Saturday, February 04, 2006

Why flow matched injectors are mandatory and not optional

So the other night a friend of mine asked me why do I try to talk all the people I tune to talk into buying flow matched injectors. I tried to explain that imprecision, combined with different conditions per cylinder (the famous #7 cyl explosion for LS1, I think the modular Fords have both 7 and 8 'pre-screwed by nature' as well) can be deadly, especially in environments extra sensitive to variances in AFR (nitrous, forced induction).
He got the point, but I don't think he was convinced that it's truly worth the extra $100-$120 that it usually costs to have them flowmatched.

So I decided to quantify some of these generalizations, for the sake of making my argument stronger.
First I wanted to see how much 'off' is my Air-Fuel Ratio going to be, with normal injectors, vs. flow-matched injectors. So I made a simple table of what ratio of air to fuel would be theoretically, vs practically, as a result of injectors just being cheap and 'good enough' for normal applications.
11units of air to 0.95units of fuel(i was aiming to the dangerous side of AFR to demonstrate my point in a more dramatic manner) and 11.0 target AFR yielded 11.58AFR. that's almost 0.6AFR of precision and unpredictability that we just gave up on cheaper injectors, and that's with perfect air measurment!
That got me thinking: If this is the acceptable, normal scenario, how much worse can it be? Injectors say +/-5%. what if we get two injectors in the set that are on the very ends of their imprecision margin? That'd be 10% difference in flow, how bad would this be, especially if they ended up on cylinders that are naturally richer/leaner? Airflow is also a bitch to measure, what if we don't get that precisely on the money? What if the 'lean' cylinder will get a 'weak' injector, and we're not measuring air precisely enough on top of that?
So I started explanding my tiny spreadsheet, and I came up with a bunch of scenarios demonstrating various situations that can occur as a result of imprecise air and/or fuel delivery:

So the scary scenario is the last table, when you combine 10% imprecisions of air and fuel. instead of a target 11.0AFR, you end up with 13.44AFR.
Scary enough for you?
That not only proves that these extra $100 on flow matched injectors are truly an insurance, but also makes a very strong case for having your VE/MAF calibrations done perfect, as apparently being within 5% is not good enough, if you intend to have a truly reliable, predictable setup.

Hope this puts the whole 'why tune and retune' argument in a much better perspective.


At 2:38 AM, February 24, 2006, Blogger ClubsportR8 said...

Hey bud.
A good point made here.
What about cylinder to cylinder variances like the infamous number 7 lean condition?
Wouldnt it be good to know the flow rates of the injectors and arrange them to suit the engine.
(ie) Use the higest flowing one on cylinder 7 etc so that afr per cylinder is optimised.
In which case having differences would be good as you could tune fueling to each cylinder.

At 10:36 AM, March 07, 2006, Anonymous joecar said...

Good work, excellent research, I appreciate what you write;
How lean does 7 get compared to the others, and wnat causes this (is it firing order related or manifold geometry...?)...?


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