qwiketz

Hi guys. We're working on tuning my ysi supercharged vette. We have the idle awesome and part throttle the a/f is where it needs to be. In addition, we're not seeing any knock; the timing is perfect.

We're having a little trouble with the fueling. I previously was running 38's and a fmu and now I'm running 60's and no fmu. Fuel pressure used to go over 90+ psi. Now it's steady at 45 psi. I thought that the fuel pressure was supposed to rise at a even rate with boost. So, I thought that with 15psi of boost, I'd have 60psi of fuel pressure. Unfortunately that isn't happening. What would cause this?

I have a 92 with the fmu still connected (except with no vacuum or boost source). I don't know if something happened to not cause the fuel pressure to rise or what. I may end up reusuing the fmu except at a super low rate like 1.66 or 2 to 1. Injectors are maxed out frm 4000 rpm up.

tjwong

You need a boost referenced pressure regulator. Normally those are a 1:1 ratio, meaning that for every pound of boost you get a equal increase in fuel pressure. Your FMU was probably a 10:1 unit which gives a 10 PSI increse in fuel pressure for every one pound of boost. I ran Glens car making 620 RWHP on a T trim with no boost refeerence, fuel pressure was set at a steady 50 PSI using 50lb/hr injectors.

qwiketz

okay. So the stock one isn't boost referenced? I was under the impression that is was, so that would explain why the fuel pressure isn't rising. It's not that something isn't working; just that it wasn't even designed to be boost referenced.

wow, it looks like I'm going to make some serious power. At this point, I don't believe that the fuel pressure is going away on the top end but I will be hooking up a boost a pump this week to assure that I have more fuel (should give me about 30% more fuel volume). I have it sitting around so no biggie.

The boost on this car should end up being very similar to glenn's. It should hit about 15lbs at 6000 rpm.

mn_vette

The stock regulator IS a 1:1 regulator. Exactly what you need. Make sure that the boost/vaccume line is hooked up to the end of the regulator properly.

Remember, when the car is at idle it actually sees vaccume so the fuel pressure should be less. When you set your pressure you should have the car idling and remove the boost/vaccume line from it.

You may be having some issues with a leak in the hose or possibly on the regulator. Check the hose first, maybe run a totally different vaccume line to the regulator and see if this effects the car during idle or small rev situations while the car is sitting in the garage.

qwiketz

I'm going to take a look at it this week. Hopefully an additional 15lbs of fuel pressure at the rail will get me real close to where I need to be fuel wise.

In regards to stock regulators, did they make any changes during the LTX years or are they all the same?

qwiketz

we did an injector swap and in the process, it looks like the vacuum/boost line to the regulator came undone. I fixed it. I haven't tried running the car yet, but I'm guessing that this is exactly what was causing my problem.

qwiketz

I reconnected the line and all is well. Fuel pressure is rising as boost rolls in.

mn_vette

Its amazing how one small thing can really screw with a car. Be sure to post the new and improved dyno numbers

anesthes

I don't understand why you are just running a 1:1. The only thing I can think of is that your MAF based and your tuning a lot of it based on airflow.

Us MAP guys are gonna have a 100kpa limit, so it's either switch to a 2-bar map which I did for years and it sucks, or go to a FMU with a 10:1 raise or something close.

-- Joe

qwiketz

If I went to a 10 to 1, I'd be seeing 189 psi of fuel pressure. The pumps stop working at about 110 psi.

I'll probably go to a 3 to 1 fmu ratio which would give me a more fuel pump friendly 90 psi. A 2 to 1 would give me 75 psi. The two to one is probably an ideal comprimise between fuel pump and injector capability.

anesthes

I hear ya. Your running lots of boost. (15psi).

I used to run an FMU on my firebird, but then I got a bigger blower. Went with $58 2 bar and large injectors. In theory thats the way to go, but on a batch fire setup with large injectors it is hard to get stable idle pulsewidths without just running way to rich.

So now that I'm gonna be using a blower on the vette, but only making about 6-7psi of boost (motor is 10:1 compression) an FMU should work fine.. A Walbro 255 HP (high pressure) should be ok to 100psi and still produce decent volume at that high pressure.

-- Joe

qwiketz

Greg has the flow charts on various fuel pumps on his website. This is a pretty good resource. The website is www.blowerworks.net .

I ended up buying a 3 to 1 disc for my fmu. I currently have the bleed off valve, so I'll probably end up making it a 1.5 to 1 or 2 to 1 step up ratio. This would put me at 75 psi of rail pressure for 2 to 1 or about 67 psi with 1.5.

I'm going to try not to use the fmu, but if I need it, I'll have the proper gear to create the step up ratio that I'll need.

My idle and part throttle response is excellent currently so I'd like to leave the base fuel pressure where it is.