With a 4.44 rear gear and 205/60-13 the speeds at 800 RPM's would likely be about 65 in 2nd, 95 in third and about 120 in 4th, that to me seems like it would be revving pretty hard on the freeway.


Tom

Do they have freeways in South Africa?

I drive my A14 at 4500 RPM on the freeway. It's right in the fat part of the torque curve.

Well, currently her diff and gearbox both whine like crazy if I get anywhere close to 120km/h. Not to mention she almost runs out of revs. I replaced the diff and gearbox oil and added a tad of ATF to clean up. I've also noticed her current speedo is quite a bit out. She's currently fitted with 175/75/ R13's as I bought her, but somehow when the speedo says I'm doing 120km/h, I'm actually doing 106km/h according to my gps.

We have plenty of space for speeding including freeways, but our speed limit will nowhere exceed 120km/h.

higher diff ratios (higher numerical ratio, lower gearing) will produce better acceleration, but you can eventually reach a point where a couple of things will start to happen. The first is a little less of an issue with dattos like this, but it is basically that the torque multiplication is sufficient enough (and that's combined with the fact a supercharged motor will have as much as double the torque of an NA one) that it just wants to break loose in first (even without dropping the clutch, you can have enouggh torque/torque multiplication that it will exceed available grip). SO a higher geared diff (lower numerical ratio) in such a case will actually help lessen such an issue and allow you to actually use full throttle all the way through first gear. SO technically it 'shouldn't' be quicker, but sometimes, just by virtue of being able to use full throttle in first, it actually can be.

On a turbo setup (not that anyone was discussing it here!) - it goes even further. A supercharger has boost at any given rpm (consistent with the superchargers boost curve in that particular application) whenever it gets full (or lots of ) throttle opening. So you have that power available for use in first and second etc. A turbo setup needs two things - sufficient exhaust flow (so rpm related of course) and time to spool up. Now modern turbos aren't bad at all at spooling up but _none_ of them are instantaneous in the real world. With low diff gears (higher numerical ration) the gearing is so low that even if you floor it off the mark, the low gearing lets the motor 'race' right through first gear at it will be at the redline, without actually having time to reach full boost (and hence full power). Then you are back off the throttle, which spools it down a little more, then into 2nd and finally boost starts building and will (typically) see full boost before the next gear change. But a _lot_ of potential is wasted in first gear and that first upshift, not having access to full boost/power. If you went with higher gears (lower numerical ratio) in the diff, well now the motor is loaded down a bit more, and it will spend longer in first gear and _now_ the motor has a chance to see full boost and actually utilise it, and it will now happily run out to the redline at full boost. But insteda of having to shift to 2nd at say, 40km/h (hypothetical figure) - it's powering through with those higher diff gears all teh way to 50+ km/h before hitting that redline. so in a sense whilst the torque multiplication from the diff gears is lessened, the actual torque produced at the motr and sent through to the tyres is higher than the lower geared example.

In other words forced induction can throw a curve ball as far as results vs predicted results go.

Technically (and this is a result of decades of observation by car gurus, not my own crackpot theory) - turbo lag issues as discussed above notwithstanding - well technically, if traction was perfect, then the 'best' diff gearing for the 1/4 mile is actually easy enough to work out - what you need to achieve is having the car passing through the traps (the end of the 1/4 mile distance) with the engine rpm sitting at 5-10% (closer to 5 generally) higher rpm than the engine makes peak power at. So if the motor made peak hp at 5000rpm for example, through testing, you'd want to go to the diff gears that have it runnning through the traps in top gear (be it 4th or 5th if you have a 4 or 5 speed box) at 5250rpm (5% of 5000 is 250, 5000 plus 250 is 5250).

That'll (with ideal traction) get you the best results for non turbo (and possibly non supercharged, but again traction is the factor that will influence whether supercharged motors is applicable) engines in the 1/4mile, which is not a bad way to get close to the best street performance for typical driving.

The only drawback with such diff gearing is that it will tend to have the motor revving higher for steady state highway cruising than some of us would care for. The datsun motors can handle some impressive rpm levels for long durations, but it will factor into eventual engine life and fuel economy and (exhaust system particulars dependant) driver comfort. A couple of optoins there - obviously just go with a diff ratio that is ok on those fronts in general. The other option is to run a 5 speed and choose diff gears to have you through the traps at that peak hp +5% rpm range in _4th_ gear. now you have 5th as more of a highway cruise gear.

Based on your broad outline here, and being particularly mindful of how much extra torque a supercharged motor will have, I reckon you might end up happier with a little less severe diff gearing - instead of 4.44 - perhaps 4.1 would be a better all round compromise?


I am assuming here that it will be dual purpose and see road use as well as the weekend circuit race work. I also note that the above 1/4mile related tech isn't quite as 100% optimal for circuit racing. If the budget allowed for it, a closer ratio gear set (if not massively closer ratio, at least partway improving the huge difference in rpm drop from 1-2 and 2-3 (to a lesser extent I suppose) vs the rpm drop from 3-4. You'd tend to want to be able to have a gear spread and diff ratio so that you'd have a 'gear' for each corner a circuit presented. If you have to bog down too low in rpm to negotiate one particular slow and tight corner you'll lose time. If you have to alternatively go to a lower gear but then have the motor screaming too high (or nearly too high) by mid corner and on corner exit, you'll lose time too (or have to upshift too soon during corner exit, which just 'doesn't work' for the most part as far as ideal traction/corner exit/smoothness etc goes.

So in that case the individual gear ratios in the gearbox can be more important (in some ways) than the diff ratio is. You'd also want to look at how many circuits there are (and I confess I don't know how many there are in SA, let alone how many you will likely be competing at) - but basically you'd have to look at realistic top speeds your car will be doing toward the end of the fastest straight on any given track. You'd ideally want to actually be right on the edge of the rev limiter kicking in just prior to where you would start braking. You'd also have to look at the circuit generally (for example if there's just one straight, but mostly it is short straights and tight corners) you might have to have it on the redline earlier down that straight and the time you'd lose on that straight would be less than what you'd lose if you were bogging down the engine in all the tight corners. On the other hand something with big big straights (something like Bathurst/mt Panorama in Australia - massive straight literally coming down the side of a (small by international standards) mountain/hilltop . There you'd probably sacrifice a little bit in the twisty sections because the straight literally allows speeds in excess of 250km/h (although I do note for accuracy that current highest level racing in AUS already mandates the diff ratio to be used for each specific circuit))

Thank You Mr. McKenzie,

You have certainly cleared up my understanding, and have interpretted my intentions very accurately! ;) I am trully thankful for all the input I get from all you gentlemen!

If I may, I'd like to vividly illustrate my intentions one last time, and I'd greatly value any advice -

Lucy is her name. She'd be an everyday driver, taking me to town across 8km of dirt road, 6km of bad tar, and 28km of freeway on a daily basis. On the weekends, she gets treated with some slicks similarly sized to her everyday wheels, and if I'm able, some extra boost for the circuit or the opportune Quarter Mile. I'd like her to top out at 220km/h-240km/h, 250km/h optimistically, on wheels(inc. tire Profile) of the up to 23".

Engine
- Seeing as its a stroked motor, I'd like to utilize a supercharger and see how much TORQUE I can squeeze out of her. I'm not to sure on the boost levels I want yet. I'd rather have a lower peek and a broader power band. I'll be treating her to 12mm head studs and a few other little things.

Drivetrain - She pulls of beautifully. She accelerates enormously fast (ate an opel astra OPC at a green light this morning) even with her 3.9:1 diff ratio, but her diff shakes vigorously at 120km/h, whining like crazy. The gearbox itself grinds quite a bit at that speed too, being in top gear (4th). So I'm considering a 5 speed. Just not sure what to go for. This leaves me with her diff. I got told to have her old diff serviced and that should suffice, but even I know the old diff won't take 300nm.

I'm having her rear springs retentioned and an extra leaf put in to take up some of the twist on the axle.

And Lastly, I was hoping for at least 225's on all four, inside the rolled out arches, even if it meant modifying the suspension. I'm just not sure on how to modify the front suspension. Or will 205's suffice for my 250nm-300nm goal?

Bart, Tom and John are right.
I'm running 4.88 gears with a 5 speed. The 5th gear overdrive is great for the highway but 1st gear is to short for drag racing. A 4 speed with a taller 1st gear would suit me better on the track. I like driving my car off the track so its a trade off I guess.
But with a higher rev'ing KA, that may change my situation.
I hope to see these changes.

JMAC - As always, such an informative response. I actually copied and pasted it for future reference. Well put!