CLASS RACER FORUM - Launch RPM

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- - Launch RPM (https://classracer.com/classforum/showthread.php?t=64036)

Quote:

Originally Posted by james schaechter (Post 518033)

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...

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

Grant

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.

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

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

Quote:

Originally Posted by weedburner (Post 518100)

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

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

Quote:

Originally Posted by weedburner (Post 518059)

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

Quote:

Originally Posted by james schaechter (Post 518106)

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

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

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!

Quote:

Originally Posted by james schaechter (Post 518111)

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

Kevin