JDM gear ratios visualized

[josby]

One of the first things I noticed when I bought my 5-speed NSX was how long the gears feel. I heard the JDM gearset mentioned a lot, so I found the old FAQ entry that has the ratios and speeds (https://web.archive.org/web/20161117235936/http://www.nsxprime.com/wiki/Gear_Ratios), but reading is not quite the same as seeing, so I plotted it:


It's disappointing that there's no improvement for the 1->2 shift point, but getting into the powerband sooner in 2nd would be nice, and I never have a need to accelerate hard above interstate speeds so the shortened 2->3 (83mph to 74mph) shift is no concern.

I'd heard so many people on YouTube mention the JDM gears allowing the engine to stay in VTEC that I was a little surprised to see that's not actually true. Urban legend, I guess.

Doing both the JDM gears and the NSX-R 4.235 final drive helps a little more:


...the 1->2 shift happens at 45mph instead of 47, and 2->3 at 71mph instead of 83.

Also interesting seeing just how closely 1st, 2nd, 3rd, and 4th of the USDM 6-speed match the JDM 5-speed:

 

[josby]

I also plotted a comparison with the stick shift car I'm used to driving, a 2004 Mazdaspeed Miata:



It's clear from that why the NSX gears feel so long. I'm used to 1st gear redlining at 30mph, not 47, and having to shift into 4th at speeds where the NSX is only halfway through 2nd!

 

[docjohn]

I like my shorties and R r&p....

 

[asylum]

JDM gears are more than 0-100 mph times. On the street, with crossings, roundabout and tight turns, most accelerations are from 15 - 30 mph in 2nd gear (except if you are a wild animal and put the 1st) and this is exactly where the stock looooong 2nd is a nightmare. I really look forward to installing my JDM gears.

 

[jrho]

I was always under the impression that the shorter gears would keep the powerband within VTEC range in between shifts but I guess not! Regardless similar to @asylum I can't wait to have my JDM gears installed too!

 

[Honcho]

JDM gears are more than 0-100 mph times. On the street, with crossings, roundabout and tight turns, most accelerations are from 15 - 30 mph in 2nd gear (except if you are a wild animal and put the 1st) and this is exactly where the stock looooong 2nd is a nightmare. I really look forward to installing my JDM gears.

This right here is exactly the main benefit of the short gears.

Always fun to see a gear ratio thread- nice job with the plots! I've owned a US, US+4.23 and now a JDM+4.23. I 100% agree that the shorter 2nd is so nice to have during regular street driving. You're always using it and it makes the car feel snappy and responsive. Combined with the 4.23 it's even better.

On the racing circuit, I found the US+4.23 to be ideally suited for the corners at Watkins Glen and HPR here in Colorado. At stock or near-stock power levels, you're only going to get into 4th on the long straights, so from a track perspective, there isn't much difference between the 6-speed and the JDM 5-speed, since you're mostly using 2nd and 3rd.

 

[stuntman]

The JDM gears really improve the cars performance below ~95mph. Above that and the US ratios are actually closer together and result in better lap times and acceleration. The JDM gears also keep the engine in VTEC "more" (below 95mph), but does not prevent the engine from falling out of VTEC.

For NA cars, the 6-speed is more ideal, for high powered cars, the 5-spd is stronger.

 

[josby]

JDM gears are more than 0-100 mph times. On the street, with crossings, roundabout and tight turns, most accelerations are from 15 - 30 mph in 2nd gear (except if you are a wild animal and put the 1st) and this is exactly where the stock looooong 2nd is a nightmare.

That's a great point! That is one place I've noticed it most, slowing down to take a standard 90 degree turn from one road to another at an intersection. 2nd gear is usually too high for that so the car bogs. Going into 1st would be ok except that it requires more time and pressure on the shifter that makes it just not very comfortable. Either way it's an unsatisfying experience, although I imagine you get used to it.

 

[Wild Turkey]

This is such an awesome post. It explains why I never felt much of a difference in driving between my 5 speed and my 6 speed. 5-speed has JDM gears and I don't drive over 160 kph that much.

 

[sca037]

Great graphic visual aides, thanks!

 

[BBVNSX]

NSX Redline is 8.000 rpm but rev limiter is like ~8.300 rpm, so with JDM gears if you change gears (2nd to 3rd) close to the rev limiter you probably will stay in VTEC

 

[MotorMouth93]

You also have to remember that the VTEC engagement point is 5800-6000ish but it doesn't change back to the low cam until you go below around 5200, at least with the stock programming. (I think, its been a while since I've looked at that code)

I'm curious how much longer the JDM gears/R final drive will be a viable upgrade path. Parts are getting really hard to find and the people that do have them are gouging hard. At sub $2000 I think its a worthwhile upgrade but above that it starts to approach the not worth it threshold pretty quickly. Especially when you look at how much you can sell a 5 speed box for vs how much it costs to put together a 6 speed, but even that might be nearly impossible due to parts availability soon.

 

[Honcho]

You also have to remember that the VTEC engagement point is 5800-6000ish but it doesn't change back to the low cam until you go below around 5200, at least with the stock programming. (I think, its been a while since I've looked at that code)

I'm curious how much longer the JDM gears/R final drive will be a viable upgrade path. Parts are getting really hard to find and the people that do have them are gouging hard. At sub $2000 I think its a worthwhile upgrade but above that it starts to approach the not worth it threshold pretty quickly. Especially when you look at how much you can sell a 5 speed box for vs how much it costs to put together a 6 speed, but even that might be nearly impossible due to parts availability soon.

Yeah sadly, that window is closing. IIRC from my logs (it's been a while since I looked since the tune is so good now), even with the JDM + 4.23 the car drops out of VTEC on the 1-2 shift for about 500ms. It's imperceptible though. More of a theoretical problem TBH.

 

[MotorMouth93]

I'd probably lower the vtec disengagement threshold a tiny bit to counteract it.

 

[Honcho]

I'd probably lower the vtec disengagement threshold a tiny bit to counteract it.

I can't remember if it was due to the car going into closed loop suddenly on the early tunes or not. I can log it again just to see.

 

[josby]

NSX Redline is 8.000 rpm but rev limiter is like ~8.300 rpm, so with JDM gears if you change gears (2nd to 3rd) close to the rev limiter you probably will stay in VTEC

You also have to remember that the VTEC engagement point is 5800-6000ish but it doesn't change back to the low cam until you go below around 5200, at least with the stock programming. (I think, its been a while since I've looked at that code)

Ah, interesting! I graphed that and yes, if you wait to shift until 8300 RPM and if VTEC doesn't go back to the low cam until 5200 RPM, then the JDM gears do keep you in VTEC while the USDM do not:

 

[vf2ss]

Nice visuals ... it'd be interesting to see JDM + 4.23 Final to 8300 RPM

 

[SWFL_NSX]

Can somebody walk me through the math of torque increase of a type-r final drive and short gears vs. a SOS supercharger?

I'm horrible at both math and physics, but it looks like going from a USDM to a type R gets you an increase of roughly 5% (4.062 vs 4.235) while the supercharger gets you an increase of roughly 24% at 2k RPM and increases from there.

Dyno chart here: https://www.scienceofspeed.com/scienceofspeed-tvs-supercharger-nsx-1991-05.html?find=1992-nsx-29950

Disclosure: I DIY'd the JDM gears+type R final this summer and it improved the around-town driving a bit, but I'm a little underwhelmed.

 

[MotorMouth93]

Yeah I was pretty underwhelmed by the JDM gears and R final drive as well. The 1-2 shift feels better of course but thats about it. A supercharger is absolutely a better way to make the car faster, but also very expensive.

 

[Honcho]

Torque is force, i.e., the actual force you feel pushing you back in your seat when you accelerate. Horsepower is a measure of work, which is force over time. With lower gearing, the transmission is able to transmit more force to the wheels at a given speed. The engine doesn't actually become more powerful, but the gearset is able to utilize more of the engine's torque output compared to taller gears.

At a 1:1 gear ratio, the engine is turning once for every turn of the drive axle. If our engine produces 1 lb/ft of torque, then the tires are putting 1 lb/ft down (assuming no drivetrain or friction losses) to the pavement.

If you add "short gears" and increase the gear ratio to 2:1, the engine is turning twice for every turn of the drive axle. The engine still makes 1 lb/ft of torque, but you now get two engine spins for every spin of the tires. Thus, we're imparting 2 lb/ft of force to the drive axle teeth instead of one. This is why heavy trucks gear down for uphill climbs.

Additionally, this is why dyno tuners try to run as close to a 1:1 ratio as possible to tune an engine. At lower gear ratios, the engine is applying more force to the axle than it actually makes, so the measure is off. You're not really seeing what the engine is putting out- you're seeing torque multiplication from the gears.

So, in your example, you would have to figure out the final, final gear ratio of the transmission (including multiplying by the individual gear selected) to see how many times the engine is turning for each turn of the axle- it isn't just the R&P ratio. Then, you'd have to measure the actual torque at the axle on a dyno. This is your baseline. Then, calculate your new gear ratio and multiply that % difference to the torque measure and you'll see how much more torque the new gears can put down through the axle. It's not even close to what a CTSC will do, since the supercharger adds huge amounts of torque to the engine output versus gears, which just multiply the existing torque slightly more.