fmu's

[3 eyed snake]

could the wrong fmu cause a back-fire ? when you press the gas all the back-firing goes away... i'm freak'n frustrated...

 

[Daggar]

If the FMU is ramping up the fuel too fast, then yes... it can cause a backfire. Are you sitting stationary and revving the motor up to redline time and again? If so then stop doing that. lol The FMU is designed to increase fuel pressure for load conditions not no load conditions. If it's backfiring when you're moving and under power then the ratio of the FMU is too high for your injectors and application.

 

[3 eyed snake]

i have 24 lbs injectors in it now. would you reccomend stepping up to 30's or 36 pounders ? its has a vortec s-trim pushing 9 lbs. of boost. My friend suggested larger injectors. Also if i keep the gas pedal at a certain position it wont do it. never at idle only at speed when i let off the gas pedal....

 

[Daggar]

Where did the FMU come from? What ratio is it? What's your motor combo?

My guess is that it's a 10:1 ratio and that the 24lb injectors are too large to be used with that particular FMU.

Post your mods and we'll see what's what.

 

[3 eyed snake]

i bought the car second hand but i do know it has a cam, bored 30 over ( 306 ), shorty equal length headers, 4.10 gears, stock t-5, cat-less h-pipe that flows into flowmasters then dumps... also has jr heads . it sounds killer until it backfires and people start running fo rther lives... lol

 

[3 eyed snake]

its a vortec fmu. but i think the guy said that they sent the wrong one or he bought the wrong one. he said the bad boy dyno's in the 430's...

 

[Daggar]

Do you have a Vortech supercharger installed as well? How much boost? Was it ever dynotuned with that FMU installed?

 

[alanw6]

I believe a 12:1 FMU comes with the base S-trim kit (for 19 lb. injectors). If you have a 10:1, that should be correct for 24 lb injectors.

IMO, 24 lb squirters on that setup is a little short. Look into ditching the FMU and get a proper set of injectors, MAF, and a tuning device/chip.

You won't miss the FMU when it's gone.

 

[3 eyed snake]

hey daggar you on this morning?

 

[Daggar]

Yep... I'm here. Just answered your PM. lol

 

[Daggar]

I believe a 12:1 FMU comes with the base S-trim kit (for 19 lb. injectors). If you have a 10:1, that should be correct for 24 lb injectors.

IMO, 24 lb squirters on that setup is a little short. Look into ditching the FMU and get a proper set of injectors, MAF, and a tuning device/chip.

You won't miss the FMU when it's gone.

Alan is right. Properly sized injectors and a custom tune are infinitely more adjustable and accurate than an FMU and under size injectors. The higher you press the fuel pressure up, the more inefficient the spray pattern of an injector becomes. FMUs also have no regard whatsoever of air/fuel ratio through the power range. They do what they do and AFR is either within range or it's not. The only way to correct that (if it's not) is with a (wait for it)... custom tune! If you in a spot where you need that tune anyway then it may as well be with an injector that correctly sized for your application.