Go Back  RX7Club.com - Mazda RX7 Forum > Tech and Performance > Race Car Tech
Reload this Page >

I know, another exhaust question, what is to small?

Race Car Tech Discuss anything related to road racing and auto X.

I know, another exhaust question, what is to small?

Old 09-29-09, 03:37 PM
  #1  
Full Member
Thread Starter
iTrader: (2)
 
Join Date: Jun 2006
Location: Portland OR
Posts: 241
Likes: 0
Received 0 Likes on 0 Posts
I know, another exhaust question, what is to small?

Yes i did searching but i never really found an answer that i was looking for.

What i have is a 82 with a 12a street port with a Weber 48 IDA and i'm planning on redoing the exhaust this winter.

it has a RB header with 2" tubes going into a 3" flange and running 3" all the way back to into a 3" magnaflow with no cats.

what i use the car for is rally racing, pretty much rapped out all the time on stage.

So my question is, how small is to small? I'm thinking of replacing the 3" with 2.5" for some better ground clearance, but it seems with all i am reading the norm for racing is 3".

But then I have had guys tell me 3" is over kill on anything NA.

What says you?
mellow65 is offline  
Old 10-01-09, 09:38 PM
  #2  
80's car lover
5 Year Member
 
Join Date: Jul 2008
Location: Lemont, IL
Posts: 145
Likes: 0
Received 0 Likes on 0 Posts
I'm no rotary exhaust expert but I know that a single 3" exhaust on my 5.7L camaro is plenty to support 300hp+. A lot of high horsepower 3rd & 4th gen camaros run a single 4" system. I've been debating on what size exhaust to put on my stock 12a and Im thinking 2.25" is plenty, for you 2.5" is probably more then enough with your mods. Velocity is better then volume and as Smokey Yunik said (paraphrase) "Anyone who tells you they know anything about exhaust is full of it, nobody knows anything about exhaust, but I will tell you that too small is better then too big".
88blkiroc is offline  
Old 10-01-09, 11:10 PM
  #3  
Full Member
Thread Starter
iTrader: (2)
 
Join Date: Jun 2006
Location: Portland OR
Posts: 241
Likes: 0
Received 0 Likes on 0 Posts
yeah, my thoughts were the 3" is over kill, but they rotarys really move some air and spinning 7000+ rpms on a somewhat normal bases i didn't want to choke it.

i'm going to run one more event with the 3" and then redo it this winter. but i'm hoping to get some good input about what is to big and what is to small.
mellow65 is offline  
Old 10-02-09, 11:28 AM
  #4  
Armchair engineer
iTrader: (2)
 
j9fd3s's Avatar
 
Join Date: Mar 2001
Location: http://www.k2rd.com
Posts: 24,599
Likes: 0
Received 123 Likes on 112 Posts
with a port and a carb, you're right at the power level where it might matter. if it was stock port, 2.5" would be fine, bridgeport needs the 3"

streetport? dunno. i suspect you'd loose a minimal amount of power, like 2-5hp, but i could be wrong
j9fd3s is online now  
Old 10-04-09, 07:25 PM
  #5  
Rotary Enthusiast
iTrader: (1)
 
Join Date: Jul 2004
Location: Rocket City, Alabama
Posts: 1,035
Likes: 0
Received 1 Like on 1 Post
Run a pair of 2" long primary tubes. Put the collector just behind the rear axle, and then your 3" muffler.
This will give you more ground clearance, and it makes good power too.

On my road racer, I cut out and raised the floor on the right side, roughly from the passenger seat front mount on back. This let me tuck my pipes up higher and get more clearance.
speedturn is offline  
Old 10-04-09, 07:59 PM
  #6  
Full Member
Thread Starter
iTrader: (2)
 
Join Date: Jun 2006
Location: Portland OR
Posts: 241
Likes: 0
Received 0 Likes on 0 Posts
Originally Posted by speedturn View Post
Run a pair of 2" long primary tubes. Put the collector just behind the rear axle, and then your 3" muffler.
This will give you more ground clearance, and it makes good power too.

On my road racer, I cut out and raised the floor on the right side, roughly from the passenger seat front mount on back. This let me tuck my pipes up higher and get more clearance.
i'm assuming you are talking to run the 2 2" pipes under the rear axle. if so, no can do, that sucker would be torn off after the first stage.

although this does give me some ideas. i'm happy with how the 3" runs over the rear axle i'm just not happy with the from the header to the rear axle section. so i might do something like a double 2" to the rear axle and then the 3" over the axle and out the back.

thanks
mellow65 is offline  
Old 10-05-09, 12:54 PM
  #7  
Rotary Enthusiast
iTrader: (3)
 
mustanghammer's Avatar
 
Join Date: Nov 2006
Location: Parkville, Mo
Posts: 1,102
Likes: 0
Received 21 Likes on 17 Posts
For what it is worth I have not seen a 12A or 13B street port SCCA E Production car with less than a 3" exhaust. These cars also run a Weber 48 IDA. We have 6 of these cars in the KC area and I have studied the all of the first gens at the Runoffs when they were in Topeka.

When Mandeville dyno'd my friends street port 13B he recomended a 25" primary with a Burns merge into a 3" exhaust.

With a close ratio box these cars are rarely below 6500 and most are redlining at 9500.

Have you thought about using oval tubing for ground clearance?
mustanghammer is offline  
Old 10-05-09, 07:17 PM
  #8  
Full Member
Thread Starter
iTrader: (2)
 
Join Date: Jun 2006
Location: Portland OR
Posts: 241
Likes: 0
Received 0 Likes on 0 Posts
Originally Posted by mustanghammer View Post
For what it is worth I have not seen a 12A or 13B street port SCCA E Production car with less than a 3" exhaust. These cars also run a Weber 48 IDA. We have 6 of these cars in the KC area and I have studied the all of the first gens at the Runoffs when they were in Topeka.

When Mandeville dyno'd my friends street port 13B he recomended a 25" primary with a Burns merge into a 3" exhaust.

With a close ratio box these cars are rarely below 6500 and most are redlining at 9500.

Have you thought about using oval tubing for ground clearance?
well that's good stuff to know.

no i haven't really even given a thought to oval tubing, i wouldn't even know where the heck to get the stuff
mellow65 is offline  
Old 10-05-09, 11:19 PM
  #9  
Rotary Enthusiast
iTrader: (3)
 
mustanghammer's Avatar
 
Join Date: Nov 2006
Location: Parkville, Mo
Posts: 1,102
Likes: 0
Received 21 Likes on 17 Posts
Originally Posted by mellow65 View Post
well that's good stuff to know.

no i haven't really even given a thought to oval tubing, i wouldn't even know where the heck to get the stuff
My local Circle Track speedshop sells it - straights and bends. Googled and dr. gas came up - aluminized mild steel and stainless. He also has a conversion chart from round to oval. Dr. Gas invented the x-pipe crossover that is used in NASCAR.

www.drgas.com

There are probably other sources.
mustanghammer is offline  
Old 10-06-09, 11:33 AM
  #10  
Full Member
Thread Starter
iTrader: (2)
 
Join Date: Jun 2006
Location: Portland OR
Posts: 241
Likes: 0
Received 0 Likes on 0 Posts
wow that's pretty spendy stuff.

i think with how much you have to replace an exhaust on a rally car i'm going to go a different approach, i cheaper approach.

thanks anyways.
mellow65 is offline  
Old 10-06-09, 11:46 AM
  #11  
Armchair engineer
iTrader: (2)
 
j9fd3s's Avatar
 
Join Date: Mar 2001
Location: http://www.k2rd.com
Posts: 24,599
Likes: 0
Received 123 Likes on 112 Posts
you could do the 2x2" pipes, go to 3 and up and over the axle...

the 2x2 can tuck in pretty close, and 3" over the axle minimizes the power loss from the bends, but since its over, should clear everything?
j9fd3s is online now  
Related Topics
Thread
Thread Starter
Forum
Replies
Last Post
Th0m4s
Build Threads
25
02-26-19 02:04 AM
astrum
2nd Generation Specific (1986-1992)
24
11-15-17 08:44 AM


Thread Tools
Search this Thread
Quick Reply: I know, another exhaust question, what is to small?


Contact Us - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service

© 2019 MH Sub I, LLC dba Internet Brands

We are a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for us to earn fees by linking to Amazon.com and affiliated sites.
 
  • Ask a Question
    Get answers from community experts
Question Title:
Description:
Your question will be posted in: