Fiat 2000TC exhaust system design queries

[Slawo2000]

Hello Guy

I have some questions regarding exhaust system for 2000TC engine. I've decided to design it myself with 3D software. As space in engine bay in my car is quite limited it's not an easy task, however I managed to get following lengths for primaries:
1. 558.0 mm
2. 553.6 mm
3. 556.6 mm
4. 548.9 mm

I didn't work yet with secondaries, but I expect this to be a little easier.
My question is: how much length deviation between primary pipes can we tolerate? In my case it's 9.1 mm difference between first and fourth. Is it acceptable? How much deviation can we tolerate for secondaries? Are they less "sensitive"?
I have attached a few pictures of how my exhaust manifold would look like. Parameters would be following:
- primaries ID 38-40 mm, length around 560 mm
- secondaries ID 44-46 mm, length around 300 mm
- tailpipe ID 54-56 mm
Engine spec: CR10.8, valves 45/38, DCOE45 chokes 36 mm, inlet cam GC3A, forged pistons.

Kind regards
Slawo

[Guy Croft]

MODEL POST!


Your deviance on length won't do any harm, in fact if you've measured the chords right (see dwg) it's pretty good! The lengths and ratio py to sy are fine.

A very nicely presented dwg by the way, makes a big difference. I have annotated it with some suggestions on radii, I can foresee considerable flwo loss on some of the bends, suggest you take a close look at improving the chord rad/pipe dia ratio.

The secondaries could do with being bigger on ID, I suggest 1.25 x min primary ID.

Hope this helps,

G

[Slawo2000]

Thank you very much for your reply and suggestions on the drw. I will try to improve curvature, however as I mentioned, due to limited space it will be very difficult.
I have one more question regarding silencer. I would like to use only one (car will be used only in rallies), but again available space allows only for use silencer with max. length 560 mm and max. OD 140 mm. Will that be fine? How important is the volume of silencer?

Thanks
Slawo

[Guy Croft]

On a straight through item like the one below, it actually doesn't matter whether the silencer is oval or cylindrical but to be effective, ie: trap white noise and minimise back - or static- pressure, the volume of the container (which is filled with absorption material like fibreglass) that surrounds the inner perforated sleeve has to be pretty big. The problem with having the distance from sleeve to the wall of the casing too small is likely that the damping is reduced and reflected waves upset the behaviour of the unit. I have seen disastrous results from overly small silencers. The perforation size is critical too, from memory it's 3.1mm and the holes should preferably be punched from gas side to filling side so a sharp edge is generated on the filling side, something few attend to these days I imagine. A single silencer should always be as close to the rear of the car as possible so that it vents right to atmosphere without any tailpipe restriction.

Just based on my own exp I recommend that (ignore the thickness of the metal..) the OD of the cylinder be a minimum of 2.8 x internal diameter of the sleeve, which in the case of a 2.25" ID sleeve (which is what you need) is 6.3" and on an oval item the overall arc length of the casing viewed along the axis of the sleeve should have a ratio of 9 x the ID - or more in both cases.

To be on the safe side it's best to buy silencers that are race proven and which, typically, someone like me recommends. I suggest that rather than making one you get onto George Polley Motorsport at:

http://www.polleymotorsport.co.uk/polley/index.asp

And buy one of their AX891 Hot Rod silencers, they are very keenly priced at 15.5" x 8" x 4", see below.

If anyone knows of a more, er, 'scientific' approach to silencer design, I'm 'all ears'!

Hope this helps,

G