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Anonymous
Guest
HI , JUST WONDERING, CARBS HAVE RATINGS 300,500,600 CFM ETC,WHAT WOULD THE RATING OF AN XF/EFI BE..... THANKS
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Anonymous
Guest
It is hard to think of an EFI setup the same way. A carb it rated to a certain CFM because it has to be able to supply the amount of fuel to suit roughly that much air. It has to be able to atomise that fule properly etc.
With EFI you think more of how much of a restriction it is because the amount of fuel is infinitely adjustable with injector size and programming.
I know of one car that ran a 500 2 barrell holley then was converted to an XF efi manifold with a modified XF throttle body.
The engine was making 200 rear wheel horsepower with a mild port job, decent cam and custom exhaust and extractors. EFI was controlled by an autronic.
The engine increased 20 rear wheel horsepower with the addition of the EFI and was far more drivable lower in the rev range.
With my EF falcon dual runner manifold I did flow tests.
My head flowed 240cfm @ 25 inches.
With the manifold and throttle body added it lost 11 cfm. So flow reduced by around 5%
I know a set of webbers with ram tubes bolted onto a datsun head dropped flow by 4cfm and there is not much restriction in a webber. I would hate to see how much flow is lost through a holley. They don't even have a ram tube style thing leading into them.
Apparently those big bellmounth shaped air filters are the best addition to a holley carby for improving air flow btw.
With EFI you think more of how much of a restriction it is because the amount of fuel is infinitely adjustable with injector size and programming.
I know of one car that ran a 500 2 barrell holley then was converted to an XF efi manifold with a modified XF throttle body.
The engine was making 200 rear wheel horsepower with a mild port job, decent cam and custom exhaust and extractors. EFI was controlled by an autronic.
The engine increased 20 rear wheel horsepower with the addition of the EFI and was far more drivable lower in the rev range.
With my EF falcon dual runner manifold I did flow tests.
My head flowed 240cfm @ 25 inches.
With the manifold and throttle body added it lost 11 cfm. So flow reduced by around 5%
I know a set of webbers with ram tubes bolted onto a datsun head dropped flow by 4cfm and there is not much restriction in a webber. I would hate to see how much flow is lost through a holley. They don't even have a ram tube style thing leading into them.
Apparently those big bellmounth shaped air filters are the best addition to a holley carby for improving air flow btw.
We're well overdue for a serious discussion (in the mainstream section, not Oz cars) on how developed airflow can influence performance. It's not a BS-free zone yet.
XT500 - I would like to see what ideas you have. Saves me having to pull out my engineering fluid dynamics books, and then get it tragically wrong!
Cheers, Adam.
XT500 - I would like to see what ideas you have. Saves me having to pull out my engineering fluid dynamics books, and then get it tragically wrong!
Cheers, Adam.
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Anonymous
Guest
Sorry mate we don't get into that stuff until much later on. I'm just starting 2nd year now. You might find something on the web somewhere but it is often unreliable and/or out of date.
What I'd like to see done - not that it would necessarily do much - is instead of having a butterfly type throttle plate, use a similar mechanism as used in a camera aperture - an "iris" type of thottling valve so that you don't interrupt the natural swirl that develops in the flow of incoming air. I think, if you designed the plenum correctly, you would see some improvement in mid to high range torque, and of course high range HP.
What I'd like to see done - not that it would necessarily do much - is instead of having a butterfly type throttle plate, use a similar mechanism as used in a camera aperture - an "iris" type of thottling valve so that you don't interrupt the natural swirl that develops in the flow of incoming air. I think, if you designed the plenum correctly, you would see some improvement in mid to high range torque, and of course high range HP.
aussie7mains
Famous Member
OK
First off gents ill let you know where Im coming from, 30 years auto trade work, 25 years of sub professional engineering work, some airflow testing, lotsa trial and error.
You need to remember that your dealing with a compressable medium here so flow isnt like water in a pipe.
consider reynolds number when trying to decide what sort of flow your getting, automotive heads NEVER get laminar flow, its always turbulent ie reynolds numbers above 2000.
Any airflow testing should only be considered as an indicator of relative restriction, not a hard and fast rule, thus 240 cfm (which is VERY good for a 47mm valve) when compared to a 2v cleveland head which flows about the same, but only work reworked with 4v valves, gives you an idea that the OHC head is a goodie and can probably only be made worse by porting.
Likewise the airflow of a carby or throttle in the case of EFI is only measured at a given DP and airflow is entire dependant on that.
Now what I do to compare airflows between carbs and throttles is to forget the airflow figure, but work out the cross sectional areas of the throttles, this also works for sizing mixers with LPG.
So a 60mm throttle could be around the same as two 42mm throttles which is about the size of a 500 two barrel holley, which when allowance for the different test [ressures between two and four barrel carbs is about 400cfm @ 25inches.
So rememeber that its all relative, the 250-2v heads is better than a 250-2v, the crossflow better again, the OHC still better.
A7m
First off gents ill let you know where Im coming from, 30 years auto trade work, 25 years of sub professional engineering work, some airflow testing, lotsa trial and error.
You need to remember that your dealing with a compressable medium here so flow isnt like water in a pipe.
consider reynolds number when trying to decide what sort of flow your getting, automotive heads NEVER get laminar flow, its always turbulent ie reynolds numbers above 2000.
Any airflow testing should only be considered as an indicator of relative restriction, not a hard and fast rule, thus 240 cfm (which is VERY good for a 47mm valve) when compared to a 2v cleveland head which flows about the same, but only work reworked with 4v valves, gives you an idea that the OHC head is a goodie and can probably only be made worse by porting.
Likewise the airflow of a carby or throttle in the case of EFI is only measured at a given DP and airflow is entire dependant on that.
Now what I do to compare airflows between carbs and throttles is to forget the airflow figure, but work out the cross sectional areas of the throttles, this also works for sizing mixers with LPG.
So a 60mm throttle could be around the same as two 42mm throttles which is about the size of a 500 two barrel holley, which when allowance for the different test [ressures between two and four barrel carbs is about 400cfm @ 25inches.
So rememeber that its all relative, the 250-2v heads is better than a 250-2v, the crossflow better again, the OHC still better.
A7m
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Anonymous
Guest
There is another thing to consider with the cross sectional area though.
There is a pressure drop across the venturi in a carby which leads to less potential for flow. The carby will actually use some of the ariflow's energy to atomise the fuel.
When measuring the cross sectional area of the holley you would have to be measuring the area of the choke or venturi not the area at the butterflies.
Also your last statement about the OHC head beiong better than the crossflow I have found to be untrue recently.
My engine is actaully a crossflow head flowing 240cfm @ 25 inches. The guy who does my cylinder head was amazed at what he got out of it. He was saying it was about 15 cfm down on the best RB26 skyline head he has ever done and is better than any SOHC falcon head he has done.
He has a touring car cylinder head which is the Chev style 18° head. it flows 315 cfm @ 700 thou lift @ 25 "
he showed me the superflow book which stated the theoretical maximum flow capability of that style head and he was flowing more than it! His head was flowing more than the theoretical maxium for a Hemi style chamber! I think they need to update their theoretical maximums!
He reckons the chamber design lends itself to better flow than the OHC engine.
There is a pressure drop across the venturi in a carby which leads to less potential for flow. The carby will actually use some of the ariflow's energy to atomise the fuel.
When measuring the cross sectional area of the holley you would have to be measuring the area of the choke or venturi not the area at the butterflies.
Also your last statement about the OHC head beiong better than the crossflow I have found to be untrue recently.
My engine is actaully a crossflow head flowing 240cfm @ 25 inches. The guy who does my cylinder head was amazed at what he got out of it. He was saying it was about 15 cfm down on the best RB26 skyline head he has ever done and is better than any SOHC falcon head he has done.
He has a touring car cylinder head which is the Chev style 18° head. it flows 315 cfm @ 700 thou lift @ 25 "
he showed me the superflow book which stated the theoretical maximum flow capability of that style head and he was flowing more than it! His head was flowing more than the theoretical maxium for a Hemi style chamber! I think they need to update their theoretical maximums!
He reckons the chamber design lends itself to better flow than the OHC engine.
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Anonymous
Guest
I wonder why that is?
the hemispherical combustion chamber of the SOHC I6 will allow fitment of larger valves, which in turn allows greater valve area, which should equate to more flow.. !
the hemispherical combustion chamber of the SOHC I6 will allow fitment of larger valves, which in turn allows greater valve area, which should equate to more flow.. !
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Anonymous
Guest
The superflow figures showe the hemispherical head having a higher theoretical maximum than the wedge head like the 18° chev but my cylinder head guy is outflowing the hemi theoretical maximum using a wedge head!
I'm sure if he spent the same amount of time mucking around with a hemi he would flow better again but all his development goes into the touring car style head as his clients are touring car teams.
I'm sure if he spent the same amount of time mucking around with a hemi he would flow better again but all his development goes into the touring car style head as his clients are touring car teams.
