since you cant keep the stock pcv setup anyways on a boosted car this is my spot for my boost gauge reference.pass side pcv on intake mani

 

I know this is an old post, but I just saw it and wanted to put in my 2 cents. Putting boost pressure only to a FPR is wrong....the purpose of running regulated fuel system is so that the pressure differential between the fuel system and manifold remains constant, so that the injector flow rate is always the same.

My explanation here will be a little bit lengthy, but hopefully will make this make sense. Let's say for a minute that you fix your fuel pressure at 58 psi on your fuel pressure gauge. When your manifold pressure is equal to atmospheric pressure (meaning you're at 0 vacuum, 0 boost), you are getting 58 psi pressure differential between the intake manifold and the fuel system.

Now, let's say that your car is sitting there idling somewhere around 16 hg-in of vacuum, which I believe is somewhere around -7 psi. Now, the pressure differential between your fuel system and intake manifold has gone up to 65 psi. You can think of this as vacuum in the intake manifold helping to "suck" fuel out of the injectors, making them flow more fuel than when the intake manifold was at atmospheric pressure.

Conversely, if your manifold pressure is positive (meaning that you're in boost), the pressure differential between the fuel system and your manifold drops. If you're at 6 psi boost, now the pressure differential is 52 psi, meaning that the injectors will flow less because the positive pressure in the manifold is pushing against the flow of fuel.

The example above is the scenario that cars with returnless fuel systems experience, which is why in the tune they use a sloped Injector Flow Rate table. The flow rate of the injectors changes with manifold pressure.


Now, let's look at a pressure regulated return style fuel system. We'll use the same base fuel pressure reference, 58 psi at 0 manifold vacuum. What the 1:1 FPR does is adjust the fuel pressure so that the differential between fuel pressure and manifold pressure remains constant. When you get into boost, fuel pressure goes up. At 6 psi boost, your fuel pressure would go up to 64 psi, so you still have the same 58 psi pressure differential. At -7 psi vacuum, your fuel pressure would go down to 51 psi, which still keeps the pressure differential between the fuel system and manifold the same.

That is the whole point of the 1:1 FPR. If you hook it up to a boost only source, you are defeating the purpose of using this type of system. It needs to see vacuum and boost, which means that it needs to be hooked up behind the throttle body.

 

Here's a visual reference for those who are more visually inclined. Pay attention primarily to the red pressure differential line.







You can see that using a boost only source really messes with the pressure differential, which means your injector flow rate table in your tune would have to be a hybrid of constant and sloped to make it work correctly. You could do it, but you would have to make provisions for it in the tune. Much easier to just change the pressure source to behind the throttle body so it sees vacuum, plus lower fuel pressure at idle will make the injectors (especially the big ones needed for FI applications) easier to tune at idle and part throttle. It's also easier on the fuel pump, since it's not having to supply fuel at quite as high of pressure.

 

Here's another opinion on the matter:

 

when you let off the gas after being wot the ecu will shut down the injectors to let the motor spin down, so the effect would be minimal.

 

exactly.with a reference before the TB your fuel pressure only changes ones youre in positive pressure, meaning it "should" be fairly easy to tune for all pressure less than 1psi.i can see how having it after the TB would be the "correct" way to do it.But it appears from a tuning standpoint and a variable standpoint,its easier to do it B4......I have mine before and the car idles right @ 14.7:1 and doesnt go stupid rich at tip in or or tip out.thats with 60# injectors and 60-65 psi base fuel pressue.

 

When you let off the gas after WOT with a turbo setup you will get a charge tube pressure spike before the BOV opens, which will then cause a fuel pressure spike, which will translate to a large increase in the fuel pressure, and a large shot of fuel, all AFTER the throttle has closed and the injectors SHOULD have cut way back. All of this will happen before the PCM has a chance to shut off the injectors.

It is NOT easier to do it before. You will have to make a custom IFR table that is a mix of both fixed and variable injector flow rates. It would be possible to tune around it, but it's not the ideal way to do it and your tune will never be quite right. From the factory all cars with a FPR are referenced AFTER the manifold, where they read vacuum.


I'm sorry guys, but on a regulated setup it is not right to allow the FPR to read only boost. You're only getting half the equation that way. The fuel system and injectors should be responding to what is happening in the manifold, because that's what is actually happening inside the engine. If you fuel based on conditions outside the manifold there will always be a margin of error, sometimes small and maybe insignificant, but sometimes very large. There is absolutely no benefit to doing it the way you guys are talking about, but there are several drawbacks.

If you guys have it set up this way and are content with it fine, my purpose is not to tell you to change your setups. My purpose is to try to avoid spreading misinformation to anyone who might turn to this thread for instruction.

 

I run 116lb (96ers @ 3bar) injectors. No problems with idle, idles at stoich -- in fact it runs at my commanded AFR mas o minus 1% across my entire VE table... My FPR is referenced from the manifold. I can't imagine how putting the FPR before or after the TB would matter at all on idle fueling -- at idle, the charge pipe and the manifold have equal vacuum.

The point about getting your reference before the TB is this -- your charge pipe sees boost when your manifold doesn't. That means your FPR is changing the fuel pressure -- whether your injectors are open or closed. More fuel pressure means a closed FPR, which means a higher volume of fuel in your lines and fuel rails.

I can't imagine anyway that this would make tuning *easier* by adding a bit of chaos to your fueling, I only see a huge *** headache.

 

Actually, vacuum is built behind the throttle body. The charge pipes should be around ambient atmospheric pressure, and the manifold would be seeing vacuum.

 

This is, of course, correct. Very tired now, boss

So actually if you are idling at 14.7 with your FPR before the TB, your idle tune is way off.

 

please explain how the idle tune can be way off if it idles where it should?850-900rpms with an A/F of 14.7-14.9 seems pretty accurate to me.and no you wont see vacuum in a charge pipe at idle.

 

you really think the PCM reacts that slowly?

 

Fuel trims and a fudged VE table will cover something like this.

Yes. Have you done any tuning or data logging? When the throttle blades shut after going WOT there is a small delay before the PCM has a chance to cut the fuel back. If you log and look at the data with a wideband connected any time the throttle closes rapidly there is a rich spike because the changes happen too fast for the PCM to keep up with.

 

If the car still has a maf on it and youre making any kind of power youll max out the maf and have to revert to running on VE anyways when you get into boost.So that just means you modify more of the ve table,which hardly seems like a big deal to me.Ive had the same tuner since ive owned my car and the tunes have always been great for me.maybe im just lucky.at least this is a sticky so people can see the ups/downs for each.

and ill let Trendsetter respond to your other reply,cause i dont believe that.

 

I am very aware of the limits of the MAF and how to run speed density. I have been doing my own tuning with a 2bar OLSD operating system in EFILive for several years. I'll tell you again what I told you before, it is possible to "kind of" tune around an issue like this. That doesn't make it right. It's possible to tune around a vacuum leak. It's possible to tune a boosted car in speed density with a 1 bar MAP sensor. It's possible to tune around an insufficient fuel system. There are many ways to tune around an improper setup and make it run well enough to not have problems most of the time. That doesn't make it right. The reason I am being so adamant about this is because it is a sticky, and it needs to have the right information in it because when a thread gets stickied, it becomes gold for those who read through it. I'm not picking you apart, and the rest of the info in this thread looks good to me, but IT IS NOT RIGHT to hook a fuel pressure regulator up to a boost only source.

Can the tune be fudged to make it work right? A lot of it, yes, but not all of it.
Is it the correct way to hook it up? NO.

How many screenshots do I need to post to prove my point? The PCM can't predict what you are going to do. All it can do is react to the inputs that it's given, and while that doesn't take a lot of time, it does take some time. There will always be a delay in a system that is reactive. Notice in the screenshots below that the magnitude of the error is relative to the change in throttle position. The smaller the change, the smaller the error. The larger the change, the larger the error.

Lean spike on throttle tip-in


Rich spike on throttle release


Lean spike on throttle tip-in


Lean spike on throttle tip-in

 

Very good explanation Thunder, I agree 100%! ALL return fuel systems from the factory have vacuum reference regulators to the manifold. This is as Thunder says to keep the pressure differential between fuel pressure and manifold pressure the same, whether your in vacuum or boost. With this setup, you have a flat line in the injector flow rate table. Return less fuel systems have a sloped IFR table to make up for the fact that fuel pressure can't be changed as manifold condition change.

If you switch to a return system with a referenced regulator, and don't let it see vacuum, you have to use fake values in the tune to get the correct fueling. Its like altering the IFR table to fake injector values to change your AFR, instead of adjusting the VE or MAF tables. Does it work? Yes, but in most all GOOD tuners eye's, it's not the correct way to tune because you are lying to the computer to get what you want out of it and the values aren't true.

A return fuel system NEEDS to see BOTH vacuum AND boost for with a flat value in the IFR table in the tune for TRUE fuel calculation in the PCM. Other wise all you have done is put a FMU on the car and are lying to the PCM to get the fuel you want.

 

so correct me if im wrong,but now youre arguing its not the "correct" way to tune?even if it works?if you do it the "right way" and you have a good a/f curve and no issues with tip in/out then its ok.but if you do it the "wrong way" and have a good a/f curve and no issues with tip in/out then its not ok?sounds like semantics,but then again im no tuner.

 

i think the whole point of these guys in this thread is to help some like myself that had asked the question where to reference some sources. DDnspider this being a sticky it should have correct information in it. THese guys are trying to not pick an arguement but show the correct [proper] way of doing this for the ppl that need to ask. I had asked this question on PT.net. I think for something to be a sticky it should have correct information in it. I had totally forgot that the Return systems do reference off manifold

 

look at the end of post #54

 

I didn't say the tune was wrong, I said that the tune had to be fudged to cover an improper hardware setup. Your tuner was working with what he was given, and sounds like he did a decent job. It will probably work very well most of the time too, but the key word there is "most". There is no way to tune around the fuel pressure spikes that you'll get from the pressure increase inside the charge tubes when the throttle snaps shut and before the blowoff valve has a chance to open to relieve that pressure. Normally it wouldn't matter, because if the FPR was plumbed into the intake manifold behind the throttle body it would not "see" this pressure spike. Maybe it's not a big enough issue for you to even worry about. If you're content with the way it runs, then by all means, leave it alone. I'm just trying to help out others who may read this thread.


Just as an example:
Say your fuel system wasn't quite up to task for your setup. When you command 11.5:1 AFR on the top end, it leans out to 12.5:1 and you can't get it any richer. So you set the PCM to command 10.0:1 AFR. Now the AFR up top reads 11.5:1, right where you want it. Does this work? Yes. Did it solve the problem? Yes. Is it the "right" way to set it up? No. Would I feel comfortable running my car like this? Hell no. I would upgrade the fuel system. Are there other people who would feel comfortable running their cars like this? Yes.

That's essentially the same type of situation we're talking about here.