Drag racing - gear change ques

240z

240z

Club Member
Well, since I can now prepare for the next drag challenge date, I was thinking about gear change techinques for the strip. Any answers may be useful for others too!

Basically, I don't know whether to stay in 3rd, or change to 4th over the 1/4. the last time I tried I had a 7200 rev limit in place, so when trying to stay in 3rd, I hit the rev limit - not good. I can set this to 8000 and therefore get over the line in 3rd. I worked this out from pluggin the data below into site:
http://www.geocities.com/z_design_studio/

Apparently the ratios for the w58 gearbox I have are:
First Gear: 3.285:1
Second Gear: 1.894:1
Third Gear: 1.275:1
Fourth Gear: 1.00:1
Fifth Gear: 0.783:1

My diff ratio is 3.9

my tyre size is 235/40/17

this gives me (in 3rd)
103@7000 rpm
117@8000 rpm

The question is do I have enough power/torque up there to compensate for my lazy change from 3rd to 4th :confused:

the dyno chart might help, but I think the guy may backed off over 6000 to avoid the rev limiter..
 
I'm no expert but...

Your torque drops off way before 6000 rpm, you want to stay in that band of torque, I'd shift up a gear.

Heres something

RPM after shift = (ratio shift into / ratio shift from) x rpm before shift

(from Auto Math handbook).

Say you shift at 5500 from 3rd to 4th you'll drop into about 4300 revs where you're making max torque.

Shift at 5000 rpm from 3rd to 4th you'll drop into about 3900 rpm making again high torque.

I'd shift at 5000 rpm between 3rd and 4th...



Now I await to be corrected, I am afterall just a desktop drag racer. Cue Mr.F and co.
 
Thanks for the reply russ, there is another factor to take into account though, although my engineering mathematics is getting a bit rusty. Because the dyno was done in fourth, it has the torque corrected for that gearing. Since third has a 1.275 ratio as opposed to 1 for 4th i think it will mean that the torque will be multiplied by this.

Therefore in third I will be getting the same torque as the max torque in 4th at 325/1.25 = 260, which I am getting at about 6300-6400 rpm.

So does a shift at about 6400 seem reasonable? or will the time to make the gear change outweigh the increase in torque?
 
What a drag

Glad to see that your having another crack at it, with any luck we will see a very very low 13 possibly a 12 something

I hang on to 3rd only because I wany to avoid the extra gear change and I am crossing the line at a shade under 99 mph (3rd with a rev limit of 7200 takes me to 105 mph)
Tyre pressures down to about 15
Burn out to clear the tyres off and hopefully to warm them up a bit
4000 rpm, hold on the clutch then feed in the power trying to avoid spin up
Gear change every time a shade before the redline, this way you keep it on cam and the engine doesnt drop of the torque

Sounds like you need that change up ---particularly with the power that you have dont forget though if that first 60 foot is bad then its game over.
 
Far as I know that is the just the torque your engine makes, regardless of gear?

You need to stay on that wave of torque, which is from 3500 to 5500 rpm. Unless your gear change is really slow I think if you worked out the best time to shift for each gear then you'd probably be faster than you currently are, and need to change into fourth earlier anyway.

I'm no expert though lol.
 
is it worth using an electronic quick shift for such a short race?

i used to use them on my race bikes, when you need to change up gear you dont close the throttle nor use the clutch you just move the gear lever and the ignition is cut for aslong as the lever is being moved allowing you to select a gear when there is no load on the gearbox. i know bike gearboxes work in a different way but the principal should be the same!....any thoughts?
 
Propa drag cars use a sequential system driven by pneumatics and a big red launch button on top of the gear shift Muhahaha

An electronic system for a 30 year old z box might be somewhat of an interesting engineering challenge.
 
SKiddell said:
An electronic system for a 30 year old z box might be somewhat of an interesting engineering challenge.
Click to expand...


looks like i have a challenge when my cars done! easy peasy, lemon squeezy:D
 
I think my Haltech also has a ignition cut for flatshifting, I'm not sure how would rig it up with a switch or something for when changing gear though..


Far as I know that is the just the torque your engine makes, regardless of gear?
Click to expand...
Yes, that is the engine torque, but gearing sends more/less torque (moment) to the wheels, so I would have more torque at the wheels in 3rd than in 4th

I've just found this that seems interesting as a kind-of aside http://www.fjr1300.info/misc/torque-power.html

I think my 60fts were about 2.3/2.2, not that great - what were your best skiddell?

thanks for the replies all!
 
It has taken me a long time (some 20 runs) to get that low with help from the Ubermeister himself, slick Pete and Mad Matt Bannister.

Get it right though Phil and your on course to take the 300 TT brigade full on.
Sub 13 here we come
 
Who you calling mad? you quarter mile Junky!
Now you've learnt how to start a race properly it's time to build that roadsports car! Next year you me DJ and CB 4 weel drifting into old hall bend at Oulton. Now wipe the driibble of your chin!!
 
LOL! Even Ive done a 2.19 in my 1500kg lardy slicktop!!!
More right foot needed!!:D
Click to expand...
hehe thanks, looks like I've got a lot of work to do.. although I'd say I need less right foot and a slower left foot! :D
 
Hi you did great on sunday, maybe an MSD ignition amp with 2 step would work for you. You would be able to hold 4000 rpm say at full throtle with the clutch down and get on boost before you leave the line then do a full throtle shift keeping the turbo spining during gear changes.
 
Thanks Gav, shame I didn't manage to bump into you on the day. The car looks incredible.. are you going to Japshow?


I am investigating as to whether I can get my haltech to have a switchable rev limit, would help a bit of the line, but to be honest I am struggling with giving it too much power off the line.. A set of those slicks you have might come in handy too ;)

then do a full throtle shift keeping the turbo spining during gear changes.
Click to expand...
Is this what is called 'flat shifting', I heard can be very hazardous for gearboxes..?

I should also be able to do an ignition cut triggered by the clutch, socan keep my foot flat to the floor, but all sounds quite tricky..

still got a lot to learn!!!!:D
 
Hi im off to the jap show in oct if you are there i can talk you through the flat shift at full throttle its not that difficult to set up or that expensive to do with the MSD kit. The slicks are reasonable 2. If the rev limit on the lower setting is set in the full torque range its not bad on clutches and gearboxes.
 
You must log in or register to reply here.
Back
Top