Mitsubishi 3000GT & Dodge Stealth Forum banner

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.
 
See less See more
#27 ·
Great post Trevor. I love seeing well written posts on this forum. It's a refreshing deviation from the norm.

I'll try to keep this short.

I chose injectors I expected to be over kill: Denso 660cc's. Using my AFPR, I set the pressure @ idle to about 49 psi. I knew this would increase my timing at all times because of the low IDC's, but that was fine with me because I'd done a lot of testing on the street and at the track with lower ignition timing maps.

At boost levels of 27 psi or less, my timing map (while under boost) was more or less 0 to 3* more advanced than stock. When I ran 31 psi with this map, I had knock counts in the mid 20's and ended up needing to make changes...

The following map was created in 2 minutes in the staging lanes while being rushed. It's far from perfect, but it kept knock counts under 10 on the highway. Sadly, on the pass following these changes, 2nd gear wasn't willing to cooperate.



On pump gas, the 660cc's were great! I never had to pull timing to run the boost / make the pump gas power I wanted.

On that note, I'm installing "780cc" (740cc) injectors to go along with my new set up and I fully intend to turn the base fuel pressure down.
 
#28 · (Edited)
Trevor said:
Yes...it reads high. How high I don't know. I'm sure all of us have heard people on the board seeing 100% (and I believe even more if I remember right) IDC's on their datalogger yet they still have a decent air/fuel ratio & have yet to hit fuel cut.
Here is a sample of one of my MirageCorp Hybrid logs with 9Bs limited to 1.00 kg/cm^2 (pressure was less at this RPM, of course) and stock fuel:

O2v Front = .90 V
O2v Rear = .92 V
Knock Count = 0
RPMs = 6844
TPS = 96%
IDC = 122%
Timing = 22 Deg


Here is an extract from later in the same log:

"Time (Seconds)","O2v Front (Volts)","O2v Rear (Volts)","Knock Count","Rpms","Tps (%)","Duty Cycle (%)","Timing (Deg)"
"64.469",".88",".90","0","3906","96","80","4"
"64.694",".88",".90","0","4125","96","89","5"
"64.959",".88",".90","0","4375","96","96","10"
"65.194",".88",".90","0","4625","96","102","12"
"65.480",".88",".90","0","4906","96","108","16"
"65.735",".88",".90","0","5188","96","113","17"
"66.000",".88",".90","0","5344","96","116","16"
"66.276",".90",".90","0","5563","96","118","17"
"66.602",".88",".90","0","5813","96","119","18"
"66.898",".88",".90","0","6156","96","122","19"
"67.847",".88",".90","0","6781","96","121","22"
"68.204",".88",".90","0","7000","96","122","23"


Yesterday I installed IPO's bolt-on FPR kit (instructions claimed the stock vacuum line would reach but it's too short, so it remains untested) and will soon install a hotwired Supra pump (I have the parts but want to test the AFPR first). Not sure I will notice any difference before I invest in a fuel controller, and possibly injectors...
 
#29 · (Edited)
So let me see if I follow and/or my own hair-brained theories are insane or not :)

I was thinking about this injector size vs. timing thing a while ago, and had come to the conclusion that running injectors that I thought would be on the small side and cranking base pressure would give me the capabilities of larger injectors without having to make excessive corrections, thus advancing timing.

So if I understand you, by cranking pressure and lowering IDC I would be in effect advancing timing just as I would if I were to use larger injectors in the first place? What I'm confused about, not being too familiar with the inner workings of any electronics, is that I was under the impression that the ECU calculated load and thus timing advance based on the MAF signal. Is this not the case? Or have I just totally misunderstood you?

I have since stopped wondering about this in regard to my own car since my overclocked ECU will do a lot to counteract timing advance from the 550's (and later 660's, going for 600AWHP), though not perfectly. Of course an AEM EMS is in the plans, I just lack the funds for now. But now you've got me all curious again :)
 
#263 ·
So if I understand you,
1) by cranking (fuel) pressure and lowering IDC I would be in effect advancing timing just as I would if I were to use larger injectors in the first place?
2) What I'm confused about, not being too familiar with the inner workings of any electronics, is that I was under the impression that the ECU calculated load and thus timing advance based on the MAF signal. Is this not the case?
doctorstupid had some good questions that I didn't see answered:

1) Reading Trevor's first post, I believe he's trying to suggest that using over-sized injectors (than your setup really requires), along with a SAFC (piggyback) to modify the MAF reading, will cause our ECU's to increase timing so much that you are losing power. So the lesson learned is to go small as possible on the injector and increase the fuel pressure to keep the ECU from overcompensating the timing advance.
2) I didn't see a response, in this thread, to how our ECUs' calculate LOAD.

The most surprising thing I read in this thread are that the (old-school?) dataloggers are not calculating IDC correctly and overstating the IDC. Hopefully my LCDBC is calculating it correctly, I will have to check with BlackStealth.
 
#30 · (Edited)
Trevor- Thanks for a really enlightening post for those like myself here with barely a modicum of understanding. I have at times wondered why some of the setups listed for cars showed monstrous injectors, and now I understand why, sort of a literal and figurative "pissing" contest. I am running stock turbos/injectors with intake/full exhaust mods/ and a Greddy boost controller soon to be hooked up. I would like to have 15G's and have a friend that has offered to do some head work at a reasonable price. I would like to at some point make 500-550 crank HP. What would you suggest and is the MAP Ecu IPO is selling a good option, or is it better to avoid piggy back options and use something like an AEM Ecu? Or do I have no F'in idea what I am talking about?LOL

Jeff
 
#31 ·
It's the EGO thing that you mentioned that makes people get injectors they won't come close to needing. "I got 720s" and such when they wouldn't tap out 550s.

I myself have been trying to steer customers away from 780s and 880s. It's starting to sink in. Back 4+ years ago 550s were the In thing and 90% of sales were 550s. Then over the years it has risen to the 720-880 area we are currently in.

Either people don't know what the HP rating per CC inj is or they plain and simple want to be able to say I have XXXcc injector no matter how negatively it effects thier power.

I would venture to say 90% of the people here don't need anything larger than 550s, and MANY need much less.

Good write up Trevor.
 
#32 · (Edited)
Crap, the first paragraphs (Thanks Trevor) make me think that going with 510cc to support my 13G upgrade wasn't too bright. However, right now with my 9bs I'm only seeing 86% avg IDC. Guess I'll have to do some WOT runs and log the results-- I'm just learning to correlate the different log functions with the engine characteristics they can help diagnose. Posts like this really help.

Maybe thats more incentive to get the 13s--> 15g'd per the post in Parts for Sale.
 
#33 ·
Agreed wholeheartedly, people keep picking such huge injectors they will never need. I got a 117 trap outa 450s, and intend to run either 450s or 510s with an afpr on my next ride. Id much rather have as small injectors as I could get by with to avoid driveability issues from deadtime adjustment problems with bigass injectors.
 
#35 ·
Thanx Trevor...i appreciate the info and help. as far as the other little things i plan on doing...fuel pump, metal tubing, better ignition, and intake. i found a shop that specializes in heads, and he is in dire need of security windows in his shop. we have worked out a fair swap, so soon i will have a nice set of heads. my goal on hp is 500awhp. so from what ive gathered from you, i think i will go with 550's. :D
 
#36 ·
Nice write-up Trevor! I would say that accurately conveys injectors vs. timing as I see it.

Picking injectors for me was a tough job. I feel like my power needs sit right on the borderline between 550s (Lucas) and 645s (your/my PTE/Delphi/Rochester 65lb-hr injectors). Scaling the 550s up to 600 would be very demanding for the fuel pump(s) at nearly 52psi base, while scaling down 645s with a ~6psi decrease in base fuel pressure might affect atomization too much.

I ended up choosing the 645s -- if they prove too large, I'll have to go smaller.

-Chris
 
#37 ·
Chris, they won't be. I really think ~660's are the perfect size injector...
 
#39 ·
I'm running base fuel pressure at 43psi on 720cc injectors with the base timing at 5 degrees.. on pump gas that gets me ~65% IDC @ 11.7:1 and I have to pull as much as 14 degrees by 5500rpm on with the e-manage

on c16 race gas I can add that 14 degrees back by setting the timing map back to 0's and get ~75% IDC @ 12.5:1. any more timing in either map and it'll set off knock. My theory has always been add as much timing as possible once the fuel is at a safe level.

so yes 720cc is overkill for pump gas but at the track it suits me well and I have room to grow when I'm ready for larger turbos. I don't regret the choice of injectors. I'd rather have more than enough then not enough and have to buy twice.

I have a fuel/timing map for pump gas and one for race gas, so it's pie to interchange.
 
#40 ·
Great post Trevor. FWIW, 90% of the reason why I got an AFPR is for timing control (increase fuel base pressure, reduce airflow single coming from ARC-II). With my plans to run W/M, I hope to be able to advance my timing at the same boost levels to get more power.
 
#41 ·
AdamVR4 said:
Chris, they won't be. I really think ~660's are the perfect size injector...
I'm hopin', but keep in mind that I want comparatively lean A/Fs most of the time (not much below 12.0:1) and plan on using water/alky at some point. I guess we'll see.
 
  • Like
Reactions: LaughnJar
#42 ·
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.
 
#43 ·
Multiades said:
I'm hopin', but keep in mind that I want comparatively lean A/Fs most of the time (not much below 12.0:1) and plan on using water/alky at some point. I guess we'll see.

I ran lean as hell all the time. I tuned for 11.7:1 on the dyno and it NEVER got richer than that ever again. :)
 
#44 ·
HellBringer said:
Although, I wouldn't recommend it for those with knock-prone stock heads.
Or, more importantly, worn, original valve stem seals. ;)
 
#45 ·
Igottabumpit.
 
#46 ·
SMTCapeCod said:
Crap, the first paragraphs (Thanks Trevor) make me think that going with 510cc to support my 13G upgrade wasn't too bright. However, right now with my 9bs I'm only seeing 86% avg IDC. Guess I'll have to do some WOT runs and log the results-- I'm just learning to correlate the different log functions with the engine characteristics they can help diagnose. Posts like this really help.
I'm currently running 510cc injectors with 13G's. I definately think going any bigger, like up to 550cc would've been stupid. As it is, I've never seen my IDC's go higher than around high 80's @ 15psi, and thats when I'm really punching it. I guess I probably could've gotten away with 450cc injectors, but I don't see it making much difference at this point.

I definately have no complaints performance-wise though. My car feels great with that setup.
 
#47 ·
bjmsam said:
O2v Front = .90 V
O2v Rear = .92 V
Knock Count = 0
RPMs = 6844
TPS = 96%
IDC = 122%
Timing = 22 Deg
450cc injectors would seem barely adequate for my 9Bs:
122% IDC * (360cc / 450cc) = 98% IDC
122% IDC * (360cc / 550cc) = 80% IDC

What am I doing wrong, here?

 
#49 ·
Trevor said:
Approximate ranges are as follows:

450's with stock FPR: 400 - 499HP
450's with adjustable FPR: 381 - 510HP
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 dynoed at 359 AWHP on pump gas with the engine knocking a little and killing the top of the pull. Calculate that out with a static DT loss of 85 HP (while not absolutely correct, I feel a static value based off of stock DT loss is more accurate than claiming 30 - 33% loss) and my car was making ~444 HP at the crank. Running in that tune pulled 109 MPH traps. Putting in race fuel and tuning it better put me to 114.75 MPH traps at the same boost level. My IDCs got down to the mid 80% range and timing peaked at 30*. In my opinion, the 450cc injectors are the perfect match for my setup. Matter of fact, there is still room for me to lean it out more, make more power, while still keeping the timing and knock in check. I'm yet to go below .92v on the logger @ 1.2 KG/CM². I should be able to get it to .90v without even thinking of knock running C12. If that doesn't get me to 116 MPH... well, so is life. ;)

I was considering upgrading to 660cc injectors soon in preperation for my next turbo upgrade (which is going to be a little less TD04 this time ;)), but I had already talked myself out of it based on the results I was seeing from the 450cc injectors and C12. If I put in anything larger than a 550cc injector the SAFC I have will not be able to lean the fuel down on C12, and even if it could get the car down to .92v, timing would go well over 30*, and that would not be good either.

550cc is the largest injector I would even consider for a DR500 / 13G car. Anything more than that is just a waste, and could lead to more harm than good.

I plan on dynoing my car again on C12 and seeing where she ends up this time. I'm will to bet I'll break the 400 AWHP mark. Add in the static DT loss, and I'll be really close to 500 HP at the crank. And since I have a stock FPR, that is right in line with your calcs.

Great post. I'm going to have fun helping to prove you right. :D
 
#50 ·
J-Ville VR-4 said:
You are using the datalogger IDC count. Its not accurate; it shows a higher IDC than you really have. Its physically impossible to have above 100%IDC.
Right. So how do folks tune for 80.2% IDC? Can the values my logger reports be simply scaled by 100/122 = 82%? Perhaps Shiver can add that factor to the next release. ;)

Would timing remain within spec if I upgraded from 9B to 13G (250 to 324 practical max CFM = 29.6%) and from 360cc to 450cc (25% + some fuel pressure)? All I'd need is a SAFC-2?
 
#51 · (Edited)
bjmsam said:
Right. So how do folks tune for 80.2% IDC? Can the values my logger reports be simply scaled by 100/122 = 82%? Perhaps Shiver can add that factor to the next release. ;)

Would timing remain within spec if I upgraded from 9B to 13G (250 to 324 practical max CFM = 29.6%) and from 360cc to 450cc (25% + some fuel pressure)? All I'd need is a SAFC-2?
"Within spec"? Timing will be higher, but not unreasonable with that injector upgrade and the necessary correction. An S-AFC can easily take care of that.

-Chris
 
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top