When talking about boost, and you say you will give the engine 7psi of boost, is that pressure before or after the carb? I have my boost meter after the carb, and the wastegate gets its pressure from above, before the carb, so i se it moves when the meter says about 5 psi when its regulated at 7psi. So where should i read the true pressure ?
When I'm talking about boost, it is the pressure in the intake plenum that I want to measure. This gives me an indication of what the engine is experiencing. I really don't care what the compressor is putting out.
I actually measured the pressure drop between the compressor outlet and the intake plenum on my system by measuring the pressure at the plenum and then at the compressor outlet port. My system shows a 2psi drop at 6.5# boost at the plenum. It's probably even more at higher boost levels (although I haven't measured it).
With my setup (SR20DE, stock MAF, and 370 injectors) most people find the air-fuel ratio leaning out at about 10psi. If I were to measure the boost at the compressor, and set it at a "safe" 9psi, I could damage the engine due to detonation because the engine would be experiencing 11+psi of boost.
lew is right, traditionally boost values are read from the intake plenum (after the throttle plate) this is what the engine is seeing. waste gates get their boost signal from the turbo outlet. There is usually a few psi pressure drop between the two signals depending your intercooler, piping and air flow.
-dave
On a blow through turbo carby system would the drop in boost pressure be contributed to the fuel cooling the compressed air down thus reducing the pressure?
On a blow through turbo carby system would the drop in boost pressure be contributed to the fuel cooling the compressed air down thus reducing the pressure?
The reduction in pressure is due to the resistance of the piping and intercooler (mostly the intercooler).
Now i have the meter before the carb and it show 2 or 3 psi more.
Th only thing is that it should put 7 psi, but it does way less, around 3 or 4.
The wastegate is opening a little early, but its not adjustable and should open at 7, maybe ill trick it in some way.
Today i managed to make the car run very well, but i still dont have much pressure and i noticed that the carb i bought has to big jets. 140 and 220, that is a bit to much, i had to close the secondary and it works better if i use both it gets too much gas.
So boyles law has nothing to do with it? I find it hard to believe its not a contributing factor.
It would be a factor in a static system. Boyles law says that if the air is cooled, the volume decreases. If you cool air in a box, the pressure in the box would drop. But air is continuously flowing in the intake system so the decrease in volume is continuously replaced. It is essentially like having a big hole in the box.
It is the restriction to flow (mostly in the intercooler) that creates a pressure drop between the compressor and the intake plenum. As the flow increases, so does the pressure drop.
Isn't there a drop in pressure inside the carb (like 2 psi maybe?).
At wide open throttle (that's when boost occurs) in a blow-through system (the carb is on the pressure side of the turbo) the restriction depends on the area of the wide open carb vs the area of the piping in and out. You would want to work with a carb which is rated close to the expected air flow under boost to minimize the restriction.
As long as you are well within the ability of the compressor to supply boost, it shouldn't make much difference. i.e. if you want the engine to experience 10psi, and there is a 5psi drop through the piping between the compressor and the intake plenum, the turbo should be capable of 15psi.
At wide open throttle (that's when boost occurs) in a blow-through system (the carb is on the pressure side of the turbo) the restriction depends on the area of the wide open carb vs the area of the piping in and out. You would want to work with a carb which is rated close to the expected air flow under boost to minimize the restriction.
As long as you are well within the ability of the compressor to supply boost, it shouldn't make much difference. i.e. if you want the engine to experience 10psi, and there is a 5psi drop through the piping between the compressor and the intake plenum, the turbo should be capable of 15psi.
Lew
I was just referring to the pressure drop in the venturi if you're trying to firgure out how much boost you're running. A former boss of mine used to turbo Corvairs and when he would looks at boost pressure, he always had to compensate for the pressure drop in the venturi and size the turbo accordingly. On a small carb I would assume it wouldn't go beyond 2-3 psi at the most. I've never seen piping after the carb on a blow through, usually the carb is bolted onto the intake manifold. I've seen it on draw through however. In that case the drop in pressure from the venturis would be irrelevent.
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