boost pressure question

[jaymz199]

Wow i was digging around the glovebox and found something that blew my mind...i found fuel pump internals o.0 they look stock so im hoping that when i get my cobb apand plugg it in and veiw that fuel pressure i will see something beautiful lol

 

[MSMS3]

All great news, but what about your boost? Did you pull the MBC and go back to just the stock ECU to evaluate your boost situation? There should be a "sticky" here on the install, so you just do the reverse.

 

[jaymz199]

No not yet im hoping to get it done tomarow

 

[lowboy72]

Also do not forget that your peak boost is determined by load. I have been tweaking my MAF calibration and I find that being in fourth gear on relatively flat surface with a somewhat normal IAT will net the best result.

 

[MSMS3]

^ +1. And not just peak boost but, how long boost is sustained over the power band. Load varies with a lot of factors, including ambient temp and humidity, altitude and particularly the gear you are in. At my tune and mod level I seem to hit highest sustained load and, therefore, boost when in 5th gear. Fourth hits very similar maximum boost but tapers a bit quicker as rpm approaches and moves past 6,000 rpm. Every car is going to be a bit different.

 

[oaklandopen]

Shew...5th is not a gear i go WOT in. I save that for 3rd and 4th....and mayyyyyybe 2nd if i know its a hot dry surface and my tires are nice and warm

 

[MSMS3]

^ lol. Yeah, to stretch her out in 5th does require touching or getting a little bit north of 120. Can't do that just anywhere and don't even need it at the dragstrip except maybe shifting from 4th into 5th through the traps. But 60 mph is right about 3,000 rpm, so it is a good "go to" gear out on the Interstate. At 70-75 mph it's a good gear to drop down into to pass a slower car.

Back on topic, I hope OP has gotten back on stock ECU boost control so he can sort out his boost issue.

 

[jaymz199]

No still havent removed the manual boost controller...simply havent had the time. but i have noticed that the car really donst spike it will hit around 15 or 16 and hold untill maybee a bit over 4k and then start to drop off. and i think im starting to understand the concept of the lud table ghat the ecu uses to determin boost pressure with would make sence of why the car loses boost as rpms increase. but the highest psi ive seen is in fourth from around 3.5k i see 17psi

 

[high3at0209]

not trying to steal the thread, but my speed 3 is stock besides a cobb sri and both resonators removed and im peaking at about 19 or 20, not sure if thats normal?

 

[b e z]

I have the same mods as you and i have only peaked at the highest like 16. Then again i am in debate if my boost gauge is crap, but il let you know when i hook up my dash hawk

 

[high3at0209]

alright sounds good, thanks

 

[b e z]

Was there a particular gear you went WOT and saw the peak so i can use the same gear or was it pretty much in every gear.

 

[oaklandopen]

Do it in 4th. Youll be going really fast but its the longest youll stay ij high boost and not have to go rediculously fast

 

[MSMS3]

Do it in 4th. Youll be going really fast but its the longest youll stay ij high boost and not have to go rediculously fast

Agree. 4th will not show as high a load, but you do not have to get to insane speeds. A pull to redline will put you at or just a bit past 100 mph, so find a safe place to do that. Also please note that the ECU will be sending the boost signal to your Dash Hawk at a slightly different point in the system, so there can be some variation between the gauge reading and the Dash Hawk, but it should not be more than about a pound or so. They probably will not be exactly the same. I do think your gauge is not reading properly since you are not getting a typical idle vacuum reading either. I suspect a bad sensor on your gauge.

 

[b e z]

Okay so i did a WOT pull in both 3rd and 4th and boost spiked to 17.25. I will do some logging when i have time to mess around with the dash hawk. After the spike it seemed to slowly go back down to 15.

 

[high3at0209]

yea its pretty much every gear after 2nd, i think its kind of weird mine is spiking at 20 instead of around 17 or 18 like yours
thanks for your info!

 

[MSMS3]

Third will not provide as much load as fourth, so fuel needs are also lower. Thus, max boost, especially sustained boost might be a bit lower in order to meet the engine's fueling requirements at the set AFR.

With stock downpipe and both cats, 17.5 might be as good as you can expect. But "spike" as you use it may not be the right term. A spike is when boost rapidly hits a higher number than your target boost and then almost instanteously drops back to target. When I think of a spike I think that boost is hitting 20-21-22 for a very quick interval and then immediately dropping to 18-19 with my mods.

A gradual drop in boost as you approach 6,000 rpm is to be expected as rpm increases and the tune is being adjusted to deal with the reality of flow limitations from the K04. For instance, the compressor map for the K04 would technically allow the compressor to work efficiently at as high as perhaps 23-24 psi back down at 3,000 rpm. For many reasons you would not want to do that. But as rpm increases and exhaust pressure increases in the exhaust manifold, you start getting more boost than the turbo can handle without producing excessive friction induced heat. the turbo becomes inefficient. Accordingly, the flow restrictions require you to adjust boost accordingly. That's why the tune is pulling boost back. But because the tune allows a bit higher boost than stock and allows the throttle body to close more slowly, you should be able to hold about 16 psi at 6,000 rpm and still also have usable boost, albeit lower, on out toward redline. I hope this makes sense. No tune can safely allow 18+ psi at 6,000 rpm and above on the stock K04 turbo.

 

[high3at0209]

my car is hitting 15 psi and then spiking to 19-20 for a quick interval like u mentioned and then dropping back down to 15