A Diff Ratio Question

[ja9]

I need some advise.
I predominately race at SMP South circuit these days and have noticed through video something I don't understand exactly. I drive a NB8A, 6 speed with a 4.1 diff ratio. My issue is coming down the hill (back part of the circuit) at the tight left hander and coming back up the hill through the right hander. On both corners my car is revving its head off at 8000+rpm. I don't want to rev the engine so hard. Now before someone suggests the obvious, I would need change in to 4th and literally straight after would need to change back. Seems like a lot of waste of time changing gears if I take this route.

So over the top of the hill I'm in 3rd at 8000+rpm, at the bottom of the hill I change into second to come up the hill. Before I get to the top of the hill I need to change to 3rd not to over rev again. I've been keeping it in 3rd round the right hander but car bogs down too much, losing a fair bit of time. My feeling is a 3.9 diff ratio would be a better choice for SMP south.

Through video I've noticed some drivers are changing gears in completely different parts of the track, later, much later. Now they may have a different diff ratio to me but is there any other factor that may be at play. Reason I'm asking is because I believe one driver in particular has a very similar set up to me (wheel size, diff ratio and 6 speed) but changes gears in completely different parts of the track. The only conclusion I can make here is that I'm wrong in assuming he has the same set up as me.

What do you guys think?

[Dan]

Now they may have a different diff ratio to me but is there any other factor that may be at play. Reason I'm asking is because I believe one driver in particular has a very similar set up to me (wheel size, diff ratio and 6 speed) but changes gears in completely different parts of the track. The only conclusion I can make here is that I'm wrong in assuming he has the same set up as me.

What do you guys think?

Speed is the factor which you didn’t mention.
- If he is running the same setup as you, is in the same gear and is at lower RPM then you are going faster around those corners.
-If you are the same speed or slower and are running at a higher RPM then he has lower gearing than you either through final drive, gearbox, wheel size or a combination.

If you are serious about changing final drive consider possibly going up a ratio as well, you might find it will allow you to get more drive in other places on the track and the amount of downshifts you need to do will remain the same (at least in that section of the track).

[mx5002]

Hey Ja9,
I run a 4.1 with a 6spd. Although mine Doesn't rev to 8k I find its faster when I shift to 4th before the off camber left at the top of the hill the car feels more stable in 4th round it then I use the free horsepower (downhill). back to 2nd to punch out of the left hairpin then into 3rd for the up Hill right hander.
Maybe a 4.3 would suit you better?

[ralt]

Hi.
If the other party seems to have better gearing it may be time to indulge in some verbal interaction with them. Who knows by getting to know them you may have a like minded person as a friend.

[Magpie]

Going to a lower diff ratio will REDUCE torque at the rear wheels, however will allow a higher top speed.

What are your corner entry/apex/exit speeds like when compared to the other person? Are the tyres/suspension setup the same as the other car? Are the engines the same?

I only have about 25 laps at SMP South so can't really comment on anything else.

[zossy1]

Hi John,

I'm wondering whether you are referring to David, or maybe to me...? Maybe someone else...?

David runs a 4.3 and I run a 4.1. Our times are similar but our cars are very different. David has a bit more torque than me and runs the 565 profile Dunlops with a spool, whereas I have the taller diff and taller tyres and run a Giken but run a higher rev limiter - meaning I am often a gear down on David (which is good in parts, bad in others).

For most, I would think a 4.3 would be the diff to have at South circuit. IMO, you will not find a 3.9 faster - probably several tents slower. It will also suck at WP and at the GP Circuit.

EDIT: Why not post some video? Might help us help you...

[1600Dave]

You don't mention what modifications your engine has (ie why you need to run out to 8000 rpm). You may find that 8000 rpm "sounds" fast, but changing up a gear may not result in any time lost (may even be faster). If you want to get serious, invest in a data logger (a QStarz GPS unit and racechrono on a mobile phone will set you back less than $100). Look at your corner speeds if you hold the gear, and corner speed if you change up a gear - I reckon you may be surprised at the results.

One example - Sunday before last, I was suffering slight overheating after 5-6 laps and resorted to short shifting for a lap or two to bring temps down. Felt painfully slow, but lap times only suffered by a second or thereabouts.

But then I am running a dead stock nb8b, which may have very different engine characteristics to yours (mine hits the limiter at 7000 for a start).

[zossy1]

Here is my video John - I change going down the hill, then hold second to the top of the hill. Big revs like you - but it works.

[youtu]http://youtu.be/0TPhqb4PAt8[/youtu]

[mazmad]

Hi John
Presume you are referring to Stu or Steve, both similar NBs both 6 spd and both at your last outing (wish I was too...).

Stu has a 4.1 if memory serves.... As Chris says, set your car up for that circuit and it will ruin it for others. Though I know of someone who was changing diffs for the Sth Circuit.

Cant honestly remember in the NB but in the NC there were a couple of spots where I would grab 4th just for a nanosecond. But it was worth it, I compared times against not doing it.

One thing I would be worried about over revving it on the downhill into the left hairpin, so changing might be prudent.
Cheers.

[Celluloid Retina]

If its my car we're talking about, I can't be sure exactly what my final drive is. Various info I have found says its either a 3.6 or a 3.9 (as per Australian NB8B delivery specs) not a 4.1. That said i've never cracked it open and looked.

http://www.miata.net/garage/KnowYourCar/S8_Gears.html

Mine is at the top of 3rd gear coming down the hill into the hairpin at SMSP south, then 2nd through to the next hairpin, dab of the brakes and carry it up the hill plucking third at the right kink.
It also has no need for 5th gear down the straight at Wakefield (where most are plucking 5th at the start/finish or before) and needs 2nd through the fishhook and turn 10 or it bogs down. So whether its a 3.6 or a 3.9 I wouldn't recommend it... its a long way between gears and generally for important corners you'll find yourself between gears. 4.3 with a six speed seems to make people happy.



last time out at SMSP South.

[hks_kansei]

If its my car we're talking about, I can't be sure exactly what my final drive is. Various info I have found says its either a 3.6 or a 3.9 (as per Australian NB8B delivery specs) not a 4.1. That said i've never cracked it open and looked.

If it's an Aussie delievered car, and still has the original diff, it will be a 3.6

The 3.9 was only on the 10AE and the Heritage models in Australia. (maybe the SE as well? not sure on the SE ratio)


If you can post up your tyre size, and exact engine RPM at a certain speed, and what gear you're in (plus if it's a 5 speed or 6 speed box), I can do the maths and work out the diff ratio.

[madjak]

Since I do a variety of events choosing an ideal diff ratio is impossible. I don't have the time to switch diffs between events so my philosophy is to have the highest ratio diff you can where you're not maxing out revs in top. In my case that's a 4.78 ratio with a 6 speed which gives me the most choices of gears for any corner that I encounter on a mixture of tracks. Sure some corners are not ideal but because the gears are so close, the next gear up still keeps you in the power band. Time lost in gear changes is made up by the greater selection of gearing at corners.

On my local track, moving from a 4.1 to a 4.7 didn't actually change the total number of shifts as you'd expect, but moved them all up one so instead of using 2nd - 5th I'm now using 3rd to 6th. These means I'm shiffting in a results in less shifting time as it's a H pattern instead.

If all you do is drive at a specific track then sure you can select the most ideal diff... but this is affected by so many factors which can't really be covered in a forum. Your power band, max revs, vehicle weight, tyres sizes and grip levels as well as aero and driving ability/style all factor on how fast you are through a certain corner and what would make for the ideal diff for that track. You can't just duplicate another setup unless you drive it first and see.

[ja9]

Thanks gents, all valid responses.

In an effort to being transparent I was talking about Stu (I hope he doesn't mind). I was always under the impression that Craig's old car (now Stu's) had the same setup as mine. I would say even Craig thought he was running 4.1's. Judging by a previous post a 3.6 or 3.9 would be correct and in sync with what I'm seeing in the video. In the last round of SMP south heat 2 at the FOSC meeting I was running 3rd behind Stu and Scott. Every lap I would lose a little time coming up the hill and out of the right hander. As mentioned I was leaving it in 3rd gear to avoid over revving but all this did was to bog the car down. Then I would have to work hard to make up the time only for it to happen again. Eventually I had some gremlin and had to pull out of the race.

Chris I would love to post a vid but don't know how and don't have any editing software to edit the early camera start out of the video. I'd be happy to come to someone's place with my GoPro to help me out and to post the vid here for anyone interested in viewing. I've just checked out your youtube post and your gear change points and revs are similar to what I'm experiencing.

Again thanks for all the replies.

- John

[mx5002]

You can learn via youtube to upload and edit videos.
However I cheat and put the Memory card in my smartphone, crop it's length down then upload it Through the YouTube app on my phone (connected to WiFi)

[mazmad]

One of the many complex conundrums to do with these bloody race cars ;)

John I think maybe you are right I did change it possibly to 3.9 not 4.1 when it had the torsen but when it went full locker.... who knows...