How do gears affect 1/4 mile MPH?
Thread Starter
Race Director
Joined: May 2001
Posts: 14,165
Likes: 9
How do gears affect 1/4 mile MPH?
A friend of mine has a project car and he just ditched his 2.73's in favor of 3.73's. He was running 84 mph in the quarter before which was about 3500 rpm's in 3rd gear. Now with the 3.73's that puts 84 mph right at 5500 RPM's which is where he's shifting.
My question is will the gears raise his MPH in the quarter? If so then he'll actually have to shift into fourth, otherwise he'll be in the same boat I'm in. In my case it makes no difference if I go for 4th or leave it in 3rd, I run the same time either way.
In any case it's got to be a vast improvement. I was just wondering what the typical MPH impact of gears is.
Thanks.
My question is will the gears raise his MPH in the quarter? If so then he'll actually have to shift into fourth, otherwise he'll be in the same boat I'm in. In my case it makes no difference if I go for 4th or leave it in 3rd, I run the same time either way.
In any case it's got to be a vast improvement. I was just wondering what the typical MPH impact of gears is.
Thanks.
Le Mans Master
Joined: Apr 2000
Posts: 6,294
Likes: 0
From: Amateur driver on open course.
Re: How do gears affect 1/4 mile MPH? (Nathan Plemons)
I went from 2:59's to 3:54's. I just looked at my slips. I went from 13.7 @101 mph to 13.30 @ 103. Picked up two mph. As for shifting, I too was shifting just before the line with my auto. I Hypertech'd 2 mph into the top of third and was okay after that.
Instructor
Joined: Feb 2003
Posts: 221
Likes: 0
From: T.Dot ON
Re: How do gears affect 1/4 mile MPH? (Nathan Plemons)
for an auto i cant say. As for a stock LT1 with a six speed, the 4.09s will probably make him shift to 5th about 20 feet from the lights which kills the mph. Either way though the mph will increase. Its not only the final gear which gives mph, each gear is shortened giving more acceleration in Each.
I always compare it to a bicycle, say a 21 speed. Now imagine starting the 1/4 in a gear you would normally use for going very fast or downhill. Imagine the guy next to you starting in a gear used to go uphill. The guy beside you will definitely accelerate way faster than you and be quite a ways ahead, and finish first too.
:cheers:
I always compare it to a bicycle, say a 21 speed. Now imagine starting the 1/4 in a gear you would normally use for going very fast or downhill. Imagine the guy next to you starting in a gear used to go uphill. The guy beside you will definitely accelerate way faster than you and be quite a ways ahead, and finish first too.
:cheers:
Race Director
Joined: Dec 2002
Posts: 13,348
Likes: 233
From: SCMR Rat Pack'r Charter Member..Great Bend KS
Re: How do gears affect 1/4 mile MPH? (Nathan Plemons)
It generally accepted that gearing affects MPH very little. It WILL help your ET's, which is what wins races. Trap speed is determined primarily by HP and vehicle weight. Want proof?
Start out in second gear. Your ET's will be way off, but your trap speed will show very little change.
also, 95aquaC4's numbers are a good illustration.
Have fun!!
Larry
code5coupe
:seeya
Start out in second gear. Your ET's will be way off, but your trap speed will show very little change.
also, 95aquaC4's numbers are a good illustration.
Have fun!!
Larry
code5coupe
:seeya
Thread Starter
Race Director
Joined: May 2001
Posts: 14,165
Likes: 9
Re: How do gears affect 1/4 mile MPH? (rocco16)
Well you see my friend has an 87 Iroc with a 305 & a T-5 tranny. The car was originally an auto, hence the 2.73 gears.
He was running 84 mph trap speeds which is exactly at the top of 3rd gear with 3.73's. I was hoping though that since he would actually be accelerating faster that he would be able to obtain a slightly higher trap speed and thus get into 4th gear a little bit, never hurts to be in the power band one more time.
The thing that still sucks is that the tranny is actually out of a V-6 car, it has an extremely high first gear (3.97) but it's a long way to second at (2.34) so it tends to want to bog in second. The gears should help that somewhat but it's still quite a jump and the 305 is lacking the torque needed to really make up such a radical difference. If he can keep from blowing the tires off of it he should be able to launch hard but then it'll still bog pretty hard in second.
He was running 84 mph trap speeds which is exactly at the top of 3rd gear with 3.73's. I was hoping though that since he would actually be accelerating faster that he would be able to obtain a slightly higher trap speed and thus get into 4th gear a little bit, never hurts to be in the power band one more time.
The thing that still sucks is that the tranny is actually out of a V-6 car, it has an extremely high first gear (3.97) but it's a long way to second at (2.34) so it tends to want to bog in second. The gears should help that somewhat but it's still quite a jump and the 305 is lacking the torque needed to really make up such a radical difference. If he can keep from blowing the tires off of it he should be able to launch hard but then it'll still bog pretty hard in second.
Safety Car
Joined: Sep 2000
Posts: 3,524
Likes: 4
From: Hinckley Ohio
Re: How do gears affect 1/4 mile MPH? (Nathan Plemons)
All else being equal a change from 2.73 to 3.73 would change the engine RPMs at 84 MPH from 3500 to 4782, not 5500.
4782=3500*(3.73/2.73)
With the 3.73 gears the speed at 5500 would be 96.6 mph.
96.6=84*(5500/4782)
This being so I am pretty sure he would still finish in 3rd gear.
But something seems wrong with your info, how did you get 5500 at 84mph??
[Modified by LT4BUD, 7:56 AM 3/6/2003]
4782=3500*(3.73/2.73)
With the 3.73 gears the speed at 5500 would be 96.6 mph.
96.6=84*(5500/4782)
This being so I am pretty sure he would still finish in 3rd gear.
But something seems wrong with your info, how did you get 5500 at 84mph??
[Modified by LT4BUD, 7:56 AM 3/6/2003]
Thread Starter
Race Director
Joined: May 2001
Posts: 14,165
Likes: 9
Re: How do gears affect 1/4 mile MPH? (LT4BUD)
Yeah you're right my info looks goofy, I must have looked at the wrong thing. Everything is correct except for the RPM's he was pulling. With 2.73's 84 mph was right around 4050 rpm's, making the same MPH with 3.73's occur at 5500 rpm's.
Rather than re-calculate everything every time I've actually made a spreadsheet that calculates everything for me, you just change the variables you want.
For example with his 2.73's, 5500 RPM is 42, 72, 115, 168, and 212 mph respectively for each gear. With 3.73's that changes to 31, 52, 84, 123, 155.
I assure you the math is accurate, you can download my spreadsheet here
http://www.wku.edu/~nathan.plemons/calc.xls
Rather than re-calculate everything every time I've actually made a spreadsheet that calculates everything for me, you just change the variables you want.
For example with his 2.73's, 5500 RPM is 42, 72, 115, 168, and 212 mph respectively for each gear. With 3.73's that changes to 31, 52, 84, 123, 155.
I assure you the math is accurate, you can download my spreadsheet here
http://www.wku.edu/~nathan.plemons/calc.xls
