2.25" Cat Back system

[moody]

I've been offered a real nice 2.25 cat back exhaust system off a mate for a real cheap price. what is everyones oppinion on a 2.25 ove a 2.5. and as for a cat conv...wats the advantage of a 2.5 hiflow...do i need 1 with a 2.5 catback system? or is the stock one fine?

also what performance and note difference is there with a hiflow cat?

[eed-250]

the note will change but i cant say the same about the performance. It wont make it any faster. and 2.5 would be a better system. i think factory is 2.25 anyway??

[AshMan]

I wouldn't of thought that the stock exhaust is much over 2.25 anyway...

I'd save your cash and get a 2.5 or 3 inch IMO.

[locknut]

I'm pretty sure the stock system is 2". A 2.25" system would be better than stock, but the main issue is a straight thru system then huge pipes. 2.5" Should be the biggest system you'd want. When you think about it, 600hp V8 Super cars run a 3" system, and we're not flowing/making that much power here.

3" System would be too big IMHO. Most likely too much loss of back pressure, and loose torque, not forgetting the massive drone.

Stick with the 2.25" if it's the right price, or there are plenty of 2.5" Kits made for the Falcon 6 that I have heard to be reasonably priced. About $300? + fitting..

[ALC]

If you keep your original cat and upgrade to a 2.25 exhaust it is still restricted back to 2 inch. My suggestion would be get the 2.25 and get a hiflo cat, even better still a set of headers. Should perform a bit better. My xh has a set of pacemaker headers, 2.5" hiflo cat and 2.5" straight through system, very loud but it goes well now. Guessing around 15-18 rwkw gain.

[moody]

alc thats the system i had lined up but with wildcat headers...i mite just go for that. except maybe a hotdog or sumthin on the tail sectio. cheers guys

[AshMan]

Originally Posted by ALC

If you keep your original cat and upgrade to a 2.25 exhaust it is still restricted back to 2 inch.

When they do the exhaust, the cut a bigger hole in the cat and weld the catback onto the cat. If you have extractors the also make the hole at the front of the cat bigger and usually put a flange just infront of it.

Originally Posted by moody

up but with wildcat headers...i mite just go for that.

wildcat make good headers IMO

[stiddy]

Not starting a flame war but - this is an incredibly over assumed and continuously stated myth. NO an engine does not require back pressure to run, and yes any back pressure WILL hurt power somewhere in the range. What is far less understood but is far more important is the way sound waves travel down the pipe. While this involves flow and pressure in many and varied ways, it has more to do with muffler choice, tuned length, volume of the pipe/system etc. for example an open chamber muffler appears like the end of a tailpipe to anything before it, thus ending the tuned length and causing a transmitted and reflected wave up and down the tuned part of the system, which at some RPM will hurt power, and at others will help due to scavenging, hence the design of extractors. You will often find that after the tuned length you can use any size pipe (as long as it doesn't restrict the flow) and power will be within 2-3hp ie open to smallest size that will still flow enough exhuast to not cause further backpressure should all be very close on the dyno. The maths required to work out exactly what will suit an engine both in terms of intake design and exhaust design is absolutely heinous, and I'de be very surprised if any "off the shelf" exhaust has had any thought put into it more than trial and error, this is what has caused the myth to build over the years as a 3" system probably doesn't outflow a 2.5" with a standard engine, hence muffler choice, extractor choice, actual tuned length etc would have had more of a bearing. However as we see more and more people moving to JMM/ITS/Alted/Forced Induction, with higher revs and higher flow perhaps the 2.5" won't flow enough and will hurt power, hence people will fit 3" and start saying 3" is the shiz and then everyone will fit a 3" regardless. </rant>

Having said all of that, you will find that the 3" systems beginning (length of extractor, 3" pipe, 3" cat) will become your tuned length, and the larger volume of the 3" pipe over the 2.5" pipe will make the 'tuned rpm' higher. Hence a little lost down low and a little gained up high. perhaps we can FINALLY put this myth to rest, and move on, that way we can all go about 'designing' our exhausts rather than just fitting said pipe size.

[AshMan]

Originally Posted by stiddy

Not starting a flame war but - ...

thanks mate, that was a really interesting read.

I think most of us are probably just posting from experience more than anything else.

Thanks for enlightening me.

Thats what this whole forum is about. Its a learning experience.

[AshMan]

sorry to hijack your post mate,

but here is the formula if anyone is interested.

Exhaust Systems Tuned Length

Lt = (Eo x Vs) / N
Lt is the tuned length, in inches
Eo is the exhaust-open period, in degrees
Vs is wave speed in feet per second (1700 ft/sec at sea level)
N is crankshaft speed, in RPM

good luck!

Edit.

Diameter
"As a guide, these are Ray's calculations for the A65 engine. He began with the knowledge that the exhaust valve opens 50 degrees before BDC. The engine capacity was 700 cc per cylinder and the RPM ran at 2,200. His calculations were as follows:

850 (180 + 50)
L = -------------- - 3 in = 223 cm = 88 inches.
2,200


To find the diameter of pipe, Ray used the following formula:
pi r^2 L = 2 * 700cc
pi r^2 * 223 cm = 1400cc
r = 1.4 cm
Diameter = 2.8 cm or 1.1 in

The nearest standard stainless steel tube available was 1.5 in."

[locknut]

Talk to your exhaust shop. There are exhaust fitters and there are mechanics. They are not the same people.

[stiddy]

I vaugly remember someone in a thread a while back and did all the maths that was said above and for a 4L and they ended up with optimum pipe diametre to be 3.1 inches.