Direct 'Dear Dave' tech letters to dave@eyesoreracing.com. Coleman will share mind-numbing details, earth-shattering revelations, and technical nerdisms in this space each month.
Low Boost BullyQ I have a '93 Mazda MX-6 that has all the bolt-ons I could find and now I'm trying to turbo it. My goal is to run about 4psi to start with. Hopefully, at that low pressure I won't need to play with the computer at all. I am going to get a Stone Mountain Racing wastegate from eBay, which is supposed to keep pressure at 3.63psi. Should I trust these numbers, or should I expect boost spikes?
I will also be using an AEM digital boost controller. Will this help keep the boost low? My understanding of boost controllers is that they only open the wastegate later and not earlier, is this true? With the AEM boost controller could you, for example, run 5psi on an 11psi wastegate?
And one last thing: when I do start cranking up the boost (which is inevitable), what should I do for engine management? I have been looking into different solutions and about the only one that seems like it would work for my car (the MX-6 wasn't exactly embraced by the aftermarket) would be a Megasquirt, but that's a full standalone unit and needs a lot of time to program and get to work. Do you know of any other cost-effective options or possibly just piggyback fuel computers? If anyone can steer me in the right direction, it's you. Zachary N. GremillionBaton Rouge, Louisiana
A First a lesson in boost controllers, then a lesson in economics.
A wastegate is a pretty simple device. It's a valve in the exhaust stream that gets opened by a diaphragm and closed by a spring. Boost pressure is fed to the diaphragm and the valve will open as soon as the diaphragm pressure pushing the valve open is stronger than the spring pressure pushing it closed. When the valve opens, it allows exhaust to go around the turbo, reducing boost, which reduces pressure on the diaphragm. Before long, everything settles out at your desired boost pressure.
Boost controllers work by interrupting that signal to the diaphragm. If the wastegate is designed, for example, to open at 6psi, and the boost controller is set to 12psi, it will block the boost signal until it approaches 12psi, then it will work to maintain 6psi at the diaphragm the whole time the manifold is at 12.
Try to go the other way and lower the boost, say, to 4psi, and the boost controller will be useless. The controller would have to send 6psi to the wastegate diaphragm when there is only 4psi in the manifold, but it has no source of 6psi air to use for this signal.
So you're right in your understanding of boost controllers, but perhaps a little confused in the basic economic principles you're applying to your project.
You don't mention a budget for this project, but I have to assume anyone trying to run 4psi on a '93 MX-6 is trying to do it on the cheap. You also don't mention what kind of turbo you're using, but any turbo sized properly for an MX-6 running 4psi will have a built-in wastegate. I'm also going to assume that turbo came from a junkyard, where it was attached to something old. Odds are good that the wastegate you already have is set to open at something between 6 and 8psi.
So your proposal is to reduce your boost by adding an $85 external wastegate that will probably cost at least $200 to install (I'm also assuming you don't weld, so feel free to subtract that if I assumed wrong). Then you're controlling it with a sophisticated $345 digital boost controller. That's $630 you're spending to avoid tuning for higher boost.
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