AH64ID

We read that boost is boost, i.e. that if you don't change anything on the intake/cam/manifold/etc that 30# of boost is the same flow of air with one turbo as the other. This is thanks to the law that states that pressure = flow x resistance. If the resistance and pressure are unchanged the flow is also unchanged. That is easy to understand.... so

This was correlated for me when I did my cam, my boost dropped at a given load from the increased flow from longer intake duration.

What got me thinking is how much cooler my truck runs with my new turbo and lower boost. I didn't change anything that would effect the resistance on the motor, so 30# should be 30#, or so I was thought.

So here is what I am wondering. Does the exhaust back pressure have much effect on the resistance to the intake charge? Lets compare 25 psi of boost with the stock turbo and the new turbo. The stock turbo took about 40 psi of drive to make 25 psi, where the new turbo takes as low as 20 psi. That is a large difference. So does the 25 psi of charge air have an easier time filling the cylinder when the exhaust pressure is lower? This would mean that the 25# with the new turbo is more air than the 25# with the stock turbo.. which I believe is happening based on the cooler EGT's with lower boost.

It makes sense when I think about it, but that's often the case until there is a 2nd opinion.

soulezoo

First thing that comes to my mind, all other things equal, is the temperature of the charge air. To exaggerate the point, 30 psi of air at say 100* has a lot more density, or air molecules, than 30psi of air at say 600*. The difference can be created from a more efficient CAC, or turbo.

Turbo maps can tell you a lot about this. For instance, in the example from the OP above, he discusses the drive pressure between the two turbos. If you look at the PR on a turbo map and see its efficiency islands, that will tell you a little about the quality of the air you will get. In the case of the stock turbo (I am guessing here as I don't have the maps in front of me) the pressure ratio is going to be toward the top of the map and begin to lose the efficiency. The turbo pushing 25 psi with lower back pressure is cruising in the fat part of the map. Less strain on the turbo too.

A turbo spinning its guts out to get that 30psi is not going to live as long and will provide hotter air than the turbo that is cruising. But with the larger turbo, you trade spool time. This is part of the turbo selection process... everything is compromise.

AH64ID

Yeah I agree. I couldn't quite make the words work to talk about intake temp. The Garrett is a much more efficient compressor. The IAT's are cooler across the board for the same boost. Even thou they are cooler it's not quite enough to fully account for the EGT change.

If we leave intake temp out, or make it constant.. does residual pressure in the cylinder have enough effect to increase the flow per psi? I am sure it does have an effect, but is cutting the DP in half for the same boost enough to noticeably increase flow? I don't see why it doesn't...

That also brings the point of cylinder pressure into play. Probably a lot higher cylinder pressure than a less efficient turbo at 30#.

Lil Dog

The efficiency of the turbo to provide the 30 psi of boost does have an effect on the outlet temperature, however, to compress air to 30 psi is a known equation and the efficiency of the turbo compressor will be less than 5% of that in heat.

The answer is in your notes already. The cam change affected the equation on the resistance end. The amount of air flow through the engine (intake through combustion and exhaust) is affected by cam duration. The residual pressure left in the cylinder after the exhaust valve closes will change the amount of air "resistance" to overcome to charge the cylinder with fresh air for combustion. 2 things affect that the most, 1 is the cam timing, 2 is the drive (TIP) pressure of the exhaust turbine.

That is where you hear the most about efficiency of the turbo as the drive pressure starts to go exponentially up over the boost air provided. The result is less exhaust gas let out of the cylinder and consequently less boost air let in during the intake stroke.

I deal with more natural gas engines that have valve overlap (exhaust and intake valves open at the same time) of up to 100° of crank rotation that the amount of boost provided greatly affects the scavenging or purging of the cylinder with fresh air for combustion. With the Cummins the overlap, as much as I can tell, is near zero. So affecting the amount of time the exhaust duration or intake duration is affects the amount of residual exhaust gas in the cylinder. The newer engines , from what I can read, are using shorter exhaust valve duration to keep exhaust gasses in the cylinder for a sort of internal EGR system for NOX control.

AH64ID

I really think that a much higher DP will defiantly effect the amount of resistance the intake charge can push thru. Even if there is no overlap and the exhaust valve closes at TDC just as the intake valve is opening the psi in the cylinder will be higher than the intake manifold and there will no flow, or an attempt at reverse flow, until the piston drops enough to get the volume of air higher and the pressure lower than the intake manifold.

In an overlap engine a higher boost than DP will move a LOT more air than higher DP than boost.

I think the cam I have has a little more overlap than stock as my IAT's rise about 6° with the EB on, where I didn't notice that on the stock cam.

Yes the new engines use an advanced and shorter exhaust duration to have an EGR effect, which is the main reason I went with a new cam.

no_6_oh_no

The thing that is not represented here, pressure = flow x resistance, and is biggest factor is air density. This is the reason 25 psi of boost is equal to 30 psi boost in different scenarios.

In that you are correct, 25 psi is NOT 25 psi.

Both the ability of the compreessor to generate denser air at lower pressures, and, the lack of DP copntribute to cylinder scavening and the efficacy of the combustion event. The denser cooler air will always win out over hotter light air, even with relatively large pressure discrepencies. Mass will always prevail.

IIRC, the cam timing is limiting the the ability to scavenge the cylinder by reducing the time the exhaust valve is open, not neccesarily the overlap. Less comnbusted air out because the vlave opens later ultimately equals less in-flow of denser air to reach cylinder fill at a given boost. More left, less room to fill.

AH64ID

I was trying to look at with the unrealistic point that air density and temp (directly related) were the same on different setups, and purely the effect of drive pressure on intake resistance (kinda how high DP blows HG's on 6.7's even thou it shouldn't matter)... But yes density is huge and the mass of air in one setup at 30# is very different than another truck at 30#.

Thanks for all the replies, its good to work the head bone every now and then.

Some people look at me funny when I run studs at 30-33# of boost, but I KNOW I have a LOT higher cylinder pressure than most do at 30# based on the airflow improvements. It's impossible to quantify but I could easily have as high of cylinder pressure as a stock cam/turbo at 40# or more, since cylinder pressure is a result of air in the cylinder not resistance in the intake.

This conversation also helps make more sense how the QSB480 makes 472bhp at 3400 rpms with under 35 psi of boost and 1301° in the manifold! How many trucks do we see with those numbers? I would like to try that tuning on my truck and see what it does for numbers.

soulezoo

There is no one trick pony for this and I try to stick to my experiences. I've talked about these things before about boost being simply a measurement of restriction. For me a change in turbos with some mild porting netted a steep gain in HP at the same boost level.
Sometimes it is just trial and error...
I made my 900 hp on DDP 120's... How many have done that? I don't know of any. Most will tell you that 180 to 200hp sticks are required for that power level. I say it is a matter of tuning; not just the programming, but managing the airflow too. It isn't all just big sticks, big programming and big turbos.

AH64ID

That's a very true statement.. One I beleive completely in.. I mean look at my mods vs my hp .

Spooler

My vote is for a decent size twin set and one heck of a ported head with shaved intake on the truck with a good cam and 90hp sticks. That's what I will do one day.

HOHN

Yes, exhaust backpressure matters. It is huge? That depends on what huge means.

Because the stock cam has very little overlap, there's not much of a chance for the high backpressure to act on the intake charge.

Step back a second and think this through with me. The engine's ability to breathe through a given cam/valve/port/etc is a function of displacement (primarily) and compression ratio (secondarily).

When the piston travels up and down, the idea is to have the maximum possible difference of volumes between the two positions. Maximum air volume at BDC and minimum volume at TDC.

With high backpressure, the engine will not exhaust spent gases as effectively, and there residual cylinder pressure at TDC is higher. This pressure expands as the piston lowers, which means the cylinder won't "suck" as hard on the port, and it will ingest intake charge less effectively. Put another way, the lb/min of fresh air into the engine will be less.

With an aftermarket cam of much higher duration, the effect could be MUCH more pronounced, as now the exhaust backpressure may have the chance to act directly to oppose the intake charge. This is because when one cylinder is parked at TDC with both valves open, some other cylinder is approaching TDC on the exhaust stroke, and it will end up pumping its exhaust into the cylinder sitting at TDC with both valves open right when that cylinder SHOULD be getting its first breath of fresh air.

Overlap is good-- but ONLY when the pressure balance across the cylinder is favorable. The more overlap, the more important it is to have backpressure be lower than boost pressure.

The corollary then is that when you have a bigger cam, any improvement in the pressure balance is magnified, and I think that's what you are seeing.


It's not the turbo that's delivering "more air". If you have 25# of boost in the manifold at a given IAT, then all the turbo is delivering the same thing.

But when the FLOW increases, that pressure will drop.

The lower boost and cooler EGTs you are seeing are a function primarily of improves pressure balance across the cylinder-- and that, mostly from a turbo that has a higher flow capacity on the hot side and doesn't create massive backpressure.

Justin

AH64ID

Great post! Thanks for your input. It pretty much puts to words what I was thinking and couldn't type!

Higher flow per psi is the end result. It's why there are 2 important things to look at on turbo MAP, PR and flow!

HOHN

Very well stated, Soul^^. Boost is restriction. As are head ports and anything else that prevents infinite flow.

It's easy to get lost in the more is better school of thought. The truth is, unless you invest in MASSIVE charge air cooling, more boost past about 50psi or so is often a step backwards. A well-tuned rig SHOULD be able to make 600hp with 50psi of boost. With very effective CAC, you can make 600hp with 40psi of boost.


That's why I think Nitrous is the best all around way to shoot for big power-- even a small shot helps a ton at lower levels. It gives massive improvements in charge air cooling and in O2 content for minimal pumping loss penalties.

Twin turbos with very large gates set to bypass a lot of flow with a large hit of nitrous is the best approach for mega-HP. Pumping losses for twins at 45psi are low, and that big hit of nitrous can go a long way.

Turbines and compressor only operate well within a somewhat narrow pressure range. Keep them in the range and use the nitrous to shoot for the moon.

PS-- lower compression ratios and all that really help

AH64ID

Exactly! It's why with a more efficient setup you can move a higher mass of air at lower boost. And why I was asking the question, does higher DP than boost create a "measurable" amount of restriction.

60 lb/min at 30 psi is better than 60/lb a min at 35 psi, and in the end it's still 60 lb/min.

Lil Dog

If lbs/min are at standard conditions, then that would be a true statement. Air density has to be constant for that to hold true.