Ben's Z Posted October 12, 2012 Share Posted October 12, 2012 Have a line on a 3.90 diff. Is this too much gear with a 77 5 speed with plans to run the stock turbo? Quote Link to comment Share on other sites More sharing options...
Leon Posted October 12, 2012 Share Posted October 12, 2012 https://www.google.com/#hl=en&output=search&sclient=psy-ab&q=3.9+turbo+site:hybridz.org&oq=3.9+turbo+site:hybridz.org&gs_l=hp.3...527.7184.0.7456.26.22.0.0.0.0.737.3791.3j8j3j0j1j1j1.17.0.les%3B..0.0...1c.1.bxx77lg3_wo&pbx=1&bav=on.2,or.r_gc.r_pw.r_qf.&fp=3711ae5bed1ccc88&bpcl=35277026&biw=1088&bih=563 Quote Link to comment Share on other sites More sharing options...
SleeperZ Posted October 12, 2012 Share Posted October 12, 2012 With a stock L28ET and my T5, 3.90 made 1st gear almost useless, and I had to shift into 5th to make to the end of the 1/4 mile. I like a 3.54 much better. Quote Link to comment Share on other sites More sharing options...
Six_Shooter Posted October 12, 2012 Share Posted October 12, 2012 With a stock L28ET and my T5, 3.90 made 1st gear almost useless, and I had to shift into 5th to make to the end of the 1/4 mile. I like a 3.54 much better. This is what I'm finding, though I have the later ZX trans, which I think has a taller 1st gear, so it's only almost useless. lol I have a 3.54 to go back in, once I get the modifications to the axles done that I need to do. I had an R180 3.54 previously and prefered that gear. Quote Link to comment Share on other sites More sharing options...
280zex Posted October 13, 2012 Share Posted October 13, 2012 I have a 1983 zx 5 speed and a 3.90 rear diff. I have a t3/to4e 60 trim turbo. first gear is useless, more so in the rain. 60 mph in 5th gear = 3000 rpm. My tires are 225-50x15. A 3.54 rear diff ratio would be way better... Quote Link to comment Share on other sites More sharing options...
ArchetypeDatsun Posted October 13, 2012 Share Posted October 13, 2012 (edited) I've got a 1981 280zx L28ET 5 speed T-5, T3/TO4E Turbo with 3.90 gears and 1st gear is also useless should have kept my N/A 5 speed with 3.54. Edited October 13, 2012 by 280zx@541 Quote Link to comment Share on other sites More sharing options...
240zphilly Posted October 16, 2012 Share Posted October 16, 2012 I would just opt for an lsd in whatever gear ratio and run a taller tire... However I'm strictly into drag racing and generally driving like a douchebag on the streets. If your use is different that may effect your decision. Quote Link to comment Share on other sites More sharing options...
78zstyle Posted October 18, 2012 Share Posted October 18, 2012 I have a 4.11 with a stock turbo. First gear is beyond useless. I'm going N/A. Not because of the diff. But now I don't need to change diffs, I'm a speed freak. Quote Link to comment Share on other sites More sharing options...
Gollum Posted October 19, 2012 Share Posted October 19, 2012 I'm running a stockish turbo motor with a 3.54 and later ZX trans. 1st gear was almost useless, and with my new curb weight, next time I drive the car it WILL be useless. That doesn't mean don't do the turbo swap of course... That's a stupid aspect to get caught up on. Just change the diff down the road when you get to it. Still a hoot to drive. Quote Link to comment Share on other sites More sharing options...
NewZed Posted October 19, 2012 Share Posted October 19, 2012 When you all are reporting "useless" are you talking about when accelerating only? Otherwise, when putting around town, off-boost, isn't the car essentially a low compression NA engine, which could use the same ratios as any other NA engine? If you get a diff that makes first gear useful for getting on the boost, won't the car be boggy in normal city traffic? Just wondering if I'm missing something about how the turbo engines work. The factory set the cars up with about a 12 ratio, first gear times diff. 3.54 x 3.321 (11.75), 3.9 x 3.062 (11.94). Quote Link to comment Share on other sites More sharing options...
SleeperZ Posted October 19, 2012 Share Posted October 19, 2012 Not boggy at all. The taller gears present load to the engine, so if your turbo is moderately responsive, you get better response with taller gears. There is a reason the turbo ZX got a 3.54 and the N/A ZX got the 3.90. Quote Link to comment Share on other sites More sharing options...
Gollum Posted October 19, 2012 Share Posted October 19, 2012 Yes and no. There's a reason the turbo cars came with the 3.54, not the 3.9 the manuals of the same years used. The Z31's were the same story. While the NA's had a 3.7 the turbo used 3.54, and when the NA switched to 3.9, the turbo switched to 3.7. Yes, a turbo engine when on low load isn't under boost, but even these OLD CRAPPY T3's can't stay out of boost when going up a hill on the freeway, no matter how hard I try. One day I even tried getting up to around 95mph before going up a huge hill on my way to work that's a solid 1/4 mile long. I hoped to carry enough speed to keep the engine out of boost.... nope. Only made it about half way before I was bouncing around .5 positive psi then gave up and gave it some go juice. Another fun story is after a friend of mine got his STI running on E85 he took me for a ride. He was putting down around 375 AWHP, and this is with a stock turbo, stock intercooler, just about stock everything minus fuel system. We were going on a little stint to show me what the car could do. He was stopped at a light at an intersection of a 4 lane road and was going to U-turn to head back to where we started. He rolls the car out in 1st, shifts to second as he starts to turn the wheel, then PUNCHES it, and lights up ALL 4 TIRES!!!! So when people want to complain about turbo lag I just laugh. He generated enough torque out of 2 liters to break 4 tires loose under 10mph in 2nd gear! It's one thing if it were 1st, but no, 2nd gear. Around town, even being super easy on the throttle my turbo S30 only uses 1st gear to get me to about 5mph, because I can lug around in 2nd gear just fine at those speeds. And if I DO want power, I can generate min boost at seemingly ANY rpm in ANY gear. If I'm in 4th cruising around town at 1500rpm and just stab the throttle I'm at 7psi (stock waste gate limit) by the time I reach 2,000rpm, which only takes a moment. In other words, even in 4th I've got plenty of "grunt" around town to get around traffic and such. The ONLY time I'd be worried about having a 1st gear in a turbo S30, is if you're one of the guys with full interior, 5 layers of sound absorption, and have huge subs in the back. In other words, keep it under 3,000 pounds and 2nd gear pulls away just fine. I estimate my car is now around 2,300 pounds, and we'll see once I get it back on the road. I doubt I'll ever see 2nd gear, and when going easy around town I'll probably skip shift to 4th. Quote Link to comment Share on other sites More sharing options...
letitsnow Posted October 19, 2012 Share Posted October 19, 2012 Around town, even being super easy on the throttle my turbo S30 only uses 1st gear to get me to about 5mph, because I can lug around in 2nd gear just fine at those speeds. And if I DO want power, I can generate min boost at seemingly ANY rpm in ANY gear. If I'm in 4th cruising around town at 1500rpm and just stab the throttle I'm at 7psi (stock waste gate limit) by the time I reach 2,000rpm, which only takes a moment. In other words, even in 4th I've got plenty of "grunt" around town to get around traffic and such. My car has never been quite that quick to get on boost, but the turbo has never sounded good and it just went out with enough shaft play to hit the housing. For me it was 2300-2400 in 5th would make 10psi, but it would take FOREVER. 4th would accelerate out of that rpm range before it hit the wastegate. It still made more than enough grunt to be fun, but now I'm hopeful that the HY35 I'm using to replace the cooked stocker will actually feel similar to my apparently slow spooling t3. I have a 3.70 diff and noticed very little change from the 3.54, 4th was plenty to go through the 1/4, something like 122mph is the theoretical top of 4th for me. I trapped 108, 1st gear would just barely spin on the street with the LSD in the dry, it certainly wasn't useless. Quote Link to comment Share on other sites More sharing options...
280zex Posted October 19, 2012 Share Posted October 19, 2012 When you all are reporting "useless" are you talking about when accelerating only? Otherwise, when putting around town, off-boost, isn't the car essentially a low compression NA engine, which could use the same ratios as any other NA engine? If you get a diff that makes first gear useful for getting on the boost, won't the car be boggy in normal city traffic? Just wondering if I'm missing something about how the turbo engines work. The factory set the cars up with about a 12 ratio, first gear times diff. 3.54 x 3.321 (11.75), 3.9 x 3.062 (11.94). I cant even get through an intersection when taking off in first with out having to shift, mabe 50feet, then shift to 2nd. IF it is even a little rainy, my Z just spins in first gear. I dont even see boost in first, or if I do it happened way to fast for me to see it. I can to do a 180 drift u-turn from a dead stop, 4kprm and drop the clutch with the wheel turned = snap u-turn spin. If I'm on an uphill intersection and taking off I have to spin the rear tires (nasty clutch dont slip). So its not that the 1st gear is useless, I just have to shift so darn soon. Quote Link to comment Share on other sites More sharing options...
Gollum Posted October 19, 2012 Share Posted October 19, 2012 My EFI was also running absurdly rich, which helps turbo spool. And yes, only making 10 psi by 2400 in 5th sounds like something is amiss. My wastegate line once came off in a spirited run and all of the sudden I was reaching 14psi in quick 2nd gear pulls to 3,500, yikes! I also had huge 3" exhaust with a straight through muffler. I didn't have a better downpipe though which I'm sure would make a difference. I also ran NO intercooler, and don't plan on it myself. I never had a LSD, which I suspect would help a LOT, but in 1st gear I could easily burn both tires at will. Getting a good launch wasn't hard but 1st gear sure went by fast, and at only stock power levels I can't image what it would be like running closer to 300whp. Quote Link to comment Share on other sites More sharing options...
letitsnow Posted October 19, 2012 Share Posted October 19, 2012 (edited) 1st does go by quick, but it's absolutely necessary when trying to go fast, driving normally, I'd shift mid-end of an intersection. In the rain, rolling burnouts from 40-60mph were pretty common in 2nd. I had a 3" exhaust with a 2.5" downpipe, at 2800 it would make full boost(15-18psi or so) and do it pretty quickly. Edited October 19, 2012 by letitsnow Quote Link to comment Share on other sites More sharing options...
Gollum Posted October 19, 2012 Share Posted October 19, 2012 What was your car's trim level letitsnow? My '75 was fairly gutted, and will now be completely gutted. I'm not even running side glass. Before, I was at 2560lbs and I could easily just let the clutch out in 2nd gear (not pop, just minor slip) and be rolling just fine. I really don't see the point in having a 1st gear if I'll spend as much time shifting out of it than I will in it. I'm still not certain what exact clutch I have in my car as I've never pulled the tranny, but I DO know that it grabs PLENTY hard at ANY rpm without noticeable slip, but is still very streetable and easy enough to feather. I'd assume it's just an aggressive organic clutch. Having very little weight in the car with a stock flywheel helps though. And as any drag car should exemplify for all of us, is that the more power/weight you have, the less you need your shorter gears to go fast. Most of the 1,000+hp cars I've seen in person either had two or three speeds in them. Of course most were auto, but some were manual still. And never once have I seen a quad digit car (in person) that had rear gears over 4.00. Maybe once I get my car back up and running I'll take some video of 2nd gear pulls, both racing launches, and tame driving roll offs. Quote Link to comment Share on other sites More sharing options...
NewZed Posted October 19, 2012 Share Posted October 19, 2012 (edited) I guess a lot depends on how and where you drive the car and where you live. I just put a 3.9 in my NA car from 3.54, with the 3.062 ZX first gear, to get more pickup from a stop and when I'm on the hilly streets of Portland OR. Otherwise, on the flat roads outside of the city it was nice to have the extra speed in first to wait on the shift to 2nd. Out on the flat plains of Texas Ben might find that a 3.9 is wasteful. In San Francisco, he might be hating a 3.54. As for why the factory put the 3.54 in the turbo cars instead the 3.9, it could be to make the car less likely to burn the tires from throttle alone in first gear. Detuning for the masses. Edited October 19, 2012 by NewZed Quote Link to comment Share on other sites More sharing options...
Leon Posted October 19, 2012 Share Posted October 19, 2012 Hell, I take off from a standstill in 2nd gear with my L24-powered Z! First is pretty much only for uphill take-offs. This is partially because my trans/diff combo isn't great (3.9 behind an early 5-speed), but these engines are freakin torquey, turbo or not. Quote Link to comment Share on other sites More sharing options...
Tony D Posted October 19, 2012 Share Posted October 19, 2012 (edited) I cant even get through an intersection when taking off in first with out having to shift, mabe 50feet, then shift to 2nd. Yep, takes you through the first timing lights and 60mph is 2 second in 2nd gear beyond that at 6500. For getting on the freeway merge, it's perfect... just about freeway speed and hit third hard on the throttle and merge ahead and pulling away... Like stated above, it all depends on your tire sizes, if you have taller tires a 3.9 isn't untoward. If you're stock height or shorter it makes for busy work taking off. One thing is for sure, you are above boost threshold immediately so you should be on-boost and if properly shifting on boost the rest of the way through all of the gears. One big reason to restricting the gear selection to 3.54 and 3.7 in a turbo is that freeway merge rationale. You won't accelerate as quickly 0-60 with three shifts, which will be required on stock size tires and a 3.9. On a 3.7 you hit 100KPH (62mph) at the top of second gear, and if you granny shift like most Americans at 6000 or before, a 3.54 is what gets you there quickest. Edited October 19, 2012 by Tony D Quote Link to comment Share on other sites More sharing options...
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