[Joshinator99]

Originally Posted by MartinLe

OK, in a perfect world, we have 75# low side and 2900# at the rail and IPW's in the mid 5's. Of the three numbers, the IPW's are the most important in keeping the motor in one piece. Now we have some fluctuation in the fuel pressure, but the IPW's stay in the happy zone. During a run, the low side hangs in there at 70#+ and IPW's are in the low 5's. At the shift into second at 7K, in that little window of rpm drop at the disengagement of the clutch, the rail pressure drops to 2000# (low side 70#) and the IPW's raise to 5.8 - 5.9 for a split second, and then rail goes up to 2800# and IPW's back to mid 5's. The motor is happy. This is my understanding.

When I installed the blower/cam package, I was discouraged from the LPE BB pump because it ran at 2175#. "We want to have the 2900# at the rail at all times." You are running a steady 2175# at the rail and the IPW's are in the 5's. How can that be argued as anything less than safe and acceptable? Some tuners will argue the 2175#. But the IPW's are in the happy zone.

As Josh stated, the higher the pressure the lower the volume. The LPE pump will run under some conditions at 2900#, but it is designed for 2175#. Why wouldn't the OEM pump push more fuel at a lower PSI?

I apologize if this is a little derail of the OP's intent, but it is about pressures.

Not sure who discouraged you from the BB HPFP, but it simply moves more fuel than the LT4 HPFP. There is a ton of confusion about that product, and LPE does not do themselves any favors as their technical support of the product is marginal at best. The size of the LT4 HPFP bore is likely not big enough to handle more volume, even at lower pressures, so in that scenario you might as well run higher pressure since it helps the injectors flow more.

At the end of the day, you’re on the right track. Keeping the IPWs in check is the name of the game. Rock solid low side pressure and bigger injectors are the foundation and the one variable you can mess with is the HPFP pressure, if you’re running the LPE version.

[MartinLe]

Originally Posted by Joshinator99

Not sure who discouraged you from the BB HPFP, but it simply moves more fuel than the LT4 HPFP. There is a ton of confusion about that product, and LPE does not do themselves any favors as their technical support of the product is marginal at best. The size of the LT4 HPFP bore is likely not big enough to handle more volume, even at lower pressures, so in that scenario you might as well run higher pressure since it helps the injectors flow more.

At the end of the day, you’re on the right track. Keeping the IPWs in check is the name of the game. Rock solid low side pressure and bigger injectors are the foundation and the one variable you can mess with is the HPFP pressure, if you’re running the LPE version.

So, even if my rail pressure fluctuates, as long as the IPW's are in the 5's, I can not be concerned?

[Joshinator99]

Originally Posted by MartinLe

So, even if my rail pressure fluctuates, as long as the IPW's are in the 5's, I can not be concerned?

Rail pressure is going to fluctuate *a little*. It’s driven by cam pulses so if you command 2175, you’ll see maybe 2050-2300 or so as it bounces around. It should bounce right back very quickly to what is commanded though. A number like 1800 is unacceptable. If it’s low for a good part of the run then you have an issue, and I find that if I get too greedy the HPFP will literally drop off like a rock and IPWs shoot way up. If you’re under 6.0ms I wouldn’t be nervous. Generally speaking, what you command for low side and HPFP pressures should be there within 5-10%.

[Megahurtz]

The info in here has been very valuable in regards to the LPE HPFP. I too was also under the impression that with the Supercharger, you need higher fuel pressure. But if 2175 PSI is sufficient and the pump flows more volume at a lower PSI, then that seems like the right thing to do with the XDI 30% injectors that I'm installing as well.

I'm looking on their website and it still says max pressure recommended is 2175. I swear I spoke to them and 2900 was acceptable. Maybe the guy on the phone didn't know or didn't care. Since it is not documented in writing, I'm inclined to believe I was falsely advised by whomever I spoke with.

I tune a bunch of cammed LT4 hpfp stuff but this would be my first LPE big bore. Fortunately I'm testing in my own car so I have plenty of time to test. I am after volume so that I can run full pump E so it seems I should stick to a lower pressure. I really appreciate all of those who has chimed in with their experience.

[CW3SF]

Originally Posted by Megahurtz

The info in here has been very valuable in regards to the LPE HPFP. I too was also under the impression that with the Supercharger, you need higher fuel pressure. But if 2175 PSI is sufficient and the pump flows more volume at a lower PSI, then that seems like the right thing to do with the XDI 30% injectors that I'm installing as well.

I'm looking on their website and it still says max pressure recommended is 2175. I swear I spoke to them and 2900 was acceptable. Maybe the guy on the phone didn't know or didn't care. Since it is not documented in writing, I'm inclined to believe I was falsely advised by whomever I spoke with.

I tune a bunch of cammed LT4 hpfp stuff but this would be my first LPE big bore. Fortunately I'm testing in my own car so I have plenty of time to test. I am after volume so that I can run full pump E so it seems I should stick to a lower pressure. I really appreciate all of those who has chimed in with their experience.

My local pumps in Colorado were e70, and not much gain above that anyways. So I always just topped off at that pump. It is very nice just pulling in and filling with ethanol from one pump.

[KingLT1]

Originally Posted by MartinLe

OK, in a perfect world, we have 75# low side and 2900# at the rail and IPW's in the mid 5's. Of the three numbers, the IPW's are the most important in keeping the motor in one piece. Now we have some fluctuation in the fuel pressure, but the IPW's stay in the happy zone. During a run, the low side hangs in there at 70#+ and IPW's are in the low 5's. At the shift into second at 7K, in that little window of rpm drop at the disengagement of the clutch, the rail pressure drops to 2000# (low side 70#) and the IPW's raise to 5.8 - 5.9 for a split second, and then rail goes up to 2800# and IPW's back to mid 5's. The motor is happy. This is my understanding.

When I installed the blower/cam package, I was discouraged from the LPE BB pump because it ran at 2175#. "We want to have the 2900# at the rail at all times." You are running a steady 2175# at the rail and the IPW's are in the 5's. How can that be argued as anything less than safe and acceptable? Some tuners will argue the 2175#. But the IPW's are in the happy zone.

As Josh stated, the higher the pressure the lower the volume. The LPE pump will run under some conditions at 2900#, but it is designed for 2175#. Why wouldn't the OEM pump push more fuel at a lower PSI?

I apologize if this is a little derail of the OP's intent, but it is about pressures.

Alot of tuners don't understand the LPE pump. LPE pump is a modified LT1 pump designed to run @ LT1 pressure which is 2175psi. It flows 10% more @ 2175 vs the LT4 @ 2900psi. I assume LPE chose to use the LT1 pump as the foundation to make a higher volume pump because the LT4 pump is limited on the size of cam lobe to drive it. The LT4 can only run up to a 38% fuel lobe. The LPE can handle a 43% lobe. So, the 10 % flow increase on the LPE is comparing the same fuel lobe. Move up to a 43% lobe then it flow significantly more than the +10% volume vs a LT4 with a 38.

Lowering the pressure on any pump will lower fuel delivery. The difference here is one is designed to operate at a lower pressure vs the other. You can crank the pressure up on the LT4. I have seen guys run 32-3400 psi. The problem is most aftermarket injectors are not stable at that high of pressure. Only the OEM LT4 injectors can tolerate that high of pressure and it's only going to squeak a little more headroom out.

[Megahurtz]

All in all the LPE pump was pretty easy to dial in. Just had to tweak the high side pressure in some areas so that the injectors played nice.

[LiqTenExp]

High side fuel pressure discussion for anyone doing their own tuning...

Something that was in my tune before I took it over it was ~3400 psi high side peak. The car has been like this for 5 years and makes great power and reaches commanded high side fuel pressure...See attachment from a dyno run back in 2021.

I'm stock ZL1 LT4 high side fuel pump, stock low side pump w/JMS FuelMax, and stock injectors. I have a cam w/larger fuel lobe, tvs 2300, and everything else you'd expect to reach 800+whp

The car has no issues. I understand the concern for injectors to misbehave above 2900 but this was a setting I never looked at touched once I took over the tune a few year ago (there wasn't issues in this area so just missed it). Peak injector DC is 5 ms, it was below 5.4 ms before I added a 50/50 meth spray. I assume if I turned down pressure that I'd need more DC to compensate... anyone experiment with this?

I guess my takeaway is it is possible to maintain those pressures with stock everything fuel system wise (w/JMS). This also might be something to "turn down" a little and see how the car responds.

[Megahurtz]

While your post is off topic, I have experience with cranking up the pressure on the stock LT4 stuff. 3400 is about the max where the stock injectors are happy. You need a cam of course to maintain that pressure. I have a few of those setups running pretty big meth kits. I just dyno tuned a couple of ZL1's like this. One was on stock blower and stock pulley that hit 798rwhp and the other one was on a smaller cam but max effort blower and pulleys and more meth and it made just shy of 900rwhp.

Both cars with stock LT4 fuel system parts, 32% fuel lobe and meth.

The setup that I started this thread about was because I wanted the maximum amount of power without a cam or meth as it is not setup for racing. It's a street car with factory tires. Car is max effort bolt on, x port blower, and moderate pulley and can run E60-E70 depending on weather.

[KingLT1]

^ I was going to say the same thing. Stock LT4 stuff can be cranked up higher but we are discussing aftermarket stuff which is designed to flow more volume with less pressure. I like volume vs pressure because the fueling stays more consistent with changes in alcohol content. The LT4 stuff cranked up is like a light switch. You either have enough fuel or it just tanks hard.

[JSH]

Originally Posted by KingLT1

You either have enough fuel or it just tanks hard.

which is what mine did on the last dyno pull, but no one sees a fueling issue in the log, so... we're dynoing again today or plan to

[Joshinator99]

Originally Posted by KingLT1

^ I was going to say the same thing. Stock LT4 stuff can be cranked up higher but we are discussing aftermarket stuff which is designed to flow more volume with less pressure. I like volume vs pressure because the fueling stays more consistent with changes in alcohol content. The LT4 stuff cranked up is like a light switch. You either have enough fuel or it just tanks hard.

Originally Posted by JSH

which is what mine did on the last dyno pull, but no one sees a fueling issue in the log, so... we're dynoing again today or plan to

If what King describes is happening to you, it will be plain as day in the logs. I’ve had it happen on both LT4 and LPE HPFPs when I got overzealous with commanded rail pressure. It TANKS and your IPWs go sky high…like 9+ MS lol. The key is to do short, one gear, test runs after you make changes…that way you minimize the chances of wrecking stuff.

[Megahurtz]

Anyone else with the LPE Big Bore have a manual? I'm seeing in the logs when I No Lift Shift at redline, the high side pressure spikes to 3600psi for a brief moment. I'm hoping this isn't an issue. I'm certainly not commanding that much pressure. It happens when the injectors close at redline before I let the clutch back out. The stock setup never did this. I don't want to blow the seal out of the pump. It took 6 months to get it from LPE due to backorder.

[Joshinator99]

Originally Posted by Megahurtz

Anyone else with the LPE Big Bore have a manual? I'm seeing in the logs when I No Lift Shift at redline, the high side pressure spikes to 3600psi for a brief moment. I'm hoping this isn't an issue. I'm certainly not commanding that much pressure. It happens when the injectors close at redline before I let the clutch back out. The stock setup never did this. I don't want to blow the seal out of the pump. It took 6 months to get it from LPE due to backorder.

What are you commanding…?