Which diff?

[zippy67roadster]

I am working on the rear axle housing for my 67 vintage racer project and I can't afford the LSD or Detroit locker right now so I am looking to run the open diff for awhile. I am going to run the stock 3 main R16 (completely rebuilt with a more aggressive cam) and a 5 speed from a U20 car. My question is, what open diff should I run? I have the stock one and many from U20 cars as well. I also have ones from late R16 cars. Is there a year that had the best ratio to run with the 5 speed?

[fj20spl311]

I would highly recommend at least 4.38 gears especially with a U20 5 speed standard gear set.

A stock U20 3.7 gears is going the wrong direction.

Look around for a PL520, PL521, PL620 Truck or PL510 wagon for lower gears.

Some have used welded or lead filled Spider gears.......I am not sure about welded gears.

[zippy67roadster]

I would highly recommend at least 4.38 gears especially with a U20 5 speed standard gear set.

A stock U20 3.7 gears is going the wrong direction.

Look around for a PL520, PL521, PL620 Truck or PL510 wagon for lower gears.

Some have used welded or lead filled Spider gears.......I am not sure about welded gears.

I'm planning on racing in V.A.R.A. and they don't allow welded diffs. The truck and wagon gears are hard to find! I am planning on going to a Detroit locker as soon as the funds allow but just want the best stock open gears I can run until then.

[Alvin]

I'm with Phil some gears would do wonders for you. Find a 4.38 or 4.62 would be killer with the 5speed.
If you can't find gears I think the 4-speed would work better with the standard 3.9 …all depends on track speeds!

ping the racers on the forum they might have extra R&Ps for you.

ps…you don't need an LSD unless the course has numerous sharp turns/sections.

[Steve49841]

Everybody's right and everybody's wrong...there is no right or perfect gear, there is just the best gear for the track(s) you are running. I went through this last year on my car that at the time had a 4:88 (1968 U20 5 speed) and a calculated top end of around 105-107 in 5th at 6500 RPM. On two mile tracks the car spent a lot of time in 5th gear and did okay but on a 4 mile track was seriously under geared. So I found a top speed calculator on line and figured a 3:90 gear would give me 109 in 4th (very close to top speed in 5th with the 4:88) and 135 in 5th theoretical top speed. The 4:88 has now been replaced with a 3:90 (thanks to Jake) and the plan now is to run a lot of 4th gear on two mile tracks and 5th on 4 mile tracks. If you think about the track length you are going to run and do the calculations you can come pretty close to the best choice for your situation. As to welding the diff...EVERYBODY welds the diff, even BRE and Sharp welded the diff, V.A.R.A is NUTS! Screw them, weld the diff....Power to the People!

[jamesw]

There is a 4.38 complete unit on Ebay for $300 right now.

[SLOroadster]

With an R16, think short gears, but not too short. All the R16s I've driven seem to run out of breath at about 5K rpm. (they have been stock, and one stroker so take that with a heads up.) Since you are looking to run with VARA, you get Willow Springs, Streets of Willow, Buttonwillow, Chuckwalla, Las Vegas and Ontario. I don't know most of those tracks but I will say at Buttonwillow, I ran 5th gear out to about 7300 rpm with a 4.11. The rest of the track was all 3-4 with 2nd gear being used in turn 3 (I think, its a sharp slow hairpin.)

Since all the other tracks have some seriously long fast sections and your R16 isn't a full 10/10ths race motor, I'd almost say either a 4.11 or 4.38 would be about right. The 4.11 will be a bit on the long side through the slower corners, but make up for it on long fast straights (I found this when I was racing with Ron at Laguna Seca. I think he was running a 4.38 to my 4.11 and he would pull me out of the corners, but I'd catch him at the other end of the straight) since you don't exactly have a high revving motor. The 4.38 is likely a better choice, but you might run out of 5th gear at the end of the long straights. Still, even that might be on the tall side.

With my setup I rarely need 2nd gear for any of the tracks around here. I've run Laguna, Sears Point, Thunderhill, and Buttonwillow, and for the most part I run 3rd and 4th gear through most of the tracks. At Thunderhill I'll wind 5th out down the front straight, and possibly from turns 6 to 9 (I'm a lot better driver now than I was the last time I ran T-hill in the roadster so its hard to say.) At Sears Point I find myself in 5th out of turn 8 down through 9 and into 10 (I don't have the balls to try pushing 5th through 10, but I think it could be done.) Everywhere else is 3rd and 4th gear. At Buttonwillow running the LONG back straight I was redlining 5th as I entered turn 9, everywhere else was again 3-4.

Obviously you won't be spinning an R16 that high so finding the balance between pull out of a corner and top end speed is the key. I think its going to take a little experimenting. There is a guy who runs a 1600 with VARA, it might be worth trying to track him down and see what he has to say. No, I can't remember his name.

Will

[fj20spl311]

FYI

4.38 gears final ratio using a U20 5th is 3.7
4.62 gears final ratio using a U20 5th is 3.9

If you can't pull to redline in 4th, you will need lower gears.

whats the power range for your cam?

BTW,
The old racer's saying:
It's not how fast you go, but how long you go fast!

[Mike Unger]

I just glanced at the VARA rulebook and it says type of limited slip is open so weld it if you don't have a LSD. A welded rear is a very limited slip rear If somebody questions you tell them it's shimmed tight, very tight. I have the LSD in my Z shimmed until it's basically locked. I would choose the rear end based on the track with the goal of 4th being your top gear you use. That keeps the rpms of the driveshaft down. You'll probably not need more than 3 gears on track unless you have a very peaky engine. If you have a very peaky engine you'll need a very tight ratio tranny as well. You want to have enough gear so you don't run out of revs on the longest straight or ideally you run out just at the braking zone but also you should select the ratio so your in a good rpm range on the corner leading onto the longest straight. It depends on many factors and you won't know the ideal gear until you get some seat time. Joe Hauser told me he never used the overdrive in his 1600 and switched rears depending on the track. I'm fortunate in that the three main tracks that I race (Glen, VIR, Summit Point) all have long straights and fast corners leading to the straights so I don't need to switch rears in the roadster or the Z.

Mike

[fj20spl311]

Sorry,
But it does say.....Not Welded....
I don't know if that means you can't fill the spiders with lead or pin the spiders to the shaft.....

[Mike Unger]

Don't be sorry I don't have to race with VARA . I did race with HSR west last November at Chuckwalla (now SVRA who I'm familiar with). I was looking at the specific rules for production cars up to 1972. I now see what your seeing in the general rules. I'm suggesting that they aren't going to tear your car down to check the rear end to see what's in there. There's no way they will know unless they tear your car down and I seriously doubt anybody is going to do that. I would run the welded rear until I found an LSD but you should do what makes you comfortable. Others have suggested the same advice. Regardless, replace the axles with upgraded ones from Summers Brothers or a floater. I believe Les at CDM can get these SB axles for you if you don't want to order them directly. This is a weak point and is a potentially dangerous failure. Most of all get out and have some fun. That's what it's all about.

[Steve49841]

Mike nailed it...but do what makes you comfortable. Just know that with an open diff you might spend some time looking in the rear view as cars pile up behind you on corner exit. Besides you should look at the VARA rules kind of like you look at speed limit signs....a suggestion.

[zippy67roadster]

Mike nailed it...but do what makes you comfortable. Just know that with an open diff you might spend some time looking in the rear view as cars pile up behind you on corner exit. Besides you should look at the VARA rules kind of like you look at speed limit signs....a suggestion.

I love it. Mike is right though, I don't think they will have be pull my rear end to check. It's not like NASCAR where they do post race inspections! I just started to pull everything off the rear end housing today. It, like the frame, was covered in about 1/4 inch of dirt from stitting since the 80's in the Las Vegas desert. I will tack some pictures and update my build thread. I think I will weld it. That being said, what part needs to be welded? Pictures would be great!

[pebbles]

There is more than one way to skin a cat, here is one,

[Steve49841]

Welcome to the dark side.....mine is welded like the pic from Pebbles and works fine. One thing to think about when you go thru tech for your annual, is the car does not like to make turns, so when pushed by hand get some help. If you scope out the line and try to make a straight shot into the tech area or can drive it in even better. And, as a final precaution, have the tight shim story warmed up and ready to go in case some body notices the rear end binding a little bit. Something like "jeeze the guy I bought it from must have set those shims pretty tight...."HA!