racinskittle

Alright, I was thinking about this all last night and am wondering if im right on this. From time to time my car was hard to start and it idled sort of irratic with the "hack" and 450cc injectors. So I decided to take it from -35 to -37%. It got better so I left it like that.

Now Im thinking. Since our cars had 240cc injectors in them to start with. Going from 240cc to 450cc is a 47% jump in fuel. So why aren't we taking out 47% of the fuel to simulate normal injectors in the first 1500RPM's or so instead of going -35% across the board? This seems like it may make starting and idling much easier.

Any thoughts in this?

birz

I'd like to know too

TurboJesse

my friends car starts perfect and idles nicely with the hack and rc 440's. The DSM injectors do weird things sometimes

boosted_dc2

at idle you can lean it out as much as you want.....each vehicle is different. mine worked best at -37 at idle but i was running 440's. either way it never idled like stock, in fact it stalled alot and i never got it perfect. when i started it i always had to stay on the gas for 3-5 min before it would even idle on its own. Also sometimes it would idle perfect at like 750 rpm's but after driving for 20 min or so the idel would be at like 1200. I think its different for every car so just rty different settings, i was told by my tuner if you hear popping at idle then its too lean.

Landon

racinskittle

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by boosted_dc2 &raquo;</TD></TR><TR><TD CLASS="quote">at idle you can lean it out as much as you want.....each vehicle is different. mine worked best at -37 at idle but i was running 440's. either way it never idled like stock, in fact it stalled alot and i never got it perfect. when i started it i always had to stay on the gas for 3-5 min before it would even idle on its own. Also sometimes it would idle perfect at like 750 rpm's but after driving for 20 min or so the idel would be at like 1200. I think its different for every car so just rty different settings, i was told by my tuner if you hear popping at idle then its too lean.

Landon</TD></TR></TABLE>

How low have you set it? So with -37 you still had problems?


How come no one goes lower than 40%? It seems like that's what we should be doing.

kpt4321

47% more fuel flow does NOT mean -47% on the SAFC.

Think about this simplified model: Stock, you have 200 cc/min injectors. You install 400 cc/min injectors, which flow 100% more. What do you do with the SAFC? NO! You pull out 50%, because you want half the airflow since the injectors flow twice as much.

1-(1/flow increase)

For 450's, if stock was 240's, the flow increase is 87.5% more fuel (who knows where the HELL 47% came from). 1-(1/1.875) = 47%. You DO want to set the SAFC to -47% (give or take), but NOT because you have 47% more fuel! You don't! If you really had 47% more fuel, you would want a LOT less negative correction on the SAFC.

That said, I won't be responsible if any of you go out and blow you your cars. You don't want to totally compensate for your larger injectors, because you want the car to run richer at WOT than it did originally. -40% or so is probably best.

Another note:

You want to take out less fuel at the lower rpm's on the lo throttle map. 450's have a lot more deadtime than stockers, AND DSM injectors have a lot more deadtime then rockin' Honda ones. That means you need a longer puleswidth. What you want is a taper towards this 40% (or so) number.

The hi throttle map will be the large correction all the way across, plus or minus for tuning.

racinskittle

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by kpt4321 &raquo;</TD></TR><TR><TD CLASS="quote">47% more fuel flow does NOT mean -47% on the SAFC.

Think about this simplified model: Stock, you have 200 cc/min injectors. You install 400 cc/min injectors, which flow 100% more. What do you do with the SAFC? NO! You pull out 50%, because you want half the airflow since the injectors flow twice as much.

1-(1/flow increase)

For 450's, if stock was 240's, the flow increase is 87.5% more fuel (who knows where the HELL 47% came from). 1-(1/1.875) = 47%. You DO want to set the SAFC to -47% (give or take), but NOT because you have 47% more fuel! You don't! If you really had 47% more fuel, you would want a LOT less negative correction on the SAFC.

That said, I won't be responsible if any of you go out and blow you your cars. You don't want to totally compensate for your larger injectors, because you want the car to run richer at WOT than it did originally. -40% or so is probably best.

Another note:

You want to take out less fuel at the lower rpm's on the lo throttle map. 450's have a lot more deadtime than stockers, AND DSM injectors have a lot more deadtime then rockin' Honda ones. That means you need a longer puleswidth. What you want is a taper towards this 40% (or so) number.

The hi throttle map will be the large correction all the way across, plus or minus for tuning.
</TD></TR></TABLE>

Sweet! Thanks for the reply, I understand that now. I didn't know where these #'s were coming from so I figured i'd ask

As far as the 47% goes, that's the amount of fuel you'd want to take out I thought? Because it 210cc/450cc= 47%. My wording was wrong though I appologize for that. The flow is 87% more over stock. I really never thought of that.

Thanks for clearin this up for me and the explination. This is exactly why I didn't go out and do it. I actually forgot about WOT.

kpt4321

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by racinskittle &raquo;</TD></TR><TR><TD CLASS="quote">
As far as the 47% goes, that's the amount of fuel you'd want to take out I thought? Because it 210cc/450cc= 47%. My wording was wrong though I appologize for that. The flow is 87% more over stock. I really never thought of that.</TD></TR></TABLE>

That's correct, you want to "take out" 47% of the fuel. It's not because you have 47%, I just wanted to clear that up.


As a final note for people reading this, and for the archives: You DO NOT want to set your SAFC/VAFC's to -47%. You probably want somewhere around -40%.