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Bathurst Easter 2010, races and a six hour regularity relay, anyone interested i


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#81 .

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Posted 14 April 2010 - 03:10 PM

I had a 3.9 in the car for Bathurst this time around and thought it was almost perfect. I have a silvia gearbox, so the 0.76 5th gear. We were also pulling 6500ish down conrod (240.3 according to GPS) and I thought the car pulled great up the hill, I was hitting 5th just coming over the rise so it was being used for more than just the run down the hill, indicating good gearing. The gearing also worked very well across the top of the mountain with 2/3/4 all being used. It's interesting that we were getting the same downhill speed but you didn't like going up the hill. My car also weighs 1250 with a half tank and me in it, so it's 2+2 territory.

At home here I run a 4.4 and that I think would be too short every wheer and in 2008 I ran a 3.5 diff (0.83 5th gear with a standard 260 box) which was SHOCKINGLY over geared. The perfect gearbox, personally, I think would be a 4.1. There are a couple corners (Murray, Hell, Esses/Dipper/Forrest Elbow) that are about 300-400revs too low with the 3.9 in 3rd but to fast for 2nd and I think a 4.1 would be exactly on the money/power.


OK sounds like that confirms my thinking then. I too was changing to 5th just at the rise going up mountain straight. So it must just be a lack of power that is costing me in the cutting. (I only have 130HP at the rear wheels) Power is not such an issue down conrod.

#82 260DET

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Posted 14 April 2010 - 05:31 PM

OK sounds like that confirms my thinking then. I too was changing to 5th just at the rise going up mountain straight. So it must just be a lack of power that is costing me in the cutting. (I only have 130HP at the rear wheels) Power is not such an issue down conrod.


Yeh I don't see how you possibly could get anywhere  near ~240 down Conrod with 130 WHP Greg. Your front spoiler may help a bit but power comes into it a hell of a lot at those speeds. If the maths all work out and you are actually getting 240 I want to know the secret :)

Lakeside is a good comparison. Mine has 450 WHP and is good for say 220 max through the kink, thats with an entry of around 130.

#83 .

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Posted 14 April 2010 - 06:29 PM

I have a 280 gear box, a 3.9 diff and 16 inch wheels. I was doing between 6500-7000 RPM and my speedo was off the dial. Those are the facts. Make of it what ever you like.

I don't know of any other variables that can stuff that calculation but obviously something is wrong.

I am not trying to tell you what speed I was doing because none of it makes any sense to me. But lets put it this way I was catching most cars on Conrod, there were a few cars edging past me but nobody went whizzing past.

But also a certain 1600 driver (who shall remain nameless) did a GPS confirmed 180 down conrod with no aero and a bog standard 1600 with SUs producing something like 90HP.......Like I was saying the Gravity does the rest.

and by the way I do 190KM through the kink at lakeside with the same HP and that one I am sure of. Lakeside is not a good comparison at all. It was pedestrian compared to Conrod. I was building heaps of extra speed due to the drop in altitude.

#84 260DET

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Posted 14 April 2010 - 07:37 PM

Whooo, I'm not having a go at you Greg, just wondering how you get such speeds out of a S30 with so little power. There has to be a reason.

Same with your Lakeside speed. Other S30's with powerful stroker motors don't reach 200 through the kink yet their lap times are way under yours. Like I said, there must be a reason, maybe you have more power than you think.

By the way, I still have two front 280ZX lights of yours.

#85 1600dave

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Posted 14 April 2010 - 08:55 PM

But also a certain 1600 driver (who shall remain nameless) did a GPS confirmed 180 down conrod with no aero and a bog standard 1600 with SUs producing something like 90HP


That would be me. Its an L18 which runs standard (re-needled) 180B SSS SU carbs, 180B SSS .85:1 5th gearbox, 3.7 diff and 195/55/15 tyres. And 90hp is an overstatement, last dyno was for 68 hp (although it has been rebuilt with flat top pistons and more porting since then). Top speed was 181kph by Racechrono datalogger, don't know what rpm (don't have a tacho  :-[) but it wouldn't pull 5th gear, top speed was pretty much the same in 4th or 5th so there was plenty of top speed left if I could get it to rev out in 5th. It was that underpowered that it would actually start to slow slightly as it started going up the "Hump" in conrod straight, and would pick up speed again once over the hump till the chase.

I was worried about my lack of speed down conrod straight before the event, but its a real roller-coaster ride - I think the length and steepness of conrod straight really help to artificially inflate top speeds, there's no way mine would hit 181 on flat ground. It struggled to get to 140 going up mountain straight on the weekend.

#86 .

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Posted 14 April 2010 - 09:06 PM

Whooo, I'm not having a go at you Greg, just wondering how you get such speeds out of a S30 with so little power. There has to be a reason.

Same with your Lakeside speed. Other S30's with powerful stroker motors don't reach 200 through the kink yet their lap times are way under yours. Like I said, there must be a reason, maybe you have more power than you think.

By the way, I still have two front 280ZX lights of yours.


Well it was a rusty shell when I started...Maybe there's not as much metal in my car. But yeah you might be right. I have only had it on the Dyno once so its quite possible that dyno was reading low on the day.

That 190 at Lakeside was with the 4.6 diff and screaming at top revs. Thats the only reason I can confirm the speed because I know the car couldn't go faster with that diff. I guess the only peice of the puzzle that I can't confirm is the ratio of my 5th gear. There is a slim chance that the previous owner of that Gear box may have changed the standard 5th for something with less legs.

Headlights, ba who cares.....unless they are gold plated.

#87 dat2kman

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Posted 14 April 2010 - 10:33 PM

Man, I am loving this thread, see it has got you all thinking, which is absolutely fantastic!
Diffs, Boxes, ratios, wheel sizes, power.
Only thing to raise it's little head is the brick wall of aero and speed, and 260det is right on the money here.
We are all running giant windsocks large end in front, even my little dato sports2000 really feels the wind, i know, cos my helmeted head is getting that much buffeting, my neck is sore for a week after, and I know from the gearcalc program that I am doing 240, and lapping at 2 min 43.

That place is a real compromise, ie: fifht gear do you go .75 or .83? Tyre size? Diff 3.9/4.1.4.4
I was running a 610 mm diam  tyre, and a 4.4 diff .83 fifth, and safe rpm at 7600 maybe 7700, I was in fifth well before hump down Connie, and just over hump on Mountain, it all worked for me!
Excepting for Forests, all the important cornere are all UPHILL exits,  and the setup should be aimed at this. If you have to ease off throttle down Connie, so be it.
BTW i ran same set up 'cept for a 627mm tall tyre for phillip Island two weeks prior Won all four group S races and fastes was a 1.55
( Bloody Datsuns, grumbles the pommy brigade!!!)

#88 1600dave

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Posted 14 April 2010 - 10:41 PM

Its definitely got me thinking and planning for next time. Mine was way undergeared for the slog up the mountain. I was still running a 3.7 diff, and took most corners in 3rd (except for the cutting and Murray's Corner in 2nd). Unfortunately, taking corners in 3rd meant it just wouldn't pull out of the corner as the revs were too low. Hell Corner was just painfull - I took it in 3rd and was still in 3rd going over the hump near the end of mountain straight. The only issue for me running a 4.1 or 4.4 would be maybe not being able to drop all the way back to 2nd in Murray's Corner. Everywhere else, I feel I would gain from a shorter diff (or a better box without the big 2nd - 3rd gap).

And for anyone wanting to see what that small amount of power looks like from the inside, here's a few in-car videos.







#89 Whittie

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Posted 15 April 2010 - 12:12 AM

I have a 280 gear box, a 3.9 diff and 16 inch wheels. I was doing between 6500-7000 RPM and my speedo was off the dial. Those are the facts. Make of it what ever you like.


I am pulling 175hp/230Nm at the rears and only doing 240 and it was a KILLER forrest elbow. In 2008, with 165hp, I was only capable of 230 down the mountain. When I first built my motor with dished pistons it had 130hp and I can tell you that there is no WAY it would have done anywhere near 200 down that hill. Conrod is not such a huge drop as I think some people think it is, most of the drop is done from skyline to the exit of forrest elbow, it is just very long. I was hitting 5th still coming down conrod, well before the hump, and still only doing 240. I would say you have a fair few more ponies than you think to be pulling 235kph down the hill.

My speedo reads to 200 and it was about vertically down at those speeds (english car with a kph sticker over the top of the mph speedo :P) but I cannot feasibly think that the maths works out 240kph with 130hp unless you have some crazy aero mods!

Here is an excel file to calc top speeds. Enter your tyre and rim size and 6500rpm and it will tell you the speed you were doing. This does ignore tyre expansion, which probably does occur at those speeds.

http://members.iinet...speed calc.xlsx

#90 .

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Posted 15 April 2010 - 08:52 AM

Thanks but I tried that already and something is wrong. It tells me I was doing 276 KMPH thats why I have said all along I don't know what speed I was doing. 

#91 bluerat

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Posted 15 April 2010 - 10:03 AM

Here Guy's have a play with this...

http://www.locost7.info/gearcalc.php

Using my .86 5th 260Z box as an example(3.9 diff and 225/50 16's), 6500 rpm in 5th is 230kmph.

And Dave, changing the ratio to 3.7 and tyre size to 205/50/15 (Guess?) you were doing 5000 rpm at 180kmh. Changing your diff ratio to a 4.4 will give you 180 kmh at 6200 rpm. Food for thought!

#92 PZG302

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Posted 15 April 2010 - 11:55 AM

Bluerat, great program, thanks for sharing ;D

I had a quick play and it works pretty well spot on for me as I was clocked at Turn 1 at Eastern Creek at 187km/h which I am generally at peak revs in 4th with a 4.1 diff and the program shows my max speed would be 198km/h at 7200 in 4th, which to me is spot on.

I think this will definately sort the chaff from the wheat!!!!

The speeds definately show what I would expect from my car, not too fast in a straight line to everyone elses figures, but it gets there quickly and can get through corners at high speed.

#93 sco_aus

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Posted 15 April 2010 - 12:09 PM

3.7 diff, 235/45/17 and RB25DET gearbox at 7700rpm is 336km/h

#94 bluerat

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Posted 15 April 2010 - 12:24 PM

Yeah it's a very handy tool. I scored it off a mate with a clubman, hence the locost site. Trust me I played around with different ratio's for years at Symmons and it all comes down to horsepower! and where the power is made. I tried 3.9, 4.1 and 4.4 and the top speed, via GPS, only varied 2-3 kmh you need power to push that air!! the 4.4 was quickest as it got out of the corners quicker and held higher gears through the slower corners, and it was a very top heavy engine power wise. My new motor makes more power, lower in the rev's and the 4.1 is around a sec a lap quicker than the 4.4 now. It's very handy at hillclimbs working out weather to change up or hold a gear in to corners as`well.



#95 dat2kman

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Posted 16 April 2010 - 10:00 AM

3.7 diff, 235/45/17 and RB25DET gearbox at 7700rpm is 336km/h


Only if Conrod Straight is in a sealed vacuum!
Biggest issue above 250kmh is frontal aero drag.
Gnose, lowered with no gaps under car,  full undertray,side skirts, effective front splitter etc, and yes you may just see 280!
Cochranes Supertaxis do 290, GT3 Porkers do 300 they got aero packages!
Bonneville Salt flats USA have had a few Zeds on it, cant recall numbers but very fast.
So lets get you entered for FOSC Bathurst 2011 and see how you go??????

#96 Scando

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Posted 16 April 2010 - 01:14 PM

I'm keen as to do this bathurst meet one year.

For what it's worth, at Phillip island I was hitting the 6,300 rpm limit with a 3.54 diff, 0.759 5th and 255/40 17's.  the gearcalc program tells me I was doing 281km/h.  I do have a front spoiler and an undertray from the spoiler to the radiator support.  I tried the same calc with the tyre diameter as specified by Bridgestone at it came out at 278.

A guy I race against runs a VY Commodore and recently added a V8 Supercar bodykit.  It's over a second a lap quicker now but hit's lower speeds on his telemetry due to the extra drag of the spoilers.

#97 1600dave

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Posted 16 April 2010 - 02:02 PM

And Dave, changing the ratio to 3.7 and tyre size to 205/50/15 (Guess?) you were doing 5000 rpm at 180kmh. Changing your diff ratio to a 4.4 will give you 180 kmh at 6200 rpm. Food for thought!


Food for thought indeed.

My tyre size is 195/55/15, so pretty much the same as what you used for the calculation.

This is the first time I've been on a track where I've had the opportunity to think about top speeds, usually I pick a diff ratio based on getting best accelleration out of whatever corners are at the circuit.

Out of curiousity, assuming peak power on my L18 comes in around 6500 (it was around there last time it was dyno'd), would a shorter diff ratio as outlined above lead to increased top speed ? ie if it will only get to 180kmh @ 5000 rpm in 5th, by lifting revs to 6200 rpm @ 180kmh will it continue on to a higher speed due to having more power when it hits 180kmh ?

At Easter, I would just run it out down the straight till it ran out of "go forward" in 4th, then pop it into 5th, but it really didn't seem to accellerate anymore once in 5th - it was just an "overdrive" gear that brought the revs down rather than a "useable" extra gear.

Hope that made sense  ::)

Might have to set up portable datalogging next year (instead of having it stuck on the dashtop), share it between cars and have a "top speed" competition  8)

I'd really love to do both FOSC and the Nationals.....Will have to wait and see how the timing works.


And the wallet  :(. Its a real dilemna, I only intended Bathurst this year to be a one-off and back to the nationals next year, but now I just don't know what I'd rather do  :-\

#98 luvemfast

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Posted 16 April 2010 - 03:50 PM

Bluerat, great program, thanks for sharing ;D

+1
great program, will check how accurate it is over the weekend.

#99 Whittie

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Posted 17 April 2010 - 04:28 PM

I am horrifically slow at doing the videos this year!

Another one up of me following Narelle in her and Gary's 2+2






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