MONDA - the Honda Powered MX5

[lightyear]

You got a lot more revs than an Oem motor. So a 4.3 might work on a tighter track like Wakefield. A 4.1 would be around the money otherwise. My NA8B would do 206km/h in 6th on the limiter at 7,200rpm at Phillip Island for a couple of seconds at the end of the straight with 4.3 ratio with around 90rwkw. No other track here could get close to maxing out.

[Magpie]

Actually I have just been looking at the exact question...

The final ratio will only move the available torque/acceleration up or down, it will not change the point at which it is delivered. To change the point of delivery requires a gearbox ratio change.

For my NA6, the best acceleration is acheived at 6,250 with any type of rear diff ratio. In 3rd at 6,250 rpm a 4.3 diff will deliver 5% more acceleration than a 4.1 (4.42m/sec2 and 4.21m/sec2), whereas a 5.1 diff will deliver 24% more acceleration than a 4.1 (5.24m/sec2 and 4.21m/sec2). However the 5.1 will more than likley require more gear changes to keep it on song, like a 2 stroke motorbike (road bike KR250, RZ500, H2 Mach IV etc). Again horses for courses.

The other factor to consider when looking at the final drive ratio is the max speed the car can acheive based on its resistance and power. Is it sensible to have a final drive ratio that could acheive 260k but the car could only over reach 225k? Therefore why would you need gearing that puts you beyond this number?

Some people choose the final drive to have fuel economy and have the lowest possible revs at 100 kmh, hence a 3.6. A hill climb car that put acceleration above economy will probably want a 5.1 or better.

However, when you do extensive modifications to the engine a stock gearbox may no longer be suitable due to the change in the torque curve and a custom gear set chosen to make use of the new torque curve maybe required... Unless you can do the modifications that just raises the torque curve and not changes its shape.

[rascal]

The final ratio will only move the available torque/acceleration up or down, it will not change the point at which it is delivered. To change the point of delivery requires a gearbox ratio change.

This is incorrect. The torque multiplication from engine to wheels is a combination of gearbox ratio and final drive (diff) ratio. Changing either or both of these has the same net effect. (ie changing the point of delivery in your example above)

[Dan]

^Yeh, I was confused reading that too. As well as the point about acceleration increasing with a change of Final Drive.

The way I think of it is regardless of final drive the torque will be delivered at the same point in RPM, what will change with final drive is the speed at which the car is travelling at when this torque is delivered.

Therefore acceleration doesn't increase just because of the addition of a higher final drive, it is just that the higher final drive allows the cars engine to stay closer to the upper end of it’s torque curve at the same speed.

[Magpie]

rascal we need to be careful to not go off topic, this is a build thread But this is a bit nerdy so it may be tolerated...

What I'm saying is that just changing a ratio only shifts the curve up or down it does not change its shape, its shape is defined by the torque curve of the motor! The gearbox and rear dif are only a constant multipler of what the engine produces.

Acceleration m/sec2 (3rd gear, 6 speed box, my car's torque curve)

RPM 1 2 3 4 5
6000 9.58 5.78 4.19 3.20 2.55 (4.1)
6000 10.04 6.06 4.40 3.36 2.67 (4.3)
6000 11.91 7.19 5.21 3.98 3.17 (5.1)

6250 9.63 5.81 4.21 3.22 2.56
6250 10.10 6.09 4.42 3.38 2.69
6250 11.98 7.23 5.24 4.00 3.19

6500 9.37 5.65 4.10 3.13 2.49
6500 9.82 5.93 4.30 3.28 2.61
6500 11.65 7.03 5.10 3.89 3.10

The peak acceleration is at 6,250 for each diff ratio, the difference is the amount.

I have now done the same calcs but on a 5 speed box...

Acceleration m/sec2 (3rd gear, 5 speed box, my car's torque curve)
6000 7.99 4.81 3.39 2.55 2.07 (4.1)
6000 8.38 5.04 3.55 2.67 2.17 (4.3)
6000 9.93 5.98 4.21 3.17 2.58 (5.1)

6250 8.03 4.83 3.41 2.56 2.08
6250 8.42 5.07 3.57 2.69 2.19
6250 9.99 6.01 4.24 3.19 2.59

6500 7.81 4.70 3.31 2.49 2.03
6500 8.19 4.93 3.47 2.61 2.13
6500 9.72 5.85 4.12 3.10 2.52

Again the peak acceleration was still at 6,250, the difference is that the 6 speed provides greater acceleration (1.646 gearbox ratio as opposed to 1.33). This is because of the torque curve and even if you changed the gear set, the peak would still be at 6,250. To change this point would require a change to the torque curve.

I have been looking into this because of some feedback on my car and looking for theoptimun shift point. That is when changing gear the next gear you select should be developing more torque. However with my build (and rev limit) I do not have an optimun shift point, each gear change I make will always result in the next gear having less torque. Plus changing at 8,000 (in general) puts me at 5,500 (with a 4.1 diff) and this is also below the peak acceleration. What the 4.3 does is increases the RPM on the gear change for the next gear selected and brings it closer to peak acceleration. Interestingly for me this happens with about a 5.1 rear diff. The 5.1 dif also aligns with the cars max speed once resistance is taken into account.

I have attached my excel file so that people can play around. Change the values in Pink, as well as the weight in G10. Enjoy

[rascal]

To bring back to oztrackdays question on what diff ratio to use, for a track car, you usually want the shortest (numerically highest) diff ratio you can run without it negatively impacting your top speed too much. ie the 4.3
For a road car (which is how he describes this one) cruising rpms, noise, economy may be of more concern, which would suggest 3.9.

But based on OP comment of "Don't care about the Highway", I would get the 4.3 as this would give you the best acceleration of the 3 choices.
You have 15% more revs to play with compared to BP motor, and even 3.9 to 4.3 is only 10% different so MONDA with a 4.3 is still further from redline in top than BP with a 3.9..

[oztrackdays]

"The way I think of it is regardless of final drive the torque will be delivered at the same point in RPM, what will change with final drive is the speed at which the car is travelling at when this torque is delivered."

This is one of the sentences I could understand.

MONDA has the same 8200 rpm rev limit as my NA8 Racecar (with a 4.3 welded spool) and at Bathurst Downhill
wind assisted best I could do was get was get to the rev limiter in 5th ignore the awful driving
as much as you can and notice how long it takes to run out 4th and then 5th is an eternity considering
it's the 1.0 ratio.

[youtube]http://www.youtube.com/watch?v=P-KyiYVhSU4#t=298[/youtube]

I take Magpies point that at some point you have to acknowledge that the OEM 6 speed spread may not be ideal.

I think if the 4.3 means I could use 6th Gear at the few longer tracks we have like SMP,PI,Bathurst
then it might be the go. Perhaps even a 4.4 where might I get one of those I think the Mazda Bravo
or Kia Sportage have them as OEM.

[rascal]

I think if the 4.3 means I could use 6th Gear at the few longer tracks we have like SMP,PI,Bathurst
then it might be the go. Perhaps even a 4.4 where might I get one of those I think the Mazda Bravo
or Kia Sportage have them as OEM.

If you are not using/needing 6th, then your gearing is too tall, and are wasting the opportunity to run a shorter diff and gain the advantage of closer stacked gears that a shorter diff provides.

[Patrick Bramston]

Hellmun and I with 200rwkw are selecting 6th gear at 195k's at the control tower at Wakefield with 4.1 diff ratio.Not sure if that helps

[sailaholic]

I would think the 4.3 would be way too short for the green hell?


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[Magpie]

This part if the thread may need to be split out as it is a good discussion!

My calcs were looking at acceleration and not speed. Gear selection based on speed on the track without understanding the logic, at least to me is flawed.

I could re do the calcs based on speed but it would tell you very much.

At least for me both the 5 and 6 speed OEM boxes are not a good match for my torque curve. Possibly the Monda could be in the same boat.

[Hellmun]

I think if the 4.3 means I could use 6th Gear at the few longer tracks we have like SMP,PI,Bathurst
then it might be the go. Perhaps even a 4.4 where might I get one of those I think the Mazda Bravo
or Kia Sportage have them as OEM.

If you are not using/needing 6th, then your gearing is too tall, and are wasting the opportunity to run a shorter diff and gain the advantage of closer stacked gears that a shorter diff provides.

See I get that works for formula cars with sequential gearboxes but with a H pattern box changes cost time and then you've got gear/rpm issues in relation to corner speed/brake points. Two problems you can work on with the differential ratio (though obviously it's not as good as tailoring individual gears). From the torque graph on the previous page I wouldn't say the K20/24 is peaky and lacking a power band (that torque graph is awesome for N/A) so unless your running into problems at particular corners or top speed at a track I wouldn't touch it until I ran a few more tracks and identified any problem area's...plus started looking for any new issues you might cause moving RPM up X amount on a corner for a ratio change. I think Phillip island and Eastern Creek GP are the only places I'd personally hit the limiter in 6th now...or at least very close.

For me I found lap-time back with 90kw by changing my gear ratio down via tyre profile. When I moved from 225/45/15 to 225/50/15 I stopped hitting the rev limiter on the back-straight of wakefield and could use 2nd Gear through T10 (Fixed similar issues at EC too). Same diff ratio but about 5 tenths lap time improvement going the opposite way. Now with boost...changing gears is even worse due to the extra time needed to refill the intake after the BOV opens. So I've been tempted to go back to my standard 3.606 just so I'm changing gears less and can sit in the power band longer. I don't have it handy but I made a graph of 90kw N/A vs 200kw FI with speed loss per gear change. I think it was 2-3kph consistently less loss N/A, drag for the car was the same.... I was curious because I was expecting a much higher terminal speed. Best N/A terminal speed was 161kph...more than 100kw netted barely 25kph.

BTW Pat according to the GPS I'm redlining 5th at about 182kph and only peaking at 187kph on the best data log I have before T2. Your speedo would be out due to your 16's and tall slicks. Though I haven't run 14psi yet or had a full day of actually driving the car without issues... in theory with full boost and with flat shifting enabled it should pass 200kph for real though . I still need seat time badly before that.... I've been so rusty and every trackday something is wrong with the dam car and I barely do 2-3 sessions... I've had fuel leaks, coolant leaks, brake failure, electrical issues, coolant line burst, fuel pressure reg undo itself... if you got 50 laps your first day after a build that is awesome! Congrats.

[oztrackdays]

Managed GPS recorded 175 kph on approach to turn 2 but with the poor braking confidence of mystery hand me down pads and rotors.
had to brake earlier than normal so I think 180+ kph is possible into turn 2 once diff and brakes are sorted.

What speeds do you turbo boys hit into the approach for Turn 10 ie the back straight I was getting GPS speed of 152 kph
which is up on the racecar by a few k's.

[zossy1]

Managed GPS recorded 175 kph on approach to turn 2 but with the poor braking confidence of mystery hand me down pads and rotors.
had to brake earlier than normal so I think 180+ kph is possible into turn 2 once diff and brakes are sorted.

What speeds do you turbo boys hit into the approach for Turn 10 ie the back straight I was getting GPS speed of 152 kph
which is up on the racecar by a few k's.

Impressive speeds Nathan - my car is some way short of those speeds! This thing will be a weapon when tuned and sticking to the track.

[16bit]

do you have a plan for more power?

I assume you are going to get the handling and drive ability sorted first but is there a path, ie na or forced induction?