Hey all,

Rescuing my Blue 1200 Coupe from what has been it's home for the last few years....

It's got a stocko A14 in it currently, and it needs an exhaust. I've got some 4-2-1 extractors to put on.

The question then is..Is there an optimum size for the exhaust diameter, this car is for dirt/autocross and some circuit racing.

I imagine engine power will also grow over the next few years and would like to see it use the same exhaust for awhile.

So what diameter do you believe is ideal?

I would suggest you go 2" with/without crush bends depending on budget.

2" is much to big for a stocker but if ther are REAL plans for mods then a 2" will suit anything fine for the future.

a 1 3/4 will be ample big enough for a stocker that will stay that way.

Cool I was thinking 2", I can't imagine that picking an exhaust too big will effect engine performance by much at all.

I can see how a bigger exhaust will result in a slower exhaust gas velocity and the oppuunity to cool more - but don't really understand how much impact that would make.....unless anyone knows better?

I know that for ultra high performace there are all sorts of calculations and specific stuff they worry about, but for a streeter (ie not performance orientated) your main concern I believe is not bieng too small, and total noise levels.

A bigger diameter has the potential to be louder than you would like, possibly attact the wrong attention and possibly drone a little at highways speeds, but changing the muffler will usually fix that without too much drama.

My little thou with its bomber a12 and 2" straight thru and hi-flow muffler did drone and was a little noisy, but that was the intention. I used to do highway trips with earmuffs on and the stereo wound right up!!

Go with a 2" with a quiet muffler. Have them put flanges in so it can be changed out if you want more noise.

Just my opinion, and im no exhaust professional.

I read in some book, think it wAs performance 21st cent cars or similar if on a tight budget and can't afford mandrel bends, go up 1/4 inch in size press bends for equivalent flow of mandrel.
That being said 2 inch was fine on a15 but went 2.25 on l16, it was louder too

David Vizard says it makes a significant difference.

Quote:


More interesting is about the tuned-header and collector lengths:
Quote:

He recommends a sized extractor and a resonator box at the correct length. Then after the resonator it doesn't matter how long the exhaust pipe is or what muffler type is used (open chamber or glass pack). Without a resonator the type of muffler makes a huge difference on horsepower.

2" pipe is good for 100HP. More than most A14s are likely to make.

Quote:

Interesting - I wonder what the correct length would be for an A14....

Not disputing any exhaust professional but,
Remember that when it comes to race engines its a whole different ball game.

Tuned length stuff is for specific rev ranges, where the car will spend 90% of its time.

I believe for a streeter its not so important as the engine is required to preform throughout the rev range.

I reckon that the difference between a fancy tuned length set vs an off the shelf non-everything set of extractors would be un-noticable on anything but a very accurate dyno. and even then you couldnt rule out all other variables.
Conmpared to the std exhaust manifold, i would expect any set of extractors and new exhaust to preform better.

Considering that it seems that a slighly undersize system will produce better torque, (more important in a streeter than outright HP), then go no larger than a 1 & 3/4.


On a side note, I found out that 'smaller is better' is not true for diesels.
I fitted a twin 2&1/4 system for my diesel and its suffocated despite the exhaust manifolds bieng only 2" outlet. I should have gone to 3" twins.

Exactly true. He says for racing, don't worry about equal length extractors, it's a waste of time.

For a downdraft, don't mess with the length.

For an engine with twin DCOE/Mikuni/Dellorto, you have ITB - individual throttle bores - like a racing engine. So you can pick an appropriate length from the collector. This makes a huge difference on high overlap camshafts and not just a "dyno difference".

There is no "correct length" for an A14. It depends on how you build the engine. What cam do you have? Where do you want the max HP to occur? Via the length you can shift the power band somewhat.

Most of us ignore this advice and just fit an off-the-shelf exhaust setup. It takes some experimentation of dyno tuning to do it right.

But for exhaust size, the answer is pretty clear: 1.75" for mildly built A14. 2" for built A14. Larger for dedicated racing A14.

I would stick with a 2" pipe.

As an example to what a 2" exhaust could sound like on your engine with a few mods, Here is a video of my old sedan and engine.


http://www.youtube.com/watch?v=TWNlRmp6NOk

Exhaust system: Off the shelf Coby extractors, 2" mandrel bent system and one chamberd sports muffler