I got an 1987 GXL with 4 little 460cc injectors in an FC S4 NA 6-port with stock ECU and original motor.

Well, old reliable finally needs a rebuild after 25 years. So I'm gonna tear it down and build it to last another 25 solid years. You know, do it once and do it right.

I want to keep the low idle and low end torque, so I don't want to port the primary intake at all. Therefore a pair of stock 460cc primary injectors should be fine, right?

But let's say, when I get to 4000RPM, my auxilary ports open up and I want them to suck more air. Would a large street port on just the secondary ports do that for me? Would the stock ECU and stock injectors be able to keep up without running lean or detonating?

If I could port the secondary intakes to flow 20% more air, then I would need 20% more fuel. And if I throw in a pair of 550cc injectors as my secondary injectors, that would mathematically give the proper air/fuel balance, while retaining the stock ECU.

But realistically, if I could only port the secondary intake by 10% and it would actually only flow 5% more air, should I choose to run super rich with the 550cc injectors dumping more than enough gas for the short time I spend above 4000RPM?
Or is the stock ECU generous enough to cover the fuel requirements of a street ported secondary intake, provided the primary intake is still stock size.

And I got stock exhaust ports...if that matters.

Remember, I want it reliable and built to last another 25 years. I don't want it to run lean and detonate (BANG) or backfire (POP) from running too rich.

 

The stock injectors will flow more than enough to feed the power you will make with that set up.

 

You are not going to max out your injectors. No point in changing them to larger ones.

 

So stock injectors with stock ECU and stock ports are very rich. (I knew that)

And stock injectors with stock ECU and a large street port won't be too lean, then.

I know the stock injectors with a standalone can be tuned to compensate for the porting/air flow.

But I wasn't sure if the stock tune from a stock ECU could keep up.

I'm trying to avoid the exta cost of buying a standalone and the complexityand time required to tune properly.

 

You are going to get a bit of a power bump from porting on a stock ecu but it is my opinion that you can get more power out of a stock motor with a properly tuned aftermarket ecu than you can a stock ecu on a ported motor. Porting should always be the very last thing you do since it's the least important.

 

I need to rebuild now, so I have an opportunity to port the intake a little.

(I'm also upgrading to a light steel flywheel.)

Once it is all put back together it should be solid and reliable with a little power gain.

After I pay for this rebuild, I can start saving for a standalone, which will maximize the porting that I'm doing now.

Over time, I get the benefit of both.

Than a header...and re-tune

 

its debatable what rich actually means. the S4 ecu is rich, but its much leaner than the S5 ecu tune. the S4 tune will run as lean as 12.8:1 around 5500rpm, which is actually very close to ideal for power. the S5 is probably in to the 11's by then, which is rich to keep EGT's under control so the cat lasts the warranty period.

however any small porting done isn't going to make any significant difference in air flow or power, especially without an exhaust. so it should still be totally fine.

from a tuning standpoint the injectors are sized really well, and you could arguably have a better combo with a larger fuel pump, and running higher fuel pressure, but stock injectors