So I'm familiar with the equations for calculating BPW, and I even found a cool website that has a BPW calculator in it, http://members.fortunecity.com/tunemaster/gm_inj.html

Anyway, according to everything I've read nobody is really sure what, the injectors I have (Came out of a truck 350) actually flow. Some are saying they're 55# injectors @12PSI. Some are saying they're 61# @12psi, and still others claim that there's no such thing as 61# TBI injectors, and that somebody is confusing them for the 65# "Police" injectors.

So I don't even know what my baseline is to start with. It's either 61, or 55 though, because these aren't police injectors.

So I can calculate 61 for the 5.7 liter engine, and it comes out closest to the stock chip (135), at 137.

If I calculate for the 55# injector, I get 151.

Herein lies my predicament. My fuel pressure is not at 12PSI. It's at a rock steady 10PSI.

So If I recalculate a 55# @12 PSI injector, to 10 PSI, it actually becomes a 45# injector, meaning my BPW should be 185.

If I recalculate a 61# @ 12PSI injector for 10PSI, I get 50# injector, and a BPW of 167.

This is a HUGE range, obviously just guessing isn't going to do the trick.

Anybody on here that KNOWS, that can tell me what the BPW should be for 350 truck injectors running @ 10PSI of fuel pressure?

 

Honestly, don't put much stock in these BPW calculations, they will only get you "close", you then need to fine tune from there.

As an example, the calculated BPW for my engine combo is supposedly around 80 or so, my actual BPW is is the 140s.

I've seen many people mention the same, You can use the BPW calculations as a guide, but don't believe they are perfect.

 

I'm not expecting them to be perfect, but at least ballpark. The difference between 135, and 181 is pretty significant though. A difference of >25%.

I really just want to know for my own peace of mind, and so that I have a decent place to start from.

 

This is from a post by Fast355. I trust it is accurrate...

http://www.turbocity.com/default.php?cPath=16_30

 

Perfect! So now that I know they are 61# injectors, is my math correct, that at 10PSI, they should be 50#?

 

My calc shows 61 lbs injs at 10 lbs for a 350 cid has a BPC of 140.50. 7.45 grams/sec

I dont know if they mimic 50 lbs injs?

I ran 61 lbs at 13 lbs and it flows 53.5

 

Would you mind showing me your math for that? I'm not sure I get it.

The way I've read to calculate it is to take the current pressure (10), and divide by original pressure (12), which is 0.833. Then multiply by the injector flow rate (61), which results in 50.833. So technically, I should be rounding up to 51. When I calculate the BPW on this page http://members.fortunecity.com/tunemaster/gm_inj.html

for a 5.7 liter engine, with 51# injection rate, I get a BPW of 163...

What am I doing wrong?

 

Variables...

CID - # of inj's - inj size - FP - BSFC- DC% - and phase of moon/ocean tides

send an email to rfehn@amfam.com and I will send to you the Excel spreadsheet...

 

Got it! Thanks! That's why I love this forum! I've been lurking for years, but never had to ask a question until now! I'm sure there will be many more to follow though, because I'm working on my first actual tune.

I think I even figured out my high idle problem. I moved timing to 6' across the board in my advance table, between 400 and 800RPM, and it smoothed it out.

It now idles at 650 RPM, right where I wanted it.

 

How did you arrive at 6 deg in SA main table? seems very low.

 

Because before I moved from a carb to a TBI system, it was running at 6' at idle. When I moved to the TBI, I moved my timing back to 0' (that's what the stock .bin called for).

Before I moved the spark back down to 6' at idle, I was getting an idle speed of 950-1150, depending on engine temp.

Should I be looking elsewhere for the problem? My engine vacuum was rock solid at 50cm of mercury (my gauge reads cm, and I'm too lazy to do the conversion, but it's dead on where I'm used to seeing it. I'm confident there are no vacuum leaks.

I moved the BPW down to 141, as per your chart. Still getting high idle. I looked at my spark table, and they seemed pretty high for an idle speed, so I moved the idle spark speed down and everything seems good.

This engine is in a boat, because I just finished the conversion http://home.hiwaay.net/~davida1/boat.htm, after I saw another member on here mention that he did the same on his boat. So I don't have the luxury of a 02 sensor, until I can figure out how others have gotten around that...

 

Also thought I should add this:

My MAP sensor is reading about 27.5kPa when the engine idles high. After dropping the spark advance down, it reads about 35kPa.

When the engine is off, the MAP reads 85kPa.

I don't know if these are normal readings or not (I'm at about 5000' of elevation), but I do know that the engine idle speed dropped back down to about 750 or so. This is what got me looking at spark, because I noticed that at higher MAP values, the spark drops down considerably in the spark table.

I'm kinda flying by the seat of my pants here though. I've read a ton, but since I can't use o2 readings to figure out what's going on, I'm just trying different things, until I get the result I want.