Exhaust design for a pint. (Sprint, sorry, auto correct!)

[Bitsilly]

I have spent a while now trawling the inter web and have realised I am just going around in circles.
I figured if anyone can help they would be here.

My cams are to be 420's and the head will be ported to 32mm exhaust outlets, so it will be a very mildly breathed on engine for the road.
All I ask is to not suffocate the engine and not be too noisy.

So looking at scavenging the gasses out I figured there is no point going larger than 32mm ID for the manifold.
Paul Matty is no longer selling his version.
At 38mm I figured TTR system would be OTT for me.
SJ sell a system based on 35mm OD manifold, which I figured would be a close match to 32mm ID.

Presuming that the SJ manifold and centre pipe is suitable, can anyone tell me if there is any significant difference between the twin back silencers, and the single one. Both available from SJ. I do not know the diameter of the single tail pipe, but I presume it is also 35mm. As there is only one cylinder exhausting at a time I figured that would be sufficient, so I would like to go for the quietest option whichever that may be.

Finally another idea is to swop over the rear silencer to a TTR one which I have read can be quieter.

Thoughts please or perhaps constructive criticisms ??



PS I would prefer stainless even if it is noisier thinner etc.

[Bitsilly]

PPS On a rolling road, my engine spec lost 17 bhp with an old stock exhaust before it was swopped out. I have no idea about the torque, but wouldn't mind losing HP at the top end to get a little more .

[Frogelan]

It would also be useful if you were to give more details about your engine (I'm not the expert either!). As you insist on exhausts I will stick to my approach on this subject alone (manifolds / headers are even more fun) and I am assuming that your Elan has a "fruity" engine.

I had a look at what at the rule of thumb maths of the SAE as I enjoy the nerdy side of this...For my project this gives the following:

The SAE multiplier for the intake size CFM = BHP x 1.5 = 173 bhp x 1.5 = 259.5 CFM. The intake obviously conditions the exhaust...

For the exhaust CFM, SAE suggest peak BHP x 2.2 = 380. You then need to looked up the SAE table and this corresponds to a 57mm (2.25”) tail pipe. Reassuringly, this concords with most dealers' offers.

The exhaust is not the same part as the manifold / headers and I could not find the SAE doctrine. Instead, I looked at "Four Stroke Performance Tuning” by Graham Bell which is the only book that I have to hand on the subject (an old book picked up from a second hand shop!).

Bell indicates that the subject is more complex and the write up needs very careful reading. I have adopted Bell's proposals, but have not yet tested them yet. As mentioned above, without more data on your cams and objectives...most of the engineering folks here will probably wish to have more detail...

[Bitsilly]

Thanks for your efforts and taking the time to have a think.
I would guess the engine would have about 145 bhp,
Cams are 420’s
Head is gently ported to 32mm exhaust. No idea about intakes.
I will also have a slightly larger air box.
Sorry if presuming 175 bhp has mucked things up?