I think the intention is to keep 6 as OEM aero as possible. But allowing "modifications" to BTM bumpers, non-btm bumpers or complete bumper replacement would seem to allow an added lip or airdam.
The odd part to me is the rule in 5 about extending the airdam beyond/below the bumper edges. That might not be allowed in 6, but needs cleared up.

Either way, a 'heavy' BTM NB2 looks mighty strong in 6.

Ahhh, I think that's it. I didn't read the ST5 air dam rule well enough.

Agreed. I would love to make more power and run on the heavier side. Only track it might not be ideal for me is Cresson 1.7, but MSRH, CoTA, NOLA all have long straights were mo' power is ideal. VVT is definitely gonna be strong.

My long con finally paying off. Already got a 6 speed lined up. Now to figure out rear end and how much to spend making the motor rev.

ST6 rules make me thing we could run an air dam that kicks out 2" in all directions. I'm pretty positive that Greg does not actually want that though.

205s on 8" wheels tuck nicely behind an OEM NB2 bumper. No airdam means you get half your A-arm penalty back.

With a diy rules-max undertray and hood vents, it would be pretty slick still.

Oh, didn't even think about the NB2 front bumper. My NA crutch gets even worse then, BUT at least I get bump steer shims /sarcasm

Would you ever want to be on 205s though? I'm not seeing why being lighter is better, especially now with the tiered tire sizes.

If you can't make the power to 'fill out' a 225 R7 or 245 RC1 build, yes.

Assuming BTM aero and 17.8/9:1 adjusted:
2250lbs gets 125avg whp
2450lbs gets 137avg whp

NASA is doing a good thing by widening the gap from 226mm to 257mm, IMO. The 2018 rules have a sweet spot at 2400lbs opening the 257mm allowance. There is less of a bonus incentive now to run heavy, other than the old "more raw power at the same ratio" race-ability vs other heavy ST6 cars. For a TT car, I don't see much reason to ballast up heavier than an old TTE/PTE car.

Comp weight pound per tire millimeter:
2250lbs on 205s = 2.74. Current setup
2400lbs on 225s = 2.66. 2018 rule, going away.
2450lbs on 225s = 2.72. 2019 rule.

Also, it gets pretty hard to push a 225 R7 or 245 RC1 down a straightaway with only 140 peak whp.
Also, also, consumable costs go down with less weight and fun-to-drive factor goes up.

There is an opening for a big power car to fit both 5 and 6 with the addition/subtraction of the max 250lbs of ballast though. Which could be useful depending on regional car counts.

So does an NB have an advantage over NA in ST6?

Better (BTM) aero and can fit more wheel/tire?

Some initial testing (heresay) seems to indicate that at 135ish whp that a 205 on a 15x9 is faster than a 225. For an NB, what about a 225 R7 on a 15x10? (Are the 225 R7s that much wider than the 205s?) I suppose it's likely to be track dependent. For places like CPM, AMP or Barber I wouldn't be surprised if more tire was faster. Daytona or VIR? Yea, probably the "skinny" tire.

For my car I may lean toward a combo ST5 / ST6 upgrade. I'm already pretty close to the power limit in ST6, would only need to ballast up 30ish pounds. Add sway bars, change spring rates? I have a built VVT motor that just needs the head re-freshed. Could run a 93 octane turned down tune in ST6, pull ballast, add wang and front dam/splitter, and have a high power e85 tune for ST5. Depending on ballast and aero bits, the swap could be as easy as 45 minutes work? As Dan alluded, this may be the way to go here in the SE to be able to swap between the two classes depending on car turnouts.

For a points build NB1, what are cheap engine reliability/durability/performance upgrades that I can do now that I couldn't before? Flat-top? Skunk2 TB?

NB have the advantage of donor parts coming with the car that need added to an NA: 6spd is free if attached to an NB engine, Sport brakes and BP4W/BP6D come with the right base car. Plus the better NB suspension geometry.
BTM aero advantage is small, but matters. Mainly NB2 only as the lower bumper corners cover a bit more tire.

Ultimately, its geo and BTM aero.

Tire wise, I don't see 225 R7s being faster than 205R7s on an ST6 Miata. The .1 P:W credit is smaller than the dyno cushion we should be leaving. The extra 200lbs of ballast is offset by 11 whp, which is big, but you're still working with ~140whp max. Consider that you need to warm that tire up on an outlap too, and you're still going to get walked on the starts by a car that actually gets to run at 17:1 P:W.

Why only 140whp peak? Wouldn't a non massaged curve put you in the 150+range?

e: looked at an old dyno run and there is only a 3hp difference between my max and average.

Yeah, I agree with your edit there. My peak and avg are ~4hp from each other. I would guessimate that delta would increase a tad as you add power or potentially RPM.


Luke and I were discussing gearing options.... 6spd/4.3 vs 5spd/4.77.
Upside of 6spd/4.3 is you don't have to change a final-drive and the gears are set a tad closer, but is that tad closer really worth it? Downside, ~$600 transmission replacements AND I'd have to buy another MR shifter.
Upside of 5spd/4.77 $100 replacements, already have MR shifter, and gota rebuilt diff anyways. Downside, sourcing 4.77 and installing it. I might run out of gear at Daytona(?)

For the sake of discussion here, price aside, what's the best gearing option...... depends on the track?

I was thinking about going 6spd/4.1 purely b/c my car still see's street duty, but I am curious how high into 5th I'm going to get on CoTA's back straight with 4.77's. With ST5 power/aero it might become an issue. At current 136whp and 7200rpm it shouldn't be an issue at all. As for peak vs avg I don't have a print out of rpm/hp, but eyeballing it mine is about a 6whp spread using 7200rpm, but drops to 3-4whp if using 7000rpm and fewer data points. That's probably just poor tuning though, my BP4W seems to be a bit more peaky than most so think I'm missing some power below 6000rpm.

I could definitely see running out of gear at Daytona on a 5spd/4.77. Our Chump Car on the NA BP would just run out of gear at the end of the straight at Sebring (~125mph). We were making a 120-130whp.

You will run out of gear at Daytona. I almost run out of gear at VIR with a 5-speed, 4.77 and 7400rpm limit @ 133 peak HP. The 4.77 build really needs the 8k redline.

Between those two, the 6sp/4.30s are faster, but only because you didn't go short enough on the R&P with the 5sp. Once you have the 5sp geared to use 3-4-5, the RPM drops between gears between the two are within 100rpm (the 5sp actually has a small advantage on the 4-5 shift over the 6sp). The 6sp/4.30 goes much slower in gear than the 5sp/4.77 does, though, so you end up with higher average RPM and more power to the ground in the real world. In order to make up that difference, you would need to do 5.38:1 gears combined with the 5-speed.

Approximately equivalent speeds in 3rd/4th/5th:

6sp/3.303 vs 5sp/4.10
6sp/3.636 vs 5sp/4.44
6sp/3.909 vs 5sp/4.78
6sp/4.10 vs 5sp/5.13
6sp/4.30 vs 5sp/5.38

Less speed in each gear means RPMs will be higher on average, and that's important because...

...generally, the best gearing option is the one that maximizes RPM at all times. In general, if you maximize RPM, you maximize horsepower. If you want more RPM at the same speed, you need to go shorter. Therefore, generally, the best gearing option for a given track is the shortest possible option that still allows you to hit max speed on the longest straightaway.

The caveat to "generally" is that if you have a specific important corner on a track, or if you have a track with lots of slow-speed corners and then one very long straight, you may be better off going slightly longer on the R&P and sacrificing horsepower in order to eliminate some awkward shifts. The "right" way to solve this caveat would be to add RPM capability instead, though.

You can do a lot of analysis of the track, GPS speeds, etc. This is what pro teams do. Or, you can simply set the gearing so that you reach max RPM in the tallest gear at the very end of the longest straightaway, and you'll probably get 95% of the way there.

Corollary to that: The big benefit of the 6sp is the ability to achieve max speed without needlessly lengthening the R&P. Packing 4 ratios into the space of 3 means that the average RPMs are higher, and when RPMs are higher, horsepower is higher.

I bang my head against my desk every time someone says they "like long gear ratios" for this reason.

So in theory a 6-speed with the 4.77 and a high enough rev limit to not run out of gear in 6th could be good? Assuming you don't end up with super shitty shift points all over the place.

Yes.

I don't recall. I think we were on 225 Hankook RS3's on 15x8's that time with a stock ECU. Drafting was required to max out the speed just before the brake zone (so perfect gearing?).