I was searching in the archives and saw a thread where you computed pulsewidth based on BPW, INT/BLM, Coolant temp, and VE (there may have been another) for the 7747

I thought that was really neat, dont mean to expose all your secrets but I would really like to know what the math is to get that answer.

Seems like using that along with your intended rpm range of your engine would help you set a better BPW than the calculation ive seen in the hack

 

Is this what you're after? (just copied from customefis.com):

BPW = BPC * MAP * T * A/F * VE * BVC * BLM * DFCO * DE * CLT * TBM

Where

BPW - Base Pulse Width

BPC - Base Pulse Constant

MAP - Manifold Absolute Pressure

T - Temperature

A/F - Air Fuel Ratio

VE - Volumetric Efficiency

BVC - Battery Voltage Correction

BLM - Block Learn

DFCO - Decel Fuel Cutoff

DE - Decel Enleanment

CLT - Closed Loop

TBM - Turbo Boost Multiplier

Here's the link: http://www.customefis.com/GMEFI.html

 

I did the same calculation the ECM code does except in a DOS C program. Duplicated the lookup tables for inverse AFR and inverse temperature along with a 2d lookup with interpolation. I did it this way in order to duplicate a close as possible the same calculated PW the ECM would provide.

Even the factor routine is done in sixteen bit integer math. Here is a sample run where at first the input parameters are obtained followed by the output of those parameters and the PW.

RBob.

 

WOW, This thread is great. Is there an analogous calculation breakdown for 6e? And would it be possible to add a term to this equation that comes from an addition to the source code???

A little while ago I was thinking about putting a creative nitrous system on my tpi baseplate that injected dry into the runners from the cold start passages. Adding more fuel from another multiplier or adding to an existing one would be a nice way to add the fuel for this system.

 

The PW calculation in the $6E code is even more straightforward. They take the airflow and multiply it by the inverse RPM. This converts a steady air flow rate to a pulsed rate by frequency. The commanded AFR is factored in followed by the injector flow constant. Add/subtract the blm/int/proportional terms and out pops a PW.

Add another term for N2O compensation and you are done.

RBob.

 

SWEET!!!!

Ok now all I have to do is learn how to write a patch for the source code (haha!). I imagine I will have to read many more posts and as alot more questions before I can write source code but baby steps..... baby steps.

 

this is interesting information, I wish I knew this before. Just with the examples you've cited youve given me just about the information I was ultimately seeking.


Wouldn't it make sense to set up your BPW on just about any mild to moderate SBC to down around mid 70s at the most simply to ensure that you could not even if you tried making your injectors static?

It would seem to me that everything would line up better if you adjusted your BPW so that you could concievably hit 100% ve in the rpm cell where your engine reaches its peak ve without making the injectors static taking into consideration how much further those kind of ve numbers extend in the rpm range and if the pw amount of time and the actual window allotted intersect.

Lets say we put our engine figures into one of the better engine analyzers out thre and come up with VE numbers.

We get 95% ve at peak torque and that happens to be 4500 rpm. Couldnt you work out how much fuel would be required for one stroke, then determine the amount of fuel injector/pressure needed to deliver that fuel in a pulse width window being 85% less than the actual time available then subtract another 5% of fuel so that if you input 95% in the table you are actually using 95% of the time available for that particular rpm cell?

I realize that the ve to rpm window correlation cannot be made since eventually you can be at an rpm where 0 fuel is delivered because the time is so short.

Seems to me like maybe alot of tbi tuners should probably be going with a BPW in a short range dependent on where their peak torque occurs in the rpm range. Somewhere in the 70s, no?

I say this merely because you can sometimes be adding fuel and not even realize you are making your injectors static in the process since the tables will allow you to do so.

What do you think about this?

Also, rbob, did you write that program? is it available on the internet for public consumption? if not, that is cool.

 

The majority of your thoughts are correct and in the right direction (IMHO). When it comes to fueling I like to start with how much fuel is required for the level of expected HP.

As you surmised the peak RPM also plays a part. With a lessor motor with less fuel requirements the injectors do not need to be so large. As the engines peak torque will be at a lower RPM smaller injectors can also be used.

Another item to consider is the intake air temperature. With a cold air inducted engine the HP goes up. Stock type engines can retain a stovepipe heater for driveability in the winter.

As a side note the biggest problem I have had is the true flow rate of different injectors. When starting with supposed 90#'ers which are not, then it becomes difficult to determine the required PW.

Anyway, here are two samples where the intake temperature and the injector bias has been changed (perf vs stock):

There is a big difference in DC% here. The first set is a cold air intake with big injectors and high fuel pressure . The high fuel pressure is why the larger inj bias term.

The second set has a stock style pre-heat on the aircleaner and a smaller inj bias term. Note the difference in DC%.

The low perf version has a high VE% for 6000 RPM (even high for a moderate performance engine at 6000 RPM).

I can elaborate a little more later. Of course questions are welcome.

RBob.

 

cool deal, so why do you suppose no one ever hears of factoring the RPM range into things when selecting injector size?

Seems like a 200 hp engine could be short on injector even, for instance if it was a 4 cyl setup that made peak power at very high rpm. There oughta be a way to figure that in.
I suppose youd have to determine how much fuel is required for one cyl through the induction stoke at peak power and how much fuel the injector flows in msec relative to injector size or pressure.

A few more q's

about the 90lbrs, you say they arent 90, what have you typically found them to be, above 90 or below?

the IAT number you mention, I imagine that applies to the 746 mostly correct? I am aware that the 7747 makes an assumption of IAT based on cts right?

and finally, how do you determine the injector bias? I remember some talk of it wayyyyy back when. Do you basically just have to have some kind of injector test bench to figure out what kind of effect the injectors get from raised pressure?

 

I've been tracing the AE and crank fuel in the ANHT hac lately.

Need to put AE in there too.
Depending on set up, you can drive the DC up then too. That's one of the reasons Grumpy says to make sure to compare injector sizing to a stock motor. 2x power = 2x injector, or close.

I did a sizing calc on a stock L98 and my engine. I got a 4 lbs larger injector than the rated power required for the L98. So I made sure I got injectors about 4 lbs bigger for my HP. Calc link
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Also, Grumpy said that AUJP goes static around the L98 torque peak IIRC.

 

I thought I was buying 90's when in fact I wasn't, wrong number. The DarkRed/DarkBlue injectors are an 80.5lb/hr flow. Supposedly there are 90's out there, just don't know the app/number for them.

On the TBI ECMS ('8746) the IAT doesn't buy a lot. From the factory TBI engines are setup with preheat (stovepipe) and a fully water jacketed manifold. This keeps the intake temperatures stable (and high). Even the '8746 IAT fueling compensation starts at something like 48 degrees C. It usually adds fuel as the IAT goes up.

The code I use has a true IAT and CTS blend function that allows the use of a cold air intake system. Using that result for the inverse temperature PW calculation term keeps the AFR in line.

For the bias testing I used the TBI as installed on a vehicle. This provided a fuel delivery system. I then used a function generator to provide pulses to operate the injectors. At other times I used an adjustable power supply and also PROMS burned to pulse the injectors at various PW's.

As the pressure fuel increases or the injector voltage decreases the amount of time to open the injector increases.

I think the reason we don't hear much about high RPM and injectors is that not too many folks here are running EFI on high reving small blocks. I am involved with tuning such an engine now and it has been a topic of discussion on various occasions.

The problem with high RPM is the injector opening/closing time. It eats into the available time. Port and PnH's help in this regard.

RBob.

 

may I ask what were your findings concerning the opening/closing time of tbi injectors relative to pressure was?

I just purchased the same set of injectors you have (darkred/darkblue) cant wait to install them and see how she runs