Guys I was browsing the Tech Tips (C4 section) this morning when I came across a tech tip that I think needs to be discussed. I'm sure the intent of the tech tip was intended to mean well, but I would hate to see people do it then experience problems. The tech tip, and again I have nothing against the person that wrote it, but unknowing people will do more harm, than good with it.

The tip had to do with removing the vacuum line from the Fuel Pressure Regulator to run at the higher FP throughout the RPM range :eek:

Here is the link: https://www.corvetteforum.com/techti...=154&TopicID=2

There are sooo many things wrong with doing this, but the first and foremost is that your regulator serves a very important part of the injectors function. Due to to Pulse width vs fuel delivery characteristics you need to have a regulator to keep a constant pressure on the injectors.

I know someone here that has had a fuel pressure guage on their fuel rail is thinking that, wait with the vac guage hooked up, the needle jumped all over the place with the increase and decrease of throttle. Well that is exactly correct, and it should. If you unhook the manifold reference vac line and leave it referencing atomospheric then your pressure will infact stay rock steady at whatever pressure you set it, but something else is happening that you are unaware of. And that is your pressure in your manifold is also increasing and decreasing.

As the manifold vac decreases with increased RPM the pressure on the injectors also needs to increase to keep on constant pressure drop across the injectors. That is why your regualtor is needed. Here it is real simple.

At Idle the manifold draws a strong vacuum ~15". Therefore the regulator sees that vacuum and decreases the pressure on the incoming side of the injectors. As the RPMs increase the manifold pressure increases closer to atmospheric and the regulator then increases the pressure on the line to keep that same pressure difference across the injector.

This in the nutshell allows the ECM to maintain a constant injector size (or in tuning a constant injector constant) without the regulator active, you would be changing the tune as a function of your right foot. In tuning terms it would be like changing your injector constant with every move of your foot. There is nothing at all good about this scenerio. Your taking away the single most vital part of the fuel management system, and sending the ECM on a ghost chase.

I can't express how much of a :nono: this is......If you have performed this modification, please do yourself a favor and put the vacuum line back on the regulator. There is nothing at all good that can come from doing this.

I'm sure the author meant well by this instruction, but this is really something that will screw up your tune in a heartbeat.

Please understand that I am not trying to critisize others help, but this tip should be removed from that list before people get the wrong impression.

I would be happy to work with the author one on one and help him draft up a tech tip in his name on how to ensure that the regulator is working properly...with pressure comparisons relative to atomospheric and manifold to ensure that people with AFPR are getting the proper drop in pressure they should be with reference to manifold vacuum. :cheers:


[Modified by ski_dwn_it, 8:54 AM 7/8/2003]

 

after years of reading stuff on the internet, I have come to learn that 90% of it is :bs or misinformation, or just plain WRONG.

and also people on forums such as this and many others, act like a heard of sheep. 1 person does something, it becomes the latest craze until it blows up in there face.

don't get me wrong, there is good infomation out there too, but you need to be able to weed though the :bs

 

:iagree:

Well summarized. Just read the same thing in a book about Corvette fuel injection systems. Except the book took alot more space and used more technical terms :lol: The author had a whole section on dumb things people do that scew up the system.

Perhaps we can get agreement here and ask a moderator to remove the tip.

 

That one scared me too !!! :eek:

 

Again this is nothing against the author.....I'm sure he meant well.

But a Mod should really take a look at this and remove it. Its one of those easy things to do, that I can see people running out and doing, then having problems they can't figure out in the future. :nonod:

Wel have all been there. For years till I knew better I watched my Dad remove the TV cable from his tranny when racing! Still can't figure out why it never failed, but the same thing...someone told him to do it and he did. I talked him into hooking it up and he gained 4 MPH and lost .3 sec ET. :Doh:

Live and learn I suppose. :smash:

 

I support the same conclusion......Tips should only be applied if they are verifiable from more than 3 sources to accept the tip and then be applied for approval from a 4th and final Guru of that tip for validity. I never questioned it because ......who am I?

Kitt

 

Great post! You answered the part most overlook - "what are the potential ramifications of making a change that eliminates a design feature?" Thanks a lot. :flag

 

this same tip is recommended in the corvette central catalog. And a pic to boot .

 

Unreal. :crazy: :cry

 

I'm waiting for someone to start talking about an adjustable fuel pressure regualtor :D

 

You mean the "change setting at random power generator"? :D

 

Once again I re-eiterate, when my car was stock, unplugging the FPR form the vacuum line caused higher fuel pressure throught, and made a slight improvement on the low end which I could notice.. Tested it x number of times..

If you have many mods, I am sur eyou want the exact amount of fuel your engine needs to get bets performance, but for a stock motor, worse unplugging it can do is clogg your Cat (Which happened to me after 2 years of running with an open FPR).. but that too could be just a coincidence of having a 13-14year old cat in the car..

 

Unfortunately, doing this is a hit or miss thing. Some cars it may help, some it may hurt, some may not matter.

A richer mixture down low can potentially help torque. However, a richer mixture after torque peak hurts hp.

As for the at worst statement about the cats...If an unsuspecting LT1 or LT4 owner does this and clogs up one or both cats, it's potentially a $1000+ mistake if they want new factory cats vs used or aftermarket.

 

There is no reason to do this...I don't know what else I can say. You are totally and completely messing up the Fuel management of the car. ON a dyno you would see exactly what I am talking about. Your AFR would be going up and down like a roller coaster, even at part throttle, Since the injectors and the ECM to work together effectively NEED a constant pressure on the injectors.

You see its real simple when you speak about Turnbo or SC cars. They can have a reverse effect. Since they are ramming air into the intakes, rather than sucking air, they can, if their regulators are not set up properly and a correct PSI for fuel pressure is not picked to start with, can actually force air back into the fuel system from the intake.

Now conversely in N/A cars, there is an ease of fuel delivery at idle/lower RPM since there is a vacuum present on the intake side of the injector. Therefore less pressure is needed to supply the same amount of fuel. At higher RPM that vacuum decreases and pressure is becomes the same as atmospheric and hence your injectors need a little more pressure to deliver the required fuel.

Remember the key is pressure difference across the injector. If manifold pressure increases so must FP. Conversely if the manifold pressure decreases you need to lower FP. That pressure differential on the injectors needs to stay the same whether your driving a 8 sec FI car or a 15 sec FI car...There just is no shortcuts.

Typically you can expect to see a for every 20" of vac a 10psi change in FP from a regulator. In other words. If you had 15" of vac at idle, when you connect your vac line to the regulator you should expect to see about a 7psi drop in FP.

I for one was a victim of a bad regulator that wouldn't go below 38psi, no matter what I did. My atmospheric ref FP was 43psi. With ~16" vac. I would only see a drop of about 5psi when I hooked up the vac line to the regulator. Ran the car like that for over a year. Wondering why my tune would change in weird ways. Sure it ran fine and the average person would have never known better. One tell-tale improvement was at idle the car would reak of fuel and I would too. That was because the FP would not trim down a lower pressure at idle, high vac, hence too much fuel. The computer would try to trim for it, but it can only do so much.

Had a wise person explain this regulator business to me and I followed his advice. Bingo...expected tuning results, and the bonus was no more fuel smell.

You need that constant pressure differential across the injectors..... its impossible to have that when one side stays at a constant pressure and the manifold pressure is changing. That is whyat you have when you don't have the vacuum line hooked to the regulator. :leaving:

 

ski, I didn't quite get your first post, but the last one made it evident :cheers:

flow occurs over a difference in pressure. the greater the difference, the greater the flow. our fuel pressure is 43psi, referenced at atmospheric pressure, or WOT. at part throttle, there is some vacuum (less then atmospheric pressure) so the difference is greater and therefore the flow. to combat that difference, the FPR usues less psi to the injectors under vacuum (or part throttle).

so, by unhooking the vac to the FPR, you will not effect WOT conditions, but under part throttle, you'll be confusing the computer by flowing more fuel then it's expecting.

am I thinking correct here?


[Modified by Raistlin, 2:35 PM 7/8/2003]

 

I allways looked at that tech tip with :skep: . Before I do one of those projects I do a search in the archives and try to get as much info before screwing something up...

 

Funny you should mention that....

I was going to post about that myself. Something along the lines of Bogus Tech Tip, Not So Bogus"

I believe some of you guys just have too much time on your hands to ponder, speculate and think too much. Just go out to your car and do it! By the time you log on here and post your unsubstantiated remarks, you could have taken the vacuum line off the reg, plug it with a screw, drove it around the block and changed it back again. Then you could talk from experience rather than speculate on the fallacies of relevance to this or that.

I have an 87 just like the guy who did this tech tip. I know pretty much that I need more FP because of my CC 501-8 camshaft and all the porting that's been done. And I know that right after the engine was shut off the pressure at the fuel rail valve was 39 lbs which is pretty much stock. I have an AFPR I got from Summit but I haven't had the time to install it yet.

I also know that my mileage always is about the same, between 13-15 mpg on the average mileage gauge. It's been that way for about two years now and I reset it everytime I get gas. And lastly, I know that the worst thing that could happen is the engine would run a little rich until you put the FPR back in the circuit. So for 5-10 seconds worth of work, I did it. No big deal.

Just to get things going in the right direction, I reset the ECM at the same time. First thing, it started fine. Upon driving away from the garage, the off-idle stumble was diminished slightly. On a rolling stab, the 315/35/17ZR's still go up in smoke and letting the tranny shift itself, it still wiggled some going into second. So until I get better tires than those rock-hard Sumatomo's, I won't be able to tell any acceleration improvements, if any (got a set of KDW2's on the way).

As far as mileage, it works EXACTLY like that guy said it does. I've left the FPR vacuum line disconnected and plugged for about 3 weeks now. If I go out and look at the average mileage gauge, it's at about 20.3 mpg at this time. And the car runs, idles and as far as I can tell, with the same performance it has been. But the gauge says I've increased my mileage by as much as 35%!

I will say that my mechanic who rebuilt the engine and did the porting says he doesn't understand it either nor does and Chevy or Corvette buddies I have. Again, it does EXACTLY what that tech tip said it does to that year C4. About other years I dont know and don't care.

If somehow disconnecting the FPR and letting it run wide open does something to the trip and mileagle computer to change how it reads mileage and fuel consumed, I can't say anything about that either.

I'm just saying it works like that guy says it does and doesn't hurt anything.

BIG JIM


[Modified by BIG JIM 54, 12:51 PM 7/8/2003]

 

it does change the way the computer calculates milage.

injector flow is rated at a certain psi, increase the psi and the flow increases. so, when you have increased psi at part throttle (bigger injectors) the computer has to compensate by adding less fuel, therefore calculating a higher mpg.

same thing happens when you add larger injectors but don't change the injector constant in the programming.

 

Well, I hate to burst your vacuum bubble, but your making your injectors look much larger than they are represented in the calibration file, thus your reported gas milage via the dash is all hosed up. That is something that many of the people that understand how the ECM work have known happens as soon as you change the injector size (pressure or actual swap).

From a technical and a tuning perspective your way way worse off doing what you are doing. If you think it works that great....I'm not going to argue with anyone. Its your car....and how much were those new cats?

As for going and doing it......trust me I have. With Ease software logging while I did it. And I can tell you right now as I said before the computer can't adjust or every move of your right foot quick enough therefore the BLMs go from 108 (lowest) to 160 (highest) as you go through the peddles range, for normal to agressive driving. So I guess I have tried the tip, but it wasn't from reading it it was requested that I try it to better understand what was happening when I experienced the tuning problems way back when.

If you take the time to understand how the manifold pressure effect the pressure across the injectors....you will quickly understand why its needed.

Let me ask you this simple question. What is going to be able to be filled easier/quicker with a steady Fuel line pressure of 43psi. A manifold at 15" vac or a manifold at atmospere pressure?


You can almost think of the 15" vac as a suction effect on the injectors helping it draw the fuel, hence the less line pressure needed.

With the atmopheric pressure, you have a "resistance" and therefore the line pressure must overcome that pressure.

These terms here are being used very loosely to convey the idea. So please no technical corrections.

Also it doesn't matter what "Year" C4 or even C5 you have. The concepts and the injector flow characteristics are the same, as is the physics.

Go check your miles/gal the old fashion way....Travel 200 miles, fill tank up with 10 gallons = 20 miles/gal.

I think you will soon see your 20.x miles per gal is more like 15 miles per gal if your lucky.

Jim, I know the concept is a little tricky and understanding it is about as clear as mud till you really get it. But trust me your kidding yourself if you think its helping. It would be like me sitting out under your hood, with the vac line connected and turning up and down your FP via an adjustable one at will. How can that help? Only in this case your altering the manifold side, and holding the line side constant :crazy: :skep:

Not gonna work no matter how you slice it.

Sorry for the fella that reported the first post was hard to follow. I tried to make it simple and to the point and not real technical. Sometimes that doesn't always work. :D

But the way you decribed it back is perfectly accurate. You got the idea. Now just make sure you have that pressure drop on your guage when you hook up the vac line. You will definately see an improvement there.

And its should be again stressed that this will not effect WOT as you should easily still be able to scortch the tires off the car, because this doesn't effect WOT, as that is atmospheric referece. Or manifold pressure = amospheric pressure (same as with vac line off).

Where it kills you is when your manifold pressure is much less than atmospheric pressure. Then as stated before you need that regulator to cut back the line pressure the injectors see.

Again, clear as mud right. ;) :banghead:

 

:iagree: