Member
Can anyone provide some examples on what a diff ratio change has done for track lap times?
For example, if I changed from my OE 3.15:1 to a 3.38 or 3.64, what should I expect to see in performance?
I'm far less concerned with the 'stop light drag race' effects and butt-dyno claims.......but is the expense really worth it in lap times?
Help me feed my addiction.
Cheers,
Chet
end user of fossil fuels
Depends on the change and depends on the track. But, yes, a higher ratio should yield better laps times. BUT, there are better ways to improve lap times. Diff changes aren't the first on the list...seat time is. But, a good functioning LSD is a must.
It's not speed that kills, it's the speed difference that does. Obviously you aren't going fast enough.
Turning Benjamins into noise since 1997
I read a list of the 100 things you MUST do before you die. Funny, "Yelling 'HELP'" didn't make the list!
Member
Yes, this statement may be absolutely true and I can always use more seat time.Originally Posted by gobuffs
The question could have been prefaced by: If I have aproached the performance potential limitations of my current gear ratio.......what gains might be possible with a higher (numeric) ratio diff?
Note I didn't say:
"how much will my 0-60 time drop with some new LSD."
;-)
Also note that I'm not asking if a diff ratio change is the best bang for the buck or an appropriate modification for my car at this juncture. That's a different question and I can answer that for my circumstances.
So, the question still stands:
Can anyone provide quantifiable results from a diff ratio change?
Same track + same driver + X diff ratio change = Y lap time change
end user of fossil fuels
It really depends and I bet there isn't that much data out there. Going from a 3.15 to a 3.46 may give you 2 seconds on one track and cost you 5 seconds on another. If you got 2 seconds from a 3.46, and 3 seconds if you did a 3.73, but a 3.91 might net you nothing.
Really not as simple of a question as you are trying to make it out to be.
It's not speed that kills, it's the speed difference that does. Obviously you aren't going fast enough.
Turning Benjamins into noise since 1997
I read a list of the 100 things you MUST do before you die. Funny, "Yelling 'HELP'" didn't make the list!
RacerDeacon
Not a chance....at least nothing that's usable for you. Every track is different, every car has a different power curve, every car exits corners at different speeds and thus different RPM's and thus a different point in the gearing curve. A 3.23 may be faster than a 3.91 on a certain track depending on the layout and the car.
Member
Yes, sort of. Can't do it in lap time but in speed at the end of the back straight at Watkins Glen. E28 535is, stock 3.25:1 LSD, street tires, reached ~117 at the braking zone for the Bus Stop. Same car, same tires, 3.64:1 LSD, managed just about 120 mph at the same point. The 535i is a low-powered, heavy car, and the diff really made a noticeable difference in terms of getting into the powerband quicker. But, don't discount driver improvement between the two events as well.
-tammer
Spec e30 FTW!
With the stock engine in the ti (125rwhp) and stock 3.45 LSD I'd run ~2:16s at Thunderhill, depending on tires and track condition. I'd see speeds just about 100mph on the front straight.
Switching to the ti's automatic-sourced 4.44 LSD I dropped to a low 2:14 within just a few laps, and saw front straight speeds in the 106-107 range. Considering the calculated top speed of that diff on that car at 7000 in 5th was 110 or so, it was pretty optimal.
So, with not much hp on tap, it made a very significant difference. Bryan is certainly correct though, this just happened to work well in this situation, certainly YMMV.
Member
Bryan,
I understand your point. And I agree with certain aspects at least.
I figured I'd start out with general questions rather than specifics like:
'95 M3 with mostly stock powertrain and thus stock-like power/torque curves. At Mid-Ohio the stock 3.15:1 gives me less than optimal gearing for the exit of two of the most critical corners on the track as I run out of revs in 2nd, but am not really in the power band in 3rd gear. So a shorter ratio would allow a significant advantage. Same is true through turn 1 and up the hill to keyhole. 3rd gear for that short stretch is a bit of a waste of time for a small benefit in acceleration. But a shorter ratio would be a nice benefit there too. How much though? Are we talking tenths, or full second changes??
I didn't figure anyone would have data specific enough for my true question. But perhaps?
At Grattan, the stock 3.15:1 has pretty good gearing for that track. Perhaps a slight change to a 3.23 would offer a performance advantage, but in general a 3.45 or so might cause some 'wasted shifts' or get me into the scenario like above at Mid-Ohio.
Cheers,
Chet
Member
Chet, let's analyze that for a bit. You use 2nd gear in two corners? I'm guessing the carousel and the keyhole?
I keep my car in 3rd at both places, given I have a 3.2L with a 3.23:1 stock diff. I have ridden with some of the fast instructors but I've never noticed any of them shifting into 2nd gear, I doubt the racers do either.
One thing I've noticed is that if you carry more speed through certain areas you're able to stay in the higher gear (kind of a duh comment) but it applies to Mid-Ohio for sure. Downshifting seems to slow the car down too much. For example in class we discussed keeping the car in 4th through the chicane and downshifting to 3rd as you enter the keyhole or do you downshift to 3rd midway through the chicane? I do the latter but because I know I'm not carrying as much speed through t2 as others (the chicken factor). I'd rather get my downshift done and over with and then the entrance to the keyhole is simply a left foot brake and turn in maneuver.
The other is t1, I'm in 4th and would be crazy to think about a downshift there, I only need to slow the car down some and plant the nose for my turn in then it's back to WOT. Perfect corner for a small amount of left foot brake again.
I don't know I guess how do others drive Mid-Ohio with a stock diff? I know Mr. Berkowitz who is pretty darn fast there probably has much different shift points but he's using a 3.46 or 3.64 diff? What are your thoughts Scott.
Carlos.
Member
Nope, I'm in 3rd for both carousel and keyhole as well. Can't use 2nd or I'll run out of revs at a point I don't want to be up-shifting......
I stay flat in 4th through 1 (as you described), downshift to 3rd after turn 2 during braking, 3/4/5 down the main straight. Then 3rd from the end of the back straight into thunder valley where I upshift to 4th. Carry 4th until the brake zone for carousel, back down to 3rd. Upshift to 4th at roughly S/F line on pit straight. Lather, rinse and repeat.
But, I'm very near the rev limit with my 3.15:1 between Madness and Thunder Valley. I don't have the balls to stay flat through there or I'd need to upshift for sure.
Anyway, pulling 4th gear out of 1 is a bit slow, as is the left hander before Carousel. And I could definitely use a bit more torque out of Keyhole. Those damn turbo P-cars (read: Park and Shoot) are so annoying!
I only had 1 good dry session when I was there last and I was on street tires running 1:51 flat. Slow, I know.....but hey I'm out there and having fun.
Keep in mind this is a 3.15:1 diff and 3.0L '95. So the torque of the 3.2 coupled with the 3.23 diff helps you get off the same corners (if we assume same corner exit speeds).
It is good to get another person's perspective from the same track in a very similar car.
Thanks Carlos!
Cheers,
Chet
Man of fixing steel
I ran a 3.73 there and it was perfect... our cars aren't very similar though.
Member
Hey Carlos,
I have a 3.46. I drive Mid-Ohio a bit different than alot of guy's, but it seems to work for me. For the club course(chicane) I downshift to 3rd. in T3 and T13. It seems to settle the car for me and my lap times have been pretty good. Like you said in T1(4th. gear) I just tap the brakes to settle the front and go back to full throttle. I haven't driven the club course yet with the new stiffer suspension set-up so all that could change. Most of my events there this year will be the pro course which takes a complete different approach..
Member
Scott,
How is the gearing (3.46) for Putnam. I am currently in a 95 M3 with stock diff (3.15) and except for turn 7 I am in the powerband. Bounce off the rev limiter 2/3 of the way to 5 and into 9. 3rd gear from 2 to exit of 10. 4th+/-5th down the front sraight.
You had mentioned last year possibly going to 3.64 or 3.73. Is the current gearing working well that you are not going to switch??
(Congrats on the NASA win at Putnam a few weeks ago!)
Thanks,
Tom
Devil's advocate
My fastest lap at MSR was with a 2.93 open diff. The closest I've got since was within 1 second and that was with a 3.46 LSD, Weird.
Member
Thanks Tom,
I had to drive hard to keep that pesky #001 M3 behind me. He(Sean T.) did turn the fastest GTS3 laps of the weekend. I got some good breaks in traffic which really helped. Even had to use the grass to avoid/get around a couple the SM's.
The 3.46 works pretty well for me at Putnam. 5th to 4th. at T1 down to 3rd. for T2 with a quick upshift to 4th headed to T4. hold 4th to T7 down to 3rd. hold it to T8 though I'm on the limiter just before turn in. Up to 4th between T8 and T9. Back to 3rd through 9-10. I don't know if a 3.73 will make any difference in lap times, but I'm going to try one and see what happens. I can always go back. Hope that helps
Cheers, Scott B.
Member
Scott,
Thanks! That helps a lot. I am wondering the same thing as the OP.... is there really that much to gain by going to a different ratio. You are shifting quite a bit more than I am. I hate the thought of buying a new diff for $2-3K only to find out I am slower due to shifting twice as much. I am impressed you shift between 9-10, not an easy off camber turn! I shift into 4th about 30-40 yards before start finish tower (usually about 7000-7100 RPM in 3rd at that point, rev limiter 7200).
I have never felt I was out of the powerband at Mid-Ohio, and at Road America turn 5 is the only place I feel I could use a lower ratio. (There, I just need more than the 225 RW Hp I have!)
Thanks,
Tom
Last edited by Boro; 06-01-2007 at 01:56 AM.
Member
Tom,
Do you stay in 4th from Thunder Valley to the brake zone for Carousel?
So what do the IP (or similar) folks run for diff ratios at Mid-O then?
I'm with you, it is quite a sum of $$ to 'try' a few different ratios! I'd like to learn what (if any) benefit might be had in tangible #'s. Doesn't look promising.
Cheers,
Chet
Member
Tom,
I don't shift between 9-10. I upshift from 3rd. to 4th. between 8 and 9 then back to 3rd. entering 9. Remember we are racing and looking for the very last tenth of a second. In the classes I run that can be the difference between first and fifth...
Member
FWIW I am running a stock 3.2l IP class car and run 2.05's at Thunderhill. My car will "lug" out of turn 12a and I have problems between 2 and 3. (In between gears) I am switching to a 3.73 and will prob pick up 2 seconds MAX.
Questions I have are, do you have an extra gear at the end of the longest straight? Will the diff change allow you to use the extra gear long enough to make an appreciable difference? Are you between gears at more than 1 turn?
The issue at hand is that the right diff for Thill may not be the right diff for Infineon, or Reno or.... At best short of buying 3 or 4 diffs you're going to have to compromise somewhere. Not having the budget for 3 or 4 diffs is part of the reason for my diff choice. It is the best compromise for me at my local tracks.
Member
I've run both a 3.25 and a 4.10 in my street e30 M3 at GIR (euro car...originally had dogleg tranny with 3.25). There is a significant difference in lap times going to the 4.10. In my track car I have a 4.45 and it is a big difference over the 4.10.
Here's a short vid of Putnam from last Oct. with the PCA. This was my second time at Putnam and the first time in my track car. First time was in my street M3 with the 4.10. When I watched the video, I immediately could tell I was in the wrong gears at a couple of places. I should have carried 5th into T2 instead of down shifting to 4th at the entry to T1. And, I should have carried 4th through T9 and T10.
Overall you are taking advantage of the torque multiplier of the numerically higher rear end when you swap diffs. But, it may put you between gears depending on the track and turn.
This was in the instructors run group....Oh...and just ignore that little boo boo at the end.
Putnam 10 MB
Damon in STL
'88 e30 M3/M42t - GTS3 #72 - Motorcraft Ign., Volvo Injectors, Thrush Turbo Muffler, Open Source ECU, Aerospace Connectors, Lowes Polycarbonate, Alumacore Front Splitter and Rear Diffuser, Honda Radiator(s), Racer's Tape (white), Tornado, Various Stickers, Farm Implement Paint (gloss white), Nationwide Series Windshield (Fontana version), GMC Boost Solenoid
My current car: e30 M342t Evolution
Member
I'm not sure but I imagine the original poster is referring to a street/track car as opposed to a full race car (because a full race car would swap cogs depending on the course if allowed....hell, even in autox, a Bob Tunnell and the like will travel with different diffs they might change overnight depending on the course)
In general a street car will benefit from a shorter final drive as they are often geared for fuel economy, take my MCoupe which came with a 3.15...a third that was good for 110mph with 275 rear tires!!! . A 3.46 is a vast improvement on track and track data shows it clearly.
Member
I thought M Coupes came with 3.23:1 diffs like the 96+ M3s. Or did someone change yours out?
Carlos.
Member
Scott B. and Chet
Scott,
Sorry, I miss read your post... thought it was a little weird to shift b/w 9-10. I have to keep at least neutral throtle after the crest in 9 or the back end tries to lead the front onto pit lane! You must be shifting into 4th right at track out of 10.
My concern with the 3.73 is I will simply be using 4th and 5th gear the same way I use 3rd and 4th now. But I really don't know since I have never tried a different ratio. Incidentally, Fall-Line had recommended a 3.46 for my car. Your thoughts??? (stock motor with chip headers/exhaust (no cats - track only car) and larger air meter and euro air box. Probably due for a rebuild... its got 130K miles on it)
Chet,
Yes, 4th all the way through thunder valley into the braking zone into carousel. I do shift into third braking for T2 but only to be able to transfer weight better/quicker to the rear with the throttle through T3. Otherwise the rear gets a little loose right at the 2nd apex.
Thanks,
Tom
Member
S54 M Coupes came with 3.15. between the 5 speed and the politics of making sure it was not faster than the e46 m3, it made perfect sense to BMW.
hot shoe
Nope, I'm in 3rd for both carousel and keyhole as well. Can't use 2nd or I'll run out of revs at a point I don't want to be up-shifting......
I stay flat in 4th through 1 (as you described), downshift to 3rd after turn 2 during braking, 3/4/5 down the main straight. Then 3rd from the end of the back straight into thunder valley where I upshift to 4th. Carry 4th until the brake zone for carousel, back down to 3rd. Upshift to 4th at roughly S/F line on pit straight. Lather, rinse and repeat.
But, I'm very near the rev limit with my 3.15:1 between Madness and Thunder Valley. I don't have the balls to stay flat through there or I'd need to upshift for sure.
Anyway, pulling 4th gear out of 1 is a bit slow, as is the left hander before Carousel. And I could definitely use a bit more torque out of Keyhole. Those damn turbo P-cars (read: Park and Shoot) are so annoying!
I only had 1 good dry session when I was there last and I was on street tires running 1:51 flat. Slow, I know.....but hey I'm out there and having fun.
Keep in mind this is a 3.15:1 diff and 3.0L '95. So the torque of the 3.2 coupled with the 3.23 diff helps you get off the same corners (if we assume same corner exit speeds).
It is good to get another person's perspective from the same track in a very similar car.
Thanks Carlos!
Cheers,
Chet
Chet that helps, hopefully Scott will reply. My advice will be to carry more speed, before you go the 3.23 or 3.42. I believe the ITS e36 325's run out of revs in 5th down the main straight with anything shorter than 3.42's/7k rpm limit/15or16" wheels.
I recall using 2nd in the carousel (apex speed in the low 40's), and depending on the car, 2nd in the keyhole. 4th wasn't too slow for 1 & exiting thunder valley.
Once you pop for R's, you'll carry more speed, and be in the meatier part of the rev band there. At some point shorter gears could help, but I wouldn't recommend them right away.
I can remember testing shorter and taller tires. It was rare that the shorter tires worked on tracks that needed 5th gear (tested in an VW, Honda CRX, & Honda S2000). MMV - what works for me, doesn't always work for others...
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