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,
to adjust for your altitude
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
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 OF THE CORRECT LENGTH,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 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,
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
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