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Old 03-19-2005, 06:10 PM   #1 (permalink)
Trevor
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Default Tech: My take on injectors vs. timing control vs. HP with the stock ECU

All right guys. I'm kinda bored. I think I'll make a tech post with some of my philosphies. I've posted some of this before so those that were paying attention..this is probably a partial rehash. This is just my opinion on how the world works and is some insight into why I do things the way I do. If you think I'm full of crap post up...you won't hurt my feelings any. I could be wrong...but that doesn't happen very often. I'm posting this because some people don't seem to understand how to choose an injector and others seem to think that no cars can make big HP without an emanage controlling the timing. Maybe this will shed some light on the subject and give people some info that they missed. Maybe allow people to see the relationships between things. Excuse my spelling too...I'm going to be doing the mad scientist pouring out my guts thang.

First off learn the equations on this page:
http://www.rceng.com/technical.htm
I'll be using those and I'm not going to show my work. I always use 0.5155 for Brake Specific Fuel Consumption (BSFC). I base this on MANY BPU cars we have running around making roughly 399 crank HP and running mid 108 MPH traps on our stock 360cc injectors (sometimes even on 91 octane). If our BSFC was higher than that then we would not be capable of making that much power on 360's alone.

OK. So first thing we need to know is stock timing. Timing is based on load which is for all intesive puropses the MAF frequency. Load/MAF signal frequency also regulates Injector Duty Cycle (IDC). A stock 1G making 300HP crank is pushing 75.2% IDC. A stock 2G making 320 Crank HP is pushing 80.2% IDC. Personally I like using the 80.2% IDC figure because I believe that stock cars are slightly underrated (maybe 330-335 crank HP for 2G's). It splits the difference.

So OK...we put two and two together. If you have 80.2% IDC then you have the same ignition timing as a stock 2G because the load & MAF signal frequency is going to be the same. It's all tied together if you're running a stock computer. The stock ignition timing (or a slightly more timing retard) should be close to optimal. Now granted optimal ignition timing varies slightly based on rpm and fuel burn characteristics but I think we can all agree that stock timing (or a little less) is definitely in the ball park of what is good. Some will say some advance over stock is good but usually that's referring to while running race fuel.

If your IDC is greater than 80.2% that means your MAF signal frequency is higher than a stock car. When your MAF signal frequency is higher the computer interprets that as more load. When load is higher then the computer retards your timing to match. This a good thing on low quality fuel. On plain premium gas you usually make more power with the "more boost and less timing" combo providing you have enough intercooler and turbo to work efficiently at the higher boost.

If your IDC is less than 80.2% that means your MAF signal frequency is lower than a stock car. When your MAF signal frequency is lower the computer interprets that as less load. When load is lower then the computer advances your timing. It's actually quite excessive too. Even 72% IDC's can translate into WAY too much timing advance depending on how much boost you're attempting to get away with. This is a bad thing when you're not running race gas. Hell, if the advance is excessive enough it could be a bad thing even on the best race gas.

So heres the deal guys...if your MAF signal frequency is so low as to get IDCs below 80.2% at full throttle your timing is advanced further than a stock car. The lower your MAF frequency and IDC get at full throttle the further from optimum (for premium pump fuel) you timing shifts.

So how do you keep your timing in check then? Well that's easy...
Check your ego at the door and choose injectors that reflect your actual HP output then fine tune your fuel pressure to optimize them.
Its not hard to do. Look at other people dynos and trap speeds with you approximate setup. Use Jeff's site www.stealth316.com . He has an air & fuel flow calculator that is AWESOME for figuring out estimated HP numbers. You're going to need this site too in order to get your approximate uncorrected ambient air pressure for your altitude:
http://www.digitaldutch.com/atmoscalc/
Using all these tools will tell you how large of injectors you need. Don't fall into the trap of buying injectors for how much HP you hope you get. Don't fall into the "I'm going to get huge injectors now so I don't have to upgrade later" thing. It's not hard to upgrade later and you can always EASILY sell your old injectors. Also don't fall into the old addage of "trying to compete with the Jones'." Just because Jack T's car needs 720's doesn't mean you do. Approximate ranges are as follows:

360's with stock FPR: 320 - 399HP
360's with adjustable FPR: 305 - 417HP
450's with stock FPR: 400 - 499HP
450's with adjustable FPR: 381 - 510HP
550's with stock FPR: 489 - 610HP
550's with adjustable FPR: 466 - 624HP
560's with stock FPR: 498 - 621HP
560's with adjustable FPR: 474 - 635HP
645's with stock FPR: 573 - 715HP
645's with adjustable FPR: 546 - 731HP
660's with stock FPR: 587 - 732HP
660's with adjustable FPR: 559 - 748HP
720's with stock FPR: 640 - 798HP
720's with adjustable FPR: 610 - 816HP
745's with stock FPR: 662 - 826HP
745's with adjustable FPR: 631 - 845HP


The "adjustable FPR" ranges are based on base pressures of 39-45 psi. Anything under 39 psi base pressure gets you 29 psi at the rail at idle and atomization usually suffers. Maximum rail pressures are usually quoted at 75psi by the injector manufacturers. 45 psi base pressure gives you 28 pounds of boost to play with.

You want proof of what the so called "too-small" injectors are good for? Matt Monett went 10.65 & 134.44 mph on 660's with the pressure turned up. The above numbers aren't just pulled out of thin air. The injectors that some of you guys think are much too small are capable of much more than you've been led to believe by people regurgitating the same crap they heard from another idiot.

So are those numbers surprising to anyone? Keep in mind that you need to make the minimum HP for the injectors you're choosing during the worst possible conditions. Unless you want to cripple your car on daily driver fuel that means on the worst fuel you run, during the hottest weather you see, and at the highest altitudes your drive at. So...to run 720's and have good timing that means you need to make 640 crank HP if you have a stock FPR. I don't know about you guys but I don't make quite 640 HP on 94 octane here at 3075' altitude when its 112F outside. That's just not going to happen.

On another note...put your adjustable FPR to work. Use it in your math when you're figuring out how much injector you're going to need. to fine tune your IDC's and get them right where you want them. It works great.

So here's the beauty of it...if you get it right you'll be golden. Take my ride for instance. Latest tune is 38.4 psi base on 645's. That gets me a range of 542 - 676 HP. 542 HP is roughly what my car should make on 94 octane and alcohol, 17.5 psi, 112F ambient temps, here at 3075' altitude. I'll never see any more timing advance than a stock car. Obviously I've been flirting with fuel cut on the 676 HP side but at least that way I'll be running less timing which will be the safe way to go about it when you're running over 28 psi.

So like I said...if you're trying to avoid adding an emanage to your ride check your ego at the door when you're picking injectors. Don't BS yourself...you will pay for it later.

Personally I don't want timing control. I quite enjoy my two dimension fuel vs. boost tune that I have right now. Very simple. Simple is good. Throw in timing control and you now have a three dimensional setup. Do you realize how difficult that can be to get right? I have big respect for people that do good things on standalones and emanage setups. The thing is that for every person that's successful with a 3D setup like that there's 8 people that are going nowhere floundering around like a fish trying to figure it all out.

And another thing...people are always saying "those injectors are good for XX psi on that turbo." That's bull. Injectors are good for a certain amount of fuel flow and that fuel will make a certain amount of power. Now I can understand people factoring in someone's mods and saying a statement like that but we all know theres a HUGE difference between a set of 650R's at 20 psi on an otherwise stock motor and a set of 650R's at 20 psi on a motor with full intake & exhaust, manifold work, built heads, an overbore, and headers. It's apples to oranges. Injectors are good for XXX crank HP...not XX psi.

So anyway...that's my take on things. I hope this helps somebody to make the right decision while ordering injectors or tuning their car. Sorry if I went in about 10,000 directions at once.
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Last edited by Trevor; 03-19-2005 at 06:22 PM.
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