Maximum speed

[manga_blue]

All very interesting .... thanks guys. When I saw Russ's speeds I thought Yeah, that's right, he makes a tiny bit more power than me, he's peaking at 186 over the top where I do 181 there - so makes sense. Then Tony's comes in at faster than mine with a potentially milder motor and he's quicker than both Russ and me- something's wrong. Maybe Tony's got a really well made motor? I'm also thinking that Russ and Mark were almost level pegging up the PI straight last year and Mark's got a bit more under the bonnet than Russ. Turns out he's much quicker, so ???? Then greenmachine starts talking 220 indicated (= ~205 real?) and I thought he had about 105rwkw in an atmo.

So go read Guran's references then: drag is proportional to the square of velocity. I think I have 80rwkw and my guesswork dodgy calculations tell me I would need 84.4rwkw to match Russell, 88.2rwkw to match Mark and 102.6rwkw to match the GM.

I think you're right, Russell, "its simply aero drag overwhelming the hp you have". I've run out of easy cheap options to boost horsepower and CAMS 2B rules prohibit aero mods .. so all I can do is wash the car for once and drive the it harder.

BTW, there's a slight bias in the dodgy calcs. Is the NB shape more slippery than the NA?

[Hellmun]

We weren't level, I was catching, backing off and catching again... I think I would pull 2-3 car lengths on the straight ( ECU, Headers, no P/S , no A/C, no soft-top, carbon roof, 6 speed with 4.1... Etc do add up A little ). The modded NC's are only a little faster than me and they have more torque and power with only a little weight but it does hold level with a stock one. I might have a vid chasing Russ around I can show.... Will have a look on the PC tomorrow.

[NitroDann]

Mines still atmo, 1.6, Ive seen an indicated 225, not sure how fast that really is, its acurate at 100.
This is with a 6 speed and a 4.1 with 195-50r15 re001s.

Hardtop and windows up, with a back window in.

It was still accelerating slowly, but that seemed like fast enough. Just with a factory lip and horizontal ducktail.

Dann

[manga_blue]

Dann, my speedo is 7% high on 195/50r15 Re001s. So if yours is the same then 225 = 210. Either way, it's really moving for a 1.6 isn't it?

Yes, I'm a bit porky. 1033 for the car plus a few meat pies on me makes about 1125. Still got A/C, P/S, cruise control, all the padding, softtop, etc. Maybe it's not slipperiness but weight.

[Guran]

Max speed is independent of weight! Check the theory in those articles. Weight affects acceleration because the time to reach a certain speed or cover a certain distance is proportional to the weight/power ratio (kg/kW). However, the same car will reach the same max speed with and without an extra 100kg of ballast, provided the track is long enough. And there's the limiting factor: most race tracks don't have a long enough straight for any of us to hit terminal velocity ... so weight does come into this debate afterall.

[Tony]

Coincidentally, I got my car weighed recently (thinking about getting a trailer built) and it was 1066 kg with very little fuel. So with my 105 kg and 45 kg for a full tank of fuel, the gross weight would be around 1220 kg. I'm working on the fact that more mass going down hill will give it more momentum, therefore more speed!

[rascal]

All very interesting .... thanks guys. When I saw Russ's speeds I thought Yeah, that's right, he makes a tiny bit more power than me, he's peaking at 186 over the top where I do 181 there - so makes sense. Then Tony's comes in at faster than mine with a potentially milder motor and he's quicker than both Russ and me- something's wrong. Maybe Tony's got a really well made motor? I'm also thinking that Russ and Mark were almost level pegging up the PI straight last year and Mark's got a bit more under the bonnet than Russ. Turns out he's much quicker, so ???? Then greenmachine starts talking 220 indicated (= ~205 real?) and I thought he had about 105rwkw in an atmo.

So go read Guran's references then: drag is proportional to the square of velocity. I think I have 80rwkw and my guesswork dodgy calculations tell me I would need 84.4rwkw to match Russell, 88.2rwkw to match Mark and 102.6rwkw to match the GM.

I think you're right, Russell, "its simply aero drag overwhelming the hp you have". I've run out of easy cheap options to boost horsepower and CAMS 2B rules prohibit aero mods .. so all I can do is wash the car for once and drive the it harder.

BTW, there's a slight bias in the dodgy calcs. Is the NB shape more slippery than the NA?

Tony's 194 on Conrod was peak. (the last section is downhill) whereas my 185 was on the flat section just before the hump. Mine then sped up again down the other side to 198 and would have just made over 200 if I wasn't woosing it and backing off before the kink.

As Mark said, he and I were so not level pegging on PI straight At one point I looked in the mirror coming out of 11 and I was 5 car lengths ahead, and before we got to turn 12 he was on my bumper again. He was just fanning the throttle on the straight to stay behind so could watch the lines I take.

I think NBs 'are' slightly more slippery than NAs and drag makes a huge difference to top speed. I also have the small factory front lip and rear spoiler and these would contribute an extra 1 or 2 kmh top speed.

[Hellmun]

Playing around chasing Russ.


Weight is definitely a factor, use skydiving for a simple example. My force acting downwards is my mass * gravity. Therefore a change in my weight will change my total force exerted downwards. Since drag is a force(mass of air multiplied by the acceleration your body is exerting on it) by changing weight you will change the amount of negating force required to reach the same velocity. With the constant acceleration of gravity this actually means the higher weight increases your top speed. With a car in the horizontal plane the increased weight has a similar effect, it pushes the car down harder increasing vertical force, thereby increasing friction on the road and reducing the force available to overcome the aerodynamic force resisting in the horizontal plane. Being that the force is acting perpendicular it's simply a smaller factor because it's calculated at a tangent. Plus as mentioned, racetracks aren't really that big so the effect it has on reducing acceleration through drag really limits practical top speed. So I think quite relevant to discussion.

I'd still like to see a dyno chart comparison of your motor see how it's generating torque. The gearing isn't exotic (5 speed with 4.1) and I thought you had a stock shape muffler, undertray and hard-top which cover the main easy ways to mess with your aerodynamics. Being the easiest to check I'd start at the motor and then look at ride height compared to some other quicker N/A's, whether you've got sticky pads, worn sliders etc. So many small things can contribute....

[evil_weevil]

very interesting thread!
I think my speedo when I "glanced" at it last time at EC just after the 100m braking marker was 190 or so - but not sure how accurate it is either!

I think we need to get a bit more torque Manga blue! (well, thats my excuse hehe)

[greenMachine]

It has been a while, and the memory grows dim ...

A Bathurst (first FOSC meeting) the GM (NB8A) was showing around 220 down Conrod - I say 'around' because my attention was largely directed at the rapidly approaching twisty bit ... No data logging unfortunately. That was running 205/50 x 15 tyres, and a 6-speed gearbox with the 5-speed diff ratio.

At EC running the same setup, I was on the rev limiter at T1 turn in in 4th and iirc the rev limit was 7500 at that time, I can't remember what that works out at in kph.

I have one the other way and using a 195's with a 3,9 diff and the 6speed the speedo is dam near accurate where it was always showing 105 at a true 100 with the 5 speed and 4.1.so you speed may be quite a bit lower.

I have seen a genuine 180klmph in 5th in the six speed (with 102RWKW)and there was more to come in sixth so I am not sure it is running into a "wall" but just not enough power that is limiting Manga Blue.

Interesting topic.

Peter.

I take the 'wall' concept with a grain of salt, it is all relative - more power, go faster. The returns will diminish if the aero is unchanged, but there will always be some gain from more Kws. However, at these speeds nailing down all the variables can be difficult, things such as the effects of tyre degradation, temperature affecting the track conditions (grip, exit speeds), power outputs varying with temperature, wind speeds and direction altering the aero, even variability of fuel quality. Consequently, it would be very difficult to isolate all the variables and definitively state that an extra 'x'kws produced an extra 'y'kmh terminal velocity.

As my diff ratio was not changed, we simply plugged in the same speedo sender into the 6-speed, and on 205x50s it was dead accurate on my Tomtom. On Conrod, that was the limit as it held that speed for a distance, it was not going to go higher without a tailwind ... or a tow from one of the turbo cars ! At the same meeting, Turboboy (SP replica racecar) datalogged 275 kmh into the Chase However, that extra 55kmh or so probably took an extra 100rwkws ...

[greenMachine]

... Then greenmachine starts talking 220 indicated (= ~205 real?) and I thought he had about 105rwkw in an atmo.

Manga, at highway speeds my Tomtom said my speedo was spot on, double that and your guess is as good as mine . I am not going to swear on the proverbial stack of bibles that the speedo was showing 220, as I said I had my mind on other more pressing matters, but it was very close to it if it wasn't actually there. However, I can say with absolute conviction that it was bum-tightening, eye-popping, sweaty-palms-on-the-wheel FAST !!!! Yes, that was with 105rwkws.

Don't forget that this was at Bathurst, and that if you are going to find your terminal velocity it will be there. I wish I could remember what the GM was doing into T1 at PI, but the memory banks fail me

[manga_blue]

I don't doubt the 220 reading at all, GM. I know how you feel. I do try to look at the speedo on the run down into turn 1 at the Island, at least as a bit of a reference value. I can see the needle somewhere over 190 but I can't focus on it as my eyes are drawn to the road dipping ahead and then snaking up to the right and I mutter "God give me strength, don't let me lift off".

I think you're right, Guran. There is still a bit of a weight factor. I exaggerated: the speed curve is not yet completely flat, there's a tiny bit of slope left. Another 1Km and maybe I'd hit a genuine 200. The real wall is just an unacceptably low rate of acceleration.

Nevertheless you'd think that at those speeds you'd be able to ram more air in. Really, how hard does a turbo push compared to a 180kmh wind?

Dann, where are you drawing your air from?

[Benny]

I've had my SP's speedo trying to get back to 0 the hard way!

Down Wakefield's straight, I've seen around 190 before I hit the brakes.

[93_Clubman]

Does the high pressure zone in the cold air box suddenly collapse at 185?

There was some opinion along these lines, but don't know if it was established.

Nevertheless you'd think that at those speeds you'd be able to ram more air in. Really, how hard does a turbo push compared to a 180kmh wind?

Some numbers for cowl & area above it:
http://forum.miata.net/vb/archive/index ... 19529.html

Last post here suggests it's limited:
http://forum.miata.net/vb/showthread.php?t=202643

You've probably seen these, but if not:
http://autospeed.com.au/cms/title_Sitin ... ticle.html

http://autospeed.com.au/cms/title_More- ... ticle.html

[Charlie Brown]

I've had my SP's speedo trying to get back to 0 the hard way!

Down Wakefield's straight, I've seen around 190 before I hit the brakes.

Always thought SP speedos were optimistic.