Man, Im sorry guys if this has been discussed in a thread somewhere, but if you can find it, youre a better man than me. Can anyone tell me the basics of reading a turbo chart? I have NO idea whats going on and Ive tried and tried. Your help is always appreciated.
Say a chart such as this...
How to read a turbo chart..
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Shaefer Thomas
- Posts: 193
- Joined: Wed Mar 31, 2004 6:53 am
- Car: 1992 240sx
oh boy, here we go.
the y axis [pressure ratio] is the ratio of the pressure comming out of the turbo vs the air comming in. 14 psi of boost would be a pressure ratio of 2. [2x atmosphere] 28 psi would be a pr of 3.
now, the hard part. alot of flow charts [including your example] use mass airflow. in this case, the airflow depends on the displacement of the engine, its volumetric efficiency, and the pressure and temperature of the incoming air.
im assuming you want the full instruction set, and that will take some time to type.
tell me the engine your trying to match, and ill show you how it works.
the y axis [pressure ratio] is the ratio of the pressure comming out of the turbo vs the air comming in. 14 psi of boost would be a pressure ratio of 2. [2x atmosphere] 28 psi would be a pr of 3.
now, the hard part. alot of flow charts [including your example] use mass airflow. in this case, the airflow depends on the displacement of the engine, its volumetric efficiency, and the pressure and temperature of the incoming air.
im assuming you want the full instruction set, and that will take some time to type.
tell me the engine your trying to match, and ill show you how it works.
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Shaefer Thomas
- Posts: 193
- Joined: Wed Mar 31, 2004 6:53 am
- Car: 1992 240sx
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Shaefer Thomas
- Posts: 193
- Joined: Wed Mar 31, 2004 6:53 am
- Car: 1992 240sx
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hannibal
- Posts: 9680
- Joined: Wed Sep 11, 2002 2:38 am
- Car: Red Line to Glenmont
- Location: Washington DC
Ivory, ive never seen a comp map plotted like that. But I think I follow you.At 7000rpm and no boost (PR=1), a KA will flow just under 20lb/min (18??).At 7000rpm and 2bar of boost (PR=3), the KA will flow about 55lb/min (18x3). This is shown on the map by the pink line.So I guess you would just trace the line until you reach your desired PR and note the airflow and that point. For example, if youre running 7psi (almost 0.5bar --> PR=1.5), your KA would flow about 28lb/min of air at 7000rpm.
I hope that explained the lines, but I know it doesnt say anything about how theyre used.Here's a good link to read...http://turbosaturns.net/articl...s.htm
I hope that explained the lines, but I know it doesnt say anything about how theyre used.Here's a good link to read...http://turbosaturns.net/articl...s.htm
you got it.
you see, what gets the maps complicated is that the chart deals with pressure ratio. you take the ratio of the absolute pressure entering the turbo, versus the absolute pressure leaving.
psia is absolute pressure. psig is gauge pressure. gauge pressure is absolute pressure minus atmospheric pressure, its the difference between the pressure your reading and the atmospheric pressure.
if you are reading 5 psi on a gauge, and its 14.7 psia outside, then the absolute pressure is 19.7 psi.
if your at sea level, your dealing with 14.7 psia on a nice day.
say you wanted to see how much air you were pushing at 14 psi of boost at 5000 rpm.
you would follow the blue line [5k rpm] up to 2PR [14.7 psia compressor input] and see the motor is moving about 26 lbs/min at that pressure.
the pressure ratio is always slightly higher than calculation, because there exists a slight vacuum at the compressor inlet, and the pressure drop across the ic and piping.
ill explain the calculations in a minute.
that turbo is a good size for highend power. you should see boost by 3000 rpm. its just slightly too big. if you have to limit your boost below 3000 rpm [14 psi max] or it will surge.
you see, what gets the maps complicated is that the chart deals with pressure ratio. you take the ratio of the absolute pressure entering the turbo, versus the absolute pressure leaving.
psia is absolute pressure. psig is gauge pressure. gauge pressure is absolute pressure minus atmospheric pressure, its the difference between the pressure your reading and the atmospheric pressure.
if you are reading 5 psi on a gauge, and its 14.7 psia outside, then the absolute pressure is 19.7 psi.
if your at sea level, your dealing with 14.7 psia on a nice day.
say you wanted to see how much air you were pushing at 14 psi of boost at 5000 rpm.
you would follow the blue line [5k rpm] up to 2PR [14.7 psia compressor input] and see the motor is moving about 26 lbs/min at that pressure.
the pressure ratio is always slightly higher than calculation, because there exists a slight vacuum at the compressor inlet, and the pressure drop across the ic and piping.
ill explain the calculations in a minute.
that turbo is a good size for highend power. you should see boost by 3000 rpm. its just slightly too big. if you have to limit your boost below 3000 rpm [14 psi max] or it will surge.
first, you need to do some calculations.
you need to convert metric displacement to cubic inches.
2.4L x 61 [cubic inches per liter] = 146 ci [ i rounded to 145 for the graph].
now, you need the absolute temperature in rankin. rankin is on the farenheit scale. since i used 70F, you need to add 70 to 460. 460 is the absolute temperature of 0F. 460 + 70 = 530 degrees rankin.
and were using sea level [14.7 psia]
now, take your displacement in ci, and divide it by 1728. write that number down, because its constant through the calculations because displacement doesnt change. [ 0.08449074]
now, the other half of the volume flow equation is to multiply that number by one half of RPM. rpm needs to be divided in half because the KA is a 4 cycle motor and only half the displacement is pumped per revolution. for 5000 rpm, you multiply 0.08449074 by 2500 [half of 5000]
at 5000 rpm, the motor is pumping 211.22685 CFM [cubic feet per minute] you need to do that calculation for every rpm point you want to graph.
CFM= ( CID/1728)x(RPM/2)
now, you need to convert the raw CFM into a mass flow that is useful.
heres where it gets sticky, after you have calculated the CFM flow for the RPM points you want to graph, you need to calculate the mass flow for all the pressure ratios you want to see. if you use 1 2 and 3 as the PR's, you just have to plot the 3 points on the graph for the specific RPM, and connect the dots.
lets continue with 5000 RPM.
a PR of 1 is no boost. its atmospheric pressure. therefore we will use 14.7 psi as the absolute pressure since theres no compression in the turbo.
lbs/min= (absolute pressure * CFM * 29) / ( 10.73 * absolute temperature)
for a PR of 1, take 14.7 psi, multiply by 211.22685, and multiply that number by 29. you get 90046.006155
take the absolute temperature of 530 rankin [70F] and multiply by 10.73 you get 5686.9
divide 90046.006155 by 5686.9 and you get 15.8339 lbs/min
multiply the lbs/min by .85 [see explanation at bottom] =13.458815 lbs/min. therefore, the dot for 5k RPM and a PR of 1 would go on the intersection of PR 1 and 13.46lbs/min on the flow chart.
{when you figure for a PR of 2, you use 29.4 for absolute pressure.a PR of 3 is 44.1 [this is assuming 14.7 psi atmospheric pressure.]}
you need to do the first equation for every RPM point you want to graph, and you need to do the second equation for every PR you want to plot for each RPM line.
for example, i did the first equation 4 times since i calculated 4 RPM lines. next, i had to do the second equation for each PR point on each RPM line. i used 3 points [pr 1,2,3]. therfore i used the first equation 4 times, and the second equation 12 times. [3 pr points, 4 rpm lines]
the lines are formed by connecting the dots you form from the second equation.
now, after you have done all the calculations for the points you want to graph, you need to multiply the final numbers by .85 that corrects for volumetric efficiency.
after you have corrected all the points [*0.85], plot them on the flow chart. connect the dots you make.
you need to convert metric displacement to cubic inches.
2.4L x 61 [cubic inches per liter] = 146 ci [ i rounded to 145 for the graph].
now, you need the absolute temperature in rankin. rankin is on the farenheit scale. since i used 70F, you need to add 70 to 460. 460 is the absolute temperature of 0F. 460 + 70 = 530 degrees rankin.
and were using sea level [14.7 psia]
now, take your displacement in ci, and divide it by 1728. write that number down, because its constant through the calculations because displacement doesnt change. [ 0.08449074]
now, the other half of the volume flow equation is to multiply that number by one half of RPM. rpm needs to be divided in half because the KA is a 4 cycle motor and only half the displacement is pumped per revolution. for 5000 rpm, you multiply 0.08449074 by 2500 [half of 5000]
at 5000 rpm, the motor is pumping 211.22685 CFM [cubic feet per minute] you need to do that calculation for every rpm point you want to graph.
CFM= ( CID/1728)x(RPM/2)
now, you need to convert the raw CFM into a mass flow that is useful.
heres where it gets sticky, after you have calculated the CFM flow for the RPM points you want to graph, you need to calculate the mass flow for all the pressure ratios you want to see. if you use 1 2 and 3 as the PR's, you just have to plot the 3 points on the graph for the specific RPM, and connect the dots.
lets continue with 5000 RPM.
a PR of 1 is no boost. its atmospheric pressure. therefore we will use 14.7 psi as the absolute pressure since theres no compression in the turbo.
lbs/min= (absolute pressure * CFM * 29) / ( 10.73 * absolute temperature)
for a PR of 1, take 14.7 psi, multiply by 211.22685, and multiply that number by 29. you get 90046.006155
take the absolute temperature of 530 rankin [70F] and multiply by 10.73 you get 5686.9
divide 90046.006155 by 5686.9 and you get 15.8339 lbs/min
multiply the lbs/min by .85 [see explanation at bottom] =13.458815 lbs/min. therefore, the dot for 5k RPM and a PR of 1 would go on the intersection of PR 1 and 13.46lbs/min on the flow chart.
{when you figure for a PR of 2, you use 29.4 for absolute pressure.a PR of 3 is 44.1 [this is assuming 14.7 psi atmospheric pressure.]}
you need to do the first equation for every RPM point you want to graph, and you need to do the second equation for every PR you want to plot for each RPM line.
for example, i did the first equation 4 times since i calculated 4 RPM lines. next, i had to do the second equation for each PR point on each RPM line. i used 3 points [pr 1,2,3]. therfore i used the first equation 4 times, and the second equation 12 times. [3 pr points, 4 rpm lines]
the lines are formed by connecting the dots you form from the second equation.
now, after you have done all the calculations for the points you want to graph, you need to multiply the final numbers by .85 that corrects for volumetric efficiency.
after you have corrected all the points [*0.85], plot them on the flow chart. connect the dots you make.
BTW- absolute pressure in psi, absolute temperature in rankin.
rankin = farenheit temperature + 460
also, the actual pressure ratio is always higher than what you calculate [slightly] due to the slight vacuum at the inlet created by the turbo breathing in, and the pressure drop throught your intake path [piping, IC]
another words, if you calculate a PR of 2, its most likely more like 2.2
confusing, isnt it.
last, but not least, the way to find the "rating" is to find the point where the highest compressor rpm line meets the line of least efficiency. this turbo would be rated at 52lbs/min.
ill update the chart.
rankin = farenheit temperature + 460
also, the actual pressure ratio is always higher than what you calculate [slightly] due to the slight vacuum at the inlet created by the turbo breathing in, and the pressure drop throught your intake path [piping, IC]
another words, if you calculate a PR of 2, its most likely more like 2.2
confusing, isnt it.
last, but not least, the way to find the "rating" is to find the point where the highest compressor rpm line meets the line of least efficiency. this turbo would be rated at 52lbs/min.
ill update the chart.
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hannibal
- Posts: 9680
- Joined: Wed Sep 11, 2002 2:38 am
- Car: Red Line to Glenmont
- Location: Washington DC
Ive been looking for a formula used to convert cfm to lb/minbased on pressure and temp. I usually just estimate using lb/min = cfm x 0.069.Thanks!IvoryJ30t wrote:lbs/min= (absolute pressure * CFM * 29) / ( 10.73 * absolute temperature)
Question about VE...Since VE changes by rpm, wouldnt it be better to use different VE % for different rpms?I read on here a way to estimate VE by rpm. They said to look a dyno plot (a stock KA) and assume a peak VE (say 85%) occurs at the torque peak. For example, if peak torque is 145 lb-ft, that implies torque would be 170lb-ft at 100% VE, since 145 / 0.85 = 170 (written another way 170 x 0.85 = 145). You would then look at the dyno sheet and find the torque output at each rpm point. That value divided by 170lb-ft (our theoretical torque at 100% VE) would tell you the VE at that rpm. For example, if torque falls off to 120lb-ft at 6000rpm, that estimates a VE of 70% (at 6000rpm).IIRC that gives VE's of over 80% around 4000-4500rpm (near the torque peak) down to 65% near 6500-7000rpm. It drops pretty low at lower rpms (50-55% at 2500-3000rpm).
Your thoughts??
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IveBeenBad
- Posts: 1138
- Joined: Sun Jan 12, 2003 11:53 am
- Car: 1990 Nissan 240SX Fastback STOCK BIOTCH
I was bored so I made an excel worksheet...
It should work for all engines withan even number of cylinders since it asks for the # of cylinders and divides by 2.
http://members.cox.net/scott24...s.xls
It should work for all engines withan even number of cylinders since it asks for the # of cylinders and divides by 2.
http://members.cox.net/scott24...s.xls
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Shaefer Thomas
- Posts: 193
- Joined: Wed Mar 31, 2004 6:53 am
- Car: 1992 240sx
i forgot to mention, flow maps that use volume air flow [cfm] use a slightly different means to rate the compressor [max flow]
the flow "rating" is accomplished as such-
on a volume air flow chart, the compressor is rated by matching the line of least efficiency with its intersection of PR 2.0
this is in contrast with the ratings for mass airflow charts which come to their rating by matching the line of higest impeller speed with the line of least efficiency and reading the airflow at that point.
**EDITED**
the flow "rating" is accomplished as such-
on a volume air flow chart, the compressor is rated by matching the line of least efficiency with its intersection of PR 2.0
this is in contrast with the ratings for mass airflow charts which come to their rating by matching the line of higest impeller speed with the line of least efficiency and reading the airflow at that point.
**EDITED**
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IveBeenBad
- Posts: 1138
- Joined: Sun Jan 12, 2003 11:53 am
- Car: 1990 Nissan 240SX Fastback STOCK BIOTCH
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IveBeenBad
- Posts: 1138
- Joined: Sun Jan 12, 2003 11:53 am
- Car: 1990 Nissan 240SX Fastback STOCK BIOTCH
-
IveBeenBad
- Posts: 1138
- Joined: Sun Jan 12, 2003 11:53 am
- Car: 1990 Nissan 240SX Fastback STOCK BIOTCH
-
IveBeenBad
- Posts: 1138
- Joined: Sun Jan 12, 2003 11:53 am
- Car: 1990 Nissan 240SX Fastback STOCK BIOTCH
It calculates VE for each rpm point using the method I described above. Still trying to solve the high rpm delimma. Right now, it says the KA flows more at 5500rpm than redline...