Starting Line Ratio

[BBR]

CDMBill has mentioned this before and I did not know exactly how to calculate it or even what a ball park number would be. Well, calculating it is easy, (1st gear ratio x rear end ratio), but what is a good ball park number to shoot for? I have read that the 9-10 range might be about right, but idk.

C6 1st gear = 2.46
Diff ratio = 3.55
SLR = 8.73

Thinking of going to a 3.90 gear.

1st = 2.46
Diff = 3.90
SLR = 9.59

My uneducated brain says the 3.90's are moving in the right direction, as the 8.73 might be on the low side.

Any thoughts?

[TheSilverBuick]

Does tire come into play?

[BBR]

o noz another variable.

[TheSilverBuick]

I too have always heard 10 is a good number, but am unsure if tire diameter was a variable in the equation. 25"-32" is a heck of a range.

To play along for comparison.

My Firebird with 27" tire, though if I use the drag radial's it'd be 26"
1st = 2.74
Diff = 4.56
SLR = 12.49

My Skylark with a 26" tire.
1st = 2.87
Diff = 3.89
SLR = 11.16
^^ The 3.89 gears makes me cross the finishline right at my redline. Or at least if I make the mph I expect.

[BlueCuda340]

26" Tire
1st 2.45
rear 3.91
SLR=9.57

I then tried this last weekend
28" tire
1st 2.45
rear 4.57
SLR=11.19
I saw fractional if any improvement across the whole time slip(my car 60s slow so I tried this). The 28" tire takes some of the SLR away compared to the other combo but it was for sure lower. She was screaming through the traps.

[stormts71]

Is there a way to add the tire size to this ? Was thinking of going from 3.89's to 3.70 ,hits the shift lite about 1000-1100ft . If i find more power it will keep going up .
28"tire
2.471st
3.89rear
SLR=9.61

28"
2.47
3.70
SLR=9.14
not a big diff but
What kind of 60 ft. times are you talkin ? Had a 1.54 best so far .

[MR P-BODY]

o noz another variable.

Then you would have to figure in auto vs stick... a
conv have a 2X multiplier at the starting line... I dont
play the SLR... I set the conv to about 200rpm above
peak torque rpm and set the gear/tire so I cross the line
at 10% above peak HP rpm... the SLR is what it is

[BlueCuda340]

Yeah, a good converter will get past a kinda crappy SLR. I proved it a little bit with my gear swap that didn't work. I am going to try going the other direction now and see what it does. Trying a 3.23 vs 3.91. Not expecting anything really but if it works i can cruise at 75 lol.

[MR P-BODY]

Yeah, a good converter will get past a kinda crappy SLR. I proved it a little bit with my gear swap that didn't work. I am going to try going the other direction now and see what it does. Trying a 3.23 vs 3.91. Not expecting anything really but if it works i can cruise at 75 lol.

You just need to stick some kind of OD in it... thats
why I went with the 518 trans... 727 with OD.. but I
changed the input and pump so I could use a high
stall conv and eliminate the lock up and yet keep the
steeper gear in back.. in my case its a 4.10... I can still
cruise at 75 mph at 2900 rpm

[gray86hatch]

On my car slr makes a big difference.

With 373 it will barely get up on the tire 4.10 it goes 100' on the rear tires. The mph is about the same but the et drops big time. Even 3.90 slowed it down.

The converter needs to be right with any gear. Within 800-1000 of shift point. Thru the traps 2-400 more than that.

I was surprised it made that big of a difference. Almost .2 was what I found.

Tim

[stormts71]

I have over drive 200 4R drives are not a problem . It is doing what all the speed cal.. say it should just trying to get those little bits . TC is only about 3000 for the drives and it has 440ftlbs at 2800 and doesn't peek till 4800 . puts it at 5000 stall to much for the street , and no lock up .

[MR P-BODY]

I have over drive 200 4R drives are not a problem . It is doing what all the speed cal.. say it should just trying to get those little bits . TC is only about 3000 for the drives and it has 440ftlbs at 2800 and doesn't peek till 4800 . puts it at 5000 stall to much for the street , and no lock up .

I'm running a 5000 stall in my street ride... I feel it drives nicely... remember that
its only 5000 when you stomp on it... if you just roll in a small amount of pedal
its much less stall... but with the big stall you have to run a cooler

[JeffMcKC]

RPM is King, use as muck low gear as you can hook, a 4000 lbs boat Is harder to move,,,,,,,,,,,, ( sorry Bill) so you could get by with more in a 2000 lbs Mustang

[JeffMcKC]

<-------- 6800 Stall pulling a trailer radiator cooler only

[Chad's Dad]

My Camaro has 5.57 rear gears and a 2.88 first gear. I launch from 6500 to 7500 rpms depending on the hook. And it is really fn cool!