All you pervs go browse somewhere else. This post is about installing a locking differential into a Volvo 740 turbo. Irene has been making pretty good power for a little while now. 400 horsepower is my best estimate, though Broomfield’s local dyno shop said more. So it was about time to do something about the one tire burnouts I was leaving around town. I was going to be needing a right rear tire in a hurry the way things were going. And corner exits on the road race track were alot slower than they should be because the inside tire was just spinning when leaving a corner with any amount of power.

Later model Volvos got a nice locking rear differential, and I happened to find one a few months back. It is an Eaton G80, and is found in alot of american trucks and other rear drive cars. It is essentially an open differential driving around town normally, but if one tire begins to spin faster than the other, then the mechanisms inside lock up and drive both tires together. Eaton made this thing very quiet and smooth. Its not obvious that anything is different, unlike some lockers that click and clack around corners.

Here are some photos from the install:

The old diff is on the left. Notice the 96 tooth trigger wheel on the old and the 48 teeth on the new one. Thats going to be problematic. I decided to use the 3.73 ring and pinion. I had a 3.55 before. My mindset is slowly moving away from keeping the car a gas sipping hiway cruiser. So, a little higher gear ratio seemed like a good idea for launching harder and letting the motor rev a bit more freely.

Here is the new locker installed in the axle. I just used the bearings, shims and crush ring from the donor axle. I didn’t remove any bearings, just bolted it all in. I then checked the gear pattern to see how close I was. It was spot on. The backlash was perfect too. Easy, I guess Volvo holds pretty good tolerances from one axle housing to another.

It is hard to tell in this next picture. But the axles from the G80 car are about 1/4″ shorter. I tried installing my original axles, and the passenger side one doesn’t go all the way in.

After buttoning everything up, I added 1.5 quarts of Royal Purple synthetic gear lube and took it for a test drive. Everything was nice and quiet. It felt just the same as before, even the lower ratio was hardly noticeable. On the freeway, it does rev a slight bit higher. But it was really a subtle change. And as I suspected, the speedometer was reading exactly 1/2 the speed it was supposed to. Also, the ABS failure light came on, and the brakes were doing some funny things.

There were some interesting solutions suggested on Turbobricks.com about how to fix this speedometer error. One possibility was feeding the speedometer from one of the front speed sensors. Though this would essentially disable the ABS system. I decided to buy a speedometer correction box from Dakota Digital. It is the SGI-5. I popped the left rear cover panel out of the rear of the wagon and found the wires from the speed sensor there. There was also power and ground there for the power antenna. It made for a convenient installation. Now, the instructions say NOT to feed the signal to the ABS computer. I just played dumb, and left everything alone, and it all works perfectly now. Cruise control, ABS, and speedometer. Here is the SGI-5 install. It cost about $80 and works great.

Stay tuned for the juicy 2 wheel burnout pictures!! . . .

Boost control on a turbocharged car is very important. I have been running a new turbo for 2 or 3 month now and battling boost control problems the entire time. The turbo is a Garrett GT3076. To save a couple bucks, I used an Audi external wastegate. It cost me $10 at the junkyard, and it is a nice durable OEM quality wastegate. It works great, sort of. Unfortunately, it is a little small for my engine combo. The wastegate’s job is to divert exhaust gases around the turbocharger to slow it down and hence control the intake manifold pressure rise. If the port through the wastegate is too small, then it can’t flow enough exhaust to slow the turbo down, and you end up with uncontrolled manifold pressure. This is the problem I was having. I wanted to run about 10-12 psi to start out, with a new engine and turbo combo. I would get about 8 psi boost at 3500 rpm, the gate would fly open to slow the boost down, and then I would get a lazy increase in boost all the way up to about 17-18 psi at 6000 rpm. So really, at the higher turbine and compressor pressures, the wastegate was flowing enough exhaust to keep the pressure from going higher than 17psi. So, it wasn’t really a true run-away boost control problem. But This was more boost than I wanted to run at first on pump gas.

Once I had the fuel and spark maps tuned a little better, I realized that I was actually able to run 17psi without any trouble on pump gas. So, now I needed a way to make the wastegate stay closed until the pressure was forced to 17psi, then, and only then open up and bypass exhaust to give me a flat boost curve all the way up to redline. I was reaching max boost at about 4500-5000 rpm before. This was really leaving alot of usable power on the floor. I paid for an expensive ball bearing turbo for a reason. I wanted this sucker to spool up FAST!.

I wired up megasquirt’s boost control feature to try and control this wasgtegate a bit better. This entailed installing an Audi VW N75 wastegate control valve, and adding a transistorized circuits to the megasuqirt unit, and wiring it all up.

I tried open loop boost control first, just setting the N75 valve to cycle at a set duty cycle, bleeding off some pressure in the signal line. But the boost creep problem remained. I could make the boost increase, but it would overboost. And if I lowered the duty cycle, the boost would just climb slowly the same as without the controller.

I studied up on the closed loop algorithm used in megasquirt, activated it, and a couple days later, I had a reasonably controlled boost level. At first, it was a bit bumpy, overshooting a little, then bouncing up and down 4-5 psi or so. It felt really strange, with all the surging power. But the boost wasn’t running away to 20+ psi like it did sometimes.

Below is after a little more experimenting. The boost holds relatively steady now, within a couple psi. And now I get full boost of about 15psi at about 3300 rpm. Its great! It just roasts the tires in second, and sometimes third gear when the boost hits now.

Here is where I ended up to get the above boost control.