i am wondering how workshops take exhaust gas pressure readings, gauge? sensor hooked up to computer? and how much it cost to get the gear to do it, if reasonably cheap i'd wouldn't mind finding out the pressure differences between the exhaust and intake. i know that when your exhaust gass pressures are lower than the intake you will make more power than if the exhaust gas pressure is higher that the intake's pressures. so if anyone knows how to do it and the rough cost i'd appreciate finding out

on an I6 motor that has good comp on all cylinders. The true blue setup is 3in intake and a pod or panel filter).

A 2.5in mandel bent exhaust is good ebough. seacrh is your friend on this topic!!!!!!!!

remember exhuast pressure isn't always bad. If you remove it at a certain point (assuming around 4000-6000rpm if you were going to the trouble of doing this) the bottom end will suffer as gas velocity will be lower and therefore heavier gasses will have to be pushed through cats/muffs/resi's

backpressure is a falacy, exhausts work on velocity and waves, and its as simple as you said it. Lower pressure in the exhaust will encourage cylinder scavenge and initial filling. The thing here is that normally anything apart from WOT is going to mean that your exhaust is at a higher pressure than the intake, and even at WOT, there is enough boundary effect from the intake manifold runners to create a lower pressure at the intake port.

See Scientific design of intake and exhaust systems for further clarification, its a book that most will need to read several times to understand what they are on about, I know that I did.

simply size the pipe to where you want the power. smaller diameter pipe, ie the stock 2-2.25 inch press bent, is better for low down torque, perhaps effective to about 3000 rpm. at low rpm the gas is moving slower as there is less supply from the motor. The same low amount of gas moving in a larger pipe moves slower, as it cools and condenses, becoming heay, and a burden for the motor to push to the end of the system. however this changes as rpm rises, and the amount of gas needed to be dicharged increases. a 2.5 inch system will be slightly less effective down very low as the stock exhaust, but will see efficiency increase in the midrange and top end. a 3 inch system will seem somehwhat doughy initially, hitting its straps above the 3k mark. this is because of where the velocity meets the required amount to make the flow efficient.

you would only use a 3 inch on a highly worked i6 motor, it just doesnt displace enough to justify any bigger than the 2.5. the 2.5 has the best power under the curve, which is why the majority use it. the trick is to elimate the power robbing areas like flow restrictive cats and mufflers. you have to get the gas moving and keep it moving. you can have the best exhaust pipe in the world but its not going to make a worthwhile difference if you have a 3 chamber dogleg muffler on it.

the same goes for the intake. longer thinner intake means at low demand (rpm) you will get better cylinder fill (which is neccesary to create torque) as the lower volume of air increases its speed as the vaccuum works on its density. Bigger wider intake means at low rpm there is very little density and thus lower air velocity to fill the cylinder more completely, as the port is only open for a certain period lower air velocity means less air in the cylinder to expand in the resulting bang. This is also reffered to as volumetric efficiency, or the real operating volume of an engine. it is accepted that generally most engines run at around 70-80% VE, or 70 to 80% of their nominated capacity, in our case 4 litres. Boosted engines work above 100% VE hence more power and torque.

Im not an expert on this though its just what ive picked up from around the traps. If anyone cares to elaborate and correct any glaring errors please feel free

yeah, unless your trying to milk every last top end kW out of an engine (or have money and wanna do some dyno runs with a 2.5 then a 3in system and post up the graphs so we don't have to see a thread on it again) it isn't worth going bigger than 2.5in...not that you would notice a difference anywhere but the dyno anyway

From 2.5 to full 3 inch gave me more aceleration power but the car has to be tuned to suit.

do please also mention that you have headwork, a CMS cam iirc and flash tune (correct me if im wrong), plus 4.11:1 diff gears (meaning less time spent in the doughy end of a large bore system).

you would want a pretty mean cam profile/tune and to have your car sitting at 3500+ for a fair chunk of the driving time to actually feel the difference

My car is modified . I dont have a cms cam. I spend 90 perecent of my time between 1800 and 3300 rpm. I do around 700 kays a week in Sydney traffic. Im not hiding anything.Most people who know me know everything about my car.
Im not going to sit here trying to convince you that it works because I like putting skeptics like yourself in the rear mirrors by doing things that arent supposed to work to the wagon.

My car rarely sees 3500 rpm in a day because it is dailly driven...It idles at 650 rpm and pulls nicely from the lights well below 3500 rpm. On coolish mornings a slight tap of the accelerator sees some crazy low down torque come on. The addition of the 3 inch from 2.5 inch gave stronger low down to mid range power for me.. But ..do what you want...its your money.

I have been hearing this for years (decades) and I believe its just a typical jump on the band wagon mentality...

I have not seen any scientific proof that a big bore exhaust looses low down torque...

from my experience all it does is give u more top end....

haha, a bit harsh. Not arguing against you as 14.2 is a pretty schmick time. Have you considered the 2.5 system you had was s**t or rooted? muffler quality and position makes a fair difference to efficiency of a system. Only reason I ask is cause you said you noticed more down low, which (particularly on paper when 2.5 flows 2/3 of what 3in does, yet it is in the lowest volume of gas displacement for what you mentioned) doesn't quite sound right.

I know after I did the rebuild on my mates EB (runs similar times to yours) he could only manage 14.5's (old muffler) but after replacing it with a spankin new hooker (holley) one got a 14.3....I reckon atleast has a 14.1 in it though (s**t driver...was getting 98.8MPH+ trap speeds )

haha yeah...in the end its a catback exhaust....anyone who notices "HUGE IMPROVEMENTS" needs some more KY

where can i get that scientific design book i know what yuo mean having to read a book a couple of times to get your head around it. trying to work out how an lsd works without actually havng seen in one proved a mind boggler when i ws 14

jump on amazon, they should have it.

Big bore, LMFAO!! For those who dont know, there is a fair incline heading out of Liverpool on Elizabeth Dr coming up to Resovoir Rd, anyway I can dawdle up this hill at 1300rpm in 5th gear doing about 55-60kph with my 3" exhaust, and mildly lumpy cam. Cats disturb the airflow enough (even with high flow's) that there is little improvement of low end torque with the 2 1/2", I know because I had one on the car before the 3"