Centrifugal, MT82 & Axle Ratios

[blakeman8192]

Alright boys, I've got a centrifugal supercharger, MT82, with the 3.73 rear axle ratio. With my wheel and tire setup, redline for these gears is at:

1st: 52mph
2nd: 79mph
3rd: 117mph
4th: 167mph

And I've got an issue when it comes to highway pulls in Mexico starting from 60-70mph. If I downshift into 3rd, I'm at 4500RPM and not making enough boost to pull away off the jump. If I downshift into 2nd, it pulls like a freight train but I've got 10-20mph (like 1-2 seconds) before I need to shift and lose some momentum.

I'm thinking about changing my rear axle ratio to help with this, but I can't decide if I should go with 3.15 and shift into 2nd for highway pulls, or 4.11 and shift into 3rd.

Redline speeds with the 3.15 would be:

1st: 61mph (nice for 0-60s)
2nd: 94mph (good for highway pulls from 60)
3rd: 139mph (a nice long pull at high RPM/high boost)
4th: 198mph (lol)

And redline speeds with the 4.11 would be:

1st: 47mph (probably unusable at full throttle?)
2nd: 72.05mph (probably sick pulls for on-ramps/getting up to highway speeds)
3rd: 107mph (a bit longer of a pull before needing to shift vs 2nd gear with the 3.15)
4th: 152mph

The 3.15 seems a little better on paper, and I think has a small torque multiplication advantage in 2nd vs the 4.11 in 3rd. But the 4.11 in 3rd has a longer pull before needing to shift, and also would get going faster from a dig. Which one would you go with, and why?

Sponsored

 

[daSNAK3]

Subscribing.. I've been wondering if it would be worth swapping out my 3.55s for Meheeco highway pulls.

 

[robvas]

You will not like how long the gears are with 3.15

 

[Angrey]

Do you want to optimize dragy pulls and highway rips (60-130 and 60-150) or do you want to optimize 1/4 traps?

It seems like with either option you have a good 2 shift setup (60-130) either 2-3 with the 3.15 or 3-4 with the 4.10s.

The secondary factors are cruising RPM (much MUCH better with the 3.15's and much much worse with the 4.10s) and also torque/traction.

The 3.15's would be much kinder on your ears, your fuel economy and your sanity for long cruises.

The 4.11 would be torquier but basically make the first gear useless and the top 2 gears useless for anything other than cruising.

If you set aside the fuel economy/cruising concern, the answer would be to pick the one with more torque multiplication up to the point of traction.

You'll probably find that with the 4.11's that you're making too much sauce and you either have to feather the throttle in most conditions to keep from spinning which in the end makes you slower. You can always achieve faster times with mashing full effort if you have the grip than trying to find the edge of traction.

The last factor I'd say is cost. I have 409s in my car. I was one of the first to do it in the GT350. You'll probably be able to find pumpkins available in 3.15 already assembled. A 4.09/4.10/4.11 pumpkin in the Super 8.8 is pretty unicornish and most likely you'll have to buy a ring/pinion set and have a shop do the swap, which is pretty involved and expensive (if done properly by a shop who's done it before).

 

[CrackedHorn]

What tire are you running to get to 167mph in 4th with 3.73?!?!?

You should be around 115mph.

A 3.31 gear tops out at like 128mph.

 

[SheepDog]

Why is a 3.31 not on the list?

 

[blakeman8192]

What tire are you running to get to 167mph in 4th with 3.73?!?!?

You should be around 115mph.

285/35/19

I've go the MT82-D4

 

[blakeman8192]

Why is a 3.31 not on the list?

I just feel like it wouldn't make a big enough difference over the 3.73. I'll calculate the top speeds per gear with that ratio and see what it looks like.

 

[CrackedHorn]

285/35/19

I've go the MT82-D4

I always forget the drastic gear ratio difference of the 18+ cars.

 

[SheepDog]

What tire are you running to get to 167mph in 4th with 3.73?!?!?

You should be around 115mph.

A 3.31 gear tops out at like 128mph.

 

[SheepDog]

I just feel like it wouldn't make a big enough difference over the 3.73. I'll calculate the top speeds per gear with that ratio and see what it looks like.

It will if you also drop down to a 105mm pulley

 

[blakeman8192]

It will if you also drop down to a 105mm pulley

No E85 within 100 miles and I want this to be a street car
I could risk it for the biscuit with my meth kit though...

 

[SheepDog]

No E85 within 100 miles and I want this to be a street car
I could risk it for the biscuit with my meth kit though...

oh, well obviously the correct answer is- pack your stuff tonight and move to an ethanol friendly State

 

[Unas2k5]

I did a 60 pull today. I have 3.55 MT82 Stage 3 im at 4200RPM in 3rd gear at 60 and that’s well into the powerband p1X. How high do you want to be ?

 

[blakeman8192]

I did a 60 pull today. I have 3.55 MT82 Stage 3 im at 4200RPM in 3rd gear at 60 and that’s well into the powerband p1X. How high do you want to be ?

I'm only making like 4-5 PSI at 4500 RPM. Maybe I should run a tiny pulley with a wastegate instead. Probably cheaper than a pumpkin anyway.

Sponsored