bridge port and turbo?
12-03-2005, 01:07 AM
#1
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i dont remember where but i heard on a site that its not good to have a turbo-bridge ported motor... but why? that doesnt sound right. i mean its just more air right? as long as you get the fuel to back it up... i was thinking about doing a mild bridge-port and havea turbo set up next year when i get the motor done... good idea?? my town is overrun by gt's and cobra's and im tired of it
12-03-2005, 10:34 PM
#3
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Originally Posted by 86fc3sMatt' post='783402' date='Dec 2 2005, 11:17 PM
oh yeah and what about a p-port motor with turbo?? wich would creat the best power ??
Well, for p-port turbos, I've heard that you can blow boost out of the exhaust port because of the overlap. But I guess that all depends on how big your p-port is and where it's located.
Owen
12-04-2005, 04:12 PM
#4
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this gets into the area of 'its possible, but what are you after'.
There are a number of ways to view a turbocharged engine. If you just look at the turbo as a torque multiplier on top of a naturally aspirated torque curve, then it follows that any engine can be turbocharged provided it can withstand the BMEP increase without exploding.
And this is the crux. If you have a high revving engine with very high overlap that does not produce much low down torque, then adding a turbo will not overcome that fundamental torque curve. This is why in general you start with a low overlap engine with wide and flat torque curve, then add as much boost as possible. This gives the best canvas on which to build boosted power.
So the BP turbo can be made to work, but boost will tend to build later than with a street port. You also need to focus a lot harder on reducing back pressure in the exhaust and realise that the overlap means that a larger turbo is needed to deal with that additional airflow, as you end up blowing a lot of air straight through the rotor and out again. Not a bad thing per say, as you get cooling of the turbine, and if you can get the EBP below the inlet pressure, then you get pressurising of the clearance volume. In laymans terms this allows you get get more power out for the same amount of boost.
However, to get to that level you end up with a motor that is not suited for the street. Lound, late spooling and them horrendously powerful. TBH a large streetport would be a better way to go unless all the local cops are deaf or its a trailer queen.
There are a number of ways to view a turbocharged engine. If you just look at the turbo as a torque multiplier on top of a naturally aspirated torque curve, then it follows that any engine can be turbocharged provided it can withstand the BMEP increase without exploding.
And this is the crux. If you have a high revving engine with very high overlap that does not produce much low down torque, then adding a turbo will not overcome that fundamental torque curve. This is why in general you start with a low overlap engine with wide and flat torque curve, then add as much boost as possible. This gives the best canvas on which to build boosted power.
So the BP turbo can be made to work, but boost will tend to build later than with a street port. You also need to focus a lot harder on reducing back pressure in the exhaust and realise that the overlap means that a larger turbo is needed to deal with that additional airflow, as you end up blowing a lot of air straight through the rotor and out again. Not a bad thing per say, as you get cooling of the turbine, and if you can get the EBP below the inlet pressure, then you get pressurising of the clearance volume. In laymans terms this allows you get get more power out for the same amount of boost.
However, to get to that level you end up with a motor that is not suited for the street. Lound, late spooling and them horrendously powerful. TBH a large streetport would be a better way to go unless all the local cops are deaf or its a trailer queen.
12-04-2005, 07:47 PM
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Originally Posted by REZCAR' post='783573' date='Dec 3 2005, 08:34 PM
Well, for p-port turbos, I've heard that you can blow boost out of the exhaust port because of the overlap.
You'd have to have a REALLY good turbo setup for that to be a problem. Generally speaking, the exhaust pressure will be about the same as or higher than the intake pressure.
If you do blow air out the exhaust port, it probably won't hurt anything. You'd know at that point that you are evacuating ALL of the exhaust gases from the chamber. More likely, you'd only be blowing exhaust gases out and no actual intake charge.
12-05-2005, 01:15 AM
#6
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Originally Posted by heretic' post='783871' date='Dec 4 2005, 05:47 PM
You'd have to have a REALLY good turbo setup for that to be a problem. Generally speaking, the exhaust pressure will be about the same as or higher than the intake pressure.
If you do blow air out the exhaust port, it probably won't hurt anything. You'd know at that point that you are evacuating ALL of the exhaust gases from the chamber. More likely, you'd only be blowing exhaust gases out and no actual intake charge.
What sort of turbo would that be? The REALLY good one? For that to be a problem, it would be one that flows a lot at low rpm, in which case overlap duration is longer and you'd be blowing intake charge out of the exhaust port, rather than building boost charge. At higher rpms, the overlap is less and more intake charge goes where it's supposed to, into the combustion chamber. Correct?
I understand the need for a larger turbo with less backpressure, or two turbos whose combined exhaust area exceeds the area of a large single. So what kind/type of turbo would that be? This is getting interesting. Oh, I'm talking p-ports here, not bridge, if it matters any.
12-05-2005, 12:57 PM
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Originally Posted by REZCAR' post='783957' date='Dec 4 2005, 11:15 PM
What sort of turbo would that be? The REALLY good one? For that to be a problem, it would be one that flows a lot at low rpm, in which case overlap duration is longer and you'd be blowing intake charge out of the exhaust port, rather than building boost charge. At higher rpms, the overlap is less and more intake charge goes where it's supposed to, into the combustion chamber. Correct?
I understand the need for a larger turbo with less backpressure, or two turbos whose combined exhaust area exceeds the area of a large single. So what kind/type of turbo would that be? This is getting interesting. Oh, I'm talking p-ports here, not bridge, if it matters any.
think about it. the turbo is adding a restriction in the exhaust, so if anything the exhaust is going to go out the intake. a REALLY good turbo in this case would be one where the exhaust backpressure is as close to intake pressure as possible.
look at howard colemans setup, he's got 2 turbos and i think he's running about 21psi backpressure @15psi boost. imagine how big his turbos would have to be on a pp motor....
https://www.nopistons.com/forums/ind...howtopic=54978
12-05-2005, 04:18 PM
#9
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I believe the physics of the situation makes it impossible for the exhaust pressure to be less than the charge pressure regardless of boost. Is what Corky Bell says anyway. So you will always end up with exhaust gasses in the intake charge with large overlaps.
With a bridge port or p-port you will always end up with charge dilution to some point which compromises low end driveability and power. However the gains from mid to top should more than make up for it.
With a bridge port or p-port you will always end up with charge dilution to some point which compromises low end driveability and power. However the gains from mid to top should more than make up for it.
12-05-2005, 05:19 PM
#10
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Originally Posted by C. Ludwig' post='784062' date='Dec 5 2005, 02:18 PM
I believe the physics of the situation makes it impossible for the exhaust pressure to be less than the charge pressure regardless of boost.
Not necessarily. There are plenty of documented situations where exhaust manifold pressure is lower than intake manifold pressure.
Exhaust gases necessarily have a lot more velocity than the intake gases, and a lot more volume. This can be exploited.