Hypothetical Small Turbo VS Larger

[Mike T]

Given this scenario....at the track we can leave with 20psi and maintain that though the entire run. Why would it matter if we were running a 60 compressor with a 63 housing or a 70 compressor with a 85 housing? Other than the obvious of possibly more heat out of the smaller set up. Also assuming there is not excessive backpressure to reduce performance of the smaller unit.

 

[Mike T]

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?

 

[muddin'metal]

Boost is just pressure (some times restriction) and not an accurate way of measuring hp or airflow

 

[Mike T]

Yes I understand that. I would think using the same engine for this "what if scenario" would see the same type of restriction?

 

[No disintegrations]

All things equal they would be the same. But everything is a trade off so there is no such case, so charge temp and back pressure, convertor, etc. all play into it.

 

[No disintegrations]

The larger exhaust housing may spool slower early in the run, but should run down the smaller one later if enough track remaining.

 

[No disintegrations]

Consider an na engine. One in the tropical swamp ass air. One up north cool air. Both breath similar 14.7 psi at sea level
The one with cool air runs much better at the track due to more power even though pressure is the same.

 

[delcowizzid]

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

 

[Mr.Spool]

Given this scenario....at the track we can leave with 20psi and maintain that though the entire run. Why would it matter if we were running a 60 compressor with a 63 housing or a 70 compressor with a 85 housing? Other than the obvious of possibly more heat out of the smaller set up. Also assuming there is not excessive backpressure to reduce performance of the smaller unit.

the smaller turbo/ex housing is always better.the smaller unit gets to that 20psi faster so all your footbrake street runs,downshift roll hits,and throttle response over the turbo will be better.as long as there is traction.bb dbb makes it even better,the right converter even better.the better breathing motor just changes where the power is made,iron needs a little more boost than the aluminum head to make the same power etc.higher compression motor changes that number as well.

 

[Nigel]

Given this scenario....at the track we can leave with 20psi and maintain that though the entire run. Why would it matter if we were running a 60 compressor with a 63 housing or a 70 compressor with a 85 housing? Other than the obvious of possibly more heat out of the smaller set up. Also assuming there is not excessive backpressure to reduce performance of the smaller unit.

The question is academic in nature and creates a set of circumstances that would be difficult to achieve in a real test, as I know you already know by the way you asked the question. So equalizing all other variables, the difference would be zero.

 

[Mike T]

All excellent responses to my question. I have a hard time conveying my thoughts so the question may have come across as confusing. Nigel pretty much nailed it by looking past all of the common variables.

Nigel I really didn't know for certain that my thoughts were correct. Some of the info out there would lead you to believe that delivered PSI is somehow different (excluding charge temperature and drive pressure) with various turbos.

Having this simple generic fact verified will help me further understand some of the results I'm seeing with my "outdated" combo.

I completely understand that the newer billet/BB turbos spool quicker, have cooler charge temperatures and less drive pressure but for those of us that don't want to "break the bank" we have to run what we have and make the best of it.

My biggest hurdle/concern has been quick spooling with an old GTQ 70 but with a few tweaks here and there it will hold 20#s on the foot brake and seems to be over the hump at the strip. Almost went with a newer/smaller turbo but I honestly believe at this point the old journal bearing 70 (though outdated) may deliver very good results.

 

[Mr.Spool]

y biggest hurdle/concern has been quick spooling with an old GTQ 70 but with a few tweaks here and there it will hold 20#s on the foot brake and seems to be over the hump at the strip. Almost went with a newer/smaller turbo but I honestly believe at this point the old journal bearing 70 (though outdated) may deliver very good results.

sell that dinosour,and all your issues will go away,64,67,69mm,will all outspool.its so hard to get a 70gtq responsive on a v6 its a lame turbo on our motors compared to the new stuff.i can bring in a billet 69mm (90+lbs/minute)faster than a cast 66 (70+lbs/minute)gtq off iron heads.

 

[Mr.Spool]

I completely understand that the newer billet/BB turbos spool quicker, have cooler charge temperatures and less drive pressure but for those of us that don't want to "break the bank" we have to run what we have and make the best of it.

i dont see the issue with the upgrade.its not that much money compared to what problem its solves.a bb64mm with the right combo all in at 82/87/lbs a minute with supercharger like spool is tough to beat and just drives great,the turbo is always there.

 

[Mr.Spool]

The question is academic in nature and creates a set of circumstances that would be difficult to achieve in a real test, as I know you already know by the way you asked the question. So equalizing all other variables, the difference would be zero.

hmm really?so take 2 stock motors,put a 60 with a .63 housing and put a 70 with a .85 housing run them side by side on the street or track at a standstill with a capable driver and tire.is your money on the 70mm with an .85?

 

[Mr.Spool]

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

a few years ago i drove a car with iron heads at 18/19psi with a billet64 that rans 10s on drag radials shifting at 4800 rpm.also drove a 70gtq on a aluminum heads that needed way more rpm to hit 10s.

 

[Mike T]

hmm really?so take 2 stock motors,put a 60 with a .63 housing and put a 70 with a .85 housing run them side by side on the street or track at a standstill with a capable driver and tire.is your money on the 70mm with an .85?

He really isn't saying it's better. He was just answering the question in the fashion that I phrased it.

 

[Mike T]

a few years ago i drove a car with iron heads at 18/19psi with a billet64 that rans 10s on drag radials shifting at 4800 rpm.also drove a 70gtq on a aluminum heads that needed way more rpm to hit 10s.

Wouldn't all of the gains be due to the quicker spool, lower charge temps and lower drive pressure?

 

[Mr.Spool]

He really isn't saying it's better. He was just answering the question in the fashion that I phrased it.

didnt make sense to me.but hey

 

[Mr.Spool]

Wouldn't all of the gains be due to the quicker spool, lower charge temps and lower drive pressure?

quicker spool for sure as a .85 housing is useless at 4800 rpm.charge temps pushing low boost off a 70 vs a 64 is a moot point in 95% of real world.quicker spool is really more torque.the smaller better aero wheels have it, the larger wheels struggle with it.

 

[Mike T]

quicker spool for sure as a .85 housing is useless at 4800 rpm.charge temps pushing low boost off a 70 vs a 64 is a moot point in 95% of real world.quicker spool is really more torque.the smaller better aero wheels have it, the larger wheels struggle with it.

I have always found your advise to be solid and consider you one of my top 5 sources of info on here.

The way my question was worded it completely eliminated spool up, charge temp and drive pressures in order to compare apples to oranges for lack of a better way to put it. Eliminating those factors 20 pounds is 20 pounds in my book so how would the motor perform any better.