Please help diagnose fuel pressure drop under boost
 

Hey folks,
I mentioned before that my 2011 LLT with Stage 1 overkill s/c is dropping serious fuel rail pressure under load. The issue only happens at WOT between about 6,000-7,000 rpm and 8-ish lbs of boost. Fuel rail pressure at lower rpms is fine, even at WOT. I’ve had multiple tuners try and get it right, installed a BAP and had it retuned, and even added meth for giggles. Will Overkill’s tweaks to the tune seemed to focus on lowering the redline/shift points to keep the issue at bay. That resulted in noticeably less max power.

Most forum members said upgrade to the ZL1 tank fuel pump. What I don’t understand is why a tank pump upgrade would help when it is the high pressure side that’s struggling. Actual vs commanded fuel pressure from the tank pump seems solid. Any help educating me on that point is much appreciated.

Crazy that nobody came up with an easy upgrade to the LLT HPFP. I read about that Ferrari mod, potential cam changes, etc. but damn it would be nice just to install a better HPFP that bolts right up.

Thanks

Add a boost pump, (DSM has kits). Maybe Dave (DSM) can recommend a setup...

-Don

Thanks, Don. I did add a JMS boost pump, have it set to 17.48v, and my tuner adjusted the fuel calibrations but I’m still dropping pressure at the fuel rail at high rpms under boost.

The in tank fuel pump makes sure there is enough fuel being fed to the hpfp so it is not getting starved at all and can maintain the high side pressure.

Thanks for the response folks. The light bulb finally clicked for me, I think. I had been too worried about pressure on the low side but wasn’t thinking enough about volume from the low side. Makes sense if the volume is low at the tank that the HPFP wouldn’t be happy. I ordered the ZL1 pump and an access door. If only I had foresight to do the in tank pump upgrade when I did those rear cradle bushings, upper control arm bushings, yada, yada. I’ll post back on how it goes. My glass is half fuel (pun intended) that more volume will equate to higher pressure at the rail.

It should help a lot, I am currently on 18psi with full e85 and that was about the max point of the zl1 pump before pressure began to fall off

I’d be pleasantly surprised if you’re that kind of boost and power with the stock HPFP 2012SIMRS. Any cams/lobes or other mods needed to get you to 18 psi boost?
Thanks

Hey sorry I should have been a little more clear. That is with hpfp, cams, and injectors on e85 which needs a fair bit more volume. I did everything in steps and on normal gas on a stock hpfp and stock injectors, but a zl1 pump I got to right around the 10-11psi mark before needing to do more so it should be more than enough for the 8psi you are trying to run

Appreciate the info. What HPFP upgrade did you go with?

I am lfx, so I used the one available from Overkill. I do not believe there is a llt one sadly, but I can think of some other things that could help depending on your end goal

Update?
 

Hey OP, curious if installing the ZL1 pump resolved your issues? I’m currently experiencing a similar problem on the high side and was just about to pull the trigger on a WMI kit but you mentioned that didn’t seem to do the trick. I’m running Stage 3 from Overkill with his included HPFP along with modified fuel lobe and larger injectors. My shop doing the work said “Cannot finalize tuning at this time since car registers knock retard during dyno pull and then erratic fueling and excess knock retard above 6200 RPM. Most likely out of fueling based on reported injector duty cycle of over 30%.” See attached picture. This has been a bit frustrating and wondering if anyone had info for a solution, thanks!

Hey Batmaro,

The ZL1 pump did not resolve my issue. I was still dropping the same psi at high rpm/boost with it. What I did to resolve the issue was swap the Vortech pulley from Will’s standard 3.6” to a 3.7”. That chilled the Vortech out a bit and almost single handily resolved the fuel starvation from the HPFP at max speeds. I’m still running over 9 psi boost with the 3.7” pulley.

To clarify, my Alky meth kit (100%) did keep the AFRs in the safe zone, but I just didn’t feel right about having dramatic fuel pressure drop at the rail when really getting on it. That would make for a crazy oh shit moment if I somehow didn’t know that I was out of meth (unlikely, but those red leds do go out from time to time).!

Others can correct me if I’m wrong, but I’m pretty sure that your injector duty of 26.9% means that you have a ton of headroom (73.1%) left. I’d be interested to see what your tuner was logging for fuel rail pressure and injector pulse width over 6,200 rpm. My injector duty cycle was looking fine but the pulse width was in the high 6s and was basically maxed out for the stock LLT squirters. Any chance you have that rail and pulse data?

Interesting, don’t think we would’ve even considered that adjustment! My tuner is currently away racing for the weekend but I will be going by the shop on Monday and can definitely ask for some numbers. I’ve also shot Will several messages so he can weigh in on the issue since he’s been providing tuning support throughout the installation process but turnaround response time is always snail mail.

Which Alky Control meth kit did you install and why? I was browsing through different brands since Overkill offers one but there’s also others like Alky, AEM, Snow and having trouble deciding so would appreciate an opinion..

I went with Alky because of the quality of their components and reputation. Plus Julio who runs the place has excellent customer service, if you need it. I went with the trunk mounted kit. You have to get the MAF kit for the LLT, since it has no MAP:

https://www.alkycontrol.com/shop/201...-maf-trunk-kit