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Discussion in 'PSI Superchargers Tech Questions' started by max, Feb 11, 2021.
on an alcohol funny car, will lowering tire pressure help stall the tire.
Im curious about the answers you will see. But to be clear. When you say "stall" you mean slow down tire speed? Say at the hit
If you’re talking wheel speed. Less tire pressure = more wheel speed. More tire pressure= less wheel speed. Within reason of course.
Marc is correct. When you lower the tire pressure the tire cups so less footprint resulting in more tire slip/spin. Raising the tire pressure puts more contact patch on the track so more grip resulting in less spin/slip. If you have too much spin on launch then you can either change your first gear ration or pull out timing or install a leanout on a timer with a 60 just that is turned on around 1.2 seconds. This makes you run rich so less power through the 60ft
Thanks for the replies. Tire pressure is usually around 5.8. Low gear 1.60 on a good track and air. Wheel Spd at 0.10 is always about 1200 with as much as 14 degrees out of it.
What rear gear do you have and what size slicks? Are you having a problem?
Just trying to go faster. Only way to settle the car in the first few feet is to pull a heap of timing. If I get it back in hard, like a vertical line it shakes. Trying to pull less timing at the hit but keep the car settled.
Have you tried stepping the timing back in ? So if you pull 14 degrees at the hit, stepping it back in 2 degrees at a time until 14 degrees is back in at 1.2 seconds.
Yes do that plenty of times but trying to run quicker. Don’t want to pull so much timing as defeats the purpose! Looking for 0.91 60s and low 5.50s or 40s over the 1/4.
Here is a DS rpm curve I have used many times with a 4.29 and a 34.5. See how it compares with yours. I would get no tire shake with it and great acceleration
.25 sec - 1505
.5 - 2445
.75 - 3105
1.00 - 3575
1.25 - 3950
1.50 - 4410
1.75 - 4800
what are you using for timing control?
Hi Mike, thanks for the info. I will look at data and get back to you.
Stay off the bar, add tire pressure. My guess is that the cars first move is rim down, crush the tire, rebound off the bar and then roll out on a wadded up tire. I see it a lot. You can’t apply power to cupped or distorted tire, it needs to be round. Conversely you get the tire more round and more hooked to the track, you may mow through the clutch good and proper which doesn’t do you any favors in the acceleration department.
.25 sec - 2230
.5 - 2496
.75 - 2779
1.00 - 3206
1.25 - 3668
1.50 - 4207
1.75 - 4749
I think it gets going early and then falls over?
hits the bar and comes off it about 1ft out. Plants the bar hard about 40ft.
I’m guessing when it tags the bar 40’ out if you don’t have enough timing out it’s a full blown shake fest. When you get on the bar, make a gear change, etc it sends an oscillation into the tire. Sometimes it don’t matter and other times it could lead to tire shake. Might be time to try hanging some weight on the nose.
I think it's worth noting that any driveshaft line needs to not have check marks in the first .25 to really carry wheelspeed well. You can hit a 'target' at .25 but it may be spinning out of a hook. Unless you haze the tires and spin out of an early dead hook, you will most likely have 2-3 more spin hooks, eventually resutling in tireshake.
Your barrel valve and head temp have a lot to do with how your car will act early. If you have the barrel valve real lean with a high head temp, it will be very aggressive down low. Most tune ups use a sub 150 head temp at stage so you can lean on it harder down track without risk of detonation.
The problem with 14 degrees out like you have expressed is getting it back in without pissing the car off. It can potentially blow through the clutch or converter when you try to get it back in if you try to ramp too much at a time.
Max what does your timing curve look like. Can you email me a picture of what you have been using to Mike.firstname.lastname@example.org
Will, thats launch to 1sec approx. Plenty of checks. I've been trying to richen it up at stage. Will keep progressing.
Car is scaled and all weights are good. Isn't it lifting the front and getting on the bar as it doesn't have enough at 40ft? Need to spin (slightly) the tire.
Well, there’s a lot that goes into a combo and you have to be careful who you listen to. If your combo isn’t exactly the same as the person giving advice, there’s a good chance that it’s not going to work. When you start listening to several different people that have different combos and then you start to mix and match that’s what will really get in trouble. Like will stated it depends on a lot of factors including stage gpm, Head temp, if it drops cylinders or not leaving, if the driver hit his stage rpm, if the foot swap was clean, the angle of the headers, the amount of spring pressure and counterweight on the clutch, trans ratio, the cut back on your discs, front end weight, crankshaft height, etc. that all comes into play of how much you can lay down early. I remember back when A top tier blown dragster team built a new car and that car was an absolute shake fest. No data from the old car would work. They eventually got it figured out several years later I ran into the person in charge at Brainerd and we got to talking and I asked him about that car. He told me that they moved the engine a 1/4” and it fixed it. I asked him which way, but he wouldn’t answer. My guess is that car scaled good also, but it didn’t like the dynamics once it was in motion. As far as your timing curve goes, you might have to ramp some timing in and out several few times in the first second to get that tire to carry the wheelspeed and smooth out some.
I’ve messed with a lot of stuff from turbo, nitrous, blower, both big and small tire. Even though a lot of stuff doesn't directly correlate, a lot of times the principles transfer over. I like the dragster/funny cars because they are solid and what’s happening at the tire is what’s really happening, you don’t have the shock there to muddy things up. I guess what I’m trying to say is that a solid car is more pure in the sense that this change netted this result.
That being said, think about this, on a small tire car with no bars the power management is going to be something like your experiencing at the moment, leave soft and then try to get the power in as soon as possible. Common issue is that they tend to power wheelie. It’s not as noticeable now days because of the wheelie control and traction control but the solution is pretty simple, either slow the power application (which the wheelie control can do for you) or hang some weight on the nose. Header angle also has a lot to do with how it holds the front end down. I had a turbo car that we put on scales and put it up on the chip with straight exhaust, 45* up exhaust and 90* up exhaust. The difference in weight on the scales was a good bit different with 90* up turns. That car was 50/50 weight bias, was really light and used to get up on the bar real hard and was a real pain to manage. With the 90* up turns it would just teeter on the rear tires when the power came in, it would carry the front end but not quite touch the bar. We could really get up after it then, and it run real good. In the end you have to do what works best for your car and how your comfortable running the car. Hope some of that helps.