? about impeller speed
09-29-2007, 11:58 PM
#1
? about impeller speed So I'm looking at impellers speed and my cam. Does 60,000 impeller rpms equate to engine rpms the same? So if you excede the limit do you then start to lose power? So a big duration cam really isnt a good option unless you have a s/c that has a hi max impeller speed? and if you do that then your building more boost but what if you dont want more because you have to high of compression to start with?
09-30-2007, 09:15 AM
#3
Engine speed is directly related to impeller speed based on the supercharger internal transmission gearing & the pulley ratio.
Impeller redline does not have any relationship to the rpm range of the motor. A higher blower redline, just means you spin it faster to make it pump a given volume of air, you do not have to rev the motor higher, you just alter the pulley sizes.
Larger diameter blower impellers usually spin at a lower rpm than smaller ones, even though the larger sizes typically flow more air. It all relates to the impeller design & impeller tip speed. Consider the smaller sizes of most turbo's & how fast they spin (100,000+ rpm), even though they still produce a lot of airflow. Compare that to a small engine that makes 400hp at 8000rpm, compared to a large engine that makes the same power at 4000rpm. You could get similar performance with either engine, but you need different rear end gearing (roughly twice the gearing) to achieve similar results.
This is not meant to say the impeller should spin at any particular RPM, but it is a means to determine a safe pulley ratio that does not overspin the blower. Peak blower efficiency is somewhere around 80% of the blower redline. Above that the additional air also comes with the penalty of additional heat. (a good reason not to use an undersized blower)
On the flip side, a blower that is way too large, may not operate in its efficiency range & could have a soggy boost curve. (meaning it just starts to make decent boost at high rpm with very little down low). This is similar to cam selection, where you should not use a cam that has a 4200-7600rpm range on a car that will never see over 6000. You will never use the full potential & will have overall worse performance than the properly sized components.
Impeller redline does not have any relationship to the rpm range of the motor. A higher blower redline, just means you spin it faster to make it pump a given volume of air, you do not have to rev the motor higher, you just alter the pulley sizes.
Larger diameter blower impellers usually spin at a lower rpm than smaller ones, even though the larger sizes typically flow more air. It all relates to the impeller design & impeller tip speed. Consider the smaller sizes of most turbo's & how fast they spin (100,000+ rpm), even though they still produce a lot of airflow. Compare that to a small engine that makes 400hp at 8000rpm, compared to a large engine that makes the same power at 4000rpm. You could get similar performance with either engine, but you need different rear end gearing (roughly twice the gearing) to achieve similar results.
This is not meant to say the impeller should spin at any particular RPM, but it is a means to determine a safe pulley ratio that does not overspin the blower. Peak blower efficiency is somewhere around 80% of the blower redline. Above that the additional air also comes with the penalty of additional heat. (a good reason not to use an undersized blower)
On the flip side, a blower that is way too large, may not operate in its efficiency range & could have a soggy boost curve. (meaning it just starts to make decent boost at high rpm with very little down low). This is similar to cam selection, where you should not use a cam that has a 4200-7600rpm range on a car that will never see over 6000. You will never use the full potential & will have overall worse performance than the properly sized components.