Boost difference on Street port and stock twins
08-08-13, 02:59 PM
#1
Boost difference on Street port and stock twins
Engine will have a medium large streetport .
I've read that when you port the engine . the turbo's need to work harder to reach the same PSI since the intake ports are flowing more air
basic logic a big straw flows more then a thin one LOL
NOOOW with the stock turbo's its known that at 14 psi they start to shoot superheated air , and get stressed and last much less ..
so what would be the equivalent of running 14 psi on stock turbos on stock ports ,
compared to medium large street ports?
obviously if I ran them at 14 PSI they would be working much harder to produce the same pressure because of the increased flow.
I've read that when you port the engine . the turbo's need to work harder to reach the same PSI since the intake ports are flowing more air
basic logic a big straw flows more then a thin one LOL
NOOOW with the stock turbo's its known that at 14 psi they start to shoot superheated air , and get stressed and last much less ..
so what would be the equivalent of running 14 psi on stock turbos on stock ports ,
compared to medium large street ports?
obviously if I ran them at 14 PSI they would be working much harder to produce the same pressure because of the increased flow.
08-08-13, 09:38 PM
#2
So here is the compressor map for the '99 twins. There are two turbos so I suppose the mass flow would be multiplied by 2. It's the only thing we have to work with.
That box is the approximate area of the map you are operating in. You said 14psi manifold pressure, which is around 1.0 atmosphere of pressure. You also have to figure the turbo is boosting more than the manifold due to pressure drop.
You will be moving horizontally, to the right, in that boxed area. What that means is efficiency goes down (intake temps would go up some) and you could even hit the choke line where you simply can't flow as much air as the engine can at high rpm's. On the turbine side, the housings would be causing more of a restriction, possibly leading to higher temperatures. That applies to certain rpm points.
If you go up in altitude you will be especially affected because the turbo will overspin.
That box is the approximate area of the map you are operating in. You said 14psi manifold pressure, which is around 1.0 atmosphere of pressure. You also have to figure the turbo is boosting more than the manifold due to pressure drop.
You will be moving horizontally, to the right, in that boxed area. What that means is efficiency goes down (intake temps would go up some) and you could even hit the choke line where you simply can't flow as much air as the engine can at high rpm's. On the turbine side, the housings would be causing more of a restriction, possibly leading to higher temperatures. That applies to certain rpm points.
If you go up in altitude you will be especially affected because the turbo will overspin.
08-09-13, 03:58 AM
#4
Someone once tried to explain to me that a mild port on the rotary with boost, was similar to a lumpy cam in a piston engine with boost.
Bear with me here a minute;
Basically what I know from my cam grinding days is that in a piston engine, the hotter cam profile has increased overlap (reduced LSA- Lobe Separation Angle) which decreases vacuum at idle, as overlap occurs when both valves are rocking open. At this moment, boost is lost past both the valves into the exhaust. This is why a lot of piston engine "turbo-cam" profiles are very conservative and not "oomph-oomph" lumpy at idle. They are trying to make the best of the available boost and not waste it during overlap, which occurs with your hotter cam durations, (typically 280-310 degrees duration).
So now you're probably thinking "STFU about piston engines, this is a rotary forum. You have no idea what you're rambling about". But this above theory may apply to porting the rotary.
The port timing in the 13B-REW is analogous to its "cam profile". Changing the characteristics of the port timing (opening and closing of intake and exhaust events relative to each other) by doing port work (using porting templates if thats how you were to port it) will have an effect on how boost is scavenged and the efficiency with which it is used or wasted in the combustion cycle.
If you actually degree the ports as you would a camshaft in a piston engine, you can see the numbers of the stock port timing. Mazda's factory port timing for the 13B-REW looks very close to what a turbo cam profile in a piston engine should be. Mazda engineers are very clever guys. They didn't give it a lumpy idle or a hot port because they wanted to make efficient use of the boost, from as low an rpm as possible, for street use. But there was likely emissions constraints too, so the port timing is not *absolutely* optimised for efficient use of boost. There is room for improvement by porting. But going too big, causes inefficient use of boost, and sacrifices low end torque even further. So with rotary porting, bigger is not always better when it comes to efficient use of available boost. Sure it sounds cool, but its not efficient.
*I used to grind cams earlier when I played with Chrysler stuff, 318/360/383/440 etc V8's. When I didnt have access to a cam grinding machine at work, I would spec what I wanted to a company called Crow Cams. My father is rebuilding his 13B-REW in his FD at the moment, and he has been looking at port timing and degreeing the ports to see how we can take advantage of boost better, next time round*
Bear with me here a minute;
Basically what I know from my cam grinding days is that in a piston engine, the hotter cam profile has increased overlap (reduced LSA- Lobe Separation Angle) which decreases vacuum at idle, as overlap occurs when both valves are rocking open. At this moment, boost is lost past both the valves into the exhaust. This is why a lot of piston engine "turbo-cam" profiles are very conservative and not "oomph-oomph" lumpy at idle. They are trying to make the best of the available boost and not waste it during overlap, which occurs with your hotter cam durations, (typically 280-310 degrees duration).
So now you're probably thinking "STFU about piston engines, this is a rotary forum. You have no idea what you're rambling about". But this above theory may apply to porting the rotary.
The port timing in the 13B-REW is analogous to its "cam profile". Changing the characteristics of the port timing (opening and closing of intake and exhaust events relative to each other) by doing port work (using porting templates if thats how you were to port it) will have an effect on how boost is scavenged and the efficiency with which it is used or wasted in the combustion cycle.
If you actually degree the ports as you would a camshaft in a piston engine, you can see the numbers of the stock port timing. Mazda's factory port timing for the 13B-REW looks very close to what a turbo cam profile in a piston engine should be. Mazda engineers are very clever guys. They didn't give it a lumpy idle or a hot port because they wanted to make efficient use of the boost, from as low an rpm as possible, for street use. But there was likely emissions constraints too, so the port timing is not *absolutely* optimised for efficient use of boost. There is room for improvement by porting. But going too big, causes inefficient use of boost, and sacrifices low end torque even further. So with rotary porting, bigger is not always better when it comes to efficient use of available boost. Sure it sounds cool, but its not efficient.
*I used to grind cams earlier when I played with Chrysler stuff, 318/360/383/440 etc V8's. When I didnt have access to a cam grinding machine at work, I would spec what I wanted to a company called Crow Cams. My father is rebuilding his 13B-REW in his FD at the moment, and he has been looking at port timing and degreeing the ports to see how we can take advantage of boost better, next time round*
08-09-13, 08:46 AM
#5
Someone once tried to explain to me that a mild port on the rotary with boost, was similar to a lumpy cam in a piston engine with boost.
Bear with me here a minute;
Basically what I know from my cam grinding days is that in a piston engine, the hotter cam profile has increased overlap (reduced LSA- Lobe Separation Angle) which decreases vacuum at idle, as overlap occurs when both valves are rocking open. At this moment, boost is lost past both the valves into the exhaust. This is why a lot of piston engine "turbo-cam" profiles are very conservative and not "oomph-oomph" lumpy at idle. They are trying to make the best of the available boost and not waste it during overlap, which occurs with your hotter cam durations, (typically 280-310 degrees duration).
So now you're probably thinking "STFU about piston engines, this is a rotary forum. You have no idea what you're rambling about". But this above theory may apply to porting the rotary.
The port timing in the 13B-REW is analogous to its "cam profile". Changing the characteristics of the port timing (opening and closing of intake and exhaust events relative to each other) by doing port work (using porting templates if thats how you were to port it) will have an effect on how boost is scavenged and the efficiency with which it is used or wasted in the combustion cycle.
If you actually degree the ports as you would a camshaft in a piston engine, you can see the numbers of the stock port timing. Mazda's factory port timing for the 13B-REW looks very close to what a turbo cam profile in a piston engine should be. Mazda engineers are very clever guys. They didn't give it a lumpy idle or a hot port because they wanted to make efficient use of the boost, from as low an rpm as possible, for street use. But there was likely emissions constraints too, so the port timing is not *absolutely* optimised for efficient use of boost. There is room for improvement by porting. But going too big, causes inefficient use of boost, and sacrifices low end torque even further. So with rotary porting, bigger is not always better when it comes to efficient use of available boost. Sure it sounds cool, but its not efficient.
*I used to grind cams earlier when I played with Chrysler stuff, 318/360/383/440 etc V8's. When I didnt have access to a cam grinding machine at work, I would spec what I wanted to a company called Crow Cams. My father is rebuilding his 13B-REW in his FD at the moment, and he has been looking at port timing and degreeing the ports to see how we can take advantage of boost better, next time round*
Bear with me here a minute;
Basically what I know from my cam grinding days is that in a piston engine, the hotter cam profile has increased overlap (reduced LSA- Lobe Separation Angle) which decreases vacuum at idle, as overlap occurs when both valves are rocking open. At this moment, boost is lost past both the valves into the exhaust. This is why a lot of piston engine "turbo-cam" profiles are very conservative and not "oomph-oomph" lumpy at idle. They are trying to make the best of the available boost and not waste it during overlap, which occurs with your hotter cam durations, (typically 280-310 degrees duration).
So now you're probably thinking "STFU about piston engines, this is a rotary forum. You have no idea what you're rambling about". But this above theory may apply to porting the rotary.
The port timing in the 13B-REW is analogous to its "cam profile". Changing the characteristics of the port timing (opening and closing of intake and exhaust events relative to each other) by doing port work (using porting templates if thats how you were to port it) will have an effect on how boost is scavenged and the efficiency with which it is used or wasted in the combustion cycle.
If you actually degree the ports as you would a camshaft in a piston engine, you can see the numbers of the stock port timing. Mazda's factory port timing for the 13B-REW looks very close to what a turbo cam profile in a piston engine should be. Mazda engineers are very clever guys. They didn't give it a lumpy idle or a hot port because they wanted to make efficient use of the boost, from as low an rpm as possible, for street use. But there was likely emissions constraints too, so the port timing is not *absolutely* optimised for efficient use of boost. There is room for improvement by porting. But going too big, causes inefficient use of boost, and sacrifices low end torque even further. So with rotary porting, bigger is not always better when it comes to efficient use of available boost. Sure it sounds cool, but its not efficient.
*I used to grind cams earlier when I played with Chrysler stuff, 318/360/383/440 etc V8's. When I didnt have access to a cam grinding machine at work, I would spec what I wanted to a company called Crow Cams. My father is rebuilding his 13B-REW in his FD at the moment, and he has been looking at port timing and degreeing the ports to see how we can take advantage of boost better, next time round*
where it has much more advanced opening and you get the lumpy idle with a street port you almost never get a lumpy idle . but it does move the powerband up a bit. This car is basically an auto-x car. I can usually keep the rpms over 3500 unless I mess up the lap
