Turbos & intercooling

[ak diesel driver]

I went back and reread TDs responce on this thread and I think the biggest difference is that were comparing performance at a given psi. The att gives better performance at a lower psi because of volume. But once you achieve the same psi flow has to be the same because you can't flow more air thru a given size (intake) without increasing psi. That's why I think that the backpressure has become the unknown quantity.

 

[ak diesel driver]

One other thought the other factor in play here is that with more volume you get cooler air cooler air is more dense hence more power. Probably a combination.

 

[buddy]

Measuring PSI on one of those little mickey mouse cigarette lighter pumps is silly, it doesnt maintain 250psi is the issue, once you open the flood gates its compressor cant create but 40 psi or something and it struggles just to overcome your tires pressure. The ability to maintain 150psi is why a real compressor can handle the job. My 3hp compressor cant even maintain 90psi while I work, even though the tank will fill to 130psi.

All the little cars put huge pipes and muffler diameters or glasspacks on to decrease backpressure because it increases horsepower. However in a gasser V8 you lose torque that way, but diesel V8 has so much torque you wont notice.

 

[Chicago TDP]

But running an intercooler also does one major thing: cools hot air in order to have a higher density of air molecules so that there is more energy to accompany the fuel.

Yes, with the limited fuel supply though, we do not necessarily have to worry about having the densest air supply but it will help in the efficiency of the motor.

 

[ak diesel driver]

TDP got any pic of your setup?

 

[buddy]

I never said that you couldn't get more Air Mass by cooling it, that is very true, so maybe with lower IATs the ATT can push a little more air mass in the same PSI, but its still the same size (volume/flow) of air. But if youre starting with 0 degree ambient, dont think an intercooler is needed, the air is plenty dense. Adding the intercooler will also drop your boost pressure a little as you have added more volume, but gained the extra air mass/density, so thats good.

 

[gmctd]

Correct - the 6.5 is an air pump, capable of ~400cfm @ ~3000rpm - the turbocharger is an air pump - if it pumps 400cfm into the 6.5 pump, intake to exhaust airflow is 400cfm @ 0psig (15psia Baro), no matter what size turbo is puffing into the 6.5 - the turbo from the Titanic would pump no more air at 400cfm than the GM-8 - however, as airflow (volume) output of 1st air pump begins to exceed flowrate of 2nd air pump, that excess stacks up in the intake as pressure - the greater the pressure, the greater the available volume\airflow - add fuel to suit

There are only three ways to increase airflow volume thru the 6.5L pump - (figures are general to illustrate concept):

- increase displaced volume: bore x stroke -
6.5L = 400cfm @ 3000rpm .....13L = 800cfm @ 3000rpm

- increase rpm: 3000rpm = 400cfm .... 6000rpm = 800 cfm

- increase inlet pressure with a 'charger:
3000rpm @ 15psia (Baro) = 400cfm
3000rpm @ 2xBaro (15psig Boost) = roughly 1.5 x 400cfm

Exhaust Back-Pressure is resulted from any restriction in the exhaust path, including the gas-turbine compressor motor - exhaust pressure of 2 to 3 times Boost pressure is acceptable, considering that cylinder pressures were just up around 3000psi as power stroke begins - the larger the turbine, the less restrictive the path, the less EBP, within limits: takes a lot of power to compress air - takes a lot of exhaust energy to make that power - increase turbine size above available exhaust energy level and Boost output will drop accordingly - trick is to size the turbine motor such that it will power desired Boost levels while providing least restriction - the higher the exhaust pressure at the end of the exhaust stroke, the greater the remaining junk in the cylinder from the preceding combustion event, which is the equivalent of Exhaust Gas Recirculation into the fresh intake air charge, reducing the amount of fresh O2 needed for combustion - reduce exhaust back pressure to get fresher air in the cylinder for more efficient combustion cycle

Thus, the engine with properly-sized turbo will make greater power at lower Boost pressures - the correctly-sized compressor will pump cooler air into the intake, cool air being more dense than hot air = better power from greater efficiency at lower pressures - lower compressed cylinder pressures = reduced pumping overhead from less rotational resistance during compression stroke

Far as charge-air coolers vs water mist injection, pros'n'cons are listed on vendor websites - I prefer the simpler c-a cooler, simple to install, simple to maintain, no fuss no muss - add a Provent to really clean up yer act - if I regularly towed\hauled thru Death Valley or thru the American deserts, I would prolly install wmi, butcept have it spraying over the c-a cooler, which would also help cool the engine coolant radiator, because of burning the A\C to help the driver survive the heat - would require a large water reservoir and accessible replenishing supply source

Most of the loss of pressure across a properly sized and mated to your system c-a cooler is due to the reduction in flowing temperature across the 'cooler - compressing air makes heat, heat creates expansion, a gallon of hot expanded air has less volume\mass than a gallon of cool air - which is why engineers measure airflow in mass\weight, rather than volume - street is still stuck on cfm because that's the way carburetors were sized - everyone wanted the dual-pumper 750cfm or 850cfm, or the huge 1050cfm Holley's over the girly-man 550-600cfm versions that rolled off the assembly line

 

[Turbine Doc]

Measuring PSI on one of those little mickey mouse cigarette lighter pumps is silly, it doesnt maintain 250psi is the issue, once you open the flood gates its compressor cant create but 40 psi or something and it struggles just to overcome your tires pressure. The ability to maintain 150psi is why a real compressor can handle the job. My 3hp compressor cant even maintain 90psi while I work, even though the tank will fill to 130psi.

All the little cars put huge pipes and muffler diameters or glasspacks on to decrease backpressure because it increases horsepower. However in a gasser V8 you lose torque that way, but diesel V8 has so much torque you wont notice.

Buddy give me some credit that was a conceptal analogy (evidently not as good one) using the small compressor not to be taken literally, more on it later just don't have time to expand my point right now, and yes some data on the replacement thread that shoes correlation of EBP to the GM and the ATT

 

[NVW]

When I installed the 4" exhaust it was a night and day difference, maybe less back pressure with the turbo is the same scenario. Less back pressure means the cylinders exhaust cleaner meaning a better fuel air ratio for the next cycle. To me it doesn't matter, I can feel the difference. I can appreciate anyone interested in one, they want proof and hard numbers. Sorry I can't give you those. I love mine, I have no regrets. Leo

Edit: I mean to say I don't know why or can't prove why it's better. It just pulls and accelerates better with lower exh. temps.​

 

[buddy]

I believe that the ATT is great and does as advertised and if my turbo craps out I'll get it in an instant, I just think that it is technically incorrect to say it flows more CFM at lower PSI, and dont want any misleading advertising.

gmctd explained it much better than me, Thank You much, I just go by the equations.

 

[ak diesel driver]

Ya know the more I think about buddy's comment on how much effort it takes the piston, on the exhaust stroke, to push the exhaust gas out when the back pressure is high. The more I think that it has to have an effect on engine performance. It would be interesting to see how much horsepower is required to rotate the crank with 20-30 psi of pressure in the exhaust

 

[Chicago TDP]

Ya know the more I think about buddy's comment on how much effort it takes the piston, on the exhaust stroke, to push the exhaust gas out when the back pressure is high. The more I think that it has to have an effect on engine performance. It would be interesting to see how much horsepower is required to rotate the crank with 20-30 psi of pressure in the exhaust

That is not that hard of a formula. It takes into consideration the resitance or work required to compress the gas in the area of the cylinder from the intial volume to the final volume (given the compression ratio 21.5:1 or in my case, 18:1) and then the stroke of the crank which is the "lever arm".

And looking at that thought, that makes me understand a little more in my own mind as to why it is better to have lower compression: easier to compress that last bit of air, the turbo at that point is doing the work and not the motor.

 

[davo727]

Hey Chicago TDP: How many miles do you have now on that 22-28psi motor?
Did you put cometic head gaskets in it?
Are you happy with that Holset?

 

[Slim Shady]

But the exhaust port is closed when the intake port is open right and vice versa? So the cyclinder would be the choking that doesnt change, which is why we go for higher displacement

I'm sure that youre right though, there has to be some greater flow into the cylinder allowed with the lower backpressure

Not exactly, there is overlap, where the intake and exhaust valve are open at the same time. This is where I believe the exhaust back pressure makes the most difference. If we have 20 psi exhaust pressure and 15 psi boost (these are just hypothetical numbers) when the exhaust and intake vale are open we have a detrimental pressure differential. I think the 20 psi exhaust pressure tends to fill the open atmospheric or slightly less: cylinder with spent exhaust gas until there is a balance made. Then the exhaust valve closes and there is no way to get that gas removed, Just like GMCTD said like putting egr gas in the cylinder. Now we have decreased cylinder efficiency and power. Just a semi simple explanation.

 

[Slim Shady]

[


in a WOT situation underload, you are maxing the turbo out. So if a perfect turbo for 6.5 puts out 200 CFM at 15psi, where as the stock one ran 100CFM at 15psi, the pressure of the intake will remain the same at 15psi, but the amount of air that is passing through will be doubled.


A better example may be a shop vac on blow, can put out more air cfm than an air hose set to 30psi, due to the amount of CFM that can pass through it.

I don't think I agree with that statement and it looks to me like you didn't either when you reread it. IMO as long as the opening or duct size is fixed the only way to increase cfm is to increase pressure. I think buddy is on to something with his back pressure idea. I know TD has taqlked about back pressure but either I read it to fast or he didn't carry the thought out all the way.[/quote]


My response

Most turbo cfm numbers are taken at 15 psi. This gives a consistent reference point to compare turbos. The actual cfm at lower boost points may be different. I do know that one change on a turbo affects the whole flow and boost characteristics. There is math that gives you numbers but sometimes the hunch is what makes things work. ):h

I am not saying that this turbo is perfect, but it sure does make my truck run smoother. I think GMCTD had it right, lower back pressure and slightly increased trubine and compressor efficiency makes a sweeter turbo for or truck.

 

[ak diesel driver]

I understand or at least I think I do the turbo output part. But I would like to hear more about some back pressure theories. Obviously with back pressure and valve overlap your going to have some egr effect. But after the intake valve closes now we have a cylinder filled with up to 20+ pounds of constant pressure against it and the engine has to expel it mechanically ie the piston moving up. I know a cuople of times when i've been changing springs on valves I put air pressure in the cylinder to hold the valve shut. If the piston wasn't all the way down it forced the piston down rather forcefully. Scared the crap out of me the first time. Even though we only have 20lbs +- of back pressure there's probably around 4 cylinders that are in varing stages of expelling exhaust. That seems to me like thats quite a bit of work the engine would have to do to overcome this. Anyone got any other ideas I'd sure like to hear some other ideas.

 

[Slim Shady]

I understand or at least I think I do the turbo output part. But I would like to hear more about some back pressure theories. Obviously with back pressure and valve overlap your going to have some egr effect. But after the intake valve closes now we have a cylinder filled with up to 20+ pounds of constant pressure against it and the engine has to expel it mechanically ie the piston moving up. I know a cuople of times when i've been changing springs on valves I put air pressure in the cylinder to hold the valve shut. If the piston wasn't all the way down it forced the piston down rather forcefully. Scared the crap out of me the first time. Even though we only have 20lbs +- of back pressure there's probably around 4 cylinders that are in varing stages of expelling exhaust. That seems to me like thats quite a bit of work the engine would have to do to overcome this. Anyone got any other ideas I'd sure like to hear some other ideas.

Now this is where we get into rotational mass, and inertia, some heavy topics (a little pun intended) stored energy. Like I said everything matters not just one piece of the pie. Valve overlap, inertia, rotational, mass Compression ratio, higher pressure eats up inertia and so it goes.

The long and the short of it is
Turbos artificially increase the engines normal displacement, A fixed bore and stoke have a fixed maximum volume as long as the air entering the cylinder completely fills that volume, now vale lap, duration and degree before or after TDC affect cylinder volumetric efficiency .

Starting to get the picture, now we artificially increase that volume by pressurizing the cylinder, now we have increased volume through the same valve size so we have decreased efficiency through intake and exhaust valves so we increase lift and or duration and time the valve to be open longer in a particular crank shaft rotation before or after TDC . I could go on through the whole thing but it would be redundant.

One aspect affects all aspects of the equation.

And i am not educated enough to explain it. :mad2:

 

[schiker]

What about the differential pressure or pressure gradient from inlet to outlet of the system say from just inside the air filter to say 2 ft down the down pipe.

Several posts have been talking about it and I believe its best described as "effeciency" in general.

Yep, the "positive displacement pump volume" of the engine is the same.
Its just the turbo pump thats different.

How about this.....

Given the same size piping and same pressure differential you should get the same flowrate. At least you do under choked flow conditions. But not necessarily true when flow is not choked. I don't believe the flow is choked due to valve size or any other cross sectional area along the path.

Its limited by the volume filling, frictional loss (not choked), and pressure differential across valve seat area during the cylcle and valve duration. If I remember right you can have velocity differences to flow but the same pressure and hence different CFM or flowrate in lbs mass per unit time at the same psi.

Because the exhaust stroke "expels" more air because the turbine is bigger and has less back pressure it allows more air to come into the cylinder when the intake opens. The rush of air is at higher velocity because there is more room to rush into. Or in other words with less back pressure of the new turbo once the intake valve opens more air goes into the cylinder faster because the pressure differential across the valve is higher (due to less pressure in cylinder not higher pressure in intake).

So if you measured along the path with super cool unobtainium gauges at equal lenghts along the flow path and compare the GMX to Ateam the ATeam turbo has a lower pressure gradient through both the compressor and turbine along with smoother flow through intake. Probably would be a more level graph of pressure and higher velocity than the GM X which would have larger peaks and valleys of both pressure and velocity.

The biggest thing that would jump out is you would see higher velocity at the valves seats for the A team turbo if you could measure it but the same psi at midpoints of path ie at the plenum due to the compressibility of gas. Might even measure the same pressure at the valve seat area but again the velocity is higher.

You won't measure any differences of this with a boost gauge mounted at the plenum area. But I believe the lbs mass of air going through the engine is different with the same boost pressure. Which I believe we all agree must be happening due to improved performance.

Not really any different explanation than what several others wrote just don't recall the connection or description of velocity of the flow of air and thought that might connect a dot for someone.

 

[schiker]

Its limited by the volume filling, frictional loss (not choked), and pressure differential across valve seat area during the cylcle and valve duration.

Dawpe duuu dang it frictional loss does kinda sound awful familar to choked flow maybe that doesn't help my explanation.

I guess its choked for the pressure differential in each of the 2 scenerios seperately but once you lower pressure on piston side of intake valve more flow can go through so its not choked with better exhaust expulsion again effeciency.

But I think my point still remains valid the velocities are different with the same PSI. Because its not a true cross sectional choking with same pressures front and back. You are lowering back pressure and increasing velocity with the better turbo at the same boost pressure.

Naw I am just dense and have to reword it sometimes maybe a different wording will help someone.

 

[Slim Shady]

Dawpe duuu dang it frictional loss does kinda sound awful familar to choked flow maybe that doesn't help my explanation.

I guess its choked for the pressure differential in each of the 2 scenerios seperately but once you lower pressure on piston side of intake valve more flow can go through so its not choked with better exhaust expulsion again effeciency.

But I think my point still remains valid the velocities are different with the same PSI. Because its not a true cross sectional choking with same pressures front and back. You are lowering back pressure and increasing velocity with the better turbo at the same boost pressure.

Naw I am just dense and have to reword it sometimes maybe a different wording will help someone.

Nope excellent, I just don't know how to say it in engineering terms. my brother is a mechanical engineer so I understand what I want to do just not how to get it across to him, We have some wild discussions at times, he tells me it doesn't work the way. Then he tells me, well, if I meant this than yes it may increase the performance. I guess, we have much different views of things.