Is my Final drive ratio too low in my car?

This is a hypothetical question, I would like some good input on this.

3500 lb car 850 hp 700 tq. ported p-51's high compression 514

c/6 with a (2.72:1 low first gear set) 4.56 rear gear set. this is a 12.40:1 final drive in first gear.
14x32 tire.

For example to get the fist gear FD with a 1.76 glide I would have to have a 7.04 ring and pinion in the car is this too low or way too low?

Also consider this as having all else optimum converter etcetera.

IDT-572 wrote:
This is a hypothetical question, I would like some good input on this.

3500 lb car 850 hp 700 tq. ported p-51's high compression 514

c/6 with a (2.72:1 low first gear set) 4.56 rear gear set. this is a 12.40:1 final drive in first gear.
14x32 tire.

For example to get the fist gear FD with a 1.76 glide I would have to have a 7.04 ring and pinion in the car is this too low or way too low?

Also consider this as having all else optimum converter etcetera.

There are many things at play here.
Is this to be optimized for 1/4 mile drag racing or 1/8 mile tracks...? What RPM are you seeking at the end...?
You can really never get it tooooo low IF you can "still hook it up" solid on a greasy starting line and still have enough left at the finish.

1/8th mile 7000 rpm

If I have enough hp to run let's say 113 mph (32 inch tire) 1/8th mile with a tight converter @ 7000 rpm then it would take about a 5.88 gear , ( give or take )


c/6 2.72 x 5.88 = 15.99 Fd in first gear.
Glide 1.76 x 9.08 = 15.99 FD
I think there comes a point where you don't have enough resistance to load the engine on the leave.

The point I am trying to make is, the c/6 low first gear set is not nessasary in most 3000-3500 lb BBF aplications.

My car at 3000 lb with a glide and 4.30 gear would be like a low set c/6 with a 2.78 gear............ it has went a 1.26 60 ft.

A low gear set in a C6 is hurting you terribly.may as well take off in high. You are in second before 60'. In most cases a low gear set in a pulling truck is even too low.BBF needs to use its torque and a higher gear set imo.

I've been pondering over this...... Hummmm....

Blake,

I'd say that you final drive ratio is too steep. Next question; since when did your car weigh 3500 lbs. Les?

Charlie

"This is a hypothetical question, I would like some good input on this"

c.evans wrote:Blake,

I'd say that you final drive ratio is too steep. Next question; since when did your car weigh 3500 lbs. Les?

Charlie

Big Chuck is driving it now ................

put more nitro parrafins in it and drop the shift point

Methanol Peak Torque 786 ft.lbs.
Nitro Percentage 15%

Calculated Results
Approximate Torque on 15% Nitro 857 ft.

As mentioned by RM, you will have to decide on your 1/8th rpm, and work backwards.......and run the gear that suits that.......IMO i'd be more concerned with the stall, and how it works for multiplication.

I also went a 1.26 60 ft at 3000 + lbs last year, but with only a 8.10 (3.27 gear x 2.48 1rst gear)

What I would like to come from this, is a good understandig of where you need to gear a basic 3000-3500 lb BBF with a 3 speed trans.

If you gear it to run through the traps at the shift point and have a low gear set tranny , your first gear final drive ratio is completly out of hand.

I would run this deal with about a 3.73 gear and not use 3rd gear. And you still would have trouble hooking it.

That would be like having a 1.76 glide with a 5.76 gear.

feetfirst wrote:As mentioned by RM, you will have to decide on your 1/8th rpm, and work backwards.......and run the gear that suits that.......IMO i'd be more concerned with the stall, and how it works for multiplication.

I also went a 1.26 60 ft at 3000 + lbs last year, but with only a 8.10 (3.27 gear x 2.48 1rst gear)

With that set up you probably wouldn't get into high gear at the stripe. Thats my point I am trying to make.

Now gear your car to cross the finish line in high gear at your shift point and tell me what it runs.

At 115 miles per hour, given a 32 inch diameter tire would put a 5.67 gear at about 7,100 RPM - a 5.43 gear about 6,850 RPM.

rmcomprandy wrote:At 115 miles per hour, given a 32 inch diameter tire would put a 5.67 gear at about 7,100 RPM - a 5.43 gear about 6,850 RPM.

That would give you a 15.42 first gear FD. Like a 1.76 glide car with a 8.76 rear gear. I think that would be a tad to low.

How long would a C6 last if you started in 2nd gear.. I use to start in 2nd on my bike.

lots of variables, but the low gearset is like a granny gear in a truck..............not very much distance before the rpm is maxed out...................if you get the drift.

IDT-572 wrote:

c.evans wrote:Blake,

I'd say that you final drive ratio is too steep. Next question; since when did your car weigh 3500 lbs. Les?

Charlie

Big Chuck is driving it now ................

How did I get in this mess!! Blake don't make me send Albert down there!!!!!! LOL

When I had Charlie's 533 in my car I accidentally did not pull it into low gear and launched off of the pedal no transbrake with 4:30 gears and 33 tall tires and powerglide . It had been running 5.26 -5.27 , it ran 5.37 in high all the way down the track.

If you launch with a super low gear with a 3 speed and the engine goes straight to its peak rpm the entire gear change is nothing but a waste. The time it takes it to change into second gear is almost a complete waste.

You want the engine in its highest average horsepower rpm's all the way down the track not torque rpm's .

OK so put a 3:00-3.50 in the car and use 1st and 2nd in the 1/8 and 3rd for the 1/4 mile?????

dfree383 wrote:OK so put a 3:00-3.50 in the car and use 1st and 2nd in the 1/8 and 3rd for the 1/4 mile?????

That's what I would do............

A good example is Coon's car ( I don't think he would mind me talking about it). He wants to drive it on the street some. But not lose anything at the track. It's going to have a c/4 (it's light and a 466 cid) with a stock first gear set and a 3.25 or 3.55 rear gear with 28x10.5 tires. Run it like a glide on the track and have third for the highway.

IDT-572 wrote:

dfree383 wrote:OK so put a 3:00-3.50 in the car and use 1st and 2nd in the 1/8 and 3rd for the 1/4 mile?????

That's what I would do............

A good example is Coon's car ( I don't think he would mind me talking about it). He wants to drive it on the street some. But not lose anything at the track. It's going to have a c/4 (it's light and a 466 cid) with a stock first gear set and a 3.25 or 3.55 rear gear with 28x10.5 tires. Run it like a glide on the track and have third for the highway.

That will work, but there will be some left on the table as running in 2nd vs the third gear 1-1 will eat more power up. But if its double duty then this is ok. And KY explained my "granny gear" concept well.

I race with a guy with a BBC Slowmaro(1/8 mile only). He has 4.88 rear gear and a 29.5 tire. He runs 5.0's at 3450lbs with about a 350 shot. He use to run a T400 with 4.11 rear gears and had the car setup for it and ran 5.20's with it. He switched to 1.40 gearset turboglide and had his converter tweaked a little and of course the gear change. From one weekend to the next he went from 5.20's to 5.0's with the the above changes. His 60ft went from 1.22 to 1.18 with the switch.

Blake, we have talked about this some in the past , and I am glad you started this thread.! I want to know more about this too.


Here is a example of a car that uses all three gears but is not a big block ! just a thinking example!


377 Clevland with c-4 3500 stall 9in rear with 3.50 3000lbs 26in tall tires 3000lbs has gone as fast as 6.50 101mph on motor in good air with 1.40 60ft shifting at 6500 and at the most 6900 .

So why can't you run a c-6 behind a big block and use 1st gear and do the rest of the traction issues with rear gear chang,tire size and suspension work?

its a torque thing