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Last Post 23 Nov 2012 05:16 PM by  TeamRX8
RX-8 to DSP?
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BlueVR6
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07 Nov 2008 02:19 PM
gavin wrote:

I thought part of the whole deal for moves was...first you have to build the car for the class and THEN see if its too high or too low for the classification.

Gavin

All except the x-1/9. When the BMW's started getting developed, they ran away from the little x-1/9's and even with empirical data, no one wants to consider reclassing them.

DSPBMW
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07 Nov 2008 02:33 PM

Aside from the fact the X19 is an old obscure model, it should be moved to FSP, WITHOUT A DOUBT. IMO.

The slowest S/P class should get the first attention, as pertaining to making classes faster......The X19, AE86, Neon, and some other front drivers......All to FSP.

FWIW the NEON was a little bit back, back then, but there are new things available in tires. 275 15's or 255 13's

gavin
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07 Nov 2008 03:22 PM

Fair point about classing without prime examples and fiats (although CSP(turbo neon/RSX)>DSP doesnt seem the same as RX8>DSP...but maybe thats me).

I guess the question then is really how well we all agree(esp when its your car) with the philosophical notion that you get a place to play, but not necessarily be competitive. There are tweener cars that WILL get buried in a class, too slow for one, too fast for the other.

Gavin

Chiketkd
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07 Nov 2008 04:57 PM
gavin wrote:
There are tweener cars that WILL get buried in a class, too slow for one, too fast for the other.
Very, very true. Tweener cars will always exist. At least the RX-8 has a place to play currently in BS and possibly STX.
solo-x
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08 Nov 2008 12:05 AM

So I crunched the numbers again, and the RX8 does indeed get beat pretty bad under acceleration when comparing Isley's dyno graph to the one on BimmerForums. I'll have some time on Sunday to post the theoretical acceleration at certain speeds for the two cars. It doesn't get beat as badly as my ITR does though, and it's not saddled with FWD drive. And theoretical acceleration is fine for bench racing, but it doesn't say if a car will be faster at autocrossing. You can't argue that the 8 will be able to attain higher cornering speeds and should be able to utilize more of that potential for acceleration then the e36 that's up on 2 wheels. The 8 will also slalom faster.

The e46 has a better suspension design, which combined with its power offsets the extra weight it carries. But I think its a bit early to say it is a better car then the e36. The two cars are very close and the e46 was simply driven better this year. Ya gotta be clean on those fast runs to win!

jeffh
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08 Nov 2008 12:06 AM

solo-x wrote:

Quick, rough math, assuming 210whp at 58mph and 2800lbs race weight, the 8 accelerates at .466 g's. A BMW at 200whp at 58mph and 2950lbs race weight mathematically accelerates at .426. The R, at 170whp, 58mph 2600lb race weight is accelerating at .411 g's. If you can get me a HP vs. speed csv output in 2nd gear I can plot that more accurately.

The 8 will be a little slower off slow corners that it can't go to first gear on (hrm, sounds familiar) but since it handles so much better, it'll be cornering at a higher speed. It's lighter so it will transition better. It uses the tires better so the fronts or rears won't burn off halfway through a 60 second run. Etc.

Nate, what formula did you use to calculate these numbers? HP is just a function of RPM/Torque. The peak acceleration of any car will occur at it's torque peak. Probably around 45mph for an RX-8 on 285's. The new RX-8 also has a 4.777 final drive so the overall ratio is 10.796 in 2nd gear.

The formula that I use is (rpm x overall ratio) / (tire radius in inches / 12) = wheel thrust (lbs/in), then divide the thrust by the weight in lbs. I'll then multiply by .79 gives to get closer to real world results. After taking those numbers, you can then average them over autocross speeds.

Example, copy and pasted from excel assuming a 12" tire radius (285 under load), and weights of 2850lbs for the RX-8 (power from taken from STU type dynos), 2900lbs for the BMW, hope it outputs properly:

Rx-8 Torque Thrust MPH Accel (G's)
4500 125 1349.50 30 0.38
4750 135 1457.46 32 0.41
5000 140 1511.44 34 0.43
5250 140 1511.44 36 0.43
5500 140 1511.44 37 0.43
5750 143 1543.83 39 0.44
6000 143 1543.83 41 0.44
6250 132 1425.07 42 0.40
6500 145 1565.42 44 0.44
6750 142 1533.03 46 0.43
7000 140 1511.44 47 0.43
7250 138 1489.85 49 0.42
7500 135 1457.46 51 0.41
7750 130 1403.48 53 0.40
8000 127 1371.09 54 0.39
8250 125 1349.50 56 0.38
8500 122 1317.11 58 0.37
8750 120 1295.52 59 0.37
9000 115 1241.54 61 0.35



Avg 0.41

DSP BMW Torque Thrust MPH Accel (G's)
2500 163 1350.31 22 0.37
2750 163 1350.31 25 0.37
3000 161 1331.29 27 0.36
3250 166 1378.84 29 0.38
3500 169 1397.86 31 0.38
3750 186 1540.50 34 0.42
4000 201 1662.05 36 0.45
4250 200 1654.61 38 0.45
4500 198 1645.10 40 0.45
4750 196 1626.08 42 0.44
5000 194 1607.06 45 0.44
5250 187 1550.01 47 0.42
5500 187 1546.29 49 0.42
5750 187 1546.29 51 0.42
6000 187 1546.29 54 0.42
6250 170 1405.71 56 0.38
6500 149. 1236.20 58 0.34



Avg 0.42

The most time will be made up coming off the slowest corners. Acceleration is still only one part of the equation though.

solo-x
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08 Nov 2008 07:52 AM
I was using the (365*Hp)/(weight*speed) and the final drive number that Isley posted. Used a gear vs. speed calculator to find the rpm the engine is at for a particular speed. I wasn't adjusting for real world though, just comparing the numbers. With the final drive you posted, the numbers get much closer.
Chiketkd
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08 Nov 2008 01:27 PM
solo-x wrote:
SP classing based on ST classing is absurd. ST tires tend to equalize things much more then anything else.
I'll add this Nate...

One reason why I decided to put my letter on hold is b/c I want to see how the 265 tired STX BMW's and RX-8's fair next season. While street tires can be a great equalizer, the same pros and cons between these two cars will arise. The BMW's will accelerate faster and power out of slower corners better while the RX-8's will have the edge in slaloms and transitions. 

If the RX-8's come out and dominate STX right out of the box, then consider this proposal dead. If the BMW's come to dominance in the class then it would again re-open the question in my mind does the RX-8 truly belond in DSP. 

About the E46 vs E36 thing. Maybe my statement was too rash -- I don't know enough about the pros and cons of the two cars. I did however chat with Doug and Elliot before DSP ran and while they were both had high hopes, I got the impression that their E46 was far from being "developed". Doug obviously went on to win the class this year, but it made me wonder just how much faster their E46 could have been with another 1-2 seasons of development. 

I also think the 2.5RS is a dark-horse in the class. Lipsinic has always competed on a stock motor which now has over 195K miles. With a fresher motor built for SP, I expect the car to be a contender for the top spot.
jeffh
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09 Nov 2008 05:24 PM
Chiketkd wrote:
SP classing based on ST classing is absurd. ST tires tend to equalize things much more then anything else.

One reason why I decided to put my letter on hold is b/c I want to see how the 265 tired STX BMW's and RX-8's fair next season. While street tires can be a great equalizer, the same pros and cons between these two cars will arise. The BMW's will accelerate faster and power out of slower corners better while the RX-8's will have the edge in slaloms and transitions.

How do you come to the conclusion that the BMW's will accelerate raster than the RX-8 in STX trim? Especially if you keep in mind that update/backdate is not allowed in STX therefore they are unable to swap engines and transmission/rear end ratios.

Chiketkd
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09 Nov 2008 08:13 PM
jeffh wrote:
How do you come to the conclusion that the BMW's will accelerate raster than the RX-8 in STX trim? Especially if you keep in mind that update/backdate is not allowed in STX therefore they are unable to swap engines and transmission/rear end ratios.

 

Maybe accelerate wasn't the right word as that is a function of horsepower.  But the BMW's will "pull harder" in STX trim due to torque.

Power-wise the RX-8's do have the advantage in STX trim, but if you look at the dyno graph Isley posted, the rotary engine doesn't make big power until very high in the rev band. The BMW's make more torque everywhere in the rev band and more power down low.
jeffh
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09 Nov 2008 09:03 PM
Chiketkd wrote:
Maybe accelerate wasn't the right word as that is a function of horsepower. But the BMW's will "pull harder" in STX trim due to torque.

Sorry but the numbers stay that statement is wrorg. Gearing/weight play a big factor in the "pull harder" or acceleration that you are talking about. Show me a dyno and gear ratio of an STX BMW that will out pull a new RX-8 with STX mods. [:O]

mwood
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09 Nov 2008 09:07 PM

I've driven a number of RX8's and a number of E36 and E46 BMW's...the BMW is much stronger throughout the useable rpm range associated with most autocross elements, I'd say...the RX8 can get caught out and, when it does, is painfully slow to spool back up. [:)]

jeffh
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09 Nov 2008 09:19 PM
mwood wrote:

I've driven a number of RX8's and a number of E36 and E46 BMW's...the BMW is much stronger throughout the useable rpm range associated with most autocross elements, I'd say...the RX8 can get caught out and, when it does, is painfully slow to spool back up. [:)]

Iif you graph the numbers posted earlier you will see that you have a very similiar acceleration curve between a modded RX-8 and a modded E36 DSP BWM. 2 x 3 does equal 6 even though at one point in your life you might have thought it equaled 5.

Spool back up?? We are also not talking about turbochargers here. These are 5 speed manul transmision normatlly aspirated vehicles.

mwood
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09 Nov 2008 09:33 PM
jeffh wrote:
mwood wrote:

I've driven a number of RX8's and a number of E36 and E46 BMW's...the BMW is much stronger throughout the useable rpm range associated with most autocross elements, I'd say...the RX8 can get caught out and, when it does, is painfully slow to spool back up. [:)]

Iif you graph the numbers posted earlier you will see that you have a very similiar acceleration curve between a modded RX-8 and a modded E36 DSP BWM. 2 x 3 does equal 6 even though at one point in your life you might have thought it equaled 5.

Spool back up?? We are also not talking about turbochargers here. These are 5 speed manul transmision normatlly aspirated vehicles.

By "spooling back up", I was just using a euphemism for the time it takes the RX8 to get back into it's powerband. Also, the RX8 is a six speed car, not five speed, fwiw. Numbers on a graph, the same as numbers generated on a dyno, do not give the whole story or predict with certainty what will happen in the fluid environment of an autocross. I've driven both cars, back to back at least twice (instructing), on multiple courses and multiple elements...have you? If you have, then we'll just have to say we have differently calibrated seat of the pants and I might be wrong, if you haven't then you're just looking at numbers, that may or may not be entirely representative, and adding them up to draw a "conclusion"...

jeffh
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09 Nov 2008 09:52 PM

mwood wrote:
[By "spooling back up", I was just using a euphemism for the time it takes the RX8 to get back into it's powerband. Also, the RX8 is a six speed car, not five speed, fwiw. Numbers on a graph, the same as numbers generated on a dyno, do not give the whole story or predict with certainty what will happen in the fluid environment of an autocross. I've driven both cars, back to back at least twice (instructing), on multiple courses and multiple elements...have you? If you have, then we'll just have to say we have differently calibrated seat of the pants and I might be wrong, if you haven't then you're just looking at numbers, that may or may not be entirely representative, and adding them up to draw a "conclusion"...

What you are feeling is a lack torque compared to the gear/weight of the car that you are in. A rotary engine is a combustion engine that will produce a set amount of torque at a set RPM. It does not build up torgue based of the length of time that you are on the throttle. When the RX-8 is prepared properly it will accelerate with any of the current top DSP cars at just about any speed. Until you've driven one with STX mods and a new rear end then that's all it is going to be is numbers. So untill you can prove them wrong...[;)]

mwood
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09 Nov 2008 10:34 PM
jeffh wrote:

mwood wrote:
[By "spooling back up", I was just using a euphemism for the time it takes the RX8 to get back into it's powerband. Also, the RX8 is a six speed car, not five speed, fwiw. Numbers on a graph, the same as numbers generated on a dyno, do not give the whole story or predict with certainty what will happen in the fluid environment of an autocross. I've driven both cars, back to back at least twice (instructing), on multiple courses and multiple elements...have you? If you have, then we'll just have to say we have differently calibrated seat of the pants and I might be wrong, if you haven't then you're just looking at numbers, that may or may not be entirely representative, and adding them up to draw a "conclusion"...

What you are feeling is a lack torque compared to the gear/weight of the car that you are in. A rotary engine is a combustion engine that will produce a set amount of torque at a set RPM. It does not build up torgue based of the length of time that you are on the throttle. When the RX-8 is prepared properly it will accelerate with any of the current top DSP cars at just about any speed. Until you've driven one with STX mods and a new rear end then that's all it is going to be is numbers. So untill you can prove them wrong...[;)]

Umm...I think "When" and "STX" are qualifiers in your argument significant enough to not be able to jump to "accelerate with any of the current top DSP cars" ...so, we're both unable to "prove" anything, either me, with real world experience in multiple cars of both types (not prepared to the limit of DSP, in the case of the RX8) or you, with data that you've taken from an internet thread...

btw, thanks for the tutorial on the nature of torque...of course, "torque" is really just a red herring, anyhow...it's really horsepower we are talking about...[;)]

jeffh
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09 Nov 2008 10:44 PM
mwood wrote:

Umm...I think "When" and "STX" are qualifiers in your argument significant enough to not be able to jump to "accelerate with any of the current top DSP cars" ...so, we're both unable to "prove" anything, either me, with real world experience in multiple cars of both types (not prepared to the limit of DSP, in the case of the RX8) or you, with data that you've taken from an internet thread...

btw, thanks for the tutorial on the nature of torque...of course, "torque" is really just a red herring, anyhow, right? [;)]

Actually not a "red herring" at all. Average torque, grearing, weight is what counts. Not hp/weight like most people classify cars based on. It is very simple to take something off of paper and build it when you're thorough and it's done right. Look at J-Rho for example. I've just stated what the car is capable of acceleration wise when fully prepared. There are multple dyno's availble to prove significant gains now that plug n play engine management is readily available. And now you have the new 4.777 rear which gives a 10% boost in acceleration by it's self. I also have extensive expenrience with piston and rotary engines as an owner of an RX-7. Still the fastest stock engine/turbo-ed turbo TII on top of that, that I know of. [:)]

mwood
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09 Nov 2008 11:03 PM

Actually, as we're talking about torque at the wheels (through the multiplication of the transmission and differential gearing) and not at the engine, we are talking about horsepower, by definition. Horsepower includes not only the engines torque but the total torque that gets put to the ground to accelerate the vehicle...that's why I was joshing around with "red herring" [:)]

Of course, that's my understanding, I'm no engineer and didn't even stay at a Holiday Inn Express last night...regardless, the BMW is "stronger", 'cause that's what my buttometer tells me[:P]

Anyhow, the whole question is kind of irrelevant, as the RX8 is a BSP car, huh?

jeffh
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09 Nov 2008 11:17 PM

Correct it's horsepower when measured at the wheels which is more for marketing puporses as it it's a universal number derived no matter what gear you are in and should not be used to determine a vehicle's acceleration. Thrust is the force that accelerates the vehicle but who I am to correct, I only have my AS degree in auto engineering tech and am still a few courses away from being an engineer. I do agree though that at this point it is about as irrelevant as the RX-8 in BSP or maybe more so an RSX-S in CSP. [Beer]

Chiketkd
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09 Nov 2008 11:57 PM
jeffh wrote:
I do agree though that at this point it is about as irrelevant as the RX-8 in BSP or maybe more so an RSX-S in CSP.  [Beer]
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