Hypothetical Small Turbo VS Larger

[Charles Young]

Mike T,
I really appreciate your original question. I have wondered the exact same thing many times but did not want to ask and sound like a dufus. The question and responses have been very informative to me. My TE44 is great on the street. Well, I would prefer less surge at part throttle, but it holds 25 psi for the few 1/8 runs I make. I'm looking at ported heads and a converter next so will just wait and see what the 44 will do with that combo. I'd be happy with a little lower boost/better flow. We'll see.

 

[Mike T]

Mike T,
I really appreciate your original question. I have wondered the exact same thing many times but did not want to ask and sound like a dufus. The question and responses have been very informative to me. My TE44 is great on the street. Well, I would prefer less surge at part throttle, but it holds 25 psi for the few 1/8 runs I make. I'm looking at ported heads and a converter next so will just wait and see what the 44 will do with that combo. I'd be happy with a little lower boost/better flow. We'll see.

Charles that's the reason I try to ask some of the questions that I do....I'm not the sharpest knife in the drawer myself so I need things explained in simpler terms. It really bothers me when members are afraid to ask questions in fear of being put down or embarrassed.

I think if we understand things on the basic level the more complicated conversations are easier to follow. I listen to EVERYONE because a different perspective can really open your eyes sometimes.

 

[Mike T]

its also the way it was posed,your question mimics a nitrous car which has a much higher power average throughout a run.

Very good point and very close to my point in general. Get that older turbo up on boost ASAP, keep it there and you won't get beat as bad

 

[gibbstroy]

A very interesting read. If I can add just a bit of non turbo experience to the mix maybe it will provide a helpful perspective.

In my job I do hydraulic calculations, and a way I describe a similar subject to novices is by describing water pressure within a pipe. If the pipe is 1" in diameter and the water pressure is 30 psi it will have the same reading on a pressure gauge as a 6" pipe at the same pressure. But if you look at the cross section of a 6" pipe, there will be many more"square inches" of that same pressure. So, you might be able to hold a cap on a 1" pipe at 30 psi but there isnt a man on the planet that can hold a cap on a 6" pipe at the same pressure.

Im certainly not an expert here but I imagine that the two turbos in your scenario would produce equal results given the condition of everything being equal, and starting at 20psi at launch and maintaining the same 20psi through the quarter mile. The difference in the turbos would be noticeable when the air volume requirements change, like changing cubic inches, intake or exhaust, cam's, etc... So the best turbo sizing for each engine would be dependent on the air volume that engine is capable of moving.

Hope this is a helpful contribution to the interesting topic.

 

[Mike T]

A very interesting read. If I can add just a bit of non turbo experience to the mix maybe it will provide a helpful perspective.

In my job I do hydraulic calculations, and a way I describe a similar subject to novices is by describing water pressure within a pipe. If the pipe is 1" in diameter and the water pressure is 30 psi it will have the same reading on a pressure gauge as a 6" pipe at the same pressure. But if you look at the cross section of a 6" pipe, there will be many more"square inches" of that same pressure. So, you might be able to hold a cap on a 1" pipe at 30 psi but there isnt a man on the planet that can hold a cap on a 6" pipe at the same pressure.

Im certainly not an expert here but I imagine that the two turbos in your scenario would produce equal results given the condition of everything being equal, and starting at 20psi at launch and maintaining the same 20psi through the quarter mile. The difference in the turbos would be noticeable when the air volume requirements change, like changing cubic inches, intake or exhaust, cam's, etc... So the best turbo sizing for each engine would be dependent on the air volume that engine is capable of moving.

Hope this is a helpful contribution to the interesting topic.

Your example of "more square inches" is also the reason a larger wastegate puck will open sooner than a smaller one at the same psi.

 

[Mr.Spool]

4200 @15 psi.

now how long does it take to get to full boost and at what rpm does max boost happen at?

 

[bison]

All about efficiency a bigger turbo spinning at lower speed running in a more efficient area of the compressor map is already going to make lower backpressure and a cooler intake charge add a bigger rear wheel or housing that lowers backpressure more you end up with a lot more airflow potential through the engine and more hp for the same boost

A lager turbo may be running in surge on an inefficient low rpm engine. Which definitely wouldn’t be a good thing. Air flow is unstable and way out of the efficiency island.


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[Mike T]

now how long does it take to get to full boost and at what rpm does max boost happen at?

I'm finding that it's highly dependent on driving technique and what we're calling max boost. On the street from a dead stop no brakes it takes about 2 seconds to go from 0-20psi (15-20 is almost instant). Add a little brake boost and it will go to 20 psi in under a second. Haven't run it much over 20psi but my guess is that spool time is going to be even better from 20-30psi.

This is a screen shot from the night that the video was taken. Unfortunately I didn't save the 10.88 run.
It does kind of support my "hypothetical scenario" by basically eliminating spool time from the run.

 

[Mr.Spool]

On the street from a dead stop no brakes it takes about 2 seconds to go from 0-20psi (15-20 is almost instant).

right,thats the flat spot.typically once you get over that your good.

 

[Mr.Spool]

Haven't run it much over 20psi but my guess is that spool time is going to be even better from 20-30psi.

thats where things change as 30psi hits the tire harder,you may need to then curve the boost in dow low which changes time/applied power

 

[No disintegrations]

This is the 70Q a few weeks ago. The wastegate solenoid was set to cycle @ 15 psi on launch. The ptc converter is stalling to 4200 @15 psi. This is the 5th pass on this motor and I was just looking to get into the 11's that night.
It went 10.88 @ 125

That was great looking pass!

 

[Mike T]

That was great looking pass!

The most satisfying part was putting 3 seconds and 25mph on the newer Mustang

 

[TurboTnZ06]

A lager turbo may be running in surge on an inefficient low rpm engine. Which definitely wouldn’t be a good thing. Air flow is unstable and way out of the efficiency island.


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Also 99.9% of the time we aren't racing, and you have to balance the wastegate for street use and race use. At the track I throw a different spring combo in, even with a bleeder and boost reference setup, to much spring can make the right turbo too too much turbo for street duty just cruising around.

 

[bison]

We can throw all of the latest billet wheels and ball bearing options into the mix as well.

Other than heat and performance robbing backpressure... how is 20 psi (for discussion purposes) not the same regardless of the source?

It also depends on where the 20psi is measured. If it’s at the manifold and the intercooler pressure drop isn’t considered then the turbo is being penalized. It takes exhaust pressure to do the work. No exhaust pressure=no work and no turbocharging.


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[Mike T]

It also depends on where the 20psi is measured. If it’s at the manifold and the intercooler pressure drop isn’t considered then the turbo is being penalized. It takes exhaust pressure to do the work. No exhaust pressure=no work and no turbocharging.


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Yes I should have said excessive back pressure when referring to the turbine side.

IC pressure drop brings up a question though. Is the primary reason for pressure drop the IC itself or the compressor?

 

[bison]

It also depends on where the 20psi is measured. If it’s at the manifold and the intercooler pressure drop isn’t considered then the turbo is being penalized. It takes exhaust pressure to do the work. No exhaust pressure=no work and no turbocharging.


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Yes I should have said excessive back pressure when referring to the turbine side.

IC pressure drop brings up a question though. Is the primary reason for pressure drop the IC itself or the compressor?

Pressure drop due to restriction would mostly be from poor intercooler design and or inadequate intercooler for application


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[Mike T]

Would it be fair to say that the 70Q's larger wheels in conjunction with the 85 housing will keep charge temperature and back pressure to a minimum? If so this only leaves spool up time which can definitely be overcome for the most part at the line.

If all of the above holds true I do not see how any other turbo would be that much different?
We leave the line at 20psi, 30psi should come rapidly (maybe too fast) ……..where would the potential gains be?

I have a good idea of how far it will go in the current configuration. Is bolting on the latest technology going to cut .5 or better off of the ET?

 

[Mr.Spool]

30psi should come rapidly (maybe too fast)

it doesnt always and you may not make 30 psi.no im not saying that you cant or wont, but i have seen many cars that struggle to make boost.

 

[Mr.Spool]

We leave the line at 20psi, 30psi should come rapidly (maybe too fast) ……..where would the potential gains be?

if you can leave the line on 20 and make 30+ there would only be a gain if a different turbo gets to that max boost number sooner,but from that moment of 30psi you should go no faster.

 

[Mr.Spool]

Is bolting on the latest technology going to cut .5 or better off of the ET?

hard to say,here is a bad scenerio.large turbo/low backpressure small rpm window,so when your trying to push boost your not getting it till its too late or it wont make it to the higher number.the aero is so much better on the new stuff it moves the air with less.in your case i would try and turn it up before i switched the turbo,see what it makes and go from there.