EdmontonCanada

I thought that if one disconnected the wastegate (so it doesn't open) on the high pressure turbo in a compound setup that more boost would be generated.

However, yesterday I was told the opposite...that the wastegate opening allows the exhaust gasses to bypass the high pressure turbo turbine and go "directly" to the atmospheric turbo.

Thoughts?

soulezoo

First thought is wrong. Second is correct but with a caveat...

First, let's get rid of that whole notion in your mind of "more boost". That is the first error.

"Boost" in terms of air psi, is a measure of restriction. On an otherwise stock truck, more psi means more air for the most part. Until you are overdriving your turbo, creating very hot air that has expanded to such a degree that you may be pushing more psi, but delivering less actual air to the engine... in other words you are going backwards. That's why we have compressor maps for turbos so we can gain an understanding of limits.

The second statement is correct to the extent that the wastegate allows more gasses to bypass the small turbo to get to the larger one.

Here's where TUNING your wastegate is important. If you allow the secondary turbo wastegate to open too soon, you'll have a great deal of lag trying to spool the primary. If you do it too late, you are overdriving your secondary, creating unnecessarily hot air and depriving your primary exhaust gas and heat to drive it.

This is something best played with on the track to dial it in, but a rule of thumb is somewhere around 80% of map on the secondary to start to open the wastegate. I dial mine in to open late when on the street and early on the track. When my primary spools, I'll usually break traction it hits so hard.

I didn't use psi numbers in this because everyone's set up and turbos are different and the psi becomes irrelevant except in an anecdotal sense.

For instance, at 70 psi I push more lbs/min of air than others pushing 80-90 psi. It's restriction.

soulezoo

I also want to add:

Think about the exhaust gas energy it takes to drive the turbos. If you expend all that energy driving the small turbo's turbine, how much of that energy is still left to drive the much larger turbo's turbine? Then think about that with the wastegate open.

EdmontonCanada

Thanks! I knew that higher PSI doesn't mean necessarily more power…One needs to flow more cool air...and in pursuing that side of the equation my question was to do more with how compounds work together in terms of flow. I've read a few turbocharger books but I didn't recall reading about wastegating on compounds.

And I know what you mean: my turbocharger system generates more power at 20 PSI than any single turbo truck I've ever had running the same boost pressure.

I think what I am going to do is install the AEM Tru-Boost electronic boost controller. It is the same dimensions as a 2 1/16 gauge and can control up to 75 PSI. That way I can tune the wastegate from inside the truck. It also overrides the disadvantages arising from the mechanical aspects of the wastegate.

soulezoo

I also should have mentioned all the back pressure you create. It is not reasonable to think that all that exhaust gas flowing only through the secondary turbine is then going to efficiently spin the much larger turbines on your primaries.

EdmontonCanada

You're correct. Confirmed with the manufacturer that the wastegate had been set to balance the system, as you stated, and that I should reconnect the wastegate pressure hose. It's all been combined as a complete system, calculated and tested.