112-degree LSA). I think this will really wake up the engine in the lower rpm range where I'll do most of my driving.

I'll be using the car for shows and Sunday street driving with some light to light street chirping along the way to make some noise, but no drag time and I'll see minimal highway driving. I plan to keep the TH350 but have it reinforced by a local shop.

All this in mind, I'm going to need to either bolster the 7.5 rear end (with a girdle and maybe upgraded axels) and add posi or upgrade the whole thing with a crate rear end.

I'm thinking given all the above and the 3 speed tranny, my best options for gearing is 3.42 or 3.50 or at most 3.73. Any advice or suggestions? Pros and cons of each set up?

Thanks so much for everyone's help in advance as always!

Dave


'84 SC - Hot Chocolate Metallic exterior, Dark Briar Brown interior, orig. 305ci, headers, custom dual exhaust
'02 SS Pacecar - competition yellow (TOTALLED)
2014 Porsche Panamera 4S
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Most likely the 85SC rear gears are low 3s or even a 2.73. The SPID sticker on the underside of the trunk lid may tell what is in there. Look the ones that start with letter G. Such as G80 is posi. GU4 could be 3.08 ring and pinion.

You try this, if a non-posi rear, with one rear tire off the ground count how many times the driveshaft rotates to make the tire rotate one time.
Bob

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The stock LG4 Sport Coupe came with a TH250C three speed and 2.29 standard gears, unless it was ordered with the G92 "performance ratio" 2.73 gears. Their wasn't much you could do to beef up the TH250C back in the day - it doesn't use friction elements for 2nd gear and has a thin band that was prone to snapping with much more than stock power levels. There may be something now that can be done to them, but we always swapped them out for a TH350 or TH400 (depending on the power level).

If you go with another rear-end, I ran a 3.89 with 28" tires (TH400) and it wasn't all that bad for limited highway use - that's about the equivalent of 3.55 gears with 26" tires. I personally wouldn't want to run more than 3.73s with 26" tires with a 3-speed. My old IROC had 3.23 gears running a TH350 with 26" tires and I never gave a second thought running 70-plus on the interstate with it. Ran high 1.6s 60ft times and would pull the front tire at the track with slicks also.

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Originally Posted by BadSS
The stock LG4 Sport Coupe came with a TH250C three speed and 2.29 standard gears


This is why these forums are awesome. Thank you! All these years I thought I had a TH350 when I actually have a crap TH250 under there. haha

Best bet for me is to track down a rebuilt stout TH350 I guess to mate up with the SP383. More $$$! Unless someone else has advice on a better option?

I have 25" tires so I guess nothing higher than 3.55 with a 3 speed makes sense?


'84 SC - Hot Chocolate Metallic exterior, Dark Briar Brown interior, orig. 305ci, headers, custom dual exhaust
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Here's a good calculator to use for setting up gearing for a car. The one at the top of the right column I use all the time.
http://www.wallaceracing.com/Calculators.htm

Before the purchase of the 86 SS I needed a daily while I shopped for an SS. An 85 CL was bought, pretty, gold colored, nice coffy ride, would cruise all day long at 75 on the way to Disney in FL. It had the 305 and a TH250C, and it was a dog. Only had it for a year and never checked the rear gear. If it had a low 2 that would explain it's lack of get up and go.
Also had an 85 G20 Chevy full size van, 305 with the TH350C, it came with a 2.73 in a corporate 10 bolt. For that van two junk yard 12 bolts were bought, one for the 3.31 gears and the other to have two good axle, 40 bucks invested and I did the work. Van didn't see much over 60 on the highway but the 3.31 made it a much better around town driver.

Currently running a 4.11 in the 86, but double OD fixes that,
Several decades ago ran 4.88 in the Nova with a short tire. You never forget 4500 rpm at 70 mph.

Picking rear ratio is important, gotta know where the vehicle is going to spend it's time and where the new engine is at a happy cruise rpm.
Bob

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Originally Posted by doublemonte
Thank you! All these years I thought I had a TH350 when I actually have a crap TH250 under there. haha

No problem. Well,,, it got you this far and would probably continue to keep shifting if you stayed stock. But, yeah, unfortunately a beefier transmission for the SP383 will be needed.

If you don't have the 2.73 gears and do have the 2.29s, then the effective gear ratio in second gear would be 3.48:1. Close enough to 3.55 that you could ease it into 2nd gear and run it up to highway speed to give you an idea of how the new gears would act in 3rd. Shouldn't be too bad, but with 25" tires, that's about the same as 3.73s with 26" tires. Plenty have run that combination for years!

Good luck with the new transplants!

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HI Dave,

According to the parts book, you only had 6 choices of gears in the 1984 Monte Carlo: 2.29, 2.41, 2.73, 3.08, 3.42, 3.73. It is good to see that you didn't have the option of 2.14.
http://gmpartswiki.com/getpage?pageid=173119
http://gmpartswiki.com/getpage?pageid=173121

The 2.29 (GH2) was paired with the TH200C and TH250C.
The 2.41 (GU1) was paired with the TH200C and TH250C.
The 2.73 (GU2) was paired with the TH200C, TH250C, and TH200-4R
The 3.08 (GU4) was paired with the TH250.
The 3.42 (GU6) was paired with the TH350.
The 3.73 (GT4) was paired with the TH200-4R.

You either have 2.29, 2.41, 2.73, or 3.08 gears. The 3.42 and 3.73 would have been only used in the SS model.

Marc

Last edited by T5montecarlo; 04/03/21 12:12 AM.
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Marc, thanks for posting those speedo gear charts - awesome info. You are correct - those were the only gears available in the 84 Chevy G-bodies.

However, Monte V8 Sport Coupes either had 2.29 std or 2.73 optional and El Caminos came with the 2.41 std or 3.08 optional (with the three speed TH250s). Those two with 200R4s were only available with 2.73s.

Both the V6 Monte and Elky came standard with 2.41 with the TH250s and 2.73s were optional. If you got one of the California V6s with a TH200, only the 2.73 gears were available.

Also, as you stated the SS either had 3.42s with the TH350 or 3.73s with the 2004R.

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Just for possible future reference. The 1985 Monte V8 Sport Coupes with a 3-speed (now only a TH200C) came with 2.29 std or 2.73 optional - El Caminos came with the 2.41 std or 2.73 optional. The MC with 2004Rs came with 2.41 std or 3.08 opt and the Elky came with 2.56 std and 3.08 optional.

The V6 Monte (3-speed onlyTH200C) came standard with 2.29 (no option) - Elky came with 2.41 with 2.73 as an option. If you ordered a 200R4, MC came with 2.41 and a 3.08 was optional - Elky came with 2.56 with 3.08 as an option.

The SS only listed 3.73s with the 2004R in 85.

In 86, everything stayed the same except you could no longer get a 3-speed in a MC with a V8 (still could with an Elky).

In 87 there were no changes from 86.

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Just want to share my experience. Although the car I had was 85' Gutless, it came with 307 Olds with TH-200, 2.14 out back.

First thing that went as soon as I got the car from previous owner was the TH-200. Reverse went out first. It work when engine was cold, but after minute of engine running, it wouldn't reverse at all. Soon, lost forward gear. Swapped it out to TH-350, which I was about to sell, from my previous car, 79' T/A (another nightmare car because previous owner installed aftermarket sunroof). TH-350 was rebuilt unit using B&M kit, so it worked out OK with Olds 307, but I wanted overdrive so looked for 200-4R. Gear out back was 2.14, so wasn't too bad on freeway, but it was real pig. With Olds 307 and 2.14 gear, it was embarrassing to drive. Kinda like grandma driving, but I was trying my best to get up to speed.

While I was looking for 200-4R, I scored myself with posi rear-end from SS, unknown mileage from a friend. came with 3.73 gear, so it really woke up, even with Olds 307. But now, I was spinning closed to 4000 rpm on freeway, just to keep up with everybody. I really needed that 200-4R. Soon after, I found one and slapped into the Gutless without rebuild. Trans was slipping, but it really settled everything. This was the KZF I still have.

just my experience, and hope this help in some way to make your decision. And just the reminder, not sure about you guys, but our gas price went up about $1 since January of this year.

Last edited by FinallySS; 04/05/21 02:59 AM.

1986 Silver MCSS - Poly bushing and Hotchkis spring w/Bilstein, Posi w/3.73, MSD 6-AL, E-Brock 600cfm, Aluminum 2-row radiator.
1986 Black MCSS - R.I.P............Finally.
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Originally Posted by T5montecarlo
HI Dave,

According to the parts book, you only had 6 choices of gears in the 1984 Monte Carlo: 2.29, 2.41, 2.73, 3.08, 3.42, 3.73. It is good to see that you didn't have the option of 2.14.
http://gmpartswiki.com/getpage?pageid=173119
http://gmpartswiki.com/getpage?pageid=173121

The 2.29 (GH2) was paired with the TH200C and TH250C.
The 2.41 (GU1) was paired with the TH200C and TH250C.
The 2.73 (GU2) was paired with the TH200C, TH250C, and TH200-4R
The 3.08 (GU4) was paired with the TH250.
The 3.42 (GU6) was paired with the TH350.
The 3.73 (GT4) was paired with the TH200-4R.

You either have 2.29, 2.41, 2.73, or 3.08 gears. The 3.42 and 3.73 would have been only used in the SS model.

Marc

You gonna make me look at the SPID sticker in my '87 LS. Always thought it was 3.08s w/ TH-200-4R???
Can't for the life of me locate the SPID sticker on my '87 Caballero but since I had swapped the ECM out to a 1227747, ran some datalogs and didn't hafta tweak the fuel maps much since mostly stock...anyhoo based on the datalog rpm vs mph, it's 2.41s outback. Never looked to confirm but what a downer if it's a TH-250C under there as I had an opportunity to score a dual pattern Chevy/BOP TH-350 from a buddy of mine but let it go due to having overdrive on my mind.


1985 GMC Caballero 4.3L V6 TBI 200C w/ 2.41 peg leg
86 Monte Carlo SS BBC396 M20 w/ 3.42 posi screen name whip rust i.p.
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Hey Stew,

For comparison, a friend had a 1986 Caballero with a 305H. It had a limited slip 2.41 rear with a TH200C transmission.

The SPID sticker is either in the glove box or under the console lid, if equipped. Since it was stuck to plastic, they didn't adhere for too long. The next best thing would be the build sheet, which could be almost anywhere: above fuel tank, behind the door panel, above the headliner, etc.

I wonder if there is a website or service, or if GM can provide the sticker details for a given VIN?

Marc

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Mine is an '85 V6 base with 200C, code says 2.29 but previous owner must have swapped with a different car because it has a 2.14 and I can confirm, yes, it's a dog off the line. I've kept an eye out for an earlier SS rear end, hoping for a 3.42 Posi to come along for cheap. 3.08 or 3.23 would be a good gear as well for what I use it for.

One day I'll do the full swap and have OD and a performance gear (3.73 ish, still don't like winding it out on the highway), but in the meanwhile the 5.3 swap added ~60% more torque, which helped out that tall gear quite a bit...


Shawn

'85 MC with budget 5.3L swap, TH350 with stock 2.14 rear end
It ain't much off the line, but it's nice on the highway
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Pulled for easy reference from the full official GM (Caddy, Chevy, and Olds) vehicle information kits
https://www.gmheritagecenter.com/gm-heritage-archive/vehicle-information-kits.html

[Linked Image]

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That website is an excellent resource. Thanks Kevin.

Marc

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Hi Doublemonte,

I think those cam specs are actually kind of big for the street. It might yield a roughly 4,500rpm torque peak with 112 deg LSA. With that much cam, not only will a tall gearset not be much off the line, it actually might hurt gas mileage a bit too.

As far as typical acceleration times are concerned, you can probably progress from the current low-2s well into the 3s before you reach the car's fastest times (on the street, at any rate.) This assumes a 1:1 FDR, btw. Of course, with overdrive, you could go deeper into the 3s and not hurt overall fuel economy.

Speaking of which, does gas mileage matter? With a 222/230 cam, I presume it doesn't? If it does, then you definitely want to keep the gears taller rather than shorter.

If this were my car, I'd do two things: First, create a good engine model. Second, create a good overall car/drivetrain model. Then, you can explore acceleration under typical WOT scenarios and fuel economy to some extent as well.

HTH,
MAP

Last edited by MAP; 04/06/21 07:51 PM.
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Originally Posted by T5montecarlo
That website is an excellent resource. Thanks Kevin.

Marc

No problem - glad to share.

Just when you think you know about everything, something pops up that puts you in place - lol.

I noticed on the build sheet that the 84 SS listed an option for a 2004R. I always thought all the 84 SS's came with the TH350c and 3.42 gears. Digging around a bit, Wiki states there were three made with the 2004R. I suspect the option was added on the last production run for the 84 SSs and not many knew about it,,, or maybe GM sent those to their top dealers??? Anyway, one of those would be cool to find for sure.

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I also thought 1985 was the first year for the TH200-4R with any engine in the G body.

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Originally Posted by T5montecarlo
Hey Stew,

For comparison, a friend had a 1986 Caballero with a 305H. It had a limited slip 2.41 rear with a TH200C transmission.

The SPID sticker is either in the glove box or under the console lid, if equipped. Since it was stuck to plastic, they didn't adhere for too long. The next best thing would be the build sheet, which could be almost anywhere: above fuel tank, behind the door panel, above the headliner, etc.

I wonder if there is a website or service, or if GM can provide the sticker details for a given VIN?

Marc
I looked and could not find the SPID sticker for the Cabby oh which BTW it's an '85^^^noticed my typo. It's taken me this long (not a huge priority lol) to crawl under to look so confirmed TH-200C which recently I learned shares the same 1st gear as a TH-200-4r. Of course that all changes with the numerous options of rearends and transmissions I have here. lol

The chart^^^posted above does list 3.08s as an option for '87 LG4 and I did confirm with the SPID behind the glove box.


1985 GMC Caballero 4.3L V6 TBI 200C w/ 2.41 peg leg
86 Monte Carlo SS BBC396 M20 w/ 3.42 posi screen name whip rust i.p.

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