Rear end and Injectors Questions

stock 8.8 mustang is a 28 spline

Injector HP ratings: divide flow rating by.5 and multiply the result by the number of injectors. This uses a 100% duty cycle. These ratings are for naturally aspirated engines. See my comments on pressurized induction in the paragraphs following this one.

Example:
19/.5 = 38, 38 x 8 = 304
24/.5 = 48, 48 x 8 = 384
30/.5 = 60, 60 x 8 = 480

The preferred duty cycle is about 85% maximum, so for a safety factor mulitply the final figure times .85.

Remember that the above ratings are at 39 PSI. Increasing the pressure will effectively increase the flow rating. Example: a 19 lb injector will flow 24 lbs at 63 PSI, and a 24 lb injector will flow 30 lbs at 63 PSI.


Here's where I will lose some of you for lack of understanding:

Increasing the intake manifold pressure above atmosphereic decreases the flow rating by the amount of pressure increase.
Example: injector flows 30 lbs at 39 PSI. You have 10 PSI boost, subtract the 10 PSI boost from the 39 PSI fuel pressure. That means you have 29 PSI pressure at the injector tip. Therefore, a 30 lb injector flows 25 lbs with 39 PSI base fuel pressure and 10 PSI of boost.

Fortunately, the design of the fuel pressure regulator causes pressure to increase as manifold pressure increases. The increase may not be linear though.

See http://users.erols.com/srweiss/ to get the calculators used in these examples.
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he's saying or giving you about what the listed injectors flow at 100%.....it's safer to choose injectors based on 85% of their full capacity...

30's are in your ball park depending on how close to the low side of 400 hp you are...you may need slightly more injector with a s/c.......thats one of those reasons the dyno tune is important......seems alot of guys run 36's and 42's but to do that you most definatly need a tune for it to run right..

you see barely used injectors for sale all the time, reason being is by the time they get it all together and dyno tune it, sometimes they go with bigger or smaller ones.....

as for his atmospheric pressure in the manifold subject...i believe he was saying the more pressure there is in the manifold the harder it is going to be for the injecters to freely flow their capacity.....at a given set psi, and that the fuel pressure regulator adjust the psi of the injectors especially if there's boost present.....normally aspirated engines have a vacuum or no vacuum while running, a blown or tubo'd engine has pressure or boost in the manifold....

89CobraGT said:

Lol I lost yeah there can someone give me a straight awnser lol??????

Click to expand...

PSI is a measurement in the difference of pressure between a closed system and a reference point. Most of the time that reference point is the outside air at sea level pressure.

If the outside air pressure increases, the pressure difference between inside and outside decreases. In the same manner if the outside pressure decreases (you drive up a mountain peak), the pressure difference between inside and outside increases.

In a fuel injected engine, fuel pressure at the injector tip is the difference in pressure between the inside of the fuel line and the inside of the intake manifold.

If the inside of the intake manifold has 5 PSI boost, then the 39 PSI measured against sea level pressure drops. You now have a pressure difference of (39-5) or 34 PSI. You subtract the boost pressure from the fuel pressure to get the pressure delivered to the injector tip.

The glitch in understanding the problem is the loss of pressure at the injector tip due to supercharging. For every 1 pound of boost you lose 1 pound of fuel pressure in a system with fixed fuel pressure.

As the fuel pressure at the injector tip drops off, so does the flow rate in Lbs or LPH. Increasing the pressure helps restore the flow rate.

Many supercharger packages include a FMU (fuel management unit) which helps increase the fuel pressure with the increase of boost pressure.


For the guys who are SCUBA divers, this is very familiar stuff.