Toyota MR2 Message Board - View Single Post

edmcguirk · 04-10-2007, 11:59 PM

I may have jumped the gun on fuel distribution differences. I don't remember exactly but I think the injectors are right at the intake ports. I was mistakenly thinking that fuel in the intake airstream wouldn't turn corners as well as the air can. However I think the differences in air distribution between the two manifolds would be subtle at best on a SC engine.

The SC on the other hand does produce pulses. All roots type blowers do. The lobes simply do not pass the same amount of air during each degree of rotation. Three lobe and twisted lobe rotors were created to try to cure that and several other problems inherent in the two lobed rotors.

A 1 liter pulse in a 1 liter intake manifold would almost double the pressure but a 1 liter pulse in a 10 liter intake manifold would be one tenth the size. Also if you raise the pressure from 1 BAR to 2 BAR you cut the size of the pulses in half. The SC is putting out air in pulses and the engine is taking in air in pulses and those pulses are not even remotely synchronized.

The whole point is that if the air does not move into the combustion chamber by itself, the air pressure in the intake manifold starts to rise beyond the average boost level. That is one reason you will see that the torque curve on the SC engine is broader than the the torque curve on the NA engine. The SC can breathe over a broader range than the cam lobes. The SC does not act like a NA engine running in a room that just happens to be 8 PSI over sea level air pressure.

I am not saying that long intake runners would hurt HP, I am just saying that they have less advantage on an SC engine and there are even advantages to using short runners with an SC engine.

Another example is that many tuners drop variable valve systems when they supercharge an engine. It's just not necessary with a positive displacement type blower. The torque curve gets so wide that it becomes necessary to favor the higher RPMs because low end torque is no good if you never need to run the engine that slowly (when you want to drive fast you never run the engine below 3000 RPM).

Very few of the people who run big pulleys know how low the torque curve wants to go. The stock ECU has a lean spot below 3500 when you run extra boost and many dyno runs don't bother to run below 3000 anyway.

04-10-2007, 11:59 PM	#10 (permalink)
edmcguirk No Skills Join Date: Mar 2007 Location: New Jersey Posts: 6 Thanks: 0 Thanked 0 Times in 0 Posts iTrader Rating: (0/0% )	I may have jumped the gun on fuel distribution differences. I don't remember exactly but I think the injectors are right at the intake ports. I was mistakenly thinking that fuel in the intake airstream wouldn't turn corners as well as the air can. However I think the differences in air distribution between the two manifolds would be subtle at best on a SC engine. The SC on the other hand does produce pulses. All roots type blowers do. The lobes simply do not pass the same amount of air during each degree of rotation. Three lobe and twisted lobe rotors were created to try to cure that and several other problems inherent in the two lobed rotors. A 1 liter pulse in a 1 liter intake manifold would almost double the pressure but a 1 liter pulse in a 10 liter intake manifold would be one tenth the size. Also if you raise the pressure from 1 BAR to 2 BAR you cut the size of the pulses in half. The SC is putting out air in pulses and the engine is taking in air in pulses and those pulses are not even remotely synchronized. The whole point is that if the air does not move into the combustion chamber by itself, the air pressure in the intake manifold starts to rise beyond the average boost level. That is one reason you will see that the torque curve on the SC engine is broader than the the torque curve on the NA engine. The SC can breathe over a broader range than the cam lobes. The SC does not act like a NA engine running in a room that just happens to be 8 PSI over sea level air pressure. I am not saying that long intake runners would hurt HP, I am just saying that they have less advantage on an SC engine and there are even advantages to using short runners with an SC engine. Another example is that many tuners drop variable valve systems when they supercharge an engine. It's just not necessary with a positive displacement type blower. The torque curve gets so wide that it becomes necessary to favor the higher RPMs because low end torque is no good if you never need to run the engine that slowly (when you want to drive fast you never run the engine below 3000 RPM). Very few of the people who run big pulleys know how low the torque curve wants to go. The stock ECU has a lean spot below 3500 when you run extra boost and many dyno runs don't bother to run below 3000 anyway.