A
Anonymous
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SnowDragon3 made a point a few posts back that I think has been overlooked, which is that a blow-thu configuration tends to lean out the carb as boost pressure rises. An unmodified carb can fairly easily handle up to about half a bar of boost (~7 to 8 psi) by fattening the secondaries and going down a size or two on the emulsion tubes (richening the transition circuit airflow). Beyond that boost level tuning becomes quite complicated, and should be considered a job for someone who knows what they're doing and has the proper equipment to monitor progress.
Blow-thru carbs are complicated by the nature of carbs themselves. The carb's venturis "see" airflow that is presumed to be roughly one atmosphere in pressure. As one deviates from this presumed air pressure, the performance of the carb falls off. This isn't normally a problem as the pressure drops with altitude. The engine makes less power, but the tuning stays roughly the same. But as the boost rises above atmospheric pressure, an insidious challenge for the carb develops. The venturis are designed to properly meter fuel to the air flowing through the carb at one atmosphere, but as the pressure rises above this, the venturis still "see" just that one atmosphere of airflow, so continue to deliver fuel as if the carb was still at one atmosphere. This results in a gradual leaning of the fuel/air mixture, and a rise in combustion cylinder temperatures. Detonation easily occurs and engines are destroyed.
The usual solution is to route some of that boost signal to the fuel bowl and/or to the back side of a mechanical fuel pump. This helps maintain a proper a/f ratio by pressurising the fuel -- and does help up to a point. But the bottom line is that safely achieving significant boost in a blow-thru configuration can be quite a challenge. That's why there are more than 500 members on Yahoo's blow-thru turbo group -- they're all trying to help each other get there...
8)
Draw-thru carbs are much simpler, but have their own set of challenges. The good news is that the carb never "sees" airflow above atmospheric pressure, so stays within its design configuration at all times. Draw-thrus usually employ vacuum actuated secondaries which have been somewhat fattened, making tuning much simpler than with blow-thrus. Their chief draw-back is that intercooling becomes very difficult. Once the fuel is in suspension with the air, cooling the charge causes fuel drop-out, and then leaning and inconsistent a/f rations become a problem. An often over-looked consideration is that draw-thrus also require a constant downhill slope from the carb to the intake manifold in order to prevent fuel drop-out, and the problems that come with that. In the photo of SnowDragon3's brother's car above, you can see that there is an elbow in the tube running from the carb to the compressor section of the turbo. The low point in this elbow gives the fuel a place to stagnate, complicating an already dicey situation (log intake manifold, unknown static compression ratio, etc.).
When I first began planning my turbo setup I wanted to go with a draw-thru installation due to it's simplicity. However, I couldn't make it all fit under the hood as planned, and kept ending up with a configuration similar to SnowDragon3's brother's. I was concerned about fuel drop-out and distribution issues with the log intake, though, so finally settled on the propane powered unit I am working on. Since the propane is introduced to the engine as a gas, it can't "fall out" of suspension with the air, and by sending the propane through the turbo, I'll ensure proper distribution to each cylinder. Finally, in order to assure the coldest possible intake air, I am mounting the carb/mixer upside down to draw air in from below and behind the bumper...
Blow-thru carbs are complicated by the nature of carbs themselves. The carb's venturis "see" airflow that is presumed to be roughly one atmosphere in pressure. As one deviates from this presumed air pressure, the performance of the carb falls off. This isn't normally a problem as the pressure drops with altitude. The engine makes less power, but the tuning stays roughly the same. But as the boost rises above atmospheric pressure, an insidious challenge for the carb develops. The venturis are designed to properly meter fuel to the air flowing through the carb at one atmosphere, but as the pressure rises above this, the venturis still "see" just that one atmosphere of airflow, so continue to deliver fuel as if the carb was still at one atmosphere. This results in a gradual leaning of the fuel/air mixture, and a rise in combustion cylinder temperatures. Detonation easily occurs and engines are destroyed.
The usual solution is to route some of that boost signal to the fuel bowl and/or to the back side of a mechanical fuel pump. This helps maintain a proper a/f ratio by pressurising the fuel -- and does help up to a point. But the bottom line is that safely achieving significant boost in a blow-thru configuration can be quite a challenge. That's why there are more than 500 members on Yahoo's blow-thru turbo group -- they're all trying to help each other get there...
8) Draw-thru carbs are much simpler, but have their own set of challenges. The good news is that the carb never "sees" airflow above atmospheric pressure, so stays within its design configuration at all times. Draw-thrus usually employ vacuum actuated secondaries which have been somewhat fattened, making tuning much simpler than with blow-thrus. Their chief draw-back is that intercooling becomes very difficult. Once the fuel is in suspension with the air, cooling the charge causes fuel drop-out, and then leaning and inconsistent a/f rations become a problem. An often over-looked consideration is that draw-thrus also require a constant downhill slope from the carb to the intake manifold in order to prevent fuel drop-out, and the problems that come with that. In the photo of SnowDragon3's brother's car above, you can see that there is an elbow in the tube running from the carb to the compressor section of the turbo. The low point in this elbow gives the fuel a place to stagnate, complicating an already dicey situation (log intake manifold, unknown static compression ratio, etc.).
When I first began planning my turbo setup I wanted to go with a draw-thru installation due to it's simplicity. However, I couldn't make it all fit under the hood as planned, and kept ending up with a configuration similar to SnowDragon3's brother's. I was concerned about fuel drop-out and distribution issues with the log intake, though, so finally settled on the propane powered unit I am working on. Since the propane is introduced to the engine as a gas, it can't "fall out" of suspension with the air, and by sending the propane through the turbo, I'll ensure proper distribution to each cylinder. Finally, in order to assure the coldest possible intake air, I am mounting the carb/mixer upside down to draw air in from below and behind the bumper...
