Surprise! Guess what was found inside our transmission? | Project MR2 Turbo

Update by J.G. Pasterjak to the Toyota MR2 Turbo project car
Nov 17, 2022 | Toyota, MR2, Differential, Toyota MR2, transmission, Monkeywrench Racing, Limited-Slip Differential, Quaife, Precision Transmission

Last week, before getting hurricaned by our friend Nicole, we dropped off our MR2’s E153 five-speed transaxle and a shiny new Quaife helical limited-slip diff at Precision Transmission in Ormond Beach.

Precision has a staff full of racers, from GT1 drivers to ChampCar competitors, so bring-your-own-part situations for track-focused cars don’t make them give you weird looks. What did produce a weird look, however, was what they found inside the case of our E153 five-speed transaxle.

The VIN plate and option sticker on our MR2 didn’t show the code for a factory limited-slip differential. MR2s of that era were optionally equipped with a viscous limited-slip diff, although it was a fairly rare option. But when Keary Morris at Precision opened our case, that’s exactly what he found.

Not so fast. Our unexpected find–a factory viscous limited-slip–meant we had to replace the ring gear bolts. Monkeywrench Racing has them on the shelf ready to ship.

If you recall, our MR2 had originally been engine-swapped with a fourth-gen 3SGTE engine by Prime MR2 in New Jersey. When we purchased the car from them, they told us that the great majority of their swaps used whatever transmission was in the donor car. Since our car didn’t indicate an LSD on the option plate, we assumed it didn’t have one. But at some point, either an earlier owner installed an LSD-equipped trans or the car was subjected to a previous swap of some sort. At any rate, this added some complications to our diff install.

Yes, the Quaife fits in the same space as the stock diff, whether it be open or a VLSD, so the basic geography works out fine. The complications arise in two forms.

The first involves the bolts affixing the ring gear to the differential. The Quaife is intended to replace an open diff, so it’s designed to use the ring gear bolts from the open diff. If you look at the accompanying pictures, you’ll notice the flange that the ring gear bolts to is thicker on the VLSD than on the Quaife, so the OEM VSLD bolts end up being too long.

The factory VLSD (top) versus the Quaife unit (bottom). Note the thicker flange around the middle of the stock diff. This is why you’ll need some shortened ring gear bolts for your aftermarket LSD install.

This is easily solved with a new set of open diff ring gear bolts, which we sourced from Monkeywrench Racing, the same place we got our differential from. Honestly, replacing the ring gear bolts isn’t a bad idea anyway, and at $45 for a set of OEM bolts, it was an easy decision.

The second complication is a bit more complicated, and it involves the speedometer gear driver ring that sits on the diff housing. This plastic gear drives a gearbox that spins the cable that drives the speedometer. The gears are different on the LSD and open diffs, not interchangeable, and they’re exceedingly rare in the wild since Toyota has discontinued the part.

That mustard-colored ring behind the differential is the speed ring from the stock diff. Since it’s from a factory LSD, it won’t fit the Quaife, which needs the blue plastic ring from the factory open diff. Oh well.

We also discovered that the little gearbox that drives the speedometer cable had seized, which is why we hadn’t had a working speedometer for a while. Since the MR2 has been a track-only car, though, we didn’t think much of it.

But there’s another complication that arises when there’s no functional speedometer cable, and it involves the fact that the electro-hydraulic power steering takes a speed signal from the instrument cluster. The power steering only actually works at lower speeds, and at higher speeds it shuts off completely. With no speed signal from the cluster, it doesn’t know to shut off.

“But wait,” you say. “Didn’t you just say that the speedometer hasn’t worked for a while? Wouldn’t you have noticed this in the power steering?”

Okay, yeah, technically you’re right. But, full disclosure, we hated the overly heavy power steering in the car, particularly for autocrossing. So we simply disconnected the speed sensor going to the power steering controller, and it stayed on all the time. Boom. All power all the time to turn those sticky Falkens even under heavy loads.

[How big a tire can you easily fit on an SW20 Toyota MR2? | Project Toyota MR2 Turbo]

The bottom line here: We don’t really care in general that we have no speed sensor. Our car doesn’t need a speedometer, and we want our power steering on all the time anyway, so we just saved a few bucks by not having to buy one of those speedometer drive gearbox dealies. We also saved the effort of tracking down the proper speed gear ring.

And what about the rest of the transmission, which spent a lot of time in New Jersey earlier in its life? Keary Morris said it “looked like the best thing to do would be to throw it off a boat from the outside, but once I got in there it looked as good as new.” If we’re being honest, that probably describes a lot of things from New Jersey.

This was also good news, as guts for E153 transmissions are pretty rare at this point. But the transaxles have a reputation for being robust, durable and capable of handling pretty insane power and torque levels, so the consensus is “If it’s not broken, don’t try to fix it.”

Keary did do a lot of measuring and inspecting inside the trans and found all the synchros and other measurable items to be well within spec. His expert visual inspection turned up nothing to worry about, so he’ll give it a nice cleaning and function check as it goes back together. Then he'll send us on our way with a proper torque-biasing differential installed–once we get our proper ring gear bolts, that is.

Our gear stacks, synchros and bearings showed minimal measurable wear. They’ll stay assembled on the bench and go right back in our transaxle as is.

You may be asking yourself, “Wait, why didn’t you just back away when you found a factory LSD and return the Quaife? You kept the receipt, right?” It’s a valid question, and the answer is that not all limited-slip differentials are crated the same.

The viscous limited-slip equipped from the factory uses a stack of fluid couplings on each side of the differential to drive the output axle shafts. If the speed of an axle drops below the speed of the differential, the fluid friction helps to accelerate that axle to match the speed of the diff gear.

In theory, it’s a pretty good system. With no physical coupling of the fluid clutch discs, there’s no real wear, and the nature of the fluid coupling gives a fairly variable amount of lockup.

But those strengths are also kind of weaknesses in a high-performance situation. Because the slip-limiting properties are based on fluid shear, a lot of additional heat goes into the transmission lubricant, and those fluid shear properties can vary over the temperature range of the fluid as well, meaning the action of the diff can vary as the fluid temp changes.

With a gear-driven differential like our Quaife, there’s no mechanical slip of clutches or shearing of fluid. Axle speed is regulated by driving the output shafts via spur gears with differential-mounted worm gears. The worm gears in the diff can easily turn the spur gears on the axle outputs, giving easy differential action.

But when a load is applied via differing axle torques, those spur gears push back on the worm gears. Since a spur gear is unable to efficiently drive a worm gear, that pushback basically binds the axle output, driving it in the direction that the worm gear is pushing.

So while there is pressure introduced between gear faces in a gear-driven limited-slip, there’s no real additional heat produced–by friction, as in a clutch differential, or by fluid shear, as in a VLSD. The action is also smoother, more consistent over a range of temperatures, and the diff–barring an unforeseen disaster–should never require servicing.

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