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Differential Gear Ratios Clarification


HeelToeNick
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So I posted up in a build thread and I think I may be wrong about my claim but I wanted to see how open this new ratio rule really is. 

 

In NA miatas, you get 2 different sizes of differentials and 4 different mechanical variants. 

 

NA6 has a 4.3 VLSD and 4.3 Open, 6 inch pumpkin.

NA8 has 4.1 Open, 4.1 Torsen I and 4.1 Torsen II, 7 inch pumpkin.

 

Under the new Diff ratio rule, many people are claiming the 4.3 torsen is free now because a car of the same generation came with a 4.3. However I think this is a bit of a gray area. I think the intent of the rule is to allow builders to simply bolt on differentials from different optioned vehicles and keep swaps simple. So if your 323i came with a 3.73 open you but the 325is came with a 4.1 LSD then you just swap it. I believe this assumes that the assembly is the same and strictly internals are different.

 

On a miata, the the NA6 and NA8 have completely different assemblies, driveshafts and axles required for the swap. PLUS a 4.3 torsen was only offered on 99+ cars which are a different generation. 

 

Are we saying that this ratio rule is so liberal that a diff from another generation can be swapped in because it "fits the mold?" I would think the intent of this rule means if you wanted a 4.3 ratio diff from an NA6 you would have to run the 6 inch pumpkin, longer drive shaft and axles to fit the diff. 

 

If the rule is this open than so be it but I could see this opening up issues on other platforms as well. 

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The rule intent was to make the part offered for any model of a car generation open for use. That would be all NA miatas. 

 

I think the explanation of what year ranges are considered a "generation" could stand alot of work. Maybe something graphical. This was also tied into the swap debate, as "generations" had to be determined so the same data was used for year ranges that should effectively be the same car. 

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49 minutes ago, Huggy said:

Read Petition 14, and write the BOD with your opinion on the matter.

 

Interesting petition. Similar to my stance but also different. I will have to digest this a bit and send in my opinion on the matter. Thanks for pointing me that way. 

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51 minutes ago, Black Magic said:

The rule intent was to make the part offered for any model of a car generation open for use. That would be all NA miatas. 

 

I think the explanation of what year ranges are considered a "generation" could stand alot of work. Maybe something graphical. This was also tied into the swap debate, as "generations" had to be determined so the same data was used for year ranges that should effectively be the same car. 

I agree that works for the differential and final drive gears, but your driveshaft and axles would still have to match the year of car you are claiming, correct?

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Back when I started in autocross, updates and backdates within a chassis generation were allowed with no points. One could pick a vehicle that had been produced for a number of years and cherry-pick all the good stuff to build a car that had all factory parts but hadn't been available as that combination.

 

Biggest fuel tank, lightest chassis, most hp, best transmission and/or diff, best suspension, hubs, rad, brakes, etc. For Champ, that list includes using the heaviest curb weight for swap calculation.

 

More freebies for the high VPi cars with no benefit to the low VPi cars that already have surplus points and nothing to use them on.

 

 

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1 hour ago, ABR-Glen said:

I agree that works for the differential and final drive gears, but your driveshaft and axles would still have to match the year of car you are claiming, correct?

 

Don't know. No idea if tech would include that as a common generation or not.

 

In a practical sense the 1.6 and 1.8 cars are only 50 points apart. I would think given the advantages of the 1.8 motor and the added fuel capacity would make a 1.6 car 50 points slower in speed. 

 

To run up front (at least in the east at reasonably open tracks) I would assume both 1.6 and 1.8  cars would need to be swapped to do it. The swap vpi would be of more interest to me.... which is the same for 1.6 and 1.8 NA

Edited by Black Magic
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1 hour ago, ABR-Glen said:

I agree that works for the differential and final drive gears, but your driveshaft and axles would still have to match the year of car you are claiming, correct?

 

Correct, there is no rule saying you can use axles and drive shaft from a year other than what you are claiming the car is without taking points for them. Unless you built the axles and drive shaft out of re-purposed flipped trunk lids........then they are free. 

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Quote

 

4.7. OTHER EQUIPMENT THAT DOES NOT COUNT TOWARD THE TOTAL POINT VALUE

 

4.7.8. Rear-end gear-ratios are open to all OEM differentials available for that year, make, and model or chassis generaton (excluding specialty high-performance models not listed on the VPI table).

 

 

Here is what I take from a pure reading comprehension level.

 

Rear-end gear-ratios   (Not just rear ends or gears, but the entirety of these words is the subject)

Are open    (Are zero points)

to all OEM differentials (Meaning the "rear-end gear-ratios" of OEM differentials, but again not the differentials themselves)

for that year, make and model (Condition of the rear-end gear-ratios, of the OEM differentials)

OR (Meaning one or the other)

chassis generation (Second condition of the rear-end gear-ratios, of the OEM differentials)

excluding specialty high-performance models not listed on the VPI table.  (The lack of an OR condition requires an AND condition, which is the exclusion from the OEM differential subset)

 

As such, the rule is defining the ratios that can be used, not the gears or differentials themselves.  

 

As a stand alone the rule means nothing, because a ratio is just a number however this entire rule specifically applies to ratios of the rear end since that is the subject to which the gear ratio is referring to.   

 

The rule only seems odd when applying it to specific instances, like the Miata where there are two sizes of differential within the same generation.  I assumed the rule was written for some of the pony cars where its common to swap rear end ratios.  None the less, that fact does not negate the rule as it relates to cars with different differential sizes, so long as the other rules are met.

Edited by LuckyKid
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26 minutes ago, Wittenauer Racing said:

So reading it at the "reading comprehension level" you can do aftermarket gears at a factory ratio as well? Mmmm, straight cut gear whine :) 

It seems so.  Do they make straight ring and pinions?

Edited by LuckyKid
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Is that what you rule mongers really want......A specific use of words that will stand up to an arguable legal conclusion that you think less contestable...but nothing really is....or simple common sense interpretations  that  the great majority get ...even you...as the intended message......this is why concerning rules ...less can be more....Now Daytona tomorrow !

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54 minutes ago, Wittenauer Racing said:

So reading it at the "comprehension level" you can do aftermarket gears at a factory ratio as well? Mmmm, straight cut gear whine :) 

 

we've been wanting to run straight cut gears in our trans.  we figured it would set the forums on fire the first run down pit lane.

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24 minutes ago, MoparBoyy said:

 

we've been wanting to run straight cut gears in our trans.  we figured it would set the forums on fire the first run down pit lane.

It will, and tech is waiting for it.  You might end up with the first example of tech deciding this isn't right for the series.  All hypothetical of course....

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3 minutes ago, hotchkis23 said:

It will, and tech is waiting for it.  You might end up with the first example of tech deciding this isn't right for the series.  All hypothetical of course....

 

but if its stock ratios.. its just fancy internals :)

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Just now, MoparBoyy said:

 

but if its stock ratios.. its just fancy internals :)

Oh I'm not choosing sides here, just saying what I have heard through the grapevines.  Kinda like if an aftermarket control arm is still the same "physical dimensions" but allows for more adjustment is no longer "stock"

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10 minutes ago, hotchkis23 said:

Oh I'm not choosing sides here, just saying what I have heard through the grapevines.  Kinda like if an aftermarket control arm is still the same "physical dimensions" but allows for more adjustment is no longer "stock"

 

well if they are allowing aftermarket gears in diffs.. aftermarket gears in trans;s should be allowed ;)

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10 minutes ago, Snake said:

Ratios are open, diffs are not.   It's pretty simple. You wanna change ratios, fine.  0 points.   You wanna upgrade your diff while changing your ratio, 25 points.   

 

 

Time will tell and hope your right.

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