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HX35 Twin Scroll Back Pressure.

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bastarddsm

20+ Year Contributor
5,620
1,457
Aug 26, 2003
Mendota, Illinois
So, I have a second 4bar map on the way as well as a 7blade hx35 to replace my current 8 blade. I also have a twinscroll manifold, as well as a open manifold. I also have a 12cm and 20cm divided housing. When I come home for thanksgiving break, I'm planing on doing the mother of all spool, flow, drive pressure tests.

Any way today I was bored so I plumed a boost gauge into one half of the t/s setup, the results were exactly as I suspected. Right now, the car has a 8 blade 56mm Wh1c on it, along with a 12cm t/s housing and a true t/s manifold. The runner lengths are not equal, but I doubt that effects it a whole lot. The car is 8.5:1 compression, 2.0l, has delta 272's straight up, 2gmaf, and an old school tube plenum smim. Boost is set at 25psi. I started watching the gauge at 5000rpm, and made a 3rd gear pull to 8500. As it sits the car should run hi 11's at 122mph.

Time for results:
At 5000rpm and 25psi, I saw 30psi drive
At 8000rpm and 25psi, I saw 50+psi drive. It was off the scale of the gauge, and almost into the vacuum scale. Extrapolating it out, I figure it was right around 50.

So we start at a 1.2:1 drive to intake pressure ratio, and end up at a 2:1 ratio, that is far less than optimal. Also, the drive is higher than the boost throughout the pull.

I'm going to try and make a spacer plate, and effectively "open T3" my set up and take some measurements. I might also swap on the 20cm housing and check that out as well.

So, I sourced a open T3 flange, and put that in to open it up. Ends up with almost the same drive pressure at full boost. But up until full boost, the drive stays much lower than the boost. So maybe the open T3 setup actually flows more? Maybe its drive stays lower because its laggier? I think that if the open t3 flowed better it would show up at much higher boost, maybe not even until you were in an unwastegated situation.
 
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Ok I realize im new here and everything so if im wrong im sorry but I thought the whole point of twin scroll was for the exhaust surges (for lack of a better word) to hit the turbine fans at different angles and at a given time so that it spools more efficiently? That being said wouldn't non equal length runners defeat that? Like I said if im wrong im sorry but I was curious.
 
Ok I realize im new here and everything so if im wrong im sorry but I thought the whole point of twin scroll was for the exhaust surges (for lack of a better word) to hit the turbine fans at different angles and at a given time so that it spools more efficiently? That being said wouldn't non equal length runners defeat that? Like I said if im wrong im sorry but I was curious.

The theory behind twin scroll is separating complimenting exhaust ports such that there is minimized reversion (exhaust pulse traveling back up into an adjacent cylinder and taking up space that would be better used by fresh intake air). By minimizing this effect, you essentially have more air charge and thus more power. The exhaust pulse also travels through the turbine instead of back up into an adjacent cylinder (who's exhaust valve happens to be closing), and gives more exhaust energy to spool the turbine.

The cylinders are paired 1+4 and 2+3. The cylinders are paired so that one cylinder leads the other by 2 strokes; When one cylinder is in the intake stroke, the other is in its power stroke; When one cylinder is in the compression stroke, the other is in its exhaust stroke. Thus, the exhaust valves are always closed when the other paired cylinder is in its exhaust stroke, so very little reversion is possible. The runner lengths play very little role in this. They would have to be VERY unequal for two exhaust pulses to reach the turbine close to or at the same time.


bastarddsm, I think its the other way around with the open T3. I think its laggier because the back pressure is lower, not lower back pressure because its laggier. The open T3 is definitely going to flow better because it doesn't have a restriction in the form of the divider. I wonder if the back pressure would be more noticeably lower if an open T3 turbine housing were used?

Very neat experimenting, and good to know some of these numbers. I'm interested to see how much lower the back pressure will be for the 20cm^2 housing.

The 12cm^2 holset housing isn't really small either. If its at a 2:1 ratio, I'd also wonder what back pressure some of the 16g or 14b guys are seeing at 8500rpm.
 
bastarddsm, I think its the other way around with the open T3. I think its laggier because the back pressure is lower, not lower back pressure because its laggier. The open T3 is definitely going to flow better because it doesn't have a restriction in the form of the divider. I wonder if the back pressure would be more noticeably lower if an open T3 turbine housing were used?

Very neat experimenting, and good to know some of these numbers. I'm interested to see how much lower the back pressure will be for the 20cm^2 housing.

The 12cm^2 holset housing isn't really small either. If its at a 2:1 ratio, I'd also wonder what back pressure some of the 16g or 14b guys are seeing at 8500rpm.

Here's the thing on the back pressure vs. lag deal. The back pressure is part of a 2d map indexed by boost and rpm. It carries these trends:
1. As boost increases back pressure increases.
2. As Rpm increases back pressure increases.

Obviously this is just a rough approximation of the actual system, but good enough for my purpose. Anyway, on a third gear pull your only seeing one narrow path through that map. So theres a lot of area your not seeing, so you can't directly compare the back pressure curves between the two. I'm thinking that at a given rpm the open has less back pressure, because its producing less boost, and thus less airflow. I'm sort of theorizing that its possible the open manifold into the t/s housing flows better than the full t/s setup. And I think this might not be seen unless you run the turbo to the max.

I don't feel that the restriction is in the divider. Its in the nature of the system. In the t/s each pulse flows through a 6cm nozzle, and the open has each pulse flow through a 12cm nozzle. Obviously the t/s has twice the time but does that matter? The exhaust is a pulse, and not a continuous flow. I tend to think that the open setup keeps the peak velocity below the speed of sound barrier easier. Maybe I'm wrong?

Anyway, I'm hoping to see close to a 1:1 with the 20cm housing, but I'm not holding my breath. I do know that open to open, the 12 and 20 spooled pretty much the same, and for me at least, going from open 20cm, to t/s 12 was about a 800rpm drop in spool time. 30psi@5000 vs. 4200. And going from open t3 to t/s on the 20cm housing was about a 500rpm drop 30psi@5000 vs. 4500.

I've been trying to research hard numbers on the effect of a/r on turbine efficiency. I'm thinking that if a/r doesn't significantly impact turbine efficiency, then is shouldn't effect back pressure. When I get the 20cm on I guess we will see.

I bet the 16g guys are seeing close to 3:1. I wanted to test out the QSV idea, so I blocked off one of the 12cm scrolls. Holy #### did that have a negative impact on the car. It spooled slower than the open setup with both scrolls, and had probably 75psi back pressure @20psi and 7000rpm. I'm hoping it works better with the 20cm housing.

Finally, keep in mind that back pressure is sort of a function of the efficiency of the turbine and the power required to turn the compressor. (which goes up rapidly with boost and airflow) So If you have a restrictive fmic, you should have much higher back pressure than someone with a hotpipe. I bet I have a 5psi drop across my fmic at the big end, this translates into about a 6-7hp (on 40hp) increase of power required to run the compressor. That can't be good for backpressure.

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