Launch RPM

[Sean Marconette]

Originally Posted by terry1

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

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

[weedburner]

Originally Posted by Sean Marconette

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

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

[Jim Caughlin]

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

[Sean Marconette]

Originally Posted by weedburner

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.



Grant

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.

[weedburner]

Originally Posted by Sean Marconette

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.

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.

[Sean Marconette]

Originally Posted by weedburner

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.

No problem Grant!!

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

Sean

[james schaechter]

Originally Posted by weedburner

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

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.

[weedburner]

Originally Posted by james schaechter

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.

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...



Grant

[weedburner]

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.

[james schaechter]

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