Hey guys,

I was wondering if any of you have just a rough idea of the maximum boost that the engine can produce at lower RPMs. I believe I read that you can get max boost, or within a few psi of it, at around 2,600 rpm. Does anyone have any rough guesses for the max boost they can get at 1,250 rpm? 1,500 rpm? 2,000 rpm?

I have an LNF engine in a test stand running natural gas and N-heptane and at 1,000 rpm the max boost I could get was around 1.75 psi. I had the pedal to 100% but that was the maximum the turbo could give me. Also as a bonus question I'm wondering if modifying the hptuners desired air load table would give me any extra boost at 1,000 rpm. I don't believe it would but thought I would ask as I have very little experience with that program.

Thanks in advance.

 

With the stock turbo it's pretty much all in the tuning. You can mechanically mess with the wg all you want, but without the proper tuning you'll never hit the desired boost you're looking for.

It's also much more than just messing with the DAL table. Everything needs to jive across the board throughout the multiple tables controlling boost control. Even at that, 1000rpm you're well within the surge boundaries of the turbo.

 

Max on stock tune with the 15 psi I made was at about 2100 rpm and gm advertised 2000..

Tuned I made 24 around 2800-3000 rpm

 

When at my 1000 rpm the desired boost reading I get is 198 kPa. So to me it seems like the turbo is trying to increase the MAP but is unable too.

But you are suggesting that I'm below the surge limit so my BOV shouldn't be opening so with proper tuning my turbo should be able to produce more boost. Most of the posts I have read on the subject only mention changing the DAL table. However that won't help me since my DAL is far greater than what I'm able to produce.

The only other boost tables I'm aware of are the PIDs. But that shouldn't matter for me because as long as I'm running steady state and the integral term is activated I should have no steady state error.

 

Boost is dependant on load. Not sure about the test stand you mentioned and all that, but without a load on the car (Wheels on the pavement trying to move the car forward) the car will not make much boost.

 

I was producing 110 ft-lbs of torque. I had the pedal all the way to floor and was running a lambda ratio of 1. So I could have done fuel enrichment to get a bit more power but not much.

Like I said the engine was trying to produce boost as the desired boost kept rising as I kept pushing the pedal down but it just wasn't able to accomplish anything more than 1.75 psi.

 

I assume you have a means to keep the engine at constant speed? probably a dyno?

if your desired boost was 198, I assume that "Wastegate DC" was at 95% ?

Since you're on an engine stand, is it possible to look, and can you check that the wastegate doesn't move when you load the engine? ie: at idle the wasteate is closed, then load the engine and make sure the wastegate isn't pushed open by the exhaust gases.

Assuming the wastegate is kept closed, theoretically it is possible that the engine just isn't producing enough exhaust gasses to spool up the turbine? The compressor map and the "surge line" doesn't actually say anything about the turbine side, and whether or not that exhaust flow has enough energy to spool up the compressor. I don't know if that's what's happening to your setup, but i think it's possible.

Could you add some details such as: desired boost, MAP, wastegate DC, throttle position, MAF (if you are still running one), etc.

The consensus is that the turbo compressor map for the LNF is what's attached to this link in post 7; the username of the poster may look familiar, i think the units of the compressor map are in m3/s.
https://www.cobaltss.net/forums/2-0l...-regal-274274/

if you still have the MAF on your engine stand, and if it's calibrated correctly, you could plot your own operating points and determine if the compressor is under surge (but i'm betting you would hear it if that's the case). my guess is that you don't have enough exhaust flow to spool the turbo.

 

Yes the engine is hooked up to a dyno.

I still have the stock MAF sensor and it was reading between 16 and 17 g/sec. TPS was 83%. It got to this value at about pedal position 40%, after that stepping on the pedal more just increased the boost desired (which didn't do anything). MAP was at 104 kPa (atmospheric is 92 kPa). I'm not sure what wastegate DC is and I can't seem to find the PID for it. Is Boost Desired DC the same? Because that was at 95%.

That's what I'm thinking as well. That why I asked what rough boost numbers people were able to produce at lower rpms. Is anyone capable of logging their MAP doing a few low RPM pulls? It would save me a great deal of trouble to just know rough boost / MAP values between 1,000 rpm - 2,000 rpm.

 

Drove my car yesterday (not a cobalt) but realized how ridiculous it is to ask for some one to do a 1,000 rpm - 2,000 rpm pull. You can barely get your foot down and your already out of that range. Also its a circumstance that rarely happens in anything other than first gear.

This is what happens when you spend too much time in the lab playing with engines and not enough time on the street. You get a warped sense of how cars actually operate.

 

i was just gonna say that it is unlikely anyone is looking for high torque at 1000 rpm.

when i was playing around with my car i kept pushing for more low end torque to the point where my stock clutch started slipping, and it was not nearly as low an rpm as you're looking for, i think i started slipping at 2500 rpm, 21psi boost, may be wrong on rpm can't remember now, either way, clutch slipped and i backed off the calibrations because of it.

from reading some of the forums it seems that people actually avoid high torque low rpm because it's hard on the clutch and trans.

that being said, your objective is different since you're on an engine dyno.
from the throttle position, desired boost, "desired boost DC" (i assume that's wastegate DC, you could measure it, you have a scope available), im thinking you are pushing as hard as you can, and just dont have the exhaust flow to spool up.

- Adding fuel to run slighly rich, could/should help (i think), everybody on HPtuners forums seems to love 0.88 lambda on LNFs, i prefer 0.85 .
- Low RPM, you could try to play with the cam's some, at 1000 RPM i think least overlap (so both cams parked) might help. I remember you had a pretty good thread about cam phasing a few months ago.
- if the goal is to make boost and spool up the turbo, and not to make torque, you could retard spark advance, i think i read about this as a means for race cars to burn the fuel in the exhaust rather than the combustion chamber for the purpose of adding heat and pressure for the turbine, at the cost of engine torque, until it spools up.

Disclaimer: These are just some ideas, and personal opinions, i dont actually have any real experience with what you're trying to achieve.

EDIT: one of the important calibration tables in HPTuners that controls boost/torque is the Max Airload Table (people usually call it MALT). Everyone seems to be obsessed with changing the Desired Air Load table, but i'm making the boost i want (22 psi) with an almost stock DAL table. Maybe it comes into play for more agressive builds, but for my daily driven, stock turbo, cobalt, GMPP stage kit DAL table did not need to be changed.

 

I'm not sure why I don't have WasteGate DC PID/Sensor. I can't find it in my table. Can you give me the step by step on how to locate it in VCM Scanner. To find desired boost (%) I go, PID List >> Engine >> Airflow. It actually might be because I haven't updated my VCM scanner in a few months.

I was playing with spark while the engine was running at that condition I wasn't able to raise it by much. However I think that's more because I was already sparking very late (3.5 degrees bTDC) so I couldn't really do much better than that. I'll attach some cylinder pressure shots at the bottom so you can see how late my combustion was.

Essentially I need to raise the effective compression ratio of the engine. My goal is to run natural gas through the port injectors, turn off the spark, and use N-Heptane (octane number of 0) through the direct injectors. I need the higher compression ratio because I need N-heptane to ignite quickly and set off the natural gas combustion. So essentially my direct injectors will be acting like my spark.

Where is the MALT table? I can't seem to find it in VCM editor.


Also I ran the engine today at 1,500 rpm and the max I could get was MAP of 117 kPa, so an improvement but I'm with you that the engine just isn't able to force enough air to spin the turbine. However this gives me around 2,200 kPa peak cylinder pressure with no combustion so I'm pretty close to what I need to ignite N-Heptane.

50 Cycles of Cylinder Pressure


Mean Cylinder Pressure

 

oooo, aaaa, cylinder pressure sensors, how freaking cool is that!!!

1) the PID you are recording "desired boost DC (%)" is the PID i am talking about. you already have it, sorry my posts were confusing. I call it "wastegate DC" for myself, in my head, since that's what i assume it is, and it seems directly tied to the table in VCM editor Torque Management>>Turbocharger>> Wastegate duty cycle.

2) it was just a wild guess on the spark retard thing, and im thinking really retarded, like -20 deg? or even later if you can? , purely meant to let the charge burn in the turbo to spool it.

3) MALT table is in: Torque Management >> General >> "Max Air Load Torque" (somewhere in the center of all the other buttons, under "max torque" title). Sorry my name was slightly off.

4) I assume you already know, but just in case it hasn't jumped out at you, you can also control the direct injector start timing with HPtuners, its under the Fuel>> general >> titled "injector timing", seems like you'll need that for your project. (just in case you haven't noticed it). I dont know how those tables work because i dont need that many changes for my goals, but the guys in HPtuners forums have messed with those tables and have an idea of how they work.

Another disclaimer: I am not a mechanical engineer, just a car enthusiast. Seems like you have a lab-grade setup, cylinder pressure sensors, oscilloscopes, pretty neat stuff. My point is you are way beyond my knowledge level, so take my little guesses with a grain of salt, odds are you know way more than me about engines.

Good luck and keep us updated, i find this really interesting.

EDIT: if you're close to the pressure you want, you might be able to get to your desired pressure by playing with the cams. OR, higher compression ratio, LNF is 9.2, LHU is 9.5 i think, or swap pistons?

 

The stock ECU for my engine doesn't control spark or the direct injectors. I have my own controller where I can change spark timing, direct injection duration and timing, and port injection duration and timing. The reason I had to do this is because I need to be able to change the parameters however I want while the engine is running. Stopping and reloading a new calibration into the ECU just wasn't cutting it.


I mean I'm not some tenured professor with 20 years of experience. I'm a young grad student just fumbling my way through this project. Everyone on this site and specifically you, have been a great help when I get stuck so don't sell yourself short. You can't learn everything in a class room so its nice to get input from people with tons of hands on experience and knowledge.

 

This is pretty cool considering my IC engines class had a very very old flathead engine in the lab for doing this stuff. We also had an APU, but that **** was very loud and it's just not that interesting to run.

Are you able to measure temps before and after the turbine? Maybe that could give some clues about efficiency. How about exhaust gas flow? Like they were saying, 1k rpm just may not provide enough air flow to spool the turbo up to any sort of reasonable efficiency. It's barely above idle.