Funny Car Launch RPM?

What rpm are most funny cars (with psi) launching at? We ran our car last weekend for the first time with the psi, and had trouble getting it to make a decent 60'. The motor was definitely fat, but was better every run, but it didn't seem to take off very hard. We wanted to leave about 6000, but every run he left around 6900, obviously too high. We started at 12gr total on clutch, and wound up at around 30gr, which is too high from what we've been told. The only thing we can think is that the motor was so high, even with all that weight, it wouldn't grab. The last run, with 30 gr, we took 20 thousandths off the clutch, which I think should be about half that. Anybody got an idea?

thanks
bryan brown

 

clutch

Who's clutch are you running and what size is it?
Do you know what springs are in the clutch?
Do you have the clutch manufacture's chart telling you how many turns on the static for the given spring? X number of turns = lb's of force on clutches.
Just a guess here, but it sounds like your light on your static setting. Remember the static adjustment, dictates how hard you hit the tires which can have a large effect on the 60ft times.

 

Have you thought about hiring someone to help you?

 

Crower 10.7, 2 1/2 turns. Discs aren't cut down. We're thinking about putting in 10" discs, going back to 12gr, and leaving down around 5500. Is it possible that the motor was so high at the launch(6900), that it didn't matter how much weight or spring pressure was on the clutch, the motor was going to drive through it? It didn't slip at all on the 1-2 shift, and shook so hard it sheared the blowback tube bolts. It did that on the last two runs. Another sign of too much clutch? Leaving lower, will that let the motor flash up, slinging out the fingers? The oil looked decent on the last run, so the tuneup is close, should we just start over? Does leaving that high pretty much taint all data?

bryan

 

bryanbrown

On your first post you indicated the clutch was slipping with 30g, but in your second post you said there was not clutch slippage at the 1 2 shift. Conclusion it's only slipping in high gear?? Not wanting to upset anyone but the launch rpm seems to be too high. The other thing that could be happening' is the driver resting his foot on the clutch pedal?? Once the car is in high gear the driver could be getting ready to shut the car down by putting his foot on the pedal and not even realizing it.

I would also check the spring and finger length combination. Most Crower 10.7 would use the black spring for a F/C set up. Check your chart from Crower to make sure you know what set up your dealing with. Other things to check, make sure all six finger are free and not binding, worn pins, and/or fingers, or miss alignment during re-assembly could cause this. Also make sure the pedal linkage is allowing enough free play to completely disengage the throw out bearing, especially if you have a frame mounted clutch pedal. Hope this will be of some help.

 

Bryan look at your computer graft, look to see when yor driver swaps feet that he is doing it right, you can look at the graft and see if hes letting the clutch out before he steps on the gas, or if hes stepping on the gas before he lets out the clutch both should be done at the exact same time, you should start out with a set rpm( 5.500) and stick to it till he knows what hes doing, and is consistant at it, i`v tuned cars that the drive is inconsistant with this and it is imposabel to figuer out what is wrong, by the way i learned this at Frank Hawleys driving school, and i would recomend that all new drivers spend the $5.000 and get an education from Frank, it will save you a lot of money and you wont look like a dope.

 

I'm confident our setup is correct. We've had a roots car for 20+ years, and our crew chief is very good, and has tuned a very good TAD. He doesn't have as much experience with a FC, but that is why we've tried to match our setup (at least at first) with everyone else we know. Every car is different, but we should end up close to most others. I'm brainstorming on my own here, and haven't asked him what he thinks since sunday, so he may already have the answer. Our driver has always had a problem creeping up the rpm at the launch, but with the roots, we started at a lower rpm, so it never wound up that high. He's not swapping feet wrong, the motor is that high well before launch. The clutch slippage is in low gear, not in high, though that is an interesting theory I never would of thought of. I'm still thinking that the motor is so high that the clutch can't get a hold of it, and slips that much until it can catch up. Is that a reasonable conclusion?

thanks
bryan

 

Could the trans and diff ratio combination be to tall ie total 1st gear ratio too much load?

 

The Crower pack height is set at .701 and can, like you said Mike, be checked with a caliper once the clutch is set at zero. The dial indicator has an adjustable collar and can and will get out of adjustment and should be check regularly. Once you know the gauge is right you can machine a test block with a .701 hole, then before setting the clutch just insert the gauge, and make sure it goes one revelation and back to zero then you know it’s correct.

I don’t know this but it just seems to me there must be something restricting the fingers from full travel. Leaving at that king of rpm the only thing a properly working clutch is going want to do is lock up, and if your not making huge power it will shake every nut and bolt loose on the car.


What kind of air gap are you using?? Should be .050 to .060.
Are the clutch disks being put in backwards, look for damage on the fingers or the disk hubs? Make sure you don’t have two hubs facing each other; if a floater is thin it is possible the hubs are touching and the disks are not making good contact with the floater.

 

1.64 low, 4:30 rear gear. .060-.070 gap. The clutch locked up in the middle of low, and stayed that way the whole run. The clutch is working properly, that I'm sure of. I can't get past the launch rpm, am I off base, or does that not make as big a difference as I'm thinking it could. I'm betting, that if we go back up with 30gr, and leave at 5500, it would either blow the tires away instantly, or shake its brains out.

bryan

 

if your shaking or blowing the tires off that early , sounds to me your lean. don't try to run 5.60's on the first pass. just get it from a-b then get after it.

 

Personally, I don't think the car/tune up is ready for a 1.64 trans. I agree with Bruce, just get it from a-b, then get after it. You can run a 1.72 trans with a screw and run low 70's all day. If the motor is down on power and the clutch is locked up during the 1-2 shift, the end result is tire shake. The clutch needs to slip during the shift.

 

Sounds to me like your clutch can pull high gear. Max clutch load and no mechanical help(gear reduction is taken away in high gear). If you cann't get enough clutch in low gear change low gear and make the cluch work less. You have to find the balance between low gear and clutch. Clutch must work in ALL gears. Low gear only works in low gear.

Also ...listen to your car ...not your buddies. Your buddies might be very smart but your car won't lie to you or leave out a few details!

 

So what i being said is that 1.68 or 1.72 gear is better on the engine/clutch than a 1.64? I thought that going down in ratio (numerically ex. 1.60) was less aggressive? And the higher ratio's such as 1.72 were for very good tracks (Joliet). Is this not correct?

 

So, would going down on rpm negate the need for more static? We actually had three turns, I went back and looked. The counterweight didn't come in early, it slipped for a long time in low, but once it locked up, it didn't slip on the shift, and shook hard.

bryan

 

Shake

Did the car shake before the shift or after?
At what RPM and speed did the clutch lock up?
How seconds/feet out did it shake?
What was the 60ft time on that run?
I think Will is correct, not enough drive shaft/tire speed.

 

bryan,
In regards to clutch adjustments I suggest you consult with Larry and Mick Snyder. They have the best set up out there.