A friend asked a question of me that caused me think about it and then turn to this board. How do you calculate port speed on a tubo engine?
Let's say it's 80 ci and makes 430HP @ 27# of boost. The only thing I could think of was to work backwards from HP.......using BSFC......and then assume a certain A/F ratio to get pounds of air and then convert that to CFM. 430HP requires @ .4 BSFC 172#/hr of fuel or or 2.86#/min........if a/f is 12.5 this means you are using 35.8# air/min which translates into around 468 CFM assuming the weight of air to be .0765#/CF I know there are variations between engine CFM and piston/cylinder CFM but I don't know what to use but the 468 CFM. The total area of the smallest cross sectional area of each port is approx. 1.8 sq in.
Using Maxraces' formula of Port Speed = CFM*2.4/CA, I came up with port speeds of around 624 ft/min.....which is less than I had suspected. I also used some HP and CFM formulas but they yielded radically different results because, I think, they are NA driven. As always, comments and insight is welcomed.......just trying to learn guys.
Boost and Port Speed
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Re: Boost and Port Speed
I too am very interested to see what the more experienced members of this forum have to say. I will comment thoug, that a BSFC of .4 would be unlikely for a forced induction engine- but am curious-is that the actuall measured BSFC, or and educated guess?SteveS wrote:A get pounds of air and then convert that to CFM. 430HP requires @ .4 BSFC .
Your speed is wrong because your air weight does not include density factor for boost. If not intercooled and 27 psi, efficiency at 70% and 70F for ambient, density ratio is 1.9.
Last edited by panic on Sun May 22, 2005 9:15 pm, edited 1 time in total.
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BillyShope
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First, your port speed formula yields a speed in feet per second, NOT feet per minute.
144/60 = 2.4
Your use of the BSFC for this calculation is quite clever. You would, however, need to know the WOT BSFC at the engine speed corresponding to the horsepower peak (assuming that it is at this point that you wish to know the port speed) and you must know, accurately, the F/A ratio. (I've only used the BSFC data, in conjunction with the road load torque, to calculate fuel mileage.)
I would have simply calculated the flow based on the absolute pressure ratio correction (as another poster has mentioned) and an estimate of volumetric efficiency. Interestingly, when I used 7000 rpm for the speed and 100% volumetric efficiency, I got a standard air volume flow rate almost exactly equal to your own value. But, of course, with a more realistic volumetric efficiency, I lose 100 CFM or so.
144/60 = 2.4
Your use of the BSFC for this calculation is quite clever. You would, however, need to know the WOT BSFC at the engine speed corresponding to the horsepower peak (assuming that it is at this point that you wish to know the port speed) and you must know, accurately, the F/A ratio. (I've only used the BSFC data, in conjunction with the road load torque, to calculate fuel mileage.)
I would have simply calculated the flow based on the absolute pressure ratio correction (as another poster has mentioned) and an estimate of volumetric efficiency. Interestingly, when I used 7000 rpm for the speed and 100% volumetric efficiency, I got a standard air volume flow rate almost exactly equal to your own value. But, of course, with a more realistic volumetric efficiency, I lose 100 CFM or so.
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11secAvanti
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SStrokerAce
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Plus are you looking for the peak velocity or the average?
If the cubes and port cross section have a lot to do with your velocities just like a NA motor. In a boosted application the temp has a lot more to do with power. You can easily see 100 deg change from no intercooler to a good one so this can really play havoc on a VE%. Personally I like a bigger cross section on a boosted motor than I would on the NA motor, lowering the velocity seems to help the denser air charge much better. You can find power just increasing the cross section of the cylinder head and keeping the same flow numbers, as long as the port is still healthy in terms of turbulence.
Bret
If the cubes and port cross section have a lot to do with your velocities just like a NA motor. In a boosted application the temp has a lot more to do with power. You can easily see 100 deg change from no intercooler to a good one so this can really play havoc on a VE%. Personally I like a bigger cross section on a boosted motor than I would on the NA motor, lowering the velocity seems to help the denser air charge much better. You can find power just increasing the cross section of the cylinder head and keeping the same flow numbers, as long as the port is still healthy in terms of turbulence.
Bret
I spent quite some time finding this little toy.
http://not2fast.wryday.com/turbo/glossa ... calc.shtml
If you use it for my application, the bore is 81mm, stroke 63mm, 4 cylinders, altitude 930, a/f=12, HP=430 STD, boost=27 and RPM=10800. No intercooler and no water.
http://not2fast.wryday.com/turbo/glossa ... calc.shtml
If you use it for my application, the bore is 81mm, stroke 63mm, 4 cylinders, altitude 930, a/f=12, HP=430 STD, boost=27 and RPM=10800. No intercooler and no water.
I just messed around with the calculator. came up aound 100 hp short on my combination.(Audi TT engine)SteveS wrote:I spent quite some time finding this little toy.
http://not2fast.wryday.com/turbo/glossa ... calc.shtml
If you use it for my application, the bore is 81mm, stroke 63mm, 4 cylinders, altitude 930, a/f=12, HP=430 STD, boost=27 and RPM=10800. No intercooler and no water.
27psi with no intercooler or water = detonation city. I'm curious, what engine are you working with?