Controling boost at the hit

Discussion in 'Manton Push Rods Top Alcohol Tech Questions' started by Catman, Aug 22, 2017.

  1. Catman

    Catman Member

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    I'm struggling with traction at the hit. BBC Blown Alcohol Fuel Altered 466", 14-71 HH set up. I use a 44 mag and a grid. 5800 stage RPM. About .25 to 1 second prior to releasing the clutch, the boost numbers go from 8-9 pounds to 20+. This hits the tires hard. Spikes the driveshaft speed at .6 seconds, .25 seconds, and .55 seconds so its bouncing the tires. We have raised and lowered tire pressures,( 4.8 to 5.7) raised and lowered the bar, but can't control spin. Got to looking at boost and see its way up there. thought I was using the grid to control max RPM while the blue wire is activated. Am I missing something in the grid??

    We've been .996 - 60' at 8 pounds boost, but at 20+ were 1.03 to 1.06

    Should I be looking at a throttle controller??

    Any help would be appreciated.

    Catman
     
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  2. turbo69camaro

    turbo69camaro Member

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    What kind of tire ? More pressure more traction Use the grid to limit timing for first part of the run to control wheel speed .you should be able to have 20 plus lbs at the hit and control it........And we can go for pages on your clutch set up ....what kind do you have? whats your basic clutch set up
     
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  3. Catman

    Catman Member

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    34.5-17-16 Hoosier, Taking 3* out at .1 into the run starting at 35*
    10" triple, small, small 6 turns base, 53 grams total. No disc wear between runs. (.001 - .003) 5.0 rear gear, 1.94 low
     
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  4. bob szabo

    bob szabo FC / altered

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    We added fuel to the launch. We could tune our launch very well with launch enrichment. We set the high speed to open just after the clutch locked up. We tuned the low end separately from the high end. We simply varied the combination of main bypass and high speed bypass jet sizes to meet the low end and high end targets.

    Bob Szabo
    bob@racecarbook.com
    airdensityonline.com
     
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  5. max

    max New Member

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    Try a different low gear.

    What is DS at .1, .25 and .5?
     
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  6. Catman

    Catman Member

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    .1 = 1075 .25 = 2145 .50 = 2091
    driveshaft spikes at .06( 1189), then .25 (2145), then .54 (2130)
     
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  7. Catman

    Catman Member

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  8. jay70cuda

    jay70cuda Member

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    You could raise the tire psi 1/2 lb. or
    raise wheelie bar 1/2 in
    Or take 5 out instead of 3. But you need to take power out at shift to calm that spike down
     
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  9. max

    max New Member

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    Take some base out to see if the DS at .1 is lower. Next try more timing out at the hit. Also look at low gear change.
     
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  10. Will Hanna

    Will Hanna We put the 'inside' in Top Alcohol
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    First thing, unless you are running in Denver, your low gear ratio is hyper aggressive. Especially considering the torque a BBC will make down low. A taller low gear or rear gear would make life a lot easier and probably run better. It's always going to be violent with that gearing.

    Two, the car is instantly spinning the tire, then hooking, and appears to be flexing the wheelie bar, which leads me to assume you have an old style long bar. The spin/hook "porpoising" is characteristic of the old style long bars. It is carrying that spin hook cycle through low gear. A more rigid tree mount wheelie bar with a 30-45 degree angle on the top bar would get rid of most of that as it would hit it once and go. With your gearing, it may try to smoke the tires when it hits it.

    With a blown car we don't try to control boost....we want all of it. We use timing to control wheelspeed, especially when you have a controller like a grid. If your rules allow it, the ARC module is a great add on and is super easy to program. Outside of that, 3 degrees is just a fine adjustment. I would start off with 8-10 degrees out and have it ramped back in by .5-.75. Remember the retard number is just that, a number, to achieve a desired wheelspeed.

    I would also consider taking some static out of it. That would let it try to flare the motor and slip the clutch instead of trying to spin the tire at the initial hit.
     
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  11. Catman

    Catman Member

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    Thanks Will, you are right, we have an old style long bar currently. And yes, it flexes. Have the pipe and pieces to build another . Have held up building until we could get some answwers on correct length. So instead of a 7' bar, more like 5-6' ??? More like a fuel FC bar like TJ's or Capps??
     
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  12. Will Hanna

    Will Hanna We put the 'inside' in Top Alcohol
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    In my opinion the angle of the bar is more important than the length. Your tree design will more or less dictate the length to get the right angle. What you give up as the bar gets shorter is it takes more front end travel to hit it. But the main use of the wheelie bar is to control starting line squat.

    The more rigid, the more hook you will have because you're not going to get any wheelspeed help from the spring rebound of a flexible wheelie bar. You have to control it right at the hit when you hit it, but if built right and you get the right combination of power/clutch at the hit coupled with the right tire pressure and wheelie bar height, it should just hit it once, maybe twice and be gone.

    That said with your current set up, it will probably be more prone to smoke the tires. If you don't change the gearing or clutch, you will need to have 10+ degrees out at the hit.

    Pro tip on the grid: If you can get away with it, have your 0.0 timing point at full timing, then get the power out at .01.

    How are you triggering the dark blue launch wire on the grid?
     
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  13. Dale H.

    Dale H. Member

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    I would take about 3 turns of base out of it.
    -Dale
     
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  14. Bjs344

    Bjs344 Member

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    I'm kinda surprised by the responses here. It's sticking the tire and then not making enough power to smoke the tires when it bounces. You can get rid of the bounce a few ways, but more power seems like a potential option that nobody mentioned?

    It worked better with a lower stage rpm but then I'm assuming he cranked it up in an attempt to go faster.

    The problem I see is that he didn't crank it up enough and then got sidetracked because of the bar not being able to catch the squat. I can see the potential that a little less base would give him more rpm/power to stay up on the tire at 0.5, so are you already planning for that when telling him to take timing out?
     
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  15. max

    max New Member

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    35* and only 3* out at .1 (where the damage has already been done). Add in that low gear and the thing is on kill.
     
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  16. Greg Clayton

    Greg Clayton New Member

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    Max and Will are on to it. We had similar when we had the long bar. We put some bracing in it to stiffen it , and that helped , but the short bar is the go, and with your set up have it low , so its trying to smoke the tyres. Need base out as well.
     
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  17. Catman

    Catman Member

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    We have a switch on the clutch pedal set at .000 travel so when the clutch is fully released it activates the launch retard
     
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  18. turbo69camaro

    turbo69camaro Member

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    I would run a 4.56 remove 4 turns of base add 20 CWT 6.5 air in the tire drop 10 timing .1 into run ramp it back in @ 1.5
     
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  19. Will Hanna

    Will Hanna We put the 'inside' in Top Alcohol
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    With the initial line being vertical, it's on the ragged edge of blowing the tires off. So it's hard to get enough power in that area without blowing the tires off, especially during the rebound of the bar. Back when we still had a long bar on Chris Foster's car, we had a pretty aggressive early set up in it, I had a 20 degree chip in the six shooter on at .15 for .05 to .1 seconds to kill the motor on the first big rebound. It worked, but the real fix was a better wheelie bar.
     
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  20. Catman

    Catman Member

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    Made 3 passes on Saturday, 1st pass pulled 6* at the hit then ramped 4 back in by .7. Pulled the motor down pretty hard for
    1.3. 1.02 60 and a 4.27. 2nd pass pulled 5, left higher(5800) and took 1 turn of base out. Improved.... 3 rd pass, 1. More turn of base out. 10 grams off the clutch, 1.06 60' but rear wheels tripped the lights Seems to be responding to less base, graphs looking better Still no clutch wear.
     
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