LSx - favorite diff ratio for autocross

[RebekahsZ]

My LS2/T56-M12 240z is a dual purpose car: drag and autocross. Please tell me about your gear ratios and if you like them or would change anything based on your experience. I'm hoping to get as many opnions as I can. I currently have a 3.90 and I know (think) I need to change ratios. The question I have is whether to go higher or lower? I'd love to go autox and make adjustments from there, but over the next couple of months I have too many family committments to get any autocrossing in. So, I'm bench racing....

 

Autocross specific:

Based on what I've learned so far, regardless of which gear I chose, I likely won't use 1st gear very much. IF this car is faster (in autocross) than it was with the L24, I'll have opportunities to get over into 3rd a fair amount (cause I was frequently tached out with the L24 in 2nd on straight aways and slaloms. My question is this: would it make sense to go even lower on my gearing to try to do my auto crossing in 3rd gear with an occasional straight back shift into 4th. Or, should I go higher (3.54 or more) in order to just wind out in 2nd with only an occasional akward shift up and over to 3rd?

 

I have searched and read several different listings. I look forward to getting your opinions. I tend to over-think every thing.

 

Edited to remove the word: "best."

Edited April 24, 2012 by RebekahsZ

[johnc]

You'll want to gear it for a second gear redline of 80 mph.

[JMortensen]

Two options: try to find a gear that puts 80 in the top of 2nd as John suggested, or try to find the one where you hit the end of the strip right at redline.

 

I know Cary has been using 2nd gear starts in autox and it would be great if you could drive the whole course in 2nd and not shift at all. Use a trans calculator and plug in your tire size to figure it out.

 

I expect you'll have to experiment at the drag strip to know what your trap speeds are going to be.

 

This calculator has the Camaro T56 ratios:

http://webspace.webring.com/people/cz/z_design_studio/

[RebekahsZ]

Thanks. Yeah, I was bench racing with one of those programs for a while last night. The 80mph goal helps - I really didn't know what rpm to plug in as a target. I'm gonna gear for autocross like you said because the driving is so technical. On drags, I kind of expect to be rowing thru the gears, so that will be whatever it is. I'm not trying to be national champ, just make a lot of noise and wander around the pits like I have a right to be there.

 

Is there someplace that specializes in changeing the ratios inside the T56? It would be great to make 1st a lot taller and have the jump from 5 to 6 closer....is this feasible?

 

Keith

[JMortensen]

I've heard of the Magnum T56, but I don't think they change the gear ratios. You really need separate gears on the counter shaft (or a custom counter shaft) to change the ratios, and I don't think the T56's have that and I'm not aware of anyone who makes such a thing.

http://www.keislerauto.com/

[RebekahsZ]

Does anybody make a 3.36 R200 or know what model I should search for that might have one, or do I just need to get happy with a 3.54 as the highest I can reasonably do?

[Leon]

Does anybody make a 3.36 R200 or know what model I should search for that might have one, or do I just need to get happy with a 3.54 as the highest I can reasonably do?

Look for a 260Z 2+2 with manual trans (good luck, I'd say it's the rarest US production chassis/drivetrain), or post a WTB ad. They come up from time-to-time.

[JMortensen]

79 280ZX 2+2 manual also. They are rare but they do come up.

Edited April 24, 2012 by JMortensen

[wfritts911]

I don't think you can really change the T56 gear ratios without pretty much getting a totally new transmission. As for hitting 80 in second, with your M12 gearset, you will need a numerically low gear. http://www.f-body.org/gears/ is a good gear calculator to work with. I just tested using 275/40-17 tires, which are about stock firebird height, 25.66 inches in diameter. And with a 6600 RPM redline, and your 2.07 2nd gear, you can hit 80 at 6500 rpms with a 3.0 rear gear. You would be a lot better finding a T56 with the 2.66/1.78 1st/2nd gear respectively like the F-bodys have. Using my transmission gear ratios, the same 6600 redline, and a 25.66 tall tire, you can hit 80mph at 6600 rpms with a 3.54 gear.

 

I'm not sure what the stock LS2 redline is, but 6600 RPM should be doable without any problems. But I would just play around with the gear calculator, but trying to reach 80 in second with your 2nd gear ratio will need a really numerically low gear.

 

-Will

[h4nsm0l3m4n]

It depends a lot on the tires you run. If youre stuck with 15s (somewhere around 275/35-15 race tire), theres not a lot you can do if you want to hit John's 80 mph target without spending a lot of money. A 3.36 rear end will get you to 65-70 mph (depending on you stock red line). A Cam/pushrod/valve spring kit may allow you to raise the red line to 7000+, which will get you 75+.

 

Upgrading to 16s or 17s will improve things a lot - with 275/40-17 (or, better, 315/35-17) race tires and a 3.54 rear end will get you 79 mph in 2nd gear. Whatever you end up doing take into account the tires you will end up using as they will change things quite a bit.

 

As I've mentioned before, I run 23x9 slicks on 15" wheels (more or less the same size as 275/35-15s). I start my car in 2nd and drive the whole course without shifting. It works very well and frees me from having one more thing to think about while I'm driving. In rare occurrences I bounce off the rev limiter but most of our courses are so tight its hardly ever an issue.

Edited April 24, 2012 by h4nsm0l3m4n

[RebekahsZ]

I really appreciate the input. OK: lets write off 1st gear and pretend that I have a Porche 914 transmission with the inverted shift pattern. How would I expect things to go if I kept the 3.90 gears and tried to drive the whole autocross in what is currently 3rd gear (pretending I'm in second). Heres my set up: 225/50/15 BFG R1 (24.5 inches), 2nd gear 2.07, 3rd gear 1.43, redline as needed. In slow turns I am usually busy - I have no idea how slow a slow turn is! What minimum speed and rpm should I be thinking about for most autocross tracks? It was really nice of johnc to give me a really simple max planning speed of 80mph.

 

I had been considering making my first diff my 3.70; now I' leaning toward the 3.90 instead, but planning to autox in 3rd instead of in second with a 3.54.

 

I'm honestly just trying to decide which of my diffs to put the Savage clutch disks in and build up to start with. Trying to have a decent first draft.

Edited April 24, 2012 by RebekahsZ

[74_5.0L_Z]

I have a Ford 5.0L Stroker in mine with a G-Force T5. I run Hoosier 275/35/15 tires at all four corners, and I have the 3.36 R200 differential.

 

When I had the GForce T5 built(after the stock T5 perished), I chose the common 2.95, 1.94, 1.34, 1, 0.59 gear spread that GForce offers. With this selection of gears, I often hit my rev limiter in second gear (6250 rpms and 66 mph).

 

I wish that I would have chosen their optional gear set which consists of the following spread: 2.42, 1.53, 1.23, 1, .73. With this gear selection first gear would be good to 59 mph and second gear would be good to 82 mph.

 

The GForce T5 can be made to fit the LSx engines, and is advertized to handle up to 500 lb-ft of torque and 600 hp ina 3200 lb car. What I like about them is that they only weigh 75 lbs.

[JMortensen]

Dammit Dan! Making me want a G-Force transmission now...

 

I'm actually in this same predicament with respect to the gears. I have a 4.11 R200 that I picked up thinking I was going to keep using my NA L6. My tires are 23.5" tall, and with a 6250 redline that puts me at 83 mph in the top of 3rd. So that will probably be my plan for the short term. Longer term that diff doesn't work out so well on a big track, because 5th tops out at 146 and 6th at 216. There are some tracks with LONG straights around here, so I'll be into the ridiculously tall 6th.

 

With a 3.54 2nd goes to 70 (not good) but 5th hits 169 (much better).

 

So my plan is to run the 4.11 for autox, start in 2nd or 3rd and then get a 3.54 for the track.

 

Dan, looks like a 4.11 puts you at 79 in 3rd and 3.90 which is easy to find does 83. I had to fudge the numbers on the calc a bit to come up with your redline speeds by changing the aspect ratio on the tires to 30.

Edited April 25, 2012 by JMortensen

[74_5.0L_Z]

The G-Force T5 is much stronger than the stock T5, but the weak link in these transmissions is still 3rd gear. So, at this point I plan to continue to use 2nd for the autocross.

 

Anyway, I am at the point where I think I am going to do fewer autocrosses and start doing track days. I love to autocross, but I would like to have more seat time.

[RebekahsZ]

This is fun - getting Ford and Chevy on same forum. I wish I had more experience as a driver but you guys are a big help. Keep talkin'.

 

See new WTB post in parts section for 3.36 and 4.11 R200 diffs, open or LSD. This would give me 3.54, 3.70, 3.90, 4.11 R&Ps in my shop. Hoping for a major car work binge (no sleep) this weekend.

Edited April 25, 2012 by RebekahsZ

[RebekahsZ]

Is a 4.38-ish longnose R200 available anywhere? What are all the ratios available?

[JMortensen]

4.38's came in the R31 Skylines IIRC. Stony here was importing them from Japan and selling them to ArizonaZCar, but I'm pretty sure that hasn't been going on for a long time now.

 

They do come up for sale occasionally or you might be able to find one overseas and have it imported.

[tube80z]

I think a lot of this depends on how high you rev your engine. In most of the cars I've ran we typically used a 3.70 to start with and launched in second and then used third for most of the course. I found that quicker for me than dropping to second and having to deal with managing a lot of torque at the rear wheels. It also depends on whether your willing to change diffs for track/street versus autox.

 

If you don't want to change diffs then I'd look at a lower ratio 3:55 or 3:36. If you're willing to change then maybe lower than what you have and start in second and go for third. As many others have written you want a gear that's good for 75 to 80. I wouldn't use a 4.38 for track days as they generate a lot more heat than a 4.11 or at least that's been what I've seen.

[johnc]

FYI... 5.14, 4.88. and 3.15 available new for the long nose R200.

 

http://kameariusa.com/L6_r200.php

[RACERZ]

From the stand point of RPMs normally you dont want to be at the top end of the range (depending on your set up) on most V8s peak torque is lower, by staying on the top end you will find you are dropping off the torque curve and the car will tend to be twitchy, keeping in in the middle of you torque curve will give you much better throtal responce and smooth out the car espically during transitions. We found this out when we changes form our AP Z to a BSP corvette. With a light weight chassie like the Z this will amplify the problems. You need to find what you torque curve is and how wide then find the gearing that will keep you in it. By gearing up 3.9+ it tends to put you into the(2,3,4th) gears which have broader ranges and therfore will utilize the V8s wider torque curve. For our vette we started with the 3.44s but found with the box we were using(stock gertrag) that the 4.11 was vastly better for most autoXs mover us up from 1-2 combinatios to 3-4 (mostly using 2nd in the tights and going to 3rd for long portions, excellent transitions and good acceleration onto long strights). Since the Z is so light like most other have said you will prob be able to run most of the course in 1 gear (big advantage of V8s) vs the inline 6s/4s. I would start buy pulling the numbers for you motor (if available) and start running gear combinations to see which gear ratio combinatios will keep you in the sweet range of you torque curve. Then you can change the diffs acording to the type of courses you run (tight vs long). Again like other have said looking at your transmission ratios and gaps between will determine which ratio fits you motors optimum torque. Just my humble 2 cents worth.