RowJ

Need to see if my reasoning is accurate.

Take two turbos...say an HX-35 and a HX-40.
Wind them both to say 20 psi boost, on the same truck....
And assume the air for both is equally cool and therefore equally dense.

Then...the usable of air from each, going to the engine would be identical...correct?

Now in real life...say with both turbos at 35 psi boost, the volume of air from each would still be identical...but the density of air from the HX-40 would be greater (due to more efficient turbo operation)....thus better performance...or more molecules of air for the engine to use.

So when we say a bigger turbo moves more air...what we are really saying is it moves denser air...or more efficient air...not more volume for a given boost reading??

Thought I had a handle on this concept...but I realized I never read it or heard it discussed. Am I missing anything?

RJ

ratsun

Humm, I always thought the larger turbo would actually have more volume at said PSI But now you got me wondering too???

Don M

The smaller turbo like the HX35 in your example will move less air ( mass flow ) , but could end up with the same pressure. The 35 might be able to give you 45 PSI, but much of that pressure is from the discharge temperature being so hot...the expansion of the hot air created some of the boost.

The HX0 will give you 45 PSI as well, but more of the pressure is created from the excess airflow ( mass flow ) and the restriction in front of the turbo. And much less is produced from overheated air expanding outwards.

Pressure is made from restriction. Air pressure is also made from heating the air up.

Intercooler piping, the engine, etc are all a restriction to the turbo. An extreme example of how a lower restriction would lower boost would be to use a HX35 on a 688 cubic inch engine. It would never make 45 PSI. The restriction is smaller because the engine will process so much more air.

The B-1 turbo uses a compressor wheel typically sized for a 10 to 12 litre engine. Nearly double the size of the 5.9 we have. The little 5.9 is a larger restriction and the big wheel in the B-1 quickly fills the space and begins to create pressure. The larger turbo needs less compressor wheel speed to move the same mass flow the smaller turbo does. The discharge temps stay lower and you get less "expansion pressure" to make the boost.


Don~

HOHN

RJ, the answer is-- it depends.

If you have equal discharge temps from both the HX40 and hx35, and they are BOTH at 20psi or so, then yes, they would have identical MASS flow. All we care about is the MASS of oxygen getting to the engine, as you mentioned (the "molecules").

But this is a somewhat static situation. What happens when you bolt them onto a running engine?

For most Holset turbos, 20psi is the "fat" part of the compressor map, and they are at maximum efficiency. This goes for a 35, a 40, and even an HT3b or HX60.

But clearly, an HX60 moves quite a bit more air.

Enter the variable of engine size. A certain engine will "consume" air at a rate that's determined by displacement, rpm , and volumetric efficiency (VE).

So, let's say you have a 5.9L engine turning 3200 rpm. Assuming 90% VE, then we have (359CID*3200/3456*.9= 299CFM

So, at 3200 rpm, our CTD consumes about 300CFM at zero boost.

What about a 14L ISX or somesuch? They have a lower redline of say, 2K rpm:
(14L*60CI*2000rpm/3456)*.9= 437.5CFM

So, our CTD takes 300CFM at 3200rpm (no boost, remember), compared to 437CFM for a 14L ISX at 2000rpm.



Now, back to our boost scenario. In order to build up boost, the turbo must supply air at a rate FASTER than the engine is consuming it. Sounds simple enough, right? Note that it's not just compression! The turbo can compress the air to 40psi, but if it's at a low airflow, all that will happen is the compressed air will re-expand and you have no boost.

Since atmospheric pressure is 14.7psi, let's say we want to double the flow into the engine. That means that a turbo on a CTD must supply 600CFM to build 14.7psi of boost.

But the Class 8 engine will need 874CFM! Again, SAME boost pressure!


Now we can see why HX40s and larger turbos are not so ideal for our trucks. They are designed for the same pressure as an HX35. But so is an HX60-- it, too is designed for only 20psi or so. But they obviously flow different amounts, by far.

A turbo for a CTD needs to have higher pressure capability, not just more airflow. This is why twins work so well, because you can use the Class 8 turbos for high airflow, but not have to operate them at PRs of 6 or 7 because they are on such a small engine.


Sorry to digress, but back to your original question. YES, IF an HX35 and HX40 have the SAME discharge temps at the SAME boost, then they are moving the same amount of air.


But in reality, this will never happen. Looking at the compressor maps would show you why this is true. At a constant 20psi, the HX40 should have slightly lower discharge temps. This, because its larger compressor is turning fewer RPM, and thus heats the air less.

Looking at the maps would also show a person why SURGE is so hard to control with the larger turbos, because we are trying to cram so much air into an engine that really doesn't want to breathe.

Hope this helps.

upersleder

simply stated, you can have the same pressure, but flow a larger volume of air.

take an air gun on a hose set with a regulator to 20# then take and put a needle for blowing a ball up, you still have 20#s, but not as much volume.

RowJ

So basically I'm right.
When we say a bigger turbo moves more air....what we mean is it moves better air (denser, cooler) per lb of boost....... Thus more efficient and more usable air, with no...or less "expansion pressure"...(love that term, Don), but not more volume of air.

And since we are not changing the size of the inlet to the motor, as per upersleders example, 30 lbs of boost (for example) from ANY sized turbo...is the same volume of air!

I've got it! Thanks to all for the input.

RJ