KraftDinner

Ok, for a very high HP 13B, lets say 650 rwhp you are going to need a lot of fuel. That means you will need huge injectors, like 1200cc primaries and 1600cc 2ndaries (about). But....

When you get injectors that big, doesn't that start to affect the driveability or smoothness of hte engine at all RPM loads? I've read and seem multiple injector set ups, where guys add 4 injectors into the top of the intake manifold for example.

I guess my question is, what would be the advantages/disadvantages of either set up. 4 large injectors, or 8 not so large linjectors ... both setups flowing about the same fuel.

Thanks

87GTR

iTrader: (61)

650hp on the fuel calc I used was

2 720 primary
2 1600 secondary
2 550 additional

(primary and secondary running 80% duty cycle)
( additonal running 75% duty)

NZConvertible

The problem with larger injectors is the higher weight of the pintle means they open and close slower than a smaller injector. When an injector receives an electrical current, it takes a certain amount of time to fully open. When that current is cut, the injector takes a certain amount of time to close. Obviously the shorter the pulsewidth, the greater the percentage of that time that’s actually opening and closing the injector. This is why large injectors at very small pulsewidths become inaccurate and can cause idling and low-load problems.
But at the higher pulsewidths associated with higher rpm, this problem goes away. For 86-on RX-7’s, the secondary injectors aren’t activated until 3500rpm, so huge injectors in the secondary positions is not a problem.
Mazda changed to staged injection because one injector per rotor would mean huge injectors. For example, you’d need 1400cc/min injectors on a 3rd Gen to match the current set-up!
BTW, the same problems occur when you try to run any size injector at very high duty cycles (>85%). The opening and closing times affect the very small “closed” time, again affecting accuracy.

KraftDinner

So would there be any downside to running extra injectors instead of just larger injectors (assuming the amount of fuel delivery is the same and all)?

soul assassin

when you run more injectors you will need more pressure usually. instead of feeding 4 injectors say at 80 % you are going to have 6 injectors 4 at 80% and 2 at 60% so you are spraying gas into 3 feed sources instead of two you tend to lose the volume and pressure, for the same amount of hp ad running the 4 bigger ones. thats my .02cent
as for driveability i run 1000cc primarys and have no problems with everyday driveability

soul assassin

i would run 2 850's in the primarys and ditch the addtionals
i.m.o.
the only time i would run add. injectors if i was running 2 1200cc and 2 1600cc and need more gas (methanol)

KraftDinner

87GTR
What is the link to that fuel injector calculator program you have? If it's just on your computer, can you email it to me?

SoulAssassin
So in your opinion, what is the largest injectors for both primary and secondary before driveability becomes an issue? 1200/1600? I guess the secondaries won't really affect the driveability even if they are ****** huge. But for primaries.. how big can one go with good driveability?

87GTR

iTrader: (61)

i used the 2 720's in the calc becuse I already have them in the car. but I think I will try some find some 1000's

KraftDinner
here is the calc I found if you find a better one let me know
http://www.rx7.com/cgi-local/2ndgencalc.cgi

soul assassin

i would say the largest to go is 1000cc in the primarys if driveability is a concern

rotarygod

iTrader: (1)

One thing that no one has mentioned is that fuel does not atomize as well coming out of larger injectors as they do from smaller ones. This is one reason (out of many) that Mazda went from 2 larger injectors ('84-'85) to 4 smaller ones ('86 and up). This does become less critical as the rpms get greater and greater since the velocity of air is such that the fuel gets mixed in very easily. Optimally you would want an injector farther away from the engine as the rpms increase. Look at the additional injector location on some of the 3rd gen. Greddy elbows. There are either 1 or 2 before the throttlebody. At wide open throttle this is a great place but only at higher rpms. There is more fuel being introduced in different areas in the intake tract as rpm increases. My setup just uses the stock locations with 550 primaries and 1600 secondaries and it works just fine. Both ways would work fine in the end. It just depends how much work you want to put in for that little (if any, in truth) gain you will get.

Fred

PaulC

OK what would be the suggestion for a 20b with a goal of 600-650 rwhp...... I know there are tons of other factors as well..... but lets kinda slide those aside for now.

rotarygod

iTrader: (1)

I would think that 3-550's and 3-1600's with programmable ecu would do the trick. You can never have too much fuel injector as a reserve so you can even add additionals to that if you think there may be a problem.

Freaky Monkey007

Well i have a question.. At what point do the injectors become below an acceptable duty cycle?? Or is there such a thing?