bqman

This question is for a '96 Z28 LT1 (stock):

Does anyone know how much fuel is injected per pound mass of air? If you know the air/fuel ratio that a stock LT1 comes with, that would be helpful too.

Injector Size: (17124248) 3.0gps, 22lbs/hr speed density (1992-1993), 24lbs/hr mass airflow (1994-1997)

What is 24 lbs/hr? Is it 24 lbs of air per hour per pound of fuel? It doesn't sound right, but I found these specs online and don't know what it means, more specifically the item in bold. Also what is 3.0 gps and 22 lbs/hr speed density?

What is the volume of each cylinder?

How many spark plugs spark per given stroke? Is it two spark plugs at a time or four?

bw_hunter

22 and 24 pounds per hour are the fuel injector ratings for how much fuel they can flow. 22 pounds per hour is roughly equivalent to 3.0 grams per second which is what the gps units are. Speed density is the type of air measurement the 93 F-body engine uses. It doesn't measure air flow directly, it uses other variables like manifold absolute pressure and air temperature (others as well) to calculate how much air is entering the engine and then applies the right amount of fuel.The later cars use a MAF system, mass air flow, where the system tries to actually measure the airflow into the engine.

Both system adjust the amount of fuel delivered by using lambda sensors in the exhaust (oxygen sensors) looking at the oxygen content. They convert unburned O2 content into an electrical signal and report to the computer. The computer than varies the fuel injected into the engine to adjust the oxygen content to the right value. Of course, the right value is different depending on what you're doing at the moment with your right foot...

Air/fuel rations vary all the time. You want it considerably richer than stoiciometric during the acceleration stage and rather lean during cruise...

The volume of each cylinder is 43.73 cubic inches (on a stock engine) or 716.9 cubic centimeters.

Sorry for the long answer.

rskrause

In closed loop, the air:fuel ratio is ~14.7:1. It is a mass ratio, so it means that the engine needs 14.7lbs of air per pound of fuel. Using the O2 sensor, the computer (PCM in GM speak) adjusts the injector flow rate to achieve this ratio. Injector flow rate is determined by the size of the injector, fuel pressure, and amount of time the injector is open. Injectors are rated at a stated fuel pressure and 100% duty cycle (in other words, open 100% of the time). A stock injector flows ~24lbs of fuel/h at 42psi if it is open 100% of the time.

In practice, most people don't like to run injectors at more than ~80% duty cycle, they may overheat and become erratic if operated for any length of time at over 80%. At 80% duty cycle, a 24# injector flows ~19lbs/h. A typical number for Brake Specific Fuel Consumption for a 4-cycle engine is 0.5lbs of fuel per horsepower per hour. Thus at 19#/h a single injector could support ~40hp. Eight injectors would therefore be good for about 320hp. That is the math behind the reason stock injectors are good up to hp in the low 300 range.

Hope that helps.

Rich

rskrause

If you use a fuel with a different stoichiometric ratio than gas, the amount of hp/injector varies accordingly. For example, methanol needs an AF ratio of ~6.4. That's ~2.3times as much fuel needed as with gas. So, you need injectors ~2.3 times larger. With methanol, assuming an AF ratio of 6.4:1 our 24# injector example only supports ~8hp/injector. Of course, under full load, the AF ratio also has to be richer than stoichiometric. Most NA gas engines want ~13:1 for max hp, blower motors sometimes as high as 10.5:1 (on gas). Injectors must be sized to accommodate this. Another reason to use 80% of the rated flow when calculating needed injector size.

Rich

Injuneer

As Rich noted, the target A/F ratio is 14.7:1 in closed loop (idle and part throttle/part load). The target A/F ratio in power enrichment (PE) mode in the stock LT1 programming is variable, but usually works out to about 11.7:1. Hence, the popular perception that "LT1's are programmed rich". Leaning out the PE mode target A/F ratio will generally improve HP, even on a stock engine. Something in the range of 12.8:1 to 13.2:1 should produce better power/torque, as Rich has also noted.

The 4-stroke engine takes 2 crank rotations to complete the full 4-strokes (intake-compression-power-exhaust). Hence to calculate the volume of air ingested, you multiply (displacement) x (RPM/2) x (volumetric efficiency). Select the units of measurement based on the end result you want. CFM is a useful volume flow rate end product. Convert volume flow rate to mass flow rate by using the density arrived at using the perfect gas law, based on absolute temperature and absolute pressure.

An easier way to size injectors (if that is your goal) is to assume a brake specific fuel consumption (BSFC) expressed in #/HR/HP, and multiply that times crank HP, divide that result by the number of cylinders, and divide that result by the maximum desired duty cycle. The online injector sizing calculators tend to be a bit conservative, using 0.50 #/HR/HP (NA only) and limiting the duty cycle to 80%. I've found that a simple rule-of-thumb to get you in the ballpark is to multiply crank HP x 0.07. That assumes a BSFC of 0.476, and limits the DC to 85%.

bqman

Scores of info, but it's never too much. If you've got more about mass flow and how injectors work, feel free to post. Thanks for all the replies.

I'm also curious to know compared to the cylinder volume, the combustion chamber volume is only 54 cubic inches. So if two or more cylinders are extracting air/fuel from it, the volume of the combustion chamber won't be able to support the volume the cylinders hold. Anyone know how that works?

bw_hunter

Each cylinder has it's own combustion chamber and it's 54cc not 54 ci. This is the area in the head directly above the piston. The compression ratio is the volume of the cylinder divided by the volume of the combustion chamber...it's a number like 10.4 to 1 on a stock LT1 engine. Anyway, the engine draws it fuel and air mixture through the intake valve as the piston moves down in the cylinder, essentially creating a vacuum. When the piston reaches the bottom of it's motion and reverses, the valve closes and the piston forces that fuel/air mixture into the small combustion area. Just before the piston reaches the top, the spark plug fires, causing the mixture to ignite and the burning gases force the piston during what is called the power stroke. There are a number of variables that affect power..the timing of the valve evens, the compression ratios, both static and dynamic, the absolute lift of the valve off it's seat to name a few.

I hope that gives you a basic view of what's going on in the engine. There isn't a sharing of combustion chambers, each cylinder is it's own, self contained little unit. Internal combustion engines are really big air pumps and the highest power is acheived by making the most efficient pump possible.