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  #16 (permalink)  
Old 06-25-2008, 06:20 PM
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Quote:
Originally Posted by 4 Jaw Chuck
I have seen well designed single exhaust headered cars make more power than a dual setup, particularly on turbo'ed/supercharged cars. Many dirt track cars run single exhaust setups with adjustable restriction mufflers for this reason.

The reason...exhaust pulsing is symetrical in a single exhaust and hence tuning for air fuel ratios from cylinder to cylinder is simplified.

It is correct that an engine is just an air pump...but it is an air pump with a controlled series of explosions that have to occur at the correct time individually and with the added complication of individual cyl plumbing differences. Single exhaust systems simplifies the system and allows for added tuning by changing relative lengths of cyl plumbing to be changed for specific circumstances.

Look at your firing order and you will see separated dual exhausts systems do not have symetrical exhaust pulses and one cylinder gets the short stick. Balance pipes added to dual systems improve the situation but on pressurized induction systems where mismatches on fuel air mixture can spell disaster, from cyl to cyl it is easier to tune a single exhaust engine at the edge of detonation.

Now you know why the guys with dual turbos have trouble getting a good balance from side to side in fuel/air mixtures and typically have to tune for the lean side. Usually adding a single exhaust outlet evens out the problem and allows the tuner to fine adjust the system to the ragged edge of detonation better than a dual exhaust setup.

Mainly dual exhaust systems just look cooler from a rodding standpoint, they don't necessarily add more power than a well designed single exhaust system.

BTW backpressure is something that cannot be dismissed as not necessary or desirable, some back pressure is needed to build low end torque at low speeds where cam overlap will bleed cylinder pressure. If all your looking for is a peak figure then a open exhaust will always outperform a closed exhaust system, however as most experienced rodders know its the area under the curve that counts and that means from idle to peak rpm. If adding a little restriction to the exhaust adds low rpm power with little loss up top then you have a net gain which adds up on the chronometer at the end of the strip.

Just something to think about.
Thus the advent of H or X-pipes, do not mean to step on toes but, a well designed dual with a balancing pipe will offer a good scavenging effects from the use of dual smaller pipes and balance pipe, and still have more volume to flow more at higher rpm.

You don't see many cars running 7's or 8's through a single exhaust system, or a dual without a balance pipe.

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  #17 (permalink)  
Old 06-26-2008, 12:24 AM
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Well technically a H or X piped car is a 2 into 1 into 2 exhaust system...would a single exhaust of the same flow rating as the two pipes after the X or H change the HP output?

Think about it.
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Old 06-26-2008, 06:41 PM
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It might not change the horsepower output at all, but any dork knows that the exhaust velocity will be higher in the dual exhaust, making for a torque curve that comes on sooner.

The same reason multi-valve heads are common place in modern engines, 2 small valves flow the exact same amount of air as compared to one large one, only the two valves flow that volume at a higher velocity.

Last edited by stroker_SS; 06-26-2008 at 06:47 PM.
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Old 06-27-2008, 09:26 PM
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Well,

Four valves heads get more flow area in the cylinder as compared to a two valve system, velocity is not the reason why four are better than two.

Which brings up a good point, if two exhaust valves flow better than one, why aren't they separate into the exhaust pipe or header? If separating the exhaust streams keeps velocity higher then this should hold true and all engines would be built with 8 mini pipes leading to the muffler. Truth is it was tried way back in the old days and port friction actually reduces the flow rate due to boundry wall restriction.

Fact is a single pipe flows better than a dual pipe of the same area due to boundry wall effects, most engineering books can illustrate that fact. If you look at the calculations for piping in any fluid system you will see that a single pipe of X area needs two pipes of larger area to flow the same amount of fluid. This is why you don't see a cluster of smaller pipes flowing liquids or gases in an industrial plant when one large pipe does the job better, its also cheaper!

Fluid dynamics are complicated by many factors and flow area is affected by boundary wall restriction more than one would guess, velocity does influence the equation but it favors the single larger pipe.

Don't confuse what you see at the drag strip where the operating range of the engine is from 5000-8000 rpm and dual headers dumping to atmosphere is totally acceptable from a operational standpoint since there is no need to generate torque lower than 5000 rpm.

Fact is we all live in the real world and most hotrods would do just as well with a larger single exhaust than two, but who doesn't like the look of two big pipes coming out the back of the car as opposed to one? It became a marketing gimmick way back in the 50's because of the look and persists to this day.

I guess what I am saying is there no engineering reason why a dual exhaust is better than a similar flowing single exhaust and there are many reasons why a single exhaust is better from an operational standpoint. Heck even I prefer the look of dual exhausts and the sound too!

But I don't fool myself to think that the flat black painted sleeper with the big block and 3" single exhaust is some slow poke because its walkin softly and carrying a big stick, usually the driver of such a car is some old dude like me who knows that the young guns are looking at him and thinking I can take him cause the car looks slow with that single exhaust...hehe!

Don't let marketing hype and fashionable dual tips fool ya grasshopper, there is always an old timer out there with a few tricks up his sleeve. Now you know one of them.

One day I'll have to tell ya about how we stuffed two mufflers in parallel onto a single exhaust 440 big block just so we could get the same flow rate without making a lot of noise, then again maybe not...I might get thrown out of the old timer club for giving away too many secrets.
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Old 06-27-2008, 09:33 PM
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Welcome back Chucky!
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Old 06-27-2008, 10:27 PM
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Old 06-27-2008, 10:57 PM
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4 Valve heads are for higher RPM capability, not flow capability as Chuck said. Lighter valves, retainers, locks, springs makes um sing. Valve float control.
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