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Discussion Starter #1
Trying out some MT 26 inch stiff side wall slicks for the first time on a 3600lb car and was wondering whats a good starting tire pressure.t56 with 4.10 gears making 577rwhp.
 

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13 to 14 psi. The key is to video tape the launch and adjust from there. Obviously the chassis has to be set properly as well or tire pressure really does not mean much.
 
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My car likes 12 psi on slicks and 14 psi on radials but it is 3200 lbs.

I'd start at 14 and see what happens. Have someone video it and look at the tire sidewalls.

With my setup, I get better and more repeatable 60 ft times with less air until the tires wad up or it gets loose on the big end. When they wad up consistency and handling gets worse. At the right pressure it is straight as an arrow.
 

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Discussion Starter #4
Thanks,will start around 14psi and work my way down.
 

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Thanks,will start around 14psi and work my way down.

You might work up or down with a heavy car. Heavy cars and narrower tires or weaker sidewalls need more pressure to keep the sidewalls from wadding and the to keep the car stable at the big end.
 

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Discussion Starter #6
When i ran Mt et streets my car like 15psi but the car moved around a lot on the big end.
 

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When i ran Mt et streets my car like 15psi but the car moved around a lot on the big end.
I hate that feeling.

When I had QA1 shocks I could barely hang on to the car below 15 psi pressure.

With Strange shocks it is straight as an arrow (as long as the tires don't start spinning down track).
 

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Did you get a chance to try the slicks? I'll be putting a 8.5/26" stiff sidewall Hoosier slick on the LTD when the guy who does the mounting gets back in (hopefully next week). Would love to hear your results.
 
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Thats a mean little tire! Def video tape the launch and watch in slomo. On my slow car I was at 16-18psi cold and 5000rpm launches to get them to roatate a couple times at the hit and not bog. . Im a firm believer that dead hooking a stick car is great for photos and bragging rights but it breaks sh*t!
 

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Discussion Starter #10
Got a chance Saturday to try them out but blew a headgasket on my first pass.with 14psi and a 4k launch it bog really bad with a 1.70 60ft.should have tried a 6k launch but it was the first pass this year so took it easy.yet it still broke.
 

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4k launch it bog really bad with a 1.70 60ft.should have tried a 6k launch but it was the first pass this year so took it easy.yet it still broke.
Raising staging rpm to minimize a bog basically works by packing more inertia energy into the rotating assy, which in turn forces the clutch to slip longer, to a point farther down the track before engine rpm and vehicle speed can sync…that’s the point where the clutch locks up 1:1. Because clutch lockup is delayed to a point farther down the track where the car is traveling faster, engine rpm does not drop as low as before…less noticeable bog.

Think about that for a minute though…you are adding inertia energy to the launch for the sole purpose of forcing more abuse on your clutch, basically using that added energy to force the clutch to slip longer against it’s maximum clamp pressure! Not only does this old school method add a lot of un-necessary wear to the clutch assy, it also makes the launch more violent than it needs to be. What most don’t realize is that after you have used that inertia energy to force the clutch to slip longer, that spent energy then has to be paid back in full before the engine can recover the rpm that it lost. That inertia energy transfer which initially forced the clutch to slip longer now slows the car as that energy transfer reverses, now some of the engine's power is used to recharge spent inertia energy back into the rotating assy as it gains back the lost rpm. In the end, that temporary boost of torque which forced the clutch to slip longer did not actually net you any performance gain.

Why continue the tradition of forcing the clutch to slip against it’s maximum clamp pressure? Instead, we prefer the alternate method of temporarily holding back some of the clutch’s initial clamp pressure at the throwout bearing. The clutch still slips as needed, but now it slips against much less average clamp pressure so it‘s surfaces remain cooler and wear less. The engine no longer has to lose rpm to force the clutch to slip during launch, so the chassis never sees the additional hit from that inertia energy, and the engine won’t have to pay that spent inertia energy back. This cuts the whole give/take of rotating assy inertia right out of the launch loop all together, for a smoother, less violent, and more efficient launch! No longer do the tires have to spin to minimize the bog, and less clutch wear is just icing on the cake.

There’s also that notion that you need a couple rotations of the tire off the line, the traditional way to launch a stick car on slicks with a minimum amount of bog. The engine does not typically make enough torque to break the tires loose on it’s own, needing help from a hard hitting clutch to release additional inertia energy from the engine’s rotating assy. When the tires spin the clutch does not have to slip as much which also minimizes the bog, basically trading wear/tear on the clutch for wear/tear on the tires. It’s been that way for more than 50 years now, but that’s beginning to change. We now have radials which are a much more efficient tire. Problem is, you can’t just bolt a set of radials on a traditional stick/slick setup and get all the benefits. The radials just won’t tolerate the violent hit or the same amount of wheel spin that the slicks needed to work effectively. If we eliminate the overly violent hit and excessive wheel spin, it becomes much easier to reap the benefits of radials, with a side benefit of less broken parts.
 

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That's a very good technical summary of what really happens. Few people seem to know that.
 

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Raising staging rpm to minimize a bog basically works by packing more inertia energy into the rotating assy, which in turn forces the clutch to slip longer, to a point farther down the track before engine rpm and vehicle speed can sync…that’s the point where the clutch locks up 1:1. Because clutch lockup is delayed to a point farther down the track where the car is traveling faster, engine rpm does not drop as low as before…less noticeable bog.

Think about that for a minute though…you are adding inertia energy to the launch for the sole purpose of forcing more abuse on your clutch, basically using that added energy to force the clutch to slip longer against it’s maximum clamp pressure! Not only does this old school method add a lot of un-necessary wear to the clutch assy, it also makes the launch more violent than it needs to be. What most don’t realize is that after you have used that inertia energy to force the clutch to slip longer, that spent energy then has to be paid back in full before the engine can recover the rpm that it lost. That inertia energy transfer which initially forced the clutch to slip longer now slows the car as that energy transfer reverses, now some of the engine's power is used to recharge spent inertia energy back into the rotating assy as it gains back the lost rpm. In the end, that temporary boost of torque which forced the clutch to slip longer did not actually net you any performance gain.

Why continue the tradition of forcing the clutch to slip against it’s maximum clamp pressure? Instead, we prefer the alternate method of temporarily holding back some of the clutch’s initial clamp pressure at the throwout bearing. The clutch still slips as needed, but now it slips against much less average clamp pressure so it‘s surfaces remain cooler and wear less. The engine no longer has to lose rpm to force the clutch to slip during launch, so the chassis never sees the additional hit from that inertia energy, and the engine won’t have to pay that spent inertia energy back. This cuts the whole give/take of rotating assy inertia right out of the launch loop all together, for a smoother, less violent, and more efficient launch! No longer do the tires have to spin to minimize the bog, and less clutch wear is just icing on the cake.

There’s also that notion that you need a couple rotations of the tire off the line, the traditional way to launch a stick car on slicks with a minimum amount of bog. The engine does not typically make enough torque to break the tires loose on it’s own, needing help from a hard hitting clutch to release additional inertia energy from the engine’s rotating assy. When the tires spin the clutch does not have to slip as much which also minimizes the bog, basically trading wear/tear on the clutch for wear/tear on the tires. It’s been that way for more than 50 years now, but that’s beginning to change. We now have radials which are a much more efficient tire. Problem is, you can’t just bolt a set of radials on a traditional stick/slick setup and get all the benefits. The radials just won’t tolerate the violent hit or the same amount of wheel spin that the slicks needed to work effectively. If we eliminate the overly violent hit and excessive wheel spin, it becomes much easier to reap the benefits of radials, with a side benefit of less broken parts.
OR... The OP could just add a bit of air pressure to the tres and keep the same lauch RPM.

I have a question, and it's in your last paragraph.. When you refer to how violent of a launch a radial can take, how are you gaging that? Trying to understand this.. The initial "hit" of the tire? The 60's they are capable of? I will agree with you that a radial does NOT recover well from a spin, however, as far as a violent launch, they can work just fine. I've seen too many sub 1.10 60's from a radial car.. But, the kicker here, and what a lot of people don't have access to is a properly preped track.. and another thing is the chassis is off as well.
 

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I have a question, and it's in your last paragraph.. When you refer to how violent of a launch a radial can take, how are you gaging that? Trying to understand this.. The initial "hit" of the tire? The 60's they are capable of? I will agree with you that a radial does NOT recover well from a spin, however, as far as a violent launch, they can work just fine. I've seen too many sub 1.10 60's from a radial car.. But, the kicker here, and what a lot of people don't have access to is a properly preped track.. and another thing is the chassis is off as well.
I don't think you realize just how violent the launch of a low power car can be. As an example, let's assume a car has power for 1.50 60's, but has a grabby clutch that has a capacity of 800 ft/lbs before it begins to slip. When you launch that car, the clutch is going to draw 800 ft/lbs, and the engine does not have to make 800 ft/lbs to make this possible. That clutch will draw all the torque that the engine is making at wot, then it will draw the balance of the 800 ft/lbs from stored inertia energy which will cause the rotating assy to lose rpm. That extra inertia energy makes the launch much more violent, but remember as soon engine rpm is drawn down to the point that engine rpm sync's up with vehicle speed, rpm ceases to drop and that transfer of inertia energy stops. Problem is that after you have used that inertia energy and lost the rpm, that spent energy then has to be paid back in full before the engine can recover the rpm that it lost. That inertia energy transfer which initially made the car launch harder now slows the car, as it reverses and some of the engine's power must be used to recharge spent inertia energy back into the rotating assy. In the end, that temporary torque boost from inertia energy did not actually net you any performance gain.

Understanding that, now let's compare that 1.5 60' car with a grabby clutch, to a car that has power for 1.1 second 60's but does not lose rpm when it launches. If that 1.1 60' car does not lose rpm during launch, that indicates no inertia energy was used and it launched on engine power alone...
…2800 lb Car #1 has power for 1.5 second 60’s (1.66 G‘s), which requires a 60’ average of 4648 lbs of thrust at the tire
…2800 lb Car #2 has power for 1.1 second 60’s (3.08 G’s), which requires a 60’ average of 8624 lbs of thrust at the tire

The 1.5 60' car averages 4648 lbs of thrust over the initial 60', but remember that the clutch in Car #1 draws 800 ft/lbs of energy before it begins to slip. Multiply that 800 ft/lbs by it's 1st gear ratio (3.35 for example), rear gear ratio (perhaps 4.30), factor in some drive train loss (13% sounds good) and the 28” tire’s lever moment at the contact patch, that 1.5 second 60' car easily matches the 8600 lb thrust of the 1.1 60' car during that very short period of time before the clutch locks up! That initial boost to 8600 lbs of thrust before the clutch locked up is then offset by a reduction of thrust below the average while the lost rpm is recovered. Even though thrust peaked at around 8600 lbs briefly, it's still just a 1.5 60' car. Add in violent pressure fluctuations at the contact patch from an unsorted chassis, it’s easy to see how a lower power car can easily upset a tire that's otherwise capable of amazing 1.1 second 60’s.

If that 800 ft/lb clutch in the 1.5 60' car were replaced with a 500 ft/lb version, the duration of clutch slip would be roughly 60% longer, which means the car would be traveling much faster at the point where rpm and vehicle speed finally sync up...much less bog. Now the tires only see a peak of around 5400 lbs of thrust instead of 8600 lbs in that brief period before the clutch locks up. Not only does the drivetrain see less abuse, but the engine does not lose as many rpm after launch and after the shifts...the engine will be pulling from a higher average rpm where it makes more power. It might be hard to believe, but even though the 500 ft/lb clutch slips longer and puts less stress on the drivetrain, the car will actually be quicker than it was with the 800 ft/lb clutch, and be much less likely to break the tires loose even with an un-sorted chassis.

These are not numbers from actual cars, just something i plugged in to help illustrate the concept.
 

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Just something to think about. If you allow 1 inch of circumferential sidewall yield in a bias tire, it translates to 1/4.10 = .244 inches of driveshaft yield.

If a radial tire has no yield at all (it will have some yield) all you gain is 1/4 inch of cushion on rotation.

This does not seem like enough to significantly change shock load as the engine inertia unloads. Both the radial and bias would be far far shorter yield than the yield needed to move down on the stored energy.
 

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I don't think you realize just how violent the launch of a low power car can be. As an example, let's assume a car has power for 1.50 60's, but has a grabby clutch that has a capacity of 800 ft/lbs before it begins to slip. When you launch that car, the clutch is going to draw 800 ft/lbs, and the engine does not have to make 800 ft/lbs to make this possible. That clutch will draw all the torque that the engine is making at wot, then it will draw the balance of the 800 ft/lbs from stored inertia energy which will cause the rotating assy to lose rpm. That extra inertia energy makes the launch much more violent, but remember as soon engine rpm is drawn down to the point that engine rpm sync's up with vehicle speed, rpm ceases to drop and that transfer of inertia energy stops. Problem is that after you have used that inertia energy and lost the rpm, that spent energy then has to be paid back in full before the engine can recover the rpm that it lost. That inertia energy transfer which initially made the car launch harder now slows the car, as it reverses and some of the engine's power must be used to recharge spent inertia energy back into the rotating assy. In the end, that temporary torque boost from inertia energy did not actually net you any performance gain.

Understanding that, now let's compare that 1.5 60' car with a grabby clutch, to a car that has power for 1.1 second 60's but does not lose rpm when it launches. If that 1.1 60' car does not lose rpm during launch, that indicates no inertia energy was used and it launched on engine power alone...
…2800 lb Car #1 has power for 1.5 second 60’s (1.66 G‘s), which requires a 60’ average of 4648 lbs of thrust at the tire
…2800 lb Car #2 has power for 1.1 second 60’s (3.08 G’s), which requires a 60’ average of 8624 lbs of thrust at the tire

The 1.5 60' car averages 4648 lbs of thrust over the initial 60', but remember that the clutch in Car #1 draws 800 ft/lbs of energy before it begins to slip. Multiply that 800 ft/lbs by it's 1st gear ratio (3.35 for example), rear gear ratio (perhaps 4.30), factor in some drive train loss (13% sounds good) and the 28” tire’s lever moment at the contact patch, that 1.5 second 60' car easily matches the 8600 lb thrust of the 1.1 60' car during that very short period of time before the clutch locks up! That initial boost to 8600 lbs of thrust before the clutch locked up is then offset by a reduction of thrust below the average while the lost rpm is recovered. Even though thrust peaked at around 8600 lbs briefly, it's still just a 1.5 60' car. Add in violent pressure fluctuations at the contact patch from an unsorted chassis, it’s easy to see how a lower power car can easily upset a tire that's otherwise capable of amazing 1.1 second 60’s.

If that 800 ft/lb clutch in the 1.5 60' car were replaced with a 500 ft/lb version, the duration of clutch slip would be roughly 60% longer, which means the car would be traveling much faster at the point where rpm and vehicle speed finally sync up...much less bog. Now the tires only see a peak of around 5400 lbs of thrust instead of 8600 lbs in that brief period before the clutch locks up. Not only does the drivetrain see less abuse, but the engine does not lose as many rpm after launch and after the shifts...the engine will be pulling from a higher average rpm where it makes more power. It might be hard to believe, but even though the 500 ft/lb clutch slips longer and puts less stress on the drivetrain, the car will actually be quicker than it was with the 800 ft/lb clutch, and be much less likely to break the tires loose even with an un-sorted chassis.

These are not numbers from actual cars, just something i plugged in to help illustrate the concept.


You do realize you're breaking your own argument right? First, your G numbers are "off" at least compared to what I've seen collected.. whether it being my own racepak logs or looking at other's data. Most quick, or what I consider quick radial cars are topping the g-meter at the 2.6"ish" range. At least on runs that go down.. Not saying it couldn't happen.. but I haven't seen it yet.

So if a car that 60's in the 1.1 area AVERAGES your magical 8624 lbs of thrust but the 1.5 60' car CAN PEAK at 8624 lbs of thrust.. why is it the 1.1 60' car does down, and 1.5 60' car will kick the tire? It's shocks and chassis set up..

BTW, an auto car looses RPM on lauch as well as the conveter "catches" the engine. It's that converter rollover that you work to get out so you keep the engine accelerating.

I know you're selling a gaget and whatnot.. and I won't say it doesn't work. However, how you describe thing makes me think you've never seen lauch data from cars that are getting in done on the front half.
 

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You do realize you're breaking your own argument right? First, your G numbers are "off" at least compared to what I've seen collected.. whether it being my own racepak logs or looking at other's data. Most quick, or what I consider quick radial cars are topping the g-meter at the 2.6"ish" range. At least on runs that go down.. Not saying it couldn't happen.. but I haven't seen it yet.

So if a car that 60's in the 1.1 area AVERAGES your magical 8624 lbs of thrust but the 1.5 60' car CAN PEAK at 8624 lbs of thrust.. why is it the 1.1 60' car does down, and 1.5 60' car will kick the tire? It's shocks and chassis set up..

BTW, an auto car looses RPM on lauch as well as the conveter "catches" the engine. It's that converter rollover that you work to get out so you keep the engine accelerating.

I know you're selling a gaget and whatnot.. and I won't say it doesn't work. However, how you describe thing makes me think you've never seen lauch data from cars that are getting in done on the front half.
uhhh umm...
 

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I'm posting because I wanted to follow up on the small tire/stiff sidewall thing. I did go ahead and get the 26x8.5 Hoosier C11 stiff sidewalls slicks mounted up. Let me tell you - they're fantastic. I ran 17 psi in them, and they dead hook, even at 3,500 rpm on the trans brake (which is over 13 psi of boost on the brake - see the data log below). I was able to drop .06 in the 60' over the previous Friday; and they still could've taken more.

But this is incorrect:

BTW, an auto car looses RPM on lauch as well as the conveter "catches" the engine. It's that converter rollover that you work to get out so you keep the engine accelerating.

I know you're selling a gaget and whatnot.. and I won't say it doesn't work. However, how you describe thing makes me think you've never seen lauch data from cars that are getting in done on the front half.
The only time my car's ever lost rpm on launch was when the accel pump settings led to an extreme rich or lean condition. Here's a closeup of a transbrake launch from Friday:



White line is RPM. You can see the "hit" pulled my foot off the throttle even (green line), and still the rpm didn't drop.

Going back to the slicks - they are excellent. Highly recommended. And they drive very well.
 

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I'm posting because I wanted to follow up on the small tire/stiff sidewall thing. I did go ahead and get the 26x8.5 Hoosier C11 stiff sidewalls slicks mounted up. Let me tell you - they're fantastic. I ran 17 psi in them, and they dead hook, even at 3,500 rpm on the trans brake (which is over 13 psi of boost on the brake - see the data log below). I was able to drop .06 in the 60' over the previous Friday; and they still could've taken more.

But this is incorrect:



The only time my car's ever lost rpm on launch was when the accel pump settings led to an extreme rich or lean condition. Here's a closeup of a transbrake launch from Friday:



White line is RPM. You can see the "hit" pulled my foot off the throttle even (green line), and still the rpm didn't drop.

Going back to the slicks - they are excellent. Highly recommended. And they drive very well.
I would attribute your grah to two things.. a 3.08 (if that's still the situation in the back) and the boost level you're already at as you let the button go.

I can post some UGLY ones of mine ( as soon as I figure it out..lol) Runs will be low 4.90's passes, pulling roughly 2.4 G's. with 60's in the mid 1.teen's They show a FAR different story. I wasn't ultilzing the power I had very well AT ALL!!!! I also think in a boosted application, it might be a touch less violent on the converter by comparison to a nitrous car..


But bringing back to the slick.. I've also seen them work killer..
 

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3.73's and no boost (naturally aspirated):

 
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