Ford Mustang Forum banner

Status
Not open for further replies.
1 - 4 of 4 Posts

·
Registered
Joined
·
18 Posts
Discussion Starter #1
Hey guys, I bought this 93 Mustang LX from someone who bolted a Supercharger onto its mostly stock 302 and I need your help. I'm new but learning fast. So far, this is what I got...

Accell 30lb fuel injectors
BBK adjustable fuel pressure regulator
BBK 190lph in-tank fuel pump
E303 cam
MSD ignition system w/ 6btm
Ford Racing hyd. roller lifters
Granatelli MAF
Ford Racing Headers
Vortech V-1

It's been running really rough lately, so I changed the plugs and the fuel filter. Here's where I get confused, while changing out the fuel filter, I noticed there was an in-line fuel pump installed along with the in-tank pump already there. I followed the lines to the FMU which I'm thinking about removing (along with one of the fuel pumps), but what do I do with the vacuum line connected from the top of it to the "B/R" port on the vacuum tree? It also has another line branching off of that to what I believe is the bypass valve connected to the inlet elbow.
Also, I have a vacuum line running from my throttle body to my oil fill port on the valve cover. I assume it's for the supercharger, but it's blocked off with a ball berring! Is this normal? I just wanna roast em' a bit before summer's over!! :gringreen Any help/knowledge would be greatly appreciated, thanks guys!
 

·
Registered
Joined
·
621 Posts
I've just sorted through things like that with my car.

The FMU is designed to increase fuel pressure when boost is applied. Without boost you are on the stock regulator and the FMU just bleeds back through. Depending on FMU settings, there is some restriction on the return line that regulates pressure. The amount of pressure before the FMU bleeds excess fuel pressure back to the tank depends on the FMU setting and the amount of boost. The extra pump boosts the fuel pressure (pumps in SERIES boost pressure, not volume) so the fuel could go 60 pounds or more.

The reason they do that is pretty simple.

With stock injectors one atmosphere of 14.7 pounds will push enough fuel for the system, but if you boosted it 7.35 pounds you would have a total of 1.5 atmospheres and need 1.5 times the fuel. This means if you ran 19 lb injectors at 39 pounds of fuel rail pressure, you would need at least 1.5 times 39 = 58.5 pounds of pressure. To be perfectly correct we also would also have to add back in the boost pressure on the injector, because the pressure on the port reduces the pressure delta across the injector and lowers the effective injection pressure. This means we would need 1.5*39+ 7.35 = 65.85 pounds of actual rail pressure to compensate for the boost.

The formula would be:

(B/14.7)+1) times A and then add in B to that for required rail pressure. B is boost and A is rail pressure when naturally aspirated. So 10 pounds boost would be 10/14.7 = .68 plus 1= 1.68 times 39 = 65.5 and add boost of 10 = 75.5 pounds.
With a Vortech FMU this would require a 7.5:1 disc, or the next size larger. That's why they give people a 9:1 disc in the FMU kits for 302's with about 10 pounds boost.

You have 30 pound injectors, which offsets the needed fuel 19/30 = .633 So you need .6333 times the calculated pressure because you upsized the injectors.

This is probably about a 6:1 FMU disc if you have a stock EEC.

The problem with this whole thing is you have no idea what was done to the EEC. If nothing was done to the EEC your car will be rich at all WOT speeds.

If the EEC was adjusted for the injector change by installing a larger MAF sensor (MAS) or a custom tune to the EEC, you might not need the FMU at all.

If the EEC is stock and you totally pull the vacuum line to the FMU and remove the boost pump, you can melt down the engine.

My advice to you would be to either take your car to a tuner and spend 1000 dollars or so, or buy a wide band O2 sensor that records mixture before you start playing.

I elected to buy an O2 wide band that records RPM and A/F ratio plus a Tweecer RT system. I spent almost $1000 and it took me about three weeks of reading the very poor manuals and documentation, and people that knew how to tune were generally unhelpful with only a few exceptions (pops racing, Mike Glover, and a couple others being a very helpful rarity). The fellow who owns TwEECer (Mike) was VERY helpful.

I would not give you two cents for the help from some people on one EEC tune reflector. They are generally snotty to newcomers.

This from my viewpoint as an electrical engineer and having written manuals for a hundred or more products.....the manuals really do stink. They are not good learning tools.

You have to decide whether to pay for a tune, or buy the stuff and learn to tune things yourself. If you are never going to change the engine then my advice would be to buy the wide band O2 sensor and pay someone to burn a custom chip. You would have to tell them the boost pressure, your fuel rail pressure, the injector size, the cam, the heads, and the mass air sensor information.

From that a person experienced in tuning could probably hit pretty close on a chip and you could eliminate the FMU and auxiliary pump. You would probably have to bump the injectors up a bit bigger if you lower rail pressure to stock.

Either way once you start messing with it, figure it will cost you $1000-2000 to get it right.

This is why people use the FMU with stock injectors and use the T-Rex boost pump and live with the results.

I use 30 lb injectors with more pressure, and I take the pressure up under boost with a FMU. I did this because my low speed mixture is better. I get well over 20 MPG on highway and 18 MPG mixed with full to the floor occasional urges. This is with a car that runs about 120 MPH in the 1/4 mile.

Tom
 

·
Registered
Joined
·
46,010 Posts
The auxiliary pump assists the fuel pump to feed the engine. It's usually sufficient, although going with what he said would simplify things.
 
1 - 4 of 4 Posts
Status
Not open for further replies.
Top