Airflow vs. HP
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From: Cincinnati, OH, USA
Airflow vs. HP
Hope this is "advanced" enough for the forum, couldn't decide for sure.
If an engine can suck in more air at a given point, is it safe to say it will make more power (given enough fuel/spark) at that point than the point right before where it sucks in less air?
The reason I ask is, many people on here say the stock LT1 doesn't need to/shouldn't be revved over ~5200. I went to the track yesterday for the first time in a few years and my first run I revved a bit high, since I'm not used to taking each gear all the way out.
I went as high as 5800.
Well, the thing is, from my logs the reported airflow from the MAF sensor continued to rise until I shifted (I think max was around 274). Does this mean that the engine is continuing to make more power all the way until when I shifted at 5800? It was showing much more airflow at 5400-5600 than at 5000-5200, for example.
What's the consensus?
If an engine can suck in more air at a given point, is it safe to say it will make more power (given enough fuel/spark) at that point than the point right before where it sucks in less air?
The reason I ask is, many people on here say the stock LT1 doesn't need to/shouldn't be revved over ~5200. I went to the track yesterday for the first time in a few years and my first run I revved a bit high, since I'm not used to taking each gear all the way out.
I went as high as 5800.Well, the thing is, from my logs the reported airflow from the MAF sensor continued to rise until I shifted (I think max was around 274). Does this mean that the engine is continuing to make more power all the way until when I shifted at 5800? It was showing much more airflow at 5400-5600 than at 5000-5200, for example.
What's the consensus?
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From: houston, Tx
Basically you are right. Performance engine building is centered around getting more air and fuel through an engine and burning it efficiently. This where power comes from. This means better heads and cams or power adders. This allows better rpm and/or VE or both. With better breathing you can run a bigger engine to the same rpms or the same engine to higher rpms or with really good heads and intake etc you can run a bigger engine to higher rpm! If you can do ths you make more power and go faster.
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From: Michigan is a decent state.
Different engines with different heads/cam/intake/induction type ect setups will acheive different max vol. eff. at different points.
Overall, its about getting the most power and your power comes from fuel. at a given A:f ratio, the only way to increase fuel input (and power) is to get more air in. So yes, more air in = more power in a super simplfied world.
As RPMs increase, the engine will suck air in faster - but the valves will also stay open for a shorter period of time. it becomes a really big really complicated related rates problem
you also run into other problems like reversion and valve float and others for forced induction guys- a WHOLE big list so its not as simple as "As RPMs increase, the engine will suck air in faster- but the valves will also stay open for a shorter period of time." But its a start.
Overall, its about getting the most power and your power comes from fuel. at a given A:f ratio, the only way to increase fuel input (and power) is to get more air in. So yes, more air in = more power in a super simplfied world.
As RPMs increase, the engine will suck air in faster - but the valves will also stay open for a shorter period of time. it becomes a really big really complicated related rates problem
you also run into other problems like reversion and valve float and others for forced induction guys- a WHOLE big list so its not as simple as "As RPMs increase, the engine will suck air in faster- but the valves will also stay open for a shorter period of time." But its a start.
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From: Cincinnati, OH, USA
OK, but I think you guys missed the main point/question of my post.
If my logs show that the MAF airflow continues to rise as the RPMs rise, even up to 5800 RPM, does that I mean I should shift that high or higher, even though this is a stock heads/cam engine?
Am I really making more power this high in the band? Fred, Bret, Rich, etc.?
If my logs show that the MAF airflow continues to rise as the RPMs rise, even up to 5800 RPM, does that I mean I should shift that high or higher, even though this is a stock heads/cam engine?
Am I really making more power this high in the band? Fred, Bret, Rich, etc.?
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Where you shift depends upon the torque in each gear. Usually going to a higher RPM in first is needed. 5800rpm, is pretty low. I'm not a computer guy but my guess would be that the MAF is always going to see higher airflow values at higher RPM due to the fact that the motor is pumping more air in the higher the engine speed. Even if it's makeing less power because the frictional losses are higher.
Bret
Bret
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From: Upstate NY
Re: Airflow vs. HP
Originally posted by JSK333
Hope this is "advanced" enough for the forum, couldn't decide for sure.
If an engine can suck in more air at a given point, is it safe to say it will make more power (given enough fuel/spark) at that point than the point right before where it sucks in less air?
The reason I ask is, many people on here say the stock LT1 doesn't need to/shouldn't be revved over ~5200. I went to the track yesterday for the first time in a few years and my first run I revved a bit high, since I'm not used to taking each gear all the way out. I went as high as 5800.
Well, the thing is, from my logs the reported airflow from the MAF sensor continued to rise until I shifted (I think max was around 274). Does this mean that the engine is continuing to make more power all the way until when I shifted at 5800? It was showing much more airflow at 5400-5600 than at 5000-5200, for example.
What's the consensus?
Hope this is "advanced" enough for the forum, couldn't decide for sure.
If an engine can suck in more air at a given point, is it safe to say it will make more power (given enough fuel/spark) at that point than the point right before where it sucks in less air?
The reason I ask is, many people on here say the stock LT1 doesn't need to/shouldn't be revved over ~5200. I went to the track yesterday for the first time in a few years and my first run I revved a bit high, since I'm not used to taking each gear all the way out.
I went as high as 5800.Well, the thing is, from my logs the reported airflow from the MAF sensor continued to rise until I shifted (I think max was around 274). Does this mean that the engine is continuing to make more power all the way until when I shifted at 5800? It was showing much more airflow at 5400-5600 than at 5000-5200, for example.
What's the consensus?
IMO, using MAF measured airflow is not a good way to determine shift points. Because of the shape of the torque curve, and the gear multiplication of torque, there is a point where torque to the wheels in second gear equals torque to the wheels in first gear, and you should shift. Often that point occurs above the redline in the lower gears.
There is some fairly lowcost software performance simulators which predict optimum shift points quite well. They need good hp or torque vs. rpm data, and reasonable car data. They take into account friction, driveline efficiency, aero drag and inertia, which is difficult to do just by multiplying torque points by gear ratios and plotting them, the method we used 40+ years ago.
Another good way is to perform shift point tests for each gear independently. A stopwatch and a helper are all that's needed. With an auto, you could skip the helper at least for the 1-2 and 2-3 shifts. A very accurate, recording g-meter would be still another way to determine shift points. Few of us have that option.
This is all a roundabout way of saying that shifting at max power point isn't necessarily optimum, nor is shifting at rev limiter.
My $.02
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From: Woodstock,Georgia
Re: Airflow vs. HP
Originally posted by JSK333
Well, the thing is, from my logs the reported airflow from the MAF sensor continued to rise until I shifted (I think max was around 274). Does this mean that the engine is continuing to make more power all the way until when I shifted at 5800? It was showing much more airflow at 5400-5600 than at 5000-5200, for example.
What's the consensus?
Well, the thing is, from my logs the reported airflow from the MAF sensor continued to rise until I shifted (I think max was around 274). Does this mean that the engine is continuing to make more power all the way until when I shifted at 5800? It was showing much more airflow at 5400-5600 than at 5000-5200, for example.
What's the consensus?
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From: Rhode Island
I have looked over all responces but I dont think anyone has given you a simple answer you were looking for. So I will do my best to keep this simple, your point about the mass airflow does make sence but there are to many other factors to take into consideration. I wouldnt base everthing on your mass air flow, the best way I have ever found to tune is much simpler ,get back to the track and make a base line run shifting at 5200 then each run go up 200 rpm then do the math and see if you picked up anything and at what part of th track. now obviosly dont go crazy you said you wre dealing with a stock engine you shouldnt be at 7000 rpm. hope that helps and answers your question.
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Originally posted by motorhed
I wouldnt base everthing on your mass air flow, the best way I have ever found to tune is much simpler ,get back to the track and make a base line run shifting at 5200 then each run go up 200 rpm then do the math and see if you picked up anything and at what part of th track. now obviosly dont go crazy you said you wre dealing with a stock engine you shouldnt be at 7000 rpm. hope that helps and answers your question.
I wouldnt base everthing on your mass air flow, the best way I have ever found to tune is much simpler ,get back to the track and make a base line run shifting at 5200 then each run go up 200 rpm then do the math and see if you picked up anything and at what part of th track. now obviosly dont go crazy you said you wre dealing with a stock engine you shouldnt be at 7000 rpm. hope that helps and answers your question.
I agree
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Torque, not horsepower, is directly related to an engine's ability to fill the cylinder(s) on each induction cycle. The volumetric efficiency (VE) refers to this. Therefore, max VE = max torque.
Horsepower, on the other hand, is a calculated value (HP = torque x rpm/ 5252). To the best of my knowledge, no-one has yet measured a single horsepower (blame Mr. Watt for that), yet it seems to be the overwhelming choice for comparing an engine's output.
Beyond the peak torque rpm, VE will decrease, and so will torque output. However, because because HP is based on rpm and torque, as long as rpm increases at a faster rate than the drop in torque output HP will increase. An increase in airflow, must be sufficient to delay the drop in VE at a rate less than the increase in rpm, if HP is to increase.
Simply put: an increase in airflow does not necessarily indicate an increase in HP. Short of putting the engine or car on a dyno, there is no simple way (i.e., no long, complicated measurements and formulas) to tell if HP is increasing. You would at least need to know the peak torque value(max VE) and the rpm at which it occurs and measure airflow at incremental rpm increases to make an educated guess at how rapidly VE(and torque) is decreasing relative to the increase in rpm.
Horsepower, on the other hand, is a calculated value (HP = torque x rpm/ 5252). To the best of my knowledge, no-one has yet measured a single horsepower (blame Mr. Watt for that), yet it seems to be the overwhelming choice for comparing an engine's output.
Beyond the peak torque rpm, VE will decrease, and so will torque output. However, because because HP is based on rpm and torque, as long as rpm increases at a faster rate than the drop in torque output HP will increase. An increase in airflow, must be sufficient to delay the drop in VE at a rate less than the increase in rpm, if HP is to increase.
Simply put: an increase in airflow does not necessarily indicate an increase in HP. Short of putting the engine or car on a dyno, there is no simple way (i.e., no long, complicated measurements and formulas) to tell if HP is increasing. You would at least need to know the peak torque value(max VE) and the rpm at which it occurs and measure airflow at incremental rpm increases to make an educated guess at how rapidly VE(and torque) is decreasing relative to the increase in rpm.
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From: W Hartford, CT
Simply put: an increase in airflow does not necessarily indicate an increase in HP. Short of putting the engine or car on a dyno, there is no simple way (i.e., no long, complicated measurements and formulas) to tell if HP is increasing.
Since VE is closely tied with torque, it tends to drop off after the torque peak. That means the engine is getting less air into the cylinders/revolution. If it's getting less air per rev, that means the MAF is flowing in less air per revolution, so the MAF reading per rev should drop down. Climbing rpms mean your sucking in more air per second, since the cycles are happening more often. This will counter-act the drop in VE, so the MAF should be fairly accurate. I don't know at what range it loses accuracy (/me running MAP here...)
If you (anyone here) has a datalogged dyno run, could you plot a graph with rpms on the bottom axis, and have the vertical axis be airflow/hp? If it's nearly a constant, that should help either validate this train of thought, or shoot it down.
Last edited by Z28tt; Oct 16, 2003 at 07:04 AM.
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From: houston, Tx
ZWILD1,
Most dynos measure power not torque. Some small engine dynos like a water brake Superflow measure tq and then figure hp from rpm but then some like the dynojet measure power and then figure tq from that if you have a tach pickup working.
All big dynos measure horspower. We have one here in Houston that can soak up 150 MW at full load ! That's 200,000 hp !
No one cares what the tq is anyway on most engines. You just gear it appropriately to get the working speed in rpm that you want. You could have a lot of tq at low rpm or a little tq at high rpm and still have the same power.
Most dynos measure power not torque. Some small engine dynos like a water brake Superflow measure tq and then figure hp from rpm but then some like the dynojet measure power and then figure tq from that if you have a tach pickup working.
All big dynos measure horspower. We have one here in Houston that can soak up 150 MW at full load ! That's 200,000 hp !
No one cares what the tq is anyway on most engines. You just gear it appropriately to get the working speed in rpm that you want. You could have a lot of tq at low rpm or a little tq at high rpm and still have the same power.
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From: Upstate NY
Originally posted by racer7088
ZWILD1,
Most dynos measure power not torque. Some small engine dynos like a water brake Superflow measure tq and then figure hp from rpm but then some like the dynojet measure power and then figure tq from that if you have a tach pickup working.
All big dynos measure horspower. We have one here in Houston that can soak up 150 MW at full load ! That's 200,000 hp !
No one cares what the tq is anyway on most engines. You just gear it appropriately to get the working speed in rpm that you want. You could have a lot of tq at low rpm or a little tq at high rpm and still have the same power.
ZWILD1,
Most dynos measure power not torque. Some small engine dynos like a water brake Superflow measure tq and then figure hp from rpm but then some like the dynojet measure power and then figure tq from that if you have a tach pickup working.
All big dynos measure horspower. We have one here in Houston that can soak up 150 MW at full load ! That's 200,000 hp !
No one cares what the tq is anyway on most engines. You just gear it appropriately to get the working speed in rpm that you want. You could have a lot of tq at low rpm or a little tq at high rpm and still have the same power.
At 200,000 hp I believe you are referring to very different engines from the 1200 hp or less most car guys have. Superflow's "small" engine dynos are capable of 1000+ hp, and measure torque.
While 200,000 hp internal combustion engines may exist, and are probably fascinating, they are not very applicable to automobiles, IMO.
I always thought that huge engines, like those used in very massive ocean-going ships weren't actually put on a dyno, but instead used torque measuring devices on the output shafts. Are you saying you have a 150 MW dyno? Must be a big hunk of machinery. I guess loss of the SCSC in TX had some fallout other than getting a great physisist into a major cam company. A pic of that dyno would be appreciated
Some folks do care what the torque is.
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NO, actually stroker, many automotive engine dynos use generators or eddy current type load cells that can measure power generated at a particular engine speed and backwards figures to torque or can see torgue through strain gauges of different types. The 150 MW load cell is an electrical load cell also to test large turbines at full load. There are turbine more powerful than this (up to 400 MW). Recipricating engines can go over 100,000 hp as far as I've seen.
Torque is mostly only an indicator of engine size but not power or acceleration ability. An F1 engine makes very little torque but spins nearly 20,000 rpm and it can accelerate extremely well due to it's 900+ hp. We have some large low rpm air cooled diesels that put out double the torque of an F1 engine but only make 125 hp due to the very low rpm they turn. These engine won't accelerate anything very fast.
The SF 901 has a small water brake and has limited hp capacity due to the cavitation of water at any even warm temperature. There are some stuska brakes and a big SF brake that can go pretty high in torque but only so far.
Torque is mostly only an indicator of engine size but not power or acceleration ability. An F1 engine makes very little torque but spins nearly 20,000 rpm and it can accelerate extremely well due to it's 900+ hp. We have some large low rpm air cooled diesels that put out double the torque of an F1 engine but only make 125 hp due to the very low rpm they turn. These engine won't accelerate anything very fast.
The SF 901 has a small water brake and has limited hp capacity due to the cavitation of water at any even warm temperature. There are some stuska brakes and a big SF brake that can go pretty high in torque but only so far.
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From: Upstate NY
Originally posted by racer7088
NO, actually stroker, many automotive engine dynos use generators or eddy current type load cells that can measure power generated at a particular engine speed and backwards figures to torque or can see torgue through strain gauges of different types.
Hmmm. All the eddy current and dc or ac generator dynos I have seen measure torque. The dyno frame is mounted on trunnions (bearings), and is connected to a load cell which measures force at a fixed distance giving torque. I realize you are in this field, but load cells don't measure power, they measure force.
The 150 MW load cell is an electrical load cell also to test large turbines at full load. There are turbine more powerful than this (up to 400 MW). Recipricating engines can go over 100,000 hp as far as I've seen.
Yep, these are load cells which are measuring torque, right? They are not absorbing the power.
Torque is mostly only an indicator of engine size but not power or acceleration ability. An F1 engine makes very little torque but spins nearly 20,000 rpm and it can accelerate extremely well due to it's 900+ hp. We have some large low rpm air cooled diesels that put out double the torque of an F1 engine but only make 125 hp due to the very low rpm they turn. These engine won't accelerate anything very fast.
Beg to differ. Actually 900 hp at 20,000 would only be about 236 lb-ft at the flywheel, but at 200 mph, it would be about 1825 ft-lb at the drive wheels with 26 inch tires (and neglecting driveline losses for simplicity). On the other hand, 900 hp @ 6000 rpm would be about 788 lb-ft at the flywheel, but @ 200 mph it would be the same 1825 ft-lb at the wheels.
The 20000 rpm engine runs a 7.74 overall gear ratio to get 200, but the 6000 rpm engine runs a 2.32 overall ratio @ 200. The higher gear ratio multiplies the torque more. Remember the wheels have no idea how fast the prime mover is turning.
The 20000 rpm engine in the example must accelerate about 3.33 times as fast as the 6000 rpm engine again due to the gearing. If you could package both of these engines and drivelines in the same vehicle, acceleration performance would be very similar (assuming the torque curves had the same general shape).
The SF 901 has a small water brake and has limited hp capacity due to the cavitation of water at any even warm temperature. There are some stuska brakes and a big SF brake that can go pretty high in torque but only so far.
As I said before, 1000+ hp capacity of the SF-901 for example, is right in there with what most of the folks around here have in their auto engines.
With sufficient cooling capacity for the water, the SF-901 and similar absorbers can be used for durability testing or race simulation.
I fully agree that above certain torque or hp levels a brake dynamometer is impractical. Even a 6000 hp @ 8000 rpm Top Fuel engine was a challenge to dyno when Kenny Bernstein tried it a few years ago.
Tell ya' what. Join us at the January AETC in Colorado Springs. There will be lots of dyno engineers there, and Hendrick Motorsports dyno guys are giving a presentation on measuring hp in an 800 hp Winston (Nextel?) Cup engine to an accuracy of 1 hp. Your expertise will certainly add to the fun.
My $.02
NO, actually stroker, many automotive engine dynos use generators or eddy current type load cells that can measure power generated at a particular engine speed and backwards figures to torque or can see torgue through strain gauges of different types.
Hmmm. All the eddy current and dc or ac generator dynos I have seen measure torque. The dyno frame is mounted on trunnions (bearings), and is connected to a load cell which measures force at a fixed distance giving torque. I realize you are in this field, but load cells don't measure power, they measure force.
The 150 MW load cell is an electrical load cell also to test large turbines at full load. There are turbine more powerful than this (up to 400 MW). Recipricating engines can go over 100,000 hp as far as I've seen.
Yep, these are load cells which are measuring torque, right? They are not absorbing the power.
Torque is mostly only an indicator of engine size but not power or acceleration ability. An F1 engine makes very little torque but spins nearly 20,000 rpm and it can accelerate extremely well due to it's 900+ hp. We have some large low rpm air cooled diesels that put out double the torque of an F1 engine but only make 125 hp due to the very low rpm they turn. These engine won't accelerate anything very fast.
Beg to differ. Actually 900 hp at 20,000 would only be about 236 lb-ft at the flywheel, but at 200 mph, it would be about 1825 ft-lb at the drive wheels with 26 inch tires (and neglecting driveline losses for simplicity). On the other hand, 900 hp @ 6000 rpm would be about 788 lb-ft at the flywheel, but @ 200 mph it would be the same 1825 ft-lb at the wheels.
The 20000 rpm engine runs a 7.74 overall gear ratio to get 200, but the 6000 rpm engine runs a 2.32 overall ratio @ 200. The higher gear ratio multiplies the torque more. Remember the wheels have no idea how fast the prime mover is turning.
The 20000 rpm engine in the example must accelerate about 3.33 times as fast as the 6000 rpm engine again due to the gearing. If you could package both of these engines and drivelines in the same vehicle, acceleration performance would be very similar (assuming the torque curves had the same general shape).
The SF 901 has a small water brake and has limited hp capacity due to the cavitation of water at any even warm temperature. There are some stuska brakes and a big SF brake that can go pretty high in torque but only so far.
As I said before, 1000+ hp capacity of the SF-901 for example, is right in there with what most of the folks around here have in their auto engines.
With sufficient cooling capacity for the water, the SF-901 and similar absorbers can be used for durability testing or race simulation. I fully agree that above certain torque or hp levels a brake dynamometer is impractical. Even a 6000 hp @ 8000 rpm Top Fuel engine was a challenge to dyno when Kenny Bernstein tried it a few years ago.
Tell ya' what. Join us at the January AETC in Colorado Springs. There will be lots of dyno engineers there, and Hendrick Motorsports dyno guys are giving a presentation on measuring hp in an 800 hp Winston (Nextel?) Cup engine to an accuracy of 1 hp. Your expertise will certainly add to the fun.
My $.02