gear selection for 3 speed auto

[79loserbluebu]

I've got 3.23 with my TH350. Around town it's great and on the highway it keeps up. I think I was probably around 70-75mph at 3000-3300RPM for a good half hour straight and it wasn't too bad. Made that same trip with 3.73's and it was unbearable. Around town though, the 3.73 is definately cool. My 350 is a smogged replacement engine and it launches pretty okay for being what it is.

 

[G_Body_Enthusiast]

thanks for the input, i needed to hear from people with different gears and their experiences with them.

while i love the 2004r i'm considering the 700r4 now since it has a 3.06 first gear so even if i go with a 3.08 gear i still get to 9:1 and that's just fine for me on the street. as i said the car is not going to be a daily driver as it once was so im having to really rethink the gearing of this car. originally i was going to use a 3.42 or 3.73 gear and a 2004r transmission since it has a slightly better OD gear than the 700r4 BUT that has changed so i'm looking for better acceleration and the 700r4 would do that but still give me an OD gear for highway use if i needed to use the car for that purpose. highway use is going to be minimal for the car though, at most it'll see a few miles going back and forth to the track on the highway and while i could get there on city streets it would take much longer given where the track is.

as it has been mentioned if i switch i dont want to regear the car so i'm thinking 3.23 or 3.08 is the highest/lowest i'll go most likely since it will give better performance while still allowing lower RPMs on the highway with an OD transmission. it would just be too much to have it regeared with a rebuild on the rear and a transmission at the same time in terms of cost and i certainly dont want to put an OD transmission in front of a 2.41 gear, the operating RPM would be horrible and it would suck even if off the line performance picked up a little so i need to do the gear first IMO.

3.06 x 3.23 = 9.88 and that's a good number for street/strip performance. 2.52 x 3.23 = 8.14 (rounded to the nearest hundredth). quite a difference no? either way i'll get better acceleration and that's all that really matters. i think i'll go with a 3.23 gear.

with the rear i'm looking to rebuild it with new axles, bearings, differential, gear set, everything needed to make it like new. and disc brakes while everything is apart, save some time by doing it all one time, not twice.

of course using the 700r4 opens a can of worms for me. i want to use a BOP engine so i'd need to use an adapter plate to make it work and cut the drive shaft, more money for a transmission swap. being a budget minded car enthusiast is kind of a oxymoron isnt it?

 

[79loserbluebu]

If you are going to do all that, I'd try looking for an 8.5" just incase you find one for a steal without the internals or with an open carrier. If not though, a worked up 7.5" would surely handle what you have right now

 

[Phoenyx]

A 700r4 isn't gonna give u better acceleration with that big jump between 1st and 2nd.

 

[G_Body_Enthusiast]

A 700r4 isn't gonna give u better acceleration with that big jump between 1st and 2nd.

i beg to differ:

GEAR RATIOS OF THE TH-7004R
FIRST GEAR SECOND GEAR THIRD GEAR FOURTH GEAR
3.06:1 1.63:1 1:1 0.70:1

GEAR RATIOS OF THE TH-350
FIRST GEAR SECOND GEAR THIRD GEAR FOURTH GEAR
2.52:1 1.52:1 1:1 N/A

even in second it STILL has a better gear than the th-350. RPM will be higher in 1st and 2nd for the 700r4 compared to the th-350 i have now. yeah the RPM drop will be "worse" for the 700r4 but the RPM comparing both 2nd gears shows the 700r4 as being slightly better.

http://www.700r4.com/faq/ratiocmp.shtml

go there and read. more gear, more torque being applied and we all know what more torque to the tires means.

also...

Gear ratios of the 200-4R are: 2.74 first, 1.57 second, 1.00 third, .67 fourth

still better gearing than a th350 in first AND second though not by much for 2nd. the 700r4 has a better 2nd gear than the 2004r for second which means slightly more RPM and slightly more torque being applied when it hits 2nd.

from a pure math perspective the numbers bear this out. either OD transmission will give better acceleration through more/better torque multiplication than the th-350 hands down.

 

[drogg1]

In defense of Phoenyx I would have to slightly disagree although its hard to really determine real-world acceleration from math.

The drop he is referring to is when shifting from first to second gear and is even further magnified by the fact that you are considering using 3.08 gears. If you did all of the math, you'd realize that jump might drop you out of your torque band as torque does not increase linearly. This all depends on cam specs and the RPM range you make peak power in. What I'm trying to say is if your rear gears and your first gear multiply out to be the same (for example 3.0:1 rear with 3.0:1 first or 3.6:1 rear with 2.5:1 first (rough numbers)) shifting from first to second in the 700R4 will shift to a lower RPM than shifting from first to second in a TH350.

I know however that you are choosing the OD trans and rear gears to be more street friendly so sacrifices would have to be made.

That's how I see it anyways. Feel free to correct me as I'm only using math and logic; not actually automotive knowledge.

 

[G_Body_Enthusiast]

IF i can get a 700r4 for pretty cheap i would go straight to 3.73, 4.11 maybe even 4.56 gears depending on the tire diameter.

i already know i can replicate similar RPM on the highway with a 3.73 gear and the .7 OD gear as the 2.41 non OD 1:1 gear i have now. 3.73 or 4.11 times 3.06 equals???? yeah ridiculous isnt it?

i've considered the factors of converter slippage, power loss from engine to tires, the build of the engine, etc. fact is 1.63 multiplies more torque than a 1.52 gear. 3.06 multiplies more torque than a 2.52, 2.48 or 2.74 gear. 1.63 multiplies more torque than a 1.52, 1.57 or a 1.48.

i also played with a lot of numbers and found the 700r4 has better torque multiplication by 21.4% over the th350 in first gear regardless of the rear gear being used. the 2004r has about 8.75% more than the th350 for first as well. in second gear the 700r4 gives about 7.48% more over the th350's second gear and the 2004r gives about 3.31% more over the th350's second gear.

and if you're dumb enough to run an engine and gearing that arent matched well then you deserve to have a crappy running car now dont you? but as i said i think the gear is needed first though easily i could go with the transmission first and do the gear second which would be the better way of doing it if i can get a 700r4 and get the swap done for fairly cheap (cheap being an extremely relative term)

i was mostly figuring originally in the thread on gearing with the th350 i have now. with a different transmission you have different options since the gearing is different from one to the next, especially with an OD gear.

but for the moment i'm weighing my options with this 3 speed since i never put much thought into gearing with a 3 speed, it's always been with a 4 speed OD since it allows for lower gears and more fun as a result.

make sense now? i have to weigh as many options as possible before making a decision since gearing can make a car fun or a nightmare to drive. i want to make sure i get it right and i will be talking to companies about matching parts for the engine to the gearing i believe i want along with the application of the car, etc. THEY will give me much better info about this and help me even more. right now this is a ballpark kind of thing. so i have to play with number for those 3 transmissions in all kinds of ways to see how they'll effect RPMs in final drive.

but feel free to add whatever you want to the discussion, im just saying what the numbers tell me, real world conditions effect the numbers and i realize that hence my need to get everyone's experience with different gears since all i've had in this car with this lousy 2.41 hunk of scrap metal they call a gear. but the numbers tell me the 700r4 would make a better street/strip transmission over the th350 hands down.

 

[jrm81bu]

Yes the gear ratio comparisons would seem to make the 700r4 better than a th350, yes you also run the risk of dropping out of your power band with the 1-2 shift. If the motor makes good power down fairly low it won't be a problem though.

 

[pontiacgp]

i also played with a lot of numbers and found the 700r4 has better torque multiplication by 21.4% over the th350 in first gear regardless of the rear gear being used. the 2004r has about 8.75% more than the th350 for first as well. in second gear the 700r4 gives about 7.48% more over the th350's second gear and the 2004r gives about 3.31% more over the th350's second gear.

these are interesting numbers....regarding the large difference between the 1st gear and 2d gear ratios in the 700 can that be calculated to see what effect that has on the torque?

 

[G_Body_Enthusiast]

Yes the gear ratio comparisons would seem to make the 700r4 better than a th350, yes you also run the risk of dropping out of your power band with the 1-2 shift. If the motor makes good power down fairly low it won't be a problem though.

given it will be mostly street, i want an engine with power down low since i'll be cruising on the street, right in the fat part of the torque curve. hopefully. i still need to look into cams once i select an engine. so i'm looking at largish cubic inches with a cam that gives fat torque just off the line, probably 1500-3500 is where i want the meat of it to be i think. but i'll have to talk about that another day in another thread. but at this point ill mismatch a little to get rid of the current set up, i can always rebuild an engine later to match the gearing, not too difficult.