Ram effect
02-23-2006, 01:02 AM
#1
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Ram effect I was actually wondering what speed ram air would start being effective. I was watching a show the other day and there was a guy in the wind tunnel and at 100mph they said there was 78lbs of pressure agaist him i think it was. Thats well above atmospheric pressure. Does the ram effect start at anything over atmospheric pressure. I'm guessing friction and how smooth the air flows has some effect on this though right? Sorry if I posted this in the wrong area.
02-23-2006, 03:44 AM
#2
TECH Fanatic
Take a look here http://sportrider.com/tech/146_9910_ram/
It works, look at the non ram air engine at the bottom compared to the rest.
Bottom line is lts only worth like 0.5psi or so max, though you get some effect from 70mph.
It works, look at the non ram air engine at the bottom compared to the rest.
Bottom line is lts only worth like 0.5psi or so max, though you get some effect from 70mph.
02-23-2006, 06:30 AM
#3
TECH Addict
Originally Posted by Red_Dragon
I was actually wondering what speed ram air would start being effective. I was watching a show the other day and there was a guy in the wind tunnel and at 100mph they said there was 78lbs of pressure agaist him i think it was. Thats well above atmospheric pressure. Does the ram effect start at anything over atmospheric pressure. I'm guessing friction and how smooth the air flows has some effect on this though right? Sorry if I posted this in the wrong area.
Originally Posted by 300bhp/ton
Marketers just can't resist it. Ram air! The words themselves summon up images of rushing wild beasts, or of secret military aircraft operating on futuristic principles.
Unfortunately, on most perofrmance cars, ram-air is as functional as tail fins were on cars of the ’60s.
What is it? Ram air just means using a forward-facing air intake to gain some extra intake pressure. We have all, as children, felt the pressure of moving air on our hands when we held them out the window of the family car. When moving air is brought smoothly to rest, the energy of its motion is converted into pressure. Motorcycles went through a "ram-air" period in the early 1990s, during which street bikes were equipped with the forward-facing "rocket-launcher" engine air intakes seen on many road-racing machines.
While it's appealing to imagine the forward velocity of a car being converted into free supercharge, the actual air pressure gain is extremely small at normal speeds. For example, at 150 mph, the pressure gain when air is efficiently brought to rest is 2.75 percent. Because this is a dynamic effect, it is proportional to the square of the air velocity. At a more realizable automobile speed of 75 mph, the effect (again with 100 percent efficient conversion of velocity into pressure) will be only one-quarter as great — that is, just under seven-tenths of one percent.
In fact, velocity energy is not converted into pressure at 100 percent efficiency. A figure of 75 percent efficiency is usual, which reduces our notional ram-air gain at 75 mph to one-half of one percent.
Therefore, at normal speeds, ram air is a myth. However, something much more interesting lies behind it, ignored by the advertiser's busy pen. That something is airbox resonance.
In order to implement ram air, the carburetors or throttle-bodies of our engine must seal to an airbox whose volume is large enough that the intake cycle of one cylinder cannot pull its internal pressure down significantly. Box volume is typically 10-20 times the engine's displacement. Then the forward-facing air intake is connected to the box. When this assembly is tested on the dyno — even without an external fan to simulate the high-speed rush of air past the intake — it is discovered that the engine's torque curve is greatly altered, with new peaks and hollows.
Why? The answer is airbox resonance. If you hold the mouth of an empty bottle near your open mouth as you loudly hum scales, you find that at certain “hum frequencies” the bottle reinforces your humming, which becomes louder. What is happening is that the springy compressibility of the air in the bottle is bouncing the slug of air in the bottle's neck back and forth at a particular frequency — higher if the bottle is small, lower if it is larger. Your humming is driving a rapid plus-and-minus variation of the air pressure inside the bottle.
The same thing happens inside a resonant airbox. The volume of air in the box is the “spring” in this kind of oscillator. The mass of air in the box's intake pipe is what oscillates. The “humming” that drives the oscillation is the rapid succession of suction pulses at the carb or throttle-body intakes. If the volume of the airbox and the dimensions of the intake pipe(s) are correctly chosen, the airbox can be made to resonate very strongly, in step with the engine's suction pulses. The result, when this is done correctly, is that the engine takes air from the box only during the high-pressure part of its cycle, while the box refills from atmosphere through its intake between engine suction pulses. This produces a useful gain in torque.
Using this idea, motorcycle engines have been able to realize torque increases, in particular speed ranges, of 10-15 percent. In race engines, it is usual to tune the airbox to resonate at peak-power rpm to increase top speed. For production engines, it is often more useful to tune the box resonance to fill in what would otherwise be a flat-spot in the torque curve, resulting in smoother power and improved acceleration.
Early resonant airbox systems used long intake pipes that terminated in forward-facing intakes. More recent designs do not connect the ram-air pipe to the box at all, but terminate it near the airbox entry. The actual entry pipe is a short piece of tubing with bellmouths on both ends. This is done because (a) the potential gain from actual ram air is too small to worry about, and (b) it's easier to tune the airbox with a short tube.
Where vehicle speeds are very high, gains from ram air are significant. This was discovered by Rolls-Royce in the late 1920s as the company developed its R Schneider Trophy air racing engine. At speeds above 300 mph, it was noticed that the R’s fuel mixture leaned out enough to cause backfiring. When the mixture was corrected for ram-air pressure gain, the engineers realized they had a "free" source of power. At 350 mph the gain from ram air is almost 15 percent. Similar mixture correction is necessary when ram air is used on drag-race and Bonneville cars and bikes.
Intuition suggests that a forward-facing intake made in the form of a funnel, large end foremost, should somehow multiply the pressure of the air, resulting in a much larger pressure gain at the small end. Sadly, intuition is wrong. In order to convert velocity energy into pressure, the air has to be slowed down, and this requires a duct that widens rather than narrows. Next time you fly on a commercial airliner, note that its engine intakes widen as the airflow approaches the compressor face. Such widening passages are called diffusers, and they are universally used in the conversion of velocity into pressure.
Language often plays tricks on us — especially when language is used by product advertisers. "Ram air" sounds much more appealing than "resonant airbox." Nevertheless, it is airbox resonance that actually generates a significant power gain.
------------------------------------------------------------------
Cliffnotes:
-Ram Air a myth? = NO
-Does it work on a road car = NO
-At 150mhp there is next to no gain.
-Significant gains arn't seen until 300mph+
-The air box is the key, not the ducting.
-When buying a CAI/induction kit look for the one that uses air box resonance
Edit: Additional Reading
Intake temperature is a whole different ball game.
The simple rule is:
'Cool for power (maximum charge density), hot for economy (minimum charge density to reduce losses due to throttling).'
Although in many cars the under bonnent temperatures are no where near as bad as many people beleive. This refers to a 5.3 liter Jguar XJS V12. So a big engine in a small engine bay.
The under-bonnet air temperature at idle can easily get up around 70 C but the faster the car goes the lower the air temperature falls - simply because the radiator is passing its heat to a much larger quantity of air per second - so at 80 mph. the engine is breathing air at around 45 C. That's still a bit higher than the ideal but not nearly so bad as many people think. Obviously the standard arrangement helps to maximise economy in moderate speed urban cruise without compromising top end power too much.
In setups that duct cool air from outside. The power gains from such a system are almost certainly attributed to the filter, and less restritive intake (meaning quite simply a bigger opening), and a form of air box resonance coupled with a 'cool air intake' from outside the engine bay. Sadly even at very high speeds (well over 100mph) I doubt that it has any form of 'Ram Air' effect. If you reconfigured the system to take air from the inside of a wheel arch it would produce the same results as having the intake ducts at the front of the car. The source of the air, not the location of the ducts is the important factor.
Remember the only way to get a greater volume of air into the engine is to compress it. This is what turbo and superchargers do. An air intake scoop either on the front of a car or on the bonnet will not compress the air at any speed most people are likely to travel at.
Taking the airbox resonance theory futher with the intake manifold itself by optimising the length and entry profile into each of the tracts to better exploit induced harmonic resonances in the air as it flows towards the cylinder. Any tube containing air can be made to resonate at certain critical frequencies in the manner of an organ pipe. Such is the case with the inlet tracts of an engine and if the natural resonance frequencies can be matched to the engine speed then a mild supercharging effect can be induced. Get it wrong and the reverse will apply, resulting in a loss of performance.
Unfortunately, on most perofrmance cars, ram-air is as functional as tail fins were on cars of the ’60s.
What is it? Ram air just means using a forward-facing air intake to gain some extra intake pressure. We have all, as children, felt the pressure of moving air on our hands when we held them out the window of the family car. When moving air is brought smoothly to rest, the energy of its motion is converted into pressure. Motorcycles went through a "ram-air" period in the early 1990s, during which street bikes were equipped with the forward-facing "rocket-launcher" engine air intakes seen on many road-racing machines.
While it's appealing to imagine the forward velocity of a car being converted into free supercharge, the actual air pressure gain is extremely small at normal speeds. For example, at 150 mph, the pressure gain when air is efficiently brought to rest is 2.75 percent. Because this is a dynamic effect, it is proportional to the square of the air velocity. At a more realizable automobile speed of 75 mph, the effect (again with 100 percent efficient conversion of velocity into pressure) will be only one-quarter as great — that is, just under seven-tenths of one percent.
In fact, velocity energy is not converted into pressure at 100 percent efficiency. A figure of 75 percent efficiency is usual, which reduces our notional ram-air gain at 75 mph to one-half of one percent.
Therefore, at normal speeds, ram air is a myth. However, something much more interesting lies behind it, ignored by the advertiser's busy pen. That something is airbox resonance.
In order to implement ram air, the carburetors or throttle-bodies of our engine must seal to an airbox whose volume is large enough that the intake cycle of one cylinder cannot pull its internal pressure down significantly. Box volume is typically 10-20 times the engine's displacement. Then the forward-facing air intake is connected to the box. When this assembly is tested on the dyno — even without an external fan to simulate the high-speed rush of air past the intake — it is discovered that the engine's torque curve is greatly altered, with new peaks and hollows.
Why? The answer is airbox resonance. If you hold the mouth of an empty bottle near your open mouth as you loudly hum scales, you find that at certain “hum frequencies” the bottle reinforces your humming, which becomes louder. What is happening is that the springy compressibility of the air in the bottle is bouncing the slug of air in the bottle's neck back and forth at a particular frequency — higher if the bottle is small, lower if it is larger. Your humming is driving a rapid plus-and-minus variation of the air pressure inside the bottle.
The same thing happens inside a resonant airbox. The volume of air in the box is the “spring” in this kind of oscillator. The mass of air in the box's intake pipe is what oscillates. The “humming” that drives the oscillation is the rapid succession of suction pulses at the carb or throttle-body intakes. If the volume of the airbox and the dimensions of the intake pipe(s) are correctly chosen, the airbox can be made to resonate very strongly, in step with the engine's suction pulses. The result, when this is done correctly, is that the engine takes air from the box only during the high-pressure part of its cycle, while the box refills from atmosphere through its intake between engine suction pulses. This produces a useful gain in torque.
Using this idea, motorcycle engines have been able to realize torque increases, in particular speed ranges, of 10-15 percent. In race engines, it is usual to tune the airbox to resonate at peak-power rpm to increase top speed. For production engines, it is often more useful to tune the box resonance to fill in what would otherwise be a flat-spot in the torque curve, resulting in smoother power and improved acceleration.
Early resonant airbox systems used long intake pipes that terminated in forward-facing intakes. More recent designs do not connect the ram-air pipe to the box at all, but terminate it near the airbox entry. The actual entry pipe is a short piece of tubing with bellmouths on both ends. This is done because (a) the potential gain from actual ram air is too small to worry about, and (b) it's easier to tune the airbox with a short tube.
Where vehicle speeds are very high, gains from ram air are significant. This was discovered by Rolls-Royce in the late 1920s as the company developed its R Schneider Trophy air racing engine. At speeds above 300 mph, it was noticed that the R’s fuel mixture leaned out enough to cause backfiring. When the mixture was corrected for ram-air pressure gain, the engineers realized they had a "free" source of power. At 350 mph the gain from ram air is almost 15 percent. Similar mixture correction is necessary when ram air is used on drag-race and Bonneville cars and bikes.
Intuition suggests that a forward-facing intake made in the form of a funnel, large end foremost, should somehow multiply the pressure of the air, resulting in a much larger pressure gain at the small end. Sadly, intuition is wrong. In order to convert velocity energy into pressure, the air has to be slowed down, and this requires a duct that widens rather than narrows. Next time you fly on a commercial airliner, note that its engine intakes widen as the airflow approaches the compressor face. Such widening passages are called diffusers, and they are universally used in the conversion of velocity into pressure.
Language often plays tricks on us — especially when language is used by product advertisers. "Ram air" sounds much more appealing than "resonant airbox." Nevertheless, it is airbox resonance that actually generates a significant power gain.
------------------------------------------------------------------
Cliffnotes:
-Ram Air a myth? = NO
-Does it work on a road car = NO
-At 150mhp there is next to no gain.
-Significant gains arn't seen until 300mph+
-The air box is the key, not the ducting.
-When buying a CAI/induction kit look for the one that uses air box resonance
Edit: Additional Reading
Intake temperature is a whole different ball game.
The simple rule is:
'Cool for power (maximum charge density), hot for economy (minimum charge density to reduce losses due to throttling).'
Although in many cars the under bonnent temperatures are no where near as bad as many people beleive. This refers to a 5.3 liter Jguar XJS V12. So a big engine in a small engine bay.
The under-bonnet air temperature at idle can easily get up around 70 C but the faster the car goes the lower the air temperature falls - simply because the radiator is passing its heat to a much larger quantity of air per second - so at 80 mph. the engine is breathing air at around 45 C. That's still a bit higher than the ideal but not nearly so bad as many people think. Obviously the standard arrangement helps to maximise economy in moderate speed urban cruise without compromising top end power too much.
In setups that duct cool air from outside. The power gains from such a system are almost certainly attributed to the filter, and less restritive intake (meaning quite simply a bigger opening), and a form of air box resonance coupled with a 'cool air intake' from outside the engine bay. Sadly even at very high speeds (well over 100mph) I doubt that it has any form of 'Ram Air' effect. If you reconfigured the system to take air from the inside of a wheel arch it would produce the same results as having the intake ducts at the front of the car. The source of the air, not the location of the ducts is the important factor.
Remember the only way to get a greater volume of air into the engine is to compress it. This is what turbo and superchargers do. An air intake scoop either on the front of a car or on the bonnet will not compress the air at any speed most people are likely to travel at.
Taking the airbox resonance theory futher with the intake manifold itself by optimising the length and entry profile into each of the tracts to better exploit induced harmonic resonances in the air as it flows towards the cylinder. Any tube containing air can be made to resonate at certain critical frequencies in the manner of an organ pipe. Such is the case with the inlet tracts of an engine and if the natural resonance frequencies can be matched to the engine speed then a mild supercharging effect can be induced. Get it wrong and the reverse will apply, resulting in a loss of performance.
Last edited by 300bhp/ton; 02-23-2006 at 06:44 AM.
02-23-2006, 07:34 AM
#6
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Originally Posted by MeentSS02
The key word being "internet." I wouldn't exactly call the internet a reliable source.
Trouble is you just have a bias against me, because I'd don't share your "lust" factor over dual exhaust setups on 4th gen Fbody's and you have conveniently ignored questions I asked you in a previous thread when you started whinging about one of my posts and info contain therein.
You like duals - fine I have no problem with this and I also like them. But I don't beleive they are the be all and end all, or the only option for Fbody's. You however seem not to share this view and apparantly don't like anyone offering a differing opinion even when supported by 3rd party links and justification for their claims.
02-23-2006, 11:18 AM
#7
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My own tests of ram air showed a significant 25hp gain by trap speed vs weight in the 1/4 mile.
This was in a car that consistently pulled 118 trap speed at the time of testing.
The results of this test were in humid summer heat. I feel the ram air device would've been more effective and start working earlier in cooler denser air, but my only test of it in cool air resulted in the same power gain but with some slight surging & detonation between 800 and 1320 ft of the run.
This circumstantially suggests that it was working more effectively, though not conclusive.
I think the curve is fairly steep between gains seen with slower cars compared with gains with faster cars due to the amount of time (% wise) that the ram air can be of benefit during the fixed test distance.
I don't think my ram air device was the best design possible, but I haven't had a chance to construct another design to test.
This was on a carbureted vehicle so there was no electronic adjustment in fuel for air density changes.
cliff notes:
The predictions of engineers don't always pan out in the real world.
This was in a car that consistently pulled 118 trap speed at the time of testing.
The results of this test were in humid summer heat. I feel the ram air device would've been more effective and start working earlier in cooler denser air, but my only test of it in cool air resulted in the same power gain but with some slight surging & detonation between 800 and 1320 ft of the run.
This circumstantially suggests that it was working more effectively, though not conclusive.
I think the curve is fairly steep between gains seen with slower cars compared with gains with faster cars due to the amount of time (% wise) that the ram air can be of benefit during the fixed test distance.
I don't think my ram air device was the best design possible, but I haven't had a chance to construct another design to test.
This was on a carbureted vehicle so there was no electronic adjustment in fuel for air density changes.
cliff notes:
The predictions of engineers don't always pan out in the real world.
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02-23-2006, 02:17 PM
#8
TECH Addict
I don't doubt your test results. But can you be 110% certain it was RAM AIR, as in the increase in air pressure that improved the performance.
And also be 110% certain that cool air ducts and air box resonance had ZERO affect?
And also be 110% certain that cool air ducts and air box resonance had ZERO affect?
02-23-2006, 02:35 PM
#9
Originally Posted by 300bhp/ton
Well, yes as much of the info is from Roger Bywater who is ex Cheif Engineer of Jagaur Cars and some of the info is from the chief engineer who desinged the engines on Concorde amoung other noted engineers as well. I can give you link for some of the info, but as I said it was a corrolation of info from various sources and I don't have all of them to hand at present.
Trouble is you just have a bias against me, because I'd don't share your "lust" factor over dual exhaust setups on 4th gen Fbody's and you have conveniently ignored questions I asked you in a previous thread when you started whinging about one of my posts and info contain therein.
You like duals - fine I have no problem with this and I also like them. But I don't beleive they are the be all and end all, or the only option for Fbody's. You however seem not to share this view and apparantly don't like anyone offering a differing opinion even when supported by 3rd party links and justification for their claims.
Trouble is you just have a bias against me, because I'd don't share your "lust" factor over dual exhaust setups on 4th gen Fbody's and you have conveniently ignored questions I asked you in a previous thread when you started whinging about one of my posts and info contain therein.
You like duals - fine I have no problem with this and I also like them. But I don't beleive they are the be all and end all, or the only option for Fbody's. You however seem not to share this view and apparantly don't like anyone offering a differing opinion even when supported by 3rd party links and justification for their claims.
I'll go dig up that thread and see what you asked that I didn't answer...I don't live on here 24/7 like some people (you).
02-23-2006, 02:52 PM
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Originally Posted by 300bhp/ton
I don't doubt your test results. But can you be 110% certain it was RAM AIR, as in the increase in air pressure that improved the performance.
And also be 110% certain that cool air ducts and air box resonance had ZERO affect?
And also be 110% certain that cool air ducts and air box resonance had ZERO affect?
The addition of the device was definately responsible for the change in performance and I measure air temps accurate to within 1* with a thermocouple, but I honestly have no way at all to observe or measure pulses in the air box above the carburetor. Even if I could measure them it would be extremely difficult to directly attribute them to the power differences.
The only argument I can offer is theoretical. I propose that resonant pulses would only be effective at certain RPM ranges and ineffective at others similar to the effect of exhaust pulse tuning. This would make the measured changes in power appear in some places and not in others.
My measured power increases were not RPM dependant but definately speed dependant (gains increased with speed), which brought me to the conclusion that the simpler answer of air density change in the entire system was responsible.
I can measure pressure as well, but even now I do not have a pressure transducer sensitive enough to measure the differences with any certainty at the speeds I was testing at. You might see a little change on a large vacuum gauge, but not exactly outside allowable testing error.
Sometime in the future I'll be doing the testing again and hopefully I'll have even better test equipment, and perhaps the answer will have more credibility. I can't see how I'd measure the pulsing, but if you have a good idea then share.
02-23-2006, 03:05 PM
#12
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Here's one angle on it... the SS hood has an inlet area
of about 11 square inches (at least, this is what I
recall from measuring every orifice in the tract one day
in a fit of new-owner enthusiasm). That's 0.076 sq ft.
Traveling 60MPH is 5280 ft/min. That means that the
inlet -encounters- only 400CFM of air, that is all that
would pass through a zero-air-resistance hole at speed.
So don't expect big things from it, at street speeds.
You get her up over 100MPH, maybe it starts to make
up for some suction losses in the front plumbing. Or not.
In fact, if -anyone- with a "ram air" setup can show me
a vehicle log in which MAP exceeds BARO by more than
1kPa, at WOT (a big 1%), I will be impressed.
Of course, to benefit modestly all you need is to bring
air temps down to where they do not mess with the
timing. That's a much easier mark to hit.
of about 11 square inches (at least, this is what I
recall from measuring every orifice in the tract one day
in a fit of new-owner enthusiasm). That's 0.076 sq ft.
Traveling 60MPH is 5280 ft/min. That means that the
inlet -encounters- only 400CFM of air, that is all that
would pass through a zero-air-resistance hole at speed.
So don't expect big things from it, at street speeds.
You get her up over 100MPH, maybe it starts to make
up for some suction losses in the front plumbing. Or not.
In fact, if -anyone- with a "ram air" setup can show me
a vehicle log in which MAP exceeds BARO by more than
1kPa, at WOT (a big 1%), I will be impressed.
Of course, to benefit modestly all you need is to bring
air temps down to where they do not mess with the
timing. That's a much easier mark to hit.
02-23-2006, 03:36 PM
#13
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Originally Posted by MeentSS02
Oh, and uh, while we're at it, update your website with pics of the mods ON your car, instead of just random internet pics of them before I think you are really full of ****.
"This is better and I know ALL" and then don't support or even justify their claim.
So who's more full of ****, the person who spends the time researching and finds FACTS and real life examples plus own expericance and offers it in that manor. Or poeple like you that just act like a LORD I am better?
https://ls1tech.com/forums/showthrea...78#post4310278
Try post 52!
02-23-2006, 03:47 PM
#16
TECH Addict
Originally Posted by Camaroholic
All right you two (300bhp/ton and MeentSS02), knock it off, or both of you will find yourselves sitting on the benches for a while (aka temporary ban / timeout). Play nice. 
But I'm happy, infact would prefer to call it quits.
02-23-2006, 03:53 PM
#17
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Originally Posted by jimmyblue
In fact, if -anyone- with a "ram air" setup can show me
a vehicle log in which MAP exceeds BARO by more than
1kPa, at WOT (a big 1%), I will be impressed.
a vehicle log in which MAP exceeds BARO by more than
1kPa, at WOT (a big 1%), I will be impressed.
02-23-2006, 07:08 PM
#18
I believe that the bulk of the performance gains from "ram air" are from the outside, cooler, airsouce. Kenne Belle has done some good research into the effects of air tempature and hp production. I think the airsource is 90 % of the gains at under 100 mph speeds. Nonthing empirical just common sense.
02-23-2006, 07:25 PM
#20
Originally Posted by Red_Dragon
I was watching a show the other day and there was a guy in the wind tunnel and at 100mph they said there was 78lbs of pressure agaist him i think it was. Thats well above atmospheric pressure.
Even though ram air does not result in big pressure increases at our speeds, that doesn't necessarily mean it's not effective. Without it, it's possible that you're drawing air from an area that's below atmospheric pressure.
Intuition suggests that a forward-facing intake made in the form of a funnel, large end foremost, should somehow multiply the pressure of the air, resulting in a much larger pressure gain at the small end. Sadly, intuition is wrong. In order to convert velocity energy into pressure, the air has to be slowed down, and this requires a duct that widens rather than narrows. Next time you fly on a commercial airliner, note that its engine intakes widen as the airflow approaches the compressor face. Such widening passages are called diffusers, and they are universally used in the conversion of velocity into pressure.