There has been so much talk about injectors I thought I would just tell you what I have found. I have been playing with Data master and tuner cat over the past week and was concerned about the 36# FMS injectors that where in my car. The reason I was a little concerned about them is I had to set my injector constant to 31.5 to get my BLM's where I wanted them for part throttle tune (Bellow 4000 and not WOT). The car ran perfect and made good numbers on the dyno but I kept thinking there was something wrong with the 36's. So I got a pretty good deal on a set of 42# FMSs and dropped them in with the injector constant of 42 and the BLMs where 150+. I shut the car down right away and went to 36.5 for the constant, this made it much better but still 130-140 range dropped it to 35.5 and that did the trick. I notice no difference in drivability and the car idles smooth and seems to pull the same as before. I did not tune the car originally, as I would not have a clue, so no other tables where touched. I know the car needs to get back to the dyno with a wideband to set the PE and WOT tables for proper WOT A/F but for now I am fairly happy. I will play with the VE tables as soon as the weather clears up then find a wideband ASAP. I feel that a wide range of injectors can be used as long as the car will tune with then and the DC is with in reason. I don't claim to be a tuner and I know there is a lot I probably missed with this experiment but when I get back to the dyno I will post the results.

One last thing I would prefer this not turn into the same old BS as every other injector post over the past two months, feel free to comment and I would like to here everyone’s opinions as long as they are useful and make a point and would request the Mods shut it down if something like that starts. I would love to here from some of the big motor, small injector folks about what they are doing for injector constant or if they are resolving the part throttle tuning in a different manner. I feel there are several injector sizes that will work and it may just come down to preference, I like a DC of under 80% even if it cost me a few HP.

 

You're in luck! I'm in the process of installing an LPE intake on my car and upgrading from 24# injectors to 25# (not a big jump but a different mfg too.) To get my BLM's in the 125-130 range I had to set the constant to 21# with the old injectors. We'll see what constant I have to use with the new Bosch 25#'s. I'll let you know...hopefully tomorrow.

 

That is great news, so i am not loosing my mind with the constant that on the surface seems way to small. Thanks for the insight

 

paul, thanks for sharing your observation

i claim no real knowledge on this subject either, but from a layman's point of view and as a matter of concern for safety, i too like the idea of keeping the DC at 80% or below........and not pushing the daylights out of the injectors.

 

Not sure what sorta system your running or what intake and/or if it needed removed for the install of the injectors.....

BUT to me, it sounds like you have a vacuum leak, and here is why.

If your chip was tuned for smaller injectors, when you put the bigger injectors in the car, it should have gone to a <128 reading, meaning you were rich. If your seeing >128 readings, that means that your lean, so you are correct.

By lowing the injector constant your making up for the air that is causing the "lean" condition that is NOT being MEASURED.

So by lowing the injector constant, you are making the larger injectors look smaller, and making up for the lean condition, again for the unmeasured air, and bringing the BLMs back into line.

Furthermore, I will share with you one of the comments made by a VERY knowledgable guy that has worked with EFI for a very long time and helps develope MANY (Fast and DFI systems), these are his thoughts on duty cycles. Again I am only sharing them for sake of knowledge, if people wish to argue them, go talk in the mirror to yourself, I am just sharing it, we are all big boys and girls and can decide what is right for our applications.

"Jesse, it was good talking to you today. I hope the crank trigger system will eliminate the fluctuating RPM signals you are seeing. I am confident that it will.

With regard to my theories and experience on injector sizing and how it affects performance and consistency...

At higher duty cycles, more of the finely atomized fuel being sprayed out of the injectors remains finely atomized and suspended in the incoming airstream, resulting in a finer, more even and repeatable mixture of air and fuel in the combustion chamber. Reducing the duty cycle allows the fuel less time to mix with the air and more time for the fuel to fall out of suspension, producing a less uniform, less repeatable air/fuel charge to the combustion chamber. This would, of course, have a negative impact on power, efficiency, and consistency.

To illustrate the concept a bit, think about painting something by either spraying the paint out of a gun or dumping it out of a bucket all at once. If the same amount of paint is used either way, which one will result in a more even distribution, and more importantly to a bracket racer dialing in a fuel curve, which result is easier to exactly duplicate every time you do it? Granted, there is more to fueling an engine than painting a wall, but I think a valid point is clear from the analogy.

In your case specifically the change from 30 pounders to 42 pounders isn't drastic enough to have a huge impact on what you are doing, but for us bracket racers, thousandths count. A set of 83s in this application, on the other hand, would almost surely cost you noticeable power and repeatability.

I hope this helps you out. Let me know how your data logs look after switching to a crank trigger. If you find that you need to stick to a distributor, I would look into switching to a belt drive from a chain and keeping cam endplay to less than 0.010". This will help take up much of the slop in the valvetrain that causes the jagged RPM traces you have been recording.

Craig Smith
Accel/DFI R&D Technician "

 

Jesse,
Thanks for the feed back. Just to make things a little more clear when I first installed the injectors with the old tune (31.5 injector constant) it was pig rich. The car is a LT1/4 383, so the intake stayed on, with a comp 306 an AFR 195 heads running 10.7:1 CR. I have 14 pounds of vacuum and checked for leaks a couple months back with propane and had no leaks with the same 14 pounds. Again without raising the whole injector debate I can see where a smaller injector would atomize fuel a little better but I am a very conservative person and like the DC to stay under 80%, not saying its better just my preference. I had to set the injector constant to 31.5 to get the BLMs to 128 with the 36's so the 35.5 for the 42's seems right in line. When I had the car dyno tuned for WOT the constant was set to 33 and it made great power but when I decided to get into part throttle tuning last week I noticed my BLMs where in the 130-140 range so I dropped the constant to 31.5 and they dropped right into place around the 128 mark, this was with the 36's. I know this make all of my WOT tuning pig rich but since the timing tables where set I only had to deal with fuel once I could get the car to a wideband.
Here is the dyno cart with the 36's and the constant set to 33

 

What I do to build up a injector constant base is have the car tuned before and after, when you put in the new injectors you should pick the constant that gets you closest there then tune on top of that..

It only makes since when all you've changed is injector size. I've noticed the injector constant that people thing should be used with SVO injectors is much higher than what the computer actually uses and pulls things back into a decent range.

 

Alvin,
Thanks thats just what I did and how I got into messing with the part throtle stuff in the first place. I dialed in the 36's as close as posible then swaped to the 42's. It was fairly straight forwad and basicly confirmed there was nothing wrong with the 36's. The good thing is i did find the 42's will run just fine and drivability is exactly the same. What you have noticed is the same thing I am seeing. That is what had me a bit baffled with the 36's. Everyone kept telling me the constant should be bigger because I am running 42 PSI -vs- 39 but my motor is not stock and likes a lot more fuel aparently.

 

Your ported MAF ends probally has alot to do with it also. Are your MAF tables modified from stock (they should be really with ported MAF ends)

 

I can relate an experience I just had with regards to a vacuum leak. I have had blm's in the 140-160's (worst numbers at idle, best numbers in cell 15 which is still closed loop but near entering PE). I have been convinced I have had a vacuum leak for two years but my previous mechanic assured me everything was ok, even demonstrated by shooting carb cleaner all over the intake tract to see if the idle jumped, which it did not. He was convinced everything was buttoned up.

Fast forward to this past friday. I told my new mechanic about my perceived vacuum problem. He said he would hook his smoke machine (I believe it is made by snap-on) to one of my vacuum lines and we just waited. It turned out that there was a leak on the throttle body were the name plate was connected. Obviously their is a gasket so he pulled the old out and resealed and the vaccum leak (or at least this one) disappeared. Hooked it to my scanner and the idle blms went from 160 to about 150. I think my issue now is my tune.

The point of this is sometimes you think you don't have a leak by traditional test methods but sometimes doing things a little differently (smoke machine) turns up something else.

 

Yes they are modified. I didn't make the changes MADZ28 did the entire tune via mail order to begin with. I was prety impressed with the drivability of the car and none of his tune has been changed since I firts fired the motor other then WOT (PE tables, adding fuel, and timing) on the dyno. I have no clue of the MAF tables other then they where changed from the original stock BIN. It ws part of the reason I ws going to try speed density mode first then hook up the MAF and see what hapened. If it was way out of wack I planed on reinstalling the stock values and work from there.

 

So you jsut don't know what stock values are? that should be pretty easy to find

 

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No i have the stock values I just didn't want to change Ions tables untill I had a good baseline. I have used him for every mail order tune I have ever got and have always had good luck, all my motors fire and idle darn near perfect. Now with that said if i got thecar perfect in speed density mode then it went out of wack with the MAF hooked up I planed on going back to the stock MAF tables andsee what hapened.

 

Glen,
is the smoke machine a common tool that I should be able to find someone with? I am not above looking for a vacume leak but I just cant seem to find anything to indicate I have one. With that said i do have an aftermarket TB so who knows.

 

I have been thinking about this now, it was late last night and I don'tthink it is a vacuyme problem. If it where I would think the problem would be more prevelent at idle or be bank spacific. Once the throttle is open I am creating a huge vacume leak am I not? The fact that the problem, or is it even a problem, seems to be the BLMs are off with the bigger injector constant acrros the board just seem to me like it wants more fuel.

 

I agree with Alvin. If you have a ported or a MAF that is modified at all, that changes everything. The reason is that the MAF is a calibrated device, and in the PCM there are the MAF calibration tables that match the sensor. If you modify the MAF or replace it with a so called performance aftermarket MAF, who knows what the true calibration is now.

And the makers of these MAF sensors will NOT tell you what their cal curve is, I personally doubt they even know. All they do is use OE components and glue their fancy multi-colored ends on them. The cal is off but it works good enough, in the worse case scenario the cal could be off enough where the PCM pegs its fuel trims to its limits, then the PCM sets either a rich or lean code. The customer gets P****D off, returns the sensor or exchanges it, end of story. The only people that could give you a cal curve for their sensor was Pro-M but heard the guy died and the business was shut down.

For a given amount of flow across the MAF the sensor outputs a given frequency signal, the higher the flow the higher the frequency. Now modify the MAF by gutting it, or changing the ends, now you have changed the flow characturistics. Now for the same air flow, you have a different frequency signal output which doesn't match the calibration table within the PCM. This is the cause of skewed fuel trims.

I beleive this is where your problem is from. You can change the MAF tables as well to get the fuel trims back in line. I once tuned a 95 LTx engine in a F body that the guy installed a LS1 ZO6 MAF sensor. Let me tell you that set up was a mess. It took me hours to remap the MAF curve. And in the end there really wasn't any appreciable gains with the larger ZO6 MAF. The reason being was that the original MAF flowed enough for what he had, which was a stock bottom end, slightly ported heads, CC306 cam and the usual bolt ons. Had the engine required more flow than the stock MAF could deliver then the larger MAF would have made more power. I have found that in 95% of all street driven cars with modified engine, and this includes strokers, the stock UNMODIFED MAF is adequate.

 

Tom,
Thanks for your input. So if I stick the stock MAF ends back on, thats all I did to change it in the first place, I should be able to place the stock MAF tables back in and be done with it, does this sound correct? I fully agree about not having a clue as to what the new tables should be and that is a big reason I didn't touch them after they where changed by another tuner with a lot more experience then I have. My only qestion is do you feel there is any HP with the MAF sensor with the open ends? If not I will reinstall the stock ends and replace the tables with the stock values and start from there. Also I hate to bug you with more questions but could a vacume leak cause the problems i am seeing? BTW all my Mods are in my sig

 

Update,
I stuck the stock MAF sensor back in with the stock housing and screen with the stock MAF tables and it did get a little richer but not much. I now have the 36s back in with the stock FPR and the injector constant is at 32. The BLMs are right in line 118-132 range acroos the board so i am going to call that goos for now.

 

I personally doubt that the open ends would give you any gains. I run a stock MAF in my car, and many that I have tuned also run stock sensors. To me its doubtful to have that large of a leak to cause the fuel trims to skew as much as they do, at least not without affecting something else as well.

 

Tom, Thanks agian for all the help, this place is great. Ok I wil either sell the thunder racing end or stick them back on because they are prety (LOL) but they seem to have little effect either way. I agree with you on the vacume leak just got a little concerned after a few people pointed me in that direction. The reason i am so confused is because of the amount of misinformation out there. The bigest one is the MAF tables. I would think that as long as you have a stock MAF sensor they should not have to be touched, does this sound right? My last question is with the bigger motor have you had to set the Injector constant lower to get more fuel across the board in your tuning or do you add fuel in a diferent manner and do you think i realy have a problem or I am ok with the lower constant and the BLMs in range. Thanks a gain.