Exhaust collector venturi function/advantage ??

[edjza80]

Just wondering if anyone could shed some light on the advantages of having a venturi type reduction/expansion incorprated into the design of an exhaust collector, eg:

secondarily, what should the subsequent exhasut pipe look like? ie, if the collector reduces to 3", then expands to 3.5", should the exhast then taper back to 3" thus generating a chamber, or should things remain at 3.5"

finally, does this venturi type of collector effect the calculated harmonic length of the primaries which feed the collector?

cheers
ed

 

[patprimmer]

I am not an expert in this regard, but as I understand it, a merged collector angles all the pipes together in a cone shaped formation and the inside also has a modified cone shaped projection to blend the individual pipes into the collector with constant cross sectional area so as to maintain air speed at the transition from the individual pipes to the collector. the pulses in each pipe will be maintained, but the pulses in the collector will be greatly reduced giving steady state airflow in the collector and tail pipe and muffler if fitted.

pulsing in primary pipes allows for improved exhaust scavenging of the combustion chamber at TDC overlap, but steady state flow in the tailpipe reduces back pressure.

Regards

eng-tips, by professional engineers for professional engineers
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[GregLocock]

Oddly I was just experimenting with a time domain model of a tapered junction last night, compared with a stepped junction. It tends to broaden the response out but I do not if this is by a significant amount, in the context of valve timing. The effect is to smear out the return pulse over an extra time period of 2*transition length/speed of sound compared with a stepped tube.

I'm sure that in practice the benefit of a smooth transition is of greater significance.


Cheers

Greg Locock

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[edjza80]

ok,

so im guessing that the 'bottle neck' in the collector at the point where the primaries merge is simply a consequence of the angle (and size) at which the primaries were collected? with the subsequent re-expasnion from this 'bottle neck' to x final dia then just a consequence of an expansion to the desired exhaust size over a gradual transition?

i was perhaps over thinking the design. i thought that the primaries were brought together deliberately tight, with a subsequent collector expansion as some kind of active feature of the collector. ie that the venturi shape was a design feature, rather than a design consequence

 

[o00otii]

It's not a design consequence, it is the core aspect of a merge collector. The venturi principle of the collector does indeed increase scavenging as the increased speed of flow through the venturi portion tends to "pull" the exhaust gases out as it then expansion into the larger section which effectively working as an anti-reversionary system. the larger section also reduces backpressure, typical of normal exhaust systems. they are more applical to high performance engines where the greater airflow would have a greater effect. Also, with a merge collector, the sizes of the venturi, primaries, etc. require more tayloring to specific engine combos than would a normal header collector.

burns stainless makes some nice ones.

Joe

 

[Viper488]

How important is the rotational order of primaries entering a collector? Meaning having primaries arranged in the collector in order of their firing order to yield maximum benefit from an adjacent primary's ability to assist it's neighbor.

As opposed to having the across the collector from each other.