it was the exhibits...wasn't it


Oh and everything DaveW has stated...I showed with post #8
https://www.rx7club.com/showpost.php...07&postcount=8
and post #17
https://www.rx7club.com/showpost.php...5&postcount=17

and at Last edited by dgeesaman : 1 Day Ago at 07:00 AM. Reason: nobody misspells Obi-Wan Kenobi and gets away with it!! for post #8

 

Sorry - I said I didn't thoroughly read the entire thread and I might be repetitive...

Dave

 

You are talking in absolutes as if the exhaust was pulling the air from the compressor. The turbo is pushing the air and the exhaust is restricting that movement, not the other way around. during the exaust phase the gas is pushed out by the face of the rotor, during the intake phase the rotor face moves away from the housing and creates a vacuum. At this instant that 10lbs boost is diluted as that fixed volume of air between the turbo and rotor face is spread accross a larger volume of area. Once this occurs the exhaust port has already been isolated from the intake and the only thing forcing air into the engine is rotor movement and the compressor wheel IOW: Flow.

and your example is wrong. Actually you can slap on a smaller compressor wheel, turn up the boost a notch, cool to the same temp with some magic intercooler and move less air. lol . think about it.

Boost is not flow.

 

The figures i have heard *third party information alert* are that for every ~7° (don't know Fahrenheit or Celsius- I assume F) you reduce the temperature of the intake charge you gain ~1hp. so i think the increase in flow would be the greater power gainer.

I didn't say that it would be as fast or enjoyable but at a given RPM it would produce the same amount of power.

 

Unless I'm reading your statements wrong, especially your last one, we really agree on everything. At this point it's just semantics.

I call for a truce.

Dave

 

oh no worries...I was just showing some of us are on the same page. Actually I think most of us are on the same page, but this thread is beginning to get fun.

 

Bigger compressors move more air ;o), ofcourse we are completely oversimplifying the mechanical aspects of this beast.

You said backpressure

 

Dubulup, I think you should post that again. We miss it and we're entirely too lazy to back-click. lol

 

I think we should make it the third gen bannor

 

Fair enough.

 

I think I'm going to print it out and tattoo it on my ***.

 

Actually I just had never considered the reduction in back-pressure garnered by the change in turbine wheels.

but if you pressurize a void to a given pressure at a given temperature, then equalize that void with a new void the total volume of the combined void will remain the same regardless of how large the compressor is. The *only* variable will be how fast the void fills up again.

The flaw in that thought process is that the flow increases due to a reduction in back-pressure & temperature.

& neither is CFM

 

gosh, I'm such an idiot

 

Nope. The flow is a result of the compressor wheel spinning at some defined RPM and compressing air from it's large inducer opening and cramming it out of its small exducer opening. This pressurizes air moves away from the compressor wheel. The amount of air compressed and expelled is measured in CFM.

The exhaust ports is isolated from the intake port for 90% of the intake phase. I don't understand how you think a reduction in backpressure is doing anything once the exhausting rotor face is isolated?

Yes decreased backpressure will help the rotors face to expell more expended gas - and that help with VE, but that's it. IOW, decreasing back pressure just clears out more room for the next intake phase so that the compressor wheel can cram more air in.

A better flowing wheel will cram more air in. And if the air is colder, well then that's all the better....

 

So if for example we put a 1" diaphragm in the exhaust the power lost would be due to lost volumetric efficiency, you'd still be able to make boost, and the total engine flow would not be affected?

 

I can see you are not grasping this info but hopefully one day you'll enjoy a single turbo FD and won't have to stress about it any more

 

Reference Post #74:

 

This is an excellent post, and matches my understanding of the topic.

-Max

 

If that's the case, then why do the BNR/M2/Australian SP model twins make more power at comparable boost levels than the stockers? They all use the same restrictive stock exhaust manifold. A reduction in backpressure isn't the endall to a larger turbo's increased efficiency.
Sorry, but there is no hard'n'fast urban myth IAT/hp rule that applies to all turbo applications no matter what peripheral components are attached the engine.

 

Please share

 

I put an intercooler on a weed whacker and added 6HP! JK, but obviously there will be no rule of thumb that works with these units. % power gain per degree is the appropriate form of this rule.

-Max

 

If you ask me, this was the most all-encompassing post on this subect ^ It's not about "volume," it's about how volume is utilized.

 

Here's another post that matches my understanding, and is very succinct:

Here are some of my own ramblings about the pressure issue (ignoring temp) that relate to this topic as well:
http://maxcooper.com/rx7/how-to/fuel_system/why.html

-Max

 

Nice write-up with which I think we all agree, but it seems that everyone ignores that period of time when the exhaust becomes isolated from the intake tract.... My stance is that flow does matter in this window of time.

Please read posts 68, 78 and 89.

Rob

 

If one turbo flowed more (meaning actually delivered more air, not just capability) than another during the post-exhaust-port-close period, the boost pressure in the manifold would change. The scenario we are considering dictates a constant boost pressure, which is both realistic and reasonable. So, the difference in compressor flow capabilities doesn't have the effect that you think it does. If backpressure, boost pressure, and intake temps were somehow equalized, a big turbo would NOT put more air into the combustion chambers than a small turbo. (However, in reality the big turbo will need to flow more, since the backpressure is lower, allowing more flow for a given boost pressure.)

Also consider that even when the exhaust port closes, the amount of backpressure still has a lasting effect. A lot of backpressure is like leaving a pet elephant in a room. A little backpressure is like leaving a pet mouse in the room. Even after the pet owner has left, you still have to deal with their pet. You can fit more stuff into the room with a mouse than you can into the room with an elephant.

Big turbos make more power because they offer less backpressure (which in turn increases the flow at a given boost pressure) and have more efficient compressors (more oxygen molecules for a given volume).

-Max