4wd manual transmission car launch stategy and tips

As I think it's very interesting topic

because we have no ability to read current ground speed because all wheels are spinning during launch

so we cannot calculate spin and reduce torque respectively

How it's possible to do best affordable start without over shocking transmission and whole drivetrain?

Today i have found that people are using adjustable clutch release valve to let clutch slip at start

It's very interesting and useful.

Andre can you tell us what advanced techniques you have ever used on your's MMC evo drag car?

I used a clutch slipper module on my own drag EVO as well as the other fast EVOs we have built. This is a mechanical valve that restricts the hydraulic fluid flow and limits how fast the clutch can release. We made these from a modified boost control valve as it offered very fine adjustment to suit the conditions of the track.

Real St Performance did a cool little video piece on a couple of options here: https://www.facebook.com/real.street/videos/492838757547797/

Note that if you need to use your clutch to change gear down the track, you will need a unit like the Magnus one that has a bypass solenoid.

Andre thanks for your reply

Please tell me about how we can limit RPM during lauch on 4wd car?

I have vipec i88 ecu, and only way I have found- is using the time as one of axis to control rpm rise rate.

That's exactly it. Base your rpm rise rate off a timer function - Usually linked to a clutch switch. The M800 I used in my old drag car had a specific rpm rise rate function linked to the dual rpm limit feature which kind of achieves the same result in a different way.

Thanks for confirmaton)

Andre please tell me what kind of gearbox was installed on yours drag Evo?

I have seen #PPGDOGBOX in instagram photo comments)

But want to know if it have h pattern or sequental gear selection mechnism?

and have you ever used straingauge to tell ecu that you are shifting now?

Maybe you want to share some little secrets how to luanch and shift perfectly?

My old car was fitted with a 4 speed PPG dog box and a modified Ikeya sequential shifter. We just used a microswitch on the shifter to initiate the ignition cut event. A strain gauge would have been a viable alternative.

Hi Andre

Just wondering if the doxbox with sequential shifter is much slower to shift than a true sequential gearbox.

What shift toims did you achive?

The across gate shift from 2nd to 3rd is noticeably slower in an H pattern box that is converted to sequential operation. We weren't using a closed loop cut on my car so i can't give you an accurate shift time value, but I can say that I was using a longer timed cut on the 2nd to 3rd shift.

Okay thanks for answering Andre

this was a big help for me as i am still in the progress of finishing my build, it helped open my options to clutch modulation great post wow.

Good day gentlemen.

I'm joining the discussion rather late but the complications I am have seen to be related and unrelated at the same time. The application in question is a stock Evo 6 with a Link G4+ fitted to it. With no launch control features activated, the driver can get off the line quite well by feathering the throttle at around 5000 rpm and slipping the single disc clutch moderately. I've tried to activate the launch control with a steering wheel switch, and I couldn't have had a worse result. The ECU holds the engine at 5000 rpm very well, I've used the ignition advance table to build boost up to 15 PSI but when the driver tried to slip the clutch then delay the release of the launch control switch until the vehicle has some momentum the launches don't go so well - lots of bogging.

The engine speed drops down to the high 2400 rpm area and pulls really nice from there. Even when I ask him to slow the car down to an almost halt at the same engine speed (high 2400 area) then go WOT it pulls well. I'm a little worried the consistency (human error) of clutch slipping is becoming a problem and I may have the launch control limiter set too high. As a test (this is being worked on remotely) I plan to reduce the launch control limiter to an engine speed 500 to 750 rpm higher than what it falls back to, so 3000 to 3250 rpm area, reduce the launch control boost from 15 to 5 or 7 PSI and repeat the exercise.

Reading the previous posts, it would appear that the trick to a great launch on a 4WD platform has to do with slipping the clutch and that would make a lot of sense when you are observe the clutch assembly actuation of many cars performing at the top tier of the NHRA. The full clutch engagement happens very late down the 1/4 mile. On a side note, we've tested the car on wet tarmac by accident once with the the same setup which bogs badly in the dry and it worked well, so I've wondering about the engine not being able to overcome the traction threshold to accelerate the vehicle with adequate wheel slip from a stand still? Or perhaps I'm over thinking things.

If I can have some feedback on the general concept then that would be great, many thanks in advance!

I may be reading your post wrong but if the driver is holding the launch activated whilst trying to get off the line they will be trying to drive the car with a very retarded engine.

You should try as soon as the car is moving deactivating all timing retard and running it back at the full advance the car is tuned to

Thanks for the feedback Chris.

Should we be aiming for de-activation upon initial clutch engagement or complete clutch engagement? Because of where the ignition retard has been adjusted the engine bogs down to where the retard is zero. In the worked example a 2WD application was used where it was necessary to hold the engine speed until the vehicle has accelerated (gained momentum) to use the slip in a positive manner. However, it seems that it's different with a 4WD application in that the engine might struggle to slip the wheels thereby changing the need for limiting engine speed? My thoughts are that the engine speed is too high so the drastic drop in engine speed. I am hoping to reduce the launch limited engine speed and perhaps the target launch boost to reduce the contrast between launching and where I know it will fall back to and accelerate.

Am I thinking correctly or have I misunderstood the fundamental concepts?

Thanks again for responding Chris, cheers.

I'm going to need to look at the example you've given again but here is how I would do it:

When you're staging and wanting to build boost you have your set rev limit and retard the timing until your desired boost level is reached.

When the driver wants to go they will be wanting maximum power so you cannot be holding the timing in a retarded condition. So I would be running the full tuned timing and using the rev limit to hold the traction as soon as the clutch starts to engage.

Launch control is to get the car off the line, once it has started to move, then the LC should be disabled and engine control is either by the driver or Traction Control once the vehicle is moving.

You need the launch control function to disengage as soon as the clutch begins to engage otherwise the engine will bog as it has no real power if you're using a lot of ignition retard. Normally you would use a clutch switch so that this becomes somewhat automatic although even then tuning the switch activation point to coincide with the clutch taking up can be a little time consuming. Once the car is moving (some wheel spin is almost essential in all cases to get the car to launch) you could use an ignition retard timer function to control engine power through first gear if necessary but get the car to initially launch properly before you worry about this.

As a side note I tried a steering wheel mounted launch control button on my drag car and it was a total disaster. I could never consistently and repeatedly time dropping the clutch to releasing the button (I know it sounds really simple). A clutch switch fixed this for me.

Thank you kindly for the response, but I'm afraid that I will have to revisit phrasing my question for further assistance. The feedback given has given me a few things to consider and think about, so that was a huge plus.

On a two wheel drive, your launch control engine speed limiter can be set to low causing the engine to bog despite using an effective disengagement switch. Slipping the clutch has some influence here but I don't know enough to describe it correctly. I am assuming that there's some concern for the engine speed used on a 4WD platform as the engine's torque was to over come the engine's weight at rest as well as power all four wheels? In addition, I am assuming that a target launch control boost setting can work hand in hand with the engine speed limiter?

Many thanks once again for the feedback but I will have to put more of an effort into wording my request for assistance. Cheers!

You need to have a sufficiently high launch rpm in a 2WD or 4WD car to initially overcome the amount of grip from the rear wheels and cause some amount of wheel spin. If you have the launch rpm too low then yes, the car will bog. Slipping the clutch can help with this but the idea behind getting a good launch is to have a small and controllable amount of slip as you will actually generate better acceleration this way and achieve better grip from the tire.

The problem with slipping the clutch is that it's very hard to do it repeatably. I actually made a hydraulic unit that controlled the clutch release on my drag car to give a controllable amount of wheel slip during launch, but I only needed to use the clutch to get off the line, and my application was very specific. In general I find that I get better results by launching with some wheel spin and then using a launch limiter and/or boost control/ignition retard to control the amount of wheel spin once the car is moving.

I've got a fair bit to learn and understand with launch control strategies, but the new plan going forward would be to implement a clutch switch and ditch the steering wheel switch and progress from there onwards.

I'll try to post a data log so that additional feedback can be given. I'd love to better understand/functionally use engine speed rise rate. Really happy I was able to take the course and the benefit from the forum feedback.

The HP Academy has been pleasant as always, cheers gentlemen.

P.S. My apologies for hijacking the thread.