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Tech: My take on injectors vs. timing control vs. HP with the stock ECU

137K views 263 replies 87 participants last post by  RealMcCoy 
#1 · (Edited)
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. :p 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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#55 ·
antero said:
That math is wrong as you need to (^2) square it somewhere. Go to stealth316.com and look for his injector calculator web page.
Uhh, no you don't. We aren't talking fuel pressures here.

-Chris
 
#56 ·
ukyo said:
I'm glad you put this in to words, Trevor. I was working this out for myself. I had a few self proclaimed "experts" telling me that 450cc injectors were wrong for my setup, and that I needed 550cc injectors.
I still believe you need 550cc injectors if you really want to max out those turbos and make the most power possible. And no, I never proclaimed myself to be an expert, but I'll be sure to remember that little "slap" ...

Aside from the obvious need for additional fuel and TIMING, I simply do not like old, used injectors. Did you even have those 450cc injectors flow tested?

ukyo said:
If I want decent IDCs that will give me mid to high 20* timing, it would appear as though the limit on my car is 1.3 KG/CM² (18.5 PSI) leaned out to ~28% with C12 leaded race fuel. Leaned any more the car starts to knock, even with the leaded fuel in there.


For now I'm going to say the practical limit for my set up on C12 is 1.2 KG/CM2 leaned down to -30% correction. That is where I made the 114.75 MPH trap. I actually had a little more room to lean the fuel at that boost level, so it has the potential to net me higher traps. I don't think running higher boost is going to help me at this point until I do changes elsewhere in the system (as noted by pacman3000gt). But still... 114.75 MPH traps (with obvious room to improve with better tuning) running the stock ungutted MAS, DR500s, tiny 450cc injectors, MBC, DP and a lightweight flywheel... (basically I have ~$2,000 in the car for performance parts). IMO, that's a good return on the investment.
So, what? Now you're arguing with what you've witnessed and stated yourself?
 
#58 ·
AdamVR4 said:
I still believe you need 550cc injectors if you really want to max out those turbos and make the most power possible. And no, I never proclaimed myself to be an expert, but I'll be sure to remember that little "slap" ...
Ummm... Did I say I was talking about you??? You can assume what you like, but I assure you 1 - You don't live around here, where I get the bulk of my "expert" advise and 2 - That "slap" was not intended for you.

AdamVR4 said:
Aside from the obvious need for additional fuel and TIMING, I simply do not like old, used injectors. Did you even have those 450cc injectors flow tested?
I'm making 30* of timing at redline and I still have room to pull fuel (on race fuel). Pulling fuel, as you know, means I will hit a little more timing as well. So I still don't see how my timing is suffering??? I make mid 20* on pump gas at 1.2 KG/CM², and 30* on race fuel at the same boost level. Maybe the timing is a little low for pump gas, but it seems to be doing just fine on race fuel.

And no, I did not have the injectors flow tested, cleaned, or otherwise looked at. I've had no blatant issues with fuel devliery. I'm still running the stock 88K mile fuel filter with the stock fuel rail loop, too.

AdamVR4 said:
So, what? Now you're arguing with what you've witnessed and stated yourself?
I'm not arguing anything. I found the perfect injector for my setup. I can basically control every aspect of the tune from a piggyback with no worries of a delicate balance. If I want more timing / power, all I'm just a few gallons of race fuel and a few negative correction clicks away from having it. And all of that with no worry of going too lean, running too much timing, or running out of correction room on the piggyback.

I feel that with 550cc injectors I would have to walk a thin line between leaning it out and getting too much timing. Thus far my experience with 450cc injectors has shown me that I can lean it until I see minute knock, and still have the timing right at 30*. I haven't tried yet, but I'm sure that if I lean 550cc injectors down an additional -12% on the SAFC timing will be above 30*, and it'll still be hitting .94v (race fuel). Now you only have 5% more fuel to play with (SAFC), and your timing is starting to get too high. What do you do? Let it sit there and run on the rich side with slightly too high timing? Richen it up and kill power to get the timing back in check? Lean it some more and hope the timing doesn't go overboard? I don't have that problem with my 450cc injectors.

550cc injectors may make more sense for my setup in two scenerios:

1 - More timing at pump gas corrections.

2 - More boost needing more fuel. This is an option, but I'm running the 17 PSI that DR rated the turbos to already. I thought about running more boost, but when I turned the boost up at the track last time all I got was knock counts. O²s looked good, but it was knocking all the same. Either my turbos can't produce anything much more than 17 PSI in their range or (as pacman3000gt already noted) I need to revisit my IC setup.

I just don't see how, from those numbers, you can think I would benefit hugely from 550cc injectors???
 
#59 ·
ukyo said:
Ummm... Did I say I was talking about you??? You can assume what you like, but I assure you 1 - You don't live around here, where I get the bulk of my "expert" advise and 2 - That "slap" was not intended for you.



I'm making 30* of timing at redline and I still have room to pull fuel (on race fuel). Pulling fuel, as you know, means I will hit a little more timing as well. So I still don't see how my timing is suffering??? I make mid 20* on pump gas at 1.2 KG/CM², and 30* on race fuel at the same boost level. Maybe the timing is a little low for pump gas, but it seems to be doing just fine on race fuel.

And no, I did not have the injectors flow tested, cleaned, or otherwise looked at. I've had no blatant issues with fuel devliery. I'm still running the stock 88K mile fuel filter with the stock fuel rail loop, too.



I'm not arguing anything. I found the perfect injector for my setup. I can basically control every aspect of the tune from a piggyback with no worries of a delicate balance. If I want more timing / power, all I'm just a few gallons of race fuel and a few negative correction clicks away from having it. And all of that with no worry of going too lean, running too much timing, or running out of correction room on the piggyback.

I feel that with 550cc injectors I would have to walk a thin line between leaning it out and getting too much timing. Thus far my experience with 450cc injectors has shown me that I can lean it until I see minute knock, and still have the timing right at 30*. I haven't tried yet, but I'm sure that if I lean 550cc injectors down an additional -12% on the SAFC timing will be above 30*, and it'll still be hitting .94v (race fuel). Now you only have 5% more fuel to play with (SAFC), and your timing is starting to get too high. What do you do? Let it sit there and run on the rich side with slightly too high timing? Richen it up and kill power to get the timing back in check? Lean it some more and hope the timing doesn't go overboard? I don't have that problem with my 450cc injectors.

550cc injectors may make more sense for my setup in two scenerios:

1 - More timing at pump gas corrections.

2 - More boost needing more fuel. This is an option, but I'm running the 17 PSI that DR rated the turbos to already. I thought about running more boost, but when I turned the boost up at the track last time all I got was knock counts. O²s looked good, but it was knocking all the same. Either my turbos can't produce anything much more than 17 PSI in their range or (as pacman3000gt already noted) I need to revisit my IC setup.

I just don't see how, from those numbers, you can think I would benefit hugely from 550cc injectors???
Unless DR500's are incredibly inferior to 13G's, you're not maxing them out at 1.2 bar.

Something is wrong if you get knock beyond 1.2 bar. And it's not your IC set-up. No offense, but that's Paul's favorite recommendation to make... It's just simply not the case if you have proper fuel. 17+ psi isn't a lot of boost...

If your IC set-up is really holding you back, why have so many before you been able to make more power and run more boost through them without knock issues?

Perfect example is FastOldGuy... 13G's, stock SMIC's, 20 psi... We "hot-lapped" ;) the car on the dyno and it held 20 psi to redline with minimal knock counts (I saw 8 counts for a second above 6000 RPM) which could have undoubtedly been ironed out. It made 410 AWHP. He has oversized valves... That 20 psi number would have been higher had he had stock valves.

What kind of race gas were you running?

You mention you'll be over 30* with more fuel to play with if you run the 550's ... This sounds perfect to me. Lean it out as much as you can with the 550's and your timing may very well be at 34* at redline (arbitrary #) ... Turn the boost up HIGHER to compensate for the additional fuel and run better gas! Pure C16.

You basically wanted to know how you could make the most power... I think you can make MORE power with 550's than you can with 450's with your turbos.

If you were only running one DR500, I'd agree, 450's would be perfect :)

Or an AFPR would allow you to get the additional fuel and timing you could use beyond 1.2 bar, but something isn't right if you're seeing knock at those boost levels on race gas. Perhaps your injectors aren't flowing the same amount. I had 5000 mile old PTE 780cc injectors flowed and cleaned and one of them failed every test and flowed like shit. 5000 miles old. Just figured I'd throw that out there...
 
#61 ·
excellent explanation, Trevor.

What are your thoughts on tuning with only a safc or similar piggyback? You mentioned having 2 degrees of freedom (I assume adjustable FPR and airflow controller), allowing you to influence IDC's without changing airflow (or vice versa), and thus change your position in the stock timing map. If this isn't available, is there anything that can be done to manipulate timing and still have acceptable O2 readings?
 
#62 ·
AdamVR4 said:
Unless DR500's are incredibly inferior to 13G's, you're not maxing them out at 1.2 bar.

Something is wrong if you get knock beyond 1.2 bar. And it's not your IC set-up. No offense, but that's Paul's favorite recommendation to make... It's just simply not the case if you have proper fuel. 17+ psi isn't a lot of boost...
I've ran 1.3 KG/CM² on them with pump gas with little to no knock before... in cooler ambient temps. Maybe the fact that I was making a pass on the track about every 3 minutes near the end of the day had something to do with it knocking? I dunno. I didn't start messing with raising the boost till near the end of the day.

AdamVR4 said:
If your IC set-up is really holding you back, why have so many before you been able to make more power and run more boost through them without knock issues?
It's just a factor in the equation that I am not going to rule out. It may or may not be some or all of the problem, but it can't be ruled out. How else can my car running 1.3 KG/CM² on pump gas and barely knocking pulling 114 MPH traps in cooler temps suddenly need C12 and a much more aggressive leaning on the SAFC to reach the same traps in warmer temps?

AdamVR4 said:
Perfect example is FastOldGuy... 13G's, stock SMIC's, 20 psi... We "hot-lapped" ;) the car on the dyno and it held 20 psi to redline with minimal knock counts (I saw 8 counts for a second above 6000 RPM) which could have undoubtedly been ironed out. It made 410 AWHP. He has oversized valves... That 20 psi number would have been higher had he had stock valves.
Isn't he also running a built, over bored engine with other head work apart from the oversized valves?

AdamVR4 said:
What kind of race gas were you running?
90% C12 (I still had probably a gallon or two of pump gas in there, with an additonal 4 gallons of C12).

AdamVR4 said:
You mention you'll be over 30* with more fuel to play with if you run the 550's ... This sounds perfect to me. Lean it out as much as you can with the 550's and your timing may very well be at 34* at redline (arbitrary #) ... Turn the boost up HIGHER to compensate for the additional fuel and run better gas! Pure C16.
So where do we draw the line on timing? If 34* should be a traget, then maybe I should consider 550cc injectors. I was under the impression that over 30* was getting too high. :confused:

We'll see what happens in April. I'm going to run the car again at 1.2 KG/CM² on C12 and seeing what I can get out of it at lower than -32% corrections. I'm pretty sure it'll work it's way in to the 31* - 32* timing range. If that doesn't get me where I want to be, then sure... why not? I'll look in to 550cc injectors.

AdamVR4 said:
You basically wanted to know how you could make the most power... I think you can make MORE power with 550's than you can with 450's with your turbos.

If you were only running one DR500, I'd agree, 450's would be perfect :)
Advice noted. If nothing else, I'm going to wring these 450cc injectors out to the fullest, so there will be no questions on what they can or cannot do.

AdamVR4 said:
Or an AFPR would allow you to get the additional fuel and timing you could use beyond 1.2 bar, but something isn't right if you're seeing knock at those boost levels on race gas. Perhaps your injectors aren't flowing the same amount. I had 5000 mile old PTE 780cc injectors flowed and cleaned and one of them failed every test and flowed like shit. 5000 miles old. Just figured I'd throw that out there...
An AFPR might make more sense for me. Even though you can sell used 550cc injectors pretty easily, I'd rather not go through with getting a baby step up in injectors when they are not going to be able to match up to my future turbo plans. Oh well, at least I have two options...

The adjustment range on my MBC is almost out, so I don't know how much boost I was running on the last few passes at the track this last time. I basically was running with it shut, and was paying more attention to the RPMs (trying to stay off the rev limiter in 1st) and O² voltages than the boost gauge. I could have been peaking at 1.7 KG/CM² for all I know, and that would explain the knock. I'll never know for sure, so that is why I'm going to be sure to keep my eye on the RPMs / boost gauge this next trip and let the warning beep on the logger be my O² / knock monitor.
 
#64 ·
Bob The Great said:
excellent explanation, Trevor.

What are your thoughts on tuning with only a safc or similar piggyback? You mentioned having 2 degrees of freedom (I assume adjustable FPR and airflow controller), allowing you to influence IDC's without changing airflow (or vice versa), and thus change your position in the stock timing map. If this isn't available, is there anything that can be done to manipulate timing and still have acceptable O2 readings?
I think you're confusing something here. Changing the fuel pressure doesn't affect the position on the stock timing map -- it simply scales up and down the injector size. If you dial your car in with a certain size injectors, then bump the fuel pressure up or down, you will have to re-correct to bring the tune back to where it was before the pressure change. Think of it as "no such thing as a free lunch", but more accurately you aren't able to "cheat the system" and reduce the correction you are applying to the ECU without also reducing the amount of available fuel; it's just like having bought slightly smaller injectors in the first place.

-Chris
 
#66 ·
Multiades said:
I think you're confusing something here. Changing the fuel pressure doesn't affect the position on the stock timing map -- it simply scales up and down the injector size. If you dial your car in with a certain size injectors, then bump the fuel pressure up or down, you will have to re-correct to bring the tune back to where it was before the pressure change. Think of it as "no such thing as a free lunch", but more accurately you aren't able to "cheat the system" and reduce the correction you are applying to the ECU without also reducing the amount of available fuel; it's just like having bought slightly smaller injectors in the first place.

-Chris
ok, I am confused then. I thought the whole point of this thread was to manipulate the IDC, changing how the ECU calculates load, and changing its position in the timing map. I was asking if there is a sneaky way to manipulate the timing map position without an AFPR or stand-alone timing control. Or is it basically just see where timing falls when your O2's are acceptable, and then lean it a little more if you can get away with it?

I understand that messing with the fuel pressure will have the same effect as using differently sized injectors, but won't this also have an impact on IDC?
 
#67 · (Edited)
Bob The Great said:
ok, I am confused then. I thought the whole point of this thread was to manipulate the IDC, changing how the ECU calculates load, and changing its position in the timing map. I was asking if there is a sneaky way to manipulate the timing map position without an AFPR or stand-alone timing control. Or is it basically just see where timing falls when your O2's are acceptable, and then lean it a little more if you can get away with it?
Fuel pressure can be used as a method of "fine tuning" injector size when there is no in-between option available, if that's what you mean.

Bob The Great said:
I understand that messing with the fuel pressure will have the same effect as using differently sized injectors, but won't this also have an impact on IDC?
Yes, in that way you're absolutely right -- your original post sounds a lot more like you are trying to assert what Innovator and a couple others did a while back, that you could change the load point that the engine draws from and somehow "cheat" the correction system. He was claiming that removing the honeycombs from the stock MAS resulted in needing less S-AFC correction to obtain the same A/F (true), which had the effect of showing the ECU a higher load point (false, the MAS is simply now miscalculating the airflow through it, effectively applying some pre-correction).
 
#68 ·
Multiades said:
Yes, in that way you're absolutely right -- your original post sounds a lot more like you are trying to assert what Innovator and a couple others did a while back, that you could change the load point that the engine draws from and somehow "cheat" the correction system. He was claiming that removing the honeycombs from the stock MAS resulted in less S-AFC correction (true), which had the effect of showing the ECU a higher load point (false).
ok, I think I see what I was doing. IDC is controlled by the ECU's load calculation, not the other way around. I don't know how I confused this, but I did.

In this respect, IDC acts as a cue to the load being calculated, and the ECU's position in the timing map. By tuning the airflow frequency correction to result in slightly below 80% IDC, you can maintain a small timing advance over stock, which is optimal.

But, in doing this, you change your F/A ratio (probably a little lean, if I'm thinking correctly) and an AFPR can bump up the pressure a little bit to bring the ratio back to where it needs to be. This also fixes the amount of air the engine must be ingesting to make use of all the fuel. So, if you tune this way, but don't have enough airflow to use the injectors you chose, then you won't be able to adjust fuel pressure enough to compensate. ie. your injectors are too ginormous.

..if that made any sense at all. :)

so, to answer my own question, in tuning with only an airflow correction, I would want to get to a point of near-stoich O2 readings, then begin leaning out to lower my max IDC to as close to 80% (or a little lower, is possible) as I could, while keeping an eye on knock and EGT's. Most likely, I'll have to settle for a happy medium between slightly high IDC (and thus close to stock timing) and slightly lean F/A.

Or I could use an AFPR and raise the base pressure by enough to have both.


Here's a thought though. I know it would be a very crude way to to it, but let's say that I keep the stock TT FPR, which maintains a base pressure of 43psi, and I find that the lowest IDC I can get without knock is somewhere around 85% (too high). If I were to then switch to the N/A FPR, which has a base pressure of 47psi, I would then be able to lean out a little more to drive the IDC down without going lean enough to knock. Sound logical? However, if I can get 80% IDC and 0 knock on a TT FPR, there would be no reason to up the fuel pressure to drive IDC even further down, correct?
 
#69 ·
Bob The Great said:
ok, I think I see what I was doing. IDC is controlled by the ECU's load calculation, not the other way around. I don't know how I confused this, but I did.

In this respect, IDC acts as a cue to the load being calculated, and the ECU's position in the timing map. By tuning the airflow frequency correction to result in slightly below 80% IDC, you can maintain a small timing advance over stock, which is optimal.

But, in doing this, you change your F/A ratio (probably a little lean, if I'm thinking correctly) and an AFPR can bump up the pressure a little bit to bring the ratio back to where it needs to be. This also fixes the amount of air the engine must be ingesting to make use of all the fuel. So, if you tune this way, but don't have enough airflow to use the injectors you chose, then you won't be able to adjust fuel pressure enough to compensate. ie. your injectors are too ginormous.

..if that made any sense at all. :)
Bingo! Now you've got it. IDC as read by an accurate datalogger (they're all off by a bit AFAIK) -- or better yet, a separate circuit like a fancy digital oscilloscope -- is a very good indicator of where you are on the stock ECU maps. Just make sure your highest IDC reads 80-85% at the most extreme settings you will be driving around on (max boost you intend to run on a cold day, etc), and you will be doing as well as you can for whatever injectors you have. Obviously optimizing IDC is secondary to ensuring the A/Fs you want for your tune.


Bob The Great said:
Here's a thought though. I know it would be a very crude way to to it, but let's say that I keep the stock TT FPR, which maintains a base pressure of 43psi, and I find that the lowest IDC I can get without knock is somewhere around 85% (too high). If I were to then switch to the N/A FPR, which has a base pressure of 47psi, I would then be able to lean out a little more to drive the IDC down without going lean enough to knock. Sound logical? However, if I can get 80% IDC and 0 knock on a TT FPR, there would be no reason to up the fuel pressure to drive IDC even further down, correct?
Correct. You could get about 15cc more per injector if that is the pressure difference between the NA and TT regulators. Since you've made no mention of other injectors I assume you're talking about stock 360cc/min ones.
 
#71 ·
Multiades said:
Correct. You could get about 15cc more per injector if that is the pressure difference between the NA and TT regulators. Since you've made no mention of other injectors I assume you're talking about stock 360cc/min ones.
450cc's actually, but the same principle applies.

I was under the impression that A/F ratio was secondary to most everything else. The general rule I've always heard is that for the most power, you want to run as lean as you can get away with without running into knock or dangerously high EGT's. And now, with IDC added to the mix, That's one more thing that airflow correction would be dependant on.
 
#72 ·
HellBringer said:
I went from 550's to 720's and didn't look back. I didn't like the way the car performed above 20psi with the 550's. There wasn't enough timing advance once the IDC's started climbing. I was getting little gains going from 18psi - 22psi; just higher EGT's. When I swapped out the 550's for 720's, even on a semi-rough tune, the car felt completely different. I regularly run 16-17psi on pump gas and have never had any issues with timing-induced knock. I also never had any driveability issues - my car starts, idles and drives damn near stock. In fact, for some odd reason, I've found my 720's easier to tune than the 550's (both Denso).

I completely agree with the core subject of the initial post. However, there is merit to intentionally overkilling on injectors to boost the timing a little. Although, I wouldn't recommend it for those with knock-prone stock heads.
With this injector/FP/IDC issue in mind, I've wondered if there is a problem range of boost when running 720s but still tuning at lower boost. If tuning at lower boost and you see knock, then typically you're not going to turn up the boost. But what if turning up the boost would increase IDC thus decrease timing and get rid of the knock. I don't have the balls to just crank it up to 20+ psi and pray for high IDCs/lower timing, so I'm going to take the race gas approach. Increase the octane and see if it helps.

I still don't know if I'm battling timing or battling my heads though. I think my heads are only ported not polished (pics), so I may be getting knock at lower psi's due to the heads--which would royally suck since it was just reassembled. Actually I'm jumping the gun at the moment since I need to burn out all this old fuel (last May) and fuel stabilizer. However, I did experience knock at 15 psi previously so I'm guessing I'll have the same problem thus I'm jumping to race gas.
 
#73 ·
I assume by the pics you have no porting done, but only polishing of the combustion chambers :confused: Usually porting is more oriented to the Intake and Exhaust runners (which are not pictured), and deshrouding (usually only with bigger valves) and polishing on the Combustion Chamber.

Rob

2root4u said:
I think my heads are only ported not polished (pics), so I may be getting knock at lower psi's due to the heads--which would royally suck since it was just reassembled.
 
#74 ·
Bob The Great said:
450cc's actually, but the same principle applies.

I was under the impression that A/F ratio was secondary to most everything else. The general rule I've always heard is that for the most power, you want to run as lean as you can get away with without running into knock or dangerously high EGT's. And now, with IDC added to the mix, That's one more thing that airflow correction would be dependant on.
On 450s expect ~20cc more from each injector when going up from 43 to 47psi.

To some degree, you're right. Timing is more where the power is at, and above all running without knock is important. I personally believe that having reasonable AFs is very important to any good tune, so don't go manipulating AF trying to adjust timing or anything -- you'll get farther paying attention to AF FIRST, then doing what you can about timing.

-Chris
 
#75 ·
Multiades said:
On 450s expect ~20cc more from each injector when going up from 43 to 47psi.

To some degree, you're right. Timing is more where the power is at, and above all running without knock is important. I personally believe that having reasonable AFs is very important to any good tune, so don't go manipulating AF trying to adjust timing or anything -- you'll get farther paying attention to AF FIRST, then doing what you can about timing.

-Chris
then what, in your opinion, is the optimum ratio? and how did you come to that? It just seems counter-intuitive to me, but I guess it may be true. Obviously, knock is most important, along with EGT's below 900C. Those make up the ceiling. After that, timing seems like the most important thing to me. It is what governs peak combustion pressure, force on the piston, and torque on the crankshaft. The F/A ratio that gives you optimum timing without hitting the ceiling should make the most power.

Rob - I'm only going off what the service manual says for the FPR pressures. I would assume it to be correct.

2root4u raises an interesting point though. Is it possible to accurately tune at low boost, where IDC will be very low?
 
#76 · (Edited)
Bob The Great said:
then what, in your opinion, is the optimum ratio? and how did you come to that? It just seems counter-intuitive to me, but I guess it may be true. Obviously, knock is most important, along with EGT's below 900C. Those make up the ceiling. After that, timing seems like the most important thing to me. It is what governs peak combustion pressure, force on the piston, and torque on the crankshaft. The F/A ratio that gives you optimum timing without hitting the ceiling should make the most power.
To some degree, I would say "as lean as possible without knock". I would prefer to have an A/F around 12.5:1 at high load assuming I can run optimum timing without knock there, and I'm prepared to use water/methanol injection to get as close to that as possible. I'd prefer to stay leaner than 12.0:1, but I understand that compromises must sometimes be made.

Fact of the matter is, you can't pick and choose your timing advance with the stock ECU. The best you can do is to get injectors which are sized properly for the maximum amount of power you will be making -- as in NO LESS than 80%-ish duty cycle at the maximum airflow you intend to use.

-Chris
 
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