3.50 Final Drive in Spirit R/T

[RJ138]

I will admit I do not know enough to understand exactly what I will give up and gain from going to a 3.5 final drive instead of the 3.85. I have a Holset powered full weight Spirit R/T that moves out pretty good... Can someone put it into simple terms for me? Will it be a dog out of the hole but a highway monster? Lose some seat of the pants feel? Kill trap speeds? Asking since I want to add a limited slip differential.

Thanks,
Ryan

[iTurbo]

I had one in my Spirit R/T. It WAS a dog out of the hole, although I can't say that was entirely the fault of the 3.50 gear because it also had some camshaft problems (LWP stage I that I could never seem to dial in just right). It was great on the highway and got wonderful gas mileage. I could get over 500 miles out of a tank cruising at 75 MPH. But driving around in town was not fun. If you wanted to take off from a stop light at anything other than normal speed you had to go out with guns blazing, and that was with a stock turbo.

I would only do it again if I had a 2.5L or was expecting to run 120+ MPH in the quarter.

[Turbo3Iroc]

You will lose low end but it will pull more up top. It is usually the lighter cars that go for the taller gears though. Unless you do a lot of highway driving it is probably not the best choice. A 3.77 is a great compromise gear and pretty common for use with the OBX.

---------- Post added at 12:44 PM ---------- Previous post was at 12:42 PM ----------

I thought you might chime in Jeremy, didn't realize it would be in the same minute!

[iTurbo]

Yeah, like Kelly mentioned I will probably go back to stock 3.85 next time around with that car, 3.77 would be good too. Probably save this 3.50 A568/OBX for my Omni someday, or a 2.5L TIII build.

IIRC, you could probably pull 105 MPH trap speed winding out 3rd gear, but I'm no drag racer. I used the car primarily for my pizza delivery vehicle, and it did pull like crazy in the top end and was awesome on gas but like I said, not fun in traffic.

[RJ138]

Well I'm shooting for 400 plus whp, just need to finish dialing a few things in and its just to hot out lately to mess around with the car. I found a good deal on a 568 with quaife but it had the 3.5 final drive, is it easy to switch to a 3.77 or 3.85?

Thanks for the replies!

[iTurbo]

Depending on how that Holset spools, the 3.50 gear might be for you with 400+ on tap. Probably be able to run shorter slicks too. Is this going to be a track only car or will you be daily driving it? If you're going to drive it a lot, I would hold out for the 3.77 personally.

I wasn't aware that you could get a Quaife LSD with the 3.50 gear. As far as I know the Quaife could only be had with the 3.85 FD from the A555 and A568. I could be wrong though. I was using an OBX LSD in my 3.50 A568/A523.

It is a pain in the butt to switch out the gears. I recently tore apart an A520 in order to get the mainshaft/pinion and the ring gear out to sell to a guy doing a hybrid 3.50 trans. I had to strip down the trans almost entirely to get it out of there. Only needed a ratchet and a few sockets and a good snap ring pliers though, but as far as *me* reassembling, forget it!

[RJ138]

Car is just for fun, its not a daily driver and will see a couple track days (no good drag strips nearby). It will be mostly used to enjoy on the street, crush egos, scare the girlfriend, etc.

I think the Neon Quaife allows you to change final gears.

[moparman76_69]

I run a 3.77 in a van and it seems about right. Just over 2 grand at 70 but still has enough grunt.

[cordes]

Car is just for fun, its not a daily driver and will see a couple track days (no good drag strips nearby). It will be mostly used to enjoy on the street, crush egos, scare the girlfriend, etc.

I think the Neon Quaife allows you to change final gears.

True, but the ring gear on the 555 and 568 are integral to the diff which makes it impractical for use with a neon OBX or Quaife diff. If you had a 3.85FD 523 or 520 you could have a 3.85 trans with no problem although you would not have the more robust gears of the 555 or 568 that way.

[RJ138]

Well the trans is already assembled, it is some kind of hybrid large spline a568 with a Quaife and 3.5 final drive.

It sounds like it is not fun to change the final gear so I might pass on this deal. I don't want to lose all my low end, there already isn't much there with this turbo. On the other hand it might be worth picking up and trying out, I'm not settling at 400 whp, thats just a short term goal.

[4 l-bodies]

Ryan,
If you want to low buck a tranny. Buy a 92+ a523 and put in the OBX. You can recover some of the costs buy selling a known good a568 if your satisfied with the 3.77 ratio. I have that exact tranny in my 2.5 89 Shadow and like it a lot. With 24.1 tall tire, I'm at around 2700 RPM at 70-75 MPH. A vast improvement over the a413 which ran around 3300-3400 RPM. 4th gear is a little tall IMO for my application at the dragstrip. It feels like the car lays down a bit in forth, but I'm only running about 17 PSI so that could be it too. Traps around 104-105 MPH. My plan is to run more boost! That should fix laying down in 4th gear (lol). See you this weekend.
Todd

[Turbo3Iroc]

Todd, he would also have to have the 523 case machined for the crank sensor. Not impossible but I only know of Cliff out here that has that capability. Also 400 hp through a 523 is probably asking for disaster in a heavy car.

[RJ138]

The case on the trans that I am thinking about buying doesn't have the crank sensor hole either. Did some large spline 568s come with out the crank sensor hole? I remember someone a long time ago made a jig to drill the hole on other transmissions, maybe that will work here. Although this deal is starting to sound like a lot of work, I'm still tempted to pick it up because I would love limited slip that is all set up and ready to go.

Ahhhhhh decisions, decisions, decisions....

[Vigo]

Anybody who has a drill and some patience can do that...

[butchsuppe]

In my 86 Laser my RPM's dropped by 300 at 65 mph and only noticed very little off the line ( slower ) performance between the 3.85 and 3.50 gears.

[RJ138]

Ryan,
If you want to low buck a tranny. Buy a 92+ a523 and put in the OBX. You can recover some of the costs buy selling a known good a568 if your satisfied with the 3.77 ratio. I have that exact tranny in my 2.5 89 Shadow and like it a lot. With 24.1 tall tire, I'm at around 2700 RPM at 70-75 MPH. A vast improvement over the a413 which ran around 3300-3400 RPM. 4th gear is a little tall IMO for my application at the dragstrip. It feels like the car lays down a bit in forth, but I'm only running about 17 PSI so that could be it too. Traps around 104-105 MPH. My plan is to run more boost! That should fix laying down in 4th gear (lol). See you this weekend.
Todd

Todd if we are not too busy oggling over cars on Saturday, I might pick your brain a bit on this subject.

[86Shelby]

Jackson has a jig to drill the crank sensor hole. I rented it from him a couple years back when I built the trans for my R/T, couldn't reuse my old case since the diff pin made it's own access hole.


Edit: There is some clearance work that needs to be done on the surface the sensor mounts to when drilling a non-R/T case. You have to measure the distance between the jig and the old case real accurately. If your measurements are off the sensor will be too far away from the crank and will give a weak signal, or even no signal at all. Too close and it will grind on the crank and likely break. All said and done I think I had a little over an hour into drilling, tapping and grinding the case so everything was correct.

[Turbo3Iroc]

It needs to be surfaced and then drilled to a pretty tight tolerance. I can only assume that trans is from a van, I dont know of anything else in 90-91 that would have come with one.

[Turbo3Iroc]

Ok, I didn't know Jackson had a jig also

[RJ138]

In my 86 Laser my RPM's dropped by 300 at 65 mph and only noticed very little off the line ( slower ) performance between the 3.85 and 3.50 gears.

That is good to know, I guess if you think about it, its not like I would be going from 4.56s to 3.23s... Haha! Although we arent rocking torque monster V8s.

[cordes]

Todd, he would also have to have the 523 case machined for the crank sensor. Not impossible but I only know of Cliff out here that has that capability. Also 400 hp through a 523 is probably asking for disaster in a heavy car.

I think the 92+ 3.77FD units should be able to handle that no problem. They are very close in pitch to the 568s.

[iTurbo]

Yup I rented the jig from Jackson also. That is how I enabled my Cliff R. build A525/A555 for TIII use for my Omni GLH. Very nicely built jig Jackson, thank you!

[DoubleD]

I have an IROC R/T with a 92+ A523 (junkyard unit out of Shadow with a fresh rebuild!) in my car. I am making over 400 whp (26PSI) and I haven't had one problem with it. I drilled my case myself and didn't have any issues, all I needed was a digital caliper and the proper drill bit, took maybe 10 mins and I was good to go. I don't run slicks on my car but the 523 is holding up perfect in my car and I have even run a few auto cross events. Ryan, call me if you have any questions.
763-486-8849. I have met you at a couple of SDAC meets in town.

[RJ138]

I have an IROC R/T with a 92+ A523 (junkyard unit out of Shadow with a fresh rebuild!) in my car. I am making over 400 whp (26PSI) and I haven't had one problem with it. I drilled my case myself and didn't have any issues, all I needed was a digital caliper and the proper drill bit, took maybe 10 mins and I was good to go. I don't run slicks on my car but the 523 is holding up perfect in my car and I have even run a few auto cross events. Ryan, call me if you have any questions.
763-486-8849. I have met you at a couple of SDAC meets in town.

I don't recall seeing a black Iroc R/T at any meets? I haven't been to one in a while though. I will start showing up again from now on. I might try and source a A523 just for kicks.

This thread is because I found a deal on a transmission with Quaife already installed but has a 3.50 final drive from a 523. If it was easy to swap the ring and pinion it would be a no brainer, but I guess it is not easy. I might buy it anyway but it is going to suck if its a pooch.

[capev86]

when you put the 3.50 gear in a 91 trans, you want to use the 92-93 1st gear (3.3 instead of 3.0) and that will put you back where you were before when pulling off the line.

3.0 x 3.85 = 11.55
3.3 x 3.50 = 11.55

a 2.5 bottom end is another upgrade i would make if getting into the internals of an R/T engine. the Spirit R/T is not exactly a super light at 3000+ pounds and the extra torque will get things moving more swiftly. that and maybe a big exhaust and you'd be able to take on a Neon SRT-4.

[cordes]

when you put the 3.50 gear in a 91 trans, you want to use the 92-93 1st gear (3.3 instead of 3.0) and that will put you back where you were before when pulling off the line.

3.0 x 3.85 = 11.55
3.3 x 3.50 = 11.55

a 2.5 bottom end is another upgrade i would make if getting into the internals of an R/T engine. the Spirit R/T is not exactly a super light at 3000+ pounds and the extra torque will get things moving more swiftly. that and maybe a big exhaust and you'd be able to take on a Neon SRT-4.

You need to source the input shaft to go with that gear. It's nice if you have it lying around, but a pain if you're trying to track it down. Especially since 1st gear is pretty useless most times anyway.