gear selection for 3 speed auto

[G_Body_Enthusiast]

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?

sure 3.06 x (rear gear here) x peak torque= torque applied to the tires in first (approximately!)
then 1.63 x rear gear ratio x peak torque = torque applied to the tires in second

lets say 3.06 x 3.23 x 190 (my peak torque)=1877 ft lbs.
1.63 x 3.23 x 190 =1022 ft lbs
a net loss of 855 ft lbs from first to second.

2.52 x 3.23 x 190=1546
1.52 x 3.23 x 190=932
614 net loss from first to second.

now if they shift at the same speed the 700r4 will be at a slightly higher RPM but as it has been mentioned before the torque curve will really effect how the gearing acts. but comparing first to first and second will give you more info on this. between the two trannys there is a net loss of 331 with the th350 while comparing second gear nets a loss of 90. larger numbers like torque will make that difference even larger though the % in difference will be the same so it wont be that noticeable for a stock engine, it would be for an engine cranking out 400+ ft lbs of torque.

3.06 x 3.23 x 400=3953
2.52 x 3.23 x 400=3255
almost 700 ft lbs of torque lost just from gearing or 21.41% gain in torque with the 700r4 depending on your point of view.

1.63 x 3.23 x 400=2105
1.52 x 3.23 x 400=1963
net loss of 142 ft lbs with the th350 and 8.75% gain in torque with the 700r4. i'd say that kind of loss is important and you would feel that.

there are other equations to this but i'm not going through them all here.

 

[drogg1]

My point was: given the same torque multiplication in first gear in both transmissions (i.e. different rear gears) the 700R4 will shift into second to a lower RPM than the TH350. This may not matter depending on your torque peak but as I have read, it's best to shift just below your engine's torque peak or at least close to it. Also, in second gear, the slight multiplication advantage the 700R4's second gear has is lost, if only slightly, due to the rear gearing of the TH350:

First to second shifts at 6,000 RPM

700R4
6,000 RPM -> 3,196 RPM
TH350
6,000 RPM -> 3,619 RPM

First gear

700R4
3.0 x 3.0 x 400ft*lbs = 3,600ft*lbs
TH350
2.5 x 3.6 x 400ft*lbs = 3,600ft*lbs

Second gear

700R4
1.63 x 3.0 x 400ft*lbs = 1,956ft*lbs
TH350
1.52 x 3.6 x 400ft*lbs = 2,189ft*lbs

Once again, this is all in interest of gaining knowledge, not to insult. I'm just a curious person I guess? Maybe it's the budding engineer in me...

 

[Phoenyx]

Well now my 3.73 geared car is getting 700R4. So now I will get to see what the gear drop is like lol.

 

[G_Body_Enthusiast]

giving one transmission a higher rear gear than the other makes for an unfair comparison of the performance of the transmissions.

give em both the same rear gear and redo your math. and dont forget to show your work for full credit.

besides if you really want to give one a lower gear give it to the 700r4 since it has an OD since that would allow for lower cruising RPMs. my point is that given the OD gear i can run a lower gear and do better with it than sticking with the th350 and using a higher gear. ultimately OD trannys make cars way more fun to drive since they can be geared much differently.

it's sorta like comparing apples and oranges to begin with since the OD gear gives the 700r4 an unfair advantage.

but tell you what, run the numbers for the 2004r, it also has an advantage in gearing over the th350 in first and second.

i'm considering off the line performance and cruising RPM and unfortunately with a non OD transmission you have to make more of a sacrifice in the off the line performance for better cruising RPMs than you would with the 700r4 or 2004r for that matter.

i am not much of a fan of the 700r4, i prefer the 2004r since it has a better OD gear and the ratios are closer but the first gear ratio isnt as high and rather than getting a 2004r for a daily driver im now leaning towards the 700r4 for the better first gear.

but be fair and compare them using the same rear gear for both since i could just as easily run a 3.23 behind a 700r4 as i could a th350. im not set on any one gear set which is why i compared them from my 2.41 up to 3.73 with the 700r4 the 2004r and the th350.

and again as was stated a properly built engine could overcome the RPM drop if it's built to have lots of low down grunt to begin with. as long as it stays in the fat part of the torque curve i dont see how it would that much of a problem Vs an engine that requires a higher operating RPM range. so yes if one was an idiot and had an engine that needed higher RPMs the 700r4 would make a BAD choice.

it's not just the gearing but the engine itself. im not interested in high revving engines, remember it's a mostly street/some what strip car so i want lots of low down torque. i just think the 3.06 gear gives just enough of an advantage to be worth using given how i want to set the car up for use engine wise.

either OD transmission will do better since i wouldnt have to compromise between off the line and cruising RPM as much as i would the th350. given how much gas is the OD gear would pay off over time whereas the th350 wouldnt so much.

 

[drogg1]

Mine point in comparing different rear gears was to eliminate the difference in torque multiplication between the two transmissions. That is to say, given they both are geared to have the same torque multiplication in first gear, the TH350 will shift into a better range and so forth as stated above. Which, as I see it, make the TH350 a better WOT accelerating transmission than the 700R4.

I do believe you are correct that the nature of the 700R4 being an OD transmission makes it better for highway cruising and gas mileage. But that brings up the 2004R as you stated. I believe that in comparing the 2004R and the 700R4, both OD transmissions, the 2004R has favorable gearing for both acceleration and mileage given an equal torque multiplication in first gear.

That's just how I see it. But I do agree with you that in a street car, I would do definitely go with either of the OD transmissions if I could afford to. From there it sort of depends on preference of RPM range and off-the-line performance.