3.42 or 3.73 with TH-350
12-10-2018, 02:15 PM
#1
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3.42 or 3.73 with TH-350 It looks like I'm finally going to build the differential on my car and I'm having a hard time deciding between 3.42's or 3.73's for my 8.5 10 bolt. I understand that the cruising RPM is going to be within a few hundred of each other. The car will have a 275/60/15 tire on it and the engine is a H/C/I small block chevy with a 3600 stall with a TH-350. I've read that stalled automatic cars don't pick up much from a gear change which is why I'm asking if the 3.73's would be worth it or if I can save a few hundred RPM at cruising speeds with 3.42's. The car currently has an open rear with 2.73's and it just takes way too long to pull through gears to the point I'm going through the traps in 2nd and the down low performance isn't there. The car is not a daily and is a street car.
Thanks for the input.
Thanks for the input.
12-10-2018, 02:40 PM
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It's too bad that nobody makes a 3.55 for the 8.5", that would be a nice compromise here.
Having said that, the 3.73 wouldn't be *too* bad with that 28" tall tire, but I would probably still go with the 3.42 since you don't have an OD or lock-up (assuming it's not a TH350-C trans).
FWIW, I have a 3.08 in my 8.5" with a TH350/355ci, but I only have a 26.5" tire. I would prefer a 3.23 or 3.42 (though the former wouldn't even be worth the gear oil to swap), but I doubt I'd go to a 3.73 with this setup.
Having said that, the 3.73 wouldn't be *too* bad with that 28" tall tire, but I would probably still go with the 3.42 since you don't have an OD or lock-up (assuming it's not a TH350-C trans).
FWIW, I have a 3.08 in my 8.5" with a TH350/355ci, but I only have a 26.5" tire. I would prefer a 3.23 or 3.42 (though the former wouldn't even be worth the gear oil to swap), but I doubt I'd go to a 3.73 with this setup.
12-10-2018, 05:21 PM
#3
Taking in the entire post details.
Currently a 2.73 open rear, so carrier and gears WILL BE changed. The debate will be 3.42 or 3.73.
The car is not a daily and is a street car. Sounds like a weekend street warrior. For me it would be 3.73s'.
BUT going thru the traps in 2nd at what ??? RPM currently and with the mentioned tire size, would 3.73s' put too high a rpm thru the traps.
Didn't mention what brand 3600 stall, stall slippage/efficiency could vary with brand and effect rpm thru the traps.
2.73s' to 3.42s' would provide a 25% torque increase to the rear wheels, 2.73s' to 3.73s' would provide a 37% torque increase to the rear wheels.
Currently a 2.73 open rear, so carrier and gears WILL BE changed. The debate will be 3.42 or 3.73.
The car is not a daily and is a street car. Sounds like a weekend street warrior. For me it would be 3.73s'.
BUT going thru the traps in 2nd at what ??? RPM currently and with the mentioned tire size, would 3.73s' put too high a rpm thru the traps.
Didn't mention what brand 3600 stall, stall slippage/efficiency could vary with brand and effect rpm thru the traps.
2.73s' to 3.42s' would provide a 25% torque increase to the rear wheels, 2.73s' to 3.73s' would provide a 37% torque increase to the rear wheels.
12-10-2018, 06:26 PM
#4
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Thread Starter
It's too bad that nobody makes a 3.55 for the 8.5", that would be a nice compromise here.
Having said that, the 3.73 wouldn't be *too* bad with that 28" tall tire, but I would probably still go with the 3.42 since you don't have an OD or lock-up (assuming it's not a TH350-C trans).
FWIW, I have a 3.08 in my 8.5" with a TH350/355ci, but I only have a 26.5" tire. I would prefer a 3.23 or 3.42 (though the former wouldn't even be worth the gear oil to swap), but I doubt I'd go to a 3.73 with this setup.
Having said that, the 3.73 wouldn't be *too* bad with that 28" tall tire, but I would probably still go with the 3.42 since you don't have an OD or lock-up (assuming it's not a TH350-C trans).
FWIW, I have a 3.08 in my 8.5" with a TH350/355ci, but I only have a 26.5" tire. I would prefer a 3.23 or 3.42 (though the former wouldn't even be worth the gear oil to swap), but I doubt I'd go to a 3.73 with this setup.
Taking in the entire post details.
Currently a 2.73 open rear, so carrier and gears WILL BE changed. The debate will be 3.42 or 3.73.
The car is not a daily and is a street car. Sounds like a weekend street warrior. For me it would be 3.73s'.
BUT going thru the traps in 2nd at what ??? RPM currently and with the mentioned tire size, would 3.73s' put too high a rpm thru the traps.
Didn't mention what brand 3600 stall, stall slippage/efficiency could vary with brand and effect rpm thru the traps.
2.73s' to 3.42s' would provide a 25% torque increase to the rear wheels, 2.73s' to 3.73s' would provide a 37% torque increase to the rear wheels.
Currently a 2.73 open rear, so carrier and gears WILL BE changed. The debate will be 3.42 or 3.73.
The car is not a daily and is a street car. Sounds like a weekend street warrior. For me it would be 3.73s'.
BUT going thru the traps in 2nd at what ??? RPM currently and with the mentioned tire size, would 3.73s' put too high a rpm thru the traps.
Didn't mention what brand 3600 stall, stall slippage/efficiency could vary with brand and effect rpm thru the traps.
2.73s' to 3.42s' would provide a 25% torque increase to the rear wheels, 2.73s' to 3.73s' would provide a 37% torque increase to the rear wheels.
The car is going through the traps at 5000-5500 @ 115 with a 27 inch tall tire and 2.73's in 2nd gear. I shift the car at 6500 so I'm in no way going to run out of gear in the 1/4 with 3.73's if I ever get the urge to go again. Closest track is a solid 40 minutes away.
The converter is a Precision Industries with an STR of 2.5
37% increase is super tempting. I just don't want to hate it on the highway or feel like I left some on the table with 3.42's.
12-10-2018, 06:36 PM
#5
I have a t400 with 3.73's. My converter flashes to 3900rpm. I went from a 26" tire to 28" tall recently and my car feels slower, especially wot from a roll. In a full run it's quicker than ever on my g-tech but only because of the traction the 28x14.5 qtp's have given me. I can definitely feel it will gain a lot from giving it some more gear back, but I'm not swapping out the 3.73's for now as I'm going to be adding much more HP.
Also the highway cruise rpm from the 28" tires is so noticeable that the car feels much more stock, it's much quieter.
With the t350/400 you don't have that very low 1st and 2nd gear like a 4l60, to not slow down the car as much if you're running tall rear gears. I'd go 3.73's minimum if your tires are 28" tall.
Also the highway cruise rpm from the 28" tires is so noticeable that the car feels much more stock, it's much quieter.
With the t350/400 you don't have that very low 1st and 2nd gear like a 4l60, to not slow down the car as much if you're running tall rear gears. I'd go 3.73's minimum if your tires are 28" tall.
12-10-2018, 06:44 PM
#6
Since you've taken it to a track 40 minutes away and evidently driven it there with the 2.73s',what was the rpm at the speed you were going to get there ?
As in my post #3,the percentages of torque increase also apply to rpm at mph. Whatever your rpm answer is, multiply that figure by 1.25 to get rpm with 3.42 and by 1.36 to get rpm with 3.73.
As in my post #3,the percentages of torque increase also apply to rpm at mph. Whatever your rpm answer is, multiply that figure by 1.25 to get rpm with 3.42 and by 1.36 to get rpm with 3.73.
Last edited by FirstYrLS1Z; 12-10-2018 at 07:48 PM.
12-10-2018, 09:31 PM
#7
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That was my thought too, a 3.55 would solve this. The transmission is a th-350 so no lock up converter. Thanks for the information!
Correct, I will either be getting an Eaton Posi or a TrueTrac for a carrier.
The car is going through the traps at 5000-5500 @ 115 with a 27 inch tall tire and 2.73's in 2nd gear. I shift the car at 6500 so I'm in no way going to run out of gear in the 1/4 with 3.73's if I ever get the urge to go again. Closest track is a solid 40 minutes away.
The converter is a Precision Industries with an STR of 2.5
37% increase is super tempting. I just don't want to hate it on the highway or feel like I left some on the table with 3.42's.
Correct, I will either be getting an Eaton Posi or a TrueTrac for a carrier.
The car is going through the traps at 5000-5500 @ 115 with a 27 inch tall tire and 2.73's in 2nd gear. I shift the car at 6500 so I'm in no way going to run out of gear in the 1/4 with 3.73's if I ever get the urge to go again. Closest track is a solid 40 minutes away.
The converter is a Precision Industries with an STR of 2.5
37% increase is super tempting. I just don't want to hate it on the highway or feel like I left some on the table with 3.42's.
Having said that, the actual measurable acceleration difference in your setup between 3.42 and 3.73 isn't going to be anything huge. Those percentages above might sound like a lot of difference, but you wouldn't see huge changes in ET by swapping between those two ratios (assuming all else is equal, including traction). Gear changes within this relatively small range "feel" like a bigger deal than they actually are when looking at real world track results (especially if stall speed is optimized for the setup).
Sounds like your tire height and shift points would certainly allow for the 3.73, so it all just comes down to how you'd feel about your typical cruising rpms with either gear.
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12-12-2018, 07:26 AM
#8
On The Tree
Thread Starter
I have a t400 with 3.73's. My converter flashes to 3900rpm. I went from a 26" tire to 28" tall recently and my car feels slower, especially wot from a roll. In a full run it's quicker than ever on my g-tech but only because of the traction the 28x14.5 qtp's have given me. I can definitely feel it will gain a lot from giving it some more gear back, but I'm not swapping out the 3.73's for now as I'm going to be adding much more HP.
Also the highway cruise rpm from the 28" tires is so noticeable that the car feels much more stock, it's much quieter.
With the t350/400 you don't have that very low 1st and 2nd gear like a 4l60, to not slow down the car as much if you're running tall rear gears. I'd go 3.73's minimum if your tires are 28" tall.
Also the highway cruise rpm from the 28" tires is so noticeable that the car feels much more stock, it's much quieter.
With the t350/400 you don't have that very low 1st and 2nd gear like a 4l60, to not slow down the car as much if you're running tall rear gears. I'd go 3.73's minimum if your tires are 28" tall.
Since you've taken it to a track 40 minutes away and evidently driven it there with the 2.73s',what was the rpm at the speed you were going to get there ?
As in my post #3,the percentages of torque increase also apply to rpm at mph. Whatever your rpm answer is, multiply that figure by 1.25 to get rpm with 3.42 and by 1.36 to get rpm with 3.73.
As in my post #3,the percentages of torque increase also apply to rpm at mph. Whatever your rpm answer is, multiply that figure by 1.25 to get rpm with 3.42 and by 1.36 to get rpm with 3.73.
I tend to be a person who doesn't prefer a lot of gear, I'd rather make it up with higher stall speed (which works well in applications which utilize a lock-up feature) - especially with a trans like the 4L60E, which has a comparatively aggressive internal 1st gear of 3.06:1. But with a 2.52 1st gear in a TH350, I'd generally prefer a little more gear there as well.
Having said that, the actual measurable acceleration difference in your setup between 3.42 and 3.73 isn't going to be anything huge. Those percentages above might sound like a lot of difference, but you wouldn't see huge changes in ET by swapping between those two ratios (assuming all else is equal, including traction). Gear changes within this relatively small range "feel" like a bigger deal than they actually are when looking at real world track results (especially if stall speed is optimized for the setup).
Sounds like your tire height and shift points would certainly allow for the 3.73, so it all just comes down to how you'd feel about your typical cruising rpms with either gear.
Having said that, the actual measurable acceleration difference in your setup between 3.42 and 3.73 isn't going to be anything huge. Those percentages above might sound like a lot of difference, but you wouldn't see huge changes in ET by swapping between those two ratios (assuming all else is equal, including traction). Gear changes within this relatively small range "feel" like a bigger deal than they actually are when looking at real world track results (especially if stall speed is optimized for the setup).
Sounds like your tire height and shift points would certainly allow for the 3.73, so it all just comes down to how you'd feel about your typical cruising rpms with either gear.
12-12-2018, 08:05 AM
#9
So using the average of 2250rpm,3.42s' would be about 2800 and 3.73s' would be about 3050. Both of which are below half way of your 6500 shift point.
Back in the days of the 60s'/70s',rear gears of up to 4.88 were available from the factory and we didn't have any overdrive gears,yet we still didn't hesitate to jump on the freeways.
Back in the days of the 60s'/70s',rear gears of up to 4.88 were available from the factory and we didn't have any overdrive gears,yet we still didn't hesitate to jump on the freeways.
12-12-2018, 02:01 PM
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Thank you for all for the information. I'd love to put an over drive transmission like a 4L60 in at some point but for now I'm going to work with the TH-350. I'm going to do a 3.73 gear because the difference between the two will be small and since the car is a weekend car / just for fun I want the car to be just that, fun. I don't plan on taking it on any road trips and will only see highway times of 20 minutes at the most unless I get an urge to go back to my local track.
12-17-2018, 07:52 AM
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12-17-2018, 10:17 AM
#14
I helped my brother in law build his 68 camaro years back. Same exact tire size and we went with 3.73's. He was usually hovering around 3 grand even on the highway, never going much faster than 60mph. I think 3.73 is the way to go, but that's about as tall as I'd go.
Close friend bought a 69 Camaro with 3.73's. He didn't like the rpm so went with 3.08's. Thing was noticeably slower and ran shitty. Switched to a dual plane intake and edelbrock 650 carb, which got it's manners better but didn't help much in the speed department. Dunno the specifics with your combo, but remember that when you start going with heads, a cam and intake your gonna want to up the gears as well. I'd go 3.73's.
Close friend bought a 69 Camaro with 3.73's. He didn't like the rpm so went with 3.08's. Thing was noticeably slower and ran shitty. Switched to a dual plane intake and edelbrock 650 carb, which got it's manners better but didn't help much in the speed department. Dunno the specifics with your combo, but remember that when you start going with heads, a cam and intake your gonna want to up the gears as well. I'd go 3.73's.
12-17-2018, 01:58 PM
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I helped my brother in law build his 68 camaro years back. Same exact tire size and we went with 3.73's. He was usually hovering around 3 grand even on the highway, never going much faster than 60mph. I think 3.73 is the way to go, but that's about as tall as I'd go.
Close friend bought a 69 Camaro with 3.73's. He didn't like the rpm so went with 3.08's. Thing was noticeably slower and ran shitty. Switched to a dual plane intake and edelbrock 650 carb, which got it's manners better but didn't help much in the speed department. Dunno the specifics with your combo, but remember that when you start going with heads, a cam and intake your gonna want to up the gears as well. I'd go 3.73's.
Close friend bought a 69 Camaro with 3.73's. He didn't like the rpm so went with 3.08's. Thing was noticeably slower and ran shitty. Switched to a dual plane intake and edelbrock 650 carb, which got it's manners better but didn't help much in the speed department. Dunno the specifics with your combo, but remember that when you start going with heads, a cam and intake your gonna want to up the gears as well. I'd go 3.73's.
As mentioned above I have 3.08s in mine right now, though with a shorter (26.5") tire. It definitely doesn't run shitty this way (heads/cam/intake all modified, looser converter as well), but I do agree that a bit more gear (I'd pick 3.42) would be preferable due to the numerically low 2.52 1st gear of the TH350. The accelerator pump circuit doesn't offer a ton of adjustability on the Edelbrocks (as compared to Holley), so not a lot of options to tune out any issues from a gear ratio that might be less than ideal for the engine's powerband when using that carb. Also, if this was a manual trans car then you don't have the option of playing with stall speed as part of this matrix.
