Street Modified minimum weights for an RX-8
 

**EDITED TO SHOW CORRECT MINIMUM WEIGHTS (valid for 2010 season)**

I'm the novice coordinator for my local SCCA region and someone asked me this question as they were confused with how to calculate the minimum weights for a rotary car from the SCCA rule book. Figured I'd post the numbers so that it can be searchable on this forum.

Note: These figures are valid for the 2010 season. I'm also including the weight figures for the rumored-to-be-coming 16X renesis.

N/A
13B---2420lbs
16X---2480lbs
20B---2740lbs
26B*--3040lbs

FI - Turbo/ Supercharger
13B---2700lbs
16X---2760lbs
20B---3020lbs
26B*--3100lbs (3320lbs) <- Note: no car required to weigh more than 3100lbs per rule book


Sample calculation is shown below for a 13B with FI:

RWD: 1800lbs + 200lbs/liter

Displacement = [1.3 + 1.4 + 2x(.9)] = 4.5L

4.5 x 200 = 900

1800 + 900 = 2700lbs


*Legality of the 26B in the Street Modified class has not been established yet due to the production engine rule. 4 rotor was only used in racing, but it is comprised of parts from the 2 & 3 rotor engines. A letter will be needed for clarification.

I just looked this up in the rule book.

Page 179 sub section 3
"Rotary Engines (Wankel): These units will be classified on the basis of a piston displacement equivalent to 1.8 liters plus the volume determined by the difference between the maximum and minimum capacity of the working chamber, times the number of rotors."

(1.8 liters + 0.65 liters) X 2 rotors = 4.9 liters
1800lbs + (200lbs X 4.9 liters) = 2780lbs N/A
1800lbs + (200lbs X 6.3 liters) = 3060lbs Turbo

Per the October fasttrack 2009 rule changes
"These units will be classified on the basis of a piston displacement equivalent of 0.9 liters times the number of rotors plus the volume determined by..."

(0.9 liters X 2 rotors) + 0.65 liters = 2.45 liters
1800lbs + (200lbs X 2.45 liters) = 2290 N/A
1800lbs + (200lbs X 3.85 liters) = 2570 turbo
Subtract 200lbs if all four tires are less than 275's

Wasn't the October Fasttrack a clarification? There's no way an n/a SM RX-8 would have been required to weigh 2,780lbs in '08??

Sipe's calculations match up with the way I've seen McKee calculate minimum weights for rotaries on another site iirc. Let me edit my first post and get in touch with that novice...doh! :o:

Thanks Mark. Numbers should be fixed now - the fact that three different people came up with different numbers (of which only you were correct) tells me that novice had a great question! :lol: :lol: :lol:

Does anyone know if the proposal to not limit the number of carbon fiber panels (trunk, hood, etc) on a car was passed? I know this rule was intended to make it easier for cars to reach minimum weight in SM/SM2.

Starting on page 3 of the PDF file are the 2009 rule changes. http://www.scca.com/documents/Fastra...strack-oct.pdf The rule change in question is item 37 on page 7.

Probably why they made such a large change to the formula for 2009.

I quoted straight from the rule book. Look it up if you don't believe me. Page 179

Not doubting you - just the method of calculation. Okay this was the post I turned up from McKee on sccaforums:

http://sccaforums.com/forums/thread/300542.aspx

These are SM2 weights, but for the SM equivalent use 1,800lbs instead of 1,600lbs. However, looking at his method of calculation, it doesn't match the method used by you or Sipe??? :banghead:

Aaargh my head hurts!!! :Eyecrazy: Let me send McKee a pm...

Andy, provided me with the following response:
I'll update the first post in this thread to show these weights for an SM car. If anyone has an issue with these weight calculations, take it up with the SMAC! :lol: :lol: :lol:

P.S. Looking at these weights, a 3 rotor RX-8 would be too heavy imho to be competitive. The RX-8 "to have' probably isn't possible yet as the 16X renesis is still just a pre-production engine that was shown at a couple of auto shows. But as it is a 2 rotor, supposedly with direct injection, a nice FI set-up on that (twin-screw or turbo) would not incur much weight penalty over a 13B, and make significantly more power and torque!

maybe we're both right; IMO that's a typo, should be times not plus

1.8 - 2.0x is the typical multiplier factor for determining equivalent displacement for a rotary compared to reciprocating engines (rotary fires every compression event, reciprocating engine fires every other compression event)

I sent a PM asking for a clarification, I've never seen it done as "plus" before, this would penalize a 2 rotor more than a 3 rotor relative to displacement percentage which makes no sense

I assume those are the XP weights that use a multiplier? If so, then yes the choice WOULD be very clear.

What kills me with the SM weights is that an N/A 20B (maybe 320-350whp & 220wtq with a darn GOOD race gas tune) has to weigh 40lbs more than than a 13B w/ FI. The choice is clear as well.

P.S. As used RX-8's continue to fall in price, it may not be too long before we see a top driver make a full tilt SM build with one. It has a better suspension design than Sias's M3, and a 13B with FI could make 400+whp on pump gas.

Andy's still not doing it right. For a turbo 2 rotor first you need to find the piston displacement equivalent. To do this you multiply the number or rotors by 0.9 liters, in this case that would be 2 X 0.9 = 1.8 liters. Then you add the volume determined by the difference between the maximum and minimum capacity of the working chamber, this is 0.65 liters. They are saying chamber not chambers this is why it is 0.65 instead of 1.3. So in this case that would be 1.8 liters + 0.65 liters = 2.45 liters, so using SCCA's formula the 2 rotor rotory engines piston equivalent is 2.45 liters. Now that we know the piston equivalent we can add the turbo modifier, 2.45 liters + 1.4 liters = 3.85 liters. Then from here we do the weight formula. 1800lbs + (200lbs * 3.85 liters) = 2570lbs.

1800 + ((1.4 + 0.65 + (2 X 0.9)) X 200) = 2570 for an RX8

1600 + ((1.4 + 0.65 + (2 X 0.9)) X 200) = 2370 for an RX7

I get what you're saying but Andy is a member of the SMAC. If you feel the current formula is ambiguous, you might want to send a letter to the SEB to the attn of the SMAC.

I don't know why they made it so complicated they should have just said the piston equivalent is 2 times the advertised displacement.

+12345 The rule could definitely be made clearer... :lol: :lol: :lol:

Edit: fixed.

I can't believe I'm getting sucked into this, but...
Remember, in mathematical expressions, you always multiply before you add. Therefore the 2008 rule reads:

equivalent displacement = 1.8 + (displ/chamber * number of rotors)

The 2009 rule reads

equivalent displacement = (.9 * number of rotors) + (displ/chamber * number of rotors)

Since (displ/chamber * number of rotors) is the same as advertised displacement, this simplifies to:

1.8 + displacement (for a two rotor, same for 2008-2009), or
2.7 + displacement (for a three rotor, new for 2009).

Add 1.4 to either for forced injection.

I was unofficially told they couldn't get the weights for rotary cars where they wanted them relative to the non-rotary cars using a multiplier, so they came up with the adder thing instead

Correct Jim - the major hit in weight goes to a 3 rotor in '09. I wonder if the Strelnieks' will convert their SM2 FD to a 2 rotor like McKee? It's getting much harder to compete with a 3 rotor...
That makes sense. Oh well - a 3 rotor would be nice for SM but not at those weights...

That's what I would have thought, but with the 2009 rule, they are essentially back to a multiplier:

1.3l 2 rotor: (1.3+2*0.9)/1.3=2.4
2.0l 3 rotor: (2.0+3*0.9)/2.0=2.4

a 1.6l two rotor would change this slightly, but then they'll probably change the formula again :)

I actually proposed this adder per rotor before I left the SMAC, as it seemed to make more sense for it to add per rotor. Before, if mazda came out with a one rotor, 1.8 was still added to the base displacement. Or likewise, a 4 rotor would only have 1.8 added. Yes, it does add weight to our car, so I am putting the A/C back in, and probably adding ballast. FYI, the car calculated weight for 2008 was 2640 and for 2009 is 2820. It went across the scales at 2740 or so last year.

Erik Strelnieks

PS. Perhaps an "s" was omitted from chamber, but the intent was for the volume/displacement to be 1.3 for 2 rotors and 2.0 for 3 rotors. Also, the SMAC made the turbo factor an adder to effectively add more weight to smaller displacement motors be design.

Thanks for that clarification Erik. Good luck in '09! :cool:

Updated to show minimum weights for a 4 rotor (n/a & FI)

Somebody's been reading my SM thread on SCCAForums

I'll be submitting the 4 rotor clarification letter soon

I'm still weighing out SM vs XP or FP relative to engine size. An NA 13B PP can now generate some decent power levels coupled with a short ratio trans and the minimum chassis weight. If it doesn't pan out in SM then jumping to FP isn't too difficult. The 26B jump would require XP which is also interesting due the openness for maximum creativity.

So the real question is, can you legally get an RX-8 down to 2420# in SM trim? :pow: Not on your life.

That means chopping off almost 300# from where I was at in STU :icon_no2:

or another 200# drop down to 2220# using 275 tires .... :hahano:

I haven't written a letter on the 4rotor yet. I'm in agreement with Andy that the rules allow it under the current engine restrictions/allowances.

Oh and for the record the 2 rotor displacement is technically 1.308L

3-rotor 20B is actually 1.962 liter

4-rotor 26B is actually 2.616 liter


so you need to recalculate the weights as they're off a bit


.

To revive this SM weight thread...

Team, now that you're in the midst of STU build #2, what do you feel is the realistic lowest possible weight an RX-8 can get down to with the n/a 13B?

Btw, I wanted to mirror the engine displacements in my calcs with the same ones McKee used in his (for the 2 rotor and 3 rotor engines). I would agree that the actual displacements are slightly different as you pointed out. However, I'm not sure what's currently been done at national events -- i.e. does National Tech use 1.3L vs 1.308L in calculating the minimum weight for McKee's car in SSM (or 1.962L vs 2.0L for the Strelnieks' car)?

They should technically use 1.308. The actual displacement is what counts. However it is the competitors responsibility, both the owner and their class competition, to determine whether or not they're at the legal weight, not the tech inspectors.

I'm only at 2800 lbs this year.