dhdenney

iTrader: (53)

I've addressed this problem before but decided to search other avenues. My decision now is my tuning problems are that the injectors are too fat. I could spend a good amount of $$ to get the car tuned for my mods. I could break even on an injector swap that would probably work better with tunes I already have. Currently I have 42.5# injectors. I bought them over a couple years ago before I started this project without really knowing how fat they were. I'm either downgrading to 30# or 36# injectors. I ran a search on the site and motors similar to mine have 30# injectors and some have 36# injectors. The smaller cammed 396's have the 30# injectors. My cam is a little healthy so I said go with 36#. Then I found a 396 with a cam about the same size as mine with the 30# injectors. I have a friend with some 30# for sale but I am afraid they might be too lean. Nothing from my search gave a blatant answer.

dhdenney

iTrader: (53)

It's not hard, 30# or 36#. Someone running a stroker, what do you have?

METALBEAST

iTrader: (1)

The injector size is going to depend on the Horsepower you expect to pull with the engine. Here is an injector sizing formula I use

IS = (BSFC*FWHP/(DC*C)

Where:

IS= Injectors Size

BSFC = Brake Specific Fuel Consumption (Roughly 0.5 for NA cars)

FWHP = Flywheel Horsepower

DC = Duty Cycle ( Max Duty Cycle of 0.9)

C = Number of Cylinders

Your 396 should make 450 RWHP or 530 FWHP Easy (assuming 15% drivetrain loss)

Therefore:

IS = (0.5*530)/(0.9*8)

IS = 36.8 lb/hr

Round up to the next biggest injector size and you should be all set.

dhdenney

iTrader: (53)

Well, I've see that formula many times and played with it many times. It was never of use to me because I'm not in the position to accurately calculate BSFC and duty cycle. And alterting those numbers could sway the equation. So it made more sense to me to see what everyone else was running. So how do rely on a figure of .5 for BSFC and a .9 for duty cycle for my car? How I know those numbers aren't something else? Does a wet shot of nitrous alter the BSFC or is that only for dry shot cars since fuel comes from the fuel solenoid?

METALBEAST

iTrader: (1)

The 0.5 is actually for an NA engine. I did not see that you were running N2O in your sig. For supercharged or nitrous you would use 0.57 for your BSFC. Remember that duty cycle is the % of time that the injector is open. The MAXIMUM amount of time you want an injector open is 90 % of the time or 0.9. The formula is therefore used to find the MINIMUM injector size required for your application. I am running 30 lb/hr injectors on my vette. It is NA with heads and a mild cam.

TwoFast4Lv

iTrader: (8)

While the 42.5s are a Little big having it tuned is the true answer here. No matter what injector you use the Tune/programming must be good. I will admit for a street car I try to run the smallest injector possible for low RPM and Idle but you still have to cover the HP you are making.

I have made over 500FWHP with the stock injectors at stock fuel pressure. For a street car that only see's over 5k occaionaly that is ok. As noted above I DO max the injectors at the track. They hit 100% at 5,600 but my AFR never went over 13.0 all the way to 7,000. I would say that is the VERY furthest I would even try to push them.

As far as drivability I like to stay with GM/MSD/Holley injectors. The reason is they are easy to tune They cost more money but in the end you do not have to re-write every singal cell in the fuel map

I would suggest the MSD #2018 38Lhp injectors. You can get them from summit now for $432

http://store.summitracing.com/defaul...G&N=120+400122

Like I said they are a little more then the cheap injectors running around but they are worth it

BTW The last two sets of MSD injectors I got had the Rochestor emblem on them just like the factory injectors

dhdenney

iTrader: (53)

OK, well that makes sense on the formula. I'm as guilty as anyone for recycling info without explaining it but I had always seen that formula but didn't understand it because there was no explanation attached to it. I didn't trust it because I didn't know the logic behind it but you've explained that. However, calculating the equation, I still get that I need. 42.5# injectors (why I bought them in the first place). And I know that is too big because the motor is drowning in it's own fuel. I have run probably 10 different tunes and one of them surely would have hit close enough to tell a difference. They were all setup for smaller injectors. Therefore, I am downgrading my injectors. I have about decided 36# will be sufficient.

TwoFast4Lv

iTrader: (8)

Well you could put in a thousand tunes. If they are all setup for smaller injectors they will never work

42.5 Injectors Will work if tuned correctly.....

dhdenney

iTrader: (53)

Exactly. I'll be going to something smaller--something I have tunes for.

Beast96Z

iTrader: (21)

That is your best bet. Installing larger injectors without them being properly tuned will throw your fuel map way off. Your lucky you haven't washed your cylinders out yet. If you don't have a tune for a certain injector, don't use it. If you have diffrent tunes for 30#, buy 30's, if you have tunes for 36#, buy 36's. I'm not quite shure why you have so many tunes avalible to you, or who did them, but the only true tune is on the dyno. Then you will know what your injectors are doing and if you need to go bigger or not.

TwoFast4Lv

iTrader: (8)

Actually the only True tune is one that includes daily driving AND a dyno

I see FAR to many 'Dyno" tunes that were never setup for the street. car runs great on the dyno but like crap on the street. Blower cars are the worst !

Beast96Z

iTrader: (21)

Very true. I think you catch what I was trying to say.

METALBEAST

iTrader: (1)

My concern is that the 36 lb/hr injectors may not be able to compensate especially in a nitrous application.

When I run the numbers assuming 600 FWHP w/ nitrous

IS = (600*0.57)/(0.9*8)

IS = 47.8 lb/hr

I used the 0.57 number for BSFC since you are runnin N2O

Brake Specific Fuel Consumption is the ratio of lbs of fuel consumed per horsepower delivered. For a naturally aspirated engine, 0.5 is the the rule. But since you are running nitrous, which requires more fuel, we need to use a BSFC value of 0.55-0.6. I split the difference and went with 0.57.

I strongly recommend you spend the $400-$650 to have your 42 lb/hr injectors tuned for the nitrous.

TwoFast4Lv

iTrader: (8)

Metal beat what system are you running? All LT systems add fuel one way or another so the injectors do not need to compensate much

METALBEAST

iTrader: (1)

My vette uses the old speed density ECU while my 97 Z is the MAF OBDII system. I think I understand what your saying. The ECU will compensate to an extent for a larger or smaller injector by decreasing or increasing the injector pulse width. I just feel it would be better, especially with dhenney running nitrous to go with a bigger injector size than to go with a maxed out smaller injector.

dhdenney

iTrader: (53)

Well I figured that the nitrous kit being wet, I would get the fuel from the solenoid on the juice. I'm looking for streetability because I can't drive the damn thing on the street! One, fuel is expensive when it's going dry after 175 miles. Two, it drives like ****! I had picked up tunes on an LT1 Edit database, trying different things to see what worked a little better. It was like an epiphany the other day--my injectors were just too big for the tunes I had. I should be able to get close once I get some smaller ones, I just can't afford money for dyno and a tune.

TwoFast4Lv

iTrader: (8)

Well if you have LT1 edit just fix it?

Beast. Only One GM N2O kit for our car(all twin TB cars even) does NOT add fuel WITH the N2O. You use the fuel jet in the N2O kit to tune the fuel in the kit

Even on My NOS 5176(?) kit there is a jet to adjust fuel pressure to taylor fuel needs for my kit.

As Far as GM cars with Stock PCM's only the NOS LS1 kit is a PURE dry kit that only uses the MAF and Stock fuel curve to add the needed fuel. At this point you only need to add the fuel to cover what your car makes N/A. Then adjust the jetting in the N2O kit to get the proper fuel on the bottle.

The only real reason to take Injectors into effect for the Juice is on a pure DRY kit. Most of thise use a aftermarket System to run them

Good luck man I hope this helped some

METALBEAST

iTrader: (1)

Ah, that makes sense 2fast. Guess I should do some more research when it comes to Nitrousing LT1s.

Since 2fast indicated we only need to size for your engine's NA power cababilities:

Therefore assuming your car can put down 450 RWHP, taking into account 15 % drivetrain losses:

450/0.85 = 530 FWHP


Then:

IS = (0.45*530)/(0.9*8) (Use 0.45 for NA with excellent head flow)

IS = 33.125 lb/hr injectors

Rounding up that gives you 36 lb/hr injectors

Good Luck!