Air intake temperatures with a cup/drilled airbox versus a stock airbox.
06-04-2006, 02:22 AM
#16
Rennlist Member
Any where the motorsound cover fits into these results? Somewhere in the middle? Do I have to give up the cool sound for more power? Is the sizing of the holes in the motorsound cover right to allow the sound to release and yet not suck up too much hot air? Could they have been that clever???
Pete
Pete
06-04-2006, 09:13 AM
#17
Nordschleife Master
I will say that on a dyno the cup airbox is cover is worth 3rwhp and 4ft/tq over the stock airbox at engine RPMs higher than 5500 regardless of air temp.
06-04-2006, 05:10 PM
#18
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Geoffrey,
As we are looking at a ratio of densities at different temperatures the pressure number cancels so we are back to the absolute temperature ratio .
P x V P x V
------ / ---------- = T2/T1
T1 T2
I did assume that the motor would consume the same volume of air at both temperatures.
Looks like the 3% (7-8 bhp ) loss for a 10 degree C temperature rise is true.
If the open Cup box gives more power with a higher intake temperature then a real opportunity for power increase exists with a better box design !
It would seem a much larger diameter snorkel and bigger volume lid would be the ultimate ?
Certainly food for thought !
All the best
Geoff
As we are looking at a ratio of densities at different temperatures the pressure number cancels so we are back to the absolute temperature ratio .
P x V P x V
------ / ---------- = T2/T1
T1 T2
I did assume that the motor would consume the same volume of air at both temperatures.
Looks like the 3% (7-8 bhp ) loss for a 10 degree C temperature rise is true.
If the open Cup box gives more power with a higher intake temperature then a real opportunity for power increase exists with a better box design !
It would seem a much larger diameter snorkel and bigger volume lid would be the ultimate ?
Certainly food for thought !
All the best
Geoff
06-19-2006, 03:01 PM
#20
Race Car
I did a test like this for my 964-powered 1972. It was interesting. I set up an aluminum heat shield (a five-sided box, basically) around my aftermarket intake so that it could only pull air through the grille, and then monitored temps inside the shielded area and also in the engine compartment itself.
Under some conditions, in steady-RPM street driving, I was able to see as much as a 25 degree difference between engine compartment temps and my new 'cold intake' temps, so I thought I was off to a good start.
But then I ran the same tests on the track, where the driving wasn't steady-RPM cruising, and the temperature difference immediately dissolved away. Driven aggressively, the engine compartment dropped down to within 0-3 degree of ambient temperatures. It's probably not too big a leap to say the efficiency of the main blower goes up dramatically at higher rpms, sucking the warm air out of the engine compartment faster than the engine can re-heat it.
So if we're considering temperature-control changes for the sake of horsepower, I'd submit that there's very little to gain, unless you've got a way to cool the air below ambient. The nice thing about the design of the air-cooled Porsche motor is that when you actually need the power, it solves the temperature problem itself.
My conclusion? If a solution to the air-intake-temp 'problem' adds weight or in any way restricts airflow to the motor, it is probably doing more harm than good. The stock box is a very good design. (I just wish we could have fit it into my early tub's engine compartment.)
Under some conditions, in steady-RPM street driving, I was able to see as much as a 25 degree difference between engine compartment temps and my new 'cold intake' temps, so I thought I was off to a good start.
But then I ran the same tests on the track, where the driving wasn't steady-RPM cruising, and the temperature difference immediately dissolved away. Driven aggressively, the engine compartment dropped down to within 0-3 degree of ambient temperatures. It's probably not too big a leap to say the efficiency of the main blower goes up dramatically at higher rpms, sucking the warm air out of the engine compartment faster than the engine can re-heat it.
So if we're considering temperature-control changes for the sake of horsepower, I'd submit that there's very little to gain, unless you've got a way to cool the air below ambient. The nice thing about the design of the air-cooled Porsche motor is that when you actually need the power, it solves the temperature problem itself.
My conclusion? If a solution to the air-intake-temp 'problem' adds weight or in any way restricts airflow to the motor, it is probably doing more harm than good. The stock box is a very good design. (I just wish we could have fit it into my early tub's engine compartment.)
06-19-2006, 08:13 PM
#21
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Jack...That little test of yours makes a hell of a lot of sense....what I would call the application of good old fashioned common sense along with some innovative engineering to come up with something more than a best guess scenario. I appreciate the trouble and effort you went through to make your point and provide us with some informative results...even if they're not "NASCAR approved" or whatever scientific stamp is supposed to be affixed to the results....Great job....and it brings some credibility and merit to the German engineering too...!! I guess they knew what they were doing, and it also shows that sometimes the aftermarket products don't always enhance performance in spite of the claims........Chuck
06-19-2006, 08:33 PM
#22
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Geoffrey touched on the important factor in this study and Jack confirmed the principle in practice. Let's make the fundamental assumption that we are only interested in performance on track, thus your 3.6 964 engine at 6000rpm will consume approximately 380cfm of air through the engine PLUS around 25cfm through the cooling fan, which equates to around 6.75cuft per second. Now in my estimation there is about 3cuft of air space in your engine bay, thus the air is being replenished at least twice per second, so any temperature increase of the air flowing into the engine is more likely to be caued by the air flowing across a greater surface area of airbox, so the argument would seem weigh in favour of runnign with the smallest air filter housing as possible ......
...so the answer (to the performance issue) is to simply measure the lap times with the different airboxes, it would seem.
...so the answer (to the performance issue) is to simply measure the lap times with the different airboxes, it would seem.
06-20-2006, 10:15 AM
#23
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Colin,
All good stuff. How about if we think about a car driven on the road , stopping at lights , crawling through towns etc.
During the summer the temperature rises are potentially going to be quite big and will need a few miles of fast driving to bring back down .
It is this, on the road, power loss that would be my concern and reason why a cone filter/ holes in air box would not appear on my list of desirable modifications.
Just my $0.02 ,
Geoff
All good stuff. How about if we think about a car driven on the road , stopping at lights , crawling through towns etc.
During the summer the temperature rises are potentially going to be quite big and will need a few miles of fast driving to bring back down .
It is this, on the road, power loss that would be my concern and reason why a cone filter/ holes in air box would not appear on my list of desirable modifications.
Just my $0.02 ,
Geoff
06-20-2006, 12:21 PM
#24
Race Car
Originally Posted by Red rooster
During the summer the temperature rises are potentially going to be quite big and will need a few miles of fast driving to bring back down.
It takes a half a second for the engine blower to get rid of all the warm air in the engine compartment. And even that is a worst-case scenario, presuming that the intake will breathe every cubic inch of the air that's under the hood before any fresh stuff makes it through the grille to the intake.
06-20-2006, 12:28 PM
#25
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Originally Posted by JackOlsen
What are you basing the "few miles" idea on?
It takes a half a second for the engine blower to get rid of all the warm air in the engine compartment. And even that is a worst-case scenario, presuming that the intake will breathe every cubic inch of the air that's under the hood before any fresh stuff makes it through the grille to the intake.
It takes a half a second for the engine blower to get rid of all the warm air in the engine compartment. And even that is a worst-case scenario, presuming that the intake will breathe every cubic inch of the air that's under the hood before any fresh stuff makes it through the grille to the intake.
06-20-2006, 12:58 PM
#26
Race Car
Well, I'm trying to keep things in perspective. We're talking about a potential 1% difference in power, based on intake temps. Does that mean anything at all when you're cruising at 3000 rpms? I mean, if the engine is putting out about 140 hp at that speed, the 1.4 hp you're losing seems pretty easily recovered by pressing the accelerator down a hair more, taking the power back from the 50% loss you're achieving by not having the throttle pressed all the way down.
And that's assuming it's possible to perceive a power loss that small.
At full throttle, in my tests, there was no significant temperature difference. But there was probably some lag time between when there was a difference and when there wasn't. Maybe an engineer out there can explain to me why that is. So yes, I can see the appeal of something that would reduce that temperature difference more quickly for driving on the streets on a hot summer day.
But again, I think some perspective is in order. That difference is going to be dropping pretty quickly, and even if the 1% difference were present at full throttle (and I suspect it's a small fraction of that), then I'd be concerned about any cold air intake's effect on the engine's ability to pull air in (through drag losses along the reconfigured intake), or -- if the solution is to try to 'scoop' air in from outside, like water-cooled muscle cars -- the effect of the solution on the car's overall aerodynamics. In other words, if you fix a half a percent's heat-related power loss by making your engine less efficient or making the car harder to push through the air, then you're probably looking at a net loss, right?
And that's assuming it's possible to perceive a power loss that small.
At full throttle, in my tests, there was no significant temperature difference. But there was probably some lag time between when there was a difference and when there wasn't. Maybe an engineer out there can explain to me why that is. So yes, I can see the appeal of something that would reduce that temperature difference more quickly for driving on the streets on a hot summer day.
But again, I think some perspective is in order. That difference is going to be dropping pretty quickly, and even if the 1% difference were present at full throttle (and I suspect it's a small fraction of that), then I'd be concerned about any cold air intake's effect on the engine's ability to pull air in (through drag losses along the reconfigured intake), or -- if the solution is to try to 'scoop' air in from outside, like water-cooled muscle cars -- the effect of the solution on the car's overall aerodynamics. In other words, if you fix a half a percent's heat-related power loss by making your engine less efficient or making the car harder to push through the air, then you're probably looking at a net loss, right?
06-20-2006, 01:10 PM
#28
Race Car
Well, don't take me too seriously. I write fiction for a living. 
It would be interesting to get some input on all of this from an engineer who actually deals with this kind of stuff. I mean, there's obviously a lag in temperature drop, and it's longer than the half second it takes to replace the engine compartment's heated air. I don't think that it's probably significant, from a horsepower point of view, but I still wish someone would explain to me why it works the way it does.
It would be interesting to get some input on all of this from an engineer who actually deals with this kind of stuff. I mean, there's obviously a lag in temperature drop, and it's longer than the half second it takes to replace the engine compartment's heated air. I don't think that it's probably significant, from a horsepower point of view, but I still wish someone would explain to me why it works the way it does.
06-20-2006, 01:26 PM
#29
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Jack,
Your concept of the engine fan clearing the engine bay of hot air isnt correct !
The fan will take air from the easiest place- though the spoiler grille . The air foward of the grill will slowly be removed and replaced by cooler air. That process will take some reasonable time with the exhaust / engine heat doing its best to keep temperatures up.
Thats why all the hoses/ flap unit etc get hot during a drive.
The power penalty for a 10 degree C temperature rise is 3 % . A 20 degreeC rise , which is easily acheived in summer traffic is 6% . That loss of performance you will feel !!
The solution is to drive at high speed in reverse !!!!!!!!!!!!
All the best
Geoff
Your concept of the engine fan clearing the engine bay of hot air isnt correct !
The fan will take air from the easiest place- though the spoiler grille . The air foward of the grill will slowly be removed and replaced by cooler air. That process will take some reasonable time with the exhaust / engine heat doing its best to keep temperatures up.
Thats why all the hoses/ flap unit etc get hot during a drive.
The power penalty for a 10 degree C temperature rise is 3 % . A 20 degreeC rise , which is easily acheived in summer traffic is 6% . That loss of performance you will feel !!
The solution is to drive at high speed in reverse !!!!!!!!!!!!
All the best
Geoff
06-20-2006, 02:21 PM
#30
Originally Posted by JackOlsen
--snip--
But again, I think some perspective is in order. That difference is going to be dropping pretty quickly, and even if the 1% difference were present at full throttle (and I suspect it's a small fraction of that), then I'd be concerned about any cold air intake's effect on the engine's ability to pull air in (through drag losses along the reconfigured intake), or -- if the solution is to try to 'scoop' air in from outside, like water-cooled muscle cars -- the effect of the solution on the car's overall aerodynamics. In other words, if you fix a half a percent's heat-related power loss by making your engine less efficient or making the car harder to push through the air, then you're probably looking at a net loss, right?
But again, I think some perspective is in order. That difference is going to be dropping pretty quickly, and even if the 1% difference were present at full throttle (and I suspect it's a small fraction of that), then I'd be concerned about any cold air intake's effect on the engine's ability to pull air in (through drag losses along the reconfigured intake), or -- if the solution is to try to 'scoop' air in from outside, like water-cooled muscle cars -- the effect of the solution on the car's overall aerodynamics. In other words, if you fix a half a percent's heat-related power loss by making your engine less efficient or making the car harder to push through the air, then you're probably looking at a net loss, right?
Allthough these ducts weren't specifically targeted at cold air intake they were likely used for general under hood temperature reduction. It gets damn hot under these Turbo charged hoods...
And here's a quick photochop of my idea...
Last edited by 951North; 06-20-2006 at 05:42 PM.