Knock after fuel rail pressure increase.

[J-CUE]

I'm working on a 2014 Sierra with a 5.3L N/A with intake only, and noticed that after increasing the fuel pressure
It started to get bad knock in the lower RPM range, around 1200 to 1600. I increased the pressure in the high load Ares, and in the 1200 to 1600 RPM area, the pressure was increased from 9.6 MPa to 15 MPa.
I ran out of time trying to dial it in but, I did get rid of the bad knock by retarding ignition timing and advancing fuel SOI in those areas for right now until I have more time to work with it.

Has anyone else experience this?

[Higgs Boson]

Leave your fuel stuff stock, there is no need to change any of it for any NA mods. Don't even need to change SOI.

[J-CUE]

I understand, and I mostly agree with you but, There is still room for improvement, albeit small for stock applications.

If you change valve timing, and ignition timing, the fuel SOI timing needs to be changed as well.



I haven’t had time to dig into my situation but, I’m assuming the knock I observed was from a combination of higher cylinder air charge because of the advanced cam timing, higher cylinder temp because of less fuel impingement from too late of fuel SOI timing, faster flame propagation because of better fuel atomization from the higher fuel rail pressure, and too much spark advance for this combination?



I have a chance to play around with a stock application and I want to find out what I can get out of it. Also to learn as much as possible with GDI engines

Before complicating the process with forced induction.



Thank you for your input, I’m learning like the rest of us, and was asking that question, to hopefully learn more.

I appreciate any and all input. Thank you.

[Higgs Boson]

The most you can advance the cam is to 0 value. If we are talking about part throttle, you can 0 out the lower RPM retard values and just 0 out the corresponding VVT Spark table cells. Your orignal post said you got knock from increasing fuel pressure down low, which is unnecessary.

There is very little benefit in changing low load / part throttle spark values, the car will run better and smoother with them stock and SOI stock and only adjust high load/RPM SOI only when needed to satisfy pulse width requirements. If you aren't running out of injection time, no need to start it sooner.

[GHuggins]

If your gaining anything much at all over stock with cam settings using a stock cam, then you must be doing something special because I've never gained anything at all on stockers by messing with cam settings. In fact I've gone in behind others and set it back to stock and made gains... Only ever seen gains dialing in a/m cams on these... Like Higg's said, you shouldn't need to mess with injection pressure. You can change soi for mpg gains, but not much for hp gains unless it's too rich, then from what I've seen you can lower soi, dial in the ve and maf and make some gains...

But what do I know?

[ecotec88fiero]

The most you can advance the cam is to 0 value. If we are talking about part throttle, you can 0 out the lower RPM retard values and just 0 out the corresponding VVT Spark table cells. Your orignal post said you got knock from increasing fuel pressure down low, which is unnecessary.

There is very little benefit in changing low load / part throttle spark values, the car will run better and smoother with them stock and SOI stock and only adjust high load/RPM SOI only when needed to satisfy pulse width requirements. If you aren't running out of injection time, no need to start it sooner.

I was recently reading a general DI tuning article and it mentioned that SOI being started too soon can cause problems. If you think about it, if you start it too soon you have a very hot piston crown(coming off the exhaust stroke) and now you are injecting much cooler high pressure fuel onto it which could cause pre igntion or even piston damage depending on load. No sure how relevant it is on the GM DI motors but it was something I thought was interesting and to keep in mind when changing the SOI if you start seeing knock.

[Higgs Boson]

I was recently reading a general DI tuning article and it mentioned that SOI being started too soon can cause problems. If you think about it, if you start it too soon you have a very hot piston crown(coming off the exhaust stroke) and now you are injecting much cooler high pressure fuel onto it which could cause pre igntion or even piston damage depending on load. No sure how relevant it is on the GM DI motors but it was something I thought was interesting and to keep in mind when changing the SOI if you start seeing knock.

even port injected motors inject fuel during overlap when the intake valve opens during the exhaust stroke so I don't think there is an issue with a hot piston. however, injecting too early with DI will send all that fuel out the open exhaust valve since you aren't spraying on a closed intake valve.

we have a thread on SOI, of course there is always room for more info, but what I found was injecting too early (still during intake stroke) made the car sluggish and can cause puddling on the piston vs injecting super late (like during the compression stroke) even at low demand times like idle or cruise, can cause crappy running as the fuel doesn't have enough time to do it's thing in the cylinder before spark occurs.

what I have found on 100% of the cars I have tuned is that they all run best, no matter what mods, if you leave SOI and spark lead bone stock everywhere except where you need to edit them. on boosted cars I move SOI as early as possible without blowing out the exhaust valve (depends on EVC) and move spark timing down for the FI application so I spread them out more. On NA cars if I advance spark lead, I also advance SOI the same amount in the same areas so I keep them the same distance from each other....generally.

[ecotec88fiero]

I wasn't sure if this applied to GM's DI motors because they have spent a lot of time and money on mixture propagation and combustion quality but it was interesting to read about. Maybe other manufactures have this problem. This article was geared a little towards DOHC import engines which may change some things due to multiple cams. Also, GM has the piston squirters on all their DI motors so maybe this keeps the pre ignition from happening.

I have read the SOI injection threads, great information there.