Waterguy

There have always been postings on this forum about boost drop at high rpm, at least with manual boost controllers. Usually, the answers revolve around weak wastegates and/or stock turbos running out of steam. As I am about to shim my wastegate, install an MBC and an aftermarket boost gauge, I have been giving this some thought.

Most boost gauges are tapped into the intake manifold (downstream of the inctercooler.) The banjo bolt for the boost controller, however, is on the turbo outlet to IC pipe. The boost controller is seeing a higher pressure than the boost gauge due to the IC pressure drop. As airflow increases at high rpm, the IC pressure drop increases.

I ran some numbers to see what the effect of this would be. If you add a mbc to a stock 951 and set it to 15 psi on the boost gauge at 4,000 rpm, I estimate that the result would be:

rpm --- outlet pressure --- IC pressure drop --- Guage Reading
4,000 ------- 17.3 ------------- 2.3 --------------- 15
5,000 ------- 17.3 ------------- 3.2 --------------- 14.1
6,000 ------- 17.3 ------------- 3.8 --------------- 13.5

This suggests to me that if everything is working absolutely right you would expect to see some pressure drop on the gauge at high rpm with a mbc. The effect would be greater with a modified 951 and/or higher boost:

rpm --- outlet pressure --- IC pressure drop --- Guage Reading
4,000 ------- 21.2 ------------- 3.2 --------------- 18
5,000 ------- 21.2 ------------- 4.3 --------------- 16.9
6,000 ------- 21.2 ------------- 4.8 --------------- 16.4

Am I missing something here, or is this old news to everyone?

NZ951

I deal in manifold absolute pressure. And I get a boost drop at high rpm with the stock turbo. It just cant push enough. I have filled in the banjo on the IC pipe.

Waterguy

I know the stock turbo runs out of air flow at about 300 rwhp, so I am not surprised that it is dropping for you. I guess I was thinking of cars closer to stock, where the turbo should not be the only problem (say less than 280 rwhp.)
How do you control boost pressure? I am guessing not mbc.

NZ951

Boost is controlled by a soleniod, nippon denso unit. Based on a MAP line... I believe this would be the most accurate mesaure you can get.

toddk911

"Am I missing something here, or is this old news to everyone?"

Nope, your not missing anyting. I have posted on this before but it did not get much attention due to the fact that I first read it at a ricer site, but then found the same info in Maximum Boost.

I have my banjo bolt set in the IC pipe AFTER the IC due to the 1-2 psi standard pressure drop. So, as you mention, if you WG sees 15 psi before the IC and opens as planned, only 14 or maybe 13 psi will actually get into the intake. Corkey Bells says that even brand new modern IC will still get about 1 psi drop, so I would assume our IC's see a good 2 psi drop, if not more.

dmoffitt

where'd you come up w/ the #s (explain please)?

Waterguy

I came up with numbers for airflow from stock or aftermarket turbo maps and boost curves. Usual corrections (turbo efficiency, intercooler efficiency, VE, ambient pressure and temp.)

The pressure drop across a stock intercooler is harder to estimate. Very few measurements seem to exist. I work from Lindsey's measurement of 27" H20 (about 1 psi) at 205 cfm. Head loss for flow of compressible fluids is difficult to estimate. I arbitrarily assume that pressure drop increases with flow velocity^1.852, which is the empirical relationship for head loss in laminar flow of incompressible fluids. This happens to match the one pressure drop measurement that I have seen quoted for the stock intercooler, which is 8 psi at 400 rwhp (estimated at 630 cfm or 43 lb/min airflow.)

This equation predicts a pressure drop across the intercooler of 3.3 psi at 250 rwhp, which is reasonably large. I could be better refined if anyone has actual pressure drop or even turbo outlet temperature measurements under known conditions.

TurboTommy

Waterguy, everything you're saying is correct; very well thought out.

There is always a reason why certain things behave the way they do. The trick is to get the logical explanations.

From the info I've gathered over time, there seems to be about 2psi pressure drop through a clean IC at 300 horsepower.

dmoffitt

waterguy, so they are calculated not measured? that was my main question...

so i guess the next question is, what's the best way to act upon this data? plumb the car differently? set up the mbc or ebc differently? suggestions?

toddk911

"so i guess the next question is, what's the best way to act upon this data? plumb the car differently? set up the mbc or ebc differently? suggestions?"

Yes.

"I have my banjo bolt set in the IC pipe AFTER the IC"

This way, the MBC is set to see the boost that is actually going into the intake. So when my boost gauge says 15psi, I know 15 psi is getting into the motor.

I would guess it is easy to measue pressure drop through an IC. Put X amount of psi into it and a guage at the other side and see what you read.

Waterguy

Thomas: Where did you gather the 2 psi @ 300 rwhp data? Sounds much better than the 8 psi @ 400 rwhp value, and I doubt that both are correct.

Dmoffitt: Yes they are calculated. It seems like they sould be easy to measure; I will work on that once I get my mbc installed. I am thinking of plumbing the boost gauge as a temporary unit, so I could compare boost before and after the intercooler. As for the best way to act on the data, NZ951 and Todd have already answered that - take the boost for the mbc/wastegate off the intake manifold side of the IC so that the boost gauge and boost controller both see the same boost that the engine sees.

Edit: Too slow, Todd beat me.

TurboTommy

Waterguy, 8 psi at 400 horsepower is about right, especially if the 400 horsepower is being achieved via higher RPM and/or more displacement (which is usually the case). Pressure drop goes up much harsher than linear after a certain flow volume. 300 hp is much easier to achieve with a proper turbo and then turning up the boost. Higher boost with relatively the same volume tends not to produce as drastic of pressure drops.

facboy

i'm confused...i thought the boost controller was just a dumb mechanical valve? it opens at a certain pressure and lets the pressure signal through to the wastegate. you adjust it so that it opens at the pressure that will give you the boost you're after. if you move the MBC elsewhere where the pressure it will see is different, then you just readjust it, and it will then reflect that difference.

most EBCs seem to be the same...you don't set it to give you 15psi of boost, you fiddle with lots of arcane variables until your boost gauge tells you that you've got the right settings. it wouldn't really matter if your boost gauge was plumbed elsewhere either, provided that you accounted for that in eg your fuel maps or whatever...eg this chip is mapped for 17psi of boost at the turbo outlet (cf 15psi at the intake manifold).

or am i all mixed up?

Waterguy

Facboy, you are right. The only thing is, as the rpm goes up, air flow to the engine increases. Therefore pressure drop across the intercooler goes up.

If you plumb things the normal way, you set the boost to some value on your boost gauge, say 15 psi at 4,000 rpm. Since your mechanical boost gauge is on the turbo side of the IC, it is actually set for some higher value of boost (who knows what? 17psi?) Then as rpms, airflow and horsepower climb the boost drop across the IC increases. The MBC holds a constant boost (17 psi?), but the motor and boost gauge see a dropping boost curve. This probably doesn't contribute to all the boost drop seen at high rpm, as the stock turbo, stock wastegate and exhaust backpressure also contribute. It is probably a factor though, and adding a TiAl wastegate doesn't cure it.

Is it a major cause for concern? no. I guess it depends on how flat a boost curve you want to see, and how much rwhp you want to achieve.

facboy

oic...could you plumb the banjo (or a smaller banjo) off the bolts where the line to the klr etc are?