guys with 20+" rims
10-05-2009, 11:40 PM
#22
i guess the argument of turbo and high gears vs low gears makes it hard. i just think my heavy *** tahoe needs everything it can get. in 4th and lockup i can get boost at 30% throttle. and when trying to merge with traffic i have to try just to not get into boost.
10-06-2009, 07:16 AM
#23
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that was always my thinking. everyone is always so inside the box to really try low gears. if you can swap gears yourself its not all that expensive. just do the back first and if ya dont like it swap it to something else.
10-06-2009, 09:10 PM
#25
right now i have the front out cause the gears and diff in it are bad. i think i'm gonna try it out. i've been doing my rear end so much here lately i feel like its my job. the swap won't be until after the first of the year. can't afford the actual rear yet.
10-07-2009, 02:08 AM
#26
Hey, sorry for showing up late to the party! 
I started off with 3.23 gears and about 400rwhp and liked them alot. On the advice of board members and general consensus I swapped to 4.10's and by that time my power had jumped to 740rwhp. I hated them, I couldn't build boost in 1st gear and spun through it anyways, second gear evaporated too quickly although I did hit full boost at the very top of it at around 70mph but would shift right after. I went down to 3.73's and traction became less of a problem, saw full boost at the top of first gear and all of 2nd gear and truck actually started to make power and feel fast. That's my personal experience, and again it's different for everyone, my truck weighed just south of 6K lbs.
Regarding this...
If the above statement is true than you could theoretically with the wastegate welded shut build full boost in Neutral and reving the engine to redline, however that wouldn't do much of anything other than hurt your engine.
In a sense some of what was stated is true but there are variables that were not taken into account in that explanation and a few things which IMO are wrong so there's a difference in opinion here. It's been a while and my memory has faded since my last big turbo project but the bottom line of what spins a turbo, regardless of what anyone else says is pressure differential across the turbine. Flow, heat, and pulses are important but they will not spin a turbo alone, pressure differential is a combination of all of those and you can achieve it anywhere in the powerband you choose to have it because it is dependent on cylinder pressures.
Engineermike is right when he states that "Mass flow is a function of rpm and throttle angle. You get more mass flow at higher rpm." However that statement only holds true in a N/A set-up where the flow is dependent on vacuum. On turbo applications flow is determined primarily by the load which dictates wastegate position. Having a constant of RPM...when an engine senses load it will close the wastgate and increase airflow which will allow the engine to add more fuel AT THE SAME RPM consequently raising cylinder pressures. Cylinder pressure is not something that is dependent on engine speed but on the amount by volume of Air/fuel that is in the cylinder. And that is why most turbo applications love load.
4.8T if you put your truck in 1st gear and drive down the street and cruise at let's say 45 mph or the top of first gear, you are not building boost and are in vacuum. If you approach a hill you'll see your boost pressures start to raise and your RPM's will remain constant, you'll see your boost continue to rise until your boost controller limit is hit at which point your RPM's will start to increase. At first your not changing RPM's your changing the cylinder pressures by adding fuel and air at the same given RPM's by changing the flow. Those raised cylinder pressures will translate to a greater pressure differential (heat, pulses, flow, etc...)across the turbine blades and "Load" your system.
Watch this video...
http://www.youtube.com/watch?v=Qzp3-pUavkU
The wastegate is closed until about the 20 second mark and for the first 16 seconds there is no load on the engine, at the 16 second mark they load the engine and you can hear the RPM's remain constant yet the turbo begins to load itself and supply more air to the engine. Starting at second 16 the truck is making more power via increased cylinder pressures due to the extra amounts of air supplied by the turbo (independent of RPM). Not until the 20 second mark do you start to hear the wastegate opening once the pre-determined max boost has been achieved and then engine start to rev.
I really need to refresh myself on Turbo theories, it's been too long
I started off with 3.23 gears and about 400rwhp and liked them alot. On the advice of board members and general consensus I swapped to 4.10's and by that time my power had jumped to 740rwhp. I hated them, I couldn't build boost in 1st gear and spun through it anyways, second gear evaporated too quickly although I did hit full boost at the very top of it at around 70mph but would shift right after. I went down to 3.73's and traction became less of a problem, saw full boost at the top of first gear and all of 2nd gear and truck actually started to make power and feel fast. That's my personal experience, and again it's different for everyone, my truck weighed just south of 6K lbs.
Regarding this...
Originally Posted by engineermike 
Think about this for a sec. Exhaust energy (enthalpy) and mass flow are what spool up a turbo.
The energy is a function of temperature. EGT is the same at low rpm and high rpm, so the gear doesn't affect it.
Mass flow is a function of rpm and throttle angle. You get more mass flow at higher rpm. So, the sooner you can get an engine to a high rpm, the more mass flow you're going to supply to the turbo, the quicker it will spool up. I used to have a sloppy-loose torque converter in my car. The turbo spooled up really quick because the converter allowed the engine to reach 4000 rpm as soon as you hit it. Of course, with boost it would blow right through it, but that's a topic for another day. . .
Taller gears may SEEM like you get boost sooner, but it's only because you get it at an earlier rpm, not in less time. Heck, if you have a 6-spd and start out in 6th gear, you might get boost at 2000 rpm, but you're better off starting off in 1st and not getting boost until 4000 rpm. I assure you 4000 rpm in 1st will come sooner than 2000 in 6th.
Think about this for a sec. Exhaust energy (enthalpy) and mass flow are what spool up a turbo.
The energy is a function of temperature. EGT is the same at low rpm and high rpm, so the gear doesn't affect it.
Mass flow is a function of rpm and throttle angle. You get more mass flow at higher rpm. So, the sooner you can get an engine to a high rpm, the more mass flow you're going to supply to the turbo, the quicker it will spool up. I used to have a sloppy-loose torque converter in my car. The turbo spooled up really quick because the converter allowed the engine to reach 4000 rpm as soon as you hit it. Of course, with boost it would blow right through it, but that's a topic for another day. . .
Taller gears may SEEM like you get boost sooner, but it's only because you get it at an earlier rpm, not in less time. Heck, if you have a 6-spd and start out in 6th gear, you might get boost at 2000 rpm, but you're better off starting off in 1st and not getting boost until 4000 rpm. I assure you 4000 rpm in 1st will come sooner than 2000 in 6th.
If the above statement is true than you could theoretically with the wastegate welded shut build full boost in Neutral and reving the engine to redline, however that wouldn't do much of anything other than hurt your engine.
In a sense some of what was stated is true but there are variables that were not taken into account in that explanation and a few things which IMO are wrong so there's a difference in opinion here. It's been a while and my memory has faded since my last big turbo project but the bottom line of what spins a turbo, regardless of what anyone else says is pressure differential across the turbine. Flow, heat, and pulses are important but they will not spin a turbo alone, pressure differential is a combination of all of those and you can achieve it anywhere in the powerband you choose to have it because it is dependent on cylinder pressures.
Engineermike is right when he states that "Mass flow is a function of rpm and throttle angle. You get more mass flow at higher rpm." However that statement only holds true in a N/A set-up where the flow is dependent on vacuum. On turbo applications flow is determined primarily by the load which dictates wastegate position. Having a constant of RPM...when an engine senses load it will close the wastgate and increase airflow which will allow the engine to add more fuel AT THE SAME RPM consequently raising cylinder pressures. Cylinder pressure is not something that is dependent on engine speed but on the amount by volume of Air/fuel that is in the cylinder. And that is why most turbo applications love load.
4.8T if you put your truck in 1st gear and drive down the street and cruise at let's say 45 mph or the top of first gear, you are not building boost and are in vacuum. If you approach a hill you'll see your boost pressures start to raise and your RPM's will remain constant, you'll see your boost continue to rise until your boost controller limit is hit at which point your RPM's will start to increase. At first your not changing RPM's your changing the cylinder pressures by adding fuel and air at the same given RPM's by changing the flow. Those raised cylinder pressures will translate to a greater pressure differential (heat, pulses, flow, etc...)across the turbine blades and "Load" your system.
Watch this video...
http://www.youtube.com/watch?v=Qzp3-pUavkU
The wastegate is closed until about the 20 second mark and for the first 16 seconds there is no load on the engine, at the 16 second mark they load the engine and you can hear the RPM's remain constant yet the turbo begins to load itself and supply more air to the engine. Starting at second 16 the truck is making more power via increased cylinder pressures due to the extra amounts of air supplied by the turbo (independent of RPM). Not until the 20 second mark do you start to hear the wastegate opening once the pre-determined max boost has been achieved and then engine start to rev.
I really need to refresh myself on Turbo theories, it's been too long
10-07-2009, 02:26 AM
#27
what is your tire size? just curious. i'm going to have to reread that in the morning after some sleep. right now with my 3.73's i have no traction issues. well that was until i broke my detroit locker. only after i build alot of boost and launch will it even spin. i have an advantage when it comes to weight transfer though. i do get what your saying as far as no load and load. i could see it going through the mountains. i could climb hills with boost in over drive with the tcc locked. at the same time it seems if you launched with boost the 4.10's would give a better 60ft also. i can't understand how people get such good 60's and i bearly get into the 1.9's. then again i broke an axle trying to launch with more boost. so i actually have no clue what it can do.
i could see me going 4.10's then not getting traction and being pissed off all the time. that would drive me to put the awd back in.
i could see me going 4.10's then not getting traction and being pissed off all the time. that would drive me to put the awd back in.
10-07-2009, 03:37 AM
#28
Formerly darynC-10/23/10
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with the new motor im still seeing around 15-17 mpg on the highway with the 22's and 3.73's around 11-13 around town
one of these days i ill put a posi and 4.10's in . but not until my g80 breaks
one of these days i ill put a posi and 4.10's in . but not until my g80 breaks
10-07-2009, 12:41 PM
#30
what is your tire size? just curious. i'm going to have to reread that in the morning after some sleep. right now with my 3.73's i have no traction issues. well that was until i broke my detroit locker. only after i build alot of boost and launch will it even spin. i have an advantage when it comes to weight transfer though. i do get what your saying as far as no load and load. i could see it going through the mountains. i could climb hills with boost in over drive with the tcc locked. at the same time it seems if you launched with boost the 4.10's would give a better 60ft also. i can't understand how people get such good 60's and i bearly get into the 1.9's. then again i broke an axle trying to launch with more boost. so i actually have no clue what it can do.
i could see me going 4.10's then not getting traction and being pissed off all the time. that would drive me to put the awd back in.
i could see me going 4.10's then not getting traction and being pissed off all the time. that would drive me to put the awd back in.
The fool proof method for figuring out what gears will work best for your combo is to go do a few pulls on the dyno with the converter locked, start your pull as low RPM-wise as you can and get a good solid graph of where your powerband lies.
Pick a point in that powerband where you want to spend most your time in and with the use of a stall (rpm related) and gears (traction related) try to match them so that you reside in that band AFTER the turbo has been loaded. If your power comes on at 3K rpm's and you have a 3K stall and 4.10 gears you'll probably spin through to about 4K rpm's at which point you'll get traction and then you'll start to load the turbo and by the time it does load you'll be at 4300-4400 rpm's and you've lost 1300-1400 rpm's worth of usable power.
That's more or less what happened when I went to 4.10's. I would blow right past the meat of my power which, due to improper set-up on my part was quite a narrow band of around 2200 rpm's.
Just remember you can't load a turbo without traction and it's harder to get traction the higher your gear. If you go with a higher gear I would suggest a smaller stall, if you go with a lower gear I would suggest a higher stall and vice versa, there's a happy medium where you'll reside right in your power band which is what you want.