boost pressure vs airflow question
12-04-09 | 01:48 PM
#26
Rotary Enthusiast
Joined: Jun 2004
Posts: 862
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From: Austin TX.
Who and how can you have more inlet pressure than exhaust backpressure, sounds like a great idea.
what does it take to do it,on a rotary.
i know ,it would require two gages one on inlet side and one before the turb.
most tests i have seen inlet was always lower than EBP, ratios around 1 to 2 ebp
what does it take to do it,on a rotary.
i know ,it would require two gages one on inlet side and one before the turb.
most tests i have seen inlet was always lower than EBP, ratios around 1 to 2 ebp
12-05-09 | 02:14 PM
#27
It is going to take a larger turbo, good manifold and wastegate design, low pressure loss intake and a little something extra to get more boost pressure than backpressure on a rotary.
Here is a set up for road race that gets 1:1 ratio at 19-20psi boost.
Well to compare with other setups you would have to be factoring in back pressure..if we wanted to raise the back press we could, and did see a 1000 rpm change in spool..but what we were working on, was achieving a one to one ratio. boost vs. back pressure. and that is what we have now, 19 psi of boost and 19-20 psi of back press. the entire system works much better, runs cooler, makes more HP, with less strain on the engine and gets better fuel economy. for us and how jack drives it works just fine. we routinely out run 650 hp vipers and maintain cooler egt's, oil temps and cooling temps then even them, and they are NA....
Twins Turbo....
Twins Turbo built Jack Mardikian road race car.
See how there is an easy merge from collector to wastegate. This will help keep EBP down.
I have thought of what else you could do to keep exhaust backpressure down and here is what I came up with for the above set up.
Larger swing valve wastegate would be better and easy to do.
Run a larger T6 framed divided exhaust manifold and as the turbo is spooling have all the exhaust go through one of the turbo exhaust manifolds runners. At full boost have the diverter open so the exhaust goes through both runners in the turbo exhaust housing and then finally when full boost is realized open the wastegate if necessary.
Or use a big VGT turbo...
Or or use a turbo that isn't powered by the exhaust gas at all, but by an external source (jet turbine or rocket fuel style powered compressor).
Here is a set up for road race that gets 1:1 ratio at 19-20psi boost.
Well to compare with other setups you would have to be factoring in back pressure..if we wanted to raise the back press we could, and did see a 1000 rpm change in spool..but what we were working on, was achieving a one to one ratio. boost vs. back pressure. and that is what we have now, 19 psi of boost and 19-20 psi of back press. the entire system works much better, runs cooler, makes more HP, with less strain on the engine and gets better fuel economy. for us and how jack drives it works just fine. we routinely out run 650 hp vipers and maintain cooler egt's, oil temps and cooling temps then even them, and they are NA....
Twins Turbo....
Twins Turbo built Jack Mardikian road race car.
See how there is an easy merge from collector to wastegate. This will help keep EBP down.
I have thought of what else you could do to keep exhaust backpressure down and here is what I came up with for the above set up.
Larger swing valve wastegate would be better and easy to do.
Run a larger T6 framed divided exhaust manifold and as the turbo is spooling have all the exhaust go through one of the turbo exhaust manifolds runners. At full boost have the diverter open so the exhaust goes through both runners in the turbo exhaust housing and then finally when full boost is realized open the wastegate if necessary.
Or use a big VGT turbo...
Or or use a turbo that isn't powered by the exhaust gas at all, but by an external source (jet turbine or rocket fuel style powered compressor).
04-10-11 | 11:23 AM
#29
Junior Member
Joined: Apr 2011
Posts: 4
Likes: 0
From: Md.
I can see how a bigger turbo can flow more air at a set psi than a smaller. But picture this example:
blow into a straw until you have 10psi pressure built up in your mouth. now, what happens when you blow harder?? does the airflow increase? yes, but can you increase the airflow without increasing pressure in your mouth? I would think not. The only way to get more airflow through the straw at a given psi, is to increase the straw size, not blowing harder. meaning, seems to me airflow is directly related to engine flow (straw size) at a fixed boost pressure, not turbo flow (blowing power)
blow into a straw until you have 10psi pressure built up in your mouth. now, what happens when you blow harder?? does the airflow increase? yes, but can you increase the airflow without increasing pressure in your mouth? I would think not. The only way to get more airflow through the straw at a given psi, is to increase the straw size, not blowing harder. meaning, seems to me airflow is directly related to engine flow (straw size) at a fixed boost pressure, not turbo flow (blowing power)
04-10-11 | 01:38 PM
#30
Thread Starter
destroy, rebuild, repeat
iTrader: (1)
Joined: May 2004
Posts: 2,991
Likes: 17
From: Charleston, SC
actually that was a bad analogy.. i understand now it is turbine backpressure that enables a bigger turbo will make more power at same psi. bigger turbine wheel, less backpressure, more top end for given psi
04-14-11 | 04:55 AM
#31
And the reason why turbine back pressure is the key point is because it has to change MASS FLOW and thus volumetric efficiency of the system.
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