couple gm 7.5 rear questions. 700r4 gear swap from 3.73

[chumscustoms]

Hope to get some help. I got a 88 monte ss with a 26 spline 7.5 rear. I am curious about swapping a 7.5 28 spline carrier from a camaro s10. And then put a set of good axles in it. Will that work?



Question number two. I'm running a 350-400 horse sbc ramjet 350. I currently have 3.73 and was thinking of swapping to 4.10.
My trans is a 700r4.

Just curious what the gearing from first to second was like with 4.10 and also possible mileage drop opinions

[quarterbooty]

I wouldn't waste your time or money putting a 7.5 behind a motor with any appreciable power. You'll blow the ring gear apart backing out of your driveway.

[chumscustoms]

Understand. On a limited budget at moment and I have a 7.5 posi carrier but it's a 28 spline.

[SRD art]

What breaks small rears is putting a lot of power to it and hooking up on hard launches. In a Pro touring type car you probably won't experience that very much. I have the stock original GM 7.5 rear still in my wagon. Has over 100K on it an has had a 400+ ft lbs at the flywheel SBC 400 for maybe 30K+ miles of that. I've both drag raced it a lot on 275mm drag radials and autocrossed it several times. I drive it everyday and take some pretty hard corners on the way to and from work. I once had a parts car that I saved the rear from because everyone said I'd blow mine apart. That was about 4 years ago and I eventually threw the spare rear away. I have a 9" collecting dust that I'll narrow and put together someday so I can go big pony LS on the motor in the future. I knew a guy that had a low 13 second El Camino daily driver that put an aftermarket center and axles in his 7.5 and swore by it. Depending on how it's built that 700R4 will likely go out before the rear, unless you're regularly drag racing with slicks and a high stall converter.

If you already have the parts, you'd just as well put it together. If you drag race it a lot I would plan on going something bigger in time, so save some $ for that broken rear rainy day.

The 700 has a very low 1st gear. In a daily driver the low 1st would make 3.73s feel sort of like 4.10s. If you were me you'd keep the 3.73s for the better fuel mileage and still have decent acceleration. 4.10s are great if you plan on drag racing it regularly. I've heard again and again the optimum gearing for a pro touring with overdrive car is usually 3.50-3.73. Ultimately you don't gear a car because bigger must be better. You have to weigh your rpm range for what you're doing most, and gear the car accordingly. I have the original 2.41s behind a TH350 in the wagon. It does barely over 2000 at 60 mph on the freeway and gets decent gas mileage. The way it's geared I can run a full autocross in 1st gear while others are usually in 2nd. It runs mid 14's @ 97 mph with a fairly mild sbc 400 at 3000 ft altitude and I've had it to 145 mph. It doesn't accelerate from a stop that fast but other than that I like the tall gears. When I get my 9" put together I'll likely change to an overdrive trans and stick with 3.50s to keep the mpg as high as possible since this is a daily driver. But you're not me, so you gotta weigh what your goals for the car are and what kind of fuel mileage you wanna hit.

If you search the internet it will be easy to find charts and calculators to determine gear ratios compared to mph and rpms. In comparison, my Nova has 4.56 gears and a 28" tire behind a TH350, which is probably close to the feel of a 700R4 with 4.10s. At the track I was doing 6000 rpms right after the 60 ft mark at around 40 mph and got there in 1.6 seconds. Low gears use up 1st gear fast. On an autocross track 4.10s' 1st gear will be basically useless once you get going. Too low of a gear can also mean huge torque multiplication and spinning tires coming out of a corner. On longer tracks with 4.10s I'd imagine you may even get into 3rd gear which is a lot of shifting and possible loss of power range which could all translate into lost time. With the charts listed below, with 4.10s I figured you'd be done with 1st gear roughly around 30+ mph.

I don't claim to be the walking truth, this is just my experience, opinion and logic talking. Someone who has a similar type of set up may be able to share some more accurate experience.

Check these out- http://www.race-track.com/news.asp?TOPIC_ID=532

http://wahiduddin.net/calc/calc_speed_rpm.htm

[SRD art]

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[SRD art]

I got your PM but the pt server is being goofy and won't let me reply for some reason. I'm pretty sure the slightly larger Camaro center swaps right over. Check maliburacing.com and gbodyforum.com to verify that, I must've read it on one of those.