a GOOD SIMPLE ANALOGY is picture a pump that pumps 200gpm out of a 2" line. A 2" butterfly valve on the end could control the flow fairly linearly from no flow to full flow. Sticking a 6" butterfly valve on the end would result in hardly any control, because full flow would occur at the valve being partialy open all the way to wide open. Your tolerANCE FOR FLOW IN USGPM WOULD BE MUCH LARGER WITH THE LARGER VALVE > -----Original Message----- > From: Greg Hermann [SMTP:bearbvd@cmn.net] > Sent: Tuesday, May 23, 2000 2:07 PM > To: gmecm@diy-efi.org > Subject: Re: Is this a good deal? > > >I don't get this argument. If your engine takes 750cfm to run at > WOT/5000 > >rpm, and 500cfm at > >WOT/3500 rpm, how can a 750cfm TB be better than a 900cfm at anything but > >WOT/5000? Anything > >under 5000 rpm and you are over-sized with a 750cfm unit, right? Is a > >900cfm unit at WOT/3500 > >worse than a 750cfm unit at WOT/3500? Or WOT/2000? > > > >Squash > > > This is the mistake of looking at this stuff based on only one point on a > curve, as is done with the cfm ratings on carbs or TB's! For most things > four barrel, the cfm is tested at 1.5" of HG pressure drop. Your best > performance will happen at a lower pressure drop. Best to figure about 1.5 > RATED cfm per PEAK horsepower that you are aiming at for a hipo engine. > Otherwise, it will be somewhat restricted at the top end. > > Having an oversized TB SHOULDN'T make any difference to an engine, other > than making the throttle feel pretty "sudden", UNLESS the injectors do not > atomize the fuel very well. With poor atomization, then the higher air > velocity will help atomize the fuel, and make the engine run enough better > to compensate for most (or maybe all) of the lost airflow. > > There will ALWAYS be some pressure drop across a carb or a TB, no matter > HOW big it is. To say otherwise indicates a pretty deep misunderstanding > of > the principals of fluid flow. Yes, there are sometimes standing wave > phenomena present which create fuel "standoff" from a carby, but these are > much better described as being anomalies resulting from a two phase flow > than as being the result of a "positive" pressure in the intake tract! > > Greg > > > > -------------------------------------------------------------------------- > -- > To unsubscribe from gmecm, send "unsubscribe gmecm" (without the quotes) > in the body of a message (not the subject) to majordomo@lists.diy-efi.org
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