Something about Pully sizes I don't understand

[Ryan@RhynoTune]

Originally posted by 2002GreyHD150


Lower pulleys are lower pulleys. The don't make less or more boost based on HD & L respectively. A quoted 6# pulley will ADD roughly 6# of boost to either engine. PERIOD. An 8# will gain how much? EIGHT POUNDS on an L on, an HD, on a Tbird S/C, on a Ford GT, heck even on a Pontiac Grand Prix... simple simple simple.

The fact that so many people beat themselves in the head over the whole L versus HD thing is a complete mystery to me. Forget about the L. It doesn't mater. It doesn't even matter if you add the L-upper for 2 more pounds of boost... the 6# lower will add SIX MORE POUNDS to what you already have.

the bigger issue is the discrepancy between one company's quoted "6# lower" versus another's which is not the same diameter. If they are different diameters they both can't be a 6# pulley.

People need to reference their pullies in terms of diameter, not gains in PSI. Bottom line is a 8.5'' pulley will not gain the same on both engines.

This is a simple calculation, the hardest part is figuring out why people can't grasp this simple concept.

7.5/2.93= 2.55 blower ratio for a Lightning @ 8 PSI
7.5/3.25= 2.30 blower ratio for a HD @ 6 PSI
8/2.93= 2.73 blower ratio w/ 8'' lower (2#) for a Lightning
8/3.25= 2.46 blower ratio w/ 8'' lower (2#) for a HD
- Time out, so with a 8'' lower, you still aren't even at the same blower speed as a stock Lightning. So how do you gain two pounds? Do I really need to go on? I guess I must.
8.5/2.93= 2.90 blower ratio w/ 8.5'' lower (4#) for a Lighting
8.5/3.25= 2.60 blower ratio w/ 8.5'' lower (4#) for a HD
- Another time out, so with a 8.5'' lower on your HD, your not even at the same rate as a Lightning with a 2lber. So you start w/ 6PSI, buy a so called 4lber, slap it on with your stock upper, and bam your at 10PSI!!! But the odd thing is, your still spinning your blower slower to get that same 10PSI than a Lightning does. It's magic
I assume after reading that, someone is saying that isn't much of a difference. These numbers are small and meaningless. This is the part where I multiply the given ratio times 5250 RPM.
5250x2.73 (L blower ratio for a 8'' lower aka 10 PSI)=14332.5
5250x2.60 (HD blower ratio for a 8.5'' lower aka 10 PSI)= 13650

So you have a difference of 900 RPM for the supposed same amount of boost. Bottom line, you do not get the same amount of increase on the two engines with the different uppers.

[glassman]

People need to reference their pullies in terms of diameter, not gains in PSI. Bottom line is a 8.5'' pulley will not gain the same on both engines.

This is a simple calculation, the hardest part is figuring out why people can't grasp this simple concept.

7.5/2.93= 2.55 blower ratio for a Lightning @ 8 PSI
7.5/3.25= 2.30 blower ratio for a HD @ 6 PSI
8/2.93= 2.73 blower ratio w/ 8'' lower (2#) for a Lightning
8/3.25= 2.46 blower ratio w/ 8'' lower (2#) for a HD
- Time out, so with a 8'' lower, you still aren't even at the same blower speed as a stock Lightning. So how do you gain two pounds? Do I really need to go on? I guess I must.
8.5/2.93= 2.90 blower ratio w/ 8.5'' lower (4#) for a Lighting
8.5/3.25= 2.60 blower ratio w/ 8.5'' lower (4#) for a HD
- Another time out, so with a 8.5'' lower on your HD, your not even at the same rate as a Lightning with a 2lber. So you start w/ 6PSI, buy a so called 4lber, slap it on with your stock upper, and bam your at 10PSI!!! But the odd thing is, your still spinning your blower slower to get that same 10PSI than a Lightning does. It's magic
I assume after reading that, someone is saying that isn't much of a difference. These numbers are small and meaningless. This is the part where I multiply the given ratio times 5250 RPM.
5250x2.73 (L blower ratio for a 8'' lower aka 10 PSI)=14332.5
5250x2.60 (HD blower ratio for a 8.5'' lower aka 10 PSI)= 13650

So you have a difference of 900 RPM for the supposed same amount of boost. Bottom line, you do not get the same amount of increase on the two engines with the different uppers.

Never one to accept things at face value
8.5/3.25 = 2.615
5250 x 2.615 = 13723
Difference is more like 600 rpm. Fair bit less than 900.

[Ryan@RhynoTune]

Not sure what you're saying, the difference between your calculation and mine is 78 RPM, and in the favor of my point.

[glassman]

5250x2.73 (L blower ratio for a 8'' lower aka 10 PSI)=14332.5
5250x2.60 (HD blower ratio for a 8.5'' lower aka 10 PSI)= 13650

So you have a difference of 900 RPM for the supposed same amount of boost. Bottom line, you do not get the same amount of increase on the two engines with the different uppers.[/QUOTE]

You wrote this not me.