matigi

Hey I'm gonna be doing a RB streetport to the intakes of my 12a, and I was just wondering if I can use the Exhust sleves as guides when porting the exhaust, Or would this cause overlap? I see many exhuast port jobs, where the port is the same size as the sleeves. Also would having a large exhaust port, and mild intakes cause any problems? And lastly, i tried searching but i'm having a hard time finding info on the best way to address the runners. Where is this info available?



Thanks alot, Matt

j9fd3s

couple quickies on the exhaust.



1. the engine has overlap already, but porting the exhaust up, adds more. little is good, too much is too much. toom much depends on the intake and exhaust setups. better they work, the more overlap you can have and not loose power/response.



2. the exhaust runner should just be polished, theres no reason not too

Nospig

The sleeves are a good guide. port parallel to the top of the sleeve. Some like to go down , i dont i think it just robs some of the power stroke , some like it wide , im not a fan , factory race housings dont go wide . Wide wxhaust ports means more of apex seal not supported , could be bad at high rpm.

Lynn E. Hanover

The store bought Exhaust ports pay little attention to all of the rules you may have heard. Sorry!



At a pressure of several hundred PSI exits the chamber at supersonic velocity and creates a very low pressure behind it (depending on the amout of backpressure available at the port opening).



This clever piece of physics accounts for the rotaries ability to produce more than 100% VE at several RPMs.



What? Yes, the Normally aspirated rotary can ingest more than its mechanical displacement under some conditions.



And the lack of an exhaust valve is one of the reasons why. When the port is opened, there is so little restriction to flow that flow is shortly above the speed of sound just a few degrees after port opening.



So, the opening line, and the radius around that line is of some importance. If you have seen pictures of the early 10A housings you would see the exhaust port is just three little drilled holes. For good street performance, the stock port is just damn hard to beat. Plus, of course, the three holes does not allow for much of a solid face on that shock wave, so muffling is not such a big problem.



Damn the muffling, full speed ahead.



Unless the engine is to be used in the scream range all of the time, almost any exhaust port treatment will work fine. Even stock. It just does not matter.



Although the rotary is a true four Otto cycle engine (like a four stroke piston engine), it tunes just like a two Otto cycle engine. More accurately a piston ported two cycle. It is utterly pointless to port the daylights out of the exhaust ports and then bolt on some crap unequil length headers, and, or, a nice quiet muffler system.



On the other hand, porting is fun. Its easy to do right (many opinions here). And it won't hurt power much if you do it wrong. (so long as the edges of the hole are rounded and smooth).



There are some who insist that the open line be curved slightly curved, with the very center of the port opening first. And these same engines often beat my car down the straightaway. I, feel that at the RPMs where the engines make racing (or impress the girlfriend) power, the difference in opening time in degrees of rotation between a straight and curved open line is not detectable. At street speeds there may well be a muffling advantage to the curved line.



I prefer to stay close to the stock open line timing wise and make a nice big radius across a straight line. Open the ends of the port slightly and radius the edges and polish the whole opening. This is for street.



For racing, copy the timing from a known good performer, and err to the conservative side. A little less timing, a little less width and so on. It is easy to go back in and grind out some more, but if you have gone too far the first time, you can only make so many lamps or fish tanks before your friends catch on.



The store bought holes have the stainless liner removed, but much of it is replaced with a custom made liner that closes up the flange hole on the outside of the hole. So, the hole is just slightly bigger than stock. And the header primary ID need be no larger than 1 7/8" or 2" tops and that is for street or full race headers.



If there is room, let the primary pipe run straight out of the port for a few inches before the first turn.



Removing the liner adds to the cooling load and from what I have seen so far, adds nothing power wise.



Oversizing the port will drop flow to subsonic right outside the engine and add to back pressure no matter how good the headers.



Thats my opinion. I could be wrong.



Lynn E. Hanover

j9fd3s

just to back lynn up. my buddies FD went to xs to be tuned and they told him it would make more power with smaller ports....