I have not installed my vortech fmu yet but when I do I am planning on running 5 psi daily and a little more at the track. I have a 12:1 fmu with stock injectors 255 fuel pump. I am wondering if I would be able to tune the fmu if my a/f were to be too rich at lower psi's. I was thinking about running a mechanical boost cotroller in the vacuum line to the fmu. This in my opinion would be a good way to have a controlled orifice which I could adjust with my boost , cutting fuel at lower psi, if need be.
Has anyone ever tried this?
What does everyone else think?

I'm not an expert on how they work, but I'm pretty certain that setup will only lean out your car. Running a MBC in line with it would make it see only a set amount of boost (or none at all depending on how it's set up) and the FMU would not be doing its job..

But I believe you can change out the disk inside to change the rate to like 10:1, 8:1, etc.. not that you'd really want to unless you used bigger injectors. Then you have to worry about too high of a fuel pressure at idle.

Well that is what I was getting at. Leaning my fuel mixture during boost if it ends up too rich without the mbc installed

Well that is what I am getting at. If my a/f is too rich when boosting on 5 psi. I will be able to turn the fuel pressure down a bit without changing disks. I think this would be a good mod for anyone. Even people who have bigger injectors but they would obviously need an afpr to deal with the idling problem.
Sorry about the previous post, I didn't think it posted.

That won't work how you envision it if you're using an MBC. if you use a bleed valve in the line, perhaps, but an MBC will just make you horribly lean (mbc closed) and then rich (boost overcomes the mbc spring).

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Maybe I don't understand completely how an mbc works. I thought that it is just a variable valve like a carburator has, with a port and tapered needle. Also how does boost overcome the spring if it is made to control boost.

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If a sentence found online has 35% misspellings or greater and includes at least two racially charged expletives, chances are it is a YouTube comment.

'95 Eclipse TurboGS (garage deco)
'95 TSi AWD (restoring a survivor)
'97 Talon ESi-T (poor impulse control)
'99 Eclipse RS-T (daily beater)
'13 Evo X (mostly stock)
'17 Sienna (Middle Aged Dad Mobile)



Factory Service Manuals: http://nawdu.de/files/

Ok, I get it, that's simple enough but if you crank it up enough it will creat a restriction in the line correct? If I were to run 5 lbs. Of boost and I only wanted the fmu to act on 3psi of that it wouldn't work?

Some manual boost controllers work like a bleed valve. So you would have to be careful which one you got if you chose to try this out. I think the problem is your not going to be rich enough to worry about. I think you might actually run a tad lean on stock injectors. Get a WB and find out how its running and go from there.

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I agree with the above. You probably are going to run lean as you approach 8-9 psi on stock injectors. That is especially true if you have the standard 255 lph instead of the high pressure version. If you find it interesting, the original FPR that came with the HRC kit had a bleed valve like this:

http://www.clippard.com/store/display_details.asp?sku=MNV-1K

The outlet had a barb, small piece of vacuum line and a plastic check valve. The valve is a needle style intended for control of air cylinders. It will it reduces the slope of your fuel pressure curve. The check valve keeps your fuel pressure normal under vacuum conditions (so you don't run richer at idle). The S-FMU had the option of using a bleed valve as well. It also had a ball/spring relief that flattened the fuel pressure curve after a certain boost was reached (like a ball/spring manual boost controller).

Corbin
'95 ESI-T

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