Correct me if I'm wrong: With the manifold air pressure type of flow measurement - the computer compares the pressure, RPM and throttle position to calculate the mass of air flowing into the engine.

Therefore, if I do anything to increase airflow, the motor will run lean? Is this right?

What can be done with this earlier system to enhance performance?

I was thinking of valve springs and 1.6 rr's as an easy upgrade without taking the whole motor apart. I've heard it's good for an honest 20hp if the calibration is right.

Most of the stuff has to remain stock due to emissions testing. (like manifolds, converters)

I'm tearing into this thing in the coming week with the whole dynaspark/miezere installation.

 

If you have a 90 thru 93 model then you have what is known as a speed density fuel strategy. The 84 C4 was a crossfire TBI system used a SD system, from 85 up through 89 were all MASS AIR FLOW or MAF strategy controls and from 90 to 93 they went back to SD then 94 and up they went back to MAF controls.

You can make several changes that will not affect the fueling. You can improve your exhaust system by adding headers and a catback, and a cold air intake, slightly larger TB (52mm). Doing these modifications does not significantly alter the VE of the engine. Meaning this will not significantly alter the fueling in a way that the ECM cannot compensate.

Now once you go in and install a different cam, now things change. Installing a cam does change the engines VE significantly and this will require a custom chip to alter the fuel curves.

 

So, when the VE doesn't increase to throw off the computer, then the power doesn't either.

If it's not going to throw any extra fuel - then there will be no additional power.

So - without planning on the computer chip enhacement - don't bother with the mods.

 

Upgrading the exhaust system will enhance the performance of the car. As will adding the 1.6 rockers. Changing the lift of the cam slightly will give you some increase in performance. Both of these mods are well withing the realm of the control of the ECM. Going with a custom chip would further enhance your mods. The ECM will adapt to the added airflow that is what the fuel trims are for. It will see that there is additional airflow by the changes in the AFR it sees via the o2 sensors. Seeing the change there will cause it to add more fuel. But within its limits.

 

Funny this should come up at this time. I have an '84 with the SD system...All stock. Over the years, I have gradually knocked 2 1/4 seconds off my 1/4 mile time. The only change to the engine is MY hand porting of the Crossfire intake manifold, and tuning. My car does not run consistently. The one puzzling anomaly, is that as the air gets better, my car slows down. *I* theorize that it is because I am running lean. When the air gets better, due to more available oxygen, the lean condition is worse and it runs slower, when it should be running faster.

I have been running 14# fuel pressure. The spec for the Crossfire is 9-13#. Last week in an effort to pass a smog test, I dropped the pressure to 10.5#. I decided to leave it that way for Friday night's racing to see if "lean is mean". My ETs were off slightly, until the DA dropped below 7200'. Then is slowed down, BIG time. I just came in from setting it at 15#, a pound higher than I've been running. Next Friday night will be interesting.

The SD system measures the manifold pressure, but especially at WOT, it has no idea of the atmospheric pressure. At so many TPS and MAP volts it goes into the PE mode which has injector pulse widths and the time curve preprogrammed. Even on carbed cars, headers can make re jetting necessary.

MY experience, confirms tjwong's statement that the modifications will make the car faster, but I think the original poster has a legitimate concern. Those modifications won't work to the maximum until the ECM is programmed to work with them. My fuel pressure adjustment is merely a crutch, to test my theory, but I'll take bets.

All this time *I* thought my car ran pretty good. It looks like I've left something on the table.

RACE ON!!!