Project Professional Awesome Time Attack Evo: Part 4 - Drivetrain

by Daniel O'Donnell

Today, boys and girls, we’re heading back into the technical side of the Professional Awesome Evo. For your reading and viewing pleasure, we’ll be showing all the secrets in the drivetrain of the fastest Limited Class time attack machine ever to compete at Buttonwillow Raceway.

When we had the Evo VII chassis, we started with the standard AYC (Active Yaw Control) rear differential and ACD (Active Center Differential) that came with the car originally, but after regularly having complications with various pumps and electronics, we decided to switch to the mechanical setup offered in the early years of USDM Evos. For us, this consists of the Wavetrac front differential we’ve had for some time, the standard OEM viscous center differential and the OEM, clutch type, rear differential. Since the switch, we couldn’t be happier with the reliability of the drivetrain and the balance it helps give the car. Diving into what puts the power to the ground, we’ll start with the clutch, head to the transmission, stop at the transfer case and work our way back to the differential.


Here's our custom Exedy twin disc clutch getting installed. We've solely used Exedy clutches in our car since the beginning of our team's time attack racing career. Reason being is that Exedy makes the OEM clutches for Evos and I personally prefer the design of the stock pull-type pressure plate over aftermarket push-type options. This has proven extremely reliable in our setup, with the addition of a MA Performance clutch fork stop and ACT Monoloc to prevent the clutch fork from over-returning and disconnecting itself from the pressure plate. This is the one major downside to pull-type clutches, but one that is easily overcome.

The clutch is a special unit made for us by the fine folks at Exedy Racing Clutch USA. We had been running an off the shelf triple solid disc cerametallic unit that was purchased in a pinch many years ago. This clutch worked flawlessly for us, but we didn’t need a clutch with a capability of holding 923 wheel torques. Due to the massive load this clutch could handle, it also weighed more than needed and could potentially apply more stress during shifts than a twin disc.


Our fully assembled Exedy twin weighs in at 23 pounds on the dot. Note the machining of the flywheel to reduce inertia.
Here's our beloved Exedy triple disc we used for many years. The weight savings going to the twin is a pretty healthy 6 pounds and 12.5 ounces. This clutch is can hold nearly 1,000 lb-ft of torque and driveability is good enough for street usage.
Here we can see a direct comparison of the weight of the flywheel with and without the machined holes.
Most of the weight savings of the entire clutch assembly comes from the deletion of the extra disc and center plate required on a triple disc. This all adds up to improved engine response and reduced shift times. Exactly what we were looking for when making the switch.
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Thursday, February 18, 2016 5:51 AM
This post should have come with an NSFW warning. This is pure, pure pornography. Don't forget to lube the shaft.

I am a bit curious as to why stock gears were used if they are prone to breaking. Surely there are hardened gearsets for the EVO out there. Was it due to cost, or with the extra prep steps does TPA think they're going to be strong enough?
Thursday, February 18, 2016 8:19 AM
@8695, the REM superfinish process significantly increases fatigue life of the gears too.

Nice stuff on the flywheel. Not only a good chunk of mass removed, but also removed from the outermost part of the flywheel which has a multiplying effect on reducing rotational inertia.
Thursday, February 18, 2016 8:44 AM
@8695Beaters, cost is of concern, without a doubt, but this transmission has proven pretty robust with the modifications shown. That being said, Limited Class rules prevent us from getting too wild with the transmission. We could go with straight cut gears, but it would be a huge investment that would then limit us if we were to ever switch and go to Unlimited Class, where a sequential box would be prefered.

@spdracerut, you know us, we look into every little detail to improve where we can!
Thursday, February 18, 2016 10:10 AM
" The entire gearset receives high intensity shot peening to raise the ultimate strength of the gearset, increasing fatigue resistance, while also reducing gear wear."

Correct me if I'm wrong, but shot peening does nothing for the ultimate strength of a material, assuming you meant ultimate tensile strength.

Having seen REM finishes before and after, it is pretty cool stuff. Keep up the good work.

Thursday, February 18, 2016 1:00 PM
@MDR I evaluated shot peened vehicle frame rails when I used to work for an OEM. Yes, there was an improvement in material properties compared to standard heat treated rails.
Thursday, February 18, 2016 3:21 PM
Shot peening will improve both yield and ultimate tensile strength to a degree, yet will extend fatigue life substantially.
Thursday, February 18, 2016 5:06 PM
@MDR. Selective shot peening can greatly increase tensile and fatigue life of a gear. If done correctly, high compressive residual stresses will remain and allow for much higher teeth loading characteristics.
Johnny Mac
Johnny Maclink
Thursday, February 18, 2016 11:11 PM
@MDR. Shot peening only increases the ultimate tensile strength in a thin layer at the surface due primarily to strain hardening caused by the high kinetic energy of the peening shot. Since this plastically deformed peened layer is usually very thin, the overall average tensile strength over the overall cross section of the material isn't increased very much (similar to what case hardening does to a part). However, the fatigue strength is improved substantially thanks to compressive stresses that are formed also in this plastically deformed peened layer. In some materials, the fatigue strength (or endurance limit) may be increased by up to 50% as well depending on the alloy and the peening process. In addition, gears can benefit from the tiny peene impressions in the surface since these micro-size impressions help to hold lubricant and thereby serve decrease the wear on the teeth.
Mike Kojima
Mike Kojimalink
Thursday, February 18, 2016 11:54 PM
Not to take anything away from this excellent work but I have had problems with clearance changes when using the REM process. WPC is less risky. You need to talk to your REM processor and make sure they understand what areas need not to be processed or what areas are sensitive to clearance change.
Friday, February 19, 2016 12:50 AM
Could the REM processes and WPC be used to compliment each other or would that just be overkill? I'm imagining a REM process followed by WPC.
Friday, February 19, 2016 6:29 AM
@Johnny Mac, thanks for taking the time to write the detailed explanation. All good information.
Mike Kojima
Mike Kojimalink
Friday, February 19, 2016 6:14 PM
REM and WPC work well together.
Monday, February 22, 2016 9:06 AM
"An interesting side note, the front differential resides in the transfer case on Evos, with the center differential residing in the transmission." - No, it most certainly does not. It resides in the transfer case. This is why the ACD transfer cases have a hydraulic fitting on them, so the ACD pump can supply pressurized ATF to the center diff's clutch pack. Evo 1-3's have the center and front differentials in the transmission.
Monday, February 22, 2016 9:25 AM
DSMraver, you are partially correct.

For ACD transfer cases, the Active Center Differential clutch pack resides in the transfer case. In our non-active setup, the center differential's viscous coupler resides in the transfer case. That being said, the physical, center differential resides in the transmission and the only differential in the transfer case is the front differential.

Check out this awesome PDF I found online that explains it all. Evolution Drivetrain PDF.
Thursday, February 25, 2016 4:15 PM
Doesn't a 1.5 or 2 way diff actually induce more instability under braking and decel since it's trying to minimize speed differential of the tires under those conditions?
Friday, February 26, 2016 8:32 AM

Think of a rear end with a spool, where there is no differential action. When you go around turns, the inside tire will chirp and make noise because it has to break traction to take the shorter path around the turn. The tire isn't happy, it wants to go straight. Now apply that to braking in a straight line, if the tires want to keep going in the same direction at the same time, you'll likely have a very stable platform.

I get what you're saying though, but I feel like it would only apply to specific applications. I am no expert, but dirt racing comes to mind. When there is a low friction surface, I could see oversteer being initiated because the inside tire could get to a point where the slip angle is great enough that friction is reduced a sizable degree and then the back end will want to come around.

In our case though, with downforce, sticky tires and only being driven in dry conditions, the more differential action we'd have, the more understeer it would likely produce. If we had a really twitchy rear end, this would probably be helpful. Thanks!
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