MSefi.com read only forum (Jan.15,2018)

I have a GM TBI from a 454. Stock fuel pressure is roughly 15-20lbs for that unit. I would like to reuse my Holley 12-920 external inline fuel pump for this project, but I'm not sure if the pressure would be too much for the Stock internal regulator on the TBI ( holley rates this pump at 100psi max). I've read another post about Matt from DIYAutoTune trying this with a Walbro pump, and he had no problems (any more info Matt?)

I've used this Holley pump on my MFI for my buick for years with no problems (which requires around 40-50lbs of fuel pressure) I removed it because it was loud and the new project I'm working on does not need a quiet pump. I would like to reuse it if possible. Would it be wise to try and use the same size fuel line for supply and return? Would this help relieve some of the extra pressure?

The pressure rating is how much pressure the fuel pump can deliver. How much it actually will depends on the regulator, and if a regulator has problems, it's from the volume and not the pressure.

Yes, we did run a 350 TBI with a Walbro GSL392 pump. Didn't have any trouble with it.

The return line can play into the equation. If it is too small it will become the "regulator" causing too much pressure, overriding the actual regulator. If you use the same size line as the stock supply/return lines for the TBI then you would eliminate that potential issue.

usagoal wrote:Would it be wise to try and use the same size fuel line for supply and return? Would this help relieve some of the extra pressure?

This would depend on the flow capacity of the FPR vs the pump vs the return line. If the stock GM return line (or any restrictions in the casting after the FPR) can outflow the FPR, then a larger line will not help.

I’m always compelled to chime in on pump pressure questions. #1 rule – pumps create flow, not pressure. #2 - Pressure is the result of restriction in a system in the face of pump flow.

A pump’s output is actually a curve of flow vs pressure, but a pump’s rating is generally given as a single point on that curve (X volume @ Y pressure), and the maximum flow is at free-flow (meaning zero pressure). Flow will always decrease with increase in pressure.

System pressure in this scenario depends on the flow capacity of the pump vs the flow capacity of the FPR.

So to add onto the first 2 responses (both of which are good, BTW) to help us visualize the results:
1. Block the return line and the system pressure (measured between the pump and regulator, and preferably as close to the TBI as possible) will shoot up to maximum pump rating (above the TBI FPR setting with a GSL392), because the FPR flowrate went to zero.
2. Restrict the return line and the pressure could be above the regulator’s setting but below the pump rating (if the FPR’s flow rating is exceeded).
3. Have a full-flow return line, but an even-higher pump volume output (and high pressure rating), and if pump output overcomes the FPR the pressure increases.
4. Have a full-flow return line, and a very high pump volume output but low pressure rating, and pump output would be ‘ceilinged’ to the FPR pressure setting.
What it all boils down to is the volume the FPR can flow before it becomes a restriction.

Based on info Matt has given us, the GM TBI FPR flows enough volume to not restrict a high-volume/high pressure pump.

Sorry to flood this topic with info.

FWIW, it depends on what year of 454 TBI unit you have. ‘Early’ ones have 90lb injectors and the FPR has a 13psi –rated spring, while ‘late’ 454 units have lower-rated injectors (like 60 lbs?) and a 30psi regulator spring.

The return fitting on the GM TBI units is smaller than the inlet (inlet - 3/8", return is 5/16"). This was quite possibly to prevent them from being plumbed backwards. The ID of the return fitting, and any restrictions in the casting after the FPR, will play a role in the pressures and flows if your pump can exceed the FPR flow capacity.

It is conceivable that if you need to increase the flow on teh return side that you could radius some edges or re-drill teh IDs larger to help a bit. I have 5-10 350 and 454 TBs at home but never really looked that closely at them to evaluate the FPR flow. Based on Matt's info (which I also asked of him some time ago) I merely cut/welded -6 JIC fittings onto the GM fittings and moved on to teh next task.

Geez, I'm like a kitten with a ball of yarn on this thing...

Here's an old thread I found, might be some more good info in it.
viewtopic.php?f=29&t=30715&p=191460#p191242

Thanks for the replies everyone. I will go with my proposed setup and see what happens. I may even install a fuel pressure gauge just for curiosity's sake. Thanks again for all the responses!

usagoal wrote:Thanks for the replies everyone. I will go with my proposed setup and see what happens. I may even install a fuel pressure gauge just for curiosity's sake. Thanks again for all the responses!

Hi !

Have you tested this setup ? I am really interested in knowing the results !