Help with Drag Racing Launch Adjustments

[06300CSRT8]

Hitting the drag strip if weather holds this sunday, the intention is to be able to drop the hammer and not pussy foot out of the hole like the last 3+ years. New tire and rear setup hopefully helps a lot, but I know some guys detune the off idle area via timing and max throttle settings. Anyone care to share some of your findings and settings? This thing is a torque monster out of the hole.

Setup is a 2.9 whipple on a 426ci low comp, 3.25 gear through a 9", 18" welds with 305/45/18 et street r tires. Makes ~820whp, 760wtq.

[mxatunerjg]

I'm a big fan of using the max throttle settings to taper the tq in with RPM. As long as it is full throttle before the lowest RPM point you hit at shifts you will not change anything other than 1st gear. Due to my background being mostly in engine building, I'm never a fan of de-tuning using ignition timing. To really make it work you have to remove quite a bit, extremely low timing is not exactly good for the engine, but it is always an option. Is there any chance you are running a VVT setup? I doubt it based on your username, but if you are I'd consider changing cam timing at low WOT rpms as well. That combined with the throttle could really soften the launch and give you the ability to ramp everything in at the exact rate your tires will hold. The upside to it is there is really no negative to doing it this way.

One thing to note. Yesterday, two of our customers broke 3.55 geared 9" pinions. One car is running a 2.9 Whipple on a 6.4. He was making 935 on a tapered in nitrous setup. The other was running around 980 HP using an F1A ProCharger and a tapered in nitrous setup. Both were making similar tq on launch in the 800 - 850 range. Neither was using any kind of "detune" leaving the line. Both cars hooked quite well running around 1.3 second 60 ft times and occasionally hitting high 1.2's. This is the first we have had issues breaking these rears in these cars so it is something we will be looking into.

[06300CSRT8]

Thanks, that is very helpful. Shot you PM on the rear end, have a few questions/concerns.

Have you ever experienced any bypass closing issues with tapered throttle settings? That was my only concern, I had closing issues on the whipple in the beginning with the bypass not getting enough of a "signal" and had to change up the hose routing as well as remove the restrictor pill from the inlet of the bypass. I am worried itll never fully close once I command full throttle.

[mxatunerjg]

I can't say I have had any issues with the bypass closing. Since it totally depends on vacuum it should not cause any issue, especially if you have the restrictor removed.

[mxatunerjg]

I was eating lunch and thinking a little more about this, how much boost are you running? Based on power numbers I'd assume you are in the 13 - 15 range. Is it possible the spring in the bypass valve actuator is not heavy enough to overcome the increased boost level, at the valve, when gradually being applied?

[06300CSRT8]

I was eating lunch and thinking a little more about this, how much boost are you running? Based on power numbers I'd assume you are in the 13 - 15 range. Is it possible the spring in the bypass valve actuator is not heavy enough to overcome the increased boost level, at the valve, when gradually being applied?

19psi now, overspinning the blower a bit hehe. Made 800whp at 16-17psi, 93 octane and meth injection. Picked up around 20whp once I redid the inlet and installed a KB 148mm throttle body, but had some slip.

I am not certain I will even have issues, was just posing the question based on previous experience. The Whipple instructions show the bypass connected to vacuum only, clearly that didn't even come close to working once I went over 14-15psi. Then I switched to vacuum/boost and it closed, but was like a turbo and would take literally a full second to close, felt like a big ol laggy turbo spooling up. Removed the bypass restrictor, issues went away.

[mxatunerjg]

Yeah you should be fine with the restrictor removed. The issue you were having is because of what I was saying in the last post. Because it was vacuum only, it was completely relying on spring pressure to overcome boost pressure...not gonna happen. The restrictor then just slowed down the process giving you that laggy feeling when you changed over. That is interesting to know, though. Can't say I've had one do the laggy thing, though I've never used the restrictor on anything with higher boost levels.

[06300CSRT8]

Thx bud appreciate the input

[Monzsta]

Sounds like the bypass could be used for launch control. Maybe rig up a solenoid and a cheap boost controller or shutdown timer to make a delay of sorts.

[mxatunerjg]

Sounds like the bypass could be used for launch control. Maybe rig up a solenoid and a cheap boost controller or shutdown timer to make a delay of sorts.

We have been down this road before. Spent a lot of time trying to come up with a way to make this work. The only way we can see it working would be to use a vacuum resevoir and a boost controller. Reason being is the bypass valve works the opposite of a wastegate actuator.

On a turbo you require boost to open a wastegate. Since the turbo is creating boost it works because the solenoid controls when and how much pressure is applied to overcome spring pressure.

Now you might say doing this on the supercharge bypass valve is the same...but it is actually just the opposite. The actuator has a spring holding it closed like a wastegate but it requires vacuum to open, not boost. The reason for this is that you are not controlling boost with a supercharger. You are contolling when boost is created, then the speed of the supercharger contolls the boost level. So anytime the throttle opens vacuum is lost to some extent. This happened until the spring pressure overcomes the vaccum applied to it.

Since vaccum is being lost you need a way to apply vaccuum that no longer exists if you are using it to contol when or how fast the supercharger comes on. This is where a vaccum resevoir comes into play. This combined with a boost solenoid could be used to hold the actuator open or even use it to gradually close the valve.

Another option we have considered is the use of a lighter spring in the actuator, but the problem is the spring also helps to hold the valve closed under boost pressure. In this case a lighter spring could be used to make it so the actuator requires lees vacuum to close. Less vacuum means more throttle more throttle so it makes it much easier to control. Then to clamp the valve closed a vacuum port after the compressor would be required to apply boost pressure to the actuator so it could assist the spring in clamping the bypass valve shut.

[06300CSRT8]

Kind of agree, we have tried this as well and it does in fact work, with just installing a duty cycle based solenoid between the bypass and the source signal, without a vacuum reservoir. It doesn't matter if its vacuum or boost that makes it open, rather its the differential pressure between whats below the bypass/wastegate versus what's being applied to the actuator. So there is the problem, with a bypass like on a roots blower, you have pressure applied to the blade of the bypass, which acts more like a throttle body rather than a true bypass like on a centri blower or a wastegate on a turbo. 12psi pushing on a bypass BLADE doesn't equate to only needing 12psi being applied to the actuator itself to stay shut, and in fact it isn't linear either. Meaning once the blade cracks open, it takes MORE and MORE pressure to close it again. A bypass on a centri and a wastegate on a turbo are like big valves or a plunger, and can be actuated just like a valve, open and shut at high rates of speed, whereas a blade style bypass must be very minutely and incrementally held open to act the same way, its very difficult.

We have never successfully controlled the total amount of boost, but instead only the ramp rate of boost. Basically by venting some of the boost going to the bypass, you can essentially HANG it open a bit longer before full boost kicks in and it snaps shut, in fact you can make it either NEVER shut or slow down the rate it does. It is damn near impossible to simply make it act as a true wastegate, i.e. max boost is 12psi and you only want 10psi. The only way I have ever seen this done is to literally use a throttle body actuator setup and control the throttle opening via a PCM.

[mxatunerjg]

Kind of agree, we have tried this as well and it does in fact work, with just installing a duty cycle based solenoid between the bypass and the source signal, without a vacuum reservoir. It doesn't matter if its vacuum or boost that makes it open, rather its the differential pressure between whats below the bypass/wastegate versus what's being applied to the actuator. So there is the problem, with a bypass like on a roots blower, you have pressure applied to the blade of the bypass, which acts more like a throttle body rather than a true bypass like on a centri blower or a wastegate on a turbo. 12psi pushing on a bypass BLADE doesn't equate to only needing 12psi being applied to the actuator itself to stay shut, and in fact it isn't linear either. Meaning once the blade cracks open, it takes MORE and MORE pressure to close it again. A bypass on a centri and a wastegate on a turbo are like big valves or a plunger, and can be actuated just like a valve, open and shut at high rates of speed, whereas a blade style bypass must be very minutely and incrementally held open to act the same way, its very difficult.

We have never successfully controlled the total amount of boost, but instead only the ramp rate of boost. Basically by venting some of the boost going to the bypass, you can essentially HANG it open a bit longer before full boost kicks in and it snaps shut, in fact you can make it either NEVER shut or slow down the rate it does. It is damn near impossible to simply make it act as a true wastegate, i.e. max boost is 12psi and you only want 10psi. The only way I have ever seen this done is to literally use a throttle body actuator setup and control the throttle opening via a PCM.

You are correct when comparing to an external wastegate, but I was comparing to an internal gate (blade style) which works just like the bypass flap on a screw or roots compressor.

Being able to actlually contol boost this way would be very difficult. Using the setup you described would definitely only control the ramp rate because of what i described assuming the spring in your actuator has enough pressure to hold the valve closed during full boost. If it does not, and it relies on boost pressure to assist in keeping it clamped shut at full boost, then a pwm solenoid tuned properly may give you the ability to contol boost. I say may because I doubt it will actually be controllable, which it sounds like that is what you found.

One part I don't think I agree with is the vacuum issue. If the actuator spring exerts enough pressure to stay closed at all boost levels (how most of the screw and roots type work in their stock form) then using a pwm solenoid without a vacuum resevoir will not be able to contol anything once boost is created. Reason being is a pressure (in this case negative) would be required to actually pull the valve open. This is the primary difference with this system vs an internally gated turbo. While the actuator itself works the same on the internal gate turbo, the diaphram is below the spring in the actuator so the boost pressure compresses the spring opening the flapper to allow the exhaust to bypass the turbine. Having a resevoir to give a reserve of vacuum to work the actuator may give enough contol to actually contol the bypass. It would be very difficult to tune the pwm solenoid, but in theory it should be possible

[06300CSRT8]

Youre forgetting that a roots blower is ALWAYS making boost, at almost full amount, which is always pushing against the blade. It is not the same as a Centri which makes next to zero boost at lower rpms. At near idle, bypass closed, I have recorded 7psi of boost on my setup. That's plenty to keep it cracked open just by venting the bypass to atmosphere.

[06300CSRT8]

And my bypass could not close using spring alone, no boost applied, once past 8psi, its not designed to stay closed at all boost levels. The boost pressure overcame the spring pressure, the bypass needed the additional pressure on the diaphragm to fully close.

So to close my thoughts...using a pwm solenoid that cycles from 0% (atmosphere pressure applied to the bypass, aka spring pressure only, which would result in it opening up from the boost the blower is making acting on the blade) to 100% (full boost applied to the bypass, along with the spring, which would result in the bypass closing entirely).

[mxatunerjg]

Yes I agree with you, if the spring alone cannot hold the bypass closed then it will act identical to the way you described. I'm assuming you are overdriving your supercharger with either a larger crank pulley, smaller supercharger pulley, or both. In that case the bypass actuator was not intended to support the boost levels you are running, so you would need to apply boost pressure to keep it closed as the supercharger builds boost. In the case of a roots or screw running a boost level that the actuator spring can overcome....things change.

Also, hopefully you didn't take that as me arguing...just conversation. I agree with everything you said, especially on a setup like you have. It is good discussion though, there are many people out there who don't really understand the workings of a wastegate, blow off valve, bypass valve etc. Maybe this will help to shed a little light on some of it for someone.

[06300CSRT8]

Yes I agree with you, if the spring alone cannot hold the bypass closed then it will act identical to the way you described. I'm assuming you are overdriving your supercharger with either a larger crank pulley, smaller supercharger pulley, or both. In that case the bypass actuator was not intended to support the boost levels you are running, so you would need to apply boost pressure to keep it closed as the supercharger builds boost. In the case of a roots or screw running a boost level that the actuator spring can overcome....things change.

Also, hopefully you didn't take that as me arguing...just conversation. I agree with everything you said, especially on a setup like you have. It is good discussion though, there are many people out there who don't really understand the workings of a wastegate, blow off valve, bypass valve etc. Maybe this will help to shed a little light on some of it for someone.

Absolutely bud, not taken the wrong way, enjoy the convo. On my 426 low comp setup, at the max rpm the blower is rated for it makes about 14-15psi. I know through many laborious convos with the Whipple guys that the stock bypass has a spring that will resist closing of the blade down to around 9in/hg, any less vacuum and the spring will snap it shut. Similar on the boost side, if the bypass isn't given boost signal (stock instructions route the bypass to the inlet of the blower which only sees vacuum or atmosphere at WOT) it will fail to close the bypass at around 7-8psi. anywhere from 9-14psi it cant shut, which isn't "overspinning" its just running more boost than a stock block is recommended for.

[mxatunerjg]

Yep, I figured you were driving the SC pretty hard. Also, with what you were saying about the way your bypass was working I knew the boost pressure had to be up pretty high.

I didn't mean driving the SC to fast by overdriving, more just driving it harder than what it was setup for out of the box. Anytime that is done the stock bypass actuator is going to be a bit of an issue.