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Dear Dave

Geek-At-Large

Dave Coleman
Dec 1, 2007
0712_sccp_01_z+dear_dave+funny_pic Photo 1/1   |   Dear Dave

Direct 'Dear Dave' tech letters to dave@eyesoreracing.com. Coleman will share mind-numbing details, earth-shattering revelations, and technical nerdisms in this space each month.

Q. Sloppy Shopper
I recently bought a used 2004 Nissan 350Z. I could only afford a used one and the only model available was an automatic with the weird manual shift mode (we can't all get what we want, just close to it). I was wondering about forced induction. I know how to drive my Z, so don't worry about telling me to buy a Miata. Is that JWT twin turbo available for the cruddy auto? And could the auto even take it? Should I settle for a supercharger from Stillen, or just a regular turbo from any other manufacturer?

Oh, by the way, please make me look like a complete idiot, not that I need to ask.Sergei CopadoAnthony, NM

A.
I don't need to make you look like an idiot, you've done that yourself. An automatic? And you claim you know how to drive? News flash: nobody can drive an automatic. They are inherently undriveable. Oh sure, you can make them go, stop and turn, but actually driving a car requires precise and immediate control of the torque hitting your drive wheels. The manual shift mode helps, as does the rev-matching downshift feature you missed out on by one model year, but ultimately the torque converter delays your inputs, softens your engine braking and basically makes it impossible to maximize the potential of any chassis.

If, by 'drive' you mean 'drag race' then please accept my apology. Your slushbox is perfectly suited to that. A Camaro would be better, but your Z will do just fine.

Now, I believe you had a question about turbocharging...

You'll be glad to know that your transmission is basically the same one used behind the VK56DE in all of Nissan and Infiniti's various Titan-based monster trucks. That means it can easily handle the 385lb-ft of torque Nissan's V8 puts out, even while towing thousands of pounds of trailers. That's the good news.

The bad news is that it can only handle that torque if the engine management for the turbo or supercharger kit doesn't lie to the transmission controller. There are several different strategies to supply the extra fuel a boosted Z will need. One is to use the stock injectors and add a secondary fuel pump to increase fuel pressure as the boost comes on. This works up to a certain power level (about 6.5psi on the JWT turbo kit) until the engine is either making more power than the over-pressurized stock injectors can deliver, or the mass airflow sensor (MAF) maxes out.

Another strategy is to put in bigger injectors, which increases fuel flow, then use a piggyback computer to reduce the MAF signal artificially. This brings off-boost fuel delivery back down to stock levels and increases the amount of airflow the system can handle before the MAF tops out.

The problem when reducing the MAF signal with an AT (besides the fact that you can't actually drive the damn thing) is that the transmission computer uses rpm and MAF to estimate how much torque the engine is making and then adjusts the shift firmness appropriately. With a trimmed MAF signal, the engine will be making far more than stock torque while the transmission thinks you're pussy-footing it. As a result, it will do a pansy-ass soft shift, which will cause the trans to slip under the huge torque load you're actually giving it.

There are three ways to avoid this slipping problem.1: Use a kit that doesn't trim the MAF signal. This includes the base, 6.5psi JWT turbo kit, but JWT warns that even this kit will cause slippage if you turn up the boost to the 7.5 or 8psi allowed by the stock injectors. Prepare to be satisfied with 6.5psi.

2: SGP Racing, in Houston, sells an upgraded 350Z transmission that uses larger servo pistons to increase band tension on the slushbox's clutches, preventing the slippage that kills the stock trans. This is a $4000 solution.

3: Sell that stupid automatic and buy a stick. If you are smart and patient with both the sale and the new purchase, there is no reason this should cost you anything, but even if you do lose a few bucks in the transaction, remember the $4000 it would take to upgrade your current transmission, not to mention that whole driving thing.

Oh, and next time you're shopping for a car and the dealer doesn't have the kind you're looking for... don't buy a car from them.

Q. 510 Evolution
I'm working on a Datsun 510 swap, but good 510 info is hard to find on the internet. To make matters worse, I'm dropping in a 4G63. This will be backed up by a Toyota R154 (Supra Turbo) transmission with a custom bellhousing.

I'm hoping for a low 12-second quarter-mile, 1g skidpad, and 110 feet braking. I figure I can get at least 300 wheel-hp and can make 400hp for dyno bragging.

What size tire will fit? I'll likely go with the Ermish Racing coilovers and brakes, and will need some idea on how low to drop it. I'm sticking with stock fenders, but will cut out the inner lip for some clearance as well as the rear control arm area. Can I mock up some wheels using cardboard or plexi or something?

Next, the stock fuel tank sucks. I refuse to use a modified stock tank since it will limit me to only 10 gallons. I looked at cutting the trunk floor and using a racing cell, but that means fuel filling issues and more headaches. I also thought about building a custom cell in the stock location. Bad idea? Any advice for internal baffles? Internal pump set-up?

What advice can you give besides 'turn and run away from this project'?Jeffrey R. BallCanon City, CO

A.
Not using a 4G63 might be a good start. While this is a great engine, converting from transverse to longitudinal, squeezing in a Supra transmission and figuring out the mating of the engine and trans are all problems you wouldn't have if you did an SR20DET. The fact that it's been done before means the kinks have been worked out. You can even buy the mounts for a few hundred bucks, saving time and headaches.

But you'll ignore that advice. In the end, you'll have no problem making the power you want with a 4G63 and having the turbo on the non-steering-wheel side will be a nice reward. For good 510 info, look no further than www.dimequarterly.com. They won't have anything on 4G63 swaps, though.

Regarding wheel clearance, the first thing to do is install the Ermish coilovers you plan to use. Then start with a smaller wheel and tire that has the correct overall diameter, then check clearances thoroughly. By that, I mean remove the springs and anti-roll bars, compress the suspension fully ('fully compressed' is likely a different position with the coilovers than with stock struts), and turn the wheel to every conceivable position, looking for the tightest spots. Then squeeze something into that gap and measure it.

Add up the inside and outside gaps, then add those to the width of the tire you're measuring. That's the widest tire you can use. Typically, the tightest spot isn't directly to the side of the widest part, so there's still some hand waving and guesswork involved. If the tight spot is something sharp, be conservative. If it's something you might be willing to rub the tire against every now and then, don't sweat it.

To figure out the required offset, subtract the bigger gap from the smaller and divide the answer by two. With 15mm of clearance on one side and 20mm on the other, you end up with 2.5mm. This is how far the offset needs to move in the direction of the bigger gap. If that 20mm was on the inside, the offset needs to be 2.5mm higher than the wheel you're measuring from.

Before settling on a size, remember to check which tires are available in that size. A smaller, stickier tire is usually the better choice.

For those not familiar with 510s, the fuel tank problem comes from both the small size of the stock tank, the unusual location (above the rear suspension, behind the rear seats), and the fact that the fuel was originally gravity fed through a straw to a mechanical pump. Fuel injection needs some re-thinking of the tank.

Here's a simple solution I've never seen executed: keep the stock tank so fill-ups will be easy. Add a small, secondary tank inside the trunk (don't cut open the floor, the 510 needs the structure) and feed it with a small, cheap electric pump designed for a carbureted car that draws from the stock tank's outlet. To keep the secondary tank from getting pressurized, run a return line back into the filler neck of the stock tank. Build a new, larger fuel line to carry fuel from the sub-tank to the engine and use the stock fuel line as a return line, which can also go to the stock filler neck.

Low 12s will be difficult with the 510's semi-trailing arm rear suspension. It squats a lot under power (even more when its been lowered) and gains a lot of camber. This tips the tires up on the inside of the tread, narrowing the contact patch and making it hard to put down big power. A re-think of the geometry may be in order. This will have implications on tire clearance.

Braking goals will be complicated by the struggle for proper bias and modulation. Plan to use a larger master cylinder to get decent pedal rigidity, add a small booster to make the effort manageable, and brace the master cylinder to the strut tower to deal with firewall flex. Ideally, you should get the brake balance as close as possible by selecting the proper piston sizes for the calipers, but this is an expensive trial-and-error process, so you'll probably just settle for an adjustable proportioning valve. Even then, 110 feet is a real accomplishment without ABS.

And the 1g thing will depend on whether you can get good tires in the size you select.

By Dave Coleman
94 Articles

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