Corvette Forum : DigitalCorvettes.com Corvette Forums banner

1 - 12 of 12 Posts

·
Super Moderator
Joined
·
8,498 Posts
Discussion Starter #1 (Edited)
you can get a very good idea if you have a muffle shop in your area that will test the back pressure in your car for you.
you can,t make intelligent decissions without facts
is exhaust back pressure killing performance ?
its a fast easy test that needs to be done and more than a few cars running older cats are having a restricted exhaust thats hurting your performance badly, and because the problem tends to build up slowly many people don,t notice the gradual power loss
first thing youll need to realize is that the TEST POINT needs to be after the header collector and in front of the CAT AND MUFFLERS. next thing you need to know is that to get a valid answer youll need to have a long connector hose so someone in the pass seat can CLOSELY WATCH the gauge as you accelerate under load (FLOOR THE CAR THRU THE FIRST 1-2 GEARS and watch the pressure surge. readings above 1psi mean somethings partly restricted, readings above 3psi are hurting your performance BADLY
the test kit is about $50.00

http://www.ntxtools.com/fuelsystem/tu24a.htm

read this
http://www.babcox.com/editorial/bf/bf19922.htm

ITS NOT RARE TO LOOSE 20% or more of your hp to a restricted cat
id highly suggest adding one of these 3" (X) to most exhaust systems
http://store5.yimg.com/I/exhaustpros_1769_9241795

http://www.uucmotorwerks.com/html_product/sue462/backpressuretorquemyth.htm

http://victorylibrary.com/mopar/header-tech-c.htm


http://www.burnsstainless.com/TechArticles/Theory/theory.html

http://grapeaperacing.com/GrapeApeRacing/tech/exhaustheaders.cfm
the short answer is ANY BACK PRESSURE IS BAD!
what your trying to do with a correctly designed exhaust is to have the majority of the exhaust system act like extended collectors on the headers, in effect useing the inertia of the hot exhaust pulses from each cylinder to cause a cyclic low pressure wave to be timed to assist the next firing cylinders to scavage the cylinders

heres the info you need,
length

http://www.engr.colostate.edu/~allan/fluids/page7/PipeLength/pipe.html
dia.

http://victorylibrary.com/mopar/header-tech-c.htm


to adjust for your altitude
http://rshelq.home.sprynet.com/density_altitude.htm

and if you want to get it perfect you should usr your DYNAMIC COMPRESSION RATIO VOLUME in the formula at the rpms for peak hp that your engine makes

http://cochise.uia.net/pkelley2/DynamicCR.html
most of the time that comes out close to the same as the static cylinder volume at the peak torque f.y.i.

LET ME ASK THIS QUESTION?
WHATS THE FIRST THING YOU DO AT THE TRACK TO YOUR EXHAUST SYSTEM?
you open the headers to have less back pressure correct!!
BACK PRESSURE IS BAD, BAD,BAD, the idea has gotten around that you need back pressure to build torque, FALSE!!!!!! that IDEA was prevalent because if you stick too large of an exhaust pipe on a low rpm engine it loses the ability to scavage the cylinders with the exhaust gas pulse in the exhaust with stock exhaust manifolds, some how the old wifes tale got around that you needed back pressure...FALSE.. what you need was EFFECTIVE CYLINDER SCAVAGEING which the smaller tail pipe dia. was provideing by acting like the collector on a set of headers!if you have headers , especially full length headers with a merge type collector you can,t make the exhaust too large,THINK ABOUT IT! the first thing you do at the track is un-cork your headers....why? because YEARS OF TESTING PROVES, less BACK PRESSURE MAKES MORE HP WITH HEADERS!!!! now some one is saying yeah but thats at the track and your running the engine at 5000rpm-6500rpm , well true and a good point because we typically only run 1000rpm-3000rpm on the street but look here,
http://www.engr.colostate.edu/~allan/fluids/page7/PipeLength/pipe.html

what it shows clearly is the collector on a low rpm engine needs to be about 10ft long for max effect so you need an exhaust that works with a collector thats about 20" long at 6000rpm and one 10 feet long at idle, THATS WHERE THE IDEA OF TOO LARGE A EXHAUST comes from but as long as you follow these rules youll be fine, READ THIS,

http://victorylibrary.com/mopar/header-tech-c.htm


btw Im writeing this like this to get you guys to think about it then we will look into the answers of how to get both ends of the rpm range covered and yes Im leaveing out some info on purpose





is exhaust back pressure killing performance ?
its a fast easy test that needs to be done and more than a few cars running older cats are having a restricted exhaust thats hurting your performance badly, and because the problem tends to build up slowly many people don,t notice the gradual power loss
first thing youll need to realize is that the TEST POINT needs to be after the header collector and in front of the CAT AND MUFFLERS. next thing you need to know is that to get a valid answer youll need to have a long connector hose so someone in the pass seat can CLOSELY WATCH the gauge as you accelerate under load (FLOOR THE CAR THRU THE FIRST 1-2 GEARS and watch the pressure surge. readings above 1psi mean somethings partly restricted, readings above 3psi are hurting your performance BADLY
the test kit is about $50.00
http://www.jdsdiagnostic.com/eptspage.htm

ITS NOT RARE TO LOOSE 20% or more of your hp to a restricted cat
your unlikely to lose much if any hp especially if you install an (H) pipe just before the reduction in tail pipe dia. and use a gradual reducer to change from 3' to 2.5" mostly because the exhaust gases have cooled a great deal by the time they reach that point in your exhaust system and with that heat loss a reduction in voluum and potential back pressure



http://www.jegs.com/cgi-bin/ncommerce3/ProductDisplay?prrfnbr=1313&prmenbr=361


http://www.jegs.com/cgi-bin/ncommerce3/ProductDisplay?prrfnbr=1306&prmenbr=361

http://store.yahoo.com/exhaustpros/hustxpi.html
http://store5.yimg.com/I/exhaustpros_1775_13682224
 

·
DC Crew
Joined
·
53,220 Posts
That is all really good reading.

I agree 100% on the cats blocking HP. I gutted my cats and had a seat-of-the-pants improvement.

I am going to junk my corsa's next year and do long tube headers and a custom 3" exhaust all the way down.:thumbsup:
 

·
Registered
Joined
·
2,409 Posts
Patrick96LT4 said:
That is all really good reading.

I agree 100% on the cats blocking HP. I gutted my cats and had a seat-of-the-pants improvement.

I am going to junk my corsa's next year and do long tube headers and a custom 3" exhaust all the way down.:thumbsup:
Interesting tidbit about the cats and emissions. The IL state readings from my 72 compared to my 95 Vette and 97 T-Bird are almost identical at idle and at 35 mph. The 72 doesn't have the more rigorous test as the late models so the higher rpm's can't be compared.
 

·
Registered
Joined
·
379 Posts
hey pat a guy here in town has a 93 manual and he just got corsas and gutted the cats yesterday so im assuming thats a lot what urs sounds like. i noticed though that with the corsas it sounds exactly like my muffler eliminators inside and out. we each rode in the others car and drove the others as well and we could tell much of a difference inside or out. when he got his cats gutted today it still sounds exactly the same INSIDE. sounds just like it did with just the corsas. but when your outside the car and it goes by you in 2nd its screaming. sounds pretty good when you have someone rev it up to 4k and you are standing behind the car. I do have to admit i thought having corsas and gutted cats would be louder than that period though. dont get me wrong it sounds GOOD i just thought with no cats the volume would be more. Did you gut ur cats or have straight pipe put in? i wonder if that makes a difference. we didnt really feel much of a SOTP improvement in his either after the corsas or the gutted cats although it feels like it pulls a bit better in higher RPMs he tells me. maybe you noticed more in your cause its an LT4. Who knows. just thought i would throw my obsservations in. please feel free to respond and tell me what you think
 

·
DC Crew
Joined
·
53,220 Posts
Agreed.

My Corsa exhaust got wicked-crackly-badass loud on the outside when I gutted the cats, but never increased in volume whatsoever on the inside. Which is COOL!

I had muffler eliminators twice on LT4 cars.... they sounds great, but I had a lot of interior droan... but not with the Corsa's.

I can't get over how wicked the car sounds at wide open throttle. It's like a "hide your women and children" sound.

In fact, I can make people jump off the sidewalk if I stab the gas on the street.

Damn hilarious.:laughing:
 

·
Registered
Joined
·
965 Posts
i cant WAIT untill i get my new muffler emliminators !!!!!:D :thumbsup: :thumbsup:
 

·
Registered
Joined
·
965 Posts
Patrick said:
That is all really good reading.

I agree 100% on the cats blocking HP. I gutted my cats and had a seat-of-the-pants improvement.

I am going to junk my corsa's next year and do long tube headers and a custom 3" exhaust all the way down.:thumbsup:
let me have your corsas :)
 

·
Registered
Joined
·
63 Posts
Interesting reading. Thanks for sharing. I have stock exhaust but I have a feeling it’s not going to stay that way for too much longer....................:D
 

·
Registered
Black 08, A&A TI supercharger. Kooks headers. CCW wheels. Much more
Joined
·
12,668 Posts
Het Patrick.
Can't you keep the corsa-craps and just hollow them out?
 
1 - 12 of 12 Posts
Top