1985 GT - Test your mustang knowledge, guess how fast i'll run 1/4 mile

[RBatson]

Originally Posted by Unit 5302

If your springs allow for "too much" squat, you waste energy lifting wheels off the ground rather than applying it to forward acceleration.

I'm just curious, how much time have you spent at the drag strip?

[tireburner163]

Rear springs should be a little stiffer for a drag car but many people think that the car should "squat" during the launch so they install softer rear springs on the car to accomplish this. The only cars that should be squatting during the launch are those cars that shock the tires very hard (5-speed cars) and with enough power where the car will overpower the tires. Prostock cars actually squat during launch because they develop enough torque and hit the tires so hard that the squatting helps absorb some of the shock and keeps from overpowering the tires. But, setting up the car to squat is not a function of the springs. It is the position of the Instant Center (IC) that determines how the car squats. The STOCK mounting points for the control arms already cause the car to squat which can unload the tires. So in order to compensate for this squatting the springs need to be stiff to transfer the force to the tires.
If the rear end squats that means that the rear springs/shocks are not pushing the rear tires into the ground. They are just compressing. For the most part you don't want the rear to squat because you want some resistance from the springs/shocks to transfer the force/weight to the tires.

Example:
Make a coil spring out of a coat hanger and place it on a bathroom scale. Push on the spring. The scale is not going to read much because you are not transferring any force to the scale through the softer spring. Now take the spring out of your car and place it on the scale and do the same thing. The scale will read more pressure because it IS transferring more force through the spring.

[tireburner163]

Originally Posted by RBatson

I'm just curious, how much time have you spent at the drag strip?

I ask the same question to you, sir.

[Unit 5302]

Originally Posted by RBatson

I'm just curious, how much time have you spent at the drag strip?

I don't go to the drag strip at all. It's not really part of my interest anymore, and when it was somewhat, there were still other things I was more interested in.

That being said, I wasn't indicating 4 banger springs are better/worse/the same. I'm explaining tireburner163's logic using some rudimentary physics.

I find the conceptual side of automotive engineering less expensive than the direct application.

[Dark_5.0]

I have 4 banger springs up front and V8 springs in the back and I yanked a 1.59 60ft on 8 inch slicks last night.

I prefer to walk the walk rather than talk the talk.

[84LX89GT]

Cool, i'll probably just run the 1st time with the 3.73 rear axle (allegedly 3.73) with stock control arms and lowering springs, then for the next time out i'll swap the 4.10 8.8" rear out of my LX and install the granatelli arms i got with GT rear springs and see what happens.
---update, the car is running and driving and i drove about 100 miles to start breaking the clutch in----
currently needs fixing: high idle, probably vacuum leak or carburetor - won't spend too much time doesn't affect 1/4 mile. I need to swap the stock shifter in - the summit shifter SUCKS!!!!! - all gauges are erratic or just don't plain work, i'm going to make an aluminum plate for the dash and install gauges in that. - Shake at freeway speeds, probably (hopefully) just unbalanced tires.
I have phone dial wheels for the slicks and i'll mount those up soon.
Also found out (found ID on carb) that carb is a 3310 Holley 750 cfm vacuum secondary....that explains the stumble/bog at part throttle or at mid-rpm.
I pulled out the MSD distributor and found that when i got it the timing was retarted to 6* initial so it would idle at 900 rpm, the distributor curve was the slowest possible (comes w/dist) heaviest springs (advance full @ 5500 rpm) and the total advance bushing was 21* (27* total advance w/initial). I changed the curve to hit full @3000 rpm, 36* total advance with 11* initial, made a huge difference in torque and power in general.
-things are progressing but i've had to start almost from the ground up with this stupid thing