I have a set of Nathan's MPI Twins. Ck my sig. They are the HTT 64/71/13 HTT SS and the S400 80mm modified. Pls add me to the list.

 

Alright...

Mr. CTD06 is now #12

Mr. OT-OF-Here is now Mr. lucky #13...

What a great initial response by everyone!

 

Now what we need to do is start some good discussion here about the compound turbos, what makes them work, what makes them work better and how to dial them in and etc. It was mentioned in another post about discussing drive pressure. That would be a good start, but any technical discussion, I believe, would be of great help to all involved. Nathan, (Idaho CTD), would you care to help us out here to get started? Let's not forget to have some fun along the way...

cquestad: i might even help bankroll that bet for you... there Phil, how's that??

 

If we round up the fuel cost...I will go and defend "our" honor!

 

I just did that round trip last month... on the fuel cost...

 

OK-- talked me into it...

Of course, I'll go to Idaho first just to see that part of the country again...

 

alright I'll make you a deal. I'll meet you in Dallas instead of Houston


What's wrong with seeing Texas again? We all know its the greatest state! Theres a reason the state flag flies at the same height as the american flag!

 

Nothing wrong with seeing Texas, just part of the trip... as long as it is not West Texas Only the folks in Texas think it's the greatest state, and it is then only the folks who haven't been anywhere else!!! (I think the folks in New York City think they are the greatest state)

 

Hey Phil, How about Oklahoma???

I gotta stay away from those darned Cabelas and Bass Pro Shop stores you have, they cost me more money than my CTD!!!

That's just WRONG!!

 

THeres a Bass Pro in Tulsa Ok now. Come do it here

 

What do you think Phil, Tulsa?? How about Denver? Gotta get you some culture!

 

yup, ranks right up there with the communist republik of california.....J/K....We all know you guys cant remember which is which....So you just fly them even...Hell I cant say much, the tards in MT where i am from flew the Mcdonalds flag above the state flag while i was home on leave.....

 

For proper compounding the small turbo (secondary) typically needs to make a minimum of 10psi. This can vary slightly with the size of the secondary turbo but in general 10psi is about right. This holds true to well over 70psi and the reason is so the secondary can process the amount of air being force fed to it. After about 70-80psi typically you need to increase the small turbo boost in order to prevent over working the primary. Granted this depends on the primary but from my experience it's best to keep the primary boost no more than 40psi.

In general at peak boost the primary should make right at 1/2 of the total boost up to the 70-80psi range mentioned earlier. At spool up you want a small amount of overlap (this is the range from when the primary starts to make boost until the secondary stops adding boost) in order to prevent midrange lag. The more overlap the better the bottom end and midrange because the secondary is doing more work creating boost quicker. This is better for towing and light to light racing (no one here does that though ) but it has a tendency to choke the top end creating high drive pressures.

I guess I should have mentioned that compound turbos work by multiplying pressure ratios the same as a two stage air compressor. For example at 70psi of total boost the pressure ratio (PR) is 5.93 PR. In order to calculate the secondary boost (it cant be measured) you need to know the primary boost. If your making half of your 70psi from the primary the pressure ratio is 3.48 at 35psi. So 5.93 divided by 3.48 equals 1.7. At 10psi of boost the secondary pressure ratio is 1.7 (It changes with atmosperic pressure. For this example I used 14.2). So in this case 35psi and 10psi equals 70psi as strange as it seems.

There is some info to chew on.

 

Thank you Nathan! A great post

 

OK, so let me tell you what I've observed and see if you think this is good performance. For instance, cruising at 65-70 I get a total of 4-5 psi of which 1-1.5 psi comes from the bottom BHT3B. I have a boost gauge installed in the cold pipe between top and bottom turbos and another boost gauge installed at the intake horn. When getting on it I typically see something like this: at 10 psi total, 2-3 is from bottom; at 20 psi total, 5-7 is from bottom; at 30 psi, 12-15 is from bottom and at 40 psi, 18-22 is from bottom. Wastegate on the top turbo is set to start to open at 25 and is fully open at 28. I know you like to set your twins around 20. I have not measured above 43 psi total, I don't think I have enough fueling to get above that presently. The "pop-off"valves are set at 52psi...

What do you think?