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  • CPI vs WMI FAQ

    CPI Specific Discussion Thread:

    WMI Specific Discussion Thread:

    CPI vs WMI?

    We get a lot of customer questions on the difference between WMI (water/meth injection) and CPI (chargepipe port injection) so I figured I’d do a short write-up I can refer them to in the future.

    TLDR: If you have easy E85 access CPI is generally a better choice. If you don’t have easy E85 access the WMI will provide similar power gains with a few more health risks, engine risks, and potential headaches.

    WMI adds a separate parallel fuel system to the vehicle which injects a mixture of methanol and water in to the chargepipe. This arrangement serves three basic purposes.

    1) To increase the effective octane level as methanol/water mix has a higher octane than normal petroleum.
    2) To augment the fueling system by injecting fuel in addition to the factory fuel system.
    3) To lower intake temps through evaporative cooling, the injected mixture evaporates in the chargepipe and intake thus absorbing some of the charge air heat and reducing the effective charge air temperature.

    CPI on the other hand expands the factory fuel system by introducing a 7th fuel injector, placed in to the chargepipe. This arrangement serves purpose 2) and 3) above. But does not address purpose 1), increasing octane. Instead with CPI you run a much higher E85 mixture than you could otherwise run, increasing octane that way.

    So practically speaking, what are the differences?

    Before we discuss performance and install differences it’s important to address general health safety differences:

    1) Methanol, even when mixed with water, is highly toxic to humans. And placing a WMI tank inside your car can result in dangerous evaporative fumes entering the cabin. As fluid is drawn from a WMI tank air must be let in to take its place and as a result WMI tanks can’t be perfectly sealed. To address this risk BMS developed a one way sealed vent cap which we strongly suggest for any WMI tank placed inside a vehicle cabin. This allows air to be drawn in to the tank but prevents dangerous methanol vapors from escaping. I’ve noticed some enthusiasts running black plastic caps are completely unaware of the health risks methanol fumes present.
    2) Methanol, even when mixed with water, is highly flammable. Managing fire risk is always an important part of any WMI kit. Having flammable fluid stored and routed inside the cabin increases fire risks.
    3) With CPI you’re not adding an extra gas tank so there is no increased potential for toxic fumes in the cabin and much less increased fire risk because all fuel lines remain in front of the engine bay firewall. In addition we use braided steel lines for our CPI kits providing a similar level of protection as the factory fuel lines.

    What are the control differences?

    WMI) The primary control mechanism for WMI is to alter the speed of the WMI pump, which allows you to change the injection pressure at the WMI nozzle. Although not particularly precise this lets you roughly alter the volume of the fluid injected. But by slowing down the WMI pump you decrease the injection pressure which also has a negative impact on fluid atomization. So there is a very narrow window of fluid control. In addition WMI kits usually include a cut off solenoid which is used to turn off WMI flow quickly to prevent fluid dripping in to the chargepipe or fluid being siphoned through the system by vacuum when not being injected.

    CPI) The primary control mechanism for CPI is altering the dutycycle to a fuel injector. We use a Bosch 950cc fuel injector which can provide a full range of flow from 0-100% without a negative impact on atomization. Since CPI runs off the factory fuel pressure system injection pressure remains constant. With CPI you can run much more precise fuel mapping.

    What are the performance differences?

    To benefit from CPI you need E85. If you don’t have easy E85 access then CPI isn’t going to provide much benefit and you’re better off with WMI.

    For fluid volume each CPI injector is good for around 950 ml/min of fuel, which is equivalent to around 2 BM7 nozzles (440ml/min each), and more than a single BM12 nozzle (750ml/min).

    WMI has a higher injection pressure (~180psi vs ~70psi) and thus can achieve a higher overall atomization level at full pressure. Although in practice the spray patterns are fairly similar as each nozzle or injector is designed to atomize at a specific pressure.

    All considered in my experience power potential between them is roughly equivalent. With CPI running at least E45 is generally enough to hit 500whp+ and max out the factory Stinger turbos. Similarly WMI running twin BM7 nozzles (on pump gas) or a single BM12 nozzle (on an E30 mix) can clear 500whp.

    How is reliability between the two?

    This is where you get in to some interesting differences. CPI is inherently very simple so there are fewer mechanical points of failure.

    1) CPI will never independently run out of fluid since it’s using your factory gas tank.
    2) Fuel flowing through CPI passes through the factory fuel filter assembly so the injector is no more likely to clog than your factory fuel injectors are.
    3) CPI will never independently run out of fuel pressure since it’s run off the factory fuel pump already powering the engine.
    4) The only point of failure is the single fuel injector, there is no additional solenoid to fail/clog.
    5) CPI uses stainless steel lines which are unlikely to ever leak, melt, come loose, degrade over time, etc.
    6) CPI does not require on any “quick connect” press fittings which are notoriously dangerous especially in the engine bay. Note the BMS WMI kit also does not use any “quick connect” fittings in the engine bay.

    WMI will have a 1 or 2 gallon tank capacity which must be monitored and refilled when low. Running the tank too low can result in air getting trapped in the pump which requires a manual purge with nozzle removed to clear before full flow can be restored. Using a single BM12 (750ml/min) nozzle 1 gallon will last around 5 minutes at full flow, and 2 gallons around 10 minutes at full flow.

    What are the install differences?

    Physical installation for WMI is a little more involved as it requires mounting a tank in the trunk and routing power and fluid lines through the vehicle interior to the engine bay.

    WMI requires a 1/8” NPT bung for the nozzle. This can be done via tapping the factory charge pipe, an aftermarket charge pipe with a bung welded on, or via the very clean BMS WMI nozzle adapter which places the nozzle where the factory charge pipe MAP sensor goes.

    CPI also requires a 1/8” NPT bung but cannot use the MAP sensor adapter.

    Note the BMS intercooler includes a 1/8” NPT bung perfectly suited for either.

    In either case we suggest placing the nozzle or injector as far away from the throttle body as practical to allow for better fluid atomization and distribution.

    Electronics required for each are relatively similar. If using a JB4 for example our FSB box plugs in to the JB4, is attached to 12v, ground, and then either the CPI injector or the WMI pump and solenoid.

    For those not using a JB4 there are a variety of control systems out that can run WMI, although they don’t monitor flow and cannot provide any practical tuning safety. To control CPI without a JB4 you can use any standalone fuel injector controller, such as split second. In theory you could also go really simple with an 8-10psi Hobbs switch although I wouldn’t suggest it as CPI would always be 100% flow or nothing.

    How is engine safety handled between the two?

    Ultimately this is up to the tuner and control system. For those running the JB4 we take a fairly similar approach between them.

    WMI) We monitor pump injection pressure and overlay that data with bank to bank fuel trims to establish whether WMI is flowing as expected. Boost is only increased when we’re confident WMI is flowing properly and reduced when it isn’t. This avoids “tip in knock” common with “spray and pray” WMI approaches, where the tuning is the same regardless of WMI flow. Note some systems include a parameter called “WMI status” which customers often confuse with flow monitoring. This parameter does not tell you whether flow is actually occurring, only that flow is being requested.

    CPI) We monitor current at the CPI injector to determine that it’s operating normally and overlay that data with bank to bank fuel trims to establish whether CPI is flowing as expected. In addition the CPI kit includes a flex fuel sensor which allows the JB4 to read your precise E85 mixture and adjust CPI volume requirements based on the E85 mixture in the tank. This allows customers the flexibility of switching between pump gas and various E85 mixtures without having to retune the vehicle each tank.

    Note that CPI is inherently safer since more of the octane is traveling through the primary fuel system, and less of the octane is supplementing externally, relative to WMI. That means in the event of a full external fluid stoppage (worst case scenario) the CPI vehicle would drop fuel pressure but the the WMI car would also drop all its octane benefit resulting in an increased potential for catastrophic detonation. Especially with a very aggressive timing profile.

    CPI Install Guide:

    CPI Specific Discussion Thread:

    WMI Specific Discussion Thread:
    Burger Motorsports
    Home of the JB4 the worlds most popular turbocharged tuning system!

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  • #2
    Great writeup Terry!