Rear end ratio and acceleration

[rb92673]

I believe my track 745 has a 3.54 rear end. I also have a six speed CD009 in it. Would changing to a 3.73 or 4.1 have a noticeable improvement in acceleration?

My shift light is set at 6000 RPM, I think fuel/spark cut is set at 6400. Most tracks I see 5th gear once or twice a lap. Only a few times have I seen top of fifth gear and that was at Willow Spings at about 117 MPH. I have never used sixth. 3rd and 4th are my most used gears. We race Sonoma and Buttonwillow, sometimes Thunderhill. Tuned to about 200 RWHP.

My transmission ratios are:
1 3.794
2 2.324
3 1.624
4 1.271
5 1
6 0.794

 

[JohnMc]

Whatever acceleration you gain might be offset by marginally more shifting?

 

[msweeney]

Yes it will. You will end up shifting more, but as long as you're snappy with those upshifts it will offset, and carrying a higher gear has far less penalty as the final drive gets shorter (it's coincidentally almost always faster to convince amateur racers to carry a gear higher pretty much everywhere and make it work, but that's a different post).

Also, with those three tracks and 200whp, I bet 3rd and 4th are about all that get used!

 

[2manyturbos]

Going to 3.73 gears is a 5% increase in torque multiplication. Going to 4.10 gears is almost 16%. So, yes, you will see a significant improvement in acceleration. Luckily, both those ratios come in complete axles that already have G80 lockers in them. It takes just over an hour to swap out a complete rear axle.

 

[shoestring]

Yes it will. You will end up shifting more, but as long as you're snappy with those upshifts it will offset, and carrying a higher gear has far less penalty as the final drive gets shorter (it's coincidentally almost always faster to convince amateur racers to carry a gear higher pretty much everywhere and make it work, but that's a different post)

I'm listening.

 

[rb92673]

Yes it will. You will end up shifting more, but as long as you're snappy with those upshifts it will offset, and carrying a higher gear has far less penalty as the final drive gets shorter (it's coincidentally almost always faster to convince amateur racers to carry a gear higher pretty much everywhere and make it work, but that's a different post).

Also, with those three tracks and 200whp, I bet 3rd and 4th are about all that get used!

Thanks. I try and shift less and just be smoother, and with an open diff it lessens wheel spin. At Buttonwillow it is pretty much 4th except for Off Ramp and Star Mazda. Sonoma is a little bit more shifting, especially with traffic. 3rd at 4, 7, 9a, and 11, sometimes 5th coming into 9a. 5th will get used a lot more if I go to a 4.10.

Going to 3.73 gears is a 5% increase in torque multiplication. Going to 4.10 gears is almost 16%. So, yes, you will see a significant improvement in acceleration. Luckily, both those ratios come in complete axles that already have G80 lockers in them. It takes just over an hour to swap out a complete rear axle.

Thanks for those numbers. Are the 700/900 axles interchangeable on a 745?

 

[2manyturbos]

Thanks. I try and shift less and just be smoother, and with an open diff it lessens wheel spin. At Buttonwillow it is pretty much 4th except for Off Ramp and Star Mazda. Sonoma is a little bit more shifting, especially with traffic. 3rd at 4, 7, 9a, and 11, sometimes 5th coming into 9a. 5th will get used a lot more if I go to a 4.10.


Thanks for those numbers. Are the 700/900 axles interchangeable on a 745?

Yes, same exact axle housing. It will be a bolt in swap. If you seek one out the 92 and newer NA 940s come with the 4.10 ratio. 93 and newer should have a G80 carrier.

 

[VB242]

My 242 has a TR-3650 transmission with 3.37 first, 2.0 something second and a 8.8 rear with 4.10 and 26.5" tall tires, and 300+hp it's silly fast off the line. I want to install an Accusump or Moroso accumulator before I take it back to a real track. I think it's limited to 7700 rpm with soft cut at 7500. Also have Enem V16T cam.

 

[msweeney]

Thanks. I try and shift less and just be smoother, and with an open diff it lessens wheel spin. At Buttonwillow it is pretty much 4th except for Off Ramp and Star Mazda. Sonoma is a little bit more shifting, especially with traffic. 3rd at 4, 7, 9a, and 11, sometimes 5th coming into 9a. 5th will get used a lot more if I go to a 4.10.


Thanks for those numbers. Are the 700/900 axles interchangeable on a 745?

Fix the open diff while you're in there. It's going to make a HUGE difference. Do something other than welded.

 

[Boxtoy940]

I believe my track 745 has a 3.54 rear end. I also have a six speed CD009 in it. Would changing to a 3.73 or 4.1 have a noticeable improvement in acceleration?

My shift light is set at 6000 RPM, I think fuel/spark cut is set at 6400. Most tracks I see 5th gear once or twice a lap. Only a few times have I seen top of fifth gear and that was at Willow Spings at about 117 MPH. I have never used sixth. 3rd and 4th are my most used gears. We race Sonoma and Buttonwillow, sometimes Thunderhill. Tuned to about 200 RWHP.

My transmission ratios are:
1 3.794
2 2.324
3 1.624
4 1.271
5 1
6 0.794

Do you have a 1041 Rear End and if so, are you interested in selling the 3.54 Ring and Pinion GEARS if it turns out that is Actually what you have?

 

[283SD]

I ran 3.90 gears in my V8 2 door wagon when I first built it, 370+HP. Powerglide {2 speed}. It would shift out of low @ 64 MPH in under 4 seconds.
Drive topped out at 118MPH. Yes the higher # gears will go faster sooner! but you limit your top end. If you have a standard trans you will shift a lot, trade off, top end RPM, vs comfortable cruising speed. I now have 3.31 gears. Low shifts out at 75 MPH {in just over 5 seconds} top end is 146 MPH. But I have a V8, choose your preference.

 

[rb92673]

Fix the open diff while you're in there. It's going to make a HUGE difference. Do something other than welded.

It is certainly on the list.

 

[rb92673]

Do you have a 1041 Rear End and if so, are you interested in selling the 3.54 Ring and Pinion GEARS if it turns out that is Actually what you have?

No idea what rear end. I have never seen a tag on the axle that is legible. I probably wont sell either, it will probably go in the spare parts hoard. You never know what is going to break at the track.

I ran 3.90 gears in my V8 2 door wagon when I first built it, 370+HP. Powerglide {2 speed}. It would shift out of low @ 64 MPH in under 4 seconds.
Drive topped out at 118MPH. Yes the higher # gears will go faster sooner! but you limit your top end. If you have a standard trans you will shift a lot, trade off, top end RPM, vs comfortable cruising speed. I now have 3.31 gears. Low shifts out at 75 MPH {in just over 5 seconds} top end is 146 MPH. But I have a V8, choose your preference.

This car is not street legal, and it only drives on track. It will never have much more than 250 horsepower for fuel economy, and it lives at the top end of the RPM range all day long. If my calculations are correct, top speed in 6th at 6000 RPM is 135+ with a 4.11. I don't think I have enough power or aerodynamics to get to 135 MPH.

This is how the car lives it's life:

We were short shifting due to a severe case of overboosting above 5000 rpm which has been fixed.

 

[JohnLane]

Looks like fun OP!

4.1 final drive ratio will multiply torque and make ratios closer for each shift.

 

[2manyturbos]

The 4.10 will multiply torque, however, it doesn't change the step between gears at all.

 

[volvosruinedmylife]

I have a cd009 in my 242. It has 3.73 gearing. I love it. It changed the car completely. My 242 Is a street/,track car, and I love the tight gearing.

 

[rb92673]

Ok I pulled an axle from a 94 940 with G80 locker and 3.73 ratio. Now to modify the locker, any suggestions on how to modify for track use only? Any recommendations on gear oil?

 

[towerymt]

This photo is post-cutting of the counterweight thingy. But I recall a recommendation of welding that shaft in place, somehow. Because I think what happens is that weight still finds a way to contribute to problems...either with 1wd, or in our case, I believe the weight / shaft failed somehow so we got pieces of metal flying around inside the axle, we had 1wd, and we had some nasty chips in the pinion gear...but not a total failure.

I was always against a welded diff, but after I road-raced one in our lemons car, I didn't mind it. We raced on some tight courses like Summit Point Shenandoah where we might see 2nd gear, and the welded made a huge difference in traction compared to the modified G80 that just wasn't 100% reliable 2wd. There was curbing there too (video above reminded me), and that might upset the G80 a bit, but the welded was not affected at all, obviously.

The downside that we found with the welded diff was the guy that did the welding did a poor job. We had a failure of the welds during a race, but we fixed it with a borrowed 110v at the track, and it kept on going (after cleaning a bit of debris out of the housing).

I never road raced any other diffs, so that's all I have to say about that. I had a pow-r-lok clutch diff in my autocross car that I liked, but it needed more preload. It was a lot better than the front-axle (non-high-preload?) truetrac, though....those are junk unless you're mostly going straight.

 

[rb92673]

This photo is post-cutting of the counterweight thingy. But I recall a recommendation of welding that shaft in place, somehow. Because I think what happens is that weight still finds a way to contribute to problems...either with 1wd, or in our case, I believe the weight / shaft failed somehow so we got pieces of metal flying around inside the axle, we had 1wd, and we had some nasty chips in the pinion gear...but not a total failure.

I was always against a welded diff, but after I road-raced one in our lemons car, I didn't mind it. We raced on some tight courses like Summit Point Shenandoah where we might see 2nd gear, and the welded made a huge difference in traction compared to the modified G80 that just wasn't 100% reliable 2wd. There was curbing there too (video above reminded me), and that might upset the G80 a bit, but the welded was not affected at all, obviously.

The downside that we found with the welded diff was the guy that did the welding did a poor job. We had a failure of the welds during a race, but we fixed it with a borrowed 110v at the track, and it kept on going (after cleaning a bit of debris out of the housing).

I never road raced any other diffs, so that's all I have to say about that. I had a pow-r-lok clutch diff in my autocross car that I liked, but it needed more preload. It was a lot better than the front-axle (non-high-preload?) truetrac, though....those are junk unless you're mostly going straight.

Thanks. I ended up just taking some weight out of it. Mostly I wanted to try 3.7 instead of 3.5 gearing.

 

[culberro]

Ok I pulled an axle from a 94 940 with G80 locker and 3.73 ratio. Now to modify the locker, any suggestions on how to modify for track use only? Any recommendations on gear oil?

I don't think a g80 is the appropriate response to fixing an open diff. Even with mild track day abuse I've never had them be consistent or reliable.
It'll work for a bit but my guess is one endurance race per g80, going from my experience with them.