When I replaced my cats this I disassembled the exhaust for a bit of flow testing. The
standard 1991 V8 exhaust on my car pulls apart in four main sections;
1. Dual 2.5 inch diam. cats (only 10 inches long).
Mass = 3.1 kg each
2. Dual engine pipes going into shared resonator. Engine pipes reduce from 2.5 to 2 inch
diameter, then combine into single 2.5 inch diameter pipe.
Mass = 7.9 kg
3. Middle mufflers. 2.5 inch diameter single pipe splits into dual 1.75 inch diameter
pipes. Is crushed down to 1.5 inches to clear rear suspension.
Mass = 9.5 kg
4. Rear mufflers. Pipe diameter = 2 inches.
Mass = 6.1 kg each
Total mass = 3.1 + 3.1 + 7.9 +9.5 + 6.1 + 6.1 = 35.8 kg (80lbs?)
I flow tested one cat, one muffler and half of the front and middle sections. I blocked of
one side of the dual sections while testing. I also tested a 6 ft long length of 3inch
pipe for a reference. Since I had it all set up I whacked on the air filter box and AFM as
well.
The rear muffler and middle muffler had the highest resistance to flow. Then the cat and
front section.
The 6ft piece of straight through pipe hardly registered on my scale.
The AFM and airbox also flowed heaps better than the exhaust. I was sucking through the
AFM, removing the filter, snorkel and leaving the lid open made no difference (this does
not relate to on road performance). The AFM was where all the flow resistance was coming
from - not the airbox or filter at these low flows (compared to on road flows through the
engine).
So the standard cats flow ok. I will try to get some cats that perform as well or better.
Having them close to the engine manifold means they heat up quickly and reduce emissions
quickly. (Never use unleaded in a lawnmower - the emissions without a cat are worse than
leaded petrol).
Replacing the rear and middle sections of the exhaust look like giving the best bang for
your buck.
Here is a pretty bar graph of the results;
AFM is Air Flow Meter. It is nothing to do with the exhaust at all. It is the aluminium
bit between the air filter and engine. I tested it here to compare with the exhaust only
out of curiosity. I am about to post another even prettier graph to the V8 Intake
Measurement thread!
As for recommendations this is only part of the story. This graph only compares the flow
restrictions of the various components to each other and to a piece of straight through
pipe. It identifies the most restrictive parts of the exhaust. This will help if you
wanted to replace just a part of the exhaust.
I have been quoted an 18kW power increase for a complete stainless system costing $2100.
On a budget perhaps a 2.5 inch mild steel mandrel bent (not press bent) system and rear
straight through stainless steel mufflers to replace the middle and rear sections would be
the best bang for your dollar.
V8 die hard exhaust enthusiasts may be interested in what the V8 Soarer sounds like as
you remove the standard bits of the exhaust – I know I was. So as I dismantled my
standard exhaust I whipped out the old tape recorder and recorded the exhaust in
stereo........
Soarer V8 Exhaust Sounds
97% improvement in back pressure for $179 AUD. That's the Lukey Ultra Flo stainless steel
double off-set straight through muffler. I tested one last night and it looks the
business. Photos and graphs here;
Soarer Rear Muffler
Replacement
It is nearly the same shape as the original and is polished like a mirror. They look
great.
Part number is LSU17272
The first cat replacement I tested wasn't much chop. Didn't look as good as the standard cat, wasn't as big and sure enough it didn't flow as well as the 10 year old one currently on the car (pressure loss of 30 versus 24 for the old cat). I'm not going to fit no "low flow" cat. Time to look for a different one. The cat was labeled CATCO cat, USA, 2.5inch universal 4806 (cato28) stamped 6806 10 00 NAT/CA D280-5 6000.
Next cat I tried was a "High Flow Catalytic Converter". It was a Car Sound brand 2.5 inch part number 98006. The number stamped on it is CA/CE 98006 13505040. It is also made in the USA (www.car-sound.com). This one flowed much better than the first one and a bit better than the standard cats.
Standard cats
The one on the left could be used to replace the originals - it is small enough. For compliance I will fit the one on the left in place of the first resonator. This will give it three cats! Afterwards I will add a second high flow cat and remove the original cats. The larger high flow cat will not fit in the original spot - they will have to go underneath the car instead.
The end of the cat tucked up near the engine - not much room there. Looking in the end of the manifold you can see the oxygen sensor poking through clearly. The internal diameter of the manifold gets real big just before the cat.