Necking down a down pipe
Thread Starter
Joined: Nov 2009
Posts: 1,621
Likes: 31
From: THORNTON, PA (NEAR PHILLY)
Interesting b/c both cars seem to making alot of power, so its a pretty decent comparison.
Will it matter much on his car, probably not. If you can fit what you can fit, fit it. There are others though Dave that will see your post and because you are MightyMouse they will think that there is no gain in going to a larger down pipe which is not true. If you can reduce back pressure why not do it?
Just fit the largest DP you can. It's that simple really.
Just fit the largest DP you can. It's that simple really.
Thread Starter
Joined: Nov 2009
Posts: 1,621
Likes: 31
From: THORNTON, PA (NEAR PHILLY)
Will it matter much on his car, probably not. If you can fit what you can fit, fit it. There are others though Dave that will see your post and because you are MightyMouse they will think that there is no gain in going to a larger down pipe which is not true. If you can reduce back pressure why not do it?
Just fit the largest DP you can. It's that simple really.
Just fit the largest DP you can. It's that simple really.
I'm not seeing a whole lot of value in your post(s). OF COURSE you want to try and run the biggest pipe. Of course I will fit what I can fit. All Dave has stated CLEARLY several times is that its not going to make a huge difference at my level and not to kill myself over it, and now you echoed this.
Seems to me that this site (like most others) sees "race car" and "street" car as the same in terms of needs/requirements.
well then they need to 
my opinion stands that if you get BIG gains by oversizing the downpipe (on an application like the OP) then it is a result of a REAL problem upstream such as valve springs
of course going from 3" to infinity will improve the turbo's efficiency, and likely net you a fraction of a second at the drag strip like you need Martin,
if you are at the TOP LEVEL COMPETITION you have to take advantage of everything, and are willing to hack the car to get it, however its not the topic here.
but even you start with the RIGHT SIZE turbo, and with the right CID for your goals.
for example an s475 (in my opinion) is the COMPLETE WRONG TURBO FOR A STREET 5.3
THE RIGHT SIZE TURBO IS ON THE LEFT 76 vs 475
i took 5 seconds and found this snippet https://ls1tech.com/forums/12728225-post19.html
I also made 852 to the tires with a turbonetics 76mm with a 3" exhaust flange and through an slp dual 2.5" muffler if anyone is still listening
found the vid!
keep in mind this is from like 8 years ago before i knew i had the wrong size exhaust, numbers would be lower with todays rules i guess??
It would work even better on a 5.3 than it did on my 6.0
my opinion stands that if you get BIG gains by oversizing the downpipe (on an application like the OP) then it is a result of a REAL problem upstream such as valve springs
of course going from 3" to infinity will improve the turbo's efficiency, and likely net you a fraction of a second at the drag strip like you need Martin,
if you are at the TOP LEVEL COMPETITION you have to take advantage of everything, and are willing to hack the car to get it, however its not the topic here.
but even you start with the RIGHT SIZE turbo, and with the right CID for your goals.
for example an s475 (in my opinion) is the COMPLETE WRONG TURBO FOR A STREET 5.3
THE RIGHT SIZE TURBO IS ON THE LEFT 76 vs 475
i took 5 seconds and found this snippet https://ls1tech.com/forums/12728225-post19.html
I also made 852 to the tires with a turbonetics 76mm with a 3" exhaust flange and through an slp dual 2.5" muffler if anyone is still listening
found the vid!
keep in mind this is from like 8 years ago before i knew i had the wrong size exhaust, numbers would be lower with todays rules i guess??
It would work even better on a 5.3 than it did on my 6.0
I think it depends on the set up as to which turbo to run on the street. I think one of the smaller turbine s475 with a T4 housing would be just fine. A little change of the stall and it really won't feel to much different. I necked mine down on my s480 to a 4" and plan to eventually split that to duaol 3" when funds allow and run it out the back with a cut out right before the split. I honestly do not anticipate issues. After talking with a couple of people about cam selection I think that could play a role with backpressure and tubing size as well. Everything has to work together.
Thread Starter
Joined: Nov 2009
Posts: 1,621
Likes: 31
From: THORNTON, PA (NEAR PHILLY)
I did go with a t4 1.1 ar s475 for what its worth. I had this exact turbo on a stock 6.0 with a 6spd and spool was pretty darn good with the minimal tuning that I did. Granted its going on a engine that has about 40 less cubes, but I figure a convertor should really help this turbo come alive. If this 5.3 goes, I will most certainly go with a 6.0...I just needed the 5.3 and all its accessories to get the project going.
I really thought LONG and HARD about the tc76, but the small turbine seemed to turn everybody off. Although, I'm seeing some big #'s being put up by that turbo in some cases.
I really thought LONG and HARD about the tc76, but the small turbine seemed to turn everybody off. Although, I'm seeing some big #'s being put up by that turbo in some cases.
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So lol
How is a 5" full exhaust not a street car?
More street car than everybody dumping downpipes out the side of the car
Yes post turbine pressure is irrelevant, the expansion ratio of the housing doesnt do anything with that innocent post turbine pressure
How is a 5" full exhaust not a street car?
More street car than everybody dumping downpipes out the side of the car
Yes post turbine pressure is irrelevant, the expansion ratio of the housing doesnt do anything with that innocent post turbine pressure
Last edited by rotary1307cc; Mar 20, 2013 at 12:56 PM.
TECH Regular
Joined: Oct 2012
Posts: 476
Likes: 2
Since when does the pressure delta across the turbine wheel give two ***** if its a street car,race car, 700 horse or 1300 horsepower???
To the OP... necking down isnt a good thing... but it can be worked around. Ideally for your situation i would run 4 inch as far as you can, do a nice smooth conical reducer to 3.5 and fit that how you need to fit it. This method allows you to maintain as much pressure delta as possible so the wheel stays efficient. As you go back the exhaust will cool causing density to increase and airspeed to drop. The conical reducer will help accelerate the gas and hopefully prevent excessive post turbine pressure.
For every 1 psi of post turbine pressure you have, it can create 2 or 3 pre. Turbine wheels arent designed around mass flow. They work off of pressure and temperature delta. The higher these delta values, the more efficient the wheel will be.
One of the guys on the bullet posted up complaining about the numbers his full exhaust street car made.. diameter was ok... was a to the bumper system....he dropped it at the downpipe and picked up 117 rwar wheel horsepower. Its the length of run that kills you. Ill take a 3 inch 24 inch long downpipe over a 3.5 or maybe even 4 inch to the rear bumper full exhaust system any day of the week and twice on sundays.
Too much dick swinging i did blah blah 150 years ago bullshit. Theres a right way and a wrong way. The right way is whichever way makes the turbo the MOST efficient. PERIOD.
To the OP... necking down isnt a good thing... but it can be worked around. Ideally for your situation i would run 4 inch as far as you can, do a nice smooth conical reducer to 3.5 and fit that how you need to fit it. This method allows you to maintain as much pressure delta as possible so the wheel stays efficient. As you go back the exhaust will cool causing density to increase and airspeed to drop. The conical reducer will help accelerate the gas and hopefully prevent excessive post turbine pressure.
For every 1 psi of post turbine pressure you have, it can create 2 or 3 pre. Turbine wheels arent designed around mass flow. They work off of pressure and temperature delta. The higher these delta values, the more efficient the wheel will be.
One of the guys on the bullet posted up complaining about the numbers his full exhaust street car made.. diameter was ok... was a to the bumper system....he dropped it at the downpipe and picked up 117 rwar wheel horsepower. Its the length of run that kills you. Ill take a 3 inch 24 inch long downpipe over a 3.5 or maybe even 4 inch to the rear bumper full exhaust system any day of the week and twice on sundays.
Too much dick swinging i did blah blah 150 years ago bullshit. Theres a right way and a wrong way. The right way is whichever way makes the turbo the MOST efficient. PERIOD.
Teching In
Joined: Aug 2007
Posts: 36
Likes: 0
if you are going bigger on the engine size in the future than leave your salf some room to grow. dont cap the s476 go with the biggest dp pipe you can fit your engine will be happier= less maintenance for your street car. alot of 650+hp turbo street cars have 4in and up dp