Boostmaniac

I have someone that has a very good streetport that informed me that a streetport can produce just as much power as a bridge port and that bridge porting was over rated.



Now...



I am of the school of thought that bridge porting creates more port overlap, thus killing some low end power and shifting the entire power band slightly higher into the RPM's thus allowing for the production of more power.



Am I wrong here? I thought anything but a super aggresive street port wouldn't come close to a bridge port. What are your opinions on this?



Thanks guys.



-Maniac

heretic

No opinions required, this is science and has been scientifically investigated. For a given level of power, the bridge port will have a broader, lower powerband. For a turbo engine, less boost is required.

Boostmaniac

I thought the bridge port would give a narrower and higher powerband.

10secgsl

you are correct to a point,it all depends on the size of the port,port timing etc,you can get just as much out of a large streetport as you can a small bridgeport if the ports are done right.

mazdaspeed7

But everything else being the same, the small bridgeport will make more torque, and have a broader usable powerband.



Overlap = torque. Even a PP peaking at 10K rpm will make more torque at 3k rpm than a streetport. The real issue here is overlap has a serious negative effect on part throttle drivability.

BDC

+1 to that.



That's what I've seen consistently time and again.



Boostmaniac, I saw your post up on the Evil forum about this. I didn't want to respond there but I will gladly here. My thoughts and feelings on this -- the bridgeport, depending on the shape, size, heighth, and width of the cut, yields a dramatically higher VE (volumetric efficiency) of the motor. In a way, it's like virtually increasing the displacement of the engine as it enables it to be able to injest and exhale a greater mass of air per stroke. This is the idea behind doing a street (theoretically, although alot of attempts at porting are not productive). The bridgeport introduces a substantial amount of overlap into the equation. On a turbo setup, all remaining variables being constant (basically changing from a stock or street port to a BP and changing nothing else), the turbo will have an earlier boost threshold, making it hit its target boost pressure earlier and much more aggressively. The overlap in the motor, producing an intake port that is open nearly all the time, stretches the peak power band outwards and higher in RPM. The combination yields what Heretic has pointed out -- a wider powerband.



All of this means nothing however with an improperly sized turbo. The principle behind a BP for a turbo setup is for it to be able to use a larger turbo with the same "feel" of a smaller turbo on a stock/street port setup. It merely enables the possibility of higher power potential; it doesn't guarantee it. The turbocharger on a turbo'd vehicle is what is primarily responsible for the power a setup can produce. Just making a couple bridgeport cuts in some irons doesn't automatically equal higher power. It's a foundational stepping stone but not a guarantee.



That make sense?



B

kuhnke

umm are you sure about that because i thought that more overlap = more hp and less overlap = more TQ

Velocifero

I am new to the differences and rotories as a whole, so please excuse my intermediate question. What are the differences between BP and J BP? I was researching it but maybe just wasn't searching right, I assume it is a more aggressive port.

j9fd3s

the "normal" bridge port is ported up to the rotor housing, usually a small bevel is added to the rotor housing, so the port is less shrouded.



the J or jacket bridge is a port where the bridge is larger, and the cut into the rotor housing is past the water seal, and into the cooling jacket, which then must be filled.



note on the bridge ports, or even the street ports, theres no real standard. the bridges can vary a lot from very small, to huge.

heretic

STOMP STOMP STAMP STOMP STOMP



There, I hope I smashed that fallacy flat.



Torque is force. Pounds of force per foot of leverage = lb-ft, for example.



Power is using that force over time. James Watt (the steam engine guy) decided that a horse could lift 550 pounds one foot in one second. So, lifting 550lb per foot per second = one "horsepower". Convert linear motion to radial motion, divide by 550, carry the two, it equals that at one RPM, 5252lb-ft of torque is one horsepower. Or, at 5252rpm, 1lb-ft of torque is one horsepower.



The SI unit of power is the watt. Old James has a monopoly in the power buisness.



Notice, that the power is interdependent on torque and the speed at which it makes the torque. 5252lb-ft at 1rpm is the same power as 1lb-ft at 5252rpm. So, if we want more power, we can increase the torque or increase the RPM at which the torque happens



Torque is created by burning air and fuel. We get more torque by burning more air and fuel: larger engine, making it easier to get air and fuel into the same size engine (volumetric efficiency), forced induction, artificial air (nitrous oxide). More difficult, we make the existing air and fuel work harder for a bigger push: higher compression ratio, better thermal efficiency, optimal ignition timing.



Volumetric efficiency is the big one. An engine's V.E. curve is almost directly linked to its torque curve.



You're yawning, because this is all basic stuff that even magazines like Import Tuner and Sport Compact Car have covered in their tech articles. Don't worry, there's a point, and the point is:



A street port makes more power by taking the existing V.E. (torque) curve and while it does get a little larger, it gets mostly shifted to a higher RPM. The power increase is mainly due to the increase in RPM.



The bridge port, on the other hand, has a much higher V.E., almost double, and it does it everywhere. Down low. In the middle. Up top. They never stop. They make much more TORQUE than a street port does, so for a given level of power, they don't need as much RPM.



The V.E. comes from the port open time. The peakiness comes from the port closing time, which has a large effect on intake reversion at lower RPM, when the air's velocity (inertia) is relatively low. Street ports mostly make their power from extending the closing time later, which is where they get their low end power hurt. You could make a bridge port peaky like that, too, but it's not due to the overlap, it's due to the late closing.





The stomping was to kill the fallacy that low RPM engines make "torque" but high RPM engines make "power". The truth is, a lot of modifications that increase power also increase torque. The misconception stems from the fact that low RPM engines are necessarily low power engines for a given level of torque. Torque is what shoves you back in the seat, so a low RPM engine will feel a lot faster than it is, just like a high RPM engine will feel boring to drive because it never really seems to shove you back in the seat very hard. Cover ground quickly, though.