PAGE 2- MORE IMPROVEMENTS
Another bottle neck in the system is the MAF sensor on your TPI if so equipped. The MAF itself is only capable of flowing around 530 cfm stock. The way to improve airflow up to 700cfm is to remove the screens from each end of the sensor. be careful while you do this, or otherwise risk, trashing your sensor. The reason for the screens is to protect the sensing wire, which is no thicker than a human hair. But if you use a air filter, the chances of trashing the MAF is extremely remote. Another modification I read about, is removing the cooling fins, which represent a 39 cfm loss. The fins were put there to dissipate the heat from the sensor circuits, personally I would leave them alone.
A modified MAF sensors with both screens removed, the cooling fins are still intact.
Once you have increased the breathing capabilities of your engine beyond the point where the stock fuel requirements can tolerate, it is time to look at other improvements.
Increasing fuel pressure is one way to help, once you have improved the airflow to the engine. The stock tpi fuel pressure regulator puts out 35 psi at idle and then increases it in one step up to 45 psi. A adjustable regulator is one way to improve power, by increasing the pressure, more flow is available to the injectors. Under a high load demand, where the air flow is matched to the injector output, the power increase is significant. Up to a point. The ecm monitors A/F ratio via the O2 sensor, going to far will cause the ecm to detect a richer mixture and shorten the injectors "on" time. To keep the ecm from reacting it may be necessary to have a recalibrated prom chip.
A chart that shows fuel pressure vs. injector output, based on injector rating at 43.5 psi. This is based on maximum fuel pressure output, not pressure output at idle.
Fuel injector pressures
|Injector size @ 43.5 psi||Pressure (psi)|
Fuel injectors can be a candidate for replacement if you have maxed out the stock units with high performance add on's. The stock 305 tpi injectors are rated at 19 lbs/hr flow rate. This is sufficient if you are putting a TPI on a 1957 Chevy with a 283 up to 1973 El Camino with a 307. The 350 injectors are rated at 22 lbs/hr flow rate. This good for engines from ranging from a 1968 Malibu with the 327 to a 1978 Camaro with a 350. The 350 injectors with a 383 stroker motor will be maxed out and can be helped by increasing the fuel pressure slightly. The stock units will handle a change up to 10% with a MAF system, so 10% of 350 cid is 35, add this to 350 and you get 385 cid, but this is really pushing it.
Just remember, a little adjustment is good for performance, too much will wipe any gains you may want to achieve. Make your adjustments in 1/4 lb increments, check your pressure with a gauge.
To calculate needed flow rate use the following formula:
(Maximum hp) X (brake specific fuel consumption) / # of injectors= lb/hr.
The brake fuel consumption assumes a naturally aspirated engine, the amount of fuel it takes to create 1 hp. This figure is typically .45 On a supercharged engine the figure is .55
If you want to find out what size injector would need to support a 350 hp 350. Multiply 350 hp by .45, divide this figure by the number of injectors(8) and you get 20lbs/hr. the stock injectors are sufficient, up to the point where they can support nearly 400 hp.
All injectors are rated for flow at a specific pressure, usually around 43.5 lbs (13 bar). Flow rate will change with pressure change. The formula is as follows:
(New fuel pressure) / (original flow rate) X (.5) X (Original fuel pressure)= new flow rate.
For example the new fuel pressure is 40 psi and the original flow rate is 24 lbs/hr. Divide these figures gives you 1.66. Multiply this by .5 and it gives you .833. Multiply this by the original fuel pressure, (we'll use 30 psi). This will give you flow rate of 25 lbs/hr.
Make note that even though higher fuel pressure means better atomization,, it also makes the injector work harder. Increasing pressure also slows down the injector response time. Raising the pressure too high (above 10-15%) above stock will shorten the life of the injectors. Stay below 15% to be safe. Since the ecm bases its calculations on a known flow rate, any changes outside the window will mean a change in the prom calibration.
Below is a list of the injectors used in TPI systems: Fuel Injector ID's
|Year||Engine size||OE manufacture||OE number||GM part #||Lucas part #|
ECM'S and PROMS:
When you have exceeded the stock ecm's prom capability to match the modifications to your engine, a change in prom may be necessary.
A prom change also may be desirable if you are willing to maintain your vehicle better than the average person. OEM proms are calibrated to be on the "safe" side. It was programmed to allow for degrading engine components, sloppy maintenance, poor gasoline quality, and still allow the vehicle to run in a wide set of programmed parameters, and still pass emissions tests for the most part, as long as the emissions systems are intact and operating properly.. If you are willing to be meticulous in your maintenance, you can run a prom that allows a better performance edge than the average Joe.
The ecm is made up of three microprocessors, the ROM, RAM and the PROM or (EPROM).
The ROM stands for Read Only Memory. It is part of the ecm that handles the basic instructions for the ecm to follow. It is purposely placed in nonvolatile chip that cannot be erased by removing power.
The RAM, or Random Access Memory is used by the ecm for temporary memory storage. This where the math calculations take place for final instructions to calculate fuel calibrations. This also where the A/F history of the engine is kept, along with any stored faults. The ram also contains calculation tables called the integrator (INT) and the Block Learn Multiplier (BLM).
The integrator is part of the RAM that responds from signals from the O2 sensor to control fuel delivery. The total range of stored numbers can be from 0 to 255. A reading of 128 +/- 10 is considered normal readings. These readings can be obtain with a scanner tool or a PC with special software. The integrator in open loop is set at 128. When the ecm is in closed loop the INT responds to the O2 sensor readings and adjust the pulse with of the fuel injectors. If the numbers are higher than normal, the ecm is trying to compensate by enriching the mixture by increasing injector "on" time. If the INT detects lower than normal numbers, it decrease the injector "on" time to lean out the mixture.
The BLM is stored in memory and can respond to changes in A/F, even while in open loop. The BLM responds to what the INT is doing. There are 16 cells in the BLM, one axis is load, the other axis is rpm. When the BLM is putting the correct A/F ratio in the engine, all the cells will be programmed at 128. The BLM at each start up , will look at the history from the last run time and go by that. If a repair was done to correct a drivability problem, the next time the engine is started, the cells will be reprogrammed with the new readings based on the INT. The INT and BLM are related directly that the same high or low readings in one will cause a corresponding reading in the other.
This is an example of a BLM In normal operating parameters:
The prom stands for Programmable Read Only Memory. This part of the prom chip that cannot be altered by removing power. It is the "hardwire" instructions for the operating parameters of the engine. This includes information such as vehicle size and weight, wind drag, rolling resistance, emission controls, camshaft, transmission and final drive ratio. A prom chip that does not have a way to erase and change information is called a prom. The correct term for a erasable prom is EPROM. The eprom has a window that is covered by a decal to prevent accidental eraser.
To change the information on the prom, the chip is put into a device called a prom burner (eprom). The original information is read by a special software program usually supplied with the prom burner. The chip is then removed from the burner, the decal is removed and a special UV prom eraser is used to reset the tiny transistor switches to one state, effectively erasing the old information. Now , if the technician has a supply of proms, he won't have to erase the information on the original chip. He can keep it for a back up, just in case he needs to compare the information on the old chip, while he is making changes to burn a new prom. He can store the original program on a floppy disk just in case he doesn't have a spare prom. Like said earlier, some proms are a one time program only (no window) and cannot be reprogrammed. Once he makes the changes on the program he desires, he inserts the prom into the burner, and commands the burner through the software to install the new information. The prom burner, if he has the right program will calculate a new checksum so the chip will operate correctly.
Prom programming is best left to those experience in doing the program changes. An inexperience person or one who is impatient that does not take the time to learn and experiment in very small increments , will end up with a engine that performs poorly or may damaged the engine by severely leaning out A/F mixture.
What may need recalibration? Changes in manifold, camshaft and engine displacement. Usually replacement items like aftermarket air filters and throttle bodies will not necessitate a reprogrammed prom. But here in lies the differences between MAF and speed density systems. A MAF based system, is more forgiving for minor changes, any changes whatsoever on a speed density system will require a prom recalibration unless the original engine and prom match exactly.
Aftermarket chips usually come in three different levels, depending on the amount and type of modifications you have made. Street stock, street/strip and competition. The street stock is made for those with no modifications to the engine. The chip is calibrated for WOT fuel enrichment, delayed lock up at WOT and spark advancement. Normal driving and part throttle parameters are not affected. Street/Strip chips are made with more modifications made to the engine. This also may include the use of a lower temperature thermostat, coolant temperature sensor, or cooling fan temp. switch. These affect not only WOT but the entire power curve. The only drawback is these chips are not approved for highway use. Plus it will sacrifice fuel mileage for performance. The completion chips are reserved for all out racing engines.
On extremely radical engines, the stock ecm may not be adequate, and a aftermarket ecm that can be user programmed may be the only answer.
The MAF system is more forgiving in changes such as cylinder heads, exhaust manifolds, cams that don't reduce manifold vacuum more than 1 or 2", intake manifolds, throttle bodies, and displacements that don't go over 10% change. This is because the MAF system bases it fuel calibration based on the weight of the incoming air, and the weight of the air is influenced on volume, temperature, humidity and atmospheric pressure, the ecm then uses the TPS, MAT, CST and O2 to trim the A/F ratio.
With the speed density system, any changes will mean a recalibration. This is because the fuel calibration is based on manifold pressure, not air flow weight. The speed density has to assume a preprogrammed air flow map that calculates based on the MAP, TPS, CST, and MAT and uses the O2 sensor to trim the A/F ratio. Any changes that upset the preprogrammed curve will throw the entire calibration off. This will definitely mean that the engine will run poorly.
Be aware that a prom change may violate your local jurisdiction's emission laws. Especially in California these modifications are not CARB approved for pollution controlled vehicles.
To change a prom chip, you'll need to disconnect the power to the ecm. This is done by locating the power lead to the ecm, usually located by the battery. Also you will need a wrist ground strap usually found at many electronic suppliers. It has a special conductive foam pad that wraps around your wrist and a wire lead with a clip on it, that is connected to a good ground point on your car, usually the metal dash mounting frame, OR at least be sure to touch a metal spot under your dash to discharge any static electricity from your body.
Just remember the working voltages of a prom chip is 5-12V, a static discharge, even if you can't hear or feel it, is around 2,000+ volts, more than enough to destroy the prom and ecm!
Remove the ecm from its holding bracket and locate the cover that has two screws in it. Pull this cover and you will see the larger prom chip on the 1226870 ecm's, and the single large chip (memcal) on the 1986 and later ecm's. The single early prom should be carefully pried from its socket. Do not remove the smaller chip (calpak) from its socket. Be careful not be bend any of the pins! The later ecm's have two levers on each side of the memcal that is pulled outward to release the chip from the socket.
To install the early chips, look at the holder and you will see on the bottom is notches that correspond with the notches on the ecm's socket, be sure to install it in the proper orientation. If the chip will not go into the socket, DO NOT force it. Investigate why. The two likely cause is bent pins or trying to insert the prom the wrong way.
The later model ecm memcals have a special holder that has tabs that make it impossible to install wrong.
Once you have installed the chip, replace cover, install ecm in its proper mounting position. Connect battery power and then you can restart engine. The idle and performance may be rough for a few minutes, until the ecm prom's INT and BLM cells are refilled and proper fuel calculations take place. Normal driving will be sufficient to the ecm to relearn. Driving at WOT will not work, as the ecm will be in WOT enrichment, and in open loop operation and will not calculate the proper parameters.