dsm-onster
DSM Wiseman
- 8,592
- 130
- Jul 11, 2004
-
Bloxom,
Virginia
I don't have a attitude I'm sorry if it comes across that way but not my intension. I simply posted about the exhaust and to show how it looks and how it sounds and so far how it performs. The points you guys are trying to make are understood but I just don't agree sorry. I understand that your trying to educate me but I have modded a FEW cars and understand the diffrence between therory and real life especialy with dsm's. My 2gnt is my first non boosted car in about 2 and a half years(87 toyota supra x2, 90 talon tsi awd) so I have seen atleast 3 exhaust systems and the effect they have on performance. That article as informative as it is is just theory. saying 300hp is pusing it for a 3" exhaust is rediculous. There is a 11.09 sec gs-t runnig on a 3" exhaust with a custom 2.5" dp makin 443whp. Trust me I understand the theory but it's just that theory. I understand you two like to be safe rather than sorry that's why one of you is running a built motor on a 14b setup and the other is aiming the same path except a 16g but i'm not that how I mod. Thanks for the imput though
Perhaps I can give some of my "real life" to back up the theory that 300 hp is pushing it for 3-inch exhaust. Running my small 16gG, I have verified via logging that going from 3 inch to no exhaust does help. And significantly! Look at the lbs/min logged. This is in the 19-22 lb/min range. Nowhere near the 300 hp mark. Yet dropping the 3-inch exhaust made a notable difference. Both logs were done in the same hour on the same day at the same elevation.
This 3-inch exhaust consisted of NO cat and a little nothing of a resonator (glass pack) for a muffler and a resonator tip. 2.5 inch at the o2 housing transitioning before the turn in the downpipe to 3 inch via a smooth cone seen here. All mandrel bent. Great for laminar flow. But, running no exhaust was significantly better.
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Here's the spreadsheet compiling the two logs.
On the matter of diameters and volume. Diameter is the usual determinant of volume of an exhaust system. Considering that most will not alter the exit point on the car or even alter the path the pipe takes to the exit. Thus, when one refers to diameter it can be almost directly proportional to volume in a mandrel bent pipe.
HOWEVER! For the sake of n/a applications AND turbo applications any sudden change causes a negative affect that adds up as more flow (turbo) or more pulses (n/a) are exhibited.
Turbo and exhaust neck downs:
Where you have a neck down you increase velocity in a consistent flow system. A simple look at Poiseuille's Law will give a better understanding. Note the much more drastic affect that radius (diameter) has on the flow versus length. To the power of 4! Thus those small lengths of 2.75 inch piping in your 3-inch exhaust will definitely have an appreciable affect on flow. This is not theory. This has been proven law.
Poiseuille's Law was originally formulated for incompressible fluids. But where a fluid can compress, it will where there are bends and transitions. Since exhaust gases are not "Newtonian fluids", then the small length of diameter has a significant effect AND the act of transitioning has an effect. Using this formula affords the easiest "math" to explain what has been verified by experiment.
But delving deeper into non-Newtonian fluids, we see that we would need to measure the individual momentum vector of each exhaust gas particle as the medium meets a transition or bend or "Y" or end. Where there is reduction, simply the gas has a momentum vector to strike the wall of the reducer pipe. The sharper the transition, the larger the right angle component of the momentum vector is that hits the wall of the transition. This "reflected momentum" causes the gas particle to slow or even travel backwards into the pipe. This "adds up" as the flow increases. So what would minimally affect a 14b powered car, will drastically affect an evo3 16g powered car at their respective flow potentials. The more the flow, the more particles you have doing negative work on the exiting gases. Again, considering that my 3-inch mandrel bent exhaust has bends, there was a significant increase in flow removing the exhaust system all together.
N/A and exhaust neck downs:
N/A exhaust flows in pulses. Where ever there is a transition, "Y", or end some of the pulse is reflected. This causes a rise in pressure. This was noted and postulated by Helmholtz. And all good headers and intake manifolds follow this theory and thus have proven the Helmholtz resonator in the "field". Where you can tune the diameter and length of a pipe for a pulse frequency then you can use that pressure rise and pressure drop to scavenge, or suck, the gases along the pipe. ANY transition or Y will alter that pulse tune length to that point of change. Is the exhaust tuned for your ideal rpm range? Not likely. But, having transitions at random lengths, will cause the reflected pulses to work against each other. NOT GOOD!
Again, turbulence has an affect on pulsed gases in identical fashion as constant flow. This is what creates the Helmholtz effect. One good thing about the Helmholtz principles is that you can tune to harmonics. Thus you can have a consistent diameter tube that can resonate at two or three rpm points. A very long, consistent tube will not have any reflected pulses conflicting w/ each other and will still exhibit good scavenging properties. If you have points of turbulence, you have to be very careful about placement. You have lots of math ahead of you and lots of testing.
Either way, avoiding transitions is ideal. It has been suggested to take out the transitions. If you can modify the exhaust simply, why not? There's no reason to have "big boy" exhaust and not have the potential to get all the flow out of it that 3" diameter would suggest.
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but that's ok I will get that cut off and a 3" flange welded on
