Sprint Spec. Exhaust On An S3

[sk178ta]

My totally undamaged from new `67 S3 has just been fitted with a rebuilt big valve engine. I suspect the standard S3 exhaust silencer is less suitable than the later Sprint, or even S4 shape. I would not even think about cutting up the underside of the bodywork to fit the proper Sprint type.....................so..................is there some magic formula that I can present to a one-off exhaust specialist who would make an S3 shaped but Sprint spec. silencer? Alternatively, am I worrying about nothing?
Jim

[type26owner]

Hi Jim,
Can't provide you with any definitive answer but I'm willing to venture a guess. Only if you're most interested in extracting the absolute last bit of horsepower from the engine at WOT at high rpms would the exhaust system backpressure issue become important. There might be a formula though but none that comes to mind. Google using 'cfm' 'exhaust' 'backpressure' 'tuned' and so on..... The other good resource is to ask that question in the Automotive section of the Eng-Tips forum. Lots of the folks that post there are the guys doing it for real for the major manufacturers.

I just replaced my original exhaust system because the baffles inside the muffler canister were rattling around loose and buzzing loudly much to my annoyance. The replacement muffler was stainless steel and it reduced the tailpipe sound level by about 2/3s. After a month or so you get use to the NEW music. What I've grown to appreciate is the induction system is the loudest source of noise. Webers really are nice to listen to and hear them sing. The replacement wasn't exactly the same as the original muffler but one has to make due with what's available these days.

[sk178ta]

Thanks for that Keith. The question was prompted as, at tickover, the revs. rise slightly if the silencer (muffler in U.S. speak!) is removed, suggesting that for a given amount of fuel, the engine is less efficient with it connected. More comments most welcome.
Thanks again,
Jim

[type26owner]

Jim,
The exhaust gasflow dynamics are a rather interesting subject. One which is not well understood nor appreciated. The backpressure and velocity is dominated by the temperature of the gases. Higher temperature means higher pressure and velocity for the most part. The exhaust system is a heat exchanger which extracts the heat very efficiently from the escaping gases. By the time they are disgorged they're slightly above the ambient and the velocity is slowest. So where it's hottest in the manifold the flow is the fastest due to the high pressures. This is why having no leaks at the collector and the head is crucial to tuning the carbies. That's where the pressure is still rather high. Some backpressure is beneficial for various reasons such as scavenging depending on how the engine is being used. Does that make sense for a nutshell description?

If removing the old silencer has that much influence at tickover I'd be mighty suspicious that it's partially plugged up.

My advice is to fix any leaks on the manifold and not sweat the exact sizing of the rest of the exhaust system. Guessing the slight cfm increase of the new engine does not warrant upsizing the exhaust system for regular type driving on the public roads.

Hopefully I've understood your question and have answered it in a way that makes sense to you.