Sierra GT

Hey guys, A few weeks ago a rebuilt 350 went into my truck (95 Sierra 1500). We put a 206/212 112LSA cam in it, bumped the compression a bit, and it's running with a set of headers and a bigger Y Pipe (stock was pretty sad). Anyways, it runs great and I've gotten it fairly well tuned, but i'm running out of fuel at 3000rpm. My VE table is sitting at 100% at 2800rpm 90-100 MAP. So the way I see it I have two options. I could throw a set of early 454 injectors in there and tune for that, or bump my stock 61lb injectors to 20 pounds and tune for that. My question is, which should I go for? I can get the 454 injectors easily, and it would let me keep my stock pump and not have to fart around with the pressure... but would they flow clean enough at low RPM's? I'm running $0D on a 16197427 and I remember you guys saying something about async or sync pulses causing problems with pulse width on big injectors? What do you mean? I don't really want to bump the pressure with my injectors because I need to get the regulator, the pump, and crawl under the damn thing. Not that it's too much work, but the closer to stock parts I can stay the better (in case a fuel pump goes like 5 years down the road and I can't get one anymore). What would you guys recommend? Any input would be appreciated! Thanks, John Oh yeah, I forgot to ask about how to tune for the bigger injectors. I've got my off idle table so close right now, I'm actually quite proud of myself. If I go with the bigger injectors as an example... that's 61lbs vs 90lbs, a 32% increase. I would change the grams/sec from 7.69 to 11.34 based on this calculation? gms/sec = (lbs/hr / 3600) * 453.6 Am I correct in then taking my fuel tables and taking 32% away from them? I've got some areas at low RPM's and low MAP that are less than 32 to begin with. What would I do then?

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Ronny

I dont think you are running out of fuel in the VE tables. You need to determine whether your duty cycle of injs at WOT is approaching 100%. If so yes you need more FP or larger inj size. I have the same 100 in my VE tables today and I just raised the BPC so as to bring the VE table under 100 in that 3000 rpm area. Are you monitoring the DC in your logs?

I will relook at my DC% after my WB is repaired later this month to be certain I am not going lean on A/F. In the mean time I wont mess with WOT blasts other that a couple secs.

Sierra GT

Hi Ronny,

Thanks for posting back. According to Datamaster my duty cycle is about 30 at 3000rpm at 90 MAP. It doesn't make much sense, considering my fuel table is at 100%. I remember reading somewhere on here that my PCM uses a different firing mode which causes datamaster to misreport by half... but that would still only bring us to a 60% duty cycle.

There's no setting for BPC (what exactly is BPC?) in my tunerpro, all I have is grams/sec. How do I readjust that to allow more fuel? I'm a little lost.

Thanks!
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I just thought of something else, if I change the grams/sec without changing the duty cycle... will that throw off everything else like pump shot?

Ronny

OK you have the newer ECU so I dont have any idea. If the DC is not reporting accurately I guess I would get a factor known to convert to real numbers. maybe someone can confirm?

I thought a 1995 truck has a higher capacity fuel pump? my understanding 1995-1996 uses 28-32 lbs per a GM shop[ manual I read once. why do you think you need to replace pump? I had a TBI pump I replaced with TPI. you may want to add the FP gauge.
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check out the stickys on the forum. I think there is info that will help you regarding your ECM and the terms. not sure how many of us use it. it may be better than my 8746.

Sierra GT

Hi Ronny,

Yeah it's the newer ECU, so a lot of the info floating around doesn't seem to apply 100%. 1995 5.7L truck is still same old TBI with 12 pounds pressure. Currently that's what I'm set at with the stock regulator. I think the 28-32 lbs was for 1994-1995 454 engines, but I'm not sure.

Anyways, I am at a bit of a loss with the duty cycle thing in datamaster, and I have no idea what redoing the flowrate in the computer without changing the pressure will do to all the other tables.

Anyone?

Thanks,

John

Ronny

OK. i was refering to 7.4L as that was my tbi from 1995-1996. so 5.7 runs less FP. i wonder if the 7.4L uses a tpi pump ???? might make an easy swap for you. check ebay on a new one. would not hurt to keep as a spare if $$$ was right.

getsideways

if you change to 90lb injecters you need to change the grams/sec to match the new injectors. do you have a fuel pressure gauge to verfy psi? the ve table should stay pretty close regardless of the injector size as long as you have it set right in the bin. i think you accell enrichment will need to be adjusted because they are pw based. larger injector opened for the same amount of time is more fuel

Sierra GT

Okay I did some poking around and I can't figure out the 454 injectors. Some sites say 75lbs, some say 80 or 85, then 90. Is there a better and more definitive resource out there for injector numbers?

getsideways, if I change the grams/sec, won't I have to drop the VE tables by the same amount? Not too sure how that will work because i have some values in my table at 20 and 30 MAP that would end up being less than zero.

getsideways

no, you shouldnt need to change the ve tables much, if any at all. the ve tables are volumetric efficiancy tables. as long as your injector costant and engine size are correct. and they were set correct with your old settings that you tuned with, it shouldnt change. the ve tables will be most effected by cam changing, new heads, intake manifold, etc. its very important that you injector constant and cylinder displacemant constant be set right.

Sierra GT

Ahh, that makes a lot more sense. Some parts of the table at like 3000rpm and up are already at 100% though, what happens with those if I don't change it? Do they get a full blast of fuel with the new injectors?

Ronny

7.4L injs are 45(1995-1996) 80 (what i run GM part # 2560) and 90 lbs. 80 and 90 are late and early model i believe for 7.4L.

Sierra GT

Hi Ronny,

Thanks for the info! Do you have any issues with those 80lb injectors flowing cleanly at low RPM? I did a bit of googling and I couldn't find a vehicle application for the part # you gave me. Would a 91 or so 1 ton 454 get me the right injectors?

Thanks!

Ronny

i bought mine from GMpartsdirect.com. i am uncertain what year they were from. i will guess late model. part #.......2560 is 80 lbs. yes issues with very small pulse width in my 350 cid. i idle in "synch" mode only. allowing asynch caused the surging idle as it would cycle synch asynch synch at idle in log.

also i am about to add the VAFPR which will improve my idle quality and allow a larger pulse width at lower fuel pressure and provide 22.5 lbs at WOT.

Sierra GT

How bad is the idle when you use synch mode? I'm not going to be making a whole lot of power, maybe just north of 250 horsepower. Do you think it would be best if I try bumping the pressure to 18lbs or something with the injectors I have?

Thanks,

John

getsideways

maybe you should think of it like this. an engine is an air pump. depending on its size, camsize, head flow, etc it needs a certain amount fuel. lets say a 350ci at 3000 rpm would be 75% efficient. the computer calculates how much fuel the engine needs. it can regulate this because you told it in grams/sec how much fuel it can feed at a given pulse width. if you change the grams/sec in the bin because you put bigger injectors in it, at the 3000rpm and 75%efficiency, the amount of fuel needed would be the same but the computer knows to shorten the pulse width to give it the same amount of fuel. so wherever you have 100 at in your ve table, the computer will calcluate how much fuel it needs as if the engine were 100% efficient, and give it that much fuel. whether you are maxing out your injecters is determined by how big your injectors are and how much fuel pressure you have.

Sierra GT

Ahh, that makes a lot more sense to me. I guess it makes the most sense to bump the pressure for now and if need be throw a better pump in the tank. I just really liked the idea of running the 454 injectors because they come in and out of there really easily.