I am looking to replace the exhaust on my 1999 225 TTQ with something that has a nice deep note, is quiet and of course no drone. I know that many of the after market manufacturers offer 3" systems, with a small number sticking with 2.5". I must admit 3" sounds a little on the large side for a 1.8 litre engine fitted with one of the smallest hair dryers on the planet.
Bigger isn't always better, and manufacturers of the 3" after market systems acknowledge low down torque loss by printing things like (a direct quote).... "Midrange and top end power increases dramatically while almost no low end power loss is felt". I spend plenty of time in traffic where low down torque is nice. I don't spend much time bouncing off the rev limiter, so driveability at the expense of a couple of horses is fine.
Unable to decide between the two sizes, I decided to go back to my old friend mathematics to see what the numbers show before I go ahead and order the components. The results may surprise some of you, and I am sure that some will not agree. I am not trying to start an argument or detract from anyone's product, just looking for the optimal result, and thought that someone else might be interested.
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Many years ago in the 70's, a dude called David Vizard did a whole lot of work around engine efficiency and horsepower using a flow bench and a dyno. He came up with many good ideas including anti-reversion cones, and the turbo muffler (yes, he invented it!) He also came up with some very useful rules of thumb for inlet and exhaust design. Google him and you will find some excellent reading on max hp and efficiency. Although his later work was with V8's, his early work was with british 4 cylinder engines.
One of his rules, 2.2cfm/hp, calculates the exhaust volume in CFM an exhaust is required to flow for a given hp output, where the hp is measured (or estimated) with zero back pressure.
For my TT Quattro, I estimate 240hp with zero back pressure on a good day, so the required CFM for my exhaust is 528CFM
Another rule of thumb he discovered was the volume a length of pipe can flow, based on it's cross sectional area. At 10.5in of mercury, a length of pipe will flow 115 CFM per sq in. Relating this back to my TT, the factory downpipe has two 47mm OD pipes entering the cats.
Downpipe size = 2 x 47mm OD, so the flow rate = 534CFM. We know the downpipe has a number of bends in it, so it will flow less than straight lengths of pipe. You can see however that the two 47mm OD pipes are pretty well matched to the 528CFM required to flow 240hp (I know, I am ignoring turbulence generated where it splits into 2 at the top of the downpipe)
Now to look at the required pipe diameter for the rest of the system:
2.50" tube flows 507CFM
2.75" tube flows 620CFM (pity you can't get this size!)
3.00" tube flows 744CFM
There are numerous factors that do have an effect....gas temperature is a big one. For every litre of ambient temp air that is drawn into an engine, over 4 litres of gas is expelled through the exhaust port. As this gas cools through the exhaust system, it reduces in volume, which reduces the effective back pressure.
All this number crunching has me thinking I could keep my existing downpipe, merge it into a Magnaflow single metallic catalytic converter (#59926 - 600CFM), exit the cat with an almost straight length of 2.5" pipe (I don't need a bend behind the cat as I don't have a post cat sensor) down to a 2.5" Magnaflow straight through resonator (#10426), and on to a Magnaflow Camaro type 2.5" in & 2 x 2.5" out muffler (#12265). I have a couple of Borla 4.5" tips that will finish off back end nicely. As you can probably guess, I like Magnaflow products.
Pipework will be aluminiumized steel or 409 stainless with mandrel bends.
I would appreciate any constructive comments before I go ahead and order the components.
By the way, I have a Modshack VTDA and a Powergasket fitted, and don't intend to do much more.
Note: all pipe calculations assume:
- 16 gauge pipe ( ID = OD - 3.3mm)
- 100% volumetric efficiency
