beatnu

just done these ports ;the biggest ones; had it in the car runnig went hard but hard to start the motor just wondering what you all think thanks

RX200013B

what are we doing w/ the engine? pp, bp, sp, turbo?

beatnu

bp na into ae old 323

Lynn E. Hanover

OK, so what are the open and close numbers?



Is this a race car? Will there be much muffling (back pressure) involved?



The rotary exhaust open and close events tend to be quite modest compared to a piston engine. The engines tune like 2 cycle dirt bike engines, and are more sensitive to exhaust back pressure than piston engines.



The engines also suffer from poor torque at low RPM, and fooling with exhaust port timing generally makes this problem worse.



Once the throttle is open about half way, exhaust gas volume is high enough that flow rates are supersonic. So the amount of back pressure becomes a big factor in how much gas volume will escape. In the ideal engine, all of the exhaust and a bit of the intake mixture would go out the exhaust port, providing a low pressure in the intake to help gain intake velocity.



Then some of the intake mixture would be pushed back into the engine by a reflected wave, just like a dirt bike. The effect would be used to pump up the worst part of the torque curve. At other RPM it might actually reduce torque.



So exhaust porting is a bit dicey, and a little bit of grinding goes a very long way.



Early opening increases velocity but directly removes torque. (less time with high pressure pushing on the rotor face).



A wider port increases flow volume without changing port timing and raises RPM where flow goes supersonic.



A later closing increases overlap. Increases intake dilution with exhaust gasses, at all but the highest RPM and ruins low speed torque.



None of this matters if the headers are poorly done, or have head pipes too big. You want supersonic flow right into the collector. So 1 7/8" to 2" ID and at least 24" of identical length tubing to the collector.



The Racing Beat bridgeport engines have exhaust port timing: 88 BBDC-57 ATDC.



The factory PPort housings had: 73 BBDC-65 ATDC. Quite modest compared to the bridgeported exhaust timing.



Go to Paul Yaws web page and read all of the Tech articles and notice the list of port timings for various engines.



Depending on what you want the engine to do, keep in mind that street transmissions have very wide spreads between gears.

Racing transmissions have very small spreads between gears. The street trans may drop 2000 RPM on a shift. The racing trans (usually you can pick anything you want) may drop only 900 to 1000 RPM on a shift. So the racing trans can be used with a very high HP engine that has a very limited (narrow) power band. Maybe only 1,200 RPM. The driver needs to shift very precisley to keep the car from going flat between shifts. This same engine would be a dog in a street car. In the long off power periods between shifts the kid down the street with half the HP will drive around you.



Build for the widest power band you can, and not the highest peak HP. Unless you just dyno race.



Lynn E. Hanover

j9fd3s

early OPEN also looses bsfc, as you can see by the numbers the factory race engine (and the rx8 engines) open LATER than everything else.



the factory race engine also closes later, but its used with an exhaust so loud you can't hear it... on a street car, overlap is hard because mufflers add backpressure...

beatnu

pretty much the Racing Beat bridgeport exhaust port timing: 88 BBDC-57 ATDC. i had the biggest ones in my motor i just had running went hard but was hard to start i was wondering if to much overlap would create a non starting problem by haveing to much scavenging

Lynn E. Hanover

At cranking speed, only the compression seal and the intake closing point affect mixture compression. The rotor has dozens of gaps in its sealing system, where the piston engine has just one per ring, and in most cases that ring has a back up just below it. In the olden days where I come from, there used to be two sets of side seals where there is just one today.



There has always been a poor starting performance problem. RX-2 and RX-3 had a bottle on the fire wall that was supposed to be filled with antifreeze. At low temps some antifreeze would be injected into the intake to unstick the frost stuck apex seals, and take up a bit of volume to improve sealing and increase compression. Heat of compression is needed to vaporize the fuel into a gas like mixture. Just fuel droplets can be ignited, but offer so little surface area that it just burns, and not explodes as you need for a crisp start.



The later the closing point, the smaller the volume of fuel air mixture being compressed. Starting at say, 9.7 to one and going backwards. So even a stock engine does not have a true 9.7 to one at cranking speed. So at 50 degrees it less than 9.7 and your new street port is closing at 70 degrees, so you have what? 7.5 to one?



So since there are engines built every day with late closing intake ports, how do people start those engines?



You need great sealing. Side seal end gaps of .002" or less. I use zero. I use Redline 2 cycle oil in a premix at one ounce per gallon of fuel (93 octane no alcohol). The premix helps seal up the rotor like a piston engine.



You need to spin the engine up fast with a clean starter and a fully charged battery with enough plate area to maintain 9.5 volts during cranking. Or, add a jump battery. The race car has a built in port for jump battery connection.



A tired battery, poor dirty connections, and the original starter make for hard starting. The slower the engine turns, the longer it has time to leak down, and reduce the chance for a start.



Take that nasty old starter apart and shine up the comutator with 400 wet or dry paper. Sand the glaze off the ends of the brushes. Clean and relube the bearings. Wire brush the conductor posts.



If it starts easy when you bump start it, but not when you crank it, then its not spinning fast enough for the rate of leakage it has.



For starting a tired engine, mix 1/4 motor oil with 3/4 fuel in an oil squirt can. Shake well and squirt some into each housing before cranking.



Lynn E. Hanover



Picture is a 4360 28 cylinder 3,000 HP aircraft engine.

And you thought you had starting problems.