Rear Mounted vs. Front: pros and cons
08-13-2012, 08:21 PM
#41
0.0 BAR
Join Date: Aug 2012
Location: alberta
Posts: 7
Harmonic tuning...
It basically works like this... if a positive pressure pulse is sent down a tube, the moment it reaches the end of the tube, a negative pulse of almost equal amplitude, get's sent back up the tube...at more or less the speed of sound. Heat, gas pressure, velocity, etc etc etc, plays a role in this speed, complicated, not going to go there...
now calculate how far the pulse will travel at more or less the speed of sound, in the time it will take the exhaust valve to close again. Devide the distance in 2 and you have the length of each exhaust runner for the engine to be tuned at that RPM.
don't worry about the positive pulses, they still carry on after going through the collector.
exactly the same for the inlet side, there it's just a negative suction pulse getting sent out from the valves, through the inlet runners, ITB's and when it gets to the mouth of the ram-tubes, a positive pulse goes back in, getting to the valves just as they are about to close... ramming that extra little bit of fuel-charge in past the valves before they seal shut....
then you can start to play with cones, like in the exhaust of a 2-stroke engine... Now you can stretch that rpm range out, giving you a much more manageable power-band, so to speak.
it is this harmonic tuning, that requires a NA exhaust manifold to be equal length, so that the pulses are evenly spaced at their tuned RPM range.
It basically works like this... if a positive pressure pulse is sent down a tube, the moment it reaches the end of the tube, a negative pulse of almost equal amplitude, get's sent back up the tube...at more or less the speed of sound. Heat, gas pressure, velocity, etc etc etc, plays a role in this speed, complicated, not going to go there...
now calculate how far the pulse will travel at more or less the speed of sound, in the time it will take the exhaust valve to close again. Devide the distance in 2 and you have the length of each exhaust runner for the engine to be tuned at that RPM.
don't worry about the positive pulses, they still carry on after going through the collector.
exactly the same for the inlet side, there it's just a negative suction pulse getting sent out from the valves, through the inlet runners, ITB's and when it gets to the mouth of the ram-tubes, a positive pulse goes back in, getting to the valves just as they are about to close... ramming that extra little bit of fuel-charge in past the valves before they seal shut....
then you can start to play with cones, like in the exhaust of a 2-stroke engine... Now you can stretch that rpm range out, giving you a much more manageable power-band, so to speak.
it is this harmonic tuning, that requires a NA exhaust manifold to be equal length, so that the pulses are evenly spaced at their tuned RPM range.
08-14-2012, 04:25 AM
#42
0.5 BAR
Join Date: Jun 2006
Posts: 189
No no... if the pulse hits a flat surface, it gets reflected back, still as a positive pulse... for port scavenging on the exhaust side, you do not want a positive pulse to be coming back while the port is still open, as that would actually hinder the gasses from being pushed out.
08-14-2012, 09:39 AM
#43
0.0 BAR
Join Date: May 2012
Location: Knoxville, TN
Posts: 27
This discussion on Miata.net has a little bit of helpful info mixed in with all of the spastic crisis remarks.
What are downsides to Rear Mount - MX-5 Miata Forum
To summarize what they were trying to tell me at the end, the cooler the boosted air is, the denser it is. 15psi of cooler air is better than 15psi of hotter air because there is more air in denser air. This, of course has nothing to do with the front vs. rear mount debate but the majority of folks contributing to that thread knew how to read instructions, install a kit and back up whatever their turbo hero's had to say. Then Corky chimed in and slapped their pee-pee to some extent.
What are downsides to Rear Mount - MX-5 Miata Forum
To summarize what they were trying to tell me at the end, the cooler the boosted air is, the denser it is. 15psi of cooler air is better than 15psi of hotter air because there is more air in denser air. This, of course has nothing to do with the front vs. rear mount debate but the majority of folks contributing to that thread knew how to read instructions, install a kit and back up whatever their turbo hero's had to say. Then Corky chimed in and slapped their pee-pee to some extent.
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