| sammyg2 |
04-23-2010 07:49 AM |
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Originally Posted by McLovin
(Post 5312406)
He did not exist, because you can't turbo a 914.
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LOL, did a search and found a couple of his gems. That was a long time ago:
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I think this was just a test to see how long I could keep my mouth shut. Well I can't, so I am going to throw my opinions in the pile.
Yep. A pull through turbo system is not the best. It works, it's simpler (read cheaper) to build and install, but it has limitations and problems. One is the fuel mixture problem. The turbo acts as a centrifuge and separates the air/fuel at low velocities. A smaller problem is the inability to use an intercooler. The big one that I am fighting now is the fact that the turbo sees not only pressure, but vacuum. Neither are a big deal IF the turbo seal works correctly. If the turbo is shut off when hot the oil cooks, and can leave a layer of carbon on and around the seal. This intermitant smoking can lead to some very embarrassing moments.
Have you ever been lined up in pre-grid and look back to see every one behind you coughing? Kind of hard on the ego, especially when the problem only happens three times over two months and you can't figure out why.
Some turbo's don't even have a seal. they have a "piston ring" (This type can not be used in a draw through system).
Under high vacuum, the oil tends to be drawn into the intake and cause smoking. See, I knew my rings were o.k.
Under pressure, a leaking seal can allow air into the oil (imitates blow by, see rings). A separation canister will take care of that. Just a tin can in the oil return line with a vent on top. I have been mulling over a lot of ideas on how to build a "perfect system". All it takes is cubic dollars. It starts with a push through turbo, into an intercooler, down through the stock throttle body, and then into the stock intake manifold. The injectors may have to be replaced with larger capacity injectors. The brains are an SDS programmable system with a mapp sensor, and a crank fired ignition run off a hall sensor with magnets installed in the outer edge of the flywheel. It also includes a knock sensor to further retard timing when needed. Total cost should be about $2600 not including a stock D-jet engine or labor. Even more hp could be obtained with an additional injector mounted in the plenum used to add a water/methanol mixture under real high boost. Fast, but not really streetable, IMO. BTW, at 7 psi boost the intake charge would gain around 100 degrees over ambient and only after prolonged full throttle operation (based on turbo software), which is not usually the case on the street. An intercooler is really neat, but would probably only lower the temperature about 50 0r 60 degrees in this case. Would that help? Sure, a little. But intercoolers really start to make a big difference at higher rates of compression. A high boost 914 engine can be a grenade unless it is a billet motor with all the goodies. Way out of my league. Back to reality. There are two real limiting factors as far as boost go. Heat and fuel. Air cooled motors don't like extra heat, and hp means heat. It can be controlled, as long as we don't get greedy. Fuel quality today sucks (did I mention I work for an oil company?) Detonation under full boost also sucks. If it's a street engine, you gotta keep the boost down, unless you have very elaborate computer controlled everything. Sorry for going on so long, but this is a subject that I have been obsessed with for six months now.
And no, I don't think I have it all figured out. I still have several bugs in my turbo system that are driving me nuts, but hopefully I will get it eventually.
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Quote:
CARBURETION OPERATION AND THEORY
FUEL DELIVERY
The fuel pump pressurizes the fuel to around 3 to 5 psi for carburetors. The fuel flows through a filter (hopefully) and into the carburetor float bowl chamber. As this chamber fills, a float on a pivot rises with the level. As it gets high enough, it closes a valve, shutting off the flow of fuel. When the level drops, the float valve opens again and lets in more fuel.
IDLE CIRCUIT
The engine wants to run fast all the time. The throttle plate controls the engine by restricting how much air it gets.
The engine tries to pull more air in but the carburetor won’t let it at idle. This pulling causes vacuum, an area of low pressure in the manifold. There is a small hole in each of the throats of the carburetor just below the throttle plates. These holes are connected to a passageway that goes up, then back down to the float bowl. The vacuum pulls fuel from the float bowl, up through a restriction orifice (jet), and down into the manifold. Most carburetors have a needle adjusting screw that adjusts how much fuel flows into the throat. Many also have small holes in the passageway that let air in so that it mixes with the fuel and starts to atomize it so that it is well mixed as it enters the manifold. Some have idle mixture screws that adjust how much air is allowed into the passageway.
TRANSITION CIRCUIT
As the throttle plates (butterflies) start to open, they expose a second set of small holes that are also tied into the idle jet passageway. As the butterfly passes these holes, the vacuum pulls fuel through them and mixes it with the higher velocity air that is rushing to squeeze through the small opening. Remember, there is low pressure below the throttle plates, but normal atmospheric pressure above them (except for full throttle high rpm conditions). Some carburetors have one transition port, some have as many as three or four. Their purpose is to try and correctly meter the amount of fuel so that the right ratio of fuel to air is maintained. Most driving is done in this circuit (Except for some sawz-all wielding friends of mine).
ACCELERATOR PUMP
When the throttle is opened rapidly, the manifold vacuum can suddenly drop to almost zero. No vacuum, no fuel gets sucked into the engine, no go. Just a big bog.
The accelerator pump is a band aid for this problem. As the throttle is opened, a piston or diaphragm is actuated, displacing fuel and pumping it right into the throat of the carburetor like a squirt gun. It mixes with the air and burns to keep the engine happy. Since the air is not moving very fast during this condition, the fuel has a tendency to puddle on the manifold floor or stick to the walls. V-8’S have this problem, but not as much as a single carburetor on a 914. The v-8’s use hot water or oil to heat the floor of the manifold to keep the fuel mixed up. It works, but is a major compromise. One of the hardest set ups to get dialed in is the Holley bug spray carburetor on a 914 4 cylinder engine. There is no heat riser to warm the manifold and the runners are long, so the fuel doesn’t stay suspended very well. Some people swear by this set up, some swear at it. Anyway, because the fuel doesn’t stay mixed very well during this low vacuum, low rpm condition, the carburetor has to add extra fuel so that we are sure to get enough into the motor. Not good for emissions or economy. Carburetor systems that have a single barrel for each cylinder and are mounted directly in line with the intake ports are the least susceptible to this, except for fuel injected motors. There are some problems with this type of carburetor set up (like reversion) but that is in the next class. Fuel injection is awesome.
MAIN CIRCUIT
As the engine rpm rises, the air that is flowing through the carburetor barrel has to go through a section that is a little smaller than the rest of It (venturi). The air really wants to get through, so it has to speed up a little so that all the air can get by. So it speeds up and squeezes though the tight spot, then the opening gets bigger again. The air is still going fast and straight. Right at the tight spot there is a small area that has lower pressure than the rest of the barrel. This is caused by the air trying to keep going straight, and the velocity makes it not want to bend back to fill the entire barrel. This is called the Bernoulli principle (I hope I spelled that right). Right there where the pressure is lowest, there is a small port, or passageway that is connected to the float bowl. The fuel is pulled from the float bowl, up the passageway, mixed with a little air, squeezed through the main jet, and goes into the carburetor throat. Some carburetors have a booster venturi, but they just do the same thing the same way.
Carburetors are simple things, but try to do an impossible job. That is, add just enough fuel to maintain a perfect air/fuel mixture, no matter how much the throttle is opened, the engine rpm, the air temperature, the engine temperature, The fuel level in the throat bowl, the list goes on and on. My father told me a long time ago that carburetor is a French word which loosely translated means “compromise”. He may be right. Early fuel injection systems had as much as 30 different "maps" or compensating circuits depending on input conditions, modern systems can have millions of "maps" due to the multiplication of all the variables.
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