Turbo sizing for compounds

[Charles]

Okay well that answers that question, so here's another lol. I'm pretty clear on how to tune the wastegate for the atmospheric charger, but how do I tune the one for the high pressure turbo?

A good rough starting point is to simply split the boost to each charger. And contrary to intuition, that does not mean dividing the boost in two. For instance, if you wanted say 60lbs on the manifold and wanted to start by dividing the work between chargers, you would do as follows. If you're already up on this, skip it, otherwise read along.

60psi, is really what is called gauge pressure. A gauge is calibrated to read 0 psi at 1 atmosphere of pressure (the pressure on a standard day at sea level). Well, in reality, that one atmosphere is really exerting 14.7psi at sea level. This is called absolute pressure. You may have seen these two units show up as psig and psia respectively.

At sea level, 0psig (psi gauge) is equal to 14.7psia (psi absolute).

So..... back to 60psi...

That is 60psig. In absolute terms, that is 60 + 14.7 = 74.7psia. Gauge pressure, plus the 14.7psi of normal atmospheric pressure the gauge is zeroed at.

So you want 74.7psia on the manifold. Well, you started at 14.7psia coming into your compressor system at the first stage (atmosphere) charger. You then want to compress this a certain number of times so that it will be made more dense and will fit into the FINITE volume your engine displaces. How many times? Well, 74.7 divided by 14.7 to be exact, or 5.08 times. That is called the pressure ratio, abbreviated PR. This is the vertical (Y) axis on a compressor map...

So we need 5.08:1 compression on the incoming air. Since each compressor multiplies incoming pressure, you take the square root of 5.08 for an even split, yielding 2.25. That is the pressure ratio (PR) that each compressor needs to do in order to be doing half the work. But what does this mean to you? You don't have a gauge that will read the absolute pressure delta across each charger.... so you need real numbers. Okay, so lets make some numbers a regular boost gauge can see. You just need a regular boost gauge on the first stage (interstage) and one on the manifold itself. We already know what we're shooting for on the manifold, 60psig, gauge pressure. But what about the one on the first stage?

Well, the PR is 2.25:1. So if you've got 14.7psia to start with, and you multiply it 2.25 times, you get 33.07psia. But that's absolute pressure.... So back the 14.7psia back out, 33.07psia - 14.7psia = 18.38psig.

So your first stage needs to make ~18lbs in order to be perfectly balanced, in terms of work at 60lbs on the manifold. Lower than people would ever think. This is also why the first stage needs to be much bigger than most people think.

If for instance, you instead split the gauge pressure reading in half, and ran 30lbs on the first stage, the first stage would be doing a PR of 3.04. 30 + 14.7 divided by 14.7. With 60 on the manifold the total is still 5.08, so you have to multiply the 3.04 from the first stage by something in the second to equal 5.08 at the manifold. So, 3.04 x Second stage = 5.08. Solve for the variable and you get 5.08/3.04 = second stage, or 1.67 PR for the second stage.

So how much work is the second stage doing? In the first case both chargers were doing about 18lbs. In this case the first stage would be doing 30, while the second stage would be doing 1.67, or 1.67 x 14.7 which is 24.5psia, subtract out the 14.7 for the atmosphere, and you're looking at 9.8psig. So the second stage would be making 10lbs.....

Not exactly balanced....

Now balanced isn't exactly ideal, but it's a great starting point. After that ideally you would measure intake air temps after each charger and while running at your target manifold pressure you would shift the first stage up while bringing the second stage down and vice-versa until the lowest intake air temps were achieved for a given manifold pressure. Bingo.... that's your MOST efficient point to run a pair of compressors for a given manifold pressure. Leave all the maps and theory at home....

 

[mcnance865]

That was awesome to read. That is serious knowledge. Thanks Charles!

 

[ja_cain]

Nice to have engineering types breaking it down for the laymen. Awesome job!

Sent from my Pixel 2 XL using Tapatalk

 

[Strokinbronco]

Okay so, say I want to run 100 psi manifold pressure and I split it equally that'd be 26psi per charger right. I know I can reference the gate for the atmospheric off the charge pipe between the two stages but what about the high pressure? Do I reference it off the turbo like a traditional gate even though it'll see much higher pressures?

 

[Charles]

Okay so, say I want to run 100 psi manifold pressure and I split it equally that'd be 26psi per charger right. I know I can reference the gate for the atmospheric off the charge pipe between the two stages but what about the high pressure? Do I reference it off the turbo like a traditional gate even though it'll see much higher pressures?

I never recommend using boost to OPEN a gate. When you pop, melt or blow off the signal line (and you will), your gate doesn't open...

Put enough spring to make the gate undershoot your desired boost then use regulated air to the top port to RAISE boost to what you desire. If you blow a line the truck sets down.

You can either use a simple twist knob air pressure regulator or a pwm solenoid to run the top port.

I ran the spring this way and actually ran a 4 port solenoid that could shift from holding full manifold pressure on the top port when building boost all the way to running full manifold pressure to the bottom port to shove them all the way open when needed.

As far as where to source the signal.... the manifold always has more pressure, and quicker pressure.

 

[Strokinbronco]

So you're saying to let drive pressure open the gates and use manifold pressure to hold them shut to the desired pressure?

 

[Charles]

So you're saying to let drive pressure open the gates and use manifold pressure to hold them shut to the desired pressure?

Yes. That's fail SAFE.


Another option is to run the same spring pressure (lower than desired boost) And then use regulated air on top to bring the desired boost slightly above your target when the bottom port is disconnected (and I emphasize slightly...) and then use a hallman controller to bring pressure to the bottom port to pop the gate open right at desired boost. This is an all manual, non-electronic control scheme that is highly reliable, super fast to boost and still largely failsafe in that if you blow the line to the bottom port you only overshoot by the amount you dialed into the regulator on the top port. This also lets your "spring" basically be an airspring that you can infinitely adjust by changing the regulator setting to the top port.

I ran that setup for quite a while before running the 4 port solenoids off my PCS controller. The solenoids were similar in speed and control to the regulator/hallman combo but lacked reliability. They would get clogged with the inevitable soot blowing back down the lines and would have to be oiled and replaced from time to time. Usually a couple years or so of daily driving, so not bad, but the manual stuff seemed immune.

The electronic control could get to desired boost faster, and stay dead on desired boost, but required very accurate tuning tables and something on the truck to actually run them obviously. The manual stuff was nearly as quick to boost, and pretty damn good on staying put throughout the rpm range without much bs.

 

[Strokinbronco]

Charles what's your opinion on port/ polish and camshaft options? I'd really like to be over the 700hp mark fuel only but is that a necessary step? Swamps is currently building the engine and Johnny says the last they tested there wasn't much difference with the available cams and some actually lost power. As far as porting I could see a gain there but at $1600 is the gain worth the price?

 

[Zeb]

Charles what's your opinion on port/ polish and camshaft options? I'd really like to be over the 700hp mark fuel only but is that a necessary step? Swamps is currently building the engine and Johnny says the last they tested there wasn't much difference with the available cams and some actually lost power. As far as porting I could see a gain there but at $1600 is the gain worth the price?

Did they say whose/what cam(s) lost power over a stock cam?

 

[Strokinbronco]

No, he refrained from pointing the finger at any one in particular but he said even the grinds they offer could hurt performance below 3k rpm

 

[Zeb]

No, he refrained from pointing the finger at any one in particular but he said even the grinds they offer could hurt performance below 3k rpm

10-4.
It seems as tho the “non emissions” motors (7.3 Powerstroke, 5.9 Cummins) benefit the least from a cam in the ‘normal’ diesel rpm range.
My experience has been the 6.0 and 6.4 can benefit considerably more from the correct cam than the earlier engines.

 

[sootie]

I find it hilarious that big name shops steer people away from cams/head work....shows how much they actually know.

I'll give you a hint-the fastest fuel only seven threes have extensive head work and experimental cams in them...

 

[Zeb]

I find it hilarious that big name shops steer people away from cams/head work....shows how much they actually know.

I'll give you a hint-the fastest fuel only seven threes have extensive head work and experimental cams in them...

At lower levels of performance I can see the cost/benefit ratio not being worth it, but when going all out I can’t believe they say that.
On the same train of thought, how many of these big name shops (Swamps, etc) have a truck/engine that is competitive in the upper level of drag racing or sled pulling?

 

[Charles]

I don't have much experience gaining anything worthwhile through heads, cams, valves, intake or anything else. I squeeze the air harder if I need more oxygen and that never fails. Moving more air is hard. Moving heavier air is not.

The only 7.3 I'm aware of doing anything of substance in the airflow department would be something like Ben Miller or hypermax in the 2000+ hp range fuel only. I have no idea what they might be doing to positively affect volumetric efficiency, but they would be who I would ask if I cared.

A number of cams have lost power despite having great SOTP feel. Anybody suggesting that they have meaningful gains through head or cam work on a 7.3 I greet with serious skepticism. I would say that 50 to 80rwhp (dynoed) from head/cam changes to be a very good job on these engines. That's not ever really appealed to me as being worthwhile considering the negatives usually involved.

The heads on the engine I just fired last week have some in chamber and port work so when I start driving around I can see if there's any drivability change, although I also went to B code VOP, so it will be hard to tell which is which and losses down low may be masked by the VOP.

 

[Strokinbronco]

As far as the intermediate pipe is concerned does it really matter if I run 3" vs 4"? I'm kinda tight for space and the 3" would make life a little easier.

 

[Zeb]

As far as the intermediate pipe is concerned does it really matter if I run 3" vs 4"? I'm kinda tight for space and the 3" would make life a little easier.

3” will be fine.

 

[m_j]

Did they say whose/what cam(s) lost power over a stock cam?

colt told me he supplies swamps

it would be nice if the big guys would let us know who they are using as it helps the entire 7.3 crowd

 

[Strokinbronco]

Well I got the turbo in where I want it and tacked into place, ended up using some 1 1/8 dom tubing and an awesome tubular turbo mount from maven performance. Originally I assumed the turbo was a mid frame Garrett but turns out it's actually a large frame and takes up a decent amount of real estate under the hood, also I relocated the passenger side battery to the driver's side of the engine bay and I really like the way it turned out! I also had the turbo modified to have a v band discharge and a boost reference port added.