Guran's NA6 "Little Reddy"

[NitroDann]

And you wonder why I want to get in your passenger seat

Dann

[Guran]

The Winter 2011 issue of Club Torque is now available as a PDF for download from the club website (just click on the cover image below). If anyone did not receive the printed magazine and believes they should have, please let me know.

[Guran]

A few weeks ago I showed a graph of my club trackday laptimes at Wakefield Park, as a function of the date during the year. There appeared to be a trend suggesting faster laps in winter and slower laps in summer. Originally I thought this must be due to temperature, but then this technical paper got me thinking that it might actually have more to do with atmospheric pressure. So I did some digging into the BOM archive and came up with these two new graphs covering all my dry trackdays over the last 12 months (the free BOM archive is limited to 12 months):


9am MSLP data is from the Goulburn Airport BOM site which is just over the hill from the track. Very reliable data. I've sketched in some "best fit" curves that seem to match the overall trend, and divided them into two groups depending on which tyres I was using. The one outlier at 1012hPa is from 5th Sep 2010 when the track had a trickle of water across the apexes of turns 2 and 10. It's interesting to see that the difference between RE001s and RE55s seems to be bang on 2 seconds in like-for-like conditions (which I've never actually tested on the same day).


9am MSLP data is from Badgerys Creek, which is about 15km to the south-east of Eastern Creek. I've split the data into two halves corresponding to before and after I started to take turn 1 flat throttle (seems to make 1.0-1.5s difference). In this case, I've fitted straight lines. I don't know which is correct - but more data from future trackdays will answer that.

The atmospheric pressure at both sites tends to be lower in the summer and higher in the winter, just like temperature. But the correlation against temperature was much weaker. Furthermore, MSLP can occasionally get as high as 1035hPa, especially in June and July.

Sooooo ... if the planets align and I happen to be at the track at that time, I'm predicting that I might eventually get down to something like 1:16.5 on RE001s / 1:14.5 on RE55s at Wakefield Park, and 1:58.5 at Eastern Creek . That'd be great ... but it sort of takes the gloss off it if the weather is such a strong contributor.

Maybe Dann or someone else with technical knowledge could pipe in here with a theory as to why this trend might be happening? I'm thinking that it might be related to AFR, with the engine running slightly leaner as MSLP increases. Is that feasible?

[manga_blue]

I tried to tell you last time. You're developing dodgy scientific hypotheses to explain laptime variations then massaging the stats to support each one. The one true reason for any variation is emotional, not scientific. Here's the corresponding and much more meaningful analysis of manga times.

I'm storing the helmet somewhere special in preparation for the next outing.

[NitroDann]

The afm will account for both air pressure and temperature, so it shouldnt be leaner.

But, cool air is denser, and high pressures are denser again.
What im saying is that if you get a litre of air and cool it, it will be now less than a litre(lets say 900cc), and so denser, then if you add more air, inside that 900cc space, this respresents your higher pressure. Now you have, lets say 1100cc of the original temp/pressure air in a 900cc space.

Long story short, cold air is less pressure, so having cold air And high pressure is a lot more air than you might realise.

But you wont be leaner.

You may make 2-3% more power though.



If you really want to run a 1:14, take mine for a lap sept 4th, im sure youd be able to manage it

Sorry, tired, huge ramble.

Dann

[Guran]

Phil that's just too funny. But you can keep your stinky helmet! I'm sticking with the science. BTW, I also have logged data showing faster straight-line acceleration on these high MSLP days.

Thanks Dann. Let me try again, delving into my 24-year-old memory of high school chemistry!

Ideal gas law: pressure times volume equals moles of gas times gas constant times temperature (PV=nRT). Volume is fixed via the AFM, so this converts to pressure divided by moles of gas divided by temperature (P/nT=constant). If pressure increases and/or temperature decreases, then moles of gas has to increase. In other words, on a cold day with high MSLP, the engine breaths more molecules of air (with oxygen being the important bit). The oxygen sensor initially detects a slight excess of oxygen and starts feeding in more fuel (compared with a hot day with low MSLP), to achieve the target lambda. So the engine makes more power because it's fed with more oxygen molecules and more fuel. Is that how it works?

[manga_blue]

The oxy sensor has no effective role on the track.

On my NA8 the ECU goes into open loop when RPM > 4608 or throttle position (TPS) > 50%. In open loop the ECU supplies fuel according to a pre-configured map based on AFM, TPS and intake air temp sensor (IAT) readings, rather than optimising outputs using oxy sensor values. As Dann says, the AFM and IAT outputs mean the ECU is always given an accurate reading of air mass, independent of ambient temps.

On your NA6 with an on/off TPS I'd assume that you'd go open loop above about 4500rpm or at full throttle. Since these describe your normal driving state then I'd say you'd never see closed loop or use the oxy sensor.

Without doing the calcs I'd say the variation in air mass delivery resulting from air temperature variations would absolutely dwarf those from MSLP variations.

You could try correlating laptime with ambient temps. That's not very easy because low temp means low grip. The perfect day is low temp, no wind but with full sun putting some warmth on the track surface.

[Charlie Brown]

Bryan, I can see numerous variables that you haven’t taken into account.

The effect of the high pressure system on your tyres. The higher atmospheric pressure will tend to lower the pressure within your tyres, this inturn will affect the amount of grip you have on the surface. This is not to be confused with the effect the higher atmospheric pressure has on the entire surface of the car and not just the top, thus negating any additional downward pressure.

Also you need to take into consideration the effect of methane gas being released from the local tip. Methane gas is 40% lighter than air, so the atmospheric pressure over the Creek may be lower than the pressure at Badgerys Creek.

Then you have surplus rubber from Time Attack increasing the co-efficient of friction that allows higher corner speeds.

Ah the list goes on and on. Basically I’m in agreement with Phil. Have a sh*t before the run and reduce weight and just flog the sh*t out of the car.

[evil_weevil]

Ah the list goes on and on. Basically I’m in agreement with Phil. Have a sh*t before the run and reduce weight and just flog the sh*t out of the car.

so THATS what I'm doing wrong !!!

[manga_blue]

If everyone followed your advice, CB, there'd be a methane gas hotspot right beside the track as well.

BTW Bryan, I think I've found a car that'd be quicker than yours in 1A. Yes, it's a stock 1600 NB Coupe.

http://www.goo-net.com/usedcar/spread/goo/13/700055051220100921001.html

[NitroDann]

Yeah bryan thats right.
It doesnt matter that the air is thicker or colder, the density makes the air heavier, carrying more momentum to push that flap open further.
Therefore more fuel is added, making more power.

But I reckon have a shave and go to the toilet is the easiest way to guarantee performance

Dann

[Guran]

BTW Bryan, I think I've found a car that'd be quicker than yours in 1A. Yes, it's a stock 1600 NB Coupe.

Nope. Mazda only made 187 NB Coupes, less than the 200 required to be eligible as a "Production Car". However, the regular 1.6L NB would be a contender. I just hope that no CRXs or Civics show up in 1A!

[Guran]

Thanks for the explanation Phil. Helps a lot!

On your NA6 with an on/off TPS I'd assume that you'd go open loop above about 4500rpm or at full throttle. Since these describe your normal driving state then I'd say you'd never see closed loop or use the oxy sensor.

Yep, when my throttle is pressed, it's pretty much always flat to the floor!

Without doing the calcs I'd say the variation in air mass delivery resulting from air temperature variations would absolutely dwarf those from MSLP variations.

MSLP variation from 1030hPa to 1010hPa is a 2% difference. But the AFM doesn't adjust for this does it?

Temperature variation of 30C versus 10C is a 7% difference (in Kelvin). But you say the AFM adjusts for IAT, so this should cancel out, shouldn't it?

You could try correlating laptime with ambient temps. That's not very easy because low temp means low grip. The perfect day is low temp, no wind but with full sun putting some warmth on the track surface.

Yes, I've tried that correlation but it doesn't fit anywhere near as well as MSLP does.

[Guran]

Ah the list goes on and on. Basically I’m in agreement with Phil. Have a sh*t before the run and reduce weight and just flog the sh*t out of the car.

Always!

[NitroDann]

The afm should account for a pressure difference, as air pressure effects mass per litre, hence density, more mass=more momentum to push the door open in the afm.

Dann