edfishjr

Thanks for the reply, Stephen.

I use the manufacturers power numbers as I never know what going on with different dyno plots. I found a dyno plot from Goodwin Racing. Not sure what type of dyno, but a well-known company in Miata-land. Shows their stock ND at 139 RWHP and 140 RWT. This is more than would be expected from the factory 148 lb-ft knocked down by at least 12% (best loss number I've ever seen from a reputable 3rd party) = 130 lb-ft. I figure 139 is within the range in the aftermarket dyno-world but assume the factory knows best since they have to test to a standard and spec the motor within a certain tolerance. The 209 lb-ft for the BS is right at what I'm using. i.e. 229 spec number reduced by 14% for losses.

Weights: Yes, Porsche weights are screwy. They claim to be DIN, which mandates all equipment like spares tires, etc. and gas tank 90% full, but I agree no way they are correct in many cases, even when subtracting all options, even the ones you can't not get in the US. Hey, aren't they owned by Volkswagen?

I have two sets of shocks that will probably go up for sale: Koni 3013 and the Pfadt/Ohlins I had reworked. Will let you know when available.

edfishjr

I dusted off Jrho's spreadsheet and here's my best guess at thrust curves for the ND vs 2001 BS vs. 2003 BS.

Rear tires assumed are shown.

I got an ND plot from Goodwin Racing and scaled it at 0.94 so the peak torque matched the factory figure assuming 16% loss. I did the same for both Boxsters, the 2001 plot off of Rennlist (no scaling needed) and the 2003 at 0.94 from Fabspeed.

Lots of hocus-pocus here, but I think it basically tells the story. Remember, this is me reading numbers off a chart, in one case off a video of a chart, so nobody take this too seriously. And the Boxster weight is very suspect, as I used factory spec. (Using 3,000 lbs for the BS makes the ND advantage at low speeds a bit bigger.) Also, the Boxster's wouldn't look so good if you used any other reasonable tire.... they are all bigger in diameter and hurt gearing.

All that said, Stephen was right: the ND has the dig advantage from 30 mph (most common) up to the mid-40's. The Boxster needs courses that have at least a couple of sections where the ND has to shift.

sl951

I have a very nice set of shocks for sale - perhaps the best for SCCA autox street class

https://rennlist.com/forums/parts-ma...l#post13759395

burglar

Well Ed your chart certainly looks a little more optimistic than Stephen's.

Biggest 986S wheels were 18x7.5/18x9, right? ND runs 17x7?

ND is 2400lb or so in CS trim, correct? I still think a 600lb disadvantage is too much to overcome, but I don't know much.

edfishjr

Those are the correct wheels, AFAIK. The spec ND weight is only 2332... that's what I used. So, the race weight delta is probably closer to 700 than 600.

I've concluded the BS can be competitive on a national course, but probably not the car to have, unless Grey Ghost is driving. I expect the SAC/SEB concluded the same.

PedalFaster

Ok, did some weight digging.

ND Miata with "STR bits on it, including coilovers, swaybars, wheels, tires, and lithium battery [...] stock brakes and seats [...] appropriate fuel level": 2255 lbs.

http://forum.miata.net/vb/showpost.p...3&postcount=53

STR bars, wheels, and tires will weigh a bit more than CS ones, but the lithium battery weighs quite a bit less than the stock one.

A friend of mine weighed his ND bone stock and got 2219 lbs. after removing gas, floormats, etc. His is a lower-spec car with smaller and presumably lighter stock wheels, but that calculation also didn't account for lighter aftermarket wheels or exhaust.

http://www.directedconsulting.com/st...in_Page#Weight

Race weight for pre-refresh 986S: 2920 lbs. with muffler, <2900 lbs. without muffler (not clear if legal under Street rules)

http://boxcar-racing.com/forum/index...sg4274#msg4274

Race weight for post-refresh 986S: 2922 lbs. with Hoosiers and muffler of unclear Street legality

http://www.sccaforums.com/forums/aft/35254/afpg/2

So call the weight difference 665 - 700 lbs. Ed, if I'm interpreting your posts correctly, you assumed a 579 lb. difference, which would make the ND's acceleration advantage appear smaller than it actually is.

edfishjr

New thrust chart, using Pedalfaster's weights. If you want to know what this means, I did a little calculating after estimating average g's for various sections of the chart. Kinda crude, but it resulting in the ND gaining about 0.1s when accelerating from 30 - 45mph over the 2003 BS. Then, from 45 to 55, the BS gets it all back and they are even.

So, on most courses, the BS will lose to the ND in the acceleration department, all other things being equal, if only because 30 to 45 mph accel zones are much more common. At these G levels, it takes about 1.5s to go from 30 to 45mph and another second to get to 55. Of course from 55 to 65, the Boxsters will kill. The ND will lose 2 tenths just shifting, plus it's acceleration from 55 to 65 in 3rd will be quite slow compared to the Boxsters still in 2nd.

burglar

Grey Ghost mentioned 245 square. Assuming he meant 245/40-17. Only 0.1" shorter, but 4lb lighter each in tire alone, with likely quite a bit lighter wheel. 0.5" less tread width, but the tire would be happier on a 9" wheel.

IIRC these cars are front grip limited, so seems like a better plan on paper?

edfishjr

Could very well be.

Btw, the Porsche 18" wheel was advertised to be 4 lbs lighter than the 17", but the spec writer probably got it wrong and it's 1lb per corner.

edit: what Porsche actually said was: "The new, optional 18" wheels cut nearly 24 lbs of unsprung weight from the vehicle..." Not sure what they were comparing it too... probably the older 18" wheels, not the 17's.

PedalFaster

One thing your chart doesn't show is that the 986S has a first gear good to about 40 mph. That doesn't help if you're accelerating from 35 mph since you can't hold the gear for long enough to be worthwhile, but in digs from 30 mph the Boxster driver can drop down to first and enjoy a brief acceleration advantage.

I've again never driven an ND, but judging by the numbers its first gear is too short to be usable except off the line.

edfishjr

But, then I'd have to learn to shift in the middle of an autocross!

PedalFaster

That's what happens when you drive a car with no torque.

Earlydays

When you're chasing the last pound of unsprung weight reduction, don't over look tire weight. Extreme Performance tires can vary 1-2 lbs. between manufacturers for the same size.

sjfehr

I think the grip of better tires more than offsets the weight, though, especially when the weight comes from thinks like stiffer sidewalls which complement the soft suspension on Street-class Porsches and give better performance than lighter/softer tires of similar grip despite the weight.

Tires designed to be durable enough for highway driving are often heavier than dedicated track tires; Hoosiers are awfully light compared to similar size extreme performance tires.

edfishjr

All true, but when comparing 71R to 71R that's as even per inch of tread as you can get. Earlydays has a point.

For instance, if I ran the 285-18 in the rear, wouldnt I still want the 245-17 in front for weight and diameter reasons? I figure an 18 inch wheel in back will be at least 2 lbs heavier, plus 4 lbs of tire, so That's 6 lbs min of extra unsprung weight, per side, and maybe more. If we can't fix the front grip limitation, the extra width at the rear is wasted, and it will sap acceleration and hurt grip.

So, while the super short 18 was my first impulse, it probably won't be the first choice.

It'll look cool, however.

Happy Thanksgiving, everyone.

Saturday week I'll be autocrossing an S2000 in BS and Sunday an STU 350Z. Last events of the season in these parts.