Hellcat help, someone smart explain

I am doing a Hellcat swap into a 71 Barracuda as you all know. I am using a Holley EFI computer and harness. The original blow off for the supercharger uses a Jeep throttle body that is controlled by the computer to relieve pressure back into the intake. The Holley system cannot duplicate this function. What I have worked out, and I am no engineer, is a vacuum controlled turbo blow off valve that is plumbed back to the intake just behind the engines throttle body into an extension we fabricated. Will this work or am I overlooking something? By moving the throttle body 6-8 inches farther away from the intake, will that affect anything? I've thought about this so much I am starting to second guess myself. I know one of you out there understands I hope and can explain the technical stuff for me.

I'd be interested to learn why the oem setup is that way. But wouldn't you be ok with a conventional bov that vents to atmosphere?

A Holley dominator can control the factory bypass throttle body

Sounds like a basic diverter/recric valve setup. I don't see why a speed density setup couldn't run one just like a standard BOV.

Isn’t it a bypass that opens and bypasses the rotor pack under light throttle so the supercharger isn’t trying to compress air when it’s not needed? It keeps the supercharger from heating up and working so hard (trying to suck through a partially open throttle body). It also helps gas mileage when it’s open because the blower is now just freewheeling and not compressing anything. How do you have the blow off valve hooked up? Can you set it up to open under vacuum? MMX has a vacuum operated bypass for the HC

I was told that if it blows to atmosphere I have just created the worlds biggest vacuum leak. My understanding, which could be wrong, is that the stock setup works just as WO23Coronet explained but that my Holley setup would not perform this function. L4staero, I would like to know more if you could help. And Goody, I'll do some research on your idea also. This build has been fun but so many little details, technical things, have me rethinking. I want to get it right the first time. Thanks everybody and I'll keep checking for further feedback.

something in front of the throttle blade can't create a vacuum leak. But you'd have to cork the other part I suppose and use a conventional bov. The OEM system sounds like a nifty efficiency trick, great if the aftermarket ecu can control it.

If you have the blow off valve setup to open under vacuum it looks like it would give a bypass around the rotor pack like the factory deal. Looks like it would work to me

I have it behind the throttle body. I was told it would damage the throttle body if it was in front of it at wide open then suddenly let off the throttle. I can't find a reference to an MMX bypass either.

Should work....Rich at Fastmanefi did somthing similar.

By moving the throttle body farther away from my intake opening, how will that affect performance?

I don’t think it’ll affect it at all, just gives it abit more volume before the blower.
The MMX bypass is a brand new thing and probably isn’t on their website but is on Facebook (that’s where I saw it). They showed it as part of their Hellcat blower swap to a 6.4 kit

My life has been spent as a bodyman/painter and I do have decades of experience with mechanics but this new stuff makes me think a little harder. Rich at Fastman may be who I talked to about the setup I have fabricated. I can't remember at the moment. I have talked to so many on several forums and pages, ordered a ton of parts and somehow managed to get this far. It does run with my setup but have never driven it to test the full throttle reaction. A miscalculation could be a $20,000 mistake.

I was told the Holley system can control the bypass valve. I don't know that for fact, but I do know that the MS EFI can control the bypass valve as it has been used on resto mod hellcat/Hellcrate builds.

If I'm seeing this correct, this thing will never make boost since you removed the bypass throttle body. The boost air will be returned to air inlet. Your BOV will do nothing since it already has no boost.

Not brand new, Whipple uses them.

The bypass has been hollowed out and capped off inside. Basically just using the modified body. Air goes into the supercharger, squeezed up into the plenum and down into the heads. I blocked the bypass at the bottom before the supercharger and added the tube for the BOV on the topside from the plenum. It is vacuum controlled and plumbed back behind the throttle body into the supercharger inlet. Under part throttle it should bleed off the pressure but under hard throttle it should close to build pressure. At least in my mind that's how I have it figured but just want to be sure. That's the best way I can describe it in my non technical way. That's why I'm here asking, because I know there are some really sharp people here that can tell me yes or no. I do appreciate all the input. I'm learning .

I mean it’s a new kit for MMX specifically for the Hellcat, I realize they’ve been used forever (Eaton used them way back in the 90’s and maybe even before that)

Yes the Dominator will drive both the DBW throttle body and the bypass valve, but that is NOT what you want to do. The OEM ECU keeps the bypass closed most of the time. They open the bypass to control the rate of boost and serve as a blowoff valve on throttle release. The Dominator would drive the bypass valve proportionately (in a linear fashion) which you do not want to do.

I have done two Hellcat swaps and I replaced the bypass valve with a generic vacuum operated blowoff valve. That works GREAT and since the blowoff bleeds back into the same port that the bypass valve did, it's very quiet with no blowoff hiss. The Holley tune needs to take into consideration that the boost will come on quick and timing must compensate for any condition where the engine might be lugged at a lower RPM, but that's normal stuff.

Both cars are a blast to drive.

BTW: I have done this with both stock and Magnuson blowers and I tuned both knock sensors as a precaution because increasing the boost is very easy to do. Last one was 850 HP at the rears.

If you want more info just contact me.

Thanks Rich. The smart guy helping me with this build is Bervil Hillis. He has been in contact with you and will probably be again when he sets up the tune. We have a basic tune installed and the car runs but we have not driven it yet. Just happy to hear it run for now. I'll post pisc in a bit of the BOV setup we made. I would like your input if you don't mind.

Here is the blower setup with the top off.

And here is what I built.

Here it is assembled

That'll work.

Thanks Rich.

Here's what we did. Same approach just less plumbing:

https://www.fastmanefi.com/photos?lightbox=dataItem-jjqb0gk7

AeroMonte,
I hate to say it but this seems like a wasted effort. While the supercharger might use the same size throttle body as a particular Jeep application, the logic is reversed. It is essentially programmed inverse of the inlet throttle body, so when the inlet tb sees ~4.8V it is completely open and when the bypass tb sees ~4.8V it is completely closed. Even the Whipple 4.5L supercharger for the Hellcat/Demon utilizes the stock bypass valve.

The reason that FCA does it this way is so they can program the bypass tb to completely close at a certain throttle position to build boost when they want, and when you let off the throttle, the bypass will snap open, recirculating the compressed air. "E-actuators" are the new age of wastegate actuators are the same way. The Eaton/Magnuson/Edelbrock roots superchargers use a more simple (and less expensive) pressure pneumatic actuator with a light spring that it has to overcome. It works, but isn't programmable like the one on the Hellcat/Demon superchargers.

Again, the factory system would have worked just fine.

The system will work if it is like you described. The vacuum source needs to come from after the inlet air TB. Vacuum is created when the throttle is closed, thus opening the bypass valve to reduce parasitic losses at idle or part throttle. It isn’t controlable like the factor e-bypass but it works well. The e-bypass allows the oem to manage boost to tailor the torque curve to their liking. With the added length in the ducting, Inlet pressure will be a little lower which will affect the vacuum response a litle on throttle closing. The long bypas duct will affect response also.
The valve does act as a blow off valve. It is a bypass to reduce losses. But if it fails, the unit will have a high chance of failure. A closed throttle with a closed bypass can heat up the unit and seize it in seconds especially at high speeds.
If someone is making an ecu for the hellcat, it should include the control of the bypass, IMO.

The vacuum is fed from the black rubber hose at the stock location just behind the intake inlet tube I made. This car will not be raced, just a fun cruiser. It will have a couple of dyno pulls on it though just to see where it's at and to fine tune it. I can change it back later if Holley refines their system to use the stock set up. Thanks everybody for your input.

According to Ray Barton's shop, the Dominator can do just about anything you need it to do. Personally, I chose the Mopar, unlocked PCM and wiring harness for my build. It's MADE for the Hellcat; it's unlocked and can be tuned (what's not to love). Much more simple arrangement and cleaner under the hood. Whatever the choice, your car is going to be an absolute beast given how much less it weighs compared to the factory cars they come in.

I used ms3 gold box/harnesses on all my hemi swaps. I don't know if they have them available with a program for hellcats, if not, surely they will eventually. Around 1300 for all and fully tuneable. I don't understand why people spend so much more to use a stock ecm and custom made harness of other more expensive options.

We used the Holley EFI because we have experience with it on other cars and the automatic trans can also be programmed with it. It has been a very easy swap other than this BOV issue, which I just wanted to be sure that I have it right. The Hellcat presents a few issues that other G3 swaps don't have and I have two other people helping with input on this build.

In my case, I don't like all the smaller company systems that are out there where in a few years, they may not support it any more or they may even not exist. I was going to use the Dominator until Mopar came out with the Hellcrate and the ECU / wiring harness to go with it. Unlocked and fully programmable. With what's being done by guys like HemiTuner on the factory stuff, well, that's good enough for me. Keeps my car all Mopar too. The only stuff on my car that's not Mopar are my TTI headers; Baer brakes and custom Moroso oil pan. Oh, and AlterKtion front suspension. I'm and old "old school" guy, so that pleases me.

B5 Classic, MegaSquirt has been around since 2001. For a long time, that was the choice of ECU for the Drag Week guys because you could build the box yourself and build in your own features. You can make it exotic or simple, and either way it costs far less than a Holley or similar setup. There is a MASSIVE following and awesome support either from DIYAutotune or on the MS forums/groups. The support isn't getting anything but better every day. I personally use an MS2 Extra ECU and it has all the fancy stuff that Holley, FAST, FITech, Mopar, etc have. Auto tune, customizable VE and AFR tables, AE enrichment, all the bells and whistles and it cost me $750. The MS3 Gold Gold Box and Harness kit was made specifically for the Hellcat. Remember that Mopar especially has to get their 400% markup for anything they sell.

Don't forget they are made in the USA also. I'd bet the customer support is better than the rest too.