Exhaust Primary - Secondary Length and configuration
Discussion
What method are people using to estimate the "best" primary and secondary length for a naturally aspirated engine.
no need to stick to 6-2 configuration, 6-3 and 6-1 would also physically fit
lets say for example its a 6 cylinder 3,152 cc making peak hp 330 at approx 6750 rpm 4V per cylinder, Camshaft is 250*@ 1 mm
the aim is not most peak hp but good area under the curve for the track and being able to come out of corners.
no need to stick to 6-2 configuration, 6-3 and 6-1 would also physically fit
lets say for example its a 6 cylinder 3,152 cc making peak hp 330 at approx 6750 rpm 4V per cylinder, Camshaft is 250*@ 1 mm
the aim is not most peak hp but good area under the curve for the track and being able to come out of corners.
What engine is this? A straight six? BMW?
Straight sixes run as two mirror-imaged threes, with pairs of pistons moving in synchrony, but 360 degrees out of ignition phase: 6&1, 5&2, 4&3.
So they can benefit if the primaries are linked in those pairs, AND the lenghts are equal, AND matched to a particular rpm, at which scavenging is optimal. The reflected pressure wave from the junction arrives at the opposite exhaust valve as a negative wave, assisting extractionb, or scavenging the cylinder.
That length is your choice, and depends on space available. The higher the revs at which you expect resonance, the longer the primary. For 4-6K rpm, your need 400-600mm (16-24").
The pairs of joined primaries can then come together, forming a 6-3-1 arrangement. The secondaries length is more conveninece than critical.
Other arrangments, 6-2-1 can occupy less space, but are ineffective; 6-1 is only of use for very high revs.
John
Straight sixes run as two mirror-imaged threes, with pairs of pistons moving in synchrony, but 360 degrees out of ignition phase: 6&1, 5&2, 4&3.
So they can benefit if the primaries are linked in those pairs, AND the lenghts are equal, AND matched to a particular rpm, at which scavenging is optimal. The reflected pressure wave from the junction arrives at the opposite exhaust valve as a negative wave, assisting extractionb, or scavenging the cylinder.
That length is your choice, and depends on space available. The higher the revs at which you expect resonance, the longer the primary. For 4-6K rpm, your need 400-600mm (16-24").
The pairs of joined primaries can then come together, forming a 6-3-1 arrangement. The secondaries length is more conveninece than critical.
Other arrangments, 6-2-1 can occupy less space, but are ineffective; 6-1 is only of use for very high revs.
John
it was my understanding that benfits of a 6-3 over a 6-2 are quite small and unlikely to be found without signicant testing (something i dont have the stomach for). the reason being stated was the 6-2 is still equal firing and seperated far enough for most camshafts, even though not a full 360.
what ever design produces the highest average torque is ideal
lets assume peak torque say 5000-5500rpm.
what ever design produces the highest average torque is ideal
lets assume peak torque say 5000-5500rpm.
Edited by Inline__engine on Monday 23 April 22:34
Inline__engine said:
I have that software but I'm after other people's experiences, thoughts, opinions, results from testing etc
I built a fairly complex spreadsheet for my 4 cylinder, based on a lot of reading. When I plugged in the stock dimensions, it came up with a tuned RPM within 100 of peak torque. 
The program is largely empirical therefore the test mules do matter . If you speak to Larry it doesn't explicitly differentiate between 6-3 6-2 or 6-1 as that's not until the newer version he's been working on which also works more like a simulator You only can use tri-y somewhat approximate 6-3. I was hoping others had experience trying different things and could comment
I've done my own testing on a full exhaust system (as opposed to open headers) on a different spec Inline engine but haven't been able to test the primary influence ( only collector Length was varied ) . My Findings were that you can't just blindly use the recommended lengths when using a full exhaust system. Some of the best lengths recommended are short but with full exhaust without a large termination box this does not work best. With a termination perhaps the recommended lengths may actually be the best due to acting like open headers but have yet tested this
I've done my own testing on a full exhaust system (as opposed to open headers) on a different spec Inline engine but haven't been able to test the primary influence ( only collector Length was varied ) . My Findings were that you can't just blindly use the recommended lengths when using a full exhaust system. Some of the best lengths recommended are short but with full exhaust without a large termination box this does not work best. With a termination perhaps the recommended lengths may actually be the best due to acting like open headers but have yet tested this
Edited by Inline__engine on Wednesday 25th April 23:41
Edited by Inline__engine on Wednesday 25th April 23:56
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