Ed,
It took a while, but my copy of Street Rotary - How to Build Maximum Horsepower & Reliability into Mazda's 12A, 13B & Renesis Engines finally arrived yesterday. Thanks for the recommendation. Lots of good info, everyone on the list ought to order a copy and read it from cover to cover, except for Lynn H. - he ought to write his own book on rotary engines. (I'd buy the first copy.)
As you suggested, I've been reading the chapter on exhaust systems. I found a paragraph that is right on target relating to what we've been discussing (exhaust system restriction). They compare a 2-rotor wankel to a 2 cylinder 4-stroke where both cylinders share the same exhaust port.
Quote, "...the exhaust system on a 12A or 13B rotary engine is roughly analogous to a two-cylinder piston engine in which both cylinders are served by a single exhaust port. If cylinder #1 was in overlap period, and the exhaust valve of the #2 cylinder then opened, high pressure gas would flow from the #2 to #1 cylinders. A highly restrictive exhaust system would aggravate the situation. This, the authors point out, is the major reason why a free-flowing exhaust system is so important in a rotary engine." This explains why I saw such an improvement when switching exhaust systems.
There is also much discussion on primary header length. Disregarding the "long" header system as we don't have the room, the "short" header length shown for a p-port engine is between 10 and 18 inches. So, your calculations for header length seem to be right in the ballpark. Now I have to figure how to get the three very short primary tubes of 11 3/4" to meet on the same tangent at the collector.
Finally, I liked their comment regarding the peripheral ported engines. It reads, "Traditionally relegated to speciality race cars, occasionally a peripherally ported engine finds its way onto the street in some high-performance vehicle. These engines are not, however, for the faint of heart." LOL Not to worry, my heart is strong!
Mark
On Sun, Nov 21, 2010 at 7:53 AM, Ed Anderson <eanderson@carolina.rr.com> wrote:
Mark,
I did some additional reading in the rotary book I recommended to you and a bit more head scratching on exhaust systems.
I modified the tube length formula I used earlier to compensate for the fact the rotary puts out two exhaust pulses per port per 720 deg cycle compared to 1 for the piston engine. This in effect halved the length of tube needed to get the same scavenging effect.
Also using the recommended rotary book values for area of a rotary exhaust tube , I calculated the tube diameter which came out to 1.8".
In any case, I have attached the spreadsheet with those modifications
Ed
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