Diff Ratio
 

Anybody out there know if any aftermarket companies have played around with changing the diff ratio on the 8?

Hi,

not really aftermarket, but my dealership did change in their racing cars from 4.44 to 5.2! I was offered this mod as well as the drilled rotors.
In the spring I'll install the RB flywheel with this shortened ratio rear diff. Will be a killer combo I think :D

sorry to ask a probably stupid question, but in what way does this help? Are there any disadvantages?
changing the diff ratio I mean... I heard it before, some one told me he did so to his miata, but I have no idea...

It changes your gearing and if done correctly, will increase your 0-100kph and 1/4 mile time but the trade off may be a lower top speed.

Another side effect is that at crusing speeds you'll be at a higher rpm. How bad do you like what little gas mileage you have now?

I'll trade fuel economy for increased performance anyday RG. :)
My old Nissan S14 200SX (Silvia) gave me 25-28L per 100km, do the math, you'll be shocked at the MPG figure. ;)

You used to be able to find a 5.55 ratio for the RX-7's. They would even adapt from the solid rear axle old models to the newer independent suspension ones. I wonder if they an be made to fit the RX-8? If you were to take these to the dragstrip, you'd find that your E.T. would get faster but your MPH would be slower.

feed has a set of 4.7 dif gears, they've use for the ms rx8 in the numerous best motoring events. It actually didn't really help acceleration much the ms rx8 clocked in 15.XX the stock rx8 clock 14.8x. (from best motoring dvd factory fighters). Only one run though, I think 5.2 would make more of a difference, but that would mean cruising speeds on highway at 70mph would probably be over 4500 rpms rpms :eek:.

The mph would not necessarily be lower if there was still enough headroom, rpm-wise. In fact, the mph might actually get higher if the engine were to pull to a higher rpm(more than enough to offset the ratio change) just before the traps. From a 4.44 to a 5.2 is close to a 20% change and if the engine found itself nearing the h.p. peak quicker, both e.t. and mph may be helped.

Charles

I imagine the mileage would not suffer much. Maybe 1 mile per gallon. If we ever find a good way to tackle the A/F issue, the mileage would still be better than stock if one did this upgrade.

Honestly, we need a 7th gear. That would dramatically increase mileage on the highway. That is a whole issue in itself, so I will stop.

I think the diff. ratio would be a good mod.

Thanks Nojooc, RG and the others... this is really interesting.
Actually, I remember now, that miata owner timed himself with a G-tech, both stock and with different diff ratio and he saw an improvement in performance. Maybe I can get the info here and you guys can say your comments...

As for top speed, I think its not a dramatic side effect. The car goes easily to 150mph, so, maybe lets say 140mph, is still good. I mean how many times does any of us go that fast?

Very true.... I am after an improvement in acceleration in normal real world driving, not track or quarter mile.

One thing I have been asking fellow 8 owners is what we would do with more torque to the rear wheels when we can't get the stock rear tires to hook up as it is. Unless we are talking about using R tires or drag radials, I can't see the benefits of higher rear gear ratios. I recently did some A/B testing of launch rpm's and found that I pulled a best 60' average of 2.334 when using 5K rpms. I averaged 2.38 when using higher rpms, like 7k or so. Of course, the lower launch speed leads to a less dramatic (and less abusive) launch which won't lend much to the showoff factor, but it hooks the car as well as can be expected. I am still contemplating a switch to something like a 3.9-4.0:1 gear set.

Charles

If you don't mind an off speedo, you can also fiddle with your gear ratios alot cheaper by adjusting your tire aspect.

Going to a smaller wheel (like a 17) with a lower profile will have a similar effect (though not as large) as adjusting the actual rear diffy.

Ex: Dropping from stock 18's to 235/40/17's nets you 6% overall change.

Alot of the mazda6 owners do this to improve the gearing of the 6i MTX's with low profile 16's.

Combined with performance rubber, and lighter wheels, it has a drastic effect on the overall acceleration of the vehicle. Of course I'm sure alot of people would argue the car doesn't look as aesthetically pleasing with smaller wheels, but those are the people who are usually behind your car anyway.

I used to own a mustang and I know changing the diff gear ratio's in that made a HUGE difference. We had a set of 4.10 gears, which is a little more on the extreme side, but it really helped acceleration!

Because not all acceleration takes place from a dead stop in a drag strip style launch? I can think of a lot of things that more torque would be nice for whether launch issues were addressed or not.

Yeah, you're right. Road racing is different, but I usually come around corners in second gear and I would still say that I have no problem with torque around corners. Leaving the dealership this afternoon I put on a bit of a drifting display through first and second gears. Sideways at 50 mph in an RX-8 is something to see!

So what would be the other things that more torque would be nice for?

Charles

About the stock tires not being able to hook up, I think it has to do with the bridgestones. Ok, they are not crap, but I think there are better tires in the market, with which the 8 handles and takes off much better... or maybe wider rear tires can help with this issue too...

More torque will definately help with acceleration at some speeds, like from 50mph to 70mph. I think this is the rx8's weak point.

Wheelspin isn't really an issue once you get going in a straight line. I wouldn't mind have better acceleration in every gear when rolling on full throttle...

You know what would be best? A SHORTER diff ratio and a TALLER sixth gear. Then we could have better acceleration through 1-5 and still have a freeway cruising gear in sixth. I know people have swapped gears out in other trannies, anyone hear of someone doing it in an RX-8?

~ Matt

Usually when discussing torque values we are discussing low rpm torque as mid and high rpm torque is nowhere near as difficult to increase. In fact, when raising h.p. figures we are actually raising the torque output measured at the mid and high rpm range. That might be where I am tending to misunderstand the discussion. To increase the torque level in the 3-4K range on up, I would agree has huge gains in useable performance as it is not out of the question to see 3-4K on a regular basis if I drive aggressively. In that case, all of you are correct on the matter.

Charles

I know it's been done on cars like the RSX-S, gains of like 9mpg highway. They are lucky however, since the TSX 6th fits, same engine series and all. I think this would be a great mod for highway cruising.

just something to think about....

if you made the diff actually taller you would improve highway cruising and as a side effect you would be finishing the 1/4 mile at the top of 3rd gear rather then having to shift into 4th right before you hit the stripe

I don't actually care about 1/4 mile time. What I care about is how the car feels on the road when I'm going through the gears or how the car puts down the power when I'm at the track. The 1/4 mile time, along with the 0-60 time is a nice easy way to compare cars or compare mods, but since I don't imagine I'll ever go to a drag strip, I'm not very worried about it.

~ Matt

So I'm taking it that no one here has attempted this mod?

Anybody willing to experiment and get back to us?