unit91

I read the following concerning dual exhaust:

"Having more outlet for the exhaust generally means less backpressure. A freer flowing exhaust increase horsepower and performance. However, some people put a dual exhaust on for purely cosmetic reasons, be it for looks or sound. Either way, this can actually rob power if it adds length to the exhaust gases exiting from the car. When done properly, dual (or more!) exhaust can makle a definite increase in performance."

Now, what are the benefits, if any, of putting dual exhaust on a 68 200 cu in?

Don't laugh.........I'm serious.

fastbackford351

Instead of going "brrrrrrrrrrrrbb" it'll go "BRRRRRRRRRRRBB"

unit91

So how will that be done? They run one pipe from the engine, then split it into two towards the rear?

TheLockNLoad

Done correctly you will have better breathing and better performance, if you haven't already you should check these guys out:
http://www.cliffordperformance.net/M...gory_Code=F200

67mustang302

Not needed on a 6er, but won't hurt if done right.

Backpressure is a myth. And I really wish it would ****ing stop in all these damned magazines. That was the term that was used "back in the day" to describe certain phenomena that we had no way of measuring or categorizing....and people simply assumed that some amount of residual pressure under certain conditions was providing boosts to power.

That had nothing to do with it. Real backpressure is ALWAYS bad in ANY exhaust system. The phenomena that was called backpressure, is actually a combination of pulse wave tuning and exhaust gas temperatures and velocities that we weren't able to measure until more recently (starting 15-20 years ago). But the data has been out there for years now, and magazine article writers continue to pedal the same crap that was being pedaled 40 years ago (when no one knew any better).

The point of the exhaust stroke is to get exhaust out of the cylinder by flowing it through exhaust pipes. Backpressure is a resistance to flow. So how would a resistance to flow help with increasing flow? Obviously, it doesn't.

Iskwezm

so your saying a (open)exhaust with ZERO backpressure is better then a exhaust (mufflers)with some restiction?

67mustang302

Where did I ever say open exhaust? I didn't. You need a proper exhaust system to scavenge gasses, but a proper exhaust system doesn't mean you need to have backpressure. And a lot of it depends on the setup.

Iskwezm

Explain what "proper" exhaust system is that doesnt have backpressure that isnt "open".

67mustang302

Proper pipe sizing and length controls exhaust gas temperature and pressure, and as a result, velocity. The idea is to keep the gasses moving at the highest possible velocity with the lowest possible pressure, which means keeping the temps up. If the pipes are too large, then as the gasses exit the primaries in the header they have a large area to expand into. So they expand, and as a result they cool down...so the gas density increases and the gasses slow down. Slower gasses with lower temps and higher density cause higher pressures that blowdown and the piston have to push against.

So when the system is sized down to match the engine and rpm range, the gasses don't have as much area to expand into and consequently their temperatures remain high, and velocity stays up and density and pressure stay down. The lower the pressure stays in the entire exhaust system, the greater the pressure differential exists when the exhaust valve opens.

The reality is that the larger exhaust systems (that tend to perform worse) are the ones with backpressure, because there's a pressure/density/temperature effect going on. The large system has slower moving cooler gasses that tend to bottle up in the system more. The properly sized systems (what people think of as smaller) typically perform better not because they have backpressure, but because they don't. The smaller system keeps the gasses moving and maintains a near constant low level pressure drop through the system (or parts of it).

With open headers the gas exits the collector in the the biggest area to expand to, the atmosphere. So there's never a low pressure drop at the outlet or in the system (since it doesn't have a system other than the header itself). That's fine if you are running constantly at high rpm where you're moving large amounts of exhaust gas constantly that needs to be cleared quickly, and can benefit more from pulse tuning.

That's also why as rpm goes up the entire system gets shorter and shorter. Taken to the extreme, look at a Formula 1 exhaust system. It's basically a multistep header with an engineered outlet (18,000rpm engine). They have to be made from Incolnel alloys because the temps and velocities remain so high, that anything else typically used in exhaust systems would melt.

Pretty much every street car runs better with an exhaust system because they don't turn enough rpm to be able to benefit from not having one. And the large systems are the ones that move gasses slowly and have backpressure. It's rather counter intuitive, since you'd think the smaller pipes are more restrictive; but gasses behave in very counter intuitive ways. Length also plays a role, longer systems are more restrictive. Cats can make a difference as well, some modern cars run larger pipes exiting the cat to account for the gas expansion from heat generated during the conversation process, it helps to limit pressure increases from the gasses heating up as the move down the system (which is the opposite of a car without cats, where the gas constantly is cooling as it moves down the system).

Mufflers are a huge problem for street cars though, since many of them function like a cork. Good mufflers usually offer very little resistance, but tend to be louder. So the problem is fitting a muffler that reduces sound enough without compromising performance too much.

Too small and you lose power, too big and you lose power. This link gets more into the pulse wave tuning aspect of modern high performance exhausts, but has a lot of good info. http://www.epi-eng.com/piston_engine...technology.htm

Al Newman

All that being said, in answer to your question about dual exhaust systems on your six cylinder, you will notice a change in sound, and you will THINK you have more power, but whatever power change you experience will largely be a figment of your imagination. If you want the flabby sound of a six cylinder with dual exhausts, then go for it; otherwise fagitaboudit.
Best,
Al