How does injector sizing work? Example: I have a det with a t25 and I put on 555cc injectors because I plan on upgrading my turbo. As long as I have an ecu programmed for 555cc injectors I should be OK. However how does the fuel delivery work? Do the 555 just use a lower duty cycle vs the 370cc? Is more fuel used or does it only spray what is needed to balance the mixture? So in essence both will spray the same amount since the turbo isnt looking for a lot of air?
Its pretty simple actually.
If you have say 80% duty on a 370cc injector at X rpm and Y load (say 6000rpm at 15psi boost) and you swap over to 740cc injectors, if you correct for the injector size your duty cycle will drop to 40%. this is because the 740cc injectors are 2x the size of the 370cc so to get the same fuel, A/F ratio for example, it takes less time with the injector open to spray the same amount of fuel.
basically always having more injector than needed is important.. but I do believe in matching the MAF with comparible injectors...
I.E. a Z32 MAF with 333 injectors is pointless as is 740cc injectors with a stock MAF
Why is there a choke on some headers? What does that thing do??
without getting over technical on header design and theory, the Header "choke point" on say a 4-1 does many things. Basically it helps maintain velocity of the exhaust gasses through the collector. the other half of this is if the choke is too big you can loose velocity through the collector and this will affect the pressure waves through the collector. on the other hand if its too small you can create too much backpressure and then have negative power results.
example,
I have seen a VE engine many times with a 2.25" choke make more power AND MORE TOP END POWER than a 2.375" choke. BUT when switching to bigger cams with more overlap, duration, valve open area, the results are backwards and the larger choke will make more power, and even more bottom end...
This is all the "basic simple explanation"
