Random

http://www.rceng.com/technical.htm#WORKSHEET

Using the above information. I use a .8 duty cycle (80%).


The stock 245 CC injectors are good for 152 HP at 45 PSI of pressure. and a BSFC of .50

Now I worked up the math for another 50 HP or 100 HP from our 5th injector. Assuming it has the same efficency of the stock ones...which I doubt.

To get 50 HP...you would need a 350 CC Injector with a BSFC of .65 and 65 PSI of fuel pressure.

To get 100 HP you would need a 700 CC Injector with a BSFC of .65 and 65 PSI of Fuel Pressure.

However, the stock injectors loose efficency with the boost...so we need to recalculate that.

The stock 245 cc Injectors are only good for 140 HP @ 65 PSI of pressure and a BSFC of .65

The 5th 440 cc Injector that Alpine uses can only handle 63 HP @ 65 PSI and a BSFC of .65.

So 4 injectors = 140
1 injector = 63

Total possible HP is only 205.

Can you find any flaws in my theorys/math above?

Mario

Remember the excel sheet I sent you? You can plug 4 injectors of 245 and 1 injector of 440 at 65psi its a total of 1736cc/min at 65psi. the results for different bsfc at 80% duty is as follows:

BSFC Duty Max HP
0.50 0.8 265
0.55 0.8 240
0.60 0.8 220
0.65 0.8 203
0.70 0.8 189

Turbo engines are around .6 to .65. You are exactly right in your math. Now is your fuel at 65 psi or you are substracting the boost to get the right pressure differential the injector sees. Therefore when calculating the flow in turbo engines you have to calculate the flow rate at pressure= fuel rail pressure-boost pressure (actual pressure seen by fuel). With the excel sheet you can plug and play. At the top you can play with pressure, at the left the size of the injector and the rest of the table the number of injectors of each at the given pressure. At bottom it calculates the total cc/min and # of injectors. Then calculates max HP for different bsfc at 80% duty. I have been playing with it and I confirmed the Alpine kit is lean!!! I dont like the steady pressure at the rail, I feel it is better to maintain the differential steady, boost goes up rail pressure goes up. I dont think the high pressure is good for injectors, it seems to me Alpine is pushing it to high. But again if you lower it the you go leaner. Random we have a fueling problem!!! I am analyzing different ways to solve, 1)maintain the 5th injector system but increase size if I can get the hardware to get into the fueller 2) Maintain the 5 th injector but change the injector and get a fueller box like MF2 that you can easily set 3D.. 3) Eliminate the 5th, change to larger injectors (4) and a fueling system like Haltech fuel management. 1 the cheaper and probably the worst, 3 probably the most expensive and not sure the best for street driving (still to research).

Red

There are a few things that I need to point out to both of you, and anyone else reading this thread.

First off is the theory that boost pressure counteracts fuel pressure... This is an incorrect assumption based on fluid dynamics, and I'll explain how.

Let's use an easy-to-identify example: You are driving down the highway at 75mph with the windows rolled down. If you stick your arm and hand straight out the window, there is a huge amount of pressure pushing against the "front" side of your arm. However, with all that pressure, there is also a vacuum on the backside of your arm. If you could put a pressure gauge on different parts of the circumference of your arm, you would find a pressure gradient that peaks at the "front", and drops into vacuum at the "back".

Here's what you need to notice about this scenario...

The overall pressure outside is 14.7psi -- given an altitude of exactly sea level. Thus, the pressure you are feeling on your arm is directly related to airflow being impeded. You are experiencing high pressure, but that's because you've created a boundary layer in the airflow outside.

This correlates to what you find in your intake manifold in a very similar way.

When the intake valve is completely shut (ie the piston is doing something else other than taking in fuel and air), then you have a very high static pressure against the valve because of the pressure exerted by the airflow running into the back of the valve. If you were to try injecting gasoline at that exact moment, the pressure in the manifold would counteract the fuel delivery.

However, when the valve is open, you have airflow passing by the injector. And because the injector fires almost directly at the intake valve, you actually have a slight vacuum at the nozzle on the injector.

There is pressure on the manifold, but the airflow itself creates a vacuum on the end of the nozzle. Fluid dynamics is wierd like that...

-Red-

Red

Oh, and now the second thing since that first post was all kinds of long....

The second thing is injector sizing, BSFC's, fuel pressure, and other stupid stuff like that.

It is my observation (not proven, but not tested either) that four bangers somehow benefit from a lower BSFC (higher fuel efficiency) than other motors that create similar horsepower. A lot of this could be attributed to lower intertial forces (less reciprocating components than a 6 or 8 cylinder motor), as well as a tendancy for lesser external drag (transmissions, flywheels, accessories are all generally "smaller" on a 4 banger than comparable motors of similar output)

Besides all that speculation crap, the simple fact is that there are many observable four bangers that are creating "considerable" horsepower on what would otherwise be assumed "inadequate" fuel injection. Honda B20A's making 280+ wheel horsepower while using 370cc/min injectors, B16A motors building 220+ wheel horsepower on 310cc injectors... Even Onpol over on FXTreme is making 227 wheel horsepower on his turbo setup, while using 440cc/min injectors. Joel's Starion 2.6L Turbo with a 16G upgrade making 230WHP while running on TWO 550cc/min injectors (TBI) and that's even an old motor.

By "popular" math (and the RC Engineering website), those situations shouldn't even exist. Those people using those setups should be running lean, or not even at all. But they work, and they work well, because it seems we are getting much better efficiency overall than is typically expected.

Hell, the 245cc/min injectors on the Tiburon should only be good to about 147HP (flywheel power at that!) when running 43psi and 80% duty cycle; you've got to be kidding me... While bolt-on upgrades may not improve power a whole lot, I'm willing to bet the entire gammut of I/TB/IM/cam/port+polish/H/E and my newer ECU is enough to launch my Tiburon well past the theoretical limit of 147HP on these injectors.

I wouldn't say that you don't have enough fuel, I would first take it to a dynoshop that has a gas analyzer probe to stick up the tailpipe and SEE if it's running rich or not. The last time I dyno'd my car (two years ago) it ran 147HP at the tires after REMOVING about 17% of my fuel trim with the AFC; it was previously spewing so much fuel that I had black smoke coming out the exhaust pipe.

You get the idea. Confirm that you are actually running lean, rather than assuming you are. There's no using buying four 670cc/min injectors because RC tells you that you'll need them to make 300HP.

-Red-

Red

TTT -- just wanted to make sure you saw this Random; at least the part about manifold pressure counteracting fuel pressure.

-Red-

Random

Yes, I saw it. I was wondering about that, and also at 9psi vs 55psi, I really didn't think that the boost would affect the injectors much.

Plus your comments at Fxtreme about small I-4's doing better than .65 on the "injector" worksheet.

[ July 13, 2001: Message edited by: Random ]