Going from 3.15 diff to 3.64.

[krisko]

I'll be getting my new diff this week from diffsonline. I'm curious what are impressions of going from a 3.15 (stock 95) to a 3.64 w/60% lockup. I know the acceleration out of the hole will be better but what kind of lap time improvement did that translate to? And is up or down shifting or rev matching different? And what exactly is '60% lockup'?

I was turning 1:45s at RA today...what kind of improvement could I hope for with the new diff? I'm hoping to get 1 second over a 2.5 mile course...too much to hope for? I've heard a diff comes right after track tires for preferred track car upgrade.

[vodomagoo]

Huge difference comming out of turns, i uped to one from a 3.25. Shifting is not much of a difference just happens more often. This is on a e30 with a full s50 drivetrain

[JamesM3M5]

I don't know that it will drop your laptimes by a full second, but it will help a lot. You can expect a 15.6% increase in RWTQ which is HUGE. That also corresponds to 15.6% better acceleration, because acceleration is proportional to RWTQ.

[ceegeezM3]

And what exactly is '60% lockup'

To a racer it means you can get on the power a earlier and harder than with a stock diff. This was the biggest difference I noticed - and yeah, it *will* make a pretty noticable difference in lap times over the 3.15.

[old skool]

let me know what your impression is of diffsonline. I just purchased my 3.23 from Dan and it came as just the diff. I will have to swap cover and possibly some other parts? Don't know if this is common. I paid more for a lower mile one from him with warranty and I hope I got what I paid for.

[krisko]

Originally Posted by old skool

let me know what your impression is of diffsonline. I just purchased my 3.23 from Dan and it came as just the diff. I will have to swap cover and possibly some other parts? Don't know if this is common. I paid more for a lower mile one from him with warranty and I hope I got what I paid for.

My impression so far is good. We talked extensively about what I was getting and the timeframes and he's delivered. I just got the diff today in fact but it will be a little while til it gets installed since my engine is blown up.

[ceegeezM3]

Dan is top notch. Highly recommended.

[sunir]

Dan at diffsonline is fantastic! I use a diffsonline unit in my racecar and it takes abuse lap after lap and holds up! Plus, on a sidenote, Dan is one of the only guys in automotive performance that delivers on time...like if you need something and it has to be done before a given race weekend. The customer service is great!

as for how the 3.64 feels over the 3.15...like someone just kicked you in the a$$ when you exit a turn...get on the throttle and power on outta there, unlike a sluggish stocker. IMO the diff is critical to have if you are competing, everyone out there will have one so you need it! As for dropping your laptimes, I think it will but I cannot say buy how much though...

[krisko]

Originally Posted by sunir

IMO the diff is critical to have if you are competing, everyone out there will have one so you need it! As for dropping your laptimes, I think it will but I cannot say buy how much though...

The only downside I can see is I'll max my speed only 2/3 the way down the back straight at Atlanta and Sebring and I'll surely run out at VIR. Obviously it works out if I can shorten my total time down those straights which I'm sure that diff will help me to do.

[sunir]

Krisko I thought the same as you about the lower speeds and running out of rpm...Scott Montana, Damion, and several other IP guys use a 3.64 if I recall correctly...I use a 3.73 even though it's a bit shorter...if your concerned and want best of both worlds go with a 3.64 or maybe slightly longer gear, 3.46 maybe? Trust me dude, you'll want that diff the first time a guy pulls you outta a corner you know you had him behind you on!

[krisko]

Originally Posted by Sunir

Trust me dude, you'll want that diff the first time a guy pulls you outta a corner you know you had him behind you on!

I just got my 3.64 in the mail today. It's already happened, I got in front of an NSX and a Z06 just to have them pass me exiting T7 at RA.

[Def]

Originally Posted by krisko

I just got my 3.64 in the mail today. It's already happened, I got in front of an NSX and a Z06 just to have them pass me exiting T7 at RA.

You don't need a diff.. you need a snail, a big FAT snail under the hood. Yep - a lil' boost keeps those pesky cars getting smaller in your rearview mirrors during straights guaranteed!



Seriously though - how much do these diffs go for? A friend was asking me about something for his E39 540i 6 speed and I had no idea what kind of price range. I'm assuming over $800-900 for a rebuilt diff, but could be off...

[vodomagoo]

well depending on builder price varies, diffs online has prices on there site i think. a full race 4 clutch disc build from blanton can be a considerable amount more 2 grand plus but theres no core

[1BADM3]

Originally Posted by JamesM3M5

I don't know that it will drop your laptimes by a full second, but it will help a lot. You can expect a 15.6% increase in RWTQ which is HUGE. That also corresponds to 15.6% better acceleration, because acceleration is proportional to RWTQ.

I agree with James assessment that it will not reduce your lap times one second, but I would like to clarify the torque percentage. As I explained in a separate post, saying a “poor man’s supercharger” or claiming xx% increase in rearwheel torque is the second biggest myth in motorsports. Yes, acceleration is proportional to amplified torque (engine torque multiplied by gear ratio and diff ratio). But because a lower geared final drive moves the torque peak relative to velocity and it causes earlier shifts, the lower geared diff’s frequently result in less acceleration at many speeds. If this were not the case, you could accelerate a dragster with super low geared Norelco shaver motor!

The attached graph shows the amplified torque for my M3 motor (stock except ECIS and ECU). This is actual dyno data and it uses my shift points which are optimized at 7000, 6800, 6600, and 6650 RPM. The blue bar area shows speeds bands were the 3.64 is best and the green bar area shows were the 3.15 is superior. Any speed above 30 mph is a toss-up.

Many folks will argue with physics because they know they can feel a big difference. Yes, they feel a big difference at low speeds when they are in any given gear, but they don’t realize they are ignoring in their mind the lower acceleration at all the speeds in the green zones.

Bob

[1BADM3]

To give you a feel for the laptime savings you may achieve, I will use an example and numerical computations for an NSX (which has different ring and pinion ratios and also has different gearbox ratios available).

Improving track performance (1/4 mile and especially road course) is more difficult than improving zero-to-any speed acceleration due to the fact the amount of time it takes to cover a certain distance is being measured, not the amount of time it takes to reach a certain speed.

Let me explain by using an example. According to Road & Track numbers a 1998 NSX takes 4.9 seconds to go zero-to-sixty and 13.3 seconds to cover the quarter mile. A Ford Focus takes 9.3 seconds zero-to-sixty and 16.9 seconds for the quarter mile. As you can see, the Ford Focus takes 90% more time to reach a certain speed, but it only takes 27% more time to cover the same distance.

Technically this is the case because time-to-speed is inversely proportional to acceleration, while time-to-distance is inversely proportional to the square root of acceleration. For instance if you doubled the acceleration of a vehicle, you would decrease the zero-to-speed time by 50% (1/2), but would only reduce the zero-to-distance time by 29% (1/2).

This is the primary reason low powered cars can have very good road course lap times and it is also the primary reason any changes to gearing have very small influences on lap time. Yes, the professional teams will optimize everything to save the last tenth, but this should be one of the last changes made in club racing.

How much time does it save? I picked the best possible case (starting from 30 mph) and the attached graph shows the amount of time saved after traveling a certain distance down a straight for numerous gearing combinations. The graph shows the most time can be saved by changing the gearbox (not the final drive) and even best cases only improved the time by roughly 0.15 seconds. Two ring and pinion changes are shown and they only show about 0.05 second time savings after a 0.6 mile long straight. I could show different starting speeds that resulted in time lost.

To answer the original question, I would estimate you would save about 0.35 seconds per lap at Road Atlanta with gear changes only. This does not count limited-slip improvements that may have been achieved.

Bob

[Lscman]

Originally Posted by JamesM3M5

I don't know that it will drop your laptimes by a full second, but it will help a lot. You can expect a 15.6% increase in RWTQ which is HUGE. That also corresponds to 15.6% better acceleration, because acceleration is proportional to RWTQ.

This is a terribly flawed argument for gears. Why not install 9.00-1 gears and triple your torque!! At that point, Viper GTS will be hiding from a 330i, right? You did not measure these gains on a chassis dyno, that's for sure. These theoretical improvements you offer ignore one thing....velocity (road speed).


**Rear wheel torque & HP comparisons should be made at the same velocity, not the same RPM. A car with twice the torque being applied to the chassis dyno at half the speed is going SLOWER.


Lap times will not drop if you are going slower with more torque (lol). With deeper gears, higher torque is available at the same RPM because the wheels are turning SLOWER. The car with deeper gears will be forced to upshift sooner. On each upshift, torque multiplication drops by over 20%!! Not many folks want to go slower.....even with more torque...unless they are pulling trees down with a John Deere Diesel Tractor or competing in off-road events in a Hummer or Jeep. Deeper gears can help, but at some point they simply turn 5th into 4th and 4th into 3rd etc. Not everybody wants to convert 1st into an "L" creeper gear and defeat overdrive.

I spend more time in OD than 1st.

[GUINNESS]

Stupid question... what's the difference between the 3.15 and 3.64 I guess in terms of ratio and RPMs ? Is the 3.64 shorter resulting in say 4k rpm @ 80mph in 5th as opposed to 3500ish stock?

I guess I'm asking because I would certainly be interested at a nice compromise between street and track. I'm no where near competative, but I enjoy going to the track. I can't stand being on the hwy cruising at 70 - 80 over 3k RPMs. Short of throwing a 6-speed in there, is there anything that can be done with diffs, or just leave it the way it is???

[sunir]

but this should be one of the last changes made in club racing.

I really have to disagree...I know that Lscman and/or Bob (1BadM3) and I have gone back and forth on this but I think a diff is essential in road course racing at any level be it club, SWC, GAC or whatever...Krisko (and others who want to compete at some point) most everyone has a diff, I think your gonna put yourself at a disadvantage without one...

look I don't have maps and charts and bar graphs or physics theory on this nor do I car about all that...all that I'm saying is from my expereince a good diff is critical, more so than a BBK or something like that...I think it made an improvement in my laptimes but I can't prove that so I am am not going to sit here and say that it can bring down your times for sure because I know Lsc or Bob are going to bust out the " hypotenuse of the sqaure root of the flux capacitor...yada yada yada"

look whatever ya'll do what you want...I'll stick with my diff!

[krisko]

I think the proof, besides lap times obviously, is at what point will you reach max speed down a straight. At Road Atlanta T10a, I am about 135 at the end. With a new diff, I think my 5th gear redline will be 130. If I'm 130 for 1/3 rd of the straight, then that has to be an overall improvement even though I'm slower than I was with the stock diff.

Of course the only thing that matters is lap times. Judging by info from this thread, I'm hoping the new diff and better skillz will get me consistently into the 1:44s at RA.

[1BADM3]

Originally Posted by krisko

At Road Atlanta T10a, I am about 135 at the end. With a new diff, I think my 5th gear redline will be 130.

I'm hoping the new diff and better skillz will get me consistently into the 1:44s at RA.

I didn’t realize you were hitting 135 mph on the back-straight. If this is the case, the 3.64 diff will certainly hurt your segment time on the longest straight. I change my prediction that the 3.64 diff will not measurably help you at Road Atlanta (helps a little everywhere else, but hurts on the back-straight).

Different final drive ratios will give you a few tenths depending on the track. Every car will have different optimal final drive ratios for every different track. Therefore must be changed out based on track.

You can easily get 1:44’s at Road Atlanta. This is typical of stock class (motor and weight) E36 M3’s. With more experience and your other more important mods (tires, suspension, and weight) you will be lower than that.

Bob

[krisko]

Originally Posted by 1BADM3

I didn’t realize you were hitting 135 mph on the back-straight. If this is the case, the 3.64 diff will certainly hurt your segment time on the longest straight. I change my prediction that the 3.64 diff will not measurably help you at Road Atlanta (helps a little everywhere else, but hurts on the back-straight).

Yeah, but I'm hitting 135 for about 1 second before I go max brakes. From the Porsche bridge to braking point, I probably go from about 100 to 135. I'm thinking if I can be very near 130 at the Porsche bridge and hold that, then the cumulative segment time would be reduced. Not to mention, I don't even get 5th gear down the front straight whereas with a new diff I can hopefully hit max speed (130 or so). It's all kind of speculative considering my engine is strewn across a garage right now.

[Def]

I think the biggest gain from a diff in road racing comes from alot of corners out there tend to be a bit too fast for a stock 2nd gear or too slow for a stock 3rd gear. If the guy next to you have a 3.64 and you have a 3.15 and you both exit a 50mph corner in 3rd side by side, he's going to be in front of you in about 3 seconds easy even if you have identical powerbands.

I never got a chance to test my M3 with the 225-230rwhp it probably had at Road Atlanta, but bone stock(212rwhp) I was hitting 125mph give or take a few mph right at the braking zone of 10a.

So I'm thinking a 3.64 might be a tad aggressive for a 5 speed. That's a ~15% shortening of the gears - that's very close to making your 5th gear just like your stock 4th gear.

So I have to ask you - what's the difference in a 3.64/3.73 diff and driving around with your car in one lower gear on the stock 3.15? Sounds to me like a 3.46 is what's really needed for tracks with longer straights like Road Atlanta, Road America and VIR.

What do you think is going to happen when you're nearing the braking zone of 10a and you top out at your 130mph top speed with 200-300 yards to go and the guy next to you continues to accelerate right by you? Leaving such a glaring weakness open in your setup doesn't sound like a wise decision when you're racing - but that's just me. For DEs knock yourself out - a 4.10 would make 1st gear a big smoke show too!

[krisko]

Originally Posted by Def

Sounds to me like a 3.46 is what's really needed for tracks with longer straights like Road Atlanta, Road America and VIR.

Yeah, the more I think about it I probably should have gotten the 3.46. I'm at RA a lot and at Sebring and VIR once a year so I'll lose out there a little bit.

Eventually I'll get another diff, maybe a 3.23 or 3.46 and I can swap them out as needed.

[ceegeezM3]

Yeah, the more I think about it I probably should have gotten the 3.46

Krisko, I'll trade ya. I bought a 3.46 from Dan, and it's still too long for the shorter tracks that I drive. It's only got 1 race weekend on it. I'd give you some cash too, to help fund your motor rebuild.

[krisko]

Originally Posted by ceegeezM3

Krisko, I'll trade ya. I bought a 3.46 from Dan, and it's still too long for the shorter tracks that I drive. It's only got 1 race weekend on it. I'd give you some cash too, to help fund your motor rebuild.

Hmmm, PM me with some details.