Our first test day | Project LS-Swapped 350Z

Tom
Update by Tom Suddard to the Nissan 350Z project car
Dec 18, 2021

After agonizing over our suspension geometry and modifications, our Nissan 350Z was ready for its date with a real race car engineer: Andy Hollis had agreed to join us for our LS-swapped time trial car’s first test day.

Our goal for that outing: Develop a setup notebook so we could adjust our car and make it fast at any track. Prep work done, it was time to roll the car out of the garage and into the world.

Let’s learn how to test.

 

Choosing a Venue

The first step to testing your race car? Picking a venue.

And sadly, Andy was quick to put his foot down here: “We need a skidpad.”

We instead proposed our official test track, The Florida International Rally & Motorsport Park, but he insisted on starting at a skidpad.

And we can’t really fault Andy’s reasoning here: Our goal wasn’t to “test the car.” Instead, our goal was to design, run and analyze a series of experiments to answer questions about how our car behaved on track, and what we could do to affect that.

Think of a typical corner, for example: There’s a braking zone, turn in, steady-state cornering, power application, corner exit, and finally straight-line acceleration towards the next corner. Each of those are distinct events, and while they’ll affect each other (and many chassis adjustments influence more than one), they’re each separate scenarios that we can test and optimize.

Enter the skidpad: Unlike a race track, which puts a dozen different combinations of these events together back to back each lap, a skidpad lets us isolate specific variables and run experiments.

As Andy explained, a skidpad would let us answer questions like “Does the car understeer or oversteer during steady-state cornering?” or “Does the car handle the same turning left as it does turning right?”

And, by adding an entry or exit path to the skidpad, we could then progress to questions like “How does shock tuning affect initial turn-in?”

Could we run these same tests at the FIRM? Theoretically, yes: Our AEM data system logs a ton of data, so with careful driving we could run single-corner tests and find meaning in the logs afterwards. But skidpads are more efficient, easier to isolate variables, and put less wear and tear on the car.

So we went searching for a skidpad, eventually renting Gainesville Raceway’s skidpad–home of our annual $2000 Challenge–and training course for a half day.

How much does this cost? Every area of the country is different, but you can generally expect to spend about $500 to rent something like this for a weekday morning. Split it among a few friends, and it’s a fairly affordable way to test your car.

 

Making a Plan

Skidpad rented, we needed a plan. Andy made it clear that we weren’t paying for track time to stand around: We were paying to be on track, so we needed to pull out all the stops to maximize our investment.

Out came the enclosed trailer, the full toolkit, the spares box, and even the slide-in truck camper. Any time spent at the parts store, the drive-thru, or even the distant track restroom meant burning time without collecting data on our car. 

Our goals at the skidpad were simple: Make sure the car is reliable enough for testing. Make sure none of our adjustments are cranked up to 11, meaning they’d overshadow any other changes we make. And, hopefully, end the day with an understanding of how our anti-roll bar and shock absorber adjustments affect our car’s handling.

Let’s get to work.

 

First Tests

Using our track time meant arriving with the car ready to go, too, so we did. Five minutes after parking, we had the 350Z out of the trailer, tire pressures checked, and everything warmed up so we could look for any last-minute issues.

Fortunately, our preparation paid off: We took turns with Andy lapping Gainesville’s miniature road course, then set hot tire temperatures, checked fluids, and set up the timer: We were finally ready for the skidpad.

Driving smoothly and consistently on a skidpad is a skill all by itself, so we belted Andy in to set our baseline laps. And, well, the car instantly broke.

Maybe broke is too strong of a word. But it did throw a wrench into our plans when the 350Z immediately filled its PCV catch can with oil.

LS engines are known to have oil drainback issues on track, and it seemed ours didn’t like the sustained 1+ G cornering loads from laps on the skidpad.

After thinking over our options, we chose the one that would get us back on track fastest: Simply re-routing our PCV system so it would pull from the inboard head instead of the outboard head.

By swapping the hose when we swapped directions, we were able to keep our oil in the engine without wasting precious track time. We’ll design a better fix back at the shop.

Now that we had contained the oil, Andy went back to driving in circles. And, well, it worked: We set a baseline time for a skidpad lap of 12.048 seconds.

Now we could start to make changes.

 

Adjusting Anti-Roll Bars

Baseline set, it was time to make changes: Andy recommended we stiffen up the rear anti-roll bar first, so we moved its end links from the middle position to the full-stiff position.

Our reward? A drop in time and a smaller standard deviation in laps: Our 350Z now completed the circle three-hundredths of a second faster on average.

Andy’s feedback was mixed, though: “This is harder to drive, but might be faster.”

Time for another change: We moved the front anti-roll bar to full stiff and sent Andy back out. This time, though, the clock didn’t show a change: We only dropped a few thousands of a second.

However, there was one clear improvement: The standard deviation dropped to just four-thousands of a second, meaning the car was remarkably consistent in this configuration. We’d made an improvement in drivability, if not in all-out speed.

Time for more changes: This time, we moved the rear anti-roll bar to its softest position, hoping for a clear change in the data.

But we didn’t see it. Yes, the car got slightly slower (two-tenths of a second), but it wasn’t nearly as drastic as we’d expected. Andy’s feedback from the driver’s seat was off, too: “It doesn’t seem to have changed the chassis much, if at all.”

 

Controlling Variables

We’d just made a major change and recorded a minor outcome. That generally means one of two things: Either we were failing to control a variable, or another factor was so drastic that it overshadowed our own inputs.

As Andy put it, “one of the chassis’ other knobs is cranked up to 11.” Time to figure out what had gone wrong.

We put the rear anti-roll bar back to its medium setting (our starting point), then sent Andy back out to the skidpad. Once again, we recorded no change.

Perhaps we were losing grip from our tires? After all, tires and track conditions are constantly changing, which is why we always bracket our tire tests with control runs at the end of the day. We rotated the 350Z’s tires and ran the skidpad again, which resulted in no change.

[The trick for scoring fast, inexpensive race tires | Project LS-Swapped 350Z]

Maybe we had our shocks adjusted wrong? We added lots of compression damping to each corner and ran the course again. Nope: That only cost another two-tenths, now putting us a full half-second behind our initial baseline.

 

Finding the Flaw

Then, finally, it hit us: We were in such a hurry to make the most out of our track time, we’d forgotten to confirm the basics: Our suspension wasn’t bottoming out, was it? After reviewing our trackside photography–and checking the O-rings we’d put on the shock shafts–we realized it was.

Despite all our math and custom-made shock shaft spacers, we’d transposed two measurements and effectively set up our rear suspension backwards. Our 350Z was driving around on its rear bumpstops, which explains the lack of influence our other adjustments had on the chassis.

But track rentals are merciless: We discovered the problem 20 minutes before our time was up, meaning all we could do was drive our 350Z onto the trailer. We’ll fix the problem and return to the test track next time.

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Comments
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Robbie (Forum Supporter)
Robbie (Forum Supporter) GRM+ Memberand MegaDork
12/17/21 11:53 a.m.

For the skid pad what gear do you use?

I'd think 6th (top) would give you the lowest revs and might keep your oil issues down a bit. Also you shouldn't need the acceleration capacity testing steady state cornering. 

But maybe you want to keep the revs in normal racing range since engine braking effects will be higher? But then again a high gear would reduce the effects of the driver's right foot, reducing a variable you're trying to control for.

Curious if there's a clear preference or not.

PunchyWrench - Ed Higginbotham
PunchyWrench - Ed Higginbotham UberDork
12/17/21 1:02 p.m.

Not much was tuned based on the testing, but by golly if that was a super informative update full of reminders and troubleshooting. yes

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