It's stickies, but also in post #5 of this thread.Quote:
Originally Posted by acannell
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It's stickies, but also in post #5 of this thread.Quote:
Originally Posted by acannell
Seems to me like the wastegate is the area to look at to reduce the drive pressure ratio without changing the turbo. Either in porting the internal wastegate hole or by adding an external wg that flows more.
As I asked earlier, what if (in theoretical design, not sure if it is actually possible) you could get a header and external wg ( or two) that would flow enough exhaust once open to lower the ratio below 1:1? Is this still improving things, or is it reducing the turbine drive pressure and causing boost stability issues?
Well said !.. in addition "or drive it right!"Quote:
Originally Posted by Shadow
There was also mention within the thread about opening the WG hole to reduce back pressure.
This is only true when the demanded exhaust flow exceeds available flow across the scroll area plus the WG hole, with the flapper near or at full extension.
Otherwise, all that's accomplished by doing so (opening the WG hole) is altering the operating position of the valve because the boost will learn to close the valve, to obtain the same boost previously realized with the smaller hole.
Therefore, depending on the boost being used and the resulting exhaust demands, you end up with the same back pressure until the turbine limit is exceeded.
This leads into the turbine housing 'game'.
The game consists of where the flow is desired 'WG/bypass OR turbine scroll'?
Imagine that if a very small turbine wheel is used with a stock .48 turbine housing, the flow demand will shift to the WG hole because the scroll housing area and related exducer diameter will not support the flow alone.
Likewise, the reflective approach is a larger housing and wheel which now minimizes the WG flow demand because the scroll area can now supports the required flow and excessive WG porting cannot be completely justified.
This is ultimately the difference in approach when comparing the 2.2 of "yesteryear" to the SRT 2.4 of recent year.
One realizes higher BP at low RPM to obtain quick response BUT minimizes upper RPM restriction when the WG opens.
The other maintains lower BP throughout the RPM range and relies on minimal WG activity to control the target boost level.
Two different approaches that can be set up in a way that produce the same exhaust BP levels but with completely different boost response curves.
Which approach is required is largely dictated by the size (area and wheel mass) of the compressor and the energy required to get the process started and maintained.
Answering this sort of question in a way that can direct design requires more formal analysis than I've ever seen on TM or TD. I dont think we're there yet. When you see overlaid efficiency lines on compressor AND turbine maps linking the two at certain operating points then we're there. Never seen that on an internet forum.Quote:
Originally Posted by Force Fed Mopar
But to speak about it in the usual generalities, theres no such thing as "turbine drive pressure". There is pressure drop across the turbine, and mass flow rate through the turbine. Those two things generate the shaft power to drive the compressor. If the post turbine restriction is reduced to 0 and you have essentially atmospheric right at the turbine blades at the output, I dont see why you wouldn't be able to continue to modulate turbine power by bypassing exhaust around it. Less gas through the turbine = less power. I think any sort of boost control stability problems are going to have a more complex cause.
To prove this you could split the exhaust stream in two and run two turbos each at half the power. So there shouldn't be some reason you cant control shaft power.
As stated earlier and in your last paragraph, twin scroll technology is pretty much 2 turbine housings, powering one compressor wheel.Quote:
Originally Posted by acannell
Any attempt to increase W/G flow will result in lower boost, there are No two ways around it. The only exception I know of is Dedicated W/G positioning using a deliberately Smaller turbine housing/ wheel that normal to get Great response down low, then Good top end flow when the exhaust drive pressure overruns the turbine and flows through the Dedicated W/G. AFAIK this will Not Increase top end power significantly over a properly sized turbine application, but it Will give Better response down low.Quote:
Originally Posted by Force Fed Mopar
Agreed, and back to the comment I made on Drive pressure vs lag. (now that I can type freely :))Quote:
Originally Posted by 5DIGITS
My first post (the novel) was an Inspired Mona Lisa, unfortunately that only happens once, so you guys will have to make due with the dummied down version.
In a nut shell, If you attempt to use A/R and turbine wheel size to correct drive pressure, you Will most definitely Suffer from turbo Lag. This is what most have done and were doing back when I first got involved with turbocharging.
So what did I do? I decide to use the Smallest turbine housing/ wheel combo I could in order to achieve desired Power level (Pressure differential be damned!) but make the Intake and exhaust Outlet as Efficient as I possibly could.
Did I Know I was heading down the path to 1:1 differential with Phenomenal street drivability? Nope! Had no clue. Until I installed the drive pressure gauge I would have thought the drive pressure in the Charger would have been at Least 2:1. I was Shocked to say the least, but it showed me that Not only does it work, it works Better than I had Ever imagined!
Remember, when I ran the 4" intake everyone said it was severe overkill. When I showed the 4" downpipe, same response. When I said I could make over 500WHP from the 9cm turbine housing on the holset (.65 A/R) they all said it was Way too small and that I would Lose top end power because of the tiny turbine housing and wheel.
Pretty sure, even to this day, that the Charger has one of, if not The Higher top end 1/4 mile Charger of Any 8v TD/ TM I know of. Add to this that the transient response is so mind boggling to those who I have take for rides that they say the car has No Lag, and How is this Possible.
So, Again, as much as most like to Think the charger is a laggy Drag car, it is actually the Complete opposite. Which Proves that you Can have a 1:1 differential and Great transient response IF you do it Right! (and yes, Know how to Drive a turbo car! :)) :)
And this is still on the stock exhaust manifold, correct? And stock Holset internal WG?Quote:
Originally Posted by Shadow
I know Rob still runs the ported stocker, and I "think" the internal gate too.
Correct. My own stock ported exhaust mani and stock untouched Holset W/G hole with tension on W/G actuator set to minimum.Quote:
Originally Posted by Force Fed Mopar
but I do think you have hinted to the fact that you may need to port the wastegate hole soon, as you seamed to have a small amount of boost creep when running the 12" long 4" dump...Quote:
stock untouched Holset W/G hole
Rob...where are you tapped for pre turbo pressure? At the manifold collector?
It Will creep at lower boost with the dump open. (below 25psi) but I only open the dump at the track, so really not an issue. I am considering porting it to bring down my minimum street boost setting Back to around 18psi. Right now it's around 22 and while that's fine for 93-94 octane, I get some knock on 91 and 91 is becoming the common Premium fuel around here.Quote:
Originally Posted by OmniLuvr
- - - Updated - - -
Yes, manifold collector right at the flange to the turbo.Quote:
Originally Posted by Pat
gotcha! I remember you talking about it a while ago, it was in a good thread where I learned a lot of things, very good info from you, just cant remember which one it was? (prob 4 years ago?)Quote:
It Will creep at lower boost with the dump open. (below 25psi) but I only open the dump at the track, so really not an issue
So the main factor in drive pressure seems to indeed be turbine selection...
Quote:
Originally Posted by Force Fed Mopar
THIS^^^^^Quote:
Originally Posted by BoostedDrummer
Is what I would consider the main factor in getting and keeping your drive pressure in check. You want the Greatest pressure differential right after the turbine. So whatever your drive pressure is (pre-turbine), let's say 20psi, if you could, you would shoot for Zero psi in the downpipe.
As the downpipe back pressure increases, the exhaust mani drive pressure Will Increase exponentially, causing the entire system to "back up" and lose power.
Just keep in mind that you want to attain this while keeping the intake vs exhaust mani ratio as close to 1:1 as possible.
And for those who are wondering, 1.5 : 1 is Not that bad ;) (there are Freaks out there that think 6 : 1 is OK lol)
Keep in mind that the turbine housing A/R ratio is only part of the equation when talking about turbine efficiency and how it uses the mass flow. The design of the turbine nozzle affects the angle and the velocity at which the flow hits the turbine blades. This can have a significant impact on turbo response and turbine flow/efficiency.
The turbine wheel itself is also very important as the aerodynamics of the blades can have a significant impact on mass flow out of the turbine, how efficient the gas can do work to the turbine, turbo response...and I'm sure I'm forgetting something right now.
Anyway, the point I'm trying to make is the turbine of a turbo is a system, just like everything else in the engine.
Yes good point. Has anyone seen a turbine/housing map for any of our typical turbos? I know there are compressor maps but Ive not seen a turbine map.Quote:
Originally Posted by Reaper1
The Holset EFR series has both their compressor and turbine maps on their website last I checked.
I'm still surprised that no one has really touched the subject on twin scroll stuff.