Do these numbers look right...

[chevyburnout1]

This is my first experience with compound turbos and I want to make sure I'm doing everything correct. These are my max numbers I'm getting so far:

50psi total boost
18-22psi primary
70psi drive

I'm getting these numbers in fourth/fifth topping out rpm with the wastegate completely shut. Do these sound right? Should I be opening the wastegate at all? I feel like I still don't have enough fuel to push these chargers into their efficiency but I just want to make sure everything is ok for now.

I have a .84 exhaust housing on the secondary charger and I'm debating on going smaller still to help with drivability up here in the mountains.

 

[SickLL7Crenshaw]

How big a tune was this on?

 

[Chevy1925]

Not bad. As I'm told, generally you want the big charger to be half of your total boost. You may need that wastegate to start opening if you run a much bigger tune

 

[delong_1]

I always run 2/1. Opening the gate will accomplish that

 

[chevyburnout1]

This is a 1800us tune with 60% overs dropping rail pressure to 22k. I won't have any more fuel till probably the beginning of next year when I throw on another pump. So it's maxed out for fuel for now. I just want it adjusted as efficient as possible.

 

[chevyburnout1]

Tinkered with my wastegate and got the primary around 23-25psi. Didn't take much. What I thought was a bit odd was if I opened the gate up not much more it loses a lot of power and smokes a lot. Total boost would drop as well. Is this just because the secondary isn't being driven hard enough to make good exhaust heat to push the primary?

Another question as well. I'm debating on dropping my secondary housing from a .84 divided down to a .70 divided or even a .58 divided. Would this drive the charger harder or is it just becoming a restriction then? With my fuel I have now the wastegate is barely cracking on the top of the rpm's so I figured I have a bit more room to play. For now I would rather move to the driveability area of the spectrum instead of max power, especially at this altitude and with winter coming. Drive pressure seems to follow about a 1.2-1.3 ratio on spool up/low boost.

Any of this feasable or am I expecting too much out of large twins at 5500+ft of elevation?

 

[Cknight199]

Tinkered with my wastegate and got the primary around 23-25psi. Didn't take much. What I thought was a bit odd was if I opened the gate up not much more it loses a lot of power and smokes a lot. Total boost would drop as well. Is this just because the secondary isn't being driven hard enough to make good exhaust heat to push the primary?

Another question as well. I'm debating on dropping my secondary housing from a .84 divided down to a .70 divided or even a .58 divided. Would this drive the charger harder or is it just becoming a restriction then? With my fuel I have now the wastegate is barely cracking on the top of the rpm's so I figured I have a bit more room to play. For now I would rather move to the driveability area of the spectrum instead of max power, especially at this altitude and with winter coming. Drive pressure seems to follow about a 1.2-1.3 ratio on spool up/low boost.

Any of this feasable or am I expecting too much out of large twins at 5500+ft of elevation?

Have you tried another tune that holds rail? Isn't there more hp on a tune that holds rail than a tune that doesn't if the tunes are close in pulse width?

 

[Chevy1925]

Have you tried another tune that holds rail? Isn't there more hp on a tune that holds rail than a tune that doesn't if the tunes are close in pulse width?

His rail isn't the issue

 

[Cknight199]

His rail isn't the issue

Ok my bad

But question, is it true that a tune that holds rail is better than a slightly larger tune that doesn't?

 

[chevyburnout1]

See I'm not really looking for maximum potential hp. I'm just curious if these chargers can be driven harder without having to be at full throttle all the time.

 

[Chevy1925]

Tinkered with my wastegate and got the primary around 23-25psi. Didn't take much. What I thought was a bit odd was if I opened the gate up not much more it loses a lot of power and smokes a lot. Total boost would drop as well. Is this just because the secondary isn't being driven hard enough to make good exhaust heat to push the primary?

Another question as well. I'm debating on dropping my secondary housing from a .84 divided down to a .70 divided or even a .58 divided. Would this drive the charger harder or is it just becoming a restriction then? With my fuel I have now the wastegate is barely cracking on the top of the rpm's so I figured I have a bit more room to play. For now I would rather move to the driveability area of the spectrum instead of max power, especially at this altitude and with winter coming. Drive pressure seems to follow about a 1.2-1.3 ratio on spool up/low boost.

Any of this feasable or am I expecting too much out of large twins at 5500+ft of elevation?

It's a fine line you walk to keep that gate from going too far one way or the other. Moving my regulator a 1/4 turn one way or the other can completely change my boost ratio. What your saying is the same stuff I noticed but I can get on top of the 475 much easier than what you can (obviously).

I don't have much of an answer on housings Dustin but I would be afraid of choking the big charger off by going much smaller. It just seems like you got a boost leak or something some where due to your lag. Hell Steve spools his 72 single down here like its nothing and he said that even at 7k feet where his cabin is, it's not all that bad.

What size turbine is in the 72?

 

[Harbin_22]

What size turbine wheels?

 

[Chevy1925]

Ok my bad

But question, is it true that a tune that holds rail is better than a slightly larger tune that doesn't?

It all depends. It's generally made a point that 3k or less drop in rail from demanded will get you just about the same power, bigger drop in rail than that and power can drop off. A dyno or track will tell just where is best for your truck

 

[chevyburnout1]

I noticed some soot on my passenger valve cover but could not tell at all where it was coming from. I even took our EVAP smoke machine we have here at work and shoved it in the tailpipe. Still didn't see any leaks. But I agree James for a 72mm secondary I feel like it takes a whole bunch of throttle to get going. I've pressure checked it up to about 30psi and the only leak I have is a tiny one still in my intercooler that we couldn't get a welder too, but I wouldn't think such a tiny leak would cause this big of an issue. And pressure checking any higher blows my tester off the inlet to the big charger so I can't get any higher test readings right now

Turbine is a 79mm.

 

[Chevy1925]

Wonder if your turbine is too small. I've always thought of it like a leverage affect, a bigger one would make the lack of drive pressure work more efficiently to your advantage. This is just thinking out loud though

 

[Cknight199]

Wonder if your turbine is too small. I've always thought of it like a leverage affect, a bigger one would make the lack of drive pressure work more efficiently to your advantage. This is just thinking out loud though

Larger one on the 72mm or the atmospheric charger?

 

[Chevy1925]

The 72

 

[delong_1]

Smaller housing, let the gate bypass when needed. You need to come down and ride in my truck. Spools super fast, I'm always making boost. Dd tune doesn't shift over 3200 rpm. Etc

 

[chevyburnout1]

Well I just got back from hauling my big ugly camper over Eisenhower pass and I can tell you the truck did not like it. It was a balancing act between EGT's, smoke, and engine oil temps. I know I can only expect so much out of these twins towing up at 11,000ft but I felt like that small turbo wasn't doing anything. I was having to keep rpms around 3100-3200 when I could, just to make 9-12psi total and keep egt's at 1200°. Drive pressure wasn't much more higher until getting up in elevation which I would think is understandable.

So I think I will try going with a smaller housing again. I have to call Comp Turbo anyhow as my larger charger just started throwing oil already Not sure if my oil return isn't adequate or if it was just a fluke or what :dontknow: On top of that I think I'm going to replace my intercooler and buy a V-band boost tester so I can be for sure positive I don't have any leaks at all.

 

[chevyburnout1]

Here's the big ugly thing haha