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RPM trace of soft launch #3114364 01/19/23 12:12 AM 01/19/23 12:12 AM
Joined: Jan 2003 Posts: 31,033 Oregon A AndyF OP I Win
AndyF OP I Win A Joined: Jan 2003 Posts: 31,033 Oregon	Working with a customer who has a car that launches softer than it should. The RPM trace looks odd to me. Usually the RPM trace shows a sharp corner or a small "hook" after the launch but this one is round. I'm thinking maybe the converter is too tight or perhaps the rear gear is too tall and the engine just can't move the car off the line? I'll post a picture of the data log. The log reads from left to right. The first area is the engine on the two step, then the launch, then the 1-2 shift and then the pull to the end. The second picture is the dyno sheet. The engine makes good power from 6500 to 7500 but this converter stalls around 6000 and the driver is shifting at 7000. So I'm thinking the car would go faster if he shifted at 7500 and ran it thru the lights at 7500. Maybe a higher stall converter also?

Re: RPM trace of soft launch [Re: AndyF] #3114365 01/19/23 12:14 AM 01/19/23 12:14 AM
Joined: Jan 2003 Posts: 31,033 Oregon A AndyF OP I Win
AndyF OP I Win A Joined: Jan 2003 Posts: 31,033 Oregon	For comparison, here is the same curve from a different car that I tune. This car is roughly the same power to weight ratio but it leaves a lot harder. The RPM curve is very different. I'm suspecting the torque converter but I don't really know, it could be something else. This car has a driveshaft sensor on it. The green line on the screen is the driveshaft speed. Last edited by AndyF; 01/19/23 12:15 AM.

Re: RPM trace of soft launch [Re: AndyF] #3114379 01/19/23 01:59 AM 01/19/23 01:59 AM
Joined: Aug 2003 Posts: 43,174 Bend,OR USA C Cab_Burge I Win
Cab_Burge I Win C Joined: Aug 2003 Posts: 43,174 Bend,OR USA	A little tire spin on the gear change? Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)

Re: RPM trace of soft launch [Re: AndyF] #3114380 01/19/23 02:02 AM 01/19/23 02:02 AM
Joined: Aug 2003 Posts: 43,174 Bend,OR USA C Cab_Burge I Win
Cab_Burge I Win C Joined: Aug 2003 Posts: 43,174 Bend,OR USA	Is it bogging, stumbling, at the hit and then going rich where the round part is from the squirters? Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)

Re: RPM trace of soft launch [Re: Cab_Burge] #3114403 01/19/23 07:32 AM 01/19/23 07:32 AM
Joined: Jan 2003 Posts: 8,027 Tulsa OK Bad340fish master
Bad340fish master Joined: Jan 2003 Posts: 8,027 Tulsa OK	He might try lowering the 2 step and getting a run at the converter. To me it looks like he is up high in the converter and it is starting to tighten up so it doesn't have as severe of a hook in the RPM maybe? Here is mine, RPM and Driveshaft. I stall lower and my converter flashes to 6300. 600HP smallblock. From stall speed to flash is .33 seconds The air was really good in this log, stall was 3550 the flash was 6460 which was the highest I have seen it. 68 Barracuda Formula S 340

Re: RPM trace of soft launch [Re: AndyF] #3114407 01/19/23 08:14 AM 01/19/23 08:14 AM
Joined: Jan 2003 Posts: 2,917 A shed in England Tig master
Tig master Joined: Jan 2003 Posts: 2,917 A shed in England	Originally Posted by AndyF For comparison, here is the same curve from a different car that I tune. This car is roughly the same power to weight ratio but it leaves a lot harder. The RPM curve is very different. I'm suspecting the torque converter but I don't really know, it could be something else. This car has a driveshaft sensor on it. The green line on the screen is the driveshaft speed. He's spinning the tyres, D/S trace is "off curve". Starts spinning around 1.2 and recovers by 1.7. The "zero" point (actual start of the pass) is around 0.9. This could be confirmed by a drop in accel G ("g force" on holley). Is it a bias ply tyre getting some wheelspeed, that's whats giving you the rpm "hook" the rpm drops as the tyres stick. EDIT: Below is a dead hook, it shows a nice predictable D/S curve, It's a footbrake launch, a little spin between shifts. The drop first in accel g is due to the front wheels touching down and slowing the car, the other 2 coincide with the gear shift. Last edited by Tig; 01/19/23 09:00 AM. '74 Challenger..9.46 @ 145.9 1/4, 6.001 @ 118 1/8 so far. 4023lb !!! # N/A, Marsh performance 655ci, Indy Maxx, T/R, Indy 600-13 X's, Street legal, pump gas, full interior, Cal-Tracs, mufflers, 3:73's and real 10.5 radials. 9.51 @ 142.4 1/4, 6.003 @ 114 1/8 with our old mule KB, 572-13, 580 wedge. RHD '68 Barracuda Fastback 323ci street/strip. Best ET 13.88 @ 99.03

Re: RPM trace of soft launch [Re: Tig] #3114472 01/19/23 12:36 PM 01/19/23 12:36 PM
Joined: Jan 2003 Posts: 19,363 Las Vegas Al_Alguire I Live Here
Al_Alguire I Live Here Joined: Jan 2003 Posts: 19,363 Las Vegas	The top one is likely mostly converter, gearing and carb or a combination of them. The bottom one is wheelspin. If you have the ability to see a accel G meter it will be your friend there. The drop in the driveshaft graph is a great indication that it was spinning the tire, once it bites it pulles the engine and driveshaft down. If you had a G meter to compare it may be a good place to look at the converter as well. "I am not ashamed to confess I am ignorant of what I do not know." "It's never wrong to do the right thing"

Re: RPM trace of soft launch [Re: Cab_Burge] #3114474 01/19/23 12:40 PM 01/19/23 12:40 PM
Joined: Jan 2003 Posts: 19,363 Las Vegas Al_Alguire I Live Here
Al_Alguire I Live Here Joined: Jan 2003 Posts: 19,363 Las Vegas	Originally Posted by Cab_Burge A little tire spin on the gear change? Thats not wheelspin on the shift, it is just the way a graph looks on the shift. The slight interruption as the trans goes from gear to gear. "I am not ashamed to confess I am ignorant of what I do not know." "It's never wrong to do the right thing"

Re: RPM trace of soft launch [Re: Al_Alguire] #3114497 01/19/23 02:28 PM 01/19/23 02:28 PM
Joined: Jan 2005 Posts: 3,635 Oakland, MI D dizuster master
dizuster master D Joined: Jan 2005 Posts: 3,635 Oakland, MI	I think there is some confusion, because if you read Andy's original post, the graphs are from two different cars. He's asking why the launch on the top graph as it transitions from 2 step to stall speed is so rounded off and slow to get there. The second graph from a different car is just an example of what he thought it would normally look like with a small "spike" after the two step comes off and then it settles in. (which as noted, in this case the driveshaft speed is showing a spike indicating spin also, but that's aside from the question). As mentioned getting the two step up close to the stall speed will do this. Also if it's in this high 2 step leave, and it dead hooks... it can also cause really really high load on the motor so it won't transition clean off the two step. I'd try lowering the two step first, but you might need to play with the suspension a little to help it spin. Depending on what it's doing it might like less air in the tire to let it get some wheel speed, or maybe even tighten up the rebound on the shock so it doesn't hit the tire so hard. Andy you are right it could probably benefit from a slightly looser converter, and slightly more RPM on the shift too, but I don't think that's causing the issue you're pointing out.

Re: RPM trace of soft launch [Re: dizuster] #3114565 01/19/23 05:02 PM 01/19/23 05:02 PM
Joined: Mar 2003 Posts: 1,632 Lubbock,TX D DavidDean top fuel
DavidDean top fuel D Joined: Mar 2003 Posts: 1,632 Lubbock,TX	The drive shaft speed on my 904 usually looks similar. I've played around with different chips on the 2 step and different 4 link adjustments including shocks. It might look better on the graph but the changes didn't really show up on the time card. Some set ups run quicker with a a little tire spin. My other car with a glide has a smooth graph. I forgot with out looking what the preferred starting line ratio is but the glide is 1.80 low gear. with a 4.56 rear gear. The 904 is 2.45 low with 3.91 rear. I think they're both within range. I've discussed this with several knowledgeable people including Dave Morgan. they told me some cars like a little slip. By the way the 904 (duster) is nose heavy around 60/40. The glide(Daytona) is not.

Re: RPM trace of soft launch [Re: dizuster] #3114712 01/20/23 12:59 AM 01/20/23 12:59 AM
Joined: Jan 2003 Posts: 31,033 Oregon A AndyF OP I Win
AndyF OP I Win A Joined: Jan 2003 Posts: 31,033 Oregon	Originally Posted by dizuster I think there is some confusion, because if you read Andy's original post, the graphs are from two different cars. He's asking why the launch on the top graph as it transitions from 2 step to stall speed is so rounded off and slow to get there. The second graph from a different car is just an example of what he thought it would normally look like with a small "spike" after the two step comes off and then it settles in. (which as noted, in this case the driveshaft speed is showing a spike indicating spin also, but that's aside from the question). As mentioned getting the two step up close to the stall speed will do this. Also if it's in this high 2 step leave, and it dead hooks... it can also cause really really high load on the motor so it won't transition clean off the two step. I'd try lowering the two step first, but you might need to play with the suspension a little to help it spin. Depending on what it's doing it might like less air in the tire to let it get some wheel speed, or maybe even tighten up the rebound on the shock so it doesn't hit the tire so hard. Andy you are right it could probably benefit from a slightly looser converter, and slightly more RPM on the shift too, but I don't think that's causing the issue you're pointing out. I think you are correct. It is a four link car with double adjustable coil over shocks so there is plenty of adjustment that can be made. Lowering the two step makes sense to me since right now there isn't a lot of room between the two step and the stall speed. A looser converter also makes sense as does more RPM at the shift point. I think the car is running low 9's right now but the Moroso calculator says it should run high 8's given the power to weight ratio. So the guy has a few tenths hidden in the chassis, tires, converter, etc.

Re: RPM trace of soft launch [Re: AndyF] #3114773 01/20/23 10:06 AM 01/20/23 10:06 AM
Joined: Feb 2008 Posts: 714 Central TEXAS!!!! sr4440 super stock
sr4440 super stock Joined: Feb 2008 Posts: 714 Central TEXAS!!!!	IMHO, he's completing the run 200 rpm below peak HP, if he races just 1/8 mile, I would go 10% steeper gear and let it eat! That would solve he soft 60Ft. What gear is in it now? Joe Without Data, you’re just another guy with an opinion.

Re: RPM trace of soft launch [Re: sr4440] #3114835 01/20/23 12:23 PM 01/20/23 12:23 PM
Joined: Jan 2003 Posts: 31,033 Oregon A AndyF OP I Win
AndyF OP I Win A Joined: Jan 2003 Posts: 31,033 Oregon	Originally Posted by sr4440 IMHO, he's completing the run 200 rpm below peak HP, if he races just 1/8 mile, I would go 10% steeper gear and let it eat! That would solve he soft 60Ft. What gear is in it now? Joe I think it has 3.90 gears in it right now but I'm not positive. Personally I think it needs a little more gear and a little more stall speed but I could be wrong. Might just need some chassis tuning to loosen things up a little bit.

Re: RPM trace of soft launch [Re: AndyF] #3114844 01/20/23 01:06 PM 01/20/23 01:06 PM
Joined: Jan 2003 Posts: 19,363 Las Vegas Al_Alguire I Live Here
Al_Alguire I Live Here Joined: Jan 2003 Posts: 19,363 Las Vegas	I get that its two different cars and again stand by what I posted. I see a lot of racepak logs for various people who send them to me. While gear is a good idea it still will need some converter work. A large "hump" on a graph is never a good thing and leaves alot of room to improve. A G meter is a very very helpful tool................ "I am not ashamed to confess I am ignorant of what I do not know." "It's never wrong to do the right thing"

Re: RPM trace of soft launch [Re: AndyF] #3114851 01/20/23 01:29 PM 01/20/23 01:29 PM
Joined: Jan 2003 Posts: 16,925 NC 440Jim I Live Here
440Jim I Live Here Joined: Jan 2003 Posts: 16,925 NC	Originally Posted by AndyF I think the car is running low 9's right now but the Moroso calculator says it should run high 8's given the power to weight ratio. So the guy has a few tenths hidden in the chassis, tires, converter, etc. For that combo and 2-speed trans, I think the finish line MPH will increase noticeably with the increase in shift point alone. Get the motor into the higher HP range = more MPH. And MPH at the finish line is nicer for the driver in "dial-in" racing (including index classes). 1993 Daytona, 5.50 at 130mph (1/8) 1.19 sixty ft (PG). Link to 572 B1 - Part 1

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