Power Cones

[tbishop11]

So today I heard about a device called a power cone. They fit in the muffler pipes right at the exhaust port and create the back pressure that your engine needs if your running straight pipes so your valves don't get jacked...
So please XS11 members does anybody use these, how do they work? And if they do, where do I get them, and what size? Please I'll be honest I didn't know anything about them till today and still don't know much! Please give me your thoughts on this one.
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[Steptoe]

power cones

just hit up power cones on google, seems these guys use them on their pocket bikes. they spin them up on a lathe

[oseaghdha]

Supposed to reduce reversion, I would imagine.

Valve "jacking" happens when you have a massive exhaust leak right at the head. Unless your straight pipe is 1 inch long, not to worry.

[tbishop11]

Power Cones

It sounded a little gimmickie, thanks for the help The Harley guys swear by them, I guess that should have been my first clue?
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[Ivan]

I still dont get the whole backpressure thing on a four stroke. Dont the exhaust valves negate the need for it? On a twostroke the backpressure (or shockwave) is tuned to increase cylinder scavenging and loading. Less residual exhaust and more fuel air mix is more power. It seems to me that on a four stroke, there is only so much time per stroke to move that exhaust out of the cylinder, and that backpressure would diminish the effect of valve opening overlap.

If backpressure on a 4 stroke is good, why do 3000 hp funny cars use straight pipes?

[cywelchjr]

I still dont get the whole backpressure thing on a four stroke. Dont the exhaust valves negate the need for it? On a twostroke the backpressure (or shockwave) is tuned to increase cylinder scavenging and loading. Less residual exhaust and more fuel air mix is more power. It seems to me that on a four stroke, there is only so much time per stroke to move that exhaust out of the cylinder, and that backpressure would diminish the effect of valve opening overlap.

If backpressure on a 4 stroke is good, why do 3000 hp funny cars use straight pipes?

Those straight pipes are tuned to give good scavenging. As I understand it, it's all about having the RIGHT amount of back pressure with the right size pipes to cause that same scavending pulse. IIRC the backpressure slows the pulse down so it's at the right point in the stroke cycle to help things move well.

[oseaghdha]

It's about velocity

Straight pipes work fine, but only at a specific RPM. Since the XS has no intake mainifold, using separate exhaust pipes makes it an Independent Runner system, which generally work really well only at tight RPM ranges. But the intake and exhaust have to be tuned together by diameter and length.

The "Power Cones" have some bit of truth to them, but they must be a part of a tuned system. There is a reverse exhaust pulse before the valve closes, and a reverse pulse from the closing of the intake. That one travels back thru the carburetor. See TC's video of his unfiltered carbs running.

Ivan is right about two strokes needing a system that plays games with echoes and velocity, but 4 strokes are also affected by the same dynamics. Anytime there is a velocity change, a pulse will be created. At the end of the pipe, the velocity drops (Small diameter equals high velocity, high velocity equals low pressure per Bernoulli) this creates a positive reverse pules that travels back towards the valve. So playing with the lengths and diameter will change the timing of these pulses.

With 4-2 pipes, or 4-1, the other cylinders provide pulses to help pull the exhaust out by creating low presure waves at the right time. That is what's behind the Jardine Spaghetti pipes design. For an extreme example look up a 180 degree exhaust system on a NASCAR motor. That's why they sound funny compared to street V-8s.

Manifolding the exhaust tends to dampen and even out all this pulsing.
Any exhaust sytem will compromise some rpm ranges to benefit others.

[crazy steve]

...The Harley guys swear by them, I guess that should have been my first clue?

'Power Cones' are generally a crutch used to correct problems in the exhaust design. Yes, they're popular with the V-Twin crowd (and not just the HD guys) as these guys usually buy an exhaust system based on sound and/or looks, not performance. These are used to restore low-end power. You won't find them on a race engine because all-out power is the goal here, and these will actually reduce that slightly.

'78E original owner

[DGXSER]

Note on back pressure, I am no expert, but I knwo for a FACT that on a 1988 Toyota Corrola, if it does not have the exhuast hooked up and sealed well, it will lose ALOT of power and run like CRAP. Fix the exhaust and nothing else, and back to normal (which is till not THAT much more power, it is a Corrola after all).

[Ivan]

Note on back pressure, I am no expert, but I knwo for a FACT that on a 1988 Toyota Corrola, if it does not have the exhuast hooked up and sealed well, it will lose ALOT of power and run like CRAP. Fix the exhaust and nothing else, and back to normal (which is till not THAT much more power, it is a Corrola after all).

88 would be an OBD I car. Oxygen sensor in the exhaust. No oxygen sensor feedback, or worse yet really lean signal would make it run like crap. That is not a back pressure issue.

[tbishop11]

Power Cones

I love being a member of this forum, if I wasn't I probably would have given the cones a try and it sounds like it would have been a waste, I'll stick to jetting the Ol' girl, setting off car alarms should be fun too with my two into one straight pipes!
Thanks Everybody!
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