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rnoll98:

Yeah, sidewall height to maximize flex ;)  Otherwise they'd be running 40" wheels.

This is all fun mental gymnastics. What OD tire do you run in XP?

Sidewall height is yet another variable - which, as you noted, provides invaluable flex for drag racers.  Although there's no way a higher-profile r-compound tire is going to provide sidewall flex even remotely similar to a wrinklewall drag tire, I could see where something like a 50-series tire could provide  measurable acceleration advantages vs. a 30-series tire.  However, I'll venture a guess that it would be difficult to validate a significant difference between otherwise identically constructed 30 and 35-series tires.

I'm currently running 23" diameter tires on all 4 corners of the XP car.  However, my rear fenderwells have been completely tubbed to support significantly taller/wider tires than I'm currently running.  Once I get the geometry properly corrected, I may end up going with 285's in back, which would give me another .8" of width and 1.9" of diameter to work with.

FWIW - I see no practical way to run the 285/30-18's on a 1st or 2nd gen Miata.  The restrictions on inner fenderwell modifications make it impossible to stay narrow, which creates MAJOR clearance issues in front, as the track gets wider.  That's one of the reasons why I believe the current MX-5 would be a far better platform for SM2.

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Steve Garnjobst
#75 XP
'99 Mazda Mutant

I was looking at some old twin screw s/c dyno plots for a Miata.  Using the shape of that torque curve and assuming 275/15's  + 2240lb weight, a Miata would need a torque about like this to be in the range of my car (more to be in Erik's) on the thrust curve.....

 

1. RPM           Torque     Power
2. 3500           218          146
3. 4000           245          186
4. 4500           264          226
5. 5000           277          263
6. 5500           277          290
7. 6000           271          310
8. 6500           264          327
9. 7000           252          334
10. 7500           245          350

 With such a small, light car you shouldn't *need* a thrust curve like that though, IMO.

-Andy M.

 

 

boxboy:

I was looking at some old twin screw s/c dyno plots for a Miata.  Using the shape of that torque curve and assuming 275/15's  + 2240lb weight, a Miata would need a torque about like this to be in the range of my car (more to be in Erik's) on the thrust curve.....

 

1. RPM           Torque     Power
2. 3500           218          146
3. 4000           245          186
4. 4500           264          226
5. 5000           277          263
6. 5500           277          290
7. 6000           271          310
8. 6500           264          327
9. 7000           252          334
10. 7500           245          350

 With such a small, light car you shouldn't *need* a thrust curve like that though, IMO.

-Andy M.

 

 

Andy,

I hope you are right and I don't *need* that much thrust.  I took your numbers and compared them to some dyno charts I found.  I don't have the motor in my car yet so I can't compare numbers from it, but the example I found is close to what I was shooting for/expecting.  How much less would the Miata have to weigh to get the thrust curve up there with the example numbers I used?

 

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Chris
51 SM2

Hi Chris,

 That's a nice flat torque curve you are looking at, though it is pretty conservative even by Miata standards as far as peak numbers.  Considering the accel numbers are all linear the difference in weight is going to be about the same as the difference and torque, so using your Examp Tq numbers I'd guess your car would need to weigh in the 1600-1700lb range.  I think you need to up the boost a bit.

-Andy M.

Chris,

That torque curve looks very promising. That's about what my curve looked like with my mildly SC'd 3.0 V6, but I only revved to 6500. If you can gear that right and get it to rev, it could be dangerous. While I had a less than stellar nationals in 2006, with that curve, and a 2560# car, I was consistently pax'ing in the top 5 locally, ahead of some reigning national champs. I'd agree with Andy and would ultimately probably want 50 more ft-lbs across the board, but I think that would be a great starting point and could definitely be trophy material right there. At 300# less than my MR2 was, and similar average tire width (I ran 245f/285r), I'd expect it to be very capable.

I think you're probably at the peak of increasing returns as far as power goes. The time you pick up for each additional 10hp you add will get smaller and smaller, and eventually (around 350whp) it will become a detriment to most of us human drivers. Most of us would drive a 250whp miata faster around a course than a 350whp one. I think this is something a lot of folks forget in these discussions.

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Randy Noll

Guys,

Thanks for all the encouragement!!!!!  I really think the 250 hp target is realistic for me.  Unfortunately the torque curve you we looking at is an example found at http://forum.miata.net/vb/showthread.php?t=238057  I took the HP numbers from that chart (since they crossed the 500 rpm increments at "nice" places) and calculated the torque.  Of course anytime you post a dyno plot everyone questions correction factor etc.  If I had taken Flyin' Miata's UBER ubercharger plot http://flyinmiata.com/tech/dyno_runs/2.0_uber.pdf  with 300 hp and torque over 250 ft/lbs from 3500 to 6000 it would have been close to Andy's projections up to about 6K.  After that, the FM chart starts to drop off where Andy's example keeps climbing. 

I am not building a 2.0 L stroked motor with the level of work the FM example had, so the example numbers are probably going to be pretty close to what I wind up with.

And being a human driver, I agree a 250 hp car will be plenty for me to handle. 

I should have "real" dyno numbers from my car in a month or so.  I am going to post numbers and correction factors from my tuning time.  Maybe I shouldn't tip my hand to the "competition" but I would like to see more folks come out and play.  I also want the SEB and SMAC to see the prep level etc.  I am still lobbying for a lesser FI penalty for the class.  I appreciate the challenges the SMAC faces trying to keep the rules in check and providing them with data is one way to help.  One of the things I hear back is that no one has yet run a "fully" developed Miata.  I don't know that one will ever be fully developed.  I think stopping the build budget at or at least very near what Andy's asking price is should be a reasonable measure of "fully" developed.  Not that I am suggesting a $ limit as the right measure of what should be allowed (talk about hard to measure).  I will get off the soapbox now.  Thanks for your attention.

I think this is a great discussion we have going and really do appreciate all the feedback.

 

 

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Chris
51 SM2

My car right now is around 240-250whp @ 17psi with a 99 head, Gibb SC header, 70mm TB Conversion, ATI Damper and 80mm SC pulley. With tuning it'll probably yield 270whp. On a built engine, I'd be running that little 60mm pulley you pickedup ~20psi will get you right at that 300whp mark with a good exhaust and a 70mm TB is a MUST....call up Corky Bell and get one.

But where my car is at now, isn't any where near undriveable with 225/45R15 A6s, granted I have a beefed up RX7 LSD which puts down power extremely well. I think 300whp is where you'll really start making the car a little difficult to drive on the 275s, and anything beyond 330whp is probably worthless at a 23" tire diameter. I run 3.9s with the 5 speed also, that helps keep 2nd gear manageable.

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Logan Murray
SM2 NA Miata

Father LeadFoot:

My car right now is around 240-250whp @ 17psi with a 99 head, Gibb SC header, 70mm TB Conversion, ATI Damper and 80mm SC pulley. With tuning it'll probably yield 270whp. On a built engine, I'd be running that little 60mm pulley you pickedup ~20psi will get you right at that 300whp mark with a good exhaust and a 70mm TB is a MUST....call up Corky Bell and get one.

But where my car is at now, isn't any where near undriveable with 225/45R15 A6s, granted I have a beefed up RX7 LSD which puts down power extremely well. I think 300whp is where you'll really start making the car a little difficult to drive on the 275s, and anything beyond 330whp is probably worthless at a 23" tire diameter. I run 3.9s with the 5 speed also, that helps keep 2nd gear manageable.

Excuse my ignoance. With the 3.9 final and 5 speed, what is your max mph in 2nd at what revs?

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Randy Noll

rnoll98:

Father LeadFoot:

My car right now is around 240-250whp @ 17psi with a 99 head, Gibb SC header, 70mm TB Conversion, ATI Damper and 80mm SC pulley. With tuning it'll probably yield 270whp. On a built engine, I'd be running that little 60mm pulley you pickedup ~20psi will get you right at that 300whp mark with a good exhaust and a 70mm TB is a MUST....call up Corky Bell and get one.

But where my car is at now, isn't any where near undriveable with 225/45R15 A6s, granted I have a beefed up RX7 LSD which puts down power extremely well. I think 300whp is where you'll really start making the car a little difficult to drive on the 275s, and anything beyond 330whp is probably worthless at a 23" tire diameter. I run 3.9s with the 5 speed also, that helps keep 2nd gear manageable.

Excuse my ignoance. With the 3.9 final and 5 speed, what is your max mph in 2nd at what revs?

On 275s that would be 65.5 mph at 7200 rpm.  

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Chris
51 SM2

Father LeadFoot:

and anything beyond 330whp is probably worthless at a 23" tire diameter. 

How much more power do you think you could manage with a 25" tire?

How much slower (seconds per 60-second lap) do you think a 23" tire is vs a 25" tire?

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Randy Noll

rnoll98:

Father LeadFoot:

and anything beyond 330whp is probably worthless at a 23" tire diameter. 

How much more power do you think you could manage with a 25" tire?

How much slower (seconds per 60-second lap) do you think a 23" tire is vs a 25" tire?

I run a 7600rpm redline, so I actually see ~70mph in 2nd gear.

At ~240whp and ~ 200-205 wtq and the 3.9 gearing with a Preloaded RX7 LSD (Good for Traction, Bad for Turning) its pretty good in second gear, although I really want to spin the blower up from 17psi up to 20-21psi Max. That way I'll be seeing around 12psi at 3500-3700rpm and beef up the mid range, which I typically don't dip below that area. In exiting low speed corners currently in 2nd gear the 225s can handle it, its actually higher speed corners where you get around 45-50 that it'll light up the tires full throttle in a turn, easy to modulate though and that's what I love about the SC.

How much power do I think it could handle with a 25" tire. I have never tested to know, but here is my assumption. I'm guessing that 285/30R18 could handle about 280-300 wtq "reasonably" vs. about 240 wtq on the 23" - 275s. 280 wtq on a SC'd 1.8L BP Miata engine would be about 340-350whp. Which I think is out of range on my 1.2L Twinscrew and 1.8L displacement....I think that is might be out of reach for even a 2.0L stroker with a 1.2L Twinscrew. Although I have contemplated doing a customer 1.6L Twinscrew, you'd need to run a TECIII-R to harness that power, probably 25-27psi on a 1.8L BP engine..that would be pretty neat if you could pull it off. 

Of course, all of the Torque and HP numbers I mentioned and how much tire can handle are not taking into account Aero Work. I know Chris has got the Wing, but he doesn't have the HP yet. We'll have to see when he's got some HP and that Big Wing and some front end Aero as well. That could make the 275s and the Miata  look even better as Jason had pointed out.

How much faster would a 285/30R18 be over a 275/35R15, you'd need extra HP to push the bigger tire, but traction would be great. Its 0.8" wider as well, I'd take a converative guess at 0.5-0.7 of a Second quicker. A 275/35R15 are looking to be about 1.0-1.2 Seconds quicker than the 225/45R15, from what I've read anyways.

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Logan Murray
SM2 NA Miata

Andy, do you have any datalogging charts from your Nationals runs?  I'm interested in knowing just how many times (and how long) you were at full throttle, and the highest RPMs you reached.

d 

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Dan Pedroza
CS 72/172
2008 Nationals CS first loser

Howdy,

rnoll98:

I think you're probably at the peak of increasing returns as far as power goes. The time you pick up for each additional 10hp you add will get smaller and smaller, and eventually (around 350whp) it will become a detriment to most of us human drivers. Most of us would drive a 250whp miata faster around a course than a 350whp one. I think this is something a lot of folks forget in these discussions.

Isn't traction control completely legal in SM?  I might agree with you otherwise but with that...

Mark

Dan, I didn't log any of my Nats runs this year as typically I only do that when I'm looking at tuning something.  I'd done alot of that earlier in the week trying to figure out my problems at the Pro, and already knew AFR's looked pretty good.

 In general though, I go foot to the floor off of most corners though I have to be more careful with that at HPT.  I did datalog some events earlier this year, like the SD Tour.  I think on the SD Tour course I was at WOT maybe 15% of the time.  On the West Course this year I think I hit about 8500rpm (my boost controller always records peak RPM) going back up the hill after the start slalom.  That's probably in the 68mph range.  

A good SM2 car, IMO, should have 2nd run out to 70mph, maybe a bit more.  At the rate these cars accelerate, you'll be on the limiter all the time with something that runs out in the low 60's.  At HPT, you might be fine with mid-60's but that's mainly due to the nature of the site.   

-Andy M. 

marka:

Howdy,

rnoll98:

I think you're probably at the peak of increasing returns as far as power goes. The time you pick up for each additional 10hp you add will get smaller and smaller, and eventually (around 350whp) it will become a detriment to most of us human drivers. Most of us would drive a 250whp miata faster around a course than a 350whp one. I think this is something a lot of folks forget in these discussions.

Isn't traction control completely legal in SM?  I might agree with you otherwise but with that...

Mark

Howdy Mark,

I need to drive a car equipped with TC before I can really comment. I've heard mixed reviews of it's capabilities. Sounds really neat in theory to just mash the throttle anywhere, mid corner or what not, with 600 ft-lbs on tap, and just steer where you want to go, and the car just stays online and pointed in the right direction.

Either way you're still getting diminishing returns, whether it's limited by your careful right foot or some computer, you're still approaching a point where you will never actually use all your available power. And even with TC you still have to steer, brake, slalom, etc. a car that accelerates and flat out goes that much faster. Finding that perfect slalom speed is a lot harder when you're accelerating towards it at twice the pace. Most folks I've seen end up over-braking and then doing some funky blip blip blip through the slalom. It's always funny to see the high-power guys point-and-squirt from corner to corner with all this drama and then watch a little CS or ES car carve it's way through, foot to the floor, and end up being faster. Likewise, we get a lot of S2000 guys in schools who want to keep it in first the whole way around to stay in the power band. As soon as we get them to upshift, they usually knock time off.

I guess my point, and what I was referring to above, is that in my experience, while gobs of power does require smooth throttle input, there's also additional precision and discipline required in almost every other input as well.

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Randy Noll

i tell ya what, i'm a fan of racelogic's traction control.  after driving schuster's miata and the hyman supra beast (SM), it works well powering out of corners, but it needs to be set at light sensitivity, else it just gets in the way.  Just a little assistance with feathering the throttle and it just bursts out of corners.

 

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Dan Pedroza
CS 72/172
2008 Nationals CS first loser

Sorry for the 2-month late reply Randy, but I for one am. That pesky life-thing got in the way for the three months following Nats until now. Just getting caught up on my reading.

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Paul Magee
45 SM2
Canton, OH

www.rogues-racing.com

Hi there!  My wife and are are new to autocross this year.  Because of the way my car was set up for daily driving over the past 5 years, it is classed in SM2.  It is NOT competitive with the famous (and intimidating) cars of Erik and Andy (and their spouses).  We really want to attend Nationals this year, but are 1) afraid of being embarrassed, 2) afraid of being shunned.  Our rubber size is modest and so is our budget.  Locally, autocrossing is fun and low-key.  But Nationals seems to border on "professional" and seeing massive-wheeled engine swapped RX7s with sponsor logos tattoo'd across their bodies and a "mod list" longer than some CVs makes us want to go buy a G-stock Mini or an SS Elise.

 It does seem like the buildup investment to stay in the same ballpark of the winners is somewhat prohibitive.  And the field is small enough that a new driver feels particularly exposed.

 Reading these threads make me feel a little better, though.  If we do horribly at Nationals and people ask, "What were you THINKING?!!!"  We can tell them we were trying to save the SM2 class.

DO IT !

 Actually the car isn't the only reason you will be in a different time zone.  Those guys are top drivers.  And as a bonus in the SM2 class----if you trip and fall in the grid, and hit your head on someone's air tank.  Bleeding out of the side of your head quite profusely, so much so that your wife passes out---DON'T WORRY !  Erik is I believe an emergency room doctor in real life, and in the interest