basics on carbs and fuel injection

[grumpyvette]

I get lots of questions on if MPFI (MULTI PORT FUEL INJECTION) or CARBS or TB (THROTTLE BODY) injection is better,SO I figure ID post a very brief explanation and info links,on how each system basically functions ,most fuel current injection systems but NOT all of them use direct port injection,and a fuel pressure return regulator with a return line and most of those current systems use a fuel pressure of at least 35 psi-40psi which the standard carb fuel pump with its 6-7 psi won,t supply
theres some TB injection systems that work on much less fuel pressure but in most cases still will above the 6-7 psi level.

bits of INFO

carbs

http://auto.howstuffworks.com/question377.htm

http://www.roadkill.com/~davet/moto/carb.monologue.html

http://popularhotrodding.automotive.com/60684/0802phr-how-carburetors-work/index.html

TBI
http://autorepair.about.com/cs/generalinfo/l/bldef_791.htm

http://www.goingfaster.com/spo/tbi.html

MPFI
http://www.youtube.com/watch?v=ufsOIOQmFHc

http://auto.howstuffworks.com/fuel-injection.htm

http://www.autoshop101.com/forms/h20.pdf

http://www.customefis.com/GMEFI.html

http://www.holley.com/data/Products/Technical/199R10179-2.pdf

http://www.popularhotrodding.com/enginemasters/articles/hardcore/0305em_holley/index.html

http://www.holley.com/91503201.asp

 

[grumpyvette]

as Im sure your aware the best torque will usually be with a ratio of about 12.5:1-13:1 and the most fuel efficient ratio will be closer to 14.7-15:1
carbs react to CHANGES in air flow rates, to change the fuel feed rates so theres always a delayed reaction, EFI generally uses sensors to detect BOTH changes in AIR/FUEL RATIOS in the exhaust and changes in vacuum and airflow rates
carbs generally cover up sudden increases in air flow such as stomping on the throttle to WOT from idle with a long shot of fuel from the accellerator pump and power valves adding extra fuel, neither one is a precisely metered responce.
you can tell a good deal about the conditions in an engine if you know the vacuum readings (use a gauge), and know how to read spark plugs
now I use a vacuum gauge a timing light, and an IR temp gun on the headers along with the data from reading the plugs and the a/f ratio sensors, each adds to the indications of whats going on, yes you can acchieve a 14.:1 ratio and get the engine to run decently, but in most cases it will tend to run better in the low 13:1 ratio range durring transitions and about 14:1-14.7:1 durring cruising simply because theres always a tendency to go lean durring sudden transitions with most carbs.
Id keep in mind that going rich durring rapid transitions is prefered over going lean and getting the a/f ratio stable and predictable at cruise and idle and durring mild accelleration like on the freeway,where youll spend 90% PLUS of your time is far more important than temp swings durring throttle changes, or durring WOT where running slightly richer, durring high load high rpm conditions is prefered simply because it makes better power and has less tendency to run into detonation, especially since youll spend far less time under those conditions


BTW (lots of links and sub links on carb tuning, spark plug reading, etc.)
if you do some searching on the hybridz site,
but heres a bunch

http://www.digitalcorvettes.com/forums/showthread.php?t=77602

BTW use a 190F T-stat and an extra electric fan that comes on at 200F to keep the engine coolant temp stable, try to keep the oil temp in the 215F-220F range if you can,to burn off moisture that can form acids over time in the oil, and the fuel pressure stabile at about 5 PSI,, have no more than 1-1.5 psi of back pressure in the exhaust at WOT, it makes tunning far easier, make sure the ignition timing curve is smooth & consistant, and do a leak/down test and adjust the valves, do a vacuum leak test, check the oil pressure,and fuel filter, to make sure your not working with mechanical problems.
if your tunning an n/a engine and the IR gun shows header temps over 1250F or under about 1100f once its up to temp, youll generally find vacuum leaks, ignition curve problems or a/f ratio problems

 

[grumpyvette]

BTW if your setting up a carbed vette with an electric fuel pump, you should place the pump and filter as low and as far to the rear of the car as you can BUT place it up inside the frame rails if possiable so its protected durring any potential crash or accident, ellectric pumps push fuel far more effectively than they suck fuel but they do suck fuel over a short distance just fine if the tank pick-up line and sock screen is mounted low in the tank, and IVE yet to have a problem if the lines are the correct size for the application, yes that usually means the pump and filter are at or slightly above or slightly below the frame and yes the tank might be lower, it usually has little or no effect as long as the lines are of decent internal dia. and clear and the pick-up tube and sock are in the tank sump

read

http://www.centuryperformance.com/f...tanding-fuel-pressure-and-volume-spg-140.html

http://www.centuryperformance.com/fuel-system-setup-spg-141.html

http://www.raceprecision.com/forums/viewtopic.php?f=3&t=22&p=35


http://www.chevyhiperformance.com/techarticles/0606ch_carbureted_motor_fuel_system/index.html

btw its also a good idea to use rubber mounts and a decent bracket to mount the pump and isolate the vibrations as many electric fuel pumps are noisy, and use double NYLOC NUTS or use wire tie nuts on the bolts with rubber bushings and large washers to prevent the mount bolts from comming loose due to those vibrations