injector sizing question

[graphicjunkie]

Is there any kind of calculation to help determine how large of injectors you need for a specific horse power application?

 

I've got a lead on some FORD SVO 30# injectors. Guy I know bought them, and decided that he needed more. Selling them for dirt cheap. But I want to make sure I don't waste my money either.

I'm SHOOTING for about 500hp.

 

 

thanks. I've been searching with no results.

[graphicjunkie]

wow...should have searched outside the forum....one quick hit on google...found tons.

Appears that the 30# injectors won't do the job.

 

Here is a great article I found:

 

You will need access to a dyno to properly do this.

Step 1: Determine the horsepower of your engine.

To determine the horsepower your engine is producing at the crank, you should have your vehicle dyno tested.

Step 2: Determine your approximate BSFC or Brake Specific Fuel Consumption.

BSFC is the amount of fuel used per unit of horsepower made by your engine. Again, you can determine this from dyno testing.

Step 3: Determine your injector's safe duty cycle.

The injector duty cycle is the percentage of time that the fuel injector is open vs. the total time between firing. For most OEM cars it's .8, but if you're on this forum it's safe to assume for this purpose that your SDC is actually .85, as this is pretty standard for most modified and racing cars. If you're upgrading your injectors to a higher flow rate, then it's safe to assume that .85 is your number!!!!! SDC=.85

Step 4: Calculating the injector size.

Okay, so now you have the numbers you need. You've done the dyno time, and you have your crank-hp, your aprox. BSFC, and your injector SDC. What now? Now you can use this information to calculate the correct size by using this formula:

injector size in lbs./hr.=(max. hp X BSFC) divided by (number of injectors X SDC)

So, if you have a modified Sentra, putting out 150 hp, then the equation might look like this:

(150 X .45) divided by ( 4 X .8 ) = 21.1 lb./hr.

So, what the hell do you do with lbs./hr.??? I mean, everyone else in the world has the sense to use the metric system... but your Nascar fan parts guy at the local Napa doesn't understand the simplicity of the universal metric system... so he only knows lbs./hr.... well, that's easy. Use this formula to calculate:

To convert pounds per hour to cc's per minute (like normal people would):

cc/min = (lbs./hr. X 60) divided by 6.177

so, using the example above, that would be:

(21.1 lbs./hr. X 60) divided by 6.177 = 204.9 cc/min.

 

 

Fuel injector duty cycle is the percentage of time each fuel injector is commanded to be open. This time period is known as the Injector Pulse Width, or IPW. If you know the engine speed (RPM) and the (IPW), then it is easy to calculate the duty cycle.

During normal engine operation, each fuel injector fires once per each four stroke cycle of the engine. These four cycles have a duration of two complete revolutions of the engine. For example, at 3,000 RPM it takes 40 milliseconds (ms.) for an engine to complete two revolutions (3,000 RPM / 60 = 50 revolutions per second, and the inverse of 50 is .02 second, or 20 milliseconds).

At 6,000 RPM it takes 20 milliseconds for two revolutions. If a fuel injector is activated for 15 ms. at 3,000 RPM the duty cycle is 37.5% (15 ms. / 40 ms.).

If an injector is powered for 15 ms. at 6000 RPM, then the duty cycle is 75% (15 ms/20 ms). One way to simplify this process is to multiply IPW by 5.33, then divide RPM by that number. So in this example, 15 x 5.33 = 80, and 6000 / 80 = 75%.

Notice that as engine speed rises, duty cycle also rises. So if we use the 75% duty cycle at 6,000 RPM from above (which is perfectly safe), and rev that same engine to 7,000 RPM instead, we get a duty cycle of 88%.

Many manufacturers recommend that fuel injector duty cycle be kept below 85 to 90% for extended periods. Some injectors may actually flow less above 95% duty cycle than below that value. Of course, if the listed duty cycle is greater than 100%, the fuel injector is inadequate for the application.

And here is a good interactive version:

http://www.injector.com/injectorselection.php

[graphicjunkie]

ok, now I'm confused though.

I went over to LS1tech, per a thread on here. On that website,I found a thread that was discussing mods and performance. MANY of the higher numbered hp numbers I was seeing, people were using ..... 30# injectors!

 

Doesn't make sense to me!!!

[grumpyvette]

http://forum.grumpysperformance.com/viewtopic.php?f=55&t=1200

[graphicjunkie]

thanks grumpy. I'll check it out.

[graphicjunkie]

ok, finished up what I was doing and went back to your link. Your site has a ton of links for the calculators. Awesome, thanks. seriously, every conceivable style/format for calculating it. That's cool as hell.

 

But my question has evolved from my OP... I see the calculators stating that I need more than a 30# injector, but I'm seeing thread after thread where people with modified LT1's are using 30# injectors, running well over the 380hp max that the calculators are showing for 30# use. I don't want to go too big, I read that it can cause cylinder wash, and that's bad.

 

Does elevation have any play here? SAE gets away with more/less because of air density? Just a shot in the dark there.

[jc052685]

ok, now I'm confused though.

I went over to LS1tech, per a thread on here. On that website,I found a thread that was discussing mods and performance. MANY of the higher numbered hp numbers I was seeing, people were using ..... 30# injectors!

 

Doesn't make sense to me!!!

 

well they also have 2 extra injecters flowing fuel.

 

http://www.rceng.com/technical.aspx

 

though I think they tend to calculate a larger injector than you need just slightly/

[ktm]

Power is power; for a volume of air you need a certain amount of fuel (mass) to produce a given power.

 

Stick with the calcs.

 

By the way, don't knock the US rating of fuel injectors. Fuel requirements are MASS based, not VOLUME based. The cc/min is misleading.