Mike Kojima posted on July 10, 2009 20:47
Every good project needs a strong base and our NSX is no exception. The NSX transaxle is a weak spot in the modification path. The six speed transaxle is the weaker of the two and it becomes marginal at about 400 whp. Thus we were careful to purchase an older model with a 5 speed. With its wider gears, the 5 speed can withstand about 500 whp before it becomes an iffy proposition. Second and third gear are known weak points as is the ring and pinion. Since our modest power goals hover at this point, we decided that our transaxle warranted some rework to reliably contain this power.
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| Here is an untreated NSX gear in all of its glory. Look at the sharp edges, machining marks and other flaws. This part is full of stress risers and sharp edges to dig into and weaken the adjacent gear. |
We started off with a brand new gearset from a JDM 5-speed NSX. We made this choice because the US model has a terrible gear split that greatly hurts the cars performance in the quarter mile and on a road course as well. Below we list the differences in the ratios between the two.
US Gear stack (with 4.06 stock final drive):
1st (3.071), 2nd (1.8), 3rd (1.23), 4th (0.967), 5th (0.771)
MPH at redline (8K rpm, and 265/30-18 rear tire) : 1st (48), 2nd (82), 3rd (121), 4th (153), 5th (192)
RPM drop down, shifting at redline (1-2) 4,689 (2-3) 5,467 (3-4) 6,289 (4-5) 6,378
JDM Gear stack (with 4.06 FD):
1st (3.071), 2nd (1.952), 3rd (1.4), 4th (1.033), 5th (0.771)
MPH @ Redline: 1st (48), 2nd (76), 3rd (106), 4th (144), 5th (192)
RPM drop down, shifting at redline (1-2) 5,085 (2-3) 5,738 (3-4) 5,903 (4-5) 5,971
As you can see, the JDM gearset will make a large difference, keeping the engine in its powerband better especially when you consider that the stock Vtec engagement point is 6100 rpm. The engine will stay on the high rpm cam lobes more. It has been documented that the JDM gears make for a 0.2-0.3 second difference in the quarter mile alone!
To understand what some of our work is all about you might want to refer to our article on how to build a bulletproof transmission by following this link. You might also want to read up on the WPC process here.
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| Here Billy Johnson polishes the gear teeth on this bench grinder with a scotchbrite green wheel. You don't want to polish things down to nubs, just enough to break the sharp edges. |
Since the NSX has a decent transaxle we did not opt for the full court press drastic treatment that we might give a Nissan FWD unit but something that would make a big difference in durability giving the most bang for the effort. The weak point in the NSX transaxle is the gears so we treated them to a deburring and tooth polishing. Using a scotchbrite green wheel on a bench grinder, we polished the sides and tops of the gear teeth to eliminate sharp edges and crack inducing stress risers. Polishing also helps prevent the gears from digging into each other which also greatly weakens the teeth.
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| After polishing, no more sharp edges and machine marks. Just nice and smooth. |
We also WPC treated all of the gears and shafts of the transaxle including the ring and pinion gear. WPC is the Japanese wonder metal treatment that in simplistic terms microforges the surface of the material making it stronger, much more fatigue resistant and with its silky smooth surface finish, reduces friction and heat. WPC treatment in short makes the gears have about double the life in high stress situations and in a transaxle like the NSX, reduces about 5 whp worth of friction.
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| On the left is the finished product, deburred, polished and WPC treated vs the sharp and rough stock part on the right |
We also replaced the gear hub sliders, balk rings and syncros of the transaxle with brand new genuine Acura replacement parts and WPC treated them. The smoothness of the WPC surface will help make for easy and fast shifting and the increase in surface hardness means longer life for these highly stressed parts. The WPC micro pitting means that the syncros will have good grip to help speed shifts while retaining lubricant on the surface and the increase in surface hardness ensures a long life at the same time. The smoothness of WPC surface on the outer parts of the syncros means smooth shifting due to reduced friction between the syncro, hub slider and balk ring. Our transaxle will be reassembled with new seals and with an OS GIken LSD.
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| The stock ring and pinion is on the left compared to the OS Giken part on the right. Both gears are polished, deburred and WPC treated. We choose to stick with the stock 4.06 final drive gear ratio. |
Stay tuned, in our next installment of project NSX we will cover tricks in setting up the LSD for maximum performance and long life.
Sources
M-Workz Motorsports Consulting
OS Giken
WPC Treatment
FX Motorsports Development