lsd and rpms!

[kmc63]

Looking for a little info. I finally got a subi lsd for my dime. It is a 3.70 ratio.Currently I have a stock 3.90 under the car, My question is what kind of rpm change can I expect? At 70 mph on the freeway my vg30 is turning pretty high rpms with the 3.90 (no tach) Im hoping it drops it quite a bit cause I have no problems in the torque area.Thanks.

[BJW's FiveTen's]

You will only Drop a few hundred RPM's. And If you were to change tire size you could also change your over all final drive. Taller tire lower Rpm's. Shorter tire Higher Rpm's. The 3.90 to 3.70 is not very much. If you still Had an L series it might be a different story. :D

Enjoy your Vg and Lsd. Them together sound like a good time.:eek:

[Z-train]

You're going from a 3.90 to a 3.70?The RPM change will not be even close to worth the money and effort.

[ichorid]

I dont think he's doing it JUST for the change in 70mph rpms.

 

he maybe, MAYBE wants to get better traction with a limited slip.

[Z-train]

Maybe.

[kmc63]

You guys hit it on the head. I mainly wanted the lsd for the grip,dropping the rpms is a bonus,which wont be much it sounds like. I have the car almost as I want except for the tire issue. I have 15x7" wheels with low profile kumho tires,on a nice road it drives perfect,but the minute I get on 1-5 or anywhere trucks have grooved the roadway its almost scarey. I go to change lanes and its like it dosent want to go,dosent want to,and then all at once goes its weird same with lines in the road, any uneven surface its pissing me off.

[datzenmike]

LSD won't fix a grooved road. This is a tire suspension thing. Try borrowing a different set of tires from a friend and try the grooved road again.

[kmc63]

Thanks mike, I know the two topics I brought to light in no way are related. That made me smile. I may not be the sharpest tool in the shed,but cmon now. My nephew said the same thing throw on some stock wheels and tires and see what it does,makes sense.Thanks again for your help. Im still laughing about the lsd fixing my handling problem.

[dimeofftheline]

I don't know what transmission you are running, but for argument's sake we'll say it has a .759 5th gear, which I think the VG trannies do?? We'll also say you have 205-50-15s, which have an approximate diameter of 23in.

 

With the 3.9 you are spinning 3019 rpm at 70mph in 5th

 

With the 3.7 you are spinning 2864 rpm at 70mph in 5th.

 

Total drop of 155 rpm. You also gain about 7mph in top speed at 6000 rpm (146mph vs 139mph)

[thisismatt]

3.7 new/3.9 old = 0.948717% change in rpm

[kmc63]

Thanks for the numbers you guys. I should install a tach,but my car is pretty much bone stock appearing I hate to start cluttering it up with add-ons, I still have the stock radio. I havent been able to find a replacement sound system that can be installed without cutting.which wont happen.

[DISLEXICDIME]

Thanks for the numbers you guys. I should install a tach,but my car is pretty much bone stock appearing I hate to start cluttering it up with add-ons, I still have the stock radio. I havent been able to find a replacement sound system that can be installed without cutting.which wont happen.

 

put a stock tach in .

 

as far as the radio get a under dash basket.

[kmc63]

Stock tach,yes.A stereo under the dash no. Id rather listen to the motor than have something hanging from the dash. Want to keep my car stock appearing.I think the width of the radio is less than the width of a cd?

[datzenmike]

I did some estimating yesterday and depending on wheel size guesses (unknown) I got 150 and 200 RPM for a 4-6% difference but didn't trust my math.

 

The VG is plenty powerful and could easily go down to 3.70 and much lower in a light dime. The biggest reason for this swap is the LSD for traction with both wheels. I can just imagine what the tire fires without it must be like.

[DISLEXICDIME]

a cd payer will fit in the stock radio bezel but you have to cut just buy another one so you dont have to cut the nice one. You could always just have a jack installed on the stock radio$$$$ or put a radio in the glove bow with external controlls or a cord to a ipod with in hands reach

[kmc63]

Thanks Mike you seem to know your stuff. I have done a little studying on the lsd. I was looking at a nice write-up some guy did you have probably seen I think Jeff put it on this website. Any way I saw the viscous rearend in a 3.54 ratio or something close to that but havent seen the true lsd in anything but 3.70 or 3,90 unless you get into the z car and then I guess there is a bunch of mod work which I didnt get into. Are those the only 2 bolt in options?

[datzenmike]

I think the Subaru does come with 3.90 LSD but you can swap an open 4.11 into a 3.70 or 3.90 case and make your own 4.11 LSD. Or go lower.

 

Here's a fascinating read:

 

http://legacycentral.org/library/literature/differential.htm

[kmc63]

No.No Mike I have a 3.90 in the car,just bought a 3.70 to lower my rpms a 4.11 would make my rpms even higher. I knew about the 4.11 but that is going the wrong way.

[kmc63]

Thanks for the link.That is the one I was talking about well written and very informative. A bit misleading when it comes to the part about which cars have the lsd but I figured out what he meant.

[kmc63]

NON-LSD units have the following Subaru part numbers (look at the foil sticker for part number and ratio):

7220 11010: 3.70 ratio

7220 11011: 3.70 ratio

7220 11012: 3.70 ratio

6220 06020: 3.90 ratio

6220 06021: 3.90 ratio

6220 06022: 3.90 ratio

27011 AA151: 3.90 ratio

27011 AA040: 4.11 ratio

27011 AA110: 4.11 ratio

27011 AA111: 4.11 ratio

 

according to this chart my differential is not an lsd.7220 11010

[thisismatt]

If it has the foil sticker it should say LSD or VLSD on it. Only sure way to tell is to open it.

[kmc63]

Some of the older subi rear-ends had stickers which were much smaller,And whether they were lsd or not was not designated on the sticker,the one I have has the older sticker but going by part# according to the chart it is not lsd. I will fully investigate tomorrow after work.

[kanyonkritter]

Which subi's had the LSD's that fit in the 510?

[banzai510(hainz)]

the Suby diff will have LSD on the sticker if a LSD or just hurn the half shaft one way and the other side should go the same way if not its just a 50 dollar diff. But when installed on my Banzai car I could tell the difference on the high way. With a sidedraft 4 cylinder and stock 4 speed I have to get the rpms goin to get power out of it but Youll like it since you got a torqier motor.

 

 

My stock 4 speed seemed like it had a 5th on the Hwy.

[hang_510]

http://www.angelfire.com/fl/procrastination/rear.html