SRL Diff in a SPL

[67 1/2 1600]

Anyone know if there will be any issues in putting a SRL read end in a 1600? I know it will be slower off the line and more top end, but anyone else ever try it?

Joaquin

[Daryl Smith]

First off, I have not tried it.
However, going to a 3.7 from the 3.9 is not a huge difference (~5%).
I was going to do this in my stroker with a 4 spd.

It should drop your rpm about 200 rpm @ 60 mph. And yes you may be a tad slower off the line, but I doubt you'll notice it.

I think it is an excellent upgrade as I have always found 1st in the 1600 to be too low. For a 1600 with a few upgrades, (ignition, exhaust, etc.), it is almost nescessary. (short of going to a 5 spd)

It is a straight forward swap. give it a shot and let us know what you think!

For what it's worth, IN FIRST GEAR, a 4 spd with a 3.7 rear still has a shorter final drive than a roadster 5 spd with a 4.11 rear! Which means for the same power levels the 4/3.7 would be faster off the line.

[67 1/2 1600]

I agree, 1st gear in a 4spd is way too short. I already pulled out the 3.9, im gonna swap brake cylinders and all that fun stuff before I put the 3.7 in. Oh it should be fun. Gotta got it ready for a show in sunday hopefully it iwll be a mission completed. I will let you know if I feel the difference and if its good or not....

Joaquin

[Datrock]

Joaquin, I have done it before. I installed lower profile tires on my 68 1600 and then installed the 370 gears for freeway driving. I was going 120 miles a day in the roadster. The engine sure liked the lowered rpm's. Then I went back to taller tires in the winter and the car always felt under powered. I found myself down shifting more on the hills.

Those times are behind me and now the car has a u20/5spd in it and the 390's back in the car with 195/60/14 tires. I like this combo very much...

I guess it's has to do with what kind of driving you do. I'm sure there is more freeway driving in your area so the 370's might be your best choice. But if don't want all of those other roadsters leaving you behind , I would stay with the 390 gears. they are quicker. Bill

[Daryl Smith]

Don't forget that your speedo will be off unless you change the little gear in the tranny!

[Daryl Smith]

Bill brings up a very valid point with regard to tire size.

If you want to run through the numbers, try this site:

http://www.311s.org/gear_ratio_rpm.xls

I am running 185/65/15's, with a mildly modded 1600.
(Soon to be 1800 with a 240SX 5 spd.)

[67 1/2 1600]

Got it in tonight and took it for a spin! Oh wow no more loud whining noise! Looks like I might be making it to the gardena show sunday. As for performance I hardly noticed the differnce besides it running way better, first gear was a bit extended which is real great! I cant wait to get her on the freeway! And the speedo was a bit off.. seemed to mark less mph?? dont remember.... but overall a great swap!

Joaquin

[Daryl Smith]

Yes, your speedometer will read slower than you are actually going. It is a fairly easy fix with the proper gear on the sending unit.

Drive it around for a few days, twisties, hills etc. as Bill says, then let us know if you still think it is a good upgrade, any downside.......

Also, What size tires/rims are you running?

Have fun!

[TurboRagtop]

First off, I have not tried it.
However, going to a 3.7 from the 3.9 is not a huge difference (~5%).

To focus only on a bare number like 5% is to understate the case, don't you think? If you went from 3.90 to say, a 3.23 gear, that would be "huge", but that is *only* a ~17% difference. (So ~5% is 1/3 of *huge*) And 200 rpms at freeway speed is a big enough difference to make the swap worth it, for me. At today's gas prices it's worth $.
Having said that, I would like to hear the opinions out there for a good ratio for my '66 2.3L roadster. (My sig details what powertrain I am running)
Here is what I am getting now:
1. Very short 1st gear. Sometimes I leave a stoplight in 2nd, because I hate that short shift. But that will kill the clutch, sooner or later...
2. Cruising in 5th, the engine is not working hard, but I could use a few less RPMs.
3. I would like to keep the stock rear axle assembly, but would like to have more strength back there.

Questions:
Is the SRL center section a stronger diff?
Are they like *impossible* to find?
Can we still get bearings to re-furb a worn center section?
And the $64 question: Can you buy different ratio gears to build a center section in a different ratio?

Or am I better off just building a custom 7.5" GM 10 bolt, or a 8" Ford diff?

Thanks for all input.

[Daryl Smith]

Karl,
I ain't no EKSPURT. I didn't focus on "5%", although that is a calculable, concrete number for reference. I also pointed out a slightly slower start, and the ~200 rpm drop at freeway speed (may vary with tire size), and that Joaquin should try it to see whether he likes it or not, as obviously Bill didn't like it.

Joaquin hasn't replied, good, bad, or tire size. I would assume he likes it but I could be wrong!

First off, the 3.7 rear ends should not be hard to find and I doubt are any stronger. Bearings and seals for them should also be fairly easily obtained. I don't know about gearsets, espescailly lower than the SRL 3.7. I know you could get them higher and many did for racing.

I am going to assume that your motor/transmission came from a much larger car than the roadster (T-bird turbo coupe?). If that is the case, then I think you will find it has a very low first gear (probably 3.3 to 3.5:1). The KA swappers are in the same boat, but not as much power as you. This was required to get the larger mass of the original car moving, and with a lot of that mass suddenly removed, first gear is suddenly a cakewalk, and hardly seems worth the trouble even with a 3.7 diff.

For reference the roadster 4 spd has a 3.382:1 first gear, the 5spd a 2.957:1 first gear. (calculates to a 12.6% difference, which is huge!). The 240SX tranny has a 3.321:1 first gear, and the KA a lot more torque than the U20, so this is also rather short for the roadster.

The way I see it you have some choices.

1. You can play with tire sizes (larger overall diameter works same as lower rear end ratio)

2. Lower rear end ratio. (Lowers rpm at all speeds/gears)

3. Play with the ratios in the transmission. Get a higher first gear gearset to match the torque to the mass of the new car it is in (probably around 3:1). OR find a transmission with a higher first gear to swap in (would require some digging for info, possibly on a Ford site of some sort). Maybe a "close ratio" transmission?

4. A combination of the above. You may find that with a 3.7 diff AND larger tires/wheels you may get a combo that works for you. Without having to pull the motor/transmission.

I can't say as to how much abuse the roadster diffs will take, but their should be a few people here that could.

Given your situation, I would find the ratios of your transmission, along with your tire size, and rear diff, and run them through this site ( http://www.311s.org/gear_ratio_rpm.xls ) to find something that works good for you. Compare them to the roadster #'s, and play with different tire sizes, available ratios etc. Ford/Chebby rear end ratios, etc. You should find something that is a reasonable compromise of parts.

Good luck.

A roadster nut who has analyzed and crunched some numbers on many aspects of the roadster, BUT ultimately has no concrete experience, and who didn't get to drive his car at all this summer, because the "few" modifications, which were started last January, turned into a "few" more, and well, this has to be done, and if you do this, this won't work, so that has to be changed.... and...and...well, it's not DONE YET!! DAMNIT!!

[67 1/2 1600]

I run 185/60/14 Before i had 175s?? I would get more speed on the freeway but take off would tremble a bit. Now the car takes off faster with the smaller tires.. and still manages to 3,500 rpms at about 75mph?? according to my dash but Of course its off due to smaller tires and bigger longer ration diff. Im sorry but I cannot compare with the 3.9 rear end because I never tried that one on the freeway. But other than that I can tell you that I am very satisfied with the speed I get now. at 4k rpms or so.. I can pass traffic. And I can accelerate and actually move ahead of the traffic which is really impressive becuase It never did that before. A U20 5 speed woudl be real nice because the rpms are real high. Just my two cents...

[TurboRagtop]

Thanks for the input. Very cool calculator; looks like a great tool. You know, your comments started me thinking about replacing my first gear ratio when the trans comes out this winter for new synchros. I figure with a 3.7 rear diff, new rebuild with bearings, seals etc. and the reworked trans I should be a lot happier with the combo. I might also go to 15" rear wheels; this combination will yield a steeper overall ratio. Since turbo engines build boost quicker with some resistance, they tend to like higher (numerically lower) gears to make power.

[Datzoom]

I have a 3.5 rear end in one of my cars and it's great. Nothing like cruising along at 85 in 5th with the engine loping along at 3700 rpm.

On the other end of the spectrum my other setup has a 4.38 limited slip.

[TurboRagtop]

That is what I was hoping for, someone who actually is running a 3.5 rear. For my turbo setup I think that will do even better than a 3.7, since the increased load will build boost faster.
Thanks a lot.

[67 1/2 1600]

Datzoom.. w here did you get the 3.5? Will it bolt into the roadster pumpkin?