Vented hood vs. a "cold-air" hood
06-29-2001, 06:17 AM
#21
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Direct Induction, has a multitude of benefits and few drawbacks. Let's look at the benefits first! The air which is pulled from the outside is generally cooler then the air swirlling around under the hood. This provides a denser charge of air into the intake. A cooler charge of air produces more power . Second, as the air is forced into the intake channel it has a slight venture effect, (speed up). We all know the effect of forcing air into the system, (positive pressure). Third, in an induction, hood which is specifically designed with a direct the flow of air to the throttle body or carb, there is no wasted pressure. The cooler air doesn't mix with the heat of the engine.
Now, consider the induction of a vented hood. One there is no direct path to channel incomming air to the throttle body, there is no positive pressure. It mixes with the heat of the engine which produces a much less denser charge of air. They are great for dispursing engine heat but have little to no effect on producing measurable horsepower gains.
Take for instance, Superbikes of past and now! In the 70's and 80's open velocity stacks were widely utilized because four cylinder engines were air cooled and ran hotter. Airboxes were small and restrictive. They primarily removed the air boxes to disipate heat which built up around the rear of the cylinders. The velocity stacks provided a slight venture effect but negligable horsepower gains were realized. They found as the bikes went faster air rushed by the cylinder and would produce a vacuum around the outer cylinders This caused some really sever drawbacks like throttles sticking open. This meant that they had to use much heavier throttle return springs. This was tough on racers. As the sport evolved and the advent of watercooling manfactures began to find out that they were able to find more horsepower. Primarily because the engine temptures were much more stable for longer periods of time. They began utilizing ram air scoops, and closed air boxes, much bigger also than previously utilized on street bikes. Naturally, Honda being the first. They found that as the air charge was spead up it became cooler and more dense. This allowed the engine to produce more power at speed. Do a little experiment! Wave your open hand back and forth in the air as fast as you can. Can you feel a diffrence in tempature? The answer is yes! Why do people use fans?
Now to finally answer your question if this isn't already apparent. A hood designed with cold air induction is far more condusive to producing usuable horsepower.
Drawbacks, few but I wouldn't drive in in a downpour you'd be taking a chance of hydraulicing (spelling) the motor. For more power I go with the coldair option if I could.
JRM
Now, consider the induction of a vented hood. One there is no direct path to channel incomming air to the throttle body, there is no positive pressure. It mixes with the heat of the engine which produces a much less denser charge of air. They are great for dispursing engine heat but have little to no effect on producing measurable horsepower gains.
Take for instance, Superbikes of past and now! In the 70's and 80's open velocity stacks were widely utilized because four cylinder engines were air cooled and ran hotter. Airboxes were small and restrictive. They primarily removed the air boxes to disipate heat which built up around the rear of the cylinders. The velocity stacks provided a slight venture effect but negligable horsepower gains were realized. They found as the bikes went faster air rushed by the cylinder and would produce a vacuum around the outer cylinders This caused some really sever drawbacks like throttles sticking open. This meant that they had to use much heavier throttle return springs. This was tough on racers. As the sport evolved and the advent of watercooling manfactures began to find out that they were able to find more horsepower. Primarily because the engine temptures were much more stable for longer periods of time. They began utilizing ram air scoops, and closed air boxes, much bigger also than previously utilized on street bikes. Naturally, Honda being the first. They found that as the air charge was spead up it became cooler and more dense. This allowed the engine to produce more power at speed. Do a little experiment! Wave your open hand back and forth in the air as fast as you can. Can you feel a diffrence in tempature? The answer is yes! Why do people use fans?
Now to finally answer your question if this isn't already apparent. A hood designed with cold air induction is far more condusive to producing usuable horsepower.
Drawbacks, few but I wouldn't drive in in a downpour you'd be taking a chance of hydraulicing (spelling) the motor. For more power I go with the coldair option if I could.
JRM
06-29-2001, 08:00 AM
#23
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Billy,
I believe that you'll actually want a smaller duct rather than larger for high speed work. I'm no fluid dynamics expert myself, but this comes from a study of airflow through radiators in high speed cars. The faster you go, the smaller the opening required for optimal flow/pressure. I do have access to people with CFD SW, so maybe I can get them to run some sims. A larger duct would build pressure sooner, but might be less than optimal at high speeds (although with a pressure relief valve/door built into the box, it might work o.k.)
Another example would be ram air on bikes. The duct openings on these systems are usually pretty small (on the order of a couple of square inches each) and are designed to build maximum pressure at 150 mph or so on engines producing 150-160 hp.
Your comment about being able to relieve pressure in the airbox has a ring of truth as well. Some formula type cars actually have small holes cut in the back of their intake scoops to relieve excess pressure. The mechanics of determining when and how big a hole is needed is beyond me. The ram air systems on bikes all seemed to have a slight sealing problem in Sport Rider's 1999 two part test of the technology as well.
For those wondering just how much benefit is available from a pressure standpoint, I refer you to the same Sport Rider articles I quoted.
The best bikes there began to build positive airbox pressure below 80 mph. Kawasaki was especially good, building pressure as soon as 60 mph in some cases. At high speeds (150+) these bikes were able to build as much as 20-25 mb of pressure, or about 0.3-0.4 psi. Now, that doesn't sound like a lot, until you realize that non-ram air bikes are actually suffering from slight airbox vacuums at similar speeds - on the order of -20 to -25 mb. Even if you were to only build 5 mb of pressure at 100 mph, you'd still see a differential improvement of 0.4-0.5 psi. Build 15-20 mb and now we're talking about 0.6-0.7 psi. That's worth 4%-5% more hp.
BTW, the testers specifically noted that on two Kawasakis, the one with the smaller ducts built pressure higher (28 mb vs. 23 mb), but if you look at the lower speed pressure trace, the larger duct equipped version actually had better pressure till above 100 mph or so.
UL
[QUOTE]Originally posted by WestSideBilly
[B]
The last issue deals with the airflow.
I believe that you'll actually want a smaller duct rather than larger for high speed work. I'm no fluid dynamics expert myself, but this comes from a study of airflow through radiators in high speed cars. The faster you go, the smaller the opening required for optimal flow/pressure. I do have access to people with CFD SW, so maybe I can get them to run some sims. A larger duct would build pressure sooner, but might be less than optimal at high speeds (although with a pressure relief valve/door built into the box, it might work o.k.)
Another example would be ram air on bikes. The duct openings on these systems are usually pretty small (on the order of a couple of square inches each) and are designed to build maximum pressure at 150 mph or so on engines producing 150-160 hp.
Your comment about being able to relieve pressure in the airbox has a ring of truth as well. Some formula type cars actually have small holes cut in the back of their intake scoops to relieve excess pressure. The mechanics of determining when and how big a hole is needed is beyond me. The ram air systems on bikes all seemed to have a slight sealing problem in Sport Rider's 1999 two part test of the technology as well.
For those wondering just how much benefit is available from a pressure standpoint, I refer you to the same Sport Rider articles I quoted.
The best bikes there began to build positive airbox pressure below 80 mph. Kawasaki was especially good, building pressure as soon as 60 mph in some cases. At high speeds (150+) these bikes were able to build as much as 20-25 mb of pressure, or about 0.3-0.4 psi. Now, that doesn't sound like a lot, until you realize that non-ram air bikes are actually suffering from slight airbox vacuums at similar speeds - on the order of -20 to -25 mb. Even if you were to only build 5 mb of pressure at 100 mph, you'd still see a differential improvement of 0.4-0.5 psi. Build 15-20 mb and now we're talking about 0.6-0.7 psi. That's worth 4%-5% more hp.
BTW, the testers specifically noted that on two Kawasakis, the one with the smaller ducts built pressure higher (28 mb vs. 23 mb), but if you look at the lower speed pressure trace, the larger duct equipped version actually had better pressure till above 100 mph or so.
UL
[QUOTE]Originally posted by WestSideBilly
[B]
The last issue deals with the airflow.
06-29-2001, 08:39 AM
#24
UL: I agree, a smaller cross-section is desirable. I was referring to the length of the duct, not the cross-section area, though. The main benefit of more length is that it allows more room for trapping moisture before it hits the air box.
Good reference on the bikes. Definately gives us something additional to consider
Good reference on the bikes. Definately gives us something additional to consider
06-29-2001, 09:00 AM
#25
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Having no CFD experience, this suggestion is from an asthetic point only, and it's validity will have to be argued by others. What if we put the ram air scoop further up on the hood, then ran a "pipe" below it to the stock airbox. The air comes into the scoop, does a 180 and is pushed to the front of the car, then bends right and into the airbox.
The 180 turn will probably reduce air-speed, but provides an opportunity for a fully functioning water trap to be built so only vapor gets into the airbox. The question is, will the extended piping negate the benefits of ram air or just hinder them a bit.
The 180 turn will probably reduce air-speed, but provides an opportunity for a fully functioning water trap to be built so only vapor gets into the airbox. The question is, will the extended piping negate the benefits of ram air or just hinder them a bit.
06-29-2001, 11:45 AM
#29
In another thread on the same topic, 1RandyC posted some pictures. The silver car with the small intake on the *bumper* is what I had pictured, except I had pictured it on the hood.
http://www.s2000online.com/forums/showthre...?threadid=17326
Now we just have to find out if they help and how they work.
http://www.s2000online.com/forums/showthre...?threadid=17326
Now we just have to find out if they help and how they work.
06-29-2001, 01:29 PM
#30
Registered User
Actually Brent, I was more interested in feeding a pressurized airbox, as opposed to running straight from a duct to the TB. Hence, the actual size of the path from inlet to airbox is open within space constraints.
In formula cars, where the radiators are usually enclosed in the sidepods, the duct to the radiator will actually diverge after the inlet. This is to slow airspeed and increase pressure. If you slow the airspeed enough, you can theoretically design a net zero drag cooling system (or in reality get very close) especially since you are adding energy to the flow by passing air through the radiator.
Since we have to have a filter for street apps, I'd like to see a pressurized airbox with a relatively low speed air inlet. The engine, through the TB and an intake tube/velocity stack/filter could then draw air without having to worry about unusual air movement/pressure variations and with the highest pressure possible (which gets us the greatest flow into the engine, which is where it counts).
UL
In formula cars, where the radiators are usually enclosed in the sidepods, the duct to the radiator will actually diverge after the inlet. This is to slow airspeed and increase pressure. If you slow the airspeed enough, you can theoretically design a net zero drag cooling system (or in reality get very close) especially since you are adding energy to the flow by passing air through the radiator.
Since we have to have a filter for street apps, I'd like to see a pressurized airbox with a relatively low speed air inlet. The engine, through the TB and an intake tube/velocity stack/filter could then draw air without having to worry about unusual air movement/pressure variations and with the highest pressure possible (which gets us the greatest flow into the engine, which is where it counts).
UL