pump under drive?

[wideopen231]

Spud,

If have apump too big for need. Whats best way to figure under drive ratio. I have cpl old Waterman under drive sitting here  different ratios. I have a 85. gpm pump.Too big for injected alky w/o going either bigger on main than I want or small nozzle and higher pressure. It will work great when I get set up and want to throw a high percentage load to it. Need few other parts first and thats after new trailer. May be have to have two complete systems I can swap from alky to nitro,just checking options and trying to make most versatile with fewest changes possible.
  Any rule of thumb for figuring pump out put with under drive.Linear or just totally changes chart characteristics?

[Paul New]

You could belt drive it than it would be simple to slow pump down

[dreracecar]

Over/under drives are ok for small % of change, but trimming the amount needed is plain impractile  just buy the correct pump and be done with it.   You are asking close to a 75% reduction running off the crank that spins 2X the cam speed right off the bat, and a custom drive off the cam could cost more than a new pump and may not work

[wideopen231]

have a 8.5 and looking to get down to 7 so roughly 20% flow reduction . I have a 15% and what appears to be 20 to 22% with rough check on it. I guess I could hve pump flowed and use flows at rpm pump would be at to give be ball park idea. Hoping Spud will chime in.

[dreracecar]

Go over your post again and edit, you typed in "85." GPM and that is what my comment was referring to.   I ran a 110 (13 gpm) on my car where an 80A (6.5gpm) would work. just went to a 130 main instead of a 75 main.  More parts to fail by using any type of XX drive

[wideopen231]

 I realized the typo after first reply. Hey if I had 85 gpm pump I could probably buy three or four small ones.LOL  My current set up with a 7.0 or 80A equal runs a 125 to 135 main and around 150 psi. If I ran the 8.5 pump I would like to keep main jets below .135.Reason is flow change more drastic once you past that for every .005 of jet. I like to keep my systems around 150 and main jet between .100 and .130 range  because changes are more predictable flow wise and not as big of jump. I do so by balancing my nozzle sizes,required flow for motor and bypass amount.  Most are .005 equal .2 to .3 gpm and after .130 they jump to .4 to .5 gpm per .005 I like 150 pressure range for cpl reasons. One all of my flow data is based on that(yes can do conversion for more pressure mathematically) and I have lots of data with systems over the years based on that area and it works well for me. Maybe just getting old and like the known factor more. I have a 18.5 on shelf I should sell,but if I want to throw a big blower on I have setup for that pump that ran 5.70's 20 years ago,admittedly lot of other things are different like TA/FC vs FED and PG vs lenco with 3 disc Hays,but everything else is out of the TA/FC anf in the FED. I just know better.LOL

As for drives I am already running belt drive and pulley swap would allow me to dial in ratio pretty well. I do agree l;less parts means less to break. Having one pump for two setups would allow me to get back up pump for both and know two pumps are not exactly the same but hey thats another minor tuning issue and with out those where would the fun be?