4" Exhaust....opinions

Skam9685

Skam9685

1992 Honda Prelude SI
Jun 22, 2007
157
0
0
39
Oldsmar, Florida, United States
I was browing my local forum and found this. http://www.tamparacing.com/forums/driven-concepts/443948-supra-4-aluminum-exhaust-pics-vid.html

Mind you it's on an MK4 Supra, but I'm pretty sure than can fab one up for the MK3.

I was thinking about possibly doing this eventually once I decided what to do engine wise. I believe that I read that the bigger the size the better for a turbo car because of back pressure. It seems that most people have 3" exhaust, so I was wondering how come some of you folks haven't decided to go bigger instead of 3".

Anyways, just wanted so see what you fellas thought.
 
M

Ma70.Ent

Supramania Contributor
Feb 26, 2006
1,871
1
0
NJ
I'm pretty sure larger exhausts (3.5-4.0) are only for those higher powered turbo cars, ones probably hitting over 600 rwhp. Also, it may have to do with cost of the exhaust and maybe they don't want it to be loud.
 
gtsfirefighter

gtsfirefighter

SM Expert on White trash
Sep 26, 2006
2,965
2
36
55
Weatherford, Texas, United States
Skam9685 said:
I was browing my local forum and found this. http://www.tamparacing.com/forums/driven-concepts/443948-supra-4-aluminum-exhaust-pics-vid.html

Mind you it's on an MK4 Supra, but I'm pretty sure than can fab one up for the MK3.

I was thinking about possibly doing this eventually once I decided what to do engine wise. I believe that I read that the bigger the size the better for a turbo car because of back pressure. It seems that most people have 3" exhaust, so I was wondering how come some of you folks haven't decided to go bigger instead of 3".

Anyways, just wanted so see what you fellas thought.
Click to expand...


Anything bigger than 3" would look retarded or like you're running a powerstroke diesel.
 
Adjuster

Adjuster

Supramania Contributor
Apr 8, 2005
1,166
1
0
Meridian, Idaho, United States
www.adjusteridaho.com
Look at it this way.
The stock CT has a pretty small exhaust wheel, and there is a good gain going to a 3" ID exhaust.

You are going to see a gain going to 4" ID, but only if your ratio of exhaust wheel to ID is similar. (As noted, the higher the HP goals, the more you might see a gain from a 4" exhaust.)

Other option? Just run your waste gate into it's own exhaust pipe/muffler, or run with no muffler and dump it just behind the motor at the ground. Yes, this is louder, but you just added more flow capability to your 3" exhaust when you really need it most. (When your at high boost, high rpm, and the larger pipe really earns it's keep.)


Downside? Scrapes and other clearance issues. I've built my own DP, and modified it a few times now, and it's about as close to the underside of the car as I can get it, but it still scrapes from time to time. I can only imagine what a 4" pipe would be like under there...

If the pipe was ovalized, then it would be more pratical, but ovalized pipe is not very easy to find, and not very easy to bend and weld up either, so not much of an option for the day to day user... (There is/was a guy in Europe that split his exhaust, and welded up strips down the middle to "oval" the pipes, turned out quite interesting, and it would flow quite a bit more volume than before.)

I have also thought about using thin wall square tubing.... Again, you lose easy ability to make bends however... so you could merge it into a 4" mandrel bend I suppose, and then split it into two 3" megan type ebay mufflers for the ultimate custom setup on a budget... (But you would want to use either stainless, or have it coated to resist rusting...)
 
D

dbsupra90

toonar
Apr 1, 2005
2,374
0
0
indiucky
actual dyno testing of 4" vs 3" results on a sp67, p trim from x-man

4" 19psi 518/475

change back half of exhaust to 3", thats it. lost 2 psi and a/f went rich.

3" 17psi 425/418

thats 93rwhp and 57rwtq

now, retuning since the back half exh change-

3" 17psi 445/441 tuned back to same a/f as 4"

now comparing tuned to tuned, but 2 psi less thats-

518 - 445 = 73rwhp
475 - 441 = 34rwtq

granted, it would have been nice to compare 19psi to 19psi, but the point of the dyno run was to change nothing but the exhaust and see what happens.
 
fstlane88

fstlane88

Single time!!
Apr 5, 2005
536
0
16
38
South FL
I have a 3.5" catback on my car with a 3" DP and it is LOUD. But that probably has a lot to do with the race bullet I have as my muffler....

As far as scraping goes, on steep driveways the test pipe flange where it meets the DP will scrape, and the DP sometimes scrapes on speedbumps but the 3.5" portion of the exhaust never has scraped.
 
daneyan

daneyan

Banned
Sep 1, 2005
441
0
0
39
South Florida
fstlane88 said:
I have a 3.5" catback on my car with a 3" DP and it is LOUD. But that probably has a lot to do with the race bullet I have as my muffler....

As far as scraping goes, on steep driveways the test pipe flange where it meets the DP will scrape, and the DP sometimes scrapes on speedbumps but the 3.5" portion of the exhaust never has scraped.
Click to expand...

hey who did your exhaust ? i wanna do 4'' on my car into a burns or aeroturbine muffler and was wondering who did yours?
 
fstlane88

fstlane88

Single time!!
Apr 5, 2005
536
0
16
38
South FL
Robert@935 made it, and it actually comes with a second race bullet that acts as a resonator. But, b/c I'm running a longer than normal DP I had to cut it out, and have the catback rewelded w/o it. I will be working it back in when I go single in the future, and have another DP made.
 
daneyan

daneyan

Banned
Sep 1, 2005
441
0
0
39
South Florida
fstlane88 said:
Robert@935 made it, and it actually comes with a second race bullet that acts as a resonator. But, b/c I'm running a longer than normal DP I had to cut it out, and have the catback rewelded w/o it. I will be working it back in when I go single in the future, and have another DP made.
Click to expand...
935? are u referring to a shop down here or the website where u had to order it?
 
fstlane88

fstlane88

Single time!!
Apr 5, 2005
536
0
16
38
South FL
Good to hear, nope I'm in West Palm Beach actually, farthest south I go usually is Boynton for work, or Delray for the beach.
 
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