Had the opportunity to make a nice full throttle pull today with no traffic. I had noticed previously I would hit about 15.5psi max via my scangauge. Today I saw it hit 17.6!! Would my new exhaust be the main culprit of this? I have no cats or resonators, stock size piping to stock muffler. It was also 23* today, about 10* lower than normal. Just seems a bit high, even with the JB installed.

 

typically since you put in the catless DP, your psi would lower since the car can flow more, more flow = less pressure

 

Not really.

Turbos get their power from heat energy differential across the turbine. Part of that energy is pressure. A free flowing downpipe means lower pressure on the turbine outlet so the delta (difference) in pressure across the turbine is greater. A greater pressure delta means more power through the shaft to turn the compressor on the other side. That means you make more boost.

Since the turbine exit can flow more easily, more hot gas would rather go that way (less resistance) than through the wastegate port. If the wastegate isn't dumping enough exhaust, it will struggle to regulate the boost to the original level.

I've seen plenty of applications where a big, free flowing exhaust has caused overboost problems.

 

Also a JB tune changes some of the inputs to the ecu to request more boost. I don't know what these are, since I don't have that tune, nor looked into it.


as for down pipes (catless and larger diameter), all this does is slow down the gases exiting the turbine, since it has a larger volume and cross sectional area than the turbine exit. this allows the turbo to breath easier and maintain less pressure going back into the turbine. (there is a higher pressure pre turbine than post. granted the temperature differential is negligible)


I forgot to mention that if the restriction is in the exhaust the boost pressure will go down (more flow = less restriction)


but this is all monitored by the wastegate. a mechanical wastegate requires that spring to allow the waste gate to open using the pressure differential between the hot and cold side. once the cold side is able to compress the spring the wastegate opens and allows some of the exhaust gases to escape through this than traveling through the turbine.


if the wastegate is not the proper size, or is not regulated correctly, or cannot bypass the gases appropriately, this can create an over boost. but unless the larger down pipe is blocking the flow of the wastegate, sure this can create an overboost.


but also I believe the wastegate has an electronic boost controller, the JB+ might be changing some other parameters

 

Doesn't the JB+ "fool" the ECU into seeing less boost than it really is?

 

I believe so, I have contacted Burger Tuning to see what they think. Something else I haven't though about is if it is "fooling" the ECU about boost pressure, then my Scangauge would also be wrong since it is reading what the computer sees. I'm thinking it doesn't manipulate boost pressure in that sense since it is attached to the MAF sensor. If it was on the MAP sensor then I'd be more inclined to think so. Lets' see what they say.

 

So the response from BT basically said nothing is wrong. Stock tune can see about 14psi in overboost and the JB+ can add 2-4psi so up to 18psi readings would be normal. It does not affect the boost sensor so my Scangauge readings are correct.

 

did they happen to see what the JB tune does to increase boost?

 

They did not and probably wont. But from what I gather, it must be telling the DME there is less air than there is, which would lead to more boost for a given load and leaner AF ratios. I am in no way a professional on this but thats what make sense to me given its placement.

 

I am really interested in what the JB+ does actually. Does it rescale the MAF?

 

Little update to this, I've been driving about a week without my JB+ and have noticed that my idle boost pressure if different. With the JB+ installed I would read -0.8 to -0.5 (vacuum). Without it I am seeing -1.7 to -1.4. So even at idle the map is moved along the curve and pressure are increased.

 

I have a JB+ installed to my Clubman, and I have been doing some research on how it works lately. From what I've read, the JB+ tricks the ECU into thinking the engine is at a higher altitude than it really is. At higher altitudes, the ECU will up the boost targets to overcome engine pumping losses. I am not sure what the actual trickery is, and not sure Burger would release that information, but that would also explain why the boost levels are not the same each day.

 

this makes sence, although do our cars actually know what altitude we are at???

 

The MAF signals tells the ECU which of its multiple maps to operate. The JB+ alters the MAF signal to tell the ECU to go to a different map location, and find an alternate boost target based on those maps. It a very simplistic device that bumps the performance targets without modifying the ECU program like the Manic tune. The Manic tune is probably better, but the JB+ gets the job done in a much simpler package.

 

Was there anyone on the forum that did a dyno run to find out the actual HP/TQ difference between a Stock and JB+ car? I wasn't able to find any posts.

 

I'm getting 19.5 through 4th 5th and 6th, steady vac -1.0 to -0.7, bone stock, only jb running.

 

Are you reading the boost through a proper OBDII scan tool or through an ELM327 Wifi/Bluetooth module, connected to an app like DashCommand?

Accuracy varies based on which one you are using.