hey all just a quick thought in need of a quick answer. When you do a large streetport, how much overlap is there in comparison to a bridgeport? ie, does a large streetport go BRAP BRAP BRAP like a bp does? How differently do they sound?



Searched around and couldnt find any audio or video files of a streeport, just lots of bp's....



https://www.nopistons.com/forums/pub...IR#>/bigok.gif

it is possible to create a nice amount of overlap with a very large street port. of course over lap doesn't do good for low end. but if we are talking turbo and i'm sure we are,,,, i would tell you to get a bridge to at least help out the midrange more so your power band isn't so short.... of course the turbo sizing can more often than not make the powerband with which every type of port you go with.

Streetports might get a hair bit throaty and grunty but they do not compare to BP's at all whatsoever, even the smaller BP's. I've built alot of both of these kinds of engines. The BP's, when tuned properly, don't lose alot of low-end at all, and are still surprisingly streetable and can still gain quite good gas mileage.



B

Where I still can't find any

Not necessarily true. You can get a LOT of low end torque in a high overlap engine. The trick is that you lose out on power at low throttle openings.



For the most part, the amount of overlap defines how well the engine runs at part throttle and at idle. The intake closing and exhaust opening dictate the powerband, and the amount of port timing helps dictate how much powerband there is. In order to get a good amount of port timing and still close the port early enough to make power down low, you necessarily need to have some amount of overlap.



Of course you can go overboard, in both directions, say an engine with so much overlap that it doesn't make any power even at wide open throttle, or an engine with no overlap that idles glass smooth but makes no power down low because the ports have to close so late in order to have any decent port area.



In regards to idle quality. Any time your porting, or even your exhaust system, reduces intake manifold vacuum to the point where it's below a certain threshold, it will start that wonderful cyclic misfire that we all know and love. I have seen street ported engines that carried on like a bridge port, half the time it was simply due to an overly restrictive exhaust system.

So while we're on the subject of overlap. What's the common theory of big ports with high overlap and turbos. Specifically a P-Port/turbo application. I know with piston motors high overlap isn't the hot setup on a turbo application. Is the flow advantage of a P-Port negated for a turbo application? Or is it worth it to go P-Port while keeping the port opening retarded enough to limit overlap? Or is it just adviseable to go with a bridge port? This being for a race only motor where low end power is not all important though I certainly want all the responsiveness I can get.

I can get my streetport to brap sometimes.

Rotaries aren't piston engines. You can equate some areas of port timing from piston to rotary but in this instance it doesn't fly.



In a piston engine, during the overlap period, the valves are only open a slight amount, so most of the flow is radial to the valve. The piston's height in the bore makes this worse. Because the valves are right nest to each other, it is fairly easy for the exhaust to flow right back up the intake, or vice-versa, depending on the varying pressures in the chamber, exhaust port, intake port, and which way inertia is trying to move the airflow in the ports in addition to the static pressure. The airflow stops immediately behind the intake valve after it closes, for example, and depending on the RPM and the intake resonance it might actually pull the exhaust gases into the intake when it opens back up.



Rotaries are somewhat different. During the overlap period the ports are at opposite ends of the chamber, and in a sense the airflow in the intake port in a bridge or peripheral port engine never "stops", it just switches from one chamber to the next, and if you have a good amount of inertia in the port...

So overlap in a rotary works alot better because since the ports are at oppostie ends of the rotor face whenit is in the stage of overlap, the exhaust flow going out of the chamber helps pull in air from the intake port as opposed to a psiton engine overlap where the ports are right next to each other and exhaust gasses are sucked back up into the intake?



That probably sounds confusing, I have the right idea, just having hard time putting into words......been up for 23 hours https://www.nopistons.com/forums/pub...IR#>/wacko.png

I had a Pineapple streetport that had earlier opening intake port (cut into corner seal track) and it had a pretty good BRAP BRAP idle. I later found my 2ndary throttle plates were cracked open and fixed it. The idle was less hunting, but still sounded much like my pops old 85hp 2 stroke outboard engine. If I put in some exhaust restriction to my 3 1/2" turbo back exhaust like the muffler tip silencer or my foot over the exhaust pipe it smoothed out even more.



It would have a LOT more brap brap if you combined the early opening streetport with beveled rotor edges for an even earlier opening.



This Pineapple early opening streetport had REALLY good low end as started spooling the turbo off idle. When my friend and I were trying to break in the engine the 2nd time with the turbo on it it was REALLY hard to stay out of boost. I set the peak warning of the boost gauge at ~1psi and my friend kept an eye on the Haltech MAP signal and throttle position. We were amazed to see it getting into boost ~ 1,500rpm at ~14% throttle. Once it was broken in I learned I could cruise at 5-7% throttle at 25mph in 5th; give it gas and it would just boost and go.



I believe the early opening street port has several advantages over the bridgeport when using a turbo and with anything resembling a stock intake manifold.



I am just a rotary novice, but this is my thinking right now.



Intake manifold compatability-

The intake timing is only altered tens of degrees from the early opening and possibly slightly later closing as compared to stock. This means it is MUCH better suited for the stock intake manifold lengths as far as dynamic effect tuning. A bridgeport requires such a short manifold for dynamic effect it is shorter than the stock lower intake manifold alone- like the Racing Beat bridgeport manifold.



Flow volume-

Sure you are only adding a couple mm of port area at the opening edge on the early opening streetport, but if you add that up for the full length of the 2ndary ports and primary ports the area is just as much or more than the small auxillary bridgeport that is so popular for the turbo applications AND the flow path is not as compromised as a bridgeport.



Overlap specifics-

Perhaps on a turbo application it is not all about how MUCH intake to exhaust overlap you have, but how the overlap works.



With the early opening streetport you have the advantage of less intake charge dillution than the bridgeport since you have less overlap. Compared to stock intake opening you have the advantage of less intake stroke volume increase prior to port opening during exhaust overlap as stock intake opening allows for 32 deg of intake volume increase sucking exhaust into the chamber.



And here is another possible early opening streetport dynamic.



If you carefully time your intake opening and exhaust opening you actually get the NEXT exhaust phase opening just before the intake opens.



So what?



Well, if you are already on full boost your exhaust manifold backpressure is very low with an efficient, optimized turbo system- especially if you consider the relationship as exhaust backpressure versus intake pressure and compare it to a naturally aspirated application.



Just after the pressure spike in the exhaust port caused by opening dissapates and as the exhaust pressure is "blowing down" (as the exhaust expands to the lower pressure of the exhaust manifold) the intake port opens up.



So what?



If you look at the edge of the rotor at this point there is the high pressure high volume exhaust port at the bottom with its flow already initiated out of the engine and into the exhaust manifold.



There is the intake just opening to initiate flow at the top with the rotor tip in between- at and angle, forming the conditions for a siphon effect.



Literally "sucking" the full boost pressure intake into the now expanding intake chamber. Think about it in terms of absolute pressures, not "sucking" in the traditional sense.



Add to this, the intake charge lost out the exhaust during the "next" exhaust blowdown hits the exhaust at its hottest stage just as the exhust port has opened and so expanding (burning) driving the turbo even more. With a bridgeport, the intake would have already been open for quite a while equalizing the intake to exhaust pressures with the cooler just ending exhaust pulse as the "next" exhaust opening hasn't happened yet.