EFI... Pressure or Pulse?

zfuller123

StarLite Diesel
Sep 2, 2006
429
0
16
Utah
www.starlitediesel.com
I have an interesting dillema that i wonder if some more skilled people can chime in on and maybe give some opinions with EFI Tuning.

Specifically, I would like to know what people think is better for creating more power when tuning. Rail Pressure or Pulse.

I understand that both are needed - but there are usually constraints with a single (but modified) CP3. Bottom line - with xxx amount of fuel - how best to use it to get the most bang for the buck?

Right now, on my LLY, i don't have any dyno numbers to back me up - but i'm believing that more pressure and less pulse is the ticket. I try to maintain 24.5k on the rail, or there abouts with my tuning, as it just flat seems to 'feel' better than when i suck the rail down to 22k or so with more pulse and timing.

I've tried so many combinations, and a DSP 5 switch makes experimenting even easier... but i've found what i'll call my 'threshhold' for fuel. How much pulse i can run to maintain the rail i want, versus how much pulse i can run before < 22k rail.

I am not posting my tunes as there is alot to the tunes, and alot of people have helped me get to the point i'm at with the race tunes in particular. Mostly, i'm curious what people think. I do have 30 over injectors, so i'm thinking thats part of the reason why pressure over pulse seems to feel better. Again, until dyno next week i can't back up either theory.

Thanks for any input! I know alot is open to speculation and not trying to open up any hostilities... Just see what people think, and why.

Thanks!
 

racinmike77

New member
Sep 14, 2008
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I agree. If you keep raising PW and pressure falls as a result, the fuel will not atomize due to the lack of pressure and the fuel being injected wont burn as efficiently creating excessive smoke and power loss.

Tuning a single cp3 is definately a balancing act.

That is just my opinion.
 

IdahoRob

New member
Jun 5, 2007
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The LB7/LLY's seem to run well at 21k and above, but LBZ/LMM not run well there at all.

ET seems best with a balance of both. LLY's seem quickest with a single cp3 having 21K as the lowest pressure point.
 
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TrentNell

Finally underway !!!!!
Jul 7, 2008
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slc tuah
Zach, I have tested on the dyno and track , and what Rob stated is dead on. what proved to be best on the dyno also proved to be fastest at the track. for me the most pulse i can run with out going below 21900 or so seems to be the ticket .
 
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zfuller123

StarLite Diesel
Sep 2, 2006
429
0
16
Utah
www.starlitediesel.com
Sounds like i have another 300 pulse to go then ;) I will put the tunes together for the DSP 5 switch and then next week i'll be on Matts Dyno to see where which one does the most for me at this point :D
 

JOHNBOY

< Rocking the Big Single!
Aug 30, 2006
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Saegertown, Pa
The LB7/LLY's seem to run well at 21k and above, but LBZ/LMM not run well there at all.

ET seems best with a balance of both. LLY's seem quickest with a single cp3 having 21K as the lowest pressure point.

X2
I can remeber 3 years ago when the gurus where all saying 19K was enough. My time on the dyno said other wise. 21k was the magic number for me. I just smiled to myself reading about guys with 3100 and 3200 shot sizes. Knowing I was making over 500rwhp than with just 2850.:coolspot: My how things change.

Pressure is your freind. A bigger shot size at less pressure can actually mean less injected fuel. Even worse when you increase the shot size and dont adjust the timing for it all you doing is adding fuel at the end. Which is most likely ATDC. Nothing but smoke and heat.
 

Fingers

Village Idiot
Vendor/Sponsor
Apr 1, 2008
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Food for thought: as your power goes up, so do the cylinder pressures. The flow out of the injector is influenced by the chamber pressures. Since they get well into the 3000's PSI with a moderate tune, the impact of lower rail pressure is even higher at the big end. 3000 PSI takes a bigger bite out of 18,000 PSI than 21,000 PSI (1/6th vs 1/7th)
 

dmaxfireman

'Can do' kind of guy
Apr 8, 2007
2,329
1
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CT
hmmm this just completely changed my outlook!! now i wanna pull all of my fueling components to go larger and port them out :D we need more pressure!!

pulsewidth and pressure related to volume, i guess i also need to know the combined bore or the injector nozzles too...? anybody have those calculations handy?
 

zfuller123

StarLite Diesel
Sep 2, 2006
429
0
16
Utah
www.starlitediesel.com
Well i've been trying to make sure i maintain about 24500 on the rail, and keep turning up the pulse in 50 increments or so - and i've actually gotten pretty high by my reckoning considering single cp3 and 30 over injectors.

I do know that going up about 300 more pulse drops my rail down to about mid 22k with the regulator wide open - but been trying to maintain the higher pressure so doing it in steps and messing with the regulator along the way to help.

My Truck is heavy - with me and 1/4 tank of fuel race weight is pretty close to 8200 pounds - so my 12.92 - 13.0's i've been able to pull off i figure equate to probably a 12.5 or quicker in a 1000 pound lighter truck. However, goal for mine is 12.5 on a single CP3 so trying to figure out how to get there. Still fighting air problems too.... but i think i can pull it off.
 

GMC_2002_Dmax

The Still Master
Pressure is KING,

Atomized fuel is better than poorly atomized fuel.

In my experience you can pull an LB7 down to 19,500 psi and still make over 500 RWHP on some trucks, it all depends on the pw and timing.

I like to tune for pressure if possible, but ET will tell the story at the track.

:D
 

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cmitchell17
Sep 8, 2008
251
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0
With my LLY with just a lift pump I can maintain 3000-3200us at my commanded 160mpa. The average pressure over the run will allways meet the commanded average within .1-.5mpa. This is with the regulator current modified with efilive in the higher mm3 per second cells. Without this it would not hold to 19k

How much of a useable range of crank degrees do we have for injection?
 

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cmitchell17
Sep 8, 2008
251
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0
I just took a log and noticed my commanded pressure dropped to 145mpa, when I am really commanding 160mpa. I looked and checked all my tables to make sure I did not change anything. Has anyone else seen this? Im not throwing any codes?
 

zfuller123

StarLite Diesel
Sep 2, 2006
429
0
16
Utah
www.starlitediesel.com
I just took a log and noticed my commanded pressure dropped to 145mpa, when I am really commanding 160mpa. I looked and checked all my tables to make sure I did not change anything. Has anyone else seen this? Im not throwing any codes?


LLY?? LLY or greater and there are some tables that will modify MPa based on ECT, IAT and som eother things if i remember, and they are further multiplied by other adjusting tables. Usually if i recall, this only happens below 162* ECT or above 232* ECT (will start to pull pressure back) and somewhere in the IAT tables, if i am remembering correctly, really low temps as well as really high temps will start to pull fuel out too - not pulse, but pressure.
 

zfuller123

StarLite Diesel
Sep 2, 2006
429
0
16
Utah
www.starlitediesel.com
With my LLY with just a lift pump I can maintain 3000-3200us at my commanded 160mpa. The average pressure over the run will allways meet the commanded average within .1-.5mpa. This is with the regulator current modified with efilive in the higher mm3 per second cells. Without this it would not hold to 19k

How much of a useable range of crank degrees do we have for injection?


I had to dabble with my Modded regulator - had some help from Larry on the low end of it and then just tweaked on the top end. Trying something a little further for tomorrows race to try and get around 25k and maintain it down the track if i can. Hoping it's possible - but who knows :) My idle is very impressive after Larry's help. Desired idle of 690, and logging in dynamice or fast stream it idles between 686 and 693. All day long. no more bouncing from 675 to 710 and back. I like it.
 

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cmitchell17
Sep 8, 2008
251
0
0
Yeah its a LLY. I was thinking the engine might not have been warm enough but I did not log ECT.

Its probably just because its an LLY.