Big enough injectors for my setup?
 

Hi guys,

I went to the track for the first time with my setup (but without running nitrous).

I wound out 4th gear with 3.73s to about 6500 rpm, and managed a 122 mph trap speed. Although I am happy with this, what concerned me were my injector cycles. I am running 50lb MSD injectors at the stock fuel pressure. They are clearing 100% duty cycle (more on the left bank, less on the right due to a split BLM condition). What is worse, my right side O2 sensor proceeds to drop down to about 100 at 6300+ rpm (left side stays in the 700-800s). I know you can't trust them, but when they hold steady at 900+mv and then slowly march down to 100mv, I feel that I have to give them some credence.

I would have thought 50lb injectors would easily support at least 500 rwhp (I dyno'd 470, altho that was without the crane box which really helped ease the stumbling and increase the pull on the top end).

My compression is 8.5:1, running a non-intercooled 9 lb S-trim. I see a full 9 lbs of boost, and my MAF maxes out at 5000 rpm. I have maxed out the P/E tables by 6000 rpm and when I hit the dyno a few weeks ago and hit 470 rwhp, the A/F was holding steady at 12.2 all the way to 6000 rpm. I do not think my pressure is dropping - I have a 255 lph booster pump inline with a Walbro intank, and have a gauge to monitor it.

Any suggestions? Ignore the stock O2's completely? Bump up the fuel pressure? Do I need more injector for 500 rwhp? I will hit the dyno again in a couple of weeks and see what happens to the A/F now that I have the crane box helping out.

SOrry for the long post - thanks for your help guys!

Re: Big enough injectors for my setup?
 

Hmmmm, very strange....I would think that those 50's you got in there would hold just fine. Im not all that familiar wi those brand of injectors but at whta pressure do they hold 50 lbs?. AS far as your o2's dropping down to 100....that would concern me a little bit. But im sure your o2's are directly related to a fuel problem unless that o2 stay at 100 all the time which basically means yo have a bad o2 and your car shouldnt even go into closed loop if one o2 is bad. i would monitor your fyel pressure and see if its dropping in the upper rpm range.

You need more injector if you are seeing duty cycles approaching 100%. Raising the FP is another option, but your pumps will flow less at elevated pressures. It's seems clear your car is leaning out and you need to be sure this is corrected before running it under boost. You need to be sure you aren't running out of pump as well, and therfore need a track or dyno run (preferred) with a FP gauge. My advice is to chuck the booster pump unless you end up raising the FP (not my first choice).

FWIW, my previous setup used a 255lph intank and a Kenne Bell Boost-a-Pump for 733rwhp. The BaP was essential to getting enough fuel. The in-line was a restriction and had to be removed.

Rich Krause

OK thanks for the responses guys.

I will hold off on spinning past 5500 till I get back on the dyno with a wideband. My main concern due to the split BLMs and thus the split duty cycles is that one side is running rich, pumping out the black smoke my friends saw, while the other side of the engine is running lean and about to melt a piston. What I should really do I guess is get a pair of wideband O2's (in different bungs then the stockers)with digital readouts like some of the import guys run - or get an EGT setup in each header.

My problem is that I have difficulty monitoring fuel pressure during my run - it might be easier if I pull hard on the street and get a friend to monitor that while I worry about driving.

Thanks again guys! :bow:

RICH.. what would be wrong with raising the fuel pressure? bad idea? do you think that maybe the booster pump is a restriction? whats the difference with the one he's running and the kenne bell pump. if you ditch the booster pump the 255lph couldnt keep up the the amount of fuel needed... right?

As the pressure goes up the fuel volume will go down. If your injectors are the limiting factor then raising pressure will help - but if your fuel pump is limiting you then raising the fuel pressure will actually hurt.

You can make a ghetto boost a pump with a 6V or so battery - just hook it up on a relay so your fuel pump, under WOT, sees the battery in series with the alternator voltage - 6V + ~14V = 20V. You can either pull the battery out and charge it, or hook up some sort of charging circuit also. But a 255in tank with a BAP will flow as much as you need (or should)


Chris

The boost-a-pump that Rich has (or had) is actually not a pump at all - it boosts the voltage to the pump, giving it more capacity.

I am going to get a Racetronix dual intank setup and hopefully that will supply me with enough fuel - if necessary I will add a boost-a-pump to that :D

if the injectors are the problem because they are maxed out wouldnt raising the pressure cause them to lock up. i kind of understand this but not really. wouldnt it just be alot safer to run the walbro 340 pump with 50# injectors. they are the biggest you can run with the stock PCM right? thanks for the help through this

Actually the largest 'type 4' injector' that works well with a factory LT1/LS1 ECM/PCM are our Siemens 57's.

Another option rarely discussed on the domestic sites is the use of a resistor pack with low impedance (peak-hold) injectors. It does make them a bit slower to open but it still works quite well. Our import dealers do this all the time with 72's and 83's on the FI 4-bangers.

Jack :cool:
Racetronix

You have neglected to mention what your "booster" pump is. Whose the manufacturer? Jack has told people many a time not to put other pumps "in line" after a walbro due to it become a restriction unless you have another walbro inline