Engineer/math minds HELP!!

[BlownV6]

I need a little help with a math type problem and hope somebody can help me (brain storm) this problem. My dragster currently uses a 1.80 ratio low gear in the powerglide and a 3.82 rear gear. If the car is on a terror type run it shifts into high gear at 1.5 seconds and that is right at 8000 rpm. Im looking to change the low gear planatery in trans to a 1.69 ( I would like to have a 1.55 to 1.60 but it doesnt look like that ratio is ever going to be made).

The goal here is to stretch out the distance traveled in low gear before the shift. Im thinking that it currently is about 200' out when it shifts. I would like to get to around the 330' mark!! BTW tire rollout is about 108"

Question of the day is , how much further (basically) will the car go with a 1.69 vs the 1.80 low gear still shifting at 8000 rpm???? Mike

 

[Speed_Dreamin']

I'll try...
My math assumes 200 engine revolutions (8000 rpm for 1.5 sec). You're actual engine rpm is less at 1.5 sec (unless you launch at 8k).

Trans gear = 1.80, distance = 262 ft
Trans gear = 1.69, distance = 281 ft
Trans gear = 1.60, distance = 300 ft
Trans gear = 1.55, distance = 312 ft

However, with each gear change, time to get to 8k engine rpm is going to change, changing the number of engine revolutions, changing the total distance traveled.

 

[Phoneguy]

I found this for you. Doesn't figure the footage but shows the increased MPH. HTH

Gear Ratio Calculator


Bryan

 

[BlownV6]

I'll try...
My math assumes 200 engine revolutions (8000 rpm for 1.5 sec). You're actual engine rpm is less at 1.5 sec (unless you launch at 8k).

Trans gear = 1.80, distance = 262 ft
Trans gear = 1.69, distance = 281 ft
Trans gear = 1.60, distance = 300 ft
Trans gear = 1.55, distance = 312 ft

However, with each gear change, time to get to 8k engine rpm is going to change, changing the number of engine revolutions, changing the total distance traveled.

On a couple of passes where it was on a terror the rpm at .5 seconds into the launch was 7000. On this kind of pass it runs .95 to .96 60' and 2.70 to the 330'. Thats where Im making the guess that its shifting at around 200'. The converter is slipping a fairly high amout at this point in the run and that is throwing these numbers for a loop!! Mike

 

[Alky V6]

Don't forget to figure in tire growth. The tire growth factor is going to vary with car speed.

 

[BlownV6]

Don't forget to figure in tire growth. The tire growth factor is going to vary with car speed.

This is all out of my league , thats why I posted it here for the brain trust to figure out!!! Mike

 

[Phoneguy]

Out of my league as well. Might want to get in touch with RC

Bryan

 

[Alky V6]

I'll start plugging the numbers into my drag sim if you're willing to give me some other numbers I may need.

What is the size on the tires shown on the sidewall? Make?
What is your approximate HP level?

 

[Alky V6]

I still have a file on your car saved.
What is the stall of your TC?

 

[Alky V6]

Weight still 1581?

 

[bison]

Is there any taller tires available? If you could pull about 10% out of the ring and pinion you should be close but you will work the converter a lot harder. What is the approx estimated tire height at 200-300'? Rollout calculated on a large slick isnt accurate for determining the circumference as the tire grows.

 

[Alky V6]

Drag slicks can vary in tire growth from 15% to 30%. Can you find out if your model tire is a low, medium or high growth tire? Maybe the manufacturer can even give you a rough tire growth number.

 

[Alky V6]

If you don't have a HP figure, give me the all the numbers off a good timeslip and I'll match to that to find a hp number.

 

[Speed_Dreamin']

I guess I'm bored....
With all of the variables involved, a simulation software is likely the way to go. But I do think your estimate of 200 ft at 1.5 sec is high. I plotted the your 60ft and 330ft times you gave, looks like you may be closer to ~125 ft at 1.5 sec.

 

[Hot Air]

Also consider that the car will not leave as hard with the new gear so 60' time should increase.
Conrad

 

[WH1_T-Type]

1+1=3









Wait..................crap.

 

[Blazer406]

Mike,

Sorry if you already mentioned this......but do you log the driveshaft RPM's? Obviously that would take the converter efficiency out of the mix if you knew the trans ratio.... the engine RPM's and driveshaft RPM's at each datapoint..... Only tire growth would remain... and it should be easier to make an educated guess on than the converter slip.... IMHO....

 

[Dusty Bradford]

It gets more interesting. The converter will flash slightly higher as well from the extra load the 1.69 gear will put on the converter.

 

[Alky V6]

Using 20% tire growth (med), and matching your 60' and 330' numbers, I get a 1-2 shift at the 192' mark at 8000 rpm assuming a 6640 rpm TC stall speed. This is with your current gearset.
What's your TC slip number on the top end, if you're logging it?
RPM drop on the shift?
Launching rpm?
RPM across the finish line?

 

[BlownV6]

Donnie , I cant give you some of the info you want because I dont have any 1/4 mile runs that are any good.

These pics are the Racpak data from the best run ever. The car had the front end up about 18-24" all the way thru low gear and was drifting out of the groove to the left so I had to lift and redirect it. This info is at .5 into the run and at 1.705 right as I lifted. It was also shifting at this exact moment as I had the shift timer (this was misleading) set at 1.5 seconds. The actual shift occurs about .2 of a second later than the timer set point. I run Goodyear 17 x 33.5 x 16 tires and I believe it weighs about 1725 with me in it. Thanks for all the response guys!! Hope somebody can figure this out!!Mike