mx5cartalk.com

I'm not sure where to post this. It could just as easily fit in Motorsport or Body or OT.

I'm interested to hear what max speed others have achieved and has anyone else experienced a sort of wall with it.

I've been going over old lap logs from Eastern Creek and Phillip Island. For some reason my car maxes out at 185kmh at both tracks (= 195-200 indicated on my speedo). This hasn't changed in 4 years despite ongoing tuning and tweaking of the car so that it goes a lot harder, spins out further and gets out of the corners a lot faster. In fact an extra 15 rwKw has made buggar all difference. It doesn't matter whether I hold 4th or go to 5th - same result. All that's really happening is that I keep hitting 185kmh earlier and earlier on the straights - but it's like there's a wall beyond that. There's no feeling of an ECU cut like the rev limiter cut, it just gets to that speed and won't go faster.

Seeing that Guran has logged 181 in a totally stock NA6 I'm left wondering what's going on.

The car's an NA8, totally stock bodywork and moderately modded drivetrain: std cams, std gearbox and diff, head cleanup, big valve springs, Loch Stewart CAI sucking from the cold air box through the firewall, good exhaust system, ECU mods, power good through to 7500 redline, light wheels on 205/50R15s.

Does the stock NA have an aerodynamic wall at 185?
Does the high pressure zone in the cold air box suddenly collapse at 185?
Does a stock fuel pump fail to give prolonged delivery on full throttle and high revs on these straights?
What else?

is the engine doing 7500rpm when travelling at 185kph?

I always thought the gearing run out at 180 odd kmph?

26kmh/1000rpm in 4th on those tyres. So 7100 in 4th, 5780 in 5th.

It pulls hard to 7500 in 1st, 2nd and 3rd (logged RPM, indicated as 8000+ on the tacho).

I had the Slug up to just shy of 190 on a very long slightly declining remote straight with a car full of luggage and a passenger, might have eked a bit more out of it with more road, but it wasn't going to get much faster than it was going

seems you have hit the power vs drag wall.

need more torque and rpm! time for some argo cams.

Top up or top down? Windows up/down?
All but one track day I drive top down, and on the one occasion with my top and windows up, I definitely felt an aerodynamic advantage even at 165km/h top speed.
I don't think I've been past 175km/h though ("National" layout at QR), but that was because there was not enough track to continue.

Does your car have any aerodynamic aids at the front? ie. splitter or some sort?

Phil, FWIW:
Wheels Jan '94 test of '93 Clubman @ Eastern Creek
1st gear: 56kph @ 7k rpm
2nd: 93 @ 7k
3rd: 133 @ 7k
4th: 177 @ 7k
5th: 187 @ 6k

Gear ratios (kph per 1k rpm)
1st: 3.13 (8.1)
2nd: 1.88 (13.4)
3rd: 1.33 (19.1)
4th: 1.00 (25.4)
5th: 0.81 (31.2)

Interesting, Paul. I'm slower than a stockie with 20% more power. I'm running 205s, so speed/rpm is a little higher. Speeds are off the GPS anyway.

Always top up/windows up, Andrew. I tried it at Sandown once. 14kmh slower on the back straight with the top down. From memory rain902 lost more than 5 secs by going topless for one session around PI in an NA6 stockie.

"argo cams"?????

Phil, these pages from Wheels magazine might be of some assistance in your calculations (click on thumbnails to see hi-res).

Article by Pedr Davis from Nov 1967


Article by Mike McCarthy from Feb 1993

Phil,

I also think that its simply aero drag overwhelming the hp you have.


Only tracks I've been to where I exceeded 180 is PI and Bathurst

My car at PI will only get to 186 consistently on the front straight before it starts going downhill . picks up about 6kmh on average thanks to the downhill run into Turn 1.

Even comparing multiple PI laps ranging from 2:01 thru 1:56 still shows my top speed on the front straight within 1 or 2 kmh...

At Bathurst it drops back to 185 on Conrod once the downhill flattened out leading into the hump. (which is 7m elevation rise in 200m distance)

Freakishly similar numbers there to what you found... hmm


Without significant power increase, or aero aids I dont think you'll see more than a few kms variation to your top speed either.

I have an NB8A which is pretty much standard. Only performance mods are an IL Motorsport airbox and a T&C exhaust. I use a hardtop and generally drive with the drivers side window down.

My Vmax on Conrod at Easter was 194 kph and the car was still accelerating, albeit fairly slowly by that stage. I used RaceChrono with a Qstarz BT-Q818XT receiver. The speedo was showing about 210 kph.

Given a long enough straight with my gearing, I reckon I should just about see 200 kph by the time the rev limiter kicked in. I'll give it a red hot go next year!

Phil, according to your race chrono I'm doing 190kph down EC on a slow session (this was that session I was showing Fergus in the Integra around and making sure he kept up) and I'm definitely going faster around PI. Quite a lot actually... I'd get a dyno run and see how your power:weight is going. I'm 90kw atw and 1080kg ( 1150kg plus my 70kg contribution) wet weight for a reference. That's 78.2rwkw/tonne. An NA is much lighter stock...

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.

greenMachine wrote: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.