Turbosmart E Boost Street boost controller, with "low" boost zeroed out on boost and gate pressure, which should mean set at wastegate pressure. Mine is 6# spring.

For this car:
LOW boost = MAC solenoid duty cycle of 0%, gate pressure 3#, and...
HIGH boost = MAC solenoid duty cycle of 50%, and gate pressure of 10#

Flipped truck manifold setup, is an ON3, front mount, intercooled, on a 2000 Trans Am, running on E-65.
VSRacing cast 78/75, On3 44mm wastegate, and 50mm blowoff
3" downpipe with an electric cutout at end of it, basically next to the starter.
If cutout is closed, exhaust is directed through complete stock style catback

Cutout closed, on "low" boost setting, results in 3.5-4.5# boost, steady, no climbing.
Cutout closed, on "high" boost setting, results in 11-13# boost, steady, no climbing.

Cutout OPEN, on "low" boost, will start at 6# and steadily rise through the gears (automatic trans.), to as high as 12#.
Cutout OPEN, on "high" boost, will pretty much jump to 13-13.5# and stays there throughout the "run"

Is it simply just breathing that much better with the cutout open? DEFINITELY runs a lot better open.
I originally suspected a wastegate issue, but with cutout closed, it controls the boost well, IMO.

I'm a turbo newbie for the most part, so some of the things that happen, make no sense to me. The boost steady rising with cutout open is one of those things that I don't understand.

One thing I have read, and could be an issue, as far as wastegates...is the boost controller came with small-ish hose, like maybe 5/32"? Almost looks like windshield washer fluid size hose? The hose barbs on the wastegate are 1/4". I have tried plumbing it exactly how I had my air regulator style manual controller, (Turbosmart calls "METHOD 1", which inlet of MBC was "T'ed" off of the boost pressure/lower WG port, and used outlet pressure to hold wastegate closed via the top WG port, and even changed it to a simple one hose to the bottom port of the WG, with zero change.

 

Don't worry about the vac line size, it really doesn't matter, the wastegate volume is tiny.

Are you running the wastegate back into the exhaust or does it open in atmosphere? If it isn't routed into the exhaust I could see the free flow pressure vs full exhaust differential making it work a lot better, open the cut out and that differential is gone. Turbo's tend to like big, free flowing exhaust. I am not sure what your stock system is like, but it certainly sounds like a restriction.

 

The WG is open to atmosphere.
The exhaust, is a pre-turbo leftover, and is very quiet, and probably restrictive. Single 3" into a transverse muffler, with dual 2.5" tails.

PRE-TURBO:



With the truck manifold and starter right there where the DP goes, I don't think it'd be possible to go any bigger on the downpipe size, unless of course, it exits up front somewhere.

DEFINITELY makes boost faster and pulls noticeably harder with cutout open.

I'm just puzzled why the boost rises up right with RPM, when it is open.

 

With high back pressure post turbo, pre turbo pressures are high and the gate is working under a higher pressure differential. Low pre turbo pressures equal less gate flow. Just a thought. The turbo is also less efficient with high post turbo pressures.

 

Makes sense. With that in mind, should my boost "reference" be sourced directly from the turbo? I have mine on the charge pipe, then to a vacuum tree, with one line from treet to boost controller, and Hobbs switch screwed into tree as well. I thought it'd be better source after the intercooler? which is why I welded a bung on the charge pipe...

 

Most say on the compressor housing, but I don't believe there's an issue with it referenced post cooler. It just changes boost level by the pressure drop across the cooler. I've run both.

 

As your pressure ratio increases the effect of post turbo restriction increases as well, and its 1:1 so at a 1.5 P/R the turbo sees 1.5x the resistance to flow from the exhaust. The more restriction through the turbo the more the exhaust gas is going to be forced through the wastegate. With a 75mm turbine wheel you should be running at least 3.5" for the downpipe, and keep that size as far back as possible.

The wastegate is another issue, how good is the gates flow priority? I think the exhaust restriction is just masking a wastegate problem.

 

+1. I agree with ben. With the cutout open the "exhaust" flows more, this means the wastegate has to work on wider pressure delta then the cutout closed. Once closed the pressure delta across the gate drops and the gate can bypass enough drive pressure to control boost. Either the wastegate location/priority is poor or it does not flow enough for some reason. Might be size, might be limited in travel, ect.

Boost creep is gods way of saying safety 3rd.

 

Thanks for the insight guys. I appreciate it.

I attached (hopefully did anyway) a pic of the wastegate mounting spot. It is exactly as the pic shows, aside from mine being a V band connection. No idea how/if it'd be able to get more flow.

The ON3 "kit", when bought new, came with a 70/65 turbo which had instant (but controllable) boost, on my mild 6.0, and I don't recall it steadily creeping up like it does now.

Would that be because the smaller turbo, itself, is/was a restriction?

The end goal this year, is to race it this last year of it's weekend bracket racer duty, and over the Winter, probably swap on a billet Gen 2 VS Racing 78/75 with a 4" fender /undercar exit. At that point, just a toy, and once every blue moon Test & Tune car.

When car was nitroused, last year, I had decent luck running N/A and using a 200 shot "blip" near/through the traps to screw many sandbaggers. That's why I was hoping to get the boost under stricter control, ie; use 3-4# of boost to run with, and since my boost controller has a "scramble" feature, use a 15-16# "blip" in the same way I used the nitrous.

I'm wondering if I should bolt the smaller turbo back on.

#ad

 

That position isn't brilliant, but if you could add another gate across from it I think it would work ok. The way it's setup now the only way I see getting good flow to a single gate would be to mount it directly to the exhaust housing.

 

What does your timing curve look like? With the big drop in back pressure, it most likely opens up your ign timing window, which could help your creep issue.

 

Is the exhaust turbine divided? Is the flange divided in the merge? Does the gate actually dump from both banks?

 

It's pretty steady...19.5* to 16.5* depending on boost...was 29* just before making any boost.
Running on E-65

1st two pics are open cutout on "low" boost setting...
data point at the 1>2 shift, and 2nd pic at the 2>3 shift
3rd picture is on "high" boost...in this case 13.5# and steady

#ad


#ad


#ad

 

The gate is just below the turbo flange, where both sides of crossover meet.
Nothing divided.

 

sorry...double post

 

This is basic. Any flow restriction in the intake or exhaust, will limit boost at higher rpm. In the early days of turbocharging street cars it was common to run without a wastegate. Turbine housing was selected to get the spool early enough then restriction was added (small carby or muffler) to limit boost.

In your case the wastegate is not quite large enough to control boost in every situation with an open exhaust. Adding some restriction (closed cutout) is helping the wastegate to control boost.

 

I'm already resigned to the fact that I'll just keep the cutout closed when bracket racing. Use the lower boost (5#) , with a "scramble" button on (15#), if I need that 200-ish HP nudge through the traps.

 

Just to update, last weekend was the 1st Strange Outlaw Streetcar Shootout at Cecil County Dragway.
Cutout open...2800+ DA, 122 water grains, shitty for Cecil
Launch from idle with zero boost...
Got 2 time runs
11.66@124
11.56@125
1st Round:
11.50 dial in...went 11.54@126

Better air the week before...
Test & Tune
11.47@127, leaving off idle, cutout open, 5#-9#...boost creeps starting at midway through 2nd gear
10.34@135, turned controller (EBoost Street) to boost group 2, (50% MAC solenoid duty cycle = steady 13#), 2# on launch
9.94@138, launched on 4#, MAC solenoids at 65% duty cycle = steady 16#
10.02@137, hotlapped, no changes, 16#

Car (2000 Trans Am) weighs 4,060# with me in it...2 step @ 3000, shift at 6100-6200, traps at 6800-6900.
Has 6.0, 4L80e, 9" rear, running a cast wheel VS Racing 78/75 turbo
Thinking about getting the Next Gen Billet version of the same turbo...

 

Four hundredths over and you lost in round one??!!

 

Yup...Margin Of Victory .0555, or 555/10,000ths, lol.
This guy beats me every time we race...
On a side note, leaving off idle, the car leaves like an absolute slug, then starts pulling 100 feet out. He did say it was hard to judge where we were both were at the big end, which I am happy about. I'm basically 10mph faster than the usual 11.50 car. Both of us thought we won, until his light came on.


#ad

 

just to reiterate. Exhaust gas pressure applies force to the gate, from 'below'. Exhaust gas pressure will eventually force the gate open all by itself. With a 3psi spring, I see 15psi of boost with the gate completely disconnected for example.

If you measure the gate valve area, you can calculate how much force from exhaust pressure will be applied (Force = pressure/area) to determine your gate opening point. By working backwards from logged boost pressure you can use this formula to find exhaust gas pressure, since pressure directly correlates to gate behavior.