Launch RPM
 

I run a 327 68 Camaro at 3300lbs and i have a bog at launch. M22 trans with a dana 60 rear, 5.38 gears. I currently do not run a 2 step and launch around 5K by ear. should i slip the clutch or side step at 7K? I'm wanting opinions from experience, thanks.

Re: Launch RPM
 

What kind of clutch? And why not have a 2 step? From a consistancy standpoint a 2 step will allow you to mat the gas and leave at a set RPM every time and allow you to concentrate on the tree.

A stock trans and aggressive clutch is not going to have a good outcome if it hooks. An alternative to not hurting parts, is to add air to the rear tires to aid in a somewhat controlled wheel spin. If you launch it at 7000 RPM and it hooks you are liable to find the weak link in the trans for sure. We only side step the clutch that is adjustable to slide. Even an adjustable clutch set too tight can tear up parts. Just some quick thoughts.

More information is needed on the engine and clutch. Is the car street/strip or strip only?

Sean

Re: Launch RPM
 

What is the first gear in the trans?

Re: Launch RPM
 

If you are not using a 2 step and the throttle is not to the floor the secondaries have not opened. So Back in the day (mid 70's) it took a lot of work to make the carb work right with the high pressure clutches and tires we had. Go buy a 2 step makes it easier. One less thing to worry about. Don't those chevy transmissions have a real high first gear like 2.20 or such? That is a problem also.

Re: Launch RPM
 

If you want less bog, i would recommend reducing your clutch's clamp pressure to more closely match your engine's power. That will raise your engine's average rpm, which means your engine will be able to produce more power over the same amount of time. It also reduces the amount of torque your clutch will draw on launch and after the shifts, which will help keep your M22 alive.

On launch, less clutch slows your rate of rpm loss when you dump the clutch, so it takes longer for engine rpm to drop far enough to sync up with wheel speed. By delaying the sync’ing of engine rpm and wheel speed (clutch lockup) to a point farther down the track, the car will have had more time to accelerate so it will be traveling faster. Basically, rpm won't dip as low because the car will be traveling faster when things sync up. Same thing happens after the shifts, which means recovery rpm will be better after all the shifts down the track. It all adds up to the engine being able to pack more revolutions of power production into the run. A 500 rpm improvement on after shift recovery is not uncommon with a well matched engine/clutch package.

If you simply air up the tires until they spin you will get much the same effect on launch, but there will be little if any spin after the shifts which means engine rpm would get dragged down lower after the gear changes, effectively lowering your average rpm. Adding air also reduces traction which helps your M22 live thru the launch, but since the tires won't likely spin after the shifts, your M22 would then still suffer the full hit from your current clutch.

The benefits of lower clamp are not limited to those with adjustable clutches. Fast Coyote Stock guys have it figured out, using non-adjustable diaphragm clutches with great success...

http://grannys.tripod.com/classracer101516.jpg

Re: Launch RPM
 

I believe that a M22 Muncie does have a 2.20 low gear, not even close to what you need for a decent launch on slicks. And if you are using a non adjustable "Street/strip" style of clutch, even more of an issue. Is this a bracket car, or street strip application? If its a drag car, I wouls sell the M22 before you break it, and buy a Jerico,or G Force, with a low gear in the 3.0 or 3.1 range. And an adjustable low pressure clutch setup. If its a "street" car, (kinda doubtful with that rearend gearing.), you will always be living with compromises.

Re: Launch RPM
 

What would be an acceptable drop in RPM between the launch RPM and when the clutch is released?
Terry K

Re: Launch RPM
 

Something has to slip to get a manual car launched, there are a few schools of thought...

Most common is that slicks need a few rotations of the tire off the line, which helps keep the car from bogging too hard until the car has a chance to gain some speed. Problem is the engine likely does not have enough power to break the tires loose on it's own, so the clutch needs to hit hard enough to cause some rpm drop. That rpm drop is evidence of the engine borrowing power from it's rotating assy to get the tires turning. If the tires are pretty closely matched to the conditions, it won't take much rpm drop to get the tires turning. If the tires stick too good, it's going to take more rpm drop to break them loose.

Personally I prefer to keep the tires stuck and let the clutch do the slipping, which makes it possible to leave without any bog or rpm drop at all. This maximizes power production by raising your average rpm during launch, and after all shifts made using the clutch. The fast Coyote Stock guys i mentioned above use this method.

Re: Launch RPM
 

This is a strip only car. Current clutch is a non adjustable Ram powergrip (Sintered iron/Organic). The trans has a 2.20 first gear. Engine is a 12.5:1 iron headed 327 running on VP 110. Is there a way i can get the clutch to slip more on the launch? I will try raising air pressure.

Re: Launch RPM
 

Just curious about the Coyote Stock guys. What transmission are the fast ones using? 4 speed, 5 speed?

Re: Launch RPM
 

They are allowed to select from a menu of approved 5 and 6spd transmissions and ratios. 4th is always 1:1, so they only use 1-4 and generally remove the overdrive gears. I might add that they are required to shift using the clutch, no clutchless shifts allowed. Here's a link to their '16 rules... http://www.nmradigital.com/dl/rules/cs.pdf

Permitted Manual Transmissions:
1. Tremec T5
2. Tremec T45
3. Tremec T56
4. Tremec TR3550
5. Tremec TR3650
6. Tremec TKO
7. Tremec TKO-II
8. Tremec TKO500
9. Tremec TKO600
10. Ford Getrag

Re: Launch RPM
 

Here's a few options...

1- less aggressive disc
2- smaller diameter disc or reduce the effective diameter of the disc you have
3- reduce clamp pressure by either swapping to a weaker PP/spring or shimming your existing PP away from the flywheel
4- temporarily hold back some clamp pressure with your foot, basically ride the clutch out for 5-10 feet. Adding an adjustable spring loaded pedal stop can make it easier to consistently find the sweet spot in the pedal travel
5- temporarily hold back some clamp pressure with a timed mechanical device

Re: Launch RPM
 

They are using the gforce 101A trans. All those listed above were breaking like toothpicks!!

Re: Launch RPM
 

Sorry, i copy/pasted from the original '16 rules instead of the mid-season update, didn't notice that the additions were not on there...

• G-Force –G101A, GF4A – 2.933, 1.867, 1.333, 1.000
3.000, 1.870, 1.340, 1.000
3.173, 1.960, 1.346, 1.000
3.252, 2.009, 1.380, 1.000
3.300, 2.100, 1.406, 1.000
• Jerico –DR4 -
• Liberty -LCS 5000 4-Speed**
• Andrews Transmission –A431 H-Pattern 4-speed**
**Contact NMRA Tech Department for permitted gear ratios

Re: Launch RPM
 

Terry,
On my car I keep the rpm drop to around 2000 and the clutch lockup on launch no more than 0.9 seconds. .9 seconds is too loose, and is based on the input shaft moving till the engine and input shaft rpm are the same. A general rule has been .3 to .8 on lockup. If you start getting to .3 the clutch may drag the engine down way too much and then the engine struggles to recover. This is where data acquisition is money well spent and can help you maintain clutch maintenance and performance.

Sean

Re: Launch RPM
 

I've found that clutch slip time usually overlaps a bit with tire slip time, so i'm thinking your overall slip time between the engine and track is likely longer.

I'm dead hooking with radials, so the clutch does all the work. It seems the longer i let the clutch slip the faster it goes, for me around 1 sec is a good compromise. I shoot for zero rpm drop but when a 500 drop is spread over a second or so, it makes little difference.

Grant

Re: Launch RPM
 

What's the ideal launch RPM? As much as your engine, chassis and tires will handle. In my case, the 2 step is set at 8400 but the one time that it failed and I left at well over 10,000 netted me the best 60' that I've ever had. Not brave enough to repeat that but you don't see Comp cars or Pro Stockers leaving at anything less than kill.

I am in agreement with most of the other comments here. Without a good slipper clutch and some form of data logger, you're pretty much twisting in the wind. The days of 40 pound flywheels and 3500 pound dead lock up clutches are gone unless you want to go slow and break parts. My flywheel and clutch assy weighs well under 20 pounds total. There are huge performance advantages in reducing flywheel & clutch diameters and weight.

Jim Caughlin
SS 6019

Re: Launch RPM
 

I am not sure where you think I guessed on my information? My results are from paying attention to how my clutch performs. My tires are not slipping otherwise I would see it with the data. 60' and ET are best for my combination at the info I posted. If my clutch does not lockup before .9 it shows on the time slip.

Re: Launch RPM
 

I find this very interesting. Dead hooking on radials with zero drop? And the clutch is doing the work? I would expect that the clutch is slipping or the tires are spinning. Or the chassis works extraordinarily well with radials and the engine is an absolute beast! Tell me more. What type of radials? Weight of car? and what Are your incremental Times? Thanks, Jim.

Re: Launch RPM
 

Sorry, didn't mean to imply that you were anything but accurate in what you wrote.

When i saw .3 to lockup, that usually means the tires are getting knocked loose unless there's a ton of SLR involved. If my car locked up at .3 from 1st movement and the tires were stuck, it would suck the engine all the way down to around 1800 rpm if it didn't break something first.

Re: Launch RPM
 

My personal car is a 2325 lb torque arm car on 275/60-15 M/T radials, powered by a 355 with a 2 stage crossbar backed by a Toploader 4 spd. Not really that relatable to the combos usually discussed here, as it was built for no-prep. It currently uses a 2800lb diaphragm with a full face 5135 iron disc. It NEEDS that much clamp to hold 800ft/lbs for those times when the little 355 has both kits turned on. But as you can imagine, that much clamp on an iron disc is going to hit pretty hard. Now imagine me trying to launch naturally aspirated or maybe just one kit with that much clamp on an iron disc...not pretty without the ability to temporarily dial out some clutch. I'm mostly a no-prep guy, so smooth power delivery and being able to add power as i work my way thru the gears is big to me. Here's a link to that build... http://tntrc.com

The clutch control on the above car is functionally similar to what i have supplied to many Coyote Stock and Factory Stock racers. The slip i posted earlier is from one of those guys...

http://grannys.tripod.com/classracer101516.jpg

Grant

Re: Launch RPM
 

Here's a simplified example of why you want to keep average rpm as high as practically possible-

Lets say a car gains rpm at an average rate of 6k per second in 1st gear. Let's also have the clutch lock up .5 sec into the run with the tires remaining dead hooked. If it is launched at 6k, tires are stuck, and the clutch locks up at .5 sec, rpm is going to get pulled down to 3k by .5 sec into the run. From .5 to 1.0 seconds it climbs from 3k back up to 6k, regaining the lost rpm. What all this boils down to is that during the initial 1 second after launch, the engine's average rpm was 4500 rpm, which means the engine made 75 revolutions over that 1st second of the run.

Now suppose that same car launches at 6k, but the clutch slips just enough that the engine does not lose any rpm over that same 1 sec period. Now the engine's average rpm was 6000, which means it made 100 revolutions during that initial 1 second period.

Both leave from the same rpm, but the launch that didn't lose any rpm actually packs 33% more revolutions of WOT power production into the same 1 second time period.

Re: Launch RPM
 

Pretty cool! How much rpm does it spike the tires on the gear changes? Sounds fun. Any video? Thanks, Jim

Re: Launch RPM
 

About 500 less than you would expect from the ratio change.

Re: Launch RPM
 

Typically, in a our stocker we see a 200-300 rpm spike of driveshaft rpm at each gear change.

Re: Launch RPM
 

No problem Grant!!

Your are right on the money with the SLR. The .3 definitely drags the engine rpm down

Sean

Re: Launch RPM
 

If the clutch is used on the shifts, there's little if any spike. Because the slipping is basically a hydraulically timed event that re-sets when the pedal is bounced off the stop, rpm loss after the shift occurs over an extended time period much like the launch. Rather than jerking rpm almost straight down until a PP with centrifugal can relax, the loss will look more diagonal on the graph. Because the car is gaining speed while the clutch is slipping, spreading rpm loss over more time reduces both it's quantity and intensity, making that returned energy more likely to do productive work rather than go up in a blip of wheelspin.

If you are clutchless it can still help you launch higher, but you will still need an adjustable/centrifugal to get some slip on the shifts.

Grant

Re: Launch RPM
 

I went to your link. You have done some really cool and well done modifications there. I see how you get the radials to work as well. In our world, the tradeoff is not likely to benefit us, compared to what we are able to legally run, but I can see where it is better for your combo. Well done!

Re: Launch RPM
 

x2!!!!! Very nice work, Grant. Definitely outside the box thinking. Super cool little car. Must be ridiculously fun.

Kevin