well I have the duty cycle at 100% so I don’t think it can compensate at that point. I made my gate manually adjustable long ago so I can dial it in or add a little bit more spring pressure if I ever need more power. I turned the dome screw in some last night and tightened up all my clamps to make sure it’s all sealed good. I’ll take it for a spin in a little while and see if it added some boost.

 

it still doesn't make sense. With less back pressure it's easier to make MORE boost not less for the same duty cycle. More back pressure pushes the wastegate open easier for a given spring pressure.

 

i get what you are saying. And that’s what I was thinking last night, but….say I’m making 20psi and I’ve got 30psi (I’m just throwing a number out there because I never checked it) of backpressure. Then say I lower that backpressure to 15-20psi, so now the motor is flowing more air through it. The boost should be lower because it can get in and out easier. I dunno, maybe I’m wrong. But I have no exhaust leaks I can find, so I have no other explanation.

 

I just double checked the tune on my boost controller and the duty cycle is set at 100% so I’ll have to add more dome pressure. What I did last night only added about 1/2psi in the upper end but it added 2 psi at lower boost.

 

I believe you're thinking about it backwards. How many horror stories have you heard of someone adding a cutout or dropping their exhaust and have an overboost and blow their motor? It's happened many times.

 

I was also thinking boost would be higher if back pressure was lower , but then I think a larger housing would turn the turbine slower than a smaller one with a given amount of exhaust flow so of course slower turbine = slower compressor = lower boost , plus as mentioned lower back pressure also means less restriction / more flow through the engine which would lower boost . I'm sure all these variables play on each other differently on different combos. As long as you can increase WG spring and play with the controller I'm sure you'll get it where you want it. In for results

 

Have you measured the inlet side vacuum? Is the air filter/inlet tube doing something odd?

I had a squirrel pack a air tube full of walnuts once.. Engine didnt run very well..

 

damnit. You have a point. So I was right originally in my thinking. Double doh. I’ll have to do some more checking for boost leaks. Thanks for setting me straight!

 

nah I haven’t. I guess I should. What’s acceptable?

 

Bad boy probably still got all squirrelly when you floored it though...LOL

 

Never knew where that saying came from. Learn somethin every day!

 

so I think we might both be wrong. The cutout frees up a restriction after the turbine, and the bigger housing frees up a restriction IN the housing….which will slow the gasses speed down and the flow won’t be as concentrated and pushing on the wheel as hard.

 

Not sure I'm buying that. The car isn't producing less exhaust with a larger turbine housing. It's the same amount of exhaust flow at a given psi, just with less restriction. It's the same size wheel in the turbine. So all the exhaust is still pushing the same size wheel. The larger AR just funnels the exhaust into a larger opening instead of a smaller one.

 

Garrett actually says you'll flow MORE with the larger AR.
https://www.garrettmotion.com/news/n...-turbo-system/

 

this results in less velocity = less rpm of the turbine therefore less rpm of the compressor, which would mean less boost from same amount of exhaust flow . Now if you start making more power , then you get more exhaust flow and on and on it goes.

 

Check the Garrett link I posted. They did make more power so they have more exhaust flow offsetting the reduction in velocity to an extent. There's been other examples online where someone's boost was maxed and then went to a larger AR and boost rose.

 

So I had posted about this on the turbo forums and a reply I got, that made sense was the following:


“The turbine housing is nothing more than a nozzle, designed to speed up the exhaust flow. The speed determines the angle the exhaust hits the turbine wheel. By going to a bigger nozzle (bigger a/r), the gas isn't being sped up as much as with the .96 housing, this means it's slower and therefore doesn't "drive" the turbine as hard. Which means less shaft speed, less compressor rpms. Less rpms means the compressors exducer isn't going as fast so it can't produce the same boost level as before.

The reason you gain boost with a cut out is because you are decreasing backpressure behind the turbo, allowing the turbine to get a higher expansion ratio.”

 

I guess I'm struggling to see how a DECREASE in pre turbine back pressure would result in less boost, especially with a controller that targets a set boost as a closed loop system. I can appreciate that you're blowing through a larger straw but it's into the same size wheel. If we're talking fractions of a psi then sure but multiple PSI just doesn't seem right.

 

First off - good discussion.
the garret link really just shows bigger ar = more top end power at the expense of spool rate , which is what everyone would expect . They don't mention whether boost went up or down from just the swap and I'm guessing they did what they had to get it the same.
I'll try to make this short , about 40 years ago in trade school the subject was Detroit diesel tune up procedure with a drawing of a 2 cycle dd with turbo and blower (think 6-71) so I ask the instructor -do they use a different camshaft in the turbo engines to close the exhaust valves earlier to stop the boost from going straight out the exhaust. He said - don't know and nobody ever asked that before LOL. He checked with some people and no ,Same cam. So I figured it out , the only reason those engines can build boost is by having exhaust back pressure. Anyways how does this apply to our stuff ? Lower back pressure means less exhaust left in the CC and more room for charge air which means less restriction to flow on the intake which would lower boost pressure ( not power or flow ) mostly trying to explain how kfx got lower boost after the swap . Like I said earlier, there's a lot a variables at work here , from valve events to intake / intercooler restrictions and where on the compressor map you end up when you increase flow , how much power/torque/rpm the turbine is able to produce with a different housing. Most of this over my head for sure , that's why I just try to use what others have had good results with lol and this housing swap is something I'm considering but someone has else to prove it's worth while LOL. This is my understanding of the whole thing, keep the great info coming !!

 

Which boost controller targets a specific boost ? Everything I've seen and used just goes by duty cycle with some overboost protection. I'd be very interested in something that actually targets a specific psi . As long as it's affordable