fuel system

[tyhoppy]

ok. I have a 91 eclipse auto. I read that the auto's should have fuel pressure set to 43 and manuals to the 39 or somethign like that. well. I have a computer out of a manual, I'm pretty sure, with 450cc injectors. do I set it to the 39 or the 43. and if I do have the auto computer in it with the 450 injector should i set it to the 39? Thanks. -Tyler

 

[LaN-]

what are you trying to accomplish? is your car staying auto? Why are you putting in 450s? Did you upgrade to a 14b? More details please

 

[tyhoppy]

14b in it. staying auto. there were 450s in it when i got it, but i took the computer out and put the one out of my manual 1g in it. but that manual 1g had 390s in it, so I'm not sure what injectors should be with that computer. I got a print out of the numbers on the computer so i can see what it was originally out of. if it was out of a manual i should set at 39, and if it was out of an auto i should set at 43? also, could i keep the computer and injectors if they don't match? what fp should i set it at if that's the case?

 

[snowborder714]

When you set fuel pressure, you should go by whatever ECU you have in there as it regulates the air/fuel ratio. And I don't see the problem with keeping the injectors as most DSM's came with 450's as stock injectors.

Fuel pressures:

1G auto - 43.5psi
1G manual - 37.5psi
2G - 43.5psi

All of these are taken with the vacuum line at AFPR disconnected and the car warmed up at idle.

Please use the search button as the fuel pressure numbers are on a bunch of different threads.

 

[tyhoppy]

Ok. thanks. I was just confused because of the different injectors and computer. I confirmed today that the computer is out of a MT, I'm not sure why that MT had 390s on it though. I now have the MT ecu, and 450s to match, as well as the 14b turbo. So I will set the Fuel pressure at 37.5 Being that this has 2g pistons in it, the compression ration is raised, should I increase fuel pressure to compensate for this, being that the computer puts in the amount of fuel for the lower compression stock pistons?

 

[oldman]

Ok. thanks. I was just confused because of the different injectors and computer. I confirmed today that the computer is out of a MT, I'm not sure why that MT had 390s on it though. I now have the MT ecu, and 450s to match, as well as the 14b turbo. So I will set the Fuel pressure at 37.5 Being that this has 2g pistons in it, the compression ration is raised, should I increase fuel pressure to compensate for this, being that the computer puts in the amount of fuel for the lower compression stock pistons?

NO, compression ratio has nothing to do with fuel pressure.

 

[Spyder4g64]

CR's do not affect fuel pressure. Agreed.

 

[tyhoppy]

Ok. but wouldn't higher compression need more fuel? But I guess that is what a tuner would be for.

 

[oldman]

Ok. but wouldn't higher compression need more fuel?

Nope, where did you get that from?

 

[AKGEN1DSM]

No a higher compression ratio won't need more fuel, just a good tune.

 

[oldman]

No a higher compression ratio won't need more fuel, just a good tune.

Why would he need a tune if he has a MT ECU and 450s?

 

[tyhoppy]

I was just wondering. i figured higher compression would need more fuel than stock compression.

 

[oldman]

Increased CR simply means you're increasing the engines VE, Volumetric Efficiency (no different than upgrading CAMs), by increasing amount of air intake per stroke. ECU calculates the amount of time to keep the injectors open (IPW, injector pulse width) by using the airflow readings from the MAF and RPM to estimate airflow per rev. Increased CR means higher airflow per rev at the same rpm and ECU will automatically compensate for this by increasing IPW.

ECU does not know nor care what the fuel pressure is, it will always assume that the base fuel pressure is at 37psi for 1G's (except for auto which is 43.5psi) and 43.5 for 2G's. Changing the fuel base pressure changes the injector flow rate, much like swapping in larger injectors, and will then require tuning.

New flow rate = SQRT (new pressure/old pressure) x old flow rate

For example, if you increase your base pressure to say 45psi,

New flow rate = SQRT (45psi/37psi) x 450
New flow rate = 496cc

Without proper tuning, you'll end up running pig rich because ECU operates base on the fact that you have 450s. Hope that cleared it up.

 

[architechnik]

Excellent description for the fuel ratios, oldman.

I am somewhat confused about swapping the AT and MT computers, though. I thought the two computers were not interchangeable - for the controlling the shift points or engaging the OD solenoid or something. I'm not entirely certain; could someone elaborate/explain differently?

 

[oldman]

I am somewhat confused about swapping the AT and MT computers, though. I thought the two computers were not interchangeable - for the controlling the shift points or engaging the OD solenoid or something. I'm not entirely certain; could someone elaborate/explain differently?

There is no problems as long as you swap the rest of the fuel system with it, mainly injectors and FPR, automatic transmission functions are controlled by a separate computer known as the TCU.

 

[tyhoppy]

Thanks. That explained it perfectly for me. I don't know why I wasn't thinking that it would read the increase in air flow.