First off let me add some perspective.
I have owned and driven a 1988 6.2 Suburban, 1994 c2500 454 gasser Suburban, 1995 350 gasser Yukon, 1993 4.3 V6 c1500 pickup, 1993 6.5 TD XC pickup x2, and a 1995 TD Suburban. I have also driven a 2008 CC Duramax and own a 2005.
The 454 Surburban although had some power was disappointing with a flat torque curve and never really had the feeling of opening up the quads. It was so bad for a 454 and lack of power to feel like a 454 we traded it. I didn't get into tuning and exhaust mods back in 1994. I have driven these vehicles out west in mountains the worst being the 1988 6.2 over Eisenhower Pass in Colorado. I would have killed for a turbo to clear the smoke and get it out of it's own way.
Last year I purchased a 1995 6.5 K2500 Suburban with catastrophic engine failure. The engine, turbo, and oil cooler all had to be replaced due to debris and coolant in the oil. If I had not hit the Elk with the 1993 it would have received an ATT first. As it was the Suburban got an ATT with a 6.2 repower.
The 1993 was towing a 24-28' trailer delivering auto parts and tires going through 10% grades and 8800' altitudes. Adding a Turbo Master and turning up the fuel did not change the miserable 7.5 MPG, but, did get us from 33 MPH to 43 MPH on the 10% grade. Anything over 2200 and the engine was choking up the turbo. Downshift and loose speed quickly to 2200 RPM then a slower loss of speed. I even used 4 LOW to get up the hill when we had a extreme load one day. Elevation from 2000' to 8800' caused the GM3 to go from 14 PSI to 16 PSI on the TM. I had to back the TM down (less boost and fuel at lower elevation) to stop running it beyond the choke heater RPM. It would also smoke black at 8800' till I turned both the fuel and the TM down.
Couple weeks ago after battling the last of the fuel system problems (Smoke) with the 1995 Suburban I took it to the Grand Canyon. 4' exhaust, 6 position tune, k47 airbox, Kennedy fan clutch, HO pump, Duramax fan, ATT with blanket and most of the exhaust header wrapped, 15PSI Walbro fuel pump, 6.2 heads, bottom end, and small precups, 3.73 rear. The hill that said "Turn Off AC" was fun. It would give the 454 a run for it's money. I didn't have any problem climbing the hill at excessive speeds. (Or accelerating from below 65 to above it.) No struggling to maintain the speed limit.
For the first time I am impressed with what you can get out of a 6.5 when you don't have a turbo choking the engine. The EGT's hit 1400 so I changed to a different 'lighter' tune and had 1200 EGT's. No smoke to worry about and I was at 4900' elevation. (A light haze is fine plus a 6.2 NA would have smoked more.)
I also just installed my second ATT on the 1993 Pickup with the DB2 IP - Finally! I could turn the fuel up more as I am getting 16 PSI of boost. Stomp the gas and the pickup just motors through 2nd gear. The 4.11's don't let it stay below 2000 RPM very long. Boost falls off with the throttle - something the GM3 doesn't do as it is boosting from airflow alone. A belt drive supercharger would be better than a power robbing GM3. (A turbo is supposed to boost from heat not airflow.)
A picture shows you how FUBAR the down elbow is on a GM3 and what a monster turbo the ATT is vs. the choke the engine to death GM3.
Yes, there is a trade off. The ATT is not a GM3. Seriously you don't want a GM3. Fast spooling is great except the cost is choking your engine to death over 2200 RPM. MPG's that should be 18 MPG (6.2 NA 4x4) fall off to 14 MPG or less. Trading a little time to spool the ATT is worth it because it is there for you where you need it over 2000 RPM. Below this RPM when you don't need as much power it is out of the way as much as possible. Light throttle doesn't need boost and more MPG results. Trying to run a GM3 WOT over 2200 RPM is like trying to breathe through a coffee stirrer.
So in town and low speed is the only place a GMX works well. You will not be a full boost as fast with an ATT, but, the boost will come on before you are through the intersection. Then it passes the choke PSI of the GMx of 14 PSI and keeps going to 17 PSI and more depending on how much fuel you are pouring on.
After driving a Duramax the boost, fuel, and resulting power doesn't come on right away in them either.
I have owned and driven a 1988 6.2 Suburban, 1994 c2500 454 gasser Suburban, 1995 350 gasser Yukon, 1993 4.3 V6 c1500 pickup, 1993 6.5 TD XC pickup x2, and a 1995 TD Suburban. I have also driven a 2008 CC Duramax and own a 2005.
The 454 Surburban although had some power was disappointing with a flat torque curve and never really had the feeling of opening up the quads. It was so bad for a 454 and lack of power to feel like a 454 we traded it. I didn't get into tuning and exhaust mods back in 1994. I have driven these vehicles out west in mountains the worst being the 1988 6.2 over Eisenhower Pass in Colorado. I would have killed for a turbo to clear the smoke and get it out of it's own way.
Last year I purchased a 1995 6.5 K2500 Suburban with catastrophic engine failure. The engine, turbo, and oil cooler all had to be replaced due to debris and coolant in the oil. If I had not hit the Elk with the 1993 it would have received an ATT first. As it was the Suburban got an ATT with a 6.2 repower.
The 1993 was towing a 24-28' trailer delivering auto parts and tires going through 10% grades and 8800' altitudes. Adding a Turbo Master and turning up the fuel did not change the miserable 7.5 MPG, but, did get us from 33 MPH to 43 MPH on the 10% grade. Anything over 2200 and the engine was choking up the turbo. Downshift and loose speed quickly to 2200 RPM then a slower loss of speed. I even used 4 LOW to get up the hill when we had a extreme load one day. Elevation from 2000' to 8800' caused the GM3 to go from 14 PSI to 16 PSI on the TM. I had to back the TM down (less boost and fuel at lower elevation) to stop running it beyond the choke heater RPM. It would also smoke black at 8800' till I turned both the fuel and the TM down.
Couple weeks ago after battling the last of the fuel system problems (Smoke) with the 1995 Suburban I took it to the Grand Canyon. 4' exhaust, 6 position tune, k47 airbox, Kennedy fan clutch, HO pump, Duramax fan, ATT with blanket and most of the exhaust header wrapped, 15PSI Walbro fuel pump, 6.2 heads, bottom end, and small precups, 3.73 rear. The hill that said "Turn Off AC" was fun. It would give the 454 a run for it's money. I didn't have any problem climbing the hill at excessive speeds. (Or accelerating from below 65 to above it.) No struggling to maintain the speed limit.
For the first time I am impressed with what you can get out of a 6.5 when you don't have a turbo choking the engine. The EGT's hit 1400 so I changed to a different 'lighter' tune and had 1200 EGT's. No smoke to worry about and I was at 4900' elevation. (A light haze is fine plus a 6.2 NA would have smoked more.)
I also just installed my second ATT on the 1993 Pickup with the DB2 IP - Finally! I could turn the fuel up more as I am getting 16 PSI of boost. Stomp the gas and the pickup just motors through 2nd gear. The 4.11's don't let it stay below 2000 RPM very long. Boost falls off with the throttle - something the GM3 doesn't do as it is boosting from airflow alone. A belt drive supercharger would be better than a power robbing GM3. (A turbo is supposed to boost from heat not airflow.)
A picture shows you how FUBAR the down elbow is on a GM3 and what a monster turbo the ATT is vs. the choke the engine to death GM3.
Yes, there is a trade off. The ATT is not a GM3. Seriously you don't want a GM3. Fast spooling is great except the cost is choking your engine to death over 2200 RPM. MPG's that should be 18 MPG (6.2 NA 4x4) fall off to 14 MPG or less. Trading a little time to spool the ATT is worth it because it is there for you where you need it over 2000 RPM. Below this RPM when you don't need as much power it is out of the way as much as possible. Light throttle doesn't need boost and more MPG results. Trying to run a GM3 WOT over 2200 RPM is like trying to breathe through a coffee stirrer.
So in town and low speed is the only place a GMX works well. You will not be a full boost as fast with an ATT, but, the boost will come on before you are through the intersection. Then it passes the choke PSI of the GMx of 14 PSI and keeps going to 17 PSI and more depending on how much fuel you are pouring on.
After driving a Duramax the boost, fuel, and resulting power doesn't come on right away in them either.
