NB series: know your diff ratios

vscalais · Postby **vscalais** » Fri Jun 12, 2015 4:31 pm

See http://www.miata.net/garage/KnowYourCar/S8_Gears.html

hks_kansei · Postby **hks_kansei** » Fri Jun 12, 2015 4:38 pm

I've actually got a spreadsheet somewhere that I put together.

Lists the ratios for a 5 speed and a 6 speed, diff ratios, wheel rolling circumference, and engine speeds so that you can plug in a few figures and work out the RPM difference at a certain speed, or the speed difference at a certain RPM.

I worked out that a 6 speed on my car would result in it sitting 109rpm higher at 100kph (in 6th) than my current 5 speed in 5th does.

madjak · Postby **madjak** » Fri Jun 12, 2015 4:42 pm

hks_kansei wrote:I've actually got a spreadsheet somewhere that I put together.

Lists the ratios for a 5 speed and a 6 speed, diff ratios, wheel rolling circumference, and engine speeds so that you can plug in a few figures and work out the RPM difference at a certain speed, or the speed difference at a certain RPM.

I worked out that a 6 speed on my car would result in it sitting 109rpm higher at 100kph (in 6th) than my current 5 speed in 5th does.

Any chance you can find it? I'd be interested in looking at it as I'm thinking of dropping a 4.7 in to get better gearing around my local tracks.

hks_kansei · Postby **hks_kansei** » Fri Jun 12, 2015 4:49 pm

Here's the formula I put together:

((((ROAD SPEED x 100000) / 60) / PI x ((WHEEL DIAMETER*25.4)+((PROFILE*(TYRE WIDTH/100)*2))) ) x DIFF RATIO) x GEAR RATIO = RPM

That lets you use the road speed to calculate the RPM in whatever gear, using whatever diff, and whatever sized wheels/tyres.

I've found the spreadsheet, I'll just need to tidy a few things up on it before posting it (I did it on my lunchbreak at work, so need to copy the info over at home since the work excel has all their details lol)

edit:
PM me your email and i'll forward it to you tonight.
Red cells are the calculations, Yellow cells are the entry cells (ie: change the yellow to reflect your car)

madjak · Postby **madjak** » Fri Jun 12, 2015 5:05 pm

hks_kansei wrote: PM me your email and i'll forward it to you tonight.
Red cells are the calculations, Yellow cells are the entry cells (ie: change the yellow to reflect your car)

Awesome... PM'd!

vscalais · Postby **vscalais** » Fri Jun 12, 2015 5:20 pm

PM from me too. THanks

hks_kansei · Postby **hks_kansei** » Fri Jun 12, 2015 5:24 pm

Both sent.

WHen I get a chance i'll tidy the sheet up so it's more user friendly than the current "I vomited numbers on a page" look it has now.

vscalais · Postby **vscalais** » Fri Jun 12, 2015 5:30 pm

Got it, thanks.

So let me understand. Assuming that you have a NB8A 5-speed with standard 4.1 diff and you could redline in 5th gear (say turbo'd).

A 3.6 diff would give you a higher top speed but take longer to get there?
A 4.1 diff would be our baseline in terms of top speed and time
A 4.5 diff would give you a lower top speed but quicker to get there?

Or have I got this all ass about?

hks_kansei · Postby **hks_kansei** » Fri Jun 12, 2015 5:41 pm

Nope, you've got it spot on.

The diff ratio basically is shown as 4.1:1 for a stock NB8A
That means the input of the diff turns 4.1 times to make the output (wheels) turn once.

A shorter ratio means that the input must spin more relative to the output, since RPM is limited this means your top speed is lower, BUT by it's nature it means that the torque is multiplied (a gear can be thought of as a never-ending lever, a shorter gear (ie: bigger number) means you have a longer lever)

This is why a 4x4 has a low range on the gearbox, it allows you to select a super short diff (well, sort of, it works differently to a diff, but for explanation assume it's the same), meaning it has more torque available to crawl up very steep inclines, but at the expense of speed.

The opposite is also true, a tall diff (say 3.6) means that there is less torque multiplication, so accelerations isn't as good, however top speed is higher since less RPM is lost in the diff.

hks_kansei · Postby **hks_kansei** » Fri Jun 12, 2015 5:43 pm

Actually, even simpler.

Think of diff ratios like gear ratios (just that you cant change them on the move like a gear)

A short ratio is 1st gear, from a standstill it accelerates well, but you can only reach about 60kph.
If you do the same in 2nd, being a taller gear, the acceleration will be much slower, but you will eventually reach 100kph

madjak · Postby **madjak** » Fri Jun 12, 2015 5:47 pm

vscalais wrote:Got it, thanks.

So let me understand. Assuming that you have a NB8A 5-speed with standard 4.1 diff and you could redline in 5th gear (say turbo'd).

A 3.6 diff would give you a higher top speed but take longer to get there?
A 4.1 diff would be our baseline in terms of top speed and time
A 4.5 diff would give you a lower top speed but quicker to get there?

Or have I got this all ass about?

yes... A 3.6 would drop your revs at cruise, but also make each gear change drop the revs further so you may be dropping out of the power ban each gear change. A 4.7 diff would mean crazy high revs at cruise but keep the gearing closer.

The higher ratio diff in theory accelerates faster however at some point the time it takes to shift gears slows you down more than you gain from the better ratios and better torque.

I'm looking at a 4.78 diff on a 6 speed solely because of the events I enter as most of them have slow corners that don't suit my current gears. Changing the rear up or down will fix those corners at the expense of others (potentially). The greater gear selection is useful because I have a powerband from 6250 - 8200. Anything below that is bad. Plus faster gear changes makes you feel like you're going faster!