sfi tuning question

[bonezdebomb]

i have a 95 3100 ciera and i was wondering if i could swap injectors and turn up the fuel pressure without chipping or tuning the ecu

[RedZMonte]

why? what is the reason for wanting more fuel pressure and larger injectors?

keep in mind increasing the fuel pressure will cause the system to be rich. and then the ecu will just trim the fuel to maintain the same AFR that its programmed to run.... If its too rich you will have problems idling, poor acceloration, poor fuel economy, premature failure of plugs.... Cat and possibly fuel pump... So the answer is yes and no.

[SappySE107]

You can only go a small increase in injector before the stock ECM hates it. Fuel pressure increase alone is possible, but injectors are likely no good.

[bonezdebomb]

how much of an increase can i run
i am adding more air and spark btw
i just need to know how far i can go without altering thhe ecm

[3400-95-Modified]

How do you expect to add "more" spark to the mix? (more as in advancing the spark or more as in "higher voltage" that these aftermarket coils claim)

Did you just go buy a cone air filter and MSD coils?


The ECM will trim fuel for a part throttle and idle condition to counter act injector changes and fuel pressure changes, but at WOT these are negated, so if you go to far then you'll be too rich, so its hard to find the happy medium without tuning/data logging. Also if you go to far the idle will suffer and the ECM wont run the engine properly since it can't compensate for much.

[RedZMonte]

If you post exactly what you are doing... Why you need more fuel, you will get more/better responces... I see you said your adding more air and spark.... can you clarify? better intake, plugs, wires, turbo, ect? more info you can give the more you will get back... What is the reason for the question? what mods have you done or are in the process of doing? did you mod something and now you are leaning out, pinging, not running right at idle, acceleration, high rpms....? i can give you generic responces all day that wont help you without knowing, "why?"

Unless your running high compression, turbo, ported heads, huge cam, ect... they you will not honestly need to "Compinsate" by messing with your fuel system...

[bonezdebomb]

im going to be running msd coils 8.5 mm wires ported intake manifolds 62mm tb and a cai i figured i might need a slight increase in fuel but if the stock fuel setup can handle that and possibly a mild cam then i wont mess with it. real thats kinda what im trying to avoid. if i need to adjust the fuel pressure a little thats ok but im not trying to do much mor than that

[ninjaguinea]

i have a question but not sure where to post it.
i just put a cone filter on my maf and it runs worse then it did before
anyone have any idea why that would be. plus i got an iac code (high rpm)

[RedZMonte]

sock fuel system should be fine, like mentioned above your ecu will make slight corrections to A/F mixture if the O2 sensor reads is leaning out slightly.. i would replace the fuel filter if you havnt done it recently and run a can of BG44K to make sure the Fuel injectors are clean and call it a day unless you have problems when driving (i doubt you will).. I would keep your stock coils handy... I have heard mixed reviews about the MSD coils..

[RedZMonte]

i have a question but not sure where to post it.
i just put a cone filter on my maf and it runs worse then it did before
anyone have any idea why that would be. plus i got an iac code (high rpm)

I would start a new thread and also include a more in depth description of exactly what "running worse then before" means... Symptoms (serging, sluggish, rough idle, ect...) but if you have a IAC code then i would start looking at that 1st... Clean the IAC, MAF..

S

[BillS]

I always see these sorts of queries as harking back to the old days when someone would increase cam or compression and need to rejet a carb.

They come out of a basic misunderstanding of modern injection. The ECM IS adjusting the 'jets' all the time and it is only if and when you get out of the normal duty cycle for the injectors you have in there that you need any changes.

I run a 3.4 with large TB, bunch of porting, fair bit of cam, headers, and it runs just fine within the normal duty cycle of the stock injectors (yes, it will be running them a little harder, but nowhere near the point where they are full open and running lean at full bore).

The only way you usually run into a need to change injectors is if you do something radical like a turbo - my 3.2 turbo required larger injectors.

Some of the guys on the site can probably tell you what you need to do to an NA engine to require the same sort of injector resizing. I'd expect it to invlove a lot of stick and compression, more than is usually used on a street car.

[torq455]

the stock tune is set by GM to run slightly rich. minor mods like CAI or bigger TB won't need more fuel psi or bigger injectors. some level of tuning is always recommended with any airflow change to make the most out of it.

[86FieroSEv6]

A lot of times I have people come into the shop wanting power adders because they feel their car doesn't have enough "punch", and I find out they have fuel pump issues that weren't severe enough to be obvious. I've also seen restricted fuel filters and the most forgotten filter of all . . . . those tiny little injector inlet debris screens.

[bonezdebomb]

thanks im glad to here these things now i dont have to worry about the fuel system. im also considering a little headwork a mild cam and thinner head gaskets (to increase compression) any suggestions?

[86FieroSEv6]

Haslet, eh? I'm from San Antonio originally. I don't know if I'd go for the thinner head gasket, you would be risking coolant leaks for not a lot of gain in compression. Plus, you will upset the alignment of the lower intake ports to the heads. A cam would be good, along with a nice set of rocker arms. If you want a little more compression, I would recommend a good set of pistons.