Recently I've seen some discussion about the so-called "6lb boost limitation" on 2004-2008 Ford F150 trucks equipped with a Roush supercharger. It's time to put some misinformation to rest. I'll discuss it here on this site, and invite supercharged F150 owners to post here in our forums rather than elsewhere. The level of PCM misinformation when it comes to the F150 found on some sites is astounding.

It's even worse when self-proclaimed experts without knowledge of this platform misapply information to the 3V F150 platform. The 2004+ F150 is a completely different beast when it comes to fueling, PCM control, etc. Frankly, I believe much of what you see is marketing bullshit meant to keep their name in front of you.

And the people promoting what they heard? Are you really going to take someone's word about this as gospel when they don't even have a boost gauge?

Setting The Record Straight
No such 6lb limitation exists. These are not Lightning F150 pickups! The Roushcharger boost by-pass mechanism is operated by the IMRC table in the PCM. It looks at 2 things: throttle position and RPM. The amount of boost is not a factor. I verified this many years ago. Try it yourself - disconnect the solenoid and route the boost by-pass in the traditional way. Your WOT boost will be the same. In my situation both before and after this change my truck put out between 9 to 9.5lbs of boost (depending on altitude and weather conditions) before the long tubes, a little less after I added long tube headers. Making changes to this by-pass does absolutely nothing for how much boost you will see.

Having said that, let's delve a little into the PCM, and the advantages and disadvantages of each method of activating the boost by-pass.

IMRC Table and PCM Controlled Boost Bypass

As noted previously, the IMRC table controls the boost-bypass in the "normal" Roush calibration. This has some advantages over a more traditional vacuum operated by-pass system. First, the tuner has exact control over when boost kicks in. Going to a race and need something which reacts quickly off the line? Set the throttle position very low and you've got it! Looking for good mileage while still having power on tap, this can be accomplished by allowing boost to come on quick at very low RPMs (stabbing the throttle from a stop) and high RPMs (downshifting to get you into the RPM range) - and setting the throttle threshold higher for cruising RPMs.

Another advantage comes from the VCT cam setup. The PCM is setup so the cam timing can be different depending on IMRC state. A tuner who knows his way around the F150 calibrations for the Roush can use this to their advantage to have a different CAM timing profile when there's boost verses no boost.

Traditional Vacuum Operated Boost Bypass

Like the PCM/IMRC control method, this method also has some advantages. First, the transition into boost can be smoother and a little more linear instead of an on/off situation. As well, when boost compared to the throttle position and RPM comes on *might* be more consistent than with the IMRC table method. I say "might" with the caveat that this can depend entirely on how your tuner setup the IMRC table. Mileage might suffer depending on how you drive and how the tuner setup your tune.

Now, there's also another advantage, useful to some, not useful to others. The IMRC table (and the wiring harness you unplugged) can be used to switch on nitrous oxide, methanol injection, or water injection. Plus you can have a different CAM timing profile for when any of these switch on. For many this is a huge plus.

Concerning CAM timing, since the PCM no longer "knows" when there is boost by-pass, CAM timing is done a little more traditionally: simply load vs RPM. This is instead of IMRC ON load verses timing, and IMRC off load verses timing. It's not really an issue for the most part if your tuner is any good, because load *is* an indirect indicator of boost. And boost isn't really what we are concerned with... air flow is.

Wrapping It Up
Now, the question is: should you do this modification and if so what needs to be done with the tune? If your tuner has setup the CAM tables properly then you should have no issues switching over to it. You'll be rewarded with a more consistent throttle but keep in mind mileage might suffer just a bit if you do mainly city driving and you have a heavier than average foot. Without seeing your tune, I really cannot tell you what's best for you. Try it out, go easy into the throttle and be prepared to back off (though frankly there should be no reason why it will be an issue). The majority of tunes out there from all the major tuning companies should have no problem with this. Personally, I prefer IMRC control, I have more control over what's going on this way.

If you're considering nitrous, methanol or water injection, then it's a no brainer to go with this method if you want the PCM to be able to control when these are switched on.

What about the MAF transfer in the PCM? As noted, this modification *does not increase boost so changes to the MAF transfer aren't needed. Even if it did increase boost the stock Roush intake setup is already calibrated with the MAF to handle much more boost than Roush's 6lb base without changes. At 9.5 lbs boost I'm seeing plenty of wiggle room and safety factor left when logging the MAF. The biggest issue with these Roush setups is the fuel pump. You simply need more pump when you get past about 10lbs (your tuner needs to have tweaked the fuel pump settings anyway for safety as Roush's setting max it out - Troyer does and I believe JDM does as well).

If you do see a boost increase from this mod there is something wrong with the way your tuner is setting up their tunes! One likely reason might be they are using the Roush ROM read as the base for their tunes instead of moving the Roush changes over to a Ford calibration and then applying their tuning on top of that. Or maybe they simply recommend this mod because they never figured out that it's the IMRC table controlling the boost by-pass?

Concerning the ROM's --- I'll be a bit more specific. If your Roush PCM code is for example XYC1RSCIC they should be using the XYC1 template as their base instead of XYC1RSCIC. From investigation in the early years of Roush's calibrations, it appears they have background timers/limiters such as a boost dump roughly at 5325 RPM (5316 if I recall correctly). I have never run into one with a background switch of the IMRC by-pass IMRC based on load (boost). Regardless of which of the above reasons, the tuner should be using the non-Roush template.