RPM dip going onto stop, w/ racepak graphs
 

Been having a problem with the RPMs dipping going onto the stop or right after going on. It seems random, doesn't seem weather related or any pattern really. Thinking of trying to tighten up front suspension not to bounce as much, and also thinking maybe making the first timer smaller and raise cruise rpm. Where would you guys start?

468bbc, 760 hp, glide, 4.10s, 30x9, 3370, T1 .400, cruise 4050, shift on rpm

Car is pretty good except this issue that'll change the 60ft .02-.04. Also notice at times, the transition from the falling rpm to the cruise at times will be a sharp "L" and sometimes slowly curls out, which will also change the 60. Don't know if its related or not.

Re: RPM dip doing onto stop, w/ racepak graphs
 

Whos carb and what size?
What size carb?
Electric or air stop?
Are shifting on stop or on rpm?

I dont know much but maybe i can help

ep

Re: RPM dip doing onto stop, w/ racepak graphs
 

Don’t let Pauley bullshyt you, he’s a bad man running a throttle stop.

Re: RPM dip doing onto stop, w/ racepak graphs
 

dont let 1290 bullchit you sir. i blind squirrel it most of the time.

by the looks of your graph i would say the you are shifting on rpm. the creep in it leads me to think rpm. most will say to try and get your graph to lay as flat as possible which you can do on time not on rpm so much. the creep has never bothered me that much as long as it repeats. but you definitely don't want the dip. that will make you pull your hair out. that is why i dont have hair. i would like to know also how fast your ride is wide open also.

ep

Re: RPM dip doing onto stop, w/ racepak graphs
 

gas or alcohol?

Re: RPM dip doing onto stop, w/ racepak graphs
 

Sorry for the dumb shift ? I see the shift.

ep- told you blind squirrel

Re: RPM dip doing onto stop, w/ racepak graphs
 

Timing issue?
Timing of low gear band release and Hi gear clutches apply..
Both applied at the same time could drag RPM down.
Different servo spring or adjust the spring pressure higher.
or
More clearance in the Hi clutch pack can increase time to apply and free up drag when in low gear.

Re: RPM dip doing onto stop, w/ racepak graphs
 

Not knowing the O2 readings, my guess the carb is a tick fat causing to 60ft to change. Try switching to shifting on time and try a bigger intermediate air bleed on the carb. Other fixes include raising the stop RPM, slowing stop closing speed. A smaller carb may work also. Just some thoughts

Re: RPM dip doing onto stop, w/ racepak graphs
 

You looked good at SGMP until Webb kinda ran over everyone! Myers was too.

Re: RPM dip doing onto stop, w/ racepak graphs
 

This setup is pretty similar to mine, although you have more power and are 200 lbs lighter.

Shifting on time would cost a lot of MPH. I wouldn't do that. I'd probably go up a bit on the cruise RPM to 4300 or so.

I go on the stop at .500 to get things moving. You seem to have a similar thought. Moving that timer would be a good thing to try, but I don't know which way might be helpful in this case.

Do you have a crossover hose on your carb bowl vent tubes? Maybe some fuel slosh.

Intermittent issues are always the worst.

Re: RPM dip doing onto stop, w/ racepak graphs
 

The Webbs are good people and very tough to beat. it seems i have to run Donald or Chris every week. Donald gave me the lap that I was expecting and unfortunately i missed the tree a little. 27 up front will typically not get it done so i knew at the 1000' it was not looking good for me. no choice but to quit and hope. Donald rarely makes a mistake and when i do i pay for it.

i agree with above posts. i feel rpm is too low and shifting on time will clean some things up. would like to know if it is gas or alk

ep

Re: RPM dip doing onto stop, w/ racepak graphs
 

4150 carb, Luke Seibert 1000
Electric shift
Gas, C12

Re: RPM dip doing onto stop, w/ racepak graphs
 

I'd like to go to a higher cruise but with a higher cruise I'll have to run more numbers and then I'll be coming off the stop and shift right away, so I was thinking if I do go higher cruise then try to move the first number from .4 to .3 or .25 to help keep the second number under 2.4 seconds and keep the run at the shift.

Yes, on the crossover tube.

Re: RPM dip doing onto stop, w/ racepak graphs
 

I agree on not shifting on time, with the 4.10 gear I'd have to run a really high cruise to keep it from making huge swings good to bad weather. It would put me back near 130-131.

And shifting on time I'd probably be .5 minimum which is after the dip so it probably would make the car more consistent but I don't think it'd fix that problem.

Re: RPM dip doing onto stop, w/ racepak graphs
 

if it were me i would try going on .20 shift at .70 pull rpm up to 4400. you can move the on and shift times if you like to get what you want in 60'. the shift time will typically be 2 to 1. meaning moving the shift .02 will move 60' .01. i would not get tangled up in my mph as long as it repeats and is predictable. trust me, i am not fast either but my car is relatively predictable so it doesn't matter that much to me. i will lose 3 mph in the 1/8 shifting on time vs. rpm.

make a few runs pulling the rpm up and shifting on time. see what the graph looks like and if it is consistent and exactly how much mph difference there is. if you dont like it you can always change it up. also you can move the shift time around to clean up dips and make it better.

also if you stay with rpm i would still pull the rpm up on the stop. try that and see what it looks like. i wouldnt worry about creep as long as it repeats. i had a set up one time that creeped 1000 rpm when the stop came off but it was nasty.

i agree with lenny about something on the vents to insure you are not sloshing fuel when it goes on the stop. i have a hose that runs from one vent to the other. there are 1000 ways to make a stop work. that is why you have to spend a lot of time testing and playing with things.

ep

Re: RPM dip doing onto stop, w/ racepak graphs
 

I don't think you need to worry about the stop/shift relationship with a higher cruise RPM. It still needs to load the converter and then run up to the shift point. Not much will change there.

Re: RPM dip doing onto stop, w/ racepak graphs
 

I chased that creep for a long time. Ended up being the converter

Re: RPM dip doing onto stop, w/ racepak graphs
 

I go on at .30, cruise at 4250-4300 with a CO2 stop and a really fat 30 year old carburetor. That set-up has been pretty good to me in 1/4 mile SST. 383 sbc at 2830lbs (6.30s W/O at that weight), usually 137 mph depending on weather and track.

I change mine to shift on RPM when I run 1/8th mile SST or any S/G, but I use the same .30 on and cruise speed, just add numbers on the back end. Just changing the shift to RPM usually gets me close to 6.90 given I have to go around 7.25 to the 1/8 to get me 10.90 in the 1/4 when shifting on time.

It does dip a bit, but not enough to hear or affect much. I can see it on my Mega 400 RPM playback but the resolution is not very fine. Might be worse than I think it is but I think I'd rather just not know. Ignorance is bliss.

Despite being slow, I'm more of a "get me close and figure it out before the finish line" kinda guy. Also never really had a T-stop ratio, I just try to get it close.

Re: RPM dip doing onto stop, w/ racepak graphs
 

I have the same graphs in my Roadster.. if I raise the cruise RPM, it goes away.. BUT, then I have to run more numbers in the box, and stay on the stop longer.. not a fan of that, and it cuts back a MPH or two at the top end, as you're actually shortening the track so to speak..

Adger Smith hit on an important issue, SHIFT POINT.. my stop comes on at .10, and the shift point is .20.. it makes perfect sense that the shift is causing an additional drop in RPM.. especially while taking power away.. I'm going to move the shift point out at bit to see if that's a fix.. the car I worked on this weekend has the stop come on at .10, but the shift is at .90.. graphs look great..

Having a torsion bar front end, I was assuming that the touch down after the launch may have been disrupting the floats, and causing the dip.. NOW, I'm not believing this to be the case.. raising cruise RPM doesn't change the touch down after launch.. WE'LL SEE!!!