355 Challenge Tuning - A primer

As I go through the process of sorting my car I thought I would start a thread to share all of my set-up info with the hope of making the process easier, and cheaper for everyone else.

Once getting the car back from Daytona, I put down some baseline laps on my home track. The car was still wearing the 235/295 Hoosier set-up from Daytona and no changes had been made. I don't recall my exact lap time, but something in the low 1:21s on the 1.7 CCW track.

Handling left a lot to be desired. My favorite turn is Ricochet, a downhill left hander that runs off camber through much of the turn. It's a fast 4th gear turn that I can take with a min speed of 91 mph in my Porsche. If the 355C, it was horrible. Turn-in was OK but the front end washed out from apex to track out. I could not get the power down and had trouble keeping the car on track at any sort of speed.

Elsewhere I could feel the rear wheels spinning, a similar feeling I had at Daytona. I'm pretty sure the stock shocks are dead and my tires are up in the air when they shouldn't.

So, time for changes ....

 

TIRES - While the 235/295 Hoosiers may be good for Daytona, where a small footprint is good for the high speed oval, they are not suited for a proper road course. First change was to go to 250/305 Hoosiers. The extra meat is really needed up front, and given the propensity for the rear to lead the car, a little wider in the back is certainly not going to hurt.

AERO - I added a carbon front splitter from Universal. It's a really nice piece and combined with the wider front tires, I felt this would help address the issues I was feeling with the front end. Before the front was just along for the ride, hopefully this will help keeps things planted and provide the balance I am looking for. A carbon piece like this is expensive, over $2K, but you can make your own for almost nothing. I did that on my old 911, picking up a thin piece of used carbon for $100. Trimmed it myself and bolted it on, worked beautifully for years until on off-track excursion destroyed it.

I also added some 2 inch spacers under the uprights of my rear wing, raising the wing just about to the roof line. Fchat sponsor Norwoods made them out of some sort of nylon stock, but again this is a cheap mod you can easily do yourself. The idea is to get the wing into cleaner air where it is more effective. I got the idea from seeing the same mod on Onofrio's car, so thank you to him.

Lastly, my car already had a small Gurney flap installed before I bought it. FCRA racer Paul Prideaux fabricated it out of aluminum and bolted it in. Another cheap add that increases downforce without creating drag.
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CAMBER - As an initial setting I ended up with -2.7 up front and -3.1 on the left rear and -3.4 on right rear. Camber and toe interact with one another on this car as the car does not have adjustable toe links like the Porsches I am familiar with, so there are some trade-offs here. The right rear has more camber as the bolts back there were pretty shredded and it impacted the set-up. I will need to get new hardware as these are past there useful life.

Camber is set with a series of shims. Instead of buying the factory parts, you can pick up a box of shims from McMaster Carr in various thicknesses and save some money. You will need a set like this to have the flexibility to dial in what you need.

I'll have to come back later and report on my toe as I don't have the set-up sheet in front of me.

SHOCKS - I had 2-way Moton Clubsports installed. These are the old style with 7 clicks of adjustment for compression and rebound. Not as fancy as 3-way Penskes but the price was right. I set the cannister at 185 psi and went out on the factory recommended starting position of 2/3 (C/R) to try things out ...

 

The result - best lap of 1:19.4 and was lapping over and over again in the mid 1:19s. Not bad for a first run in 40 degree air temps!! For perspective, mid 1:18s in a 355C is best in class (McKenzie, pro coaches, etc.) I think Rob Lay's best is 1:18.8?

Car felt beautifully balanced! Ricochet, which had been scary before was now a joy to drive again. No more front end wash out and I felt like I could get the power down without the wheels spinning in the air. Only handling issue I felt was under braking, the weight transition from the rear to the front was very abrupt and dramatic. The car would porpoise in the braking zones.

A new issue did arise though, as the engine would bog down at least once on nearly every lap. It felt like an electrical issue, like a bad ground or loose connection. Usually happened on left hand turns. Will have to trouble shoot this another day though.

I added one click of compression to the front and went out again. The issue on the brakes went away, so that was good. On the downside, it felt like I gave up front end grip. This could have been a result of the change or it could have been the fact that the clouds had come out and the track temps were dropping quickly. I lost about 0.5 seconds.

The track was chaning too much for any more testing. Next day will be this coming Friday. My plan is to put the front compression back down to the original settings and instead try one click of extra rebound in the rear. I will also take tire temps and plug into the SD2 to see if we can find the engine issue. Stay tuned ...

 

So far, no changes have been made to the stock components of the suspension other than the Motons. That's due in part to the fact that we did not even know what the factory components were. The Challenge spec had two differents sets of springs and two different sway bars. After 14 years and multiple owners, there was no way to know how the car is equipped today. So we measured.

Front sway bar is 24mm and rear sway is 19.5mm. I believe this is the larger set that was available from the factory.

The spring information is less straight forward. The front left maxed at 1915 and the front right at 2078. Left rear at 788 and right rear at 800. The tender springs were 678, 693, 713, and 763 in the same order.

This seems to be saying that I have the firmer set of factory springs that were rated at 2200 front and 900 rear. It also seems to say that either there was variation from the factory or my set has lost some life over the years. Or both.

This points to why teams spend so much time testing shocks and springs to get a matched set. Without testing, all we can be sure of is that we will most likely have a mismatched set.

My race engineer at Performance Speedtech still has some numbers to run, but when the analysis is complete we may decide on some different spring rates than the current set-up. Another data point, McKenzie is running a matched set of 1450/900 in his car, also with Moton Clubsports. It will be interesting to see if I also end up going softer.

The Motons were also measured and they were identical on all 4 corners. This sounds like a positive thing, but actually it may not be. With different spring rates front and rear it may make more sense to valve the shocks differently. Custom valving is an option we might explore.

All of this is being captured in various spreadsheets, including full graphs of the springs and shocks over their operating range. Once complete, I'd be happy to share these files with whomever wants them.

 

I am on stock rims, no change there nor do I feel that it is necessary to change. In theory a wider wheel could help spread the tire and minimize some tire deflection but the tires I selected do fit and the wheel sizes are within Hoosier's spec for these tires.

The original spec for tires was Pirelli slicks in 245/305 and of course that original compound is no longer manufactured. We are sponsored by Hoosier and the tire sizes I chose are actually the closest thing they have to the original spec. Not sure why 235/295 was ever chosen, they are smaller than originally intended and simply the wrong choice for our cars. That said, Universal indicated to me that tire sizes would NOT be mandated.

BTW, the 250 fronts are also the same size as Porsche Cup cars, so supply (including used take-offs) are in abundant supply. Very easy to find used rubber for practice days and save some money.

My front bumper is an aftermarket carbon piece but the air intakes are the same as stock. A sinlge fog light opening serves as an air intake, which I think is routed to the radiator on the left hand side. I believe the cooling for our brakes is via NACA ducts on the underside of the car.

 

I just checked, my after market front bumper also has the same air inlets to the left and right of the grill opening. Only the rear works off the under car ducts.

 

I thought these two pics were interesting. At Daytona I was complaining about my inability to put the power down coming out of corners. It felt like my wheels were spinning in the air. Well, they were! The rumble strip here is pretty benign, it shouldn't cause a wheel lift like this. These two pics were taken in sequence, stock shocks in place.
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Had a little set-back with tuning as I temporarily ran out of tires. I ran heat cycles 4 and 5 on the first set of stickers yesterday but they were toast, the outside edges on the passenger side (CCW track) were cording and had little grip. Hot tire pressures showed 31f/33R, so certainly not underinflated. Tire temps were within 10 degress across the tire with the centers the hottest, again no apparent issue. This is the real cost of testing, tossing a set of tires after 5 cycles is painful!!

Car went back into the shop to increase front camber to -3.0 degress on both sides and I have another set of tires being sent by courrier to arrive by 10:30am this morning, so I can go out again this afternoon.

I did try dialing back the front compression to the initial setting and increased rear rebound one click. My impression is that this setting is not as effective at controlling the front end dive on braking as changing front compression. Rear end grip also seemed compromised but that could have been the tires. Today's test will be to do half a session at standard setting and then increase front compression 1 click in the pit and go back out. This should tell me definitively if the improvement in braking is worth the potential loss of front end grip.

The engine stumble has not come back, but the SD2 threw off some error codes. So work there also continues ....

 

Ran 1 session on sticker tires today, changing the front compound from R80 to R100 to help address the excessive wear. Completed 9 laps with front compression/rebound at 2/3 and then came into the pits for temps, pressures and shock adjustment, adding a click of compression up front. Then ran 4 more laps.

Best lap on initial run was 1:19.9 with consistent 1:20.0 surrounding it. After the change I got down to 1:19.3, so it seems to confirm the second setting. Braking was dramatically improved but front end grip is compromised, so it is a trade-off. The car was more difficult to drive after the change and I was challenged as a driver to stay ahead of the car. But while not a comfortable drive, I was faster. Means I will need to practice more to eliminate some of the perceived drama behind the wheel.

Looking at the data from TraqMate, it was clear that I was much more comfortable with the car's stability under braking. I was able to brake later, more sharply, and trail brake more effectively which allowed me to pick up the throttle a hair sooner.

I've attached a pdf of the session log that the shop prepared. The missing lap times were because the transponder system got turned on late, but the TraqMate recorded all of the laps. The 1:19.9 I referenced was in lap 7.

Temps and pressures are also listed. Track surface temp was only 60 degrees so I did not get the tires up to temp as I would have expected. I also want to target a little higher pressure next time out. My target is 30-32 hot, though I tend to shoot for the low end. Want to try the higher side next time out.
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Upon closer look we were not happy that the springs were dyno'ed correctly the first time so they were redone, with sheet attached comparing the two tests.

Bottom line - I have the SOFT spring set with original rates of 1800/700. Furthermore, the front appears to be worn out, in particular the front left. The key numbers are labeled as 'average rate' in the sheet.

Waiting now for the shop's recommendationto change spring rates or not ...
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It was fuel starvation. My factory gauge was not operating correctly and I overestimated the fuel I was carrying.

I have now bypassed the factory gauge and wired the factory sender directly into my AIM data acquisition dash, using this adaptor: http://store.bimmerworld.com/fuel-level-sender-adaptor-p1689.aspx

I calibrated the first 10 gallons one gallon at a time and then used larger increments for the next 10 gallons. The dash now reports in 'Gallons remaining' instead of as a percentage left. The added accuracy should help measure fuel burn during practice and correctly calculate the required fill before a race without overfilling and carrying that weight. Not as accurate as emptying the tank after every session, but a lot easier to manage!

If anyone needs detailed directions on how to do this, hit me up and I will go into more detail.

 

Since the car is near its final state of tune I put the car back on the scales today with the expectation of adding ballast. Instead I learned the all the small weight gain items I threw back in got me right where I need to be. There is all dependednt on the accuracy of my gas gauge, but I have enough buffer to correct for error. (It is raining today so I can't drive the gas out of the tank and I am too lazy to jump the fuel pump)

Here is where I stand with full cool box and no driver, assuming 13 gallons in the tank:

TOTAL
3016
Cool Box Full
-84.5 Less Fuel
2931.5 TOTAL, No Driver or Fuel
169 Driver, Helmet, Hans
19.5 Fuel (3 gallons)
3120 Total, near end of race


Fuel starvation cuts in at 3 gallons on my home track so I am using that as my 'empty' mark. I will also get things weighed one more time at that fuel level to verify, but I am confident I am above the 3100 mark. I may even pull some interior to get the 20 lbs back.

Here are the add backs I did:

Tires - Running 250/305 versus original weight at 235/295. This is actually a big jump in unsprung weight so for those that are heavy they may want to stay with the smaller tires. This is a horrible place to add weight, so if you can get away with it you might experiement with a set-up that takes advantage of a smaller foot print.

Front splitter added
Spacers under rear wing with larger hardware
Replaced drivers seat with fiberglass halo seat versus original carbon (+13 pounds)
Heavier seat brackets
Proper window net with steel hardware
ICM Fuel sender adaptor
Wink mirror, SmartyCam camera w/ GPS, radio, extra roll bar padding installed