Diff Size and drive train loss

[Lambolica]

Taking an RA2x as an example, the 3 obvious Diffs that are commonly used are

T-series
F-series
G-series

If one were building up a RA2x with say 120rwkw would the drive train losses change significantly between the 3 series diffs, arguably a T-series would hold up to that sort of power, obviously a G will. but if you were trying to get every poofteenth of a kw out of the package is the diff size a concideration? (excluding the weight difference)

Cheers
Simon

[kugzs]

you might not only what to consider every poofteenth of a kilowatt that you can gain by choosing a certain diff, but the confidence you will have in you driveline standing up to you (repeatedly) enjoying those kilowatts.

just a thought

[Lambolica]

All of the above mentioned diffs will hold that sort of power, if the diff is in good knick

[psychofox]

I would liken it to the lightened flywheel argument. A T series is going to have the lowest reciprocating mass, and thus will allow the engine to "rev more freely". A G series, with those axles and the size of the diff will be like putting a heavier flywheel on a car. I am facing the same dillemma, I have a G series with an LSD set up for my AE86, If I put it in it will be absolutely unbreakable behind a 4A, but I think it will feel like I put a 10Kg flywheel behind it, and increase the sprung weight considerably.

[TurboRA28]

I am not really qualified on that.. But I think for sure it would make a difference! If the engine has to drive a larger heavier diff it will take more power to do that.

Interested to hear everyones comments on this!

[Hen]

The analogy to a lightened flywheel is good. On a dyno the power output should be pretty much identical between the 3 types of diff. But the lighter diff will accelerate better due to less mass/inertia to spin up.

However the mass of the moving parts of a diff is pretty close to the axis of rotation (axles especially) so it would have a pretty low inertia for a given weight. Also just ask AE86slut about the "heavyweight" Hilux diff. He claims its only about 13kg?? heavier than a T-series.

My recommendation would be T-series if 120rwkw was all it was ever going to see and you drive reasonably, F-series if its going to get more power or serious misuse.

Hen

[TurboRA28]

The hilux diff is a lot more than 13kg heavier.. I wish I wrote it all down at the time, but from memory the T series rear end came in around the mid 50s kg. The hilux was somewhere around the mid 80s, maybe even 90s? It was around a 60% increase in unsprung weight.

[kingmick]

there is a big diffrence aswell between Open centre, lsd and locked!
best way i can put it! on our race cars that have the 9inch in them and detroit lockers and the cars only weigh 1125kg it is easy to push with no centre in with one hand, with centre in and going dead straight it takes a big effort for one person to get it moving along, turn the wheel at all and you need a few people{with max effort} to push the car .
as to the drain on HP on them i used to know that sort of thing off the top of my head, lol but its been replaced with new data! ill see if i wrote it down in the race books!

[kingmick]

so as its straight on the dyno there would be much less drain on HP than when cornering in the real world!
there would be sweet FA diffrence between the above diffs on the dyno!

[Lambolica]

So in the real world the only difference is the weights (both mass and Reciprocating?

[mrshin]

Personally I wouldn't bother with the T as I know that I'd munch it to pieces, but then maybe that's just me!

[Lambolica]

Yeah but this is more a hypothetical question. Regardless I've been putting 140rwkw through a T-Series for ages with no issues.... yet....

If the figures were more like 100rwkw in a 900kg car a T-series would be fine.

Cheers
Simon

[RA23]

The analogy to a lightened flywheel is good. On a dyno the power output should be pretty much identical between the 3 types of diff. But the lighter diff will accelerate better due to less mass/inertia to spin up.

I am not so sure...
The mass of a rotating part is irrelevant, it is the rotational inertia and the rpm (actually rotational accelleration) that count. Obviously a flywheel has a large diameter (that is the whole point) so its mass is far from the axis of rotation so it has a lot of rotational inertia. Compared to the flywheel the mass of drive shaft and axles don't matter at all, because of their small diameter. On the rpm side, the gearing ratio between the flywheel and the wheels is such that the inertia of the flywheel has much more impact on accelleration than the inertia of the diff or the axles, especially in lower gears.
I left out the maths, but they can be found in any textbook on this topic.

[celica_john]

A tad off topic but I'v heard of car gaining kw with the change of diff ratios.
If you need a heavier diff maybe get a higher ratio to compensate.

[kingmick]

yes you can gain power on a chassis dyno with gearing but your not really getting any extra HP, just getting a figure that is already there, but need the gearing to show it.