I’m swapping an LS1/T56 into my ‘81 Corvette. I’m running completely new hard fuel lines (although I’ll be running the original gas tank). Am I going to need to run a return line, or will setting this up as a return less system work? If so, can I set up a return line using my existing gas tank or will I need to upgrade?

 

Assuming you are running an EFI LS swap and need 58 psi at the fuel rail you need a fuel filter regulator and some type of return line. I like and use the C5 Corvette FFR. Only a Wix or AC Delco. Mount it near the tank so the return line is fairly short.
Don't dead head a 58 psi fuel pump at the fuel rail.

 

Got it, so curve ball. I was looking at the Holley 12-313 Fuel Tank Module and it claims to eliminate the requirement for an external fuel pressure regulator or return line, as it returns the excess fuel directly into the tank. Would I be able to use this and eliminate all that, or would it cause issues/would it be better to run an inline fuel pump with a return line and regulator?

I feel like this is a better and simpler way to go, but am unsure if this is compatible with my LS1/if using this will cause other issues I’m not seeing.

 

That will work fine.

 

What type of EFI are you using to control the engine?

 

Most likely the original PCM (with VATS delete), but I’ve been eyeing the Holley Terminator.

 

The reason I ask is, with a stock PCM, the fuel injector data will be different between a return and a returnless system.

I didn't know if you knew that or not, but I at least wanted to mention it.

Holley EFI it doesn't matter.

 

I did not know this - different in what way?

 

An injector's flow rate is rated at atmosphere pressure.

When you are at WOT(in a normally asprirated engine) there is no vacuum in the intake, more importantly there is no vacuum at the bottom of the injector "refer to my first sentence"

However at idle and normal driving, there is vacuum in the intake and the bottom of the injector.

Due to the vacuum/lower atmoshperic pressure, the injector actually flows more because it doesn't have full atmospheric pressure pushing against the bottom.

To keep the engine form getting too much fuel out of the injector, with a return style system, the vacuum from the engine controls the fuel pressure regulator and lowers the fuel pressure depending on how much vacuum there is. At WOT, when there is no vacuum, the regulator doesn't lower the fuel pressure and the injector flows whatever it's rated at.

In a returnless system, the fuel pressure is a fixed pressure and doesn't change. To compensate for that, and engine vacuum, there's a table in tune, "injector flow vs vacuum' that tells the PCM at different amounts of vacuum, the injector flows more. The PCM takes this into account when doing the fueling calculations.

 

So, if I’m understanding this correctly, I’ll have to make sure the PCM is tuned according to whether I do a return or returnless system? Also, it sounds like it’s tuned for a returnless system from the factory? Am I correct?

 

"Most" of them are.

The first year for the Vette and Fbody were return systems and so were the trucks up to a certain year.

 

The engine and trans comes from an ‘04 GTO. So provided the seller didn’t pull a fast one and swap out the PCM, it sounds like I should be okay. 😅

 

Just in case you are unaware, the P59 computer from the GTO can be setup to monitor an alcohol composition sensor to run flex fuel for e85, and can also be setup for lean cruise. There are a lot of other things you can do with it, but they mostly apply to forced induction setups.