Kpa to Psi conversion at altitude

[Colorado-SHOBro]

I have an interesting issue that i've been racking my brain trying to figure out recently. Hoping someone smarter than me can lend a brain cell or two to guide me in the right direction haha.
So I've been in the process of getting my car tuned and thus reading a lot of logs and doing scanning with OBD apps. Reading MAP levels from logs i'm seeing 183kpa on average . . apply the formula to convert to PSI- 183 x .145= 26.5lbs of total boost. minus 14.7psi of atmospheric pressure gives me 11.8psi. However, i'm @ about 5500' altitude so there's nowhere near 14.7psi of atmospheric pressure here. It's more like 12psi of atmosphere(X4 reads 83kpa with engine off).
SOO . . . When converting kpa to psi should the formula actually be used as 183 x .145=23.5-12psi(atmospheric) which gives me 14.5psi. This seems to coincide closer to what i see using OBD scan apps. The car ran 10-12psi with the stock tune as it seems most stock cars do so all that leads me to believe the OBD boost values are accurate. The more i think it the more it seems i should adjust this formula to fit my altitude and relative boost above altitude.

Basically, i need to know what my ACTUAL manifold pressure is. I'll be doing AJP's wastegate mod at some point and need to know what spring pressure to aim for. Also i feel i need to understand this to be aware of how my altitude is affecting the tune and dynamics of the car.

opinions anyone?

TIA

[FoMoCoSHO]

Pressure at sea level is about 100 kpa so I typically subtract 100 then convert it to PSI.

Run baro on your config file or OBD program in KPA then you will have an exact number to subtract at all times.

[FoMoCoSHO]

If you are running Torque I'm pretty sure the boost PID compensates for your actual baro.

I think dash command uses baro in the formula as well.

[StealBlueSho]

Ask Bradley... He told me he likes the  spring 1.5 or so psi higher than what what he is tuning for. If he is tuning it, let him do the math...

Besides... There are only a couple spring rates that work if you go with the tial setup... 15.95 and 17.40... Unless you go with a different wastegate... Otherwise you need to put a MBC on the signal line like I did to dial it in better using the 15.95 setup...

But Bradley can look at the logs and tell which spring will work with the tune he is putting on it...there are more factors than just boost... Fuel pressure, wastegate duty cycle, and others...

[StealBlueSho]

One thing to keep in mind with the wastegate mod is it does put more stress on the turbos... They are fighting against a boost leak that the WG creates... I did notice that they run hotter with this mod.. which is why regardless of what spring rate you think you need, best to let your tuner decide based on what he see's...

[Colorado-SHOBro]

Pressure at sea level is about 100 kpa so I typically subtract 100 then convert it to PSI.

Run baro on your config file or OBD program in KPA then you will have an exact number to subtract at all times.

Right. 100kpa is ~14.5psi. Either way you work the math(subtract kpa first and then convert to psi or other way around) you get the same result. Good idea on adding the baro PID though i'm not sure it's gonna change much unless i'm climbing into the mountains or driving down towards sea level.

Ask Bradley... He told me he likes the  spring 1.5 or so psi higher than what what he is tuning for. If he is tuning it, let him do the math...

Besides... There are only a couple spring rates that work if you go with the tial setup... 15.95 and 17.40... Unless you go with a different wastegate... Otherwise you need to put a MBC on the signal line like I did to dial it in better using the 15.95 setup...

But Bradley can look at the logs and tell which spring will work with the tune he is putting on it...there are more factors than just boost... Fuel pressure, wastegate duty cycle, and others...

I've discussed it with him (he is tuning it) and didn't get a direct answer. I'm sure we will revisit the issue when time allots. I mainly just want the piece of mind to know what i see on OBD fusion is correct. He's telling me he's commanding 12psi and i see 14+ so it will just help me process and understand things a little better. And yes, my turbos are working damn hard to give me 14psi up here so wastegate duty cycle is in the 90% range much of the time and boost spikes are more pronounced because the turbos can't slow down quick enough . . . which all hurts fuel pressure etc. etc.
That's why i want to install the wastegate and why i'm seeking answers on this.

Thanks for the replies guys!

[SHOdded]

If the turbos are running hotter with the mod, be sure to run a top flight full synthetic oil, but also consider a better coolant to go with.

[StealBlueSho]

If the turbos are running hotter with the mod, be sure to run a top flight full synthetic oil, but also consider a better coolant to go with.

Mobile One all the way!!

[StealBlueSho]

Pressure at sea level is about 100 kpa so I typically subtract 100 then convert it to PSI.

Run baro on your config file or OBD program in KPA then you will have an exact number to subtract at all times.

Right. 100kpa is ~14.5psi. Either way you work the math(subtract kpa first and then convert to psi or other way around) you get the same result. Good idea on adding the baro PID though i'm not sure it's gonna change much unless i'm climbing into the mountains or driving down towards sea level.

Ask Bradley... He told me he likes the  spring 1.5 or so psi higher than what what he is tuning for. If he is tuning it, let him do the math...

Besides... There are only a couple spring rates that work if you go with the tial setup... 15.95 and 17.40... Unless you go with a different wastegate... Otherwise you need to put a MBC on the signal line like I did to dial it in better using the 15.95 setup...

But Bradley can look at the logs and tell which spring will work with the tune he is putting on it...there are more factors than just boost... Fuel pressure, wastegate duty cycle, and others...

I've discussed it with him (he is tuning it) and didn't get a direct answer. I'm sure we will revisit the issue when time allots. I mainly just want the piece of mind to know what i see on OBD fusion is correct. He's telling me he's commanding 12psi and i see 14+ so it will just help me process and understand things a little better. And yes, my turbos are working damn hard to give me 14psi up here so wastegate duty cycle is in the 90% range much of the time and boost spikes are more pronounced because the turbos can't slow down quick enough . . . which all hurts fuel pressure etc. etc.
That's why i want to install the wastegate and why i'm seeking answers on this.

Thanks for the replies guys!

5500 feet? Denver?

[FoMoCoSHO]

Sounds like hell for a car guy.......

A turbo upgrade is sorely needed for the high altitude guys.

[Colorado-SHOBro]

Sounds like hell for a car guy.......

A turbo upgrade is sorely needed for the high altitude guys.

yeah the local track is 5800' and DAs are 7500-9000. i'll be at the track tomorrow so we'll see how she does.
and actually hell for a car guy is being at this altitude and being NA!! stock SHOs are still running 13.8-14.2 up here. probably cause the turbos are still in there efficiency range at those boost levels. i'll have to post a log later just to show how hard the turbos are taxed at 14-15psi. going to be working on an E blend tune soon but it's all going to be timing advance to look for gains. no room for more boost.
long term i'm looking into doing the upgraded compressor wheels from eco.

[f8tlSHO]

I think I would move to somewhere closer to sea level... I think I'm at 400ft... it's great down here


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[Colorado-SHOBro]

I think I would move to somewhere closer to sea level... I think I'm at 400ft... it's great down here


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haha it's really not that bad. everyones car suffers from it so it's still a level playing field . .  unless your NA then your at a pretty big disadvantage. guys who don't know any better are puzzled why there vette's, mustangs, and srt8s get walked out by turbo cars with little to no mods. that can be priceless. whereas at sea level you might struggle to keep up with some of those cars.

[Vortech347]

I have an interesting issue that i've been racking my brain trying to figure out recently. Hoping someone smarter than me can lend a brain cell or two to guide me in the right direction haha.
So I've been in the process of getting my car tuned and thus reading a lot of logs and doing scanning with OBD apps. Reading MAP levels from logs i'm seeing 183kpa on average . . apply the formula to convert to PSI- 183 x .145= 26.5lbs of total boost. minus 14.7psi of atmospheric pressure gives me 11.8psi. However, i'm @ about 5500' altitude so there's nowhere near 14.7psi of atmospheric pressure here. It's more like 12psi of atmosphere(X4 reads 83kpa with engine off).
SOO . . . When converting kpa to psi should the formula actually be used as 183 x .145=23.5-12psi(atmospheric) which gives me 14.5psi. This seems to coincide closer to what i see using OBD scan apps. The car ran 10-12psi with the stock tune as it seems most stock cars do so all that leads me to believe the OBD boost values are accurate. The more i think it the more it seems i should adjust this formula to fit my altitude and relative boost above altitude.

Basically, i need to know what my ACTUAL manifold pressure is. I'll be doing AJP's wastegate mod at some point and need to know what spring pressure to aim for. Also i feel i need to understand this to be aware of how my altitude is affecting the tune and dynamics of the car.

opinions anyone?

TIA

Same situation here.  I'm at about 4250ft at my house our air sucks too.  The elevation is horrible for our turbo's.  My average is also about 180.  I've been as high as 190 but the waste gate duty cycle was to high and Brad was concerned about overall life of the turbos and the high intake temps.  I've taken logs in Park City which is around 8000-9000ft and I average around 170-175.  I'm all over elevation changes so mine is a tricky one to tune.  I often go to Vegas and Cali with my car.

I tune my two mustangs and it's a big difference here vs else where.  In elevation your fuel isn't the problem for max power, it's the air.

It sucks but it's just the way the cookie crumbles.  Our turbo's are too damn small to extend farther out in thin air.

Edit. My car only saw 8psi stock.  Based on Brad's calculations I'm at around 13.5psi I think.

[Vortech347]

Sounds like hell for a car guy.......

A turbo upgrade is sorely needed for the high altitude guys.

yeah the local track is 5800' and DAs are 7500-9000. i'll be at the track tomorrow so we'll see how she does.
and actually hell for a car guy is being at this altitude and being NA!! stock SHOs are still running 13.8-14.2 up here. probably cause the turbos are still in there efficiency range at those boost levels. i'll have to post a log later just to show how hard the turbos are taxed at 14-15psi. going to be working on an E blend tune soon but it's all going to be timing advance to look for gains. no room for more boost.
long term i'm looking into doing the upgraded compressor wheels from eco.

I ran a best ET of 13.54 and mph of 104.  With an unleashed tune on a shitty strategy that was dumping boost like a mofo after 5k.  I think my car will probably run about a 13.40 now that the top end boost has been ironed out.