Injectors too small?

[55Belair]

I finally got my car to the track. Starting making some headway. On the 5th run, split a mini spool in half and ate both axles.

What I did learn was that I had VE numbers at 120 to get the car to 13 to 1 AFR. This was with 35 degrees of timing. The duty cycle was 87% at the max rpm(6950). I have my fuel pressure set at 50 lbs on 42# injectors. I really need to add a couple of degrees of timing back in, but obviously this will cause it to start going lean with the current fuel settings.

My question is how much effect would raising the fuel pressure to 55 lbs. make. Will it be significant enough to bring the duty cycle numbers down say 20%? I still have not turned on the nitrous and need about 25% of the current injectors for that.

383 chevy, Dart pro 1 heads, JE pistons, Eagle rods, Scat Crank, ARP bolts throughout. Tunnel ram converted to Fuel injection with a Enderle bug catcher (1 plate blocked off for about 1800 cfm). MSD 6 AL box, msd distributor. FAST fuel injection.

 

[Chuck Leeper]

Correction??

What is the O2 correction % at WFO??
Is the fuel pressure steady at WOT?
Any idea of what HP you are making?? [Could be calculated from MPH and wt of the car]
If you raise the FP another 5psi, it will not give you a 10% increase in flow. I would suspect another set of inj's are needed, especially if you intend to turn on the N2O.

Any data logs??

 

[55Belair]

I do have data logs from the 4 runs, but I did not have O2 correction as one of the items it was monitoring.

As for fuel pressure, it does seem to be constant. The VE numbers are the same down the length of the run and the AF rations stay fairly consistent (within .3 afr).
I am running an aeromotive A1000 pump with the pre filter and post filter, Mallory fuel pressure regulator.

not sure of the horsepower, have not been able to get on the chassis dyno yet. Ran 89.23 mph in 1/8th still not tuned all the way. Car weighs 3751 with me in it.

I definitely intend to turn on the N20 once I get the motor more tuned. I currently have 175 horse jets in the system. Will go up to 275.

If I am doing my math correctly (assuming that I want an 80% duty cycle at max) I would need 59#'s of injector for the 175 horse jets and 68#'s for the 275 horse. (figured 87% of the 42# injectors as 36.5 # and aproximately 11#'s of fuel per injector for the 175 and 17#'s for fuel for the 275. Add base + nitrous fuel then divide by .8 for the 80% max)

 

[ijames]

Your 89.23 mph in the 1/8th gives about 435 rwhp. I'll defer to Chuck for the injector selection and tuning .

 

[Chuck Leeper]

Data logs??

I'd do a couple WFO street blasts once the rear end is mended and well..
Do this w/ the closed loop on, and the O2 correction logged.

You said that the A/F remained constant thruout the run, and the VE is the same. I would think the VE should be ramped down once the engine has reached max torque??

 

[55Belair]

Thanks for the replies. I will do that and let you know what I find.
I put the rear back together on Saturday and drove it around a little. but did not pull any logs.

 

[JDEstill]

55Belair, if I can toss in a few cents:

- sounds like your duty cycles are right about what I'd expect to see for a car like yours. With those heads and a tunnel ram on a n/a motor, having a 120% VE at the high rpms sounds like it might be reasonable. Like others said, if the O2 corrections aren't that big there, then there ya go - its a good number
- I think the extra 5 psi of fuel pressure will only give you 5% more fuel, not 10% (sorry Chuck). The increase isn't (pressure2/pressure1), it's the square root of (pressure2/pressure1)
- I don't like the way you've got that bit about "I had VE numbers at 120 to get the car to 13 to 1 AFR" worded... it might just be me, but it doesn't sound like you've quite got a handle on the concept. You shouldn't be playing with the VE table to try and get a particular AFR. You change the VE table to get the O2 corrections to go to zero (5% or less is what I guess most guys use). Then you use the AFR table to get the fueling where you want it.
- your injector sizing sounds right on to me, unless I also have a math problem somewhere. A set of 72's ought to give you plenty when you are spraying with the 275 jets.
- since the extra fuel pressure isn't giving you enough extra flow, don't spray it until you get some bigger injectors in there! Even at 55 psi you don't have enough injector to keep up with the 175 hp shot.

John

 

[Craig Smith]

Hard to say it better than John did.

As you stand you would probably max out the injectors with a 100 hp shot of nitrous. You also need to be careful when raising fuel pressure to gain fuel volume. At any given pressure X, a pump can produce volume of fuel Y. At some point, Y will begin to decrease as X increases. Depending on the pressure vs. volume curve of your pump, you can actually worsen a fuel delivery problem by increasing the pressure.

 

[Two Lane]

Originally posted by Craig Smith


Depending on the pressure vs. volume curve of your pump,

you can actually **worsen** a fuel delivery problem

by increasing the pressure.

This **fact** does not seem to be widely known or considered.

Thanks for pointing its importance out again!

 

[Craig Smith]

I've actually helped richen a car fighting a lean condition due to insufficient pump volume by DECREASING the pressure.

Chris, you may be well within the safe area of the curve to increase pressure and get a little more fuel out of it, but it would be advisable to check with your pump manufacturer about it first if you go this route. They can supply this curve information to you and help you determine how far you can go.

 

[Chuck Leeper]

??

Thot that's what I said...

If you raise the FP another 5psi, it will not give you a 10% increase in flow. I would suspect another set of inj's are needed, especially if you intend to turn on the N2O

 

[55Belair]

Thanks guys.

JDEstill

As for changing the VE numbers to get to AFR. This is the first time I have been able to get a full throttle pass. I have never tuned that part of my map. The rest of my map (street driving part) has been tuned to limit O2 correction. Don't see 6900 rpm on the street much.

Thanks for checking my math on the injectors. I don't really feel like throwing 600.00 at a set of injectors I don't need.


As for the pump volume curve, I am not going to push the pump.
I am convinced the injectors are too small and need to be replaced. The 24's I have will be sold. The 42's will be put on a shelf for future projects. I am going to order a set of 72's this week.

Thanks again for the help.

 

[JDEstill]

sorry Chuck, my brain must have glossed over the "not" in your sentance! My mistake!

55, sounds like a good plan, let us know how it runs.

John