winisd help

[Garrett Tait 5]

this is my first time tooling around with winisd. my fi sp4 should be here any day and i'd like to make sure my box design is okay or if i should tweak it.

 

ive seen port velocity used frequently around here and i was wondering how i can determine port velocity. is it for aeroports only or can port velocity be determined in a L slot port? can someone explain how to find out the port velocity for a L port. and before people start saying, yes i did search around a while on the forum and google and couldnt come up with the answer.

 

thanks!

[Impious 1,476]

It's in WinISD under Air Velocity.  Don't know what version you have, but in the version I use it would be Rear Port - Air Velocity

 

And yes, port velocity is relevant for any port size or shape

[Garrett Tait 5]

from there what do i do?

 

i tried putting numbers in and got something like 650 ft/s  i must be doing something wrong.

[Impious 1,476]

Adjust port area until the port velocity drops to an acceptable level.

[Briggiboy33 2]

what would an acceptable level be?

[shizzzon 461]

I was talking with Quentin about how WinISD calculates port velocity and, he can explain it better than I can, but appears that they do it incorrectly.

 

It was discovered in the chat one night when i mentioned the higher you tune, the larger the port needs to be to maintain port velocity X.

When everyone in the chat strongly objected and said it's the other way around.

[Impious 1,476]

Easy way to compare.  Download Unibox (also free) and compare results.  Not sure if Hornresp includes port velocity or not, but it would be another good one to compare against.

 

As for an acceptable value, it depends on who you talk to, the design of the port, the frequency of the peak velocity &/or velocity vs frequency.  55ft/sec is a good goal, but pretty conservative.  Under about 100ft/sec is usually acceptable for a decent sized port with good flares and a peak velocity that occurs at a lower frequency.  Generally speaker though the lower # the better, as long as it's not ridiculously low.

[Quentin Jarrell 671]

I was talking with Quentin about how WinISD calculates port velocity and, he can explain it better than I can, but appears that they do it incorrectly.

It was discovered in the chat one night when i mentioned the higher you tune, the larger the port needs to be to maintain port velocity X.

When everyone in the chat strongly objected and said it's the other way around.

The problem is all the programs figure port velocity based on small signal parameters. By the time the sub hits xmax it's cms has changed by at least 50 percent affecting piston acceleration and Fo which is directly related port velocity peak point and speed.

You're better off using Small's formula and using proper xmax figure.