First WOT scan on new motor...Problems Detected

Ok i've seen this here somewhere before but cant find it after searching for 30 minutes. What's the equation to convert Map kPa and Barometric pressure kPa into boost PSI?

First logged WOT run on the new motor. Have some weird numbers with my LTFT and STFT but I just reset them earlier today. I was just curious as to Boost PSI.

High end of the run:

Map kPa = 126
Barometric Pressure kPa = 102

So from what I've seen posted on other sites 126 - 102 = 24 x .49 = 11.76 PSI ?

IIRC MAP KPa 0-100 is negative vacuum, 100-200 is positive, 200 is ~1 atmosphere or 14.7?something PSI.

Baro is atmospheric, should always be almost 100, a reference point. If MAP = Baro you are at 0 PSI, WOT in an N/A car or just about to enter boost in a SC car.

0 MAP = -14.7 PSI (vacuum), 100 MAP = 0 PSI, 200 MAP = 14.7 PSI (boost)

If I'm way off feel free to correct me but I think that's it.

That's exactly what I've seen around on the different sites.

So the formula must be (14.7/100) ~ 14.7/(MAP-100) to get vacuum?

edit no I don't think this is right, but it's something like this.

This is going to drive me nuts. I know it was more simple than this. I have a whole yellow notepad page full of Texan math...its not looking good.


Also, trying to eliminate a fuel smell, going to get another screenshot at idle. LTFT sticks at 16.4 and STFT stays at 0 and O2 is low 400's...not sure if that's normal at idle.

You both have the right idea. Derek's numbers are close enough, but the .49 coefficient is incorrect

Understanding that barometer changes with the weather:

Atmospheric pressure at sea level = 101.33 kPa = 29.92 inHg = 14.70 PSI

Or:

Atmospheric pressure at 333 ft elevation = 100.00 kPa = 29.53 inHg = 14.50 PSI

Again, weather throws it off, even more so than elevation. I'm at about 875 ft elev, and have seen anywhere from 97 - 103 according to AutoTap.

To calculate boost: MAP - Barometric Pressure = Boost

So, your MAP was 130 kPa - 102 kPa Barometric Pressure = 28 kPa Boost

Converting 28 kPa to PSI = 4.06 PSI Boost

The conversion factor is .145

That's why 100 kPA = 14.5 PSI

4.06 psi is the first number I had but I didn't think that could be right. Does that mean that there is something wrong there?

I mean I know boost is going to be lower with Headers and the VS cam but I didn't think it was going to be that low. Then again, at that speed am I going to get an accurate measurement of boost psi?

Ok, definitely something not right. I'm having the same problem Dreww was in this thread:

https://rivperformance.editboard.com/t2083p15-faq-boost-bypass-valve-boost-control-solenoid-questions-no-boost?highlight=Low+boost

Looks like the thread has been merged with some other threads but he just kinda fell off and didn't get to what the problem was. He hints at the fact that 16.4 LTFT is a vaccum leak or something. I have all new vaccum lines but maybe my vaccum routing is wrong. BBV is working fine, I can watch the rod go in and out as I rev the engine.

Ok, definitely something not right. I'm having the same problem Dreww was in this thread:

https://rivperformance.editboard.com/t2083p15-faq-boost-bypass-valve-boost-control-solenoid-questions-no-boost?highlight=Low+boost

Looks like the thread has been merged with some other threads but he just kinda fell off and didn't get to what the problem was. He hints at the fact that 16.4 LTFT is a vaccum leak or something. I have all new vaccum lines but maybe my vaccum routing is wrong. BBV is working fine, I can watch the rod go in and out as I rev the engine.

Yeah, I'd say vac leak with +16.4 trim. Or, tune is way off.

Tune is definitely way off. I just started the tune. Still on 1.5 bin with the LQ4 Maf table only. I'm going to have to check all my vacuum lines because that is one part I had the most trouble with when I put the first motor in. There were no vacuum lines there to begin with so it was pretty much just guessing on my part.

The 16.4 LTFT is happening only at full vacuum, lowers as I lose vacuum and is going to 0 when WOT so 0 vacuum = 0 LTFT. I'm going to put a vaccum gauge on it this weekend and check my line routing.

Assuming you are running E10 (E7) fuel, change your stoich to 14.2. That will help reduce your LTFT a bit. Changing MAFs also requires retuning both VE and MAF. A VE tune will bring your MAF tune much closer as well.

LTFT and STFT are not used in heavy throttle.

Spray down your vacuum lines with carb cleaner (not soak, just mist) to see if you can find a leak that way. Also visually inspect all the rubber T fittings.

deekster_caddy wrote:

Assuming you are running E10 (E7) fuel, change your stoich to 14.2. That will help reduce your LTFT a bit. Changing MAFs also requires retuning both VE and MAF. A VE tune will bring your MAF tune much closer as well.

LTFT and STFT are not used in heavy throttle.

Spray down your vacuum lines with carb cleaner (not soak, just mist) to see if you can find a leak that way. Also visually inspect all the rubber T fittings.

On it. Yeah I'm working through Jerry's tune right now. This scan was just a baseline before I started going through it like he did. I think I found the reason for the high LTFT. Vac line to evap switch came out of the boot and was wide open. The only other thing I found that may be an issue is that the sensor on top behind the altenator (above the evap) has the vac line running to the middle port on the top of the supercharger that is normally plugged. WIth the N* I only had one port on the adapter so I had to run the line that originally went to the top port of the TB to the middle port on top of the sc...Problem is I never measured to check if vac is the same...Kinda just read it somewhere and went with it.

Checked all my vacuum lines and I can find no leaks anywhere now. Carb cleaner turned up no vacuum leaks. Routing is ok and replacing the 1 port i was short on the TB with the middle port on top of the supercharger seems to be ok because they have the same vacuum measurements.

Also put the stock tune back on and just added the LQ4 table. Went out for another run and maxed out at 133 kPa with 102 Barometric....4.5 or so PSI.

At this point I can only guess that there is something wrong with the BBV or actuator or something.

The evap line issue solved the 16.4 issue it seems but LTFT is still hovering around 14 or so at idle.

Figured the boost issue out, twas simply that I had a couple of vacuum lines routed wrong. Just went and did a quick run and at 86MPH WOT maxxed out at 11.4 psi, at 5461rpm...did see 3* of KR there but now i have a good place to start with my VE tune and just go step by step.