SATAN

iTrader: (2)

I keep reading over and over on here that if you are running X turbo, that it will choke out above Y RPM.

Why is it that so many people think engine RPM has anything to do with what will choke out a turbo?

The way I think of it is this. If you have an engine producing 500whp at 5000rpm, then it is making 500whp worth of exhaust flow. If that same engine can wrap up to 10,000rpm with a lower boost pressure and still only make 500whp, then it is still only making 500whp worth of exhaust flow. Therefore, the turbo is not going to know the difference between these two situations.

Now before you go into the argument of 10,000rpms has twice as many pulses than 5,000rpms... This is true, but that doesn't mean that the pulses have to be just as strong though. If they are half as strong, then the same exhaust flow is created and thus the same back pressure.

Basically, all I am getting at is that it is exhaust FLOW that a turbo cares about. NOT engine RPM. Therefore people should not say that "That turbo will choke out above xxxx RPM." But instead they should say things like "That turbo will choke out above xxxx horsepower".

Anyone care to agree or disagree? I would like to hear your reasoning.

tripblackls1

iTrader: (11)

Disagree, horsepower has no determination on exhaust flow , engine size , efficiency, and intake flow does though
This is why a small 4.8 will make more power out of 76mm turbo, then a 6.2 will

RedHotG8

iTrader: (1)

Higher rpm's produce more exhaust flow so your theory is incorrect and I disagree with you.

GueSS Who

disagree as well

obZidian

iTrader: (2)

Make more power? The same turbo is going to make the same amount of power. A specific turbo will have a max "power level" regardless of the engine pushing it. Now, shouldnt a larger engine at 14.7psi. theoretically produce more power than a smaller engine? With that, a turbo has a max and perhaps the power delivery is more to your liking with a smaller engine but the larger engine will generate more power or more effectively hit the turbos max power level.

I think a turbo "choking" is based on the a turbo's ability to handle the incoming air. To much air or to little is going to hurt the overall performance of a specific turbo. I'm no turbo genius but I play one on tv btw.

itsslow98

iTrader: (18)

This thread is full of fail lol

97badass

iTrader: (8)

A 4.8 engine requires y amount of air to run n/a
A 6.2 engine requires x amount of air to run n/a

A turbo capable of pushing the z amount of air through the 4.8 to make zzz horsepower will not necessarily make zzz amount of horsepower through the 6.2. It's not about the amount of psi, but more the matter of volume of air in the amount of psi. In a nutshell, you still have to push enough air to fill the 6.2 ltrs of engine plus compact more into it surpassing 0 vaccum creating boost aka psi. With a turbo psi means jack ****. It's all about volume. The faster the exhaust leaves the quicker the turbo spins. Quicker it turns the more air it pushes creating more volume. Make any sense? I'm tired...

Identical setup on a 4.8 will make more psi than it will on the 6.2 due to having 1.4 liters more air to push before it makes boost. A td03 turbo with 15 psi on an engine will make no where near as much power as say an he351cw on 15 psi on the same engine due to more air being pushed.

Professor_speed

iTrader: (8)

Agreed

gregrob

iTrader: (8)

Lol you guys need to do some research. "Exhaust flow" isnt even what spins the damn turbo.

Mr. Sir

This interests me as well. Since turbo's are run off of exhaust velocity, does the 4.8 somehow have more velocity than the 6.2? And since that scenario seems highly unlikely, how would you explain more horsepower from similar turbine speed (similar airflow)? Considering exhaust velocity is equal between the 2 engines, the compressor will be moving the same rpm for both. If anything, it'll be moving faster for the 6.2, since it'll have less restriction (less boost) working against the compressor. So does it come down to cylinder pressure? if the 4.8 has a 9.1:1 compression, and the 6.2 has a 10:1, even with less boost, the same (or greater) volume of air is being forced into the 6.2, and the final cylinder pressure would be (theoretically) similar. The last thing I can think of is that the smaller turbo becomes such an exhaust restriction that power is adversely affected. However, one would think that if 700hp worth of air were moving through a 4.8 with no restriction, a 6.2 would see no restriction until it exceeded 700hp, exhaust included. So... you guys that say the thread makes no sense... what are the accurate descriptions of turbo function? And the one thing I really want to get from this is: How does a 4.8 make more power than a 6.2? (for comparisons sake, just assume that the 6.2 is the same engine as the 4.8, but overstroked to 6.2. That way, heads/intake/exhaust remains the same. displacement and compression are the only changes. The only reason compression is different between the 2 engines is so that cylinder pressure at combustion is the same between the 2, yielding similar heat=pressure=exhaust velocity).

HydroStream6

iTrader: (16)

Pressure ratio. Read about it. Look at a map.

Sarg

iTrader: (6)

One thing to consider is where the two motors have to be in pressure ratio to make the same 500 hp. If the turbo is more efficient at a 15psi level on the 4.8 liter at 500 hp than it is at 10 psi on the 6.2 liter, there may be an efficiency loss on the 6.2 liter. The 4.8 at 15 psi could be at 76 percent efficiency where the 6.2 could be at 65 percent.

hellbents10

iTrader: (10)

S372 on a little eng vs big, I will try to make this simple

6l 8cyl 10psi equals 700hp, at 10psi compressor is not in a good spot

2l 4cyl 40psi equals 700hp, compressor is very happy up here working good in the island.

Both have 40psi of back pressure at this level to spin the bugger. Now which has the better PR and PR between the two sides? This is a very crude example so go do some more reading.

97badass

iTrader: (8)

Hellbent said what i was trying to say, I was exhausted last night.

ddnspider

iTrader: (26)

Take a look here:
http://forcedinductions.com/help.htm
Read the chart under "Selecting a Turbo for your Engine 101". A given engine will consume a given amount of air in lb/min based on pressure ratio and RPM. You then plot this consumption and pressure ratio on a compressor map and it will tell you efficiency. Better efficiency=more power and there is a point at which you're off the island because the engine consumption is too much for the turbo to handle, thus choking, so RPM DOES play a role in "choking" a turbo or in how much power the setup can make.

Sarg

iTrader: (6)

When he says "choking" I believe he is referring to the point where no more exhaust can physically flow through the housing. Not necessarily efficiency of the compressor side...at least as far as the OP's question.

ddnspider

iTrader: (26)

I would argue that its very similar, an engine consuming X amount of air is going to flow a proportional amount of air out of it. You can choke a turbo because of too small of a turbine or too small of a compressor, either way, RPM IS a factor.

mike13

iTrader: (27)

One point I would like to add is the turbine side is often overlooked when picking turbochargers. not all 76, 78, etc turbochargers are equal.

Sarg

iTrader: (6)

But to go with his original statement is it rpm or flow that chokes a turbo, it is in fact flow. On a 2.0 liter motor the rpm may be 8000 rpm to achieve the flow of exhaust that maxes out a particular turbo where a 6.0 liter may achieve that same choking exhaust flow at 4000 rpms (just picking rpms out of the air). So in fact he is correct that at a certain flow the turbo will be choked and not a particular rpm.
Now I believe why people say that at a particular rpm it will choke is because at a particular rpm on THAT motor (whatever motor is being discussed) the choke limit of the turbo will be reached.

Sweet_SS

I agree Hydro.

To everyone:
Also, a motor is only an air pump and when it comes down to forced induction, its the atmospheric pressure that a turbo is concerned with. You can have the exact setup as the next guy but every car is different and so is the elevation in other states.