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Discussion Starter #1
I'd like some input from the internet conglomerate on what steps to take next, and what results (big/small) I'm likely to see.

So! 3.0 liter on E85, 6062 Precision turbo, launching at 6-8 psi at 3000-3250 rpms. Converter is tight and long story short there isn't much I can do about it (Toyota converter). 28x11.5x16 Hoosier QTPs on FD RX7 rims. Car probably weighs 3500 lbs.

Fortune 500 coilovers. Rears are 4k springs on 8k struts, rebound set to full hard. Fronts are 12k springs on 12k struts, rebound set to full soft. 4k = 225 lbs, 12k = 675 lbs. Polyurethane bushings everywhere except front subframe, stock alignment. Compression not adjustable, only rebound. These are marketed as the 'drag' coilovers, personally I think the front springs are way too hard.



August 13th: 85* outside temp. Tires at 18 psi. Tiny burnout. Launched at 6.5 psi at 3000 rpms, 1.85 60'. Spun pretty bad at 10-15 mph, traction control kicked in (front 15 mph, rear 25 mph).

October 22: 70* outside temp. Tires at 15 psi. Tiny burnout. Launched at 6 psi at 3000 rpms, 1.84 60'. Spun pretty bad at 10-15 mph, trac kicked in (front 10 mph, rear 20 mph).

October 22: 70* outside temp. Tires at 15 psi. Big burnout (5 secs). Launched at 6 psi at 3000 rpms, 1.79 60'. Spun a bit at 30 mph, trac kicked in (front 30 mph, rear 38 mph).


I can get another 4 psi out of the antilag (on a good day, its tough to balance) so max I could launch at would be 10 psi, but its spinning in the middle of first gear....would a lower tire pressure, say 12-13 psi, solve this? Or are the front springs too tight, and moving to a 400 ish lb front spring help?

I have a dashcam video of the launch, haven't yet got a suction cup mount for my GoPro so I can point it at the tires...on my to do list.

ProEFI 128, traction control is adjustable...just unsure where the line is, between acceptable slip and too much.

Thanks!
 

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wait for others with more experience to chime in but i find those rear spring rates way too soft. my car with me weighed 2270pounds last time at the track. the car has 10kg(because of application these are similar to about a 5kg on a true rear coilover) in the rear and with some 225/50/15(~ 24"x8.5") hoosier drag radials the car leaves flat; i believe this is desirable. maybe even switch your springs (front to back) if they are same i.d. and length? i am a terrible shifter and have to shift to 2nd before the 60' and still get 1.65". i believe if i were to flat shift the car it would 1.55ish. i care about my transmission so i will wait till i have an ecu which i can pull ignition during the shift to take the load off; increase longevity.

one variable i feel confident in is that most of the problem is not the tires; that is a lot of tire for your power to weight ratio. when you get the car all dialed in it would be nice to see a more efficient tire under the car.

i will follow the thread as this topic is of interest to me.

may the laws of probability be in your favor!
 

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Whoever spec'd that spring rate combo out don't have a fricken clue.

You need somewhere in the 550lbs rear and 350lbs front.

You really need double adjustable shock valving to get the most out of your suspension. Since you only have rebound adjustment....Start by setting the front rebound near full loose and the rear rebound near full tight. You want the front to extend quickley and stay up..... You want the rear to stay tied down near its static ride height, not to squat down on soft springs.

Anything you can do to raise the rear ride height will help with increasing the A/S% which will help traction.
Will
 

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Discussion Starter #4
wait for others with more experience to chime in but i find those rear spring rates way too soft. my car with me weighed 2270pounds last time at the track. the car has 10kg(because of application these are similar to about a 5kg on a true rear coilover) in the rear and with some 225/50/15(~ 24"x8.5") hoosier drag radials the car leaves flat; i believe this is desirable. maybe even switch your springs (front to back) if they are same i.d. and length? i am a terrible shifter and have to shift to 2nd before the 60' and still get 1.65". i believe if i were to flat shift the car it would 1.55ish. i care about my transmission so i will wait till i have an ecu which i can pull ignition during the shift to take the load off; increase longevity.

one variable i feel confident in is that most of the problem is not the tires; that is a lot of tire for your power to weight ratio. when you get the car all dialed in it would be nice to see a more efficient tire under the car.

i will follow the thread as this topic is of interest to me.

may the laws of probability be in your favor!
Good to hear its not just me thinking they are too soft haha. I have a hard time buying the "squatting is what you want" theory of launching an IRS car, but again I'm no expert.

Wasn't sure what tire to buy, and I knew I'd be mad if I went too small and needed more...so I figured bigger is better haha.

Thanks!

Whoever spec'd that spring rate combo out don't have a fricken clue.

You need somewhere in the 550lbs rear and 350lbs front.

You really need double adjustable shock valving to get the most out of your suspension. Since you only have rebound adjustment....Start by setting the front rebound near full loose and the rear rebound near full tight. You want the front to extend quickley and stay up..... You want the rear to stay tied down near its static ride height, not to squat down on soft springs.

Anything you can do to raise the rear ride height will help with increasing the A/S% which will help traction.
Will
That's why I'm posting here :p

Front rebound is full loose, rear rebound is full tight. Coilovers have adjustable preload, and I'm pretty sure the springs are the same length, so would you suggest putting the 675lb in the rear on the 550lb strut, and the 225lb in the front with the 675lb strut? I think that's too much strut for that spring in the front, even if it is full loose.

These have adjustable ride height also, so maybe I'll bounce the rear up a hair. From memory, ride height doesn't have anything to do with antisquat (am I correct?), but rather suspension geometry...so upping the ride height is the easiest/only way to change the geometry, and therefore antisquat, correct?

Thanks guys!
 

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Discussion Starter #5
Looked it up: antisquat on IRS according to Milliken is tan(angle of IC)/CG height/wheelbase. Raising it an inch gives another 2% of antisquat, that's easy.
 

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i dont know how anyone can comment on spring rates without knowing the motion ratio, or even the type of car. what i have learned with IRS cars is that a tall staring line ratio is your friend. (at least with a light car). my datsun 510 uses a SLR of 5.39 and 60 fts 1.4x of the footbrake with a 24.5x9 inch tire. I tighten the compression all the way up on the shocks with a light spring (225 lb/in wheel rate). Yes my car squats hard, but weight transfer is all i have to plant the tire.

https://www.amazon.com/photos/share/FG1FrMov48GjWZTPJrUeVPwZQg5YrTKx7Exmdi53sM5
 

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Good to hear its not just me thinking they are too soft haha. I have a hard time buying the "squatting is what you want" theory of launching an IRS car, but again I'm no expert.

Wasn't sure what tire to buy, and I knew I'd be mad if I went too small and needed more...so I figured bigger is better haha.

Thanks!



That's why I'm posting here :p

Front rebound is full loose, rear rebound is full tight. Coilovers have adjustable preload, and I'm pretty sure the springs are the same length, so would you suggest putting the 675lb in the rear on the 550lb strut, and the 225lb in the front with the 675lb strut? I think that's too much strut for that spring in the front, even if it is full loose.

These have adjustable ride height also, so maybe I'll bounce the rear up a hair. From memory, ride height doesn't have anything to do with antisquat (am I correct?), but rather suspension geometry...so upping the ride height is the easiest/only way to change the geometry, and therefore antisquat, correct?

Thanks guys!
Cars kinda light for 675lbs spring on the rear IMO but if I already had it I would try it. 250lbs on the front will probably compress near coil bind.... thats going to be a ton of stored energy... you'll need some serious rebound valving to control the extension.

AS% comes from the suspension geometry partly.... yes ride height is the easiest way to alter this. Stock IRS cars have very little AS%... everything you can do to increase it helps.

You have to understand that when the rear squats on the spring you are only taking energy away from the tire at the hit. As the rear ride height decreases you also lower the suspensions AS% even more again, hurting the cars ability to plant the tire after the hit. Not to mention what the camber and toe changes do to the tires contact patch.

My Corvette went 1.2x 60fts and ran well into the [email protected]+mph foot braking when I still had the stock IRS under it. I have helped a couple other IRS guys on this board and all have been able to improve following my advice.
Will
 

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Discussion Starter #8
Cars kinda light for 675lbs spring on the rear IMO but if I already had it I would try it. 250lbs on the front will probably compress near coil bind.... thats going to be a ton of stored energy... you'll need some serious rebound valving to control the extension.

AS% comes from the suspension geometry partly.... yes ride height is the easiest way to alter this. Stock IRS cars have very little AS%... everything you can do to increase it helps.

You have to understand that when the rear squats on the spring you are only taking energy away from the tire at the hit. As the rear ride height decreases you also lower the suspensions AS% even more again, hurting the cars ability to plant the tire after the hit. Not to mention what the camber and toe changes do to the tires contact patch.

My Corvette went 1.2x 60fts and ran well into the [email protected]+mph foot braking when I still had the stock IRS under it. I have helped a couple other IRS guys on this board and all have been able to improve following my advice.
Will
Front shocks are valved for 675 springs so I think the valving will be enough there. Rears are only valved for 450 so the 675 springs are going to be a chore...hopefully it doesn't porpoise. If it DOES porpoise, would you recommend getting them revalved to match the springs on them, or revalved for a higher spring (so if I'm using a 450 lb spring, I get them revalved for a 675 spring, to ensure there's enough dampening)?

I've read most of your messages in most of the other IRS threads on here, I recognize your name and appreciate the help! I'll verify with Fortune Auto I can swap the springs (they are the same dimensions, just different wire diameters) and swap them around for the last test and tune this coming Sunday.
 

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Front shocks are valved for 675 springs so I think the valving will be enough there. Rears are only valved for 450 so the 675 springs are going to be a chore...hopefully it doesn't porpoise. If it DOES porpoise, would you recommend getting them revalved to match the springs on them, or revalved for a higher spring (so if I'm using a 450 lb spring, I get them revalved for a 675 spring, to ensure there's enough dampening)?

I've read most of your messages in most of the other IRS threads on here, I recognize your name and appreciate the help! I'll verify with Fortune Auto I can swap the springs (they are the same dimensions, just different wire diameters) and swap them around for the last test and tune this coming Sunday.
If the car porpoises you need to know if it is from a lack of rebound control on the rear shocks or if the front is topping out too fast and in turn causing the rear to unload.

This is where good video will help you.

You just need to see how it works first, but re-valving the rear may become necessary.

Also I when I read your first post I though you were racing an FD RX7 but now see that you just used the wheels....

It would help to know what chassis you are running as there may be some other fiarly easy adjustments I know of that can be made to the factory IRS geometry that will help. I've worked with a bunch of different designs E30 BMWs, FD/FC RX7's, Supras, Z Cars, 240SX, VW and C3/C4/C5 Corvettes.
Will
 

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Discussion Starter #10
Its a 1987 (MKIII) Toyota Supra. I have a GoPro, I'm buying a suction cup mount today and will be getting a video of the rear tires this Sunday if it doesn't rain.

Fortune Auto said the front and rear coils are different (I was hoping I could swap the whole spring/strut). Just for data gathering, I'll get video/times with this setup and get at least the rear re-valved over the winter then go back for more video/times. The Supra guys said that 15 psi cold is way too much, can't hurt to try lower since there isn't much else I can do right now haha.

As far as starting line ratio: 3.55 rear end, 2.804 first gear (A340E Toyota trans), 28" tall tires.

Thanks for all the info and help, I'm always interested in learning something new!
 

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I find it fascinating that the advice was to mess with suspension first, rather than tire pressure.

Thats where i would start, but i havent even lied aboit going as fast as these dudes actually go. LoL

Sent from my SM-J700P using Tapatalk
 

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I find it fascinating that the advice was to mess with suspension first, rather than tire pressure.

Thats where i would start, but i havent even lied aboit going as fast as these dudes actually go. LoL

Sent from my SM-J700P using Tapatalk
That is because, currently he's running the tire a good pressure range at 15-18psi and the car isn't responding one way or the other.....

Also there really isn't much your going to do on those tires... QTP's have an extremely soft sidewall and light construction..... It won't really matter if you run them at 7 psi v/s 25 psi... they are going to wad up like crazy with any power once the suspension is working properly and planting the tire at the hit.

May as well get the suspension doing what it's supposed to do first, then see how bad the tire wads up and decide if pressure adjustments can help.

Plus information is limited here... we got no video. Only the spring rate which I can see right off to be totally wrong for this type of application.
Will
 

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Discussion Starter #13
The way I see it, I can mess with tire pressures myself...just a balance of spinning off the line, sway on the top end, see if the tire slips on the bead etc.

I *know* my suspension isn't perfect, so I would defer to the experts here rather than fiddling with it myself for years haha.

Even with good tire pressures I'm only expecting a 1.6x 60' on a good day. The rest is left up to the suspension. Not to stereotype, but imports in general aren't known for fantastic launches and I think its because no one wants to play with the suspenion further than buying a set of coilovers that people said were good.
 

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Discussion Starter #14
In typical NY fashion Sunday got rained out and there's no more drag dates this year. This gives me two options: post a video of another car, or find a place to do it and not get in trouble (there's a few spots I can think of, just need to find a warm ish day).

The first one is easy!

That's a MKIV so not the exact same weight/distribution/etc (but close), plus mine is an auto and the video is a 6 speed, but its the exact same spring and damper rates. Squats at the launch, comes up a bit during a shift, squats when you're back into power. If it helps at all, that's great, if not, I'll GoPro it up as soon as I can.

Car in the video had a best 60' of 1.37 and 1.39, and most of his passes were below 1.5x.
 

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Go to 2 mins of this video and watch this Supra.


Notice it don't really squat the rear... it lifts the front, crushes the rear tire to the pavement and rolls out. This is the way an IRS (or any drag car for that matter) car should leave. Then watch at how well it drives after the hit.

Go back and look at the video of that other car you posted and tell me which one do you want to be strapped in?

Will
 

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Discussion Starter #16
Makes sense to me! That's Jared Holt's car I believe, very fast car.

I appreciate the help, if I can make a video today I will, if not I'll find some time soon, and then figure out how much stiffer the setup should be in the rear.
 

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Makes sense to me! That's Jared Holt's car I believe, very fast car.

I appreciate the help, if I can make a video today I will, if not I'll find some time soon, and then figure out how much stiffer the setup should be in the rear.
It's John Staats. Its a wicked fast car... been into the 7s in the 1/4.

He started racing with us in the MPC Real Street class at Mason Dixon Dragway back in 2013. If you watch the car then vs now you will see it leaves totally different now. It use to squat down and be all squirley after the hit.... Hes obviously found something with the suspension over the yrs. Althou we are friends I did not have anything to do with it. He and his crew figured it out on their own. Hes a super nice guy, if you asked him, I'm sure he would talk to you about the car.

Will
 

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Discussion Starter #19
This would've been the third time I've used my GoPro, but it broke last night when I was going over all my stuff (figures). So no videos of how the tire is reacting.

Only change to the suspension is front springs are now 6k instead of 12k (337 lbs instead of 675 lbs), still the same struts though.

August 12: 80* outside temp. Tires at 12 psi. About a 2 second burnout in first gear. Launched at 6 psi, 3000 rpm. 0.4 seconds in, rear wheels at 21 mph and front wheels at 6 mph, traction control reeled it in.

August 12: 80* outside temp. Tires at 11 psi. Half a second hazing of the tires. Launched at 4 psi and 3000 rpm. 0.5 seconds in, rear wheels at 18 mph and fronts at 7 mph so traction control kicked in a wee bit.

August 12: 80* outside temp. Tires at 11 psi. 3 second, mild burnout in first gear. Launched at 10 psi at 3200 rpms. 0.8 seconds in, my rear tires were at 28 mph and front tires were at 15 mph, so traction control went crazy to control it.



I did notice I'm getting some weird rubber balling on my tires (they look like tiny little rolls of rubber) along the width of my QTPs. Looking at the tire from the rear of the car: in the center they are horizontal to the road, and as you get closer to the edge of the tire the rubber rolls start to curve upwards.


Wish I could have a better update with video footage, but maybe the rubber balls on my tires will give a clue as to what's going on with them, or if its an alignment issue, etc.
 

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Discussion Starter #20
GoPro footage...launch is at about 1:20 in. Do you need to see contact patch as well as see how much the rear squats? In retrospect I should've adjusted the camera better.

 
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