why do some rotaries go "whap whap whap"?
07-28-08, 08:10 AM
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why do some rotaries go "whap whap whap"?
I was running 2 open primaries of approximately equal length for a while. Since I have added the turbo, I now have made a new exhaust for my S5. The turbo is positioned sideways in FRONT of the motor. The rear primary is about 15 inches long, and the front primary is only about 6 inches long (not great I know, but space was an issue). There is obviously a 2-1 collector and another larger length of pipe about 10 inches long that goes to the turbo inlet. The exhaust is only tack welded together; it is not air tight yet. I also do not have an exhaust gasket on the motor yet (I’m still in mock up phase if you couldn't tell).
Anyway, on to the question. Before when I had my 2 open primaries, the engine seemed to idle pretty typically, a nice consistent droning noise around 800 rpm. Now that I have my new header on the motor, the idle seems to have developed a sort of "oscillation" to it. I have seen videos of rotaries that seem to make this noise. As the title says, one could interoperate it as a "whap whap whap" sound, with a "whap" occurring about every half a second. My first guess was this was reversion waves periodically syncing up to the position of the engine, and once every few exhaust strokes causing a bunch of exhaust gas to back up into the chamber, causing a weak power stroke next time around. I however did measure the temperatures of the primaries. I can't let the engine get TOO hot just yet, but I recorded a temperature of around 400*F on the front primary, and 300*F on the rear. Keep in mind that the rear does have a lot more surface area exposed to ambient air. I checked my spark by disconnecting each plug one at a time while the engine was running. when I pulled out each of the plugs the engine bogged down a bit and made a very different sound, therefore I concluded that I am in fact running on both rotors, though perhaps the rear is not running as well (mis-firing?). I have both primary injectors hooked to the same driver, and I have checked the resistance on both. Basically, if one injector is opening, the other will undoubtedly be opening too, so I don't think it’s a fuel issue. I might try to put some thermal couple inside each primary about 3 or 4 cm from the exhaust ports so I can get a more accurate reading of EGT, but in the mean time, does anyone have any ideas? I’d like to think that the “whap whap whap” is typical, but I am getting a weird feeling, what with the 100 degree difference in surface temps of the primaries. Any advice would be appreciated.
Anyway, on to the question. Before when I had my 2 open primaries, the engine seemed to idle pretty typically, a nice consistent droning noise around 800 rpm. Now that I have my new header on the motor, the idle seems to have developed a sort of "oscillation" to it. I have seen videos of rotaries that seem to make this noise. As the title says, one could interoperate it as a "whap whap whap" sound, with a "whap" occurring about every half a second. My first guess was this was reversion waves periodically syncing up to the position of the engine, and once every few exhaust strokes causing a bunch of exhaust gas to back up into the chamber, causing a weak power stroke next time around. I however did measure the temperatures of the primaries. I can't let the engine get TOO hot just yet, but I recorded a temperature of around 400*F on the front primary, and 300*F on the rear. Keep in mind that the rear does have a lot more surface area exposed to ambient air. I checked my spark by disconnecting each plug one at a time while the engine was running. when I pulled out each of the plugs the engine bogged down a bit and made a very different sound, therefore I concluded that I am in fact running on both rotors, though perhaps the rear is not running as well (mis-firing?). I have both primary injectors hooked to the same driver, and I have checked the resistance on both. Basically, if one injector is opening, the other will undoubtedly be opening too, so I don't think it’s a fuel issue. I might try to put some thermal couple inside each primary about 3 or 4 cm from the exhaust ports so I can get a more accurate reading of EGT, but in the mean time, does anyone have any ideas? I’d like to think that the “whap whap whap” is typical, but I am getting a weird feeling, what with the 100 degree difference in surface temps of the primaries. Any advice would be appreciated.
07-28-08, 09:05 AM
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I've been doing some research and am starting to see the problems I might encounter with my drastically different primary lengths. Again, i'm not sure that the unequal length is what is couasing this idle charecteristic, but it seems like it will create probelms at some point. I'm going to try and think of a way to redesign the header to make the pipes equal length, but it will be challenging if not impossible to still make it fit in the car. I am thinking of installing "anti-reversion cones" in attempt to completely nullify any scavenging effects on both ports. This will not be good for peak power, but at least both rotors will have the same filling efficiency at all rpms, making tuning a lot simpler.
Does that strategy sound reasonable?
Does that strategy sound reasonable?
07-29-08, 08:08 PM
#4
Savanna Rx-7
accepting the following as guidlines:
well tuned turbo header is 14~18" in length of equal length made with 1 7/8~2" pipeing
other than spool charicteristics/maximum output the unequal length short header will make no difference in relation to a mildly tuned car (sub 450 RWHP) when you get above 450 or 500 RWHP then I have no experience.
Think about it; its no different than the greddy or hks cast manifolds, they are short unequal length on the greddy you have one runner thats about 6' long and one thats about 3" long with the HKS one is aobut 9" long and one is about 5~6" long.
If you are talking a lot longer than 18" I have no clue, many N/A sytems run extrememly long primaries almost to the rear axle in many cases. Extremly long turbo manifolds are rare and I have not personally used any so I could not elvauate their strengths or weaknesses .
Kenn
well tuned turbo header is 14~18" in length of equal length made with 1 7/8~2" pipeing
other than spool charicteristics/maximum output the unequal length short header will make no difference in relation to a mildly tuned car (sub 450 RWHP) when you get above 450 or 500 RWHP then I have no experience.
Think about it; its no different than the greddy or hks cast manifolds, they are short unequal length on the greddy you have one runner thats about 6' long and one thats about 3" long with the HKS one is aobut 9" long and one is about 5~6" long.
If you are talking a lot longer than 18" I have no clue, many N/A sytems run extrememly long primaries almost to the rear axle in many cases. Extremly long turbo manifolds are rare and I have not personally used any so I could not elvauate their strengths or weaknesses .
Kenn
07-30-08, 10:09 AM
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Kenn"
perhaps the videos i have seen of rotaries that sound like "whap whap whap" had headers as you are describing...
none the less I broke the welds connecting my primaries to my collector last night. i made a wood jig that allows me to screw down my exhaust flange/primaries, as well as my turbine housing where they will be with respect to each other. I took some measurements and placed an order at a local aircraft pipe supply store. I will give another update when i have everything cut and tacked in place...i'll try to get some pictures of 'before' and 'after' with a sound clip.
perhaps the videos i have seen of rotaries that sound like "whap whap whap" had headers as you are describing...
none the less I broke the welds connecting my primaries to my collector last night. i made a wood jig that allows me to screw down my exhaust flange/primaries, as well as my turbine housing where they will be with respect to each other. I took some measurements and placed an order at a local aircraft pipe supply store. I will give another update when i have everything cut and tacked in place...i'll try to get some pictures of 'before' and 'after' with a sound clip.
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