1G Correct fuel pressure 36.6 psi?

[RipperXX]

On a 1990 M/T turbo car, is the correct fuel pressure 36.6psi with the vacume line on?

I ask because I just installed a AFPR and it's sitting at 46psi with the line on and over 50 with it off. It has 750cc FIC injectors if that matters.

 

[GVR4592]

It's 36.3 with the line off, but anywhere around 36-37 will work.

 

[RipperXX]

ok so I set it to 36 with the line off, when I put it back on it drops to 30, is this correct? (just want to make sure for I go down the road with it)

 

[steve]

Close, what's your idle vacuum? The fuel pressure is intended to equal the base pressure + the manifold pressure. (2in hg vacuum is ~ -1psi) So, at a normal idle vacuum 18 in hg the fuel pressure should drop 9 psi with the hose attached.

Steve

 

[RipperXX]

Well I think I have a damaged intake valve or something, cause my car has had low vacume ever since the T-belt was a tooth off on both cams and I drove it for about a 1,000 miles that way :/

it reads from 10-15 vacume depending on where the rpm is at idel. Still on stock cams, so is that a little low or what? should I bump it up a tad? if so to what?


I know for one thing though with the new mods, man does it pull well to redline. for FMIC, 750cc's & DSMLink

 

[steve]

it reads from 10-15 vacume depending on where the rpm is at idel. Still on stock cams, so is that a little low or what?

Yes that idle vacuum is low.

 

[RipperXX]

I know it's low, what im asking is since it is, it seems that would throw off the the reading on the FPR, well not throw it off but not be the same flow as if I had set it with higher vacume. So what im asking is. Should I turn up the pressure some to compensate?

 

[steve]

As long as your fuel pressure = base + manifold then it's correct and you don't need to change it unless your trying to tune the car.

Steve

 

[RipperXX]

hmm it's pretty dang close then, I might bump it up a tad after I get the serial to USB connector for my laptop to use the DSMLink.

I figure with 750's and a E3 16g it shouldn't make a big difference if it's a little low anyway, they have plenty of head room to inject more fuel. Or would they? Would the computer think X is being injected when it's a little shy of it? Or would the injectors stay open a little longer to compensate?

If it wont compensate on it's own then i'll proably get picky about it, otherwise it's running perfectly fine and pulling hard to redline as is.




Oh and thanks for the help steve.

 

[steve]

I figure with 750's and a E3 16g it shouldn't make a big difference if it's a little low anyway, they have plenty of head room to inject more fuel.

Once you start swapping parts it's up to you to tune the car well enough that the ECU can stay within it's adjustment limits for closed loop and do the right things when using the open loop maps.

The fuel trims are what allow the ECU to correct in closed loop. As long as they are not pegged at their limits then the ECU is making the adjustments needed in closed loop but once you go open loop the variance the trims corrected will be reflected in the difference between the actual AFR and the programmed AFR in the ECUs maps.

I'd be more worried about why your vacuum is so low at this point since I don't see cams in your profile and your not living on some mountain top.

Steve

 

[RipperXX]

Well, I'm pretty sure it's a damaged intake valve :/ idel is a little low too, when I was adjusting the BISS screw it would change but then go back to what it was. I have a DSM manule on CD, so im going to look into that and see if theres something else i'm suposed to do when trying to adjust the idel. I think on the 2g you have to ground a pin or something. On my old 1g though I could just turn the screw.


Idel right now is around 650rpm it's low I know so thats one reasion the vacume is poor. I figure i'll bump it up to about 800rpm. And see what it reads. I seriously think theres a bad valve though cause a while back the timeing belt was off a tooth on each cam, and I drove it like that for a while :/ (yea I know, stupid)

 

[steve]

Your ECU and ISC should be working to maintain the 750 RPM idle speed that the ECU is programmed for. If it's not both need to be checked.

On a 1G to correctly adjust the BISS both the timing adjust connector and pin 10 on the DLC need to be grounded. On a 2G you need a MUT or DSMLink.

Steve

 

[Ski Bum]

Out of curiosity what would be the benefits/potential problems with raising or lowering the base fuel pressure along with adding performance modifications?

 

[steve]

Out of curiosity what would be the benefits/potential problems with raising or lowering the base fuel pressure along with adding performance modifications?

You tell me. What do you think raising or lowering the base fuel pressure does.

Steve

 

[Ski Bum]

You tell me. What do you think raising or lowering the base fuel pressure does.

Steve

If I knew what the effect was I wouldnt have asked the question. Obviously the pressure would be higher or lower depending on which adjustment is made.

Maybe I should reword it; How does raising or lowering fuel pressure affect fuel delivery in regards to injectors, timing and general tuning?

If one were to set their AFPR to 80psi, what would most likely occur. If one were to set it at 15psi what would occur. With X part is 37.5psi better or 39psi? I'm just trying to get a feel on the general affects of changing the base fuel presure.

 

[steve]

Simple. Lowering the pressure reduces the effective flow rate of the injectors by the square root of the pressure ratio, leaning out the AFR. Raising the pressure the reverse. http://www.rceng.com/technical.htm

Setting your base pressure to 80 psi is going to make it difficult to run any boost since your fuel pump isn't able to increase the pressure to track boost. In fact at 80 psi most of the pumps we run are not going to push much fuel at all since there relief valve would be open.

Both the high and low ends of the scale are going to have negative impact on the injectors spray pattern.

Everything else is secondary to the direct relationship of based pressure and injector flow rates. Changing the fuel pressure is one way to adjust the fuel trims (tune) on the car or get a little extra from your injectors before upgrading them if you have other ways to tune the car.

Steve