SFI popping at wot

[Todd King]

     I had a brief talk with Mike Lozano at Lozano Bros Porting in San 
     Antonio; they were Buick factory sponsored with the stage 2 stuff; 
     they did some engines for Indy as well as the Camel lights or GTP 
     stuff (something like that). He recalls having some popping problems 
     at one point with the indy motors (which ran SFI) under boost. Then he 
     started talking about camshaft overlap and injector phasing, saying 
     things like "...one phasing may be good for an rpm band but be bad for 
     another", "...causes leaning out by dumping fuel out the exhaust", 
     etc. He's a real nice guy but I know he is extremely busy so I didn't 
     want to push the phone chat any longer; they aren't into the street 
     stuff anyway. But some good food for thought... So then the light bulb 
     started to fizzle a bit- (bzzzzzzt-fizzle-pop-bzzzzzzttt...)  Say we 
     have a small stock inj (like a 28 lb/hr GN unit) and it needs 10ms to 
     shoot its load (sorry, better put the kids to bed now) at say 4000 
     rpm.  And say that the stock phasing fires the inj early in the inlet 
     cycle such that the overlap period is substantially covered by the inj 
     firing too. Now at this rpm the overlap period is a fairly long time 
     in ms. But the small inj spreads its shot out such that the overlap 
     period does not see that much fuel coming in, relatively. We assume 
     that some of the fuel goes right out the exh during the overlap 
     period. Then go up to a 55lb inj; suddenly the pw is 1/2 what it was 
     with the stock inj. Now the shot intensity during overlap is doubled 
     and with similar phasing we might see twice as much fuel head out the 
     exh during overlap as compared to the smaller inj. Make sense? Then to 
     make things worse, throw in a turbo with a large .82 a/r exh housing 
     and a big, efficient compressor at high airflow; now the exh manifold 
     backpressure is substantially reduced such that there is even more 
     tendency for fuel to be blown through during overlap. Now we get a 
     lean pop since a substantial amount of fuel that was supposed to be in 
     the cyl at fire time had already made a direct exit out the exh during 
     overlap. This is when people see the pops- after installing larger inj 
     and windy turbos. Hmmmm...
     
     Todd       todd_king at ccm2.jf.intel.com
     
     PS That new staged dual fuel pump setup arrived last night; it's gonna 
     be a fun weekend, full of boost. Looks like I might be "forced" to 
     slick up the car Sat night and go to where the big blocks play... ;-)