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5spd vs. 4spd.


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I have a '78 280z with an L28t and a 5 spd transmission with what i believe is a "wide ratio" (very low an tight 1st/2nd gear and a very wide spread 2nd/3rd gear). The 2nd to 3rd gear gap is so huge it takes my turbo out of boost. I am wondering if I go to a 4spd if that will help clear up that issue or what else can I do to alleviate this issue.

 

Thanks,

 

Brian

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This is something I had rest on zcarz.us

 

"Nissan called the '77-80 trans a "wide ratio" while the '81-83 was called a "close ratio" 5-speed. The early one has the lowest 1st/2nd gears for acceleration, but a wide spread between 2nd and 3rd. The later one has a taller 1st/2nd but a tighter spread between 2nd and 3rd. It also has a much taller 5th for cruising. Your choice.

 

The early 5-speed is a 4-speed with 5th and reverse sharing the same fork. He said it's common for people to "blow" 5th gear as the fork is weaker. He stated the later 5-speed is a genuine 5-speed which is stouter. I call the early one a 280a and the later a 280b to keep them straight."

 

 

My understanding would be there is another 5spd with tighter 2nd/3rd gears. Or a 4spd would help. But if the 5spd is the same 1-4, then what would be a fix for getting rid of the wide gap between 2nd and 3rd that would keep me in my boost? I am not sure of my gearing, but if that is a step closer for me I am open to any and all ideas. I'll look into the 80-83 4spd too.

 

Thanks

Edited by Hollywoodman
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You problem is engine tune/driving style, not gearing. With a 6,500rpm shift point the worst gap in the transmissions mentioned drop the rpm to 3,965 in the next gear and that's the first to second shift. The worst second to third shift will get you 4,225 in third. If you're not making boost at 4,200 rpm then you need to work on your tune.

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I have a turbonetics t3/t4 and my shift point is between 6500 and 7000. My boost really starts at about 3000rpm and when I shift from second to third it drops below 3000. I have a short throw shifter and I am confident and competent enough for quick shifts. Maybe I will try blipping the throttle on that shift. I feel blipping takes extra unneeded time to complete the shift though.

Edited by Hollywoodman
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I've seen this problem firsthand. It may not be a 4000 rpm drop, but it can be big enough to have a pretty drastic effect. I used to autox with a friend who had a 510 and we were pretty evenly matched there, but we started going to big track events and it was a different story. At one of our first events at Streets of Willow we gridded up with him in front and when we turned onto the straight to go up the hill he needed to shift to 3rd and I absolutely left him in the dust. It was the 2-3 gap that was killing his L18 with cam and 44s. After that event I gave him an old close ratio roadster trans so that he could keep up. It turned out that I still had enough power to ditch him at big tracks, but at least he wasn't falling off an rpm cliff when going for 3rd.

 

I believe Clifton had figured this out too. I know he was running the later ZX 5 speed as well in his turbo car, which is now cobramatt's insane V8 track car. There are other posts about the gear ratio spacing, I know I've commented on it quite a bit. I hate to see people go get a T5 out of a turbo 280ZX and put it in a NA car, because it has even worse spacing as I recall. The downside to the late ZX 5 speed is that 5th gear is supposed to be weak, so you might stay in 4th if you're really pushing it.

Edited by JMortensen
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I just converted my T3/T4 L28 with Isky cam from the 81-83 NA tranny to T5 as the bearings were making bad noises. I do find on the street that I need to delay shifting from 2 to 3 until a few hundred revs higher so the torque fall off in 3rd is not as noticeable, but if I shift at 6500 rpm, there is virtually no delay in getting boost when I get back to WOT in 3rd.

 

I used to run the NA tranny with a 4.11 and now run a 3.54 diff. The result is that the overall gearing for 1st and 2nd are nearly identical and 3-5 are longer. Keep in mind I'm at altitude so that only works against me further, and I'm not seeing this as a problem. FYI, when I ran the stock 4 speed with stock cam NA, the 2-3 gap was very noticeable on the track.

 

To johnc's point, something is wrong with your tune/setup if you're seeing boost fall off shifting at 6500 rpm.

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To johnc's point, something is wrong with your tune/setup if you're seeing boost fall off shifting at 6500 rpm.

Doesn't the stock L28ET pretty much stop accelerating after about 5500 rpm? No experience, but I had I thought that I'd read the cam timing was such that it wasn't worth it to try to hit 6500.

 

Nevermind, I see he's saying he's shifting at 6500 or 7000. If it has a stock cam, might check the tach accuracy.

Edited by JMortensen
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Ya, the stock L28ET does reach it's HP max at around 5300RPM or so. But it's still accelerating past that point. I have a '77 5-speed attached to my L28ET, but I'm using the stock 260 R180 (3.364). I've never really lost boost between 2nd and 3rd and I'm normally shifting somewhere around 4800-5500 when running in full boost... I have to say, swapping to a MAF made a large difference in my boost response though.

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I run a 83zx na 5spd with 3.7 rear gear .The 81-83 zx na trans had closest ratio spread and best 5th for freeway use.The original 77 5 spd had a wierd gap between 2nd & 3rd gear that was very noticeable at infineon.+ the 77 didnt have a good 5th for freeway use-only like a 400 rpm drop.Why no zxt t5?The zx na trans shifts better and has closer ratios.I built my car for open track days-not drag racing.I have a zxt t5 bellhousing-allways wanted to adapt a wc t5 out of a mustang to it

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  • 1 month later...

Just re-read your original post and want to confirm that you're referring to accelerating at WOT or close to it.

 

Also, you didn't describe the engine other than it's an L28t. What turbo, cam, boost pressure are you running?

 

Lastly, before you tune anything, post a datalog from a stop accelerating thru 3rd gear. That way we can quantitatively see how the boost is affected by the 2-3 shift.

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