hi guys im in the middle of building an 748/853 track bike and was wondering if changing the stacks is going to make any difference? ive got sps cams and ported heads ,45-50mm exhaust etc etc, in another post 1037sps said something about changing them for lower ones whats the difference if any? is it just another shiney part that looks nice but doesnt actually do anything? cheers.
they move the torque curve up or down (I can remember which does which). A lot of mullah for not much payback on a relatively standard motor. BEAST 4V Velocity Stack Set
So when MR installed my 748RS engine, I won a pair of RS velocity stacks. If you look at my 748 project posts, there is a picture and I think you will see they are quite flat. Don't know why they are considered a positive fitment but clearly it was thought to be of benefit. Andy
im using 916 throttle bodies on my 853, so should i be looking at lower stacks like yours obviously without shower injectors??
In my experience what I found best was to configure the injector wiring so that all injectors fire at the same time. I tried sequential firing as per 996 on my 853R motor ( RS flowed heads, R cams, 996 throttle bodies) and it was not as successful as all firing at once. It took a full day on dyno and trying different throttle bodies and firing patterns to confirm this.
You can fit ST2 (might be ST4) velocity stacks which are a fraction shorter. CJS built my sons race engine and these were recommended for a slight top end gain.
Here's a comparison, ST4 vs. 916 (same as 748?); I've got the TPO 'beast' stacks on my 916 and they are both longer and wider. Bike made good power on the dyno whilst mapping (CJS) but unsure as to whether they are an improvement over stock or ST4. I did intend to get back to the dyno with all three types for a 'shootout' but stuff got in the way. Still may do it someday but the bike runs really well as is and a potential couple of HP gain (or not?) isn't really worth the hassle & expense. TPO's look nice though; Shame you can't see'em when the tank's on... :Arghh:
"Here's a comparison, ST4 vs. 916 (same as 748?);" 748's are the same height, as can be seen the difference is not that great and they are harder to locate and fit as they are just short enough to make it more difficult to fit the rubber sealing ring around the outside of the stack to the airbox.
At low revs a longer trumpet is preferable as they help to smooth the flow of air into the throttle body, the disadvantage of this is it offers a little more restriction to the intake. At higher revs (and therefore higher charge speeds) smoothness of the flow is less important as volume so a shorter trumpet is preferable. For a track bike living at higher revs a shorter trumpet would make more sense. As for ST4 trumpets, they are shorter than the 916 ones for some reason, airbox packaging possibly. I have swapped my ST4 ones for longer 916 trumpets on my project not for performance but I was going to use them to fit k&n pod filters, now I'm using a pipercross setup and the longer trumpets fit the filter slightly better.
