I've done a bit of NA tuning and have searched and read alot on here about tuning the stock PCM with boost. I know alot of guys are still running stock PCM's and not blowing things up but I still have some basic questions on the matter...
Oh and I'm a turbo application if it matters

1. I've read that you only see boost at WOT. I don't see how this is possibly true--I would think while under load at 3/4 throttle (which I may see while feathering the throttle for traction) would also see some sort of positive manifold pressure. Is this just a myth? I'm almost positive with a gear such as 2.73, 3.08 or maybe even 3.23 backed by an auto tranny would load up the turbo and make boost without slamming the pedal to the floor.

2. Am I correct in my belief that if I program the spark and VE tables for 14psi (in the 90-100kPA columns of course) and then I want to back it down to 10 psi for some dumb reason, I will have to reflash for a 10psi specific tune? ...and if I do not reflash, it will run rich and probably be in need of a degree or two of spark?

3. If I run boost timing module (such as MSD 6 btm), can I just leave my spark tables as if I were running NA since the MSD should take care of the timing under boost?

4. Aren't there fuel pressure regulators which increase fuel pressure with boost? Are these safe to use for dialing in your AFR? And do they stay linear and accurate with all different boost levels seen by an engine?

 

AS FAR AS I KNOW.... the lt1 MAF will be maxed out fast and to do a SD style tune on stock pcm you lose half your resolution.

Best person to ask is the guys at pcm for less... though i believe they compensate for boost through the power enrichment tables...

im no tuner... this is jsut what i have read...

 

Turbos are different to tune than superchargers but I will take a stab at your questions.



You will see boost as low as 30 percent throttle so you can set your "Throttle position for WOT table" to 30 so the PE tables will take over.


If you keep your MAF it will compensate for lower boost up to the point it maxes at about 500hp. I still haven't figured out how the computer gets its fueling info after the MAF maxes other than the PE tables but it continues to add fuel. VE tables will do nothing (arguable) unless your running the tune with Speed Density turned on. It would gain you a little power to have two separate tunes due to additional timing but it is not necessary.


This is correct. If you don't then you have to use a fixed timing number (100kpa) for all boost levels once the MAP sensor maxes.


Adjustable boost compensating regulaters and FMU's are a very crude way to tune AFR. But it can be done. I like my alkycontrol because it increases the alky flow as boost increases instead of by RPM like the PE tables

 

Thanks, that is the kind of info I was hunting. If anyone wants to add to it, please do!

 

I'll probably get ridiculed for even suggesting this, but you COULD insert a parallel MAF sensor as a dummy to reduce the airflow through the active one by 50%. This would allow you to run much more than 500 HP without maxing out the MAF. You would obviously have to adjust the MAF calibration accordingly in the PCM. It would need custom ducting that would waste a lot of space too. Definitely a crude solution but I think it would work.

 

I've read about that but I'm not sure if anyone has actually implemented it on a running car. It seems like a viable option as long as the tuner does not expect there to be a 50% relation between the two MAFs. Air distribution would not be uniform. Also, I believe idle quality would suffer even on a stockish cam. Low airflow conditions would probably be a problem with this set up even if you got the high flow conditions dialed in.

 

You are very likely correct about the poor idle. I also agree that it wouldn't be EXACTLY 50% split in the airflow, although I think it would be pretty close, and relatively easy to account for the difference in the calibration. It was just an idea to throw out there.

 

I wish there was more info out there on the 2 or 3 bar map sensor integration. If someone gave me a map sensor that would work, I would work on trying to write a tune that worked once I get the car going and then I could do a write up on it and help out alot of people if done successfully. I'm hoping to fire this baby up in a couple weeks

 

The only thing the stock computer needs is help with boost compensated timing and fueling. These issues can easily be solved with alcohol injection kits and boost compensated timing boxes.

I know Alvin from PCM for less made a 2 bar setup but it must not have worked to well because it never caught on here. I'm really hoping someone will make a kit to adapt a LS1 pcm to our cars. They are much more boost friendly.

 

how would you wire up both mafs into the computer? it seems that you could probably run a single maf as long as exactly half of the air went around it through a seperate pipe, but that would be very difficult to dial in exactly

 

As far as #3 - these work but they may not be optimized and you cant be sure what you're getting for timing - only a dyno tune could tell you that.

For #4, the FMUs are not a good way to go, but the boost compensating regulators are a good thing if they work correctly because you are actually building boost in the intake where the injector is trying to squirt, so if the intake pressure goes up, the fuel pressure should go up at the same time to compensate. You could also just tune more fuel in there too, but you dont want to run lean either.

 

I am planning on running an alcohol injection kit, but how will this defeat the fuel issue? I understand your recommendation on the boost timing module.
Also, one of the sponsors over at ls1tech.com just asked ideas for new products. I proposed the LS1 pcm to LT1 car idea to him--hopefully he bites the bate and does somethign for us! <A HREF="http://www.ls1tech.com/forums/showthread.php?t=769876">LS1tech reference</A> okay so I never learned HTML, give me a break!

Kevin, a dynotune will be in the works once I get all this figured out. I actually do some of the basic LT1 tuning at a shop in Columbus so I guess I have "da hook up." I know it is not going to give you an optimum curve since the relationship of spark to boost is probably not linear, however, it seems better than having a set timing for all levels of boost.

I guess a boost compensating regulator is what I had in mind when I posted #4 (I did not know what the terminology was for it, but now I do!). Also, I need to learn what is the difference between an FMU and a boost compensating regulator. I assume the boost compensating regulator will increase fuel pressure as manifold pressure increases.

You are thinking like me. I don't know how you could wire up both mafs, but if you had approximately half the air flowing through the maf and half through an alternate duct, it would work. But again, flow would not be evenly dispersed through the MAF and the duct, and also at low frequencies, you would lose resolution and the car probably would not want to idle.

This has been an interesting discussion for me thus far

 

After further thought I don't think the twin MAF idea would work. Tunercats limits the maximum airflow in the MAF calibration table to 512 g/s, so you wouldn't be able to overcome that limit. I originally thought the only limit was in the sensor output frequency, but that's not the case. So much for that idea.

 

You might find more guys who are knowledgable of this stuff in the Computer Diagnostics & Programming Forum. Your questions are above my level of knowledge on the subject of tuning, however I also reccomend asking Bryan from PCM's for less. He has safely tuned my Z28 in my sig with the stock PCM. After looking and talking with most of the tuners out there for the LT1's, Bryan is the most knowledgable IMO, and will be able to do the best job. It gets really tricky with running the stock PCM on an LT1 car especially at higher boost levels where the MAF is maxed out.

 

Yeah, the bulk of my question was to figure out about the BTM and boost fuel regulator--these things do not require knowledge of calibrating the PCM. But, when it comes time for calibrating, I probably will try to talk to someone with more experience to see how they've done it.