thezonster

Preparing to do a 2.5" exhaust on my 86.5 with X-pipe, Borla mufflers, fresh O2 sensor, and fresh larger cats.
Can the stock injection system deal with the increased flow/reduced back pressure...or am I asking for trouble?

SMTCapeCod

stock...what? Which system?

SHould be well within range of adjustment, plenty of limitations on flow upstream of exhaust.

Disconnecting and reconnecting the battery may hasten the adaptive process, if so-equipped.

Stromius

You're fine! I ran the stock injectors for a few weeks with my X until I refreshed the intake and replaced them with 19# Fords (0280150943's) last week. Doing fine....carry on.

SMTCapeCod

WYAIT- depending on model year/application, consider adding O2 sensors to the list of new parts and baselining the CO/MAF settings, too.

thezonster

Did the larger injectors give noticably more power, by themselves?
How did they impact fuel mileage.
I just completed by intake refresh 2 weeks ago. So would need to go back in to do that.

SMTCapeCod

I'll just assume we are talking LH here then.

For a given amount of air, the injection system will request a given amount of fuel. When larger injectors are substituted for smaller, then the system will adapt based on 02 readings for a shorter opening period. Amount of fuel delivered is basically the same. The benefit to the other injectors-- on a stock car-- is a newer flow pattern with better atomization- a finer mist of fuel. I did a similar retrofit on my Jeep and did get a smoother idle, slightly improved mpg. On my 928, I did several other items at the same time, so I can't say for certain. I can say that I doubt there is measurable power increase unless you are replacing worn OEM type with newer injectors and thus mitigate the wear and possible occlusions.

NOte that OEM S3 injectors flow at a higher rate than OEM S4 before going after 19lbs...

17prospective buyer

+1 ^^^^^

Larger injectors won't increase performance on their own with a stock max CFM. If you calculate that your max CFM is greater than stock, (eg. supercharging/turbo, aggressive port/polish) larger injectors will of course help. Just like SMT said, larger injectors on a stock engine won't inject more fuel with stock airflow, they'll just open for a shorter duration. Could prolong the life of the solenoids though since less time open=less heat generated by the solenoid.

Speedtoys

I have Rogers EIS intake, X pipe, single 3" exhaust.

Sharktuning, Ive found that I have an injector pulse rate of 117% nearing the top end of the power band.


..so..my injectors are open 100% of the time, never turning off on 19# Ford injectors up high.


I have 24# to install once I bolt up Devek headers & exhaust. That will let me remove the point where the injectors double-fire, and have better fuel control for tuning.


FWIW..my X pipe with dual working cats, complete with 3" exahust to dual 2.5" tips..all stainless, is for sale.

Located in Santa Cruz, CA area.

SMTCapeCod

Gotta love real data....but again you're talking S4...

S3 OEM rate is 24lb IIRC?

For the S3, Porken sells add-ons to go with his chipsets to address WOT, because they can start to show lean....but his tune would already lean the mixture beyond stock I expect.

I still think, stock intake, stock programming, even with full flow exhaust still well within acceptable margins for stock S3/LH injection.

thezonster

OK then.
Thanks

IcemanG17

do it...don't worry about injectors......the stock setup has more than enough room to handle a proper dual 2.5" exhaust....you will gain HP-torque.... Remember the dual exhaust setup you have is about 52mm....or just over 2"

17prospective buyer

I always guessed that the stock diameter was like 2 1/4. Always wish i had a vernier so i could actually find out... lol.

PorKen

With a decent exhaust on a S3, at the stock fuel pressure (36psi), you will hit 100% injector duty well before the HP peak (5700 rpm). The stock torque peak at 2700 will be higher, but will be too lean for max TQ with the stock LH chip.

The S dual-single-side exhaust ('80-'86.0) is 52mm OD. The true dual-side exhaust ('86.5-up) is 55mm OD.


FWIW, I'm running around 320 rwhp sae with my '86.5 5-speed. With 24# and 55psi FPR, I log ~86% injector duty @ 12.5 AFR @ 6600 rpm.

SMTCapeCod

S3, 24lB 4-hole injectors, X-pipe, 2.5 inch via 100 cell cats, high flow resonators/muffler, S300.

Blind though-- No recent dyno or means to datalog injector demand/duty cycle yet.....rear muffler has a crossover to balance the different lengths of the two runs, and in deference to Ken's results when he installed an aft-end H.

GregBBRD

We're doing this, with my 2.5" system, on a vehicle this next Wednesday....with before and after dyno runs.

Except I wouldn't use a Borla muffler on my lawnmower. I've never had one/seen one that didn't crack and fall apart.