Short velocity stacks - cross post from Classic

While this was brought up in the classic section, thought it may have some audience here.

Today's project was installing a set of durable1 short velocity stacks. Workmanship/fit is quite good. Pretty dang inexpensive mod I couldn't resist trying and obviously easy to do. Of course, now must be EXTRA careful not to drop a nut down the carb.
On just a quick test spin, it does feel as if it's spinning faster above 5K. It's like it gets faster, faster - i.e the accell rate is noticibly improved seat of the pants up high with perhaps a little throttle response lost however in the 2500 - 3500 range (air column velocity?). Still very streetable, but a little less umph to me feeling like the Kjet throttle response. Very good, but not instantaneous down low as with the taller air horns.
Will try to dyno sometime, although Durable1 has a pre/post dyno that kinda confirms a top end improvement of about 10 hp, which is what you would expect anyway with shorter velocity stacks. Feels like it does improve high end breathing seat of the pants. A good mod for the high rpm (>5K) performance. Cheap, too.

Hope this is interesting to some; more later if anyone's interested. No connection, yada, yada.

best
rt
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Yes - it runs on 'carbs' as well...

 

An oversimplified way that I was taught was that longer flow horns tend to move the powerband down the rev range, increase low end torque and provide higher air velocities which sharpen throttle response - but they do relatively add resistance to top end flow. Shorter stacks are less resistance to high flow, and give increased top end, but at lower rpm ranges the velocities are lower dulling throttle response relatively. There are other factors of course (pulse tuning, cam profile and reversion, etc), but this is a short story.

Something that has borne out in my dyno experience and that has been talked about on the Alfa boards (www.alfabb.com) is that Webers like a good amount of space above the air entrance to give good high end flow. This is the primary advantage of these short air horns in the somewhat restictive stock Ferrari air box. You may lose some low end throttle response (lower velocities) but gain quite a bit up high. As you can see in the dyno posted in the other thread*, these jewels were worth 12 RWHP at the upper end on a chassis dyno. Subjective seat of the pants eval (or self expectational delusion) seems to bear this out on a short drive yesterday - will try to better quantify later..

best
rt

* http://ferrarichat.com/forum/showthread.php?t=47676