continued from above
This time around we started off with the goal to build a TTD car, so priorities are also different. I will explain "the holy trinity" of road course priorities in a section below.
One of the two times we ran this autocross car in TTD it was done on 265 street tires, as shown above
Still, I rushed the purchase of the E46 this time around and we initially settled for the 325 we found for less $$. Impatience costs money because now we are starting over with the "right" E46. I talked briefly about my NASA TT classing concerns with the 325 in my last (September 8th) thread update. It wasn't until after we bought the 325 that I ran
all of the TTD classing calculations and permutations, then I figured out the mistake I had made  the 330 is actually
classed better for the end goal of TTD than the 325. It has more points to play with AND a lot more power.
The BMW E36/46 base classing list (from 2015 NASA TT rule set) above might not make it obvious that the 330 is better classed, at first glance. The 325 starts out in TTF** with an assigned 3197 pound minimum weight and the E46 330 starts in TTE with a 3285 minimum. So you'd think that, once you move either car all the way up to TTD, the "lower" base classed 325 has more points to play with and a lower minimum weight, too. The numbers seem to show that, since one build is jumping up TWO classes (59 points worth of mods) and the other only ONE class up (39 points).
Quote:
8.3 UpClassing System (TTBTTF only)
Modifications and Point Assessments: If a car accrues 20 or more points it will be bumped up in Class. There is no limit—a car with a high level of modifications might move up several Classes.
20 thru 39 points  Up ONE Class
40 thru 59 points  Up TWO Classes
60 thru 79 points  Up THREE Classes
80 thru 99 points  Up FOUR Classes
100 thru 119 points  Up FIVE Classes
One (1) asterisk * on a base class assignment denotes a 7 point initial assessment, and two (2) asterisks ** denotes a 14 point initial assessment that is added to the total number of Modification Points to determine the final competition class.

 E46 325, base TTF** jumping to TTD = 45 points (A two class jump yields 59 points, minus 14 base class "penalty points")
 E46 330, base TTE jumping to TTD = 39 points (39 points yields for a single class jump)
 This gives an extra 6 points of modifications to get from their respective base classes up to TTD, in favor of the 325...
So 45 points to work with on mods for the 325 is better than 39 points on the 330, right?
Wrong. There are still two differences on the TTF** classed 325 that gobble up points we have to account for. First is that the base tire size is different for each base class  doesn't matter what the
factory tire sizes are, it is what the
initial base Letter class is assigned with.
Quote:
The following tire sizes will be used as the base tire size for each Base Class for all vehicles regardless of their OEM tire size(s) or their Final Competition Class. All vehicles in a given base class may use this tire size (or smaller) without a points assessment:
TTB: 265mm
TTC: 255mm
TTD: 245mm
TTE: 235mm
TTF: 215mm
TTG: 195mm
TTH: 175mm

So the E46 330, which has a base class in TTE, starts with a 235mm base tire. The E46 325 is classed in TTF, so it starts with a 215mm tire. I have run the numbers and we think that a 245mm tire is ideal once we end up in TTD (which I will explain later, see "the holy trinity of road course prep"), but that tire size costs points on each car
differently.
Not to mention that not all 245 tires are the same width, but that's not the point here. Let's just look at why the same 245mm width tire for a TTD build costs 6 more points for the E46 325 than the E46 330...
Quote:
Tire width points assessed or points credited are determined by the difference between the width of the largest tire on the vehicle and the assigned base tire size as follows:
Equal to or greater than: 10mm +1, 20mm +4, 30mm +7, 40mm +10, 50mm +13, 60mm +16, 70mm +19, 80mm +22, 90mm +25, 100mm +28, 110mm +31

 E46 325, 215mm to 245mm needs +30mm in width = 7 points
 E46 330, 235mm to 245mm needs +10mm in width = 1 point
 E46 330 in TTD on 245mm tire has a net gain of 6 points
This takes 6 points away from the E46 325 relative to the E46 330, each with the same end goal TTD class  since it didn't have to burn 6 points going from 215>235 base tire size. So remember: a lower base class doesn't always give you more points. The next advantage is the 330's larger factory brakes...
Since I've already tracked a 330 with the OEM sized rotors extensively I know that they can work well, and with good pads and cooling I'm fairly certain we can run this TTD build on the 330 brakes. The base e46 330 has larger brakes than even the M3 version of the E36. The smaller E46 325's rotors, however, are fully 1" smaller in diameter than the E46 330
at both ends, which means they might be marginal. We had already upgraded the JackDaniels 325 to use the larger 330 front brakes as a "cheap big brake upgrade", which costs +2 class points (same as going to any BBK). But now with the E46 330 using the same 330 rotors and calipers get's to
keep those 2 points because they are the factory Base Trim Level brakes for all 330s.
So if we add up the relative points advantages from the starting tire size (+6) and now the "free" big brakes (+2), then subtract out the 6 point difference in the 325 and 330 points (45  39) to get to TTD, we still have a
net gain of 2 points for the 330 over the 325 to get to TTD prep.
Baseline dyno of the 325 (at left) and my former 2001 330 (at right) with a cheap eBay header
Then we look at the fact that the E46 330 has a 225hp engine (M54B30) vs the 325's 184 hp engine (M54B25), so that's a gain of 41 hp. The chassis dyno charts above are from our 325 at left and my old blue 2001 BMW 330 at right. The E46 330 dyno'd (my blue 2001 33) had a very crappy eBay header and an even worse tune, but you can still see the relative differences: 211 whp for the 330 and 162 whp for the stock 325 (49 whp diff).
So the 330 is ahead on points and has a 25% head start on horsepower. WinWin.... right? And yes, this difference did come with a financial cost, as most 330s are going to sell for more than 325s, like this one did ($3500 for the 330 vs $1800 for the 325, a $1700 bump), but this 330 has fewer miles and fewer problems. And look how much fixing problems has cost on the 325.
TTB 10.50:1
TTC 12.00:1
TTD 14.25:1
TTE 16.50:1
TTF 19.50:1
The last issue had to do with getting the E46 325 to the PowertoWeight ratio limit (PtoW) for TTD class (14.25:1 pounds per whp), which is shown above. So we always try to get TTLetter cars down to their assigned
minimum weights (with driver), because that doesn't cost any points. To the E46 325 (3197 pounds) and E46 330 (3285 pounds) would be easy to hit those minimums, but with the M54B25 engine in the 325, there was going to be an epic struggle to get to the class PtoW limit. Then there is an often overlooked series of
adjustments to TTLetter class PtoW number from
Appendix B....
This is just a summary  you can click the image above for the FULL list of Appendix B loopholes. There are modifiers for body type (4 door and station wagons), transmission (sequentials take a hit), drivetrain (AWD takes a hit, FWD gets a bonus), even your final competition weight (under 3200 pounds takes a hit, over 3400 pounds gets a bonus).
And the "weight modifier" portion of Appendix B (see below) reminded me that the 325 run at minimum listed weight of 3197 (3 pounds below 3200) would have to take a 0.05 hit. Obviously it makes sense to run 3 pounds heavier than that minimum to avoid a weight modifier.
This is why "running lighter than minimum" can hit you THREE ways.... First, with a lighter given weight you have to make less power at the same PtoW ratio. Then if you run under minimum you have to take
points for every 15 pound increment under the listed weight. Thirdly, you take a
modifier hit for all weights below 3200 pounds. Yes, even if your car's assigned Minimum Weight is below 3200, you still take the modifier hit. So before you think about running lighter than Base Class number, or
any car lighter than 3200 pounds, DO THE MATH.
Check the published width numbers for various tire options, too!
This was a somewhat long explanation, but this is why SO many TTLetter class cars run a 245mm tire  that size or smaller allows them to gain +0.8 in their PtoW ratio, to allow for more power legally at the same weight. This is how we built our TTC Corvette (which runs on a 245mm Hoosier R7 tire), and it makes a BIG difference (TTC's ratio is 12.0:1 vs the 11.2:1 we run the C4 at). That assumes you can even MEET the revised horsepower goals on the adjusted PtoW ratio, with the class mod points you have. Let's calculate the modified PtoW for both TTD E46 BMW models we're looking at.
E46 TTD build starts with 14.25 (TTD limit)  0.8 (245 DOT tire modifier) =
13.45:1 PtoW
And yes, I understand the Appendix B says "+0.8" and we just
subtracted that bonus from the 14.25 number, but NASA writes the rules and does the calculations backwards. Trust me, I stayed at a Holiday Inn Express, we're doing this right (and will show examples of doing this "the NASA way" below). So with this revised PtoW limit of 13.45:1 for TTD on a 245mm tire, the two E46 options would need to hit the following wheel horsepower numbers at the base class mandated minimum weights for each:
 E46 330, 3285 lbs / 13.45 = 244.2 whp max (which we likely can hit with exhaust + CAI + tune on the 3.0L)
 E46 325, 3197 lbs / 13.45 = 237.7 whp max (doubtful we can hit that in the 2.5L)
So on paper it is possible for the E46 330 to "max the PtoW", but the 325 looks doubtful, even at a 6whp lower goal. Why? I don't think the M54B25 engine could ever get to 238 which, which is a
77 whp increase over the stock chassis dyno numbers we tested on the 325) without major internal engine mods, which we
don't have the points for. the 330's 244 whp goal is only ~35 whp more than a stock M54B30 makes, which I'm confident we can pull off with extensive exhaust, CAI and tuning mods (but no internal engine mods). We have allocated points in our 330 TTD build for these power changes.
Maxing out a car's PtoW ratio (running at the minimum weight listed and max power allowed by the ratio) is pretty important for lap times. So on top of spending points on tires, suspension, and aero you need to
also max out power to hit the limit of the adjusted PtoW for your class. It isn't even
possible in TTLetter classes for some cars  the factory engine's limitations and class points just don't always allow for it.
For the E46 TTD problem, some might argue to just run the less powerful 325 at a
lighter weight, to reach the PtoW. Main problem with that  going below minimum weight listed for a given car
costs points.
Quote:
B. WEIGHT REDUCTION:
Weight reduction points are based on the actual vehicle minimum competition weight (with driver). Removal and lightening of nonessential parts is permitted unless stated otherwise in the rules. Modification of the OEM frame, subframe, and floor pan are not permitted (see 5.2.2) Removal or lightening of engine parts is permitted only as listed elsewhere in the TT rules. The exterior surface of the roof, hood, body panels, and doors must remain their BTM size and shape unless listed otherwise in these rules. If the base weight used for base classing purposes (section 5.2.2) minus minimum competition weight (with driver*) is greater than:
5 lbs +1, 20 lbs +2, 35 lbs +3, 50 lbs +4, 65 lbs +5, 80 lbs +6, 95 lbs +7, 110 lbs +8, 125 lbs +9, 140 lbs +10, 155 lbs +11, 170 lbs +12, 185 lbs +13, 200 lbs +14, 215 lbs +15, 230 lbs +16, 245 lbs +17, 260 lbs +18, 275 lbs +19, 290 lbs +20, 305 lbs +21, 320 lbs +22, 335 lbs +23, 350 lbs +24, 365 lbs +25, 380 lbs +26, 395 lbs +27, 410 lbs +28, 425 lbs +29, 440 lbs +30, 455 lbs +31, 470 lbs +32, 485 lbs +33, 500 lbs +34, 515 lbs +35, etc…

So while TTLetter classes allows pretty liberal weight removal mods without burning "mod points", any time TTLetter cars are raced BELOW their Base Class stated minimum weight (shown in the base classing list), they have to take POINTS for every 15 pound increment below minimum weights (see chart above). Which means you again miss your PtoW limit and/or run out of points for proper suspension, aero, power or tire mods.
continued below