86ZZ4

car is an 86 Z, with ZZ4 ,19# svo injector,stock 1985 305 prom...was checking WOT o2 readings and was getting around 920mv@ 5000rpm,and Pulse width was 11.1ms.Car runs great ,no problems at WOT etc.At what PW am i going to start having problems?And if it is too high at WOT,only fix is prom change or injector change?Thanks in advance for any help!!!!

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86 z28 zz4 ssm lift bars ,3.73 simesed runners and a traction deficient 13.707@101.02,on a 2.16 60 ft on 295/50/15 junk street rubber ,best MPH run 13.82@102.7,69 AMX,390 4 speed sticky tires and other stuff..12.94@102.3

kdrolt

: so it's a 350.
: so your have SMALL injectors for a 350

: stock ECM so your really have problems; incorrect fuel-air maps compounded by the tiny injectors.

You need 24#/hr injectors IMO, and you need something other than the stock 305 ECM. At a minimum you need one from a 350 car, and even that won't solve the problems. Without knowing ANYTHING else, I'd say you are running lean most everywhere except part-throttle. IF you are lucky, you are using a MAF system and not speed density, because the MAF will at least TRY to supply the fuel to match the airflow.

At 5000 rpm in any 4 stroke engine:

rpm/[(60 secs per minute)*(2 revs per ignition event)]

Using your example:

5000/(60*2) = 41.66 intake event per second

and I only need to consider this for one cylinder of the engine.

so 1/41.66 = the time duration for one event or 0.024 sec which is 24 milliseconds for an intake event to occur at 5000 rpm.

This also represents the limit for the injector to stay open (100% duty cycle). If your injectors are open for roughly half that time, you have a problem.

You 11.1 ms is not even close to 20-24 ms (typical numbers for a L98 or LT1 injector max pulse width) and you do have a 350 (same size at an L98 or LT1). So you have a problem. What is the impedance of a 19#/hr SVO injector? How much does it differ than the stock 305 injector? I think that your injector impedance may not be matched to what the ECM is expecting, and therefore the duration is far too low.

Looking further, let's look at your injectors:

(19 #fuel/hr)*0.90*8/.50 = 273.6 hp

where 0.90=90% and that's the assumed volumetric efficiency of the engine (a high assumption but we'll leave it at that), 8 comes from 8 injectors, and we assume that they are at 100% duty cycle, and the 0.50 is the BSFC (brake specific fuel consumption) in lbs of fuel per (hour*hp).

Using the above numbers, it shows that your engine, if it breathed at 90% VE, and had a decent cam, would make a peak of 274 fwhp using 19#/hr injectors. That also assumes that you are running a fuel pressure consistent with 19#/hr fuel delivery. If you have higher than stock fuel pressure, the 19#/hr injectors will deliver more than 19# of fuel.

But your injector are not opening for their full duration (11 ms instead of around 24 ms) so your effective duty cycle is around 11/24, or only 46% of what it should be. Powerwise, that would mean you are running dangerously lean at large rpms/WOT.

I can't explain why it runs great, with no problems.

The numbers above make no sense (to me, given your engine), and the fact that you said you are using a stock ECM/PROM with a ZZ4 and with only 19#/hr injectors troubles me.

Probably anything above idle.

unless I did something stupid in the above, I don't think you will ever have to worry about a pw that's too large --- I think the problem is that it is waaay tooo low.

I almost decided not to reply to your question because the above is so far out of line with where it should be, that I was afraid I have goofed. ANd I may have because I haven't sanity-checked injector pulse widths in a long time.

But if I am right, you may kill your engine, so I am going to post this anyway. It might help prevent you from learning a very expensive lesson.

The quick band aid is to increase your fuel pressure, so that you don't lean out and blow your engine. But the right fix is injectors and an ECM suited to your 350. I don't think the O2 sensor data is going to help you much because you need a wide band O2 sensor to measure the WOT condition accurately. An air-fuel analyzer on a car chassis dyno would be better for you to help dial in your fuel needs.

A siamesed TPI on a 350 with a ZZ4 cam (same as ZZ3, 208 deg intake, 221 deg exhaust, and something like 0.474 and .510 inches lift) should make 300 fwhp and 350 fw ftlbs, but you don't have enough fuel to support that because your injectors are PROBABLY much too small. In an Fcar, you only need around 240-250 fwhp to dip under 14 sec in the quarter, and since you are doing just that with a 350, then I can see evidence of fuel starvation n your timeslip.

If the ZZ4 was making 300 hp and 350 ftlbs torque, then you'd be running close to a stock LT1 4th gen Fcar, and right behind a LS1 Fcar (i.e. low 13s).

At a minimum, you need 24#/hr injectors and you need a custom ECM. In the meantime, you can increase the fuel pressure to help not killing the engine.

I also advise you to search the archives because your's is not a new problem, and you will prevent getting people angry at you if you do your homework by reading up on this before you ask more.

Other comments on my post are most welcome, because I can't help further with this one, beyond what I've written above. - Ken


[This message has been edited by kdrolt (edited October 27, 2001).]

Grim Reaper

You are currently running at 93% Duty Cycle...and that is not even cconsidering if your AF Ratio is too rich or too lean.

At 6,000 rpm, the engine rotates once every 10 ms. At 5,000 RPM it rotates once every 12 ms. At 11.1 ms @ 5,000 rpm you are at 93% Duty Cycle. You should not exceed 80%.

If your fuel pressure is low say 42 psi, an AFPR could increase it to 50 psi so your Duty Cycle would decrease to the max 80% level, but you also have to adjust the eprom to compensate for the increase fuel or you'll still fire at 11.1 ms @ 5,000 and run too rich.

SMasterson

Quote:
"which is 24 milliseconds for an intake event to occur at 5000 rpm.

This also represents the limit for the injector to stay open (100% duty cycle). If your injectors are open for roughly half that time, you have a problem.

You 11.1 ms is not even close to 20-24 ms
"

But, don't the injectors fire twice per intake event on the L98TPI?

Just trying to understand the thread myself since I"m in the process of PROM tuning now.



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SMasterson

86ZZ4

Thanks Glen,i was thinking I was past the 80% figure just wasnt sure.I also havea a set of 24#'s but it gets back to the stock e-prom thing ,it was kinda blubery at idle,I might put them in and live with it till i can get a new prom burned.
Kdrolt,thanks for the replybut i think you missed on the math.The car is not starving for fuel with a 900mv plus at WOT.The Et's are low due to street tires that just boil for the first 30-40 ft with a feathered 2.16-2.2 60's.At the track i go to Lt1 cars are mid -low 14 cars,Ls1 stock are 13.40-13.50.Must be a really good track by you.Anyways thanks for the replys guys

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86 z28 zz4 ssm lift bars ,3.73 simesed runners and a traction deficient 13.707@101.02,on a 2.16 60 ft on 295/50/15 junk street rubber ,best MPH run 13.82@102.7,69 AMX,390 4 speed sticky tires and other stuff..12.94@102.3

Grim Reaper

Actually, the injectors fire 8 times within an operating cycle. The Injector Pulse Width displayed on a scan tool at a particular rpm/load represents the total time the injector was open during one revolution of the engine. That is why you only have 12ms @ 5,000 rpm @ 100% DC. Or 9.6ms @ 5,000 rpm @ 80% DC.

I should point out that 19# SVO injectors are like 21# @ 42 psi, so at 46 psi you should be just at 80% DC with a slight adjustment in the eprom.

Getting the eprom burning equipment and compensating via fuel pressure and the eprom is far cheaper than changing the injectors - and you STILL have to change the eprom to make it work properly anyways. Get the eprom equipment first - might not have to buy injectors.

[This message has been edited by Glenn91L98GTA (edited October 27, 2001).]

Grim Reaper

Simple, get the prom burning equipment. Modify the eprom initially for the 1989 MAF code, disable the CS injector, and then "tweak" away.

Once you have the equipment, you can get really fanatical; try different spark and fuel curves, to see which performs the best. Optimize for Highway Mode, trying to get the best gas mileage you can; and so forth.

Compared to what I use to have to do in the "good ol' days", its faster, easier, more versatile AND I don't get my hands dirty like I did in the "good ol' days".

kdrolt

Now I know where my math was wrong. I was thinking about a sequential port fuel injection, not a batch mode. (LT1 cars max out near 20 ms for an injector pulse)... so I did goof as it applies to a batch TPI. (Thx Smasterson & Glenn). Make the 24 ms a 12 ms event then, by correcting for the factor of two. 11.1/12 = 92.5 % duty cycle (as Glenn said) so that's not max'd out but it is close.

And my comment re the 19# injectors on a 350 still hold. For it to run usefully, the FP would have to be bumped up a lot... and so the fuel economy at part throttle will be lousy, and you may also be running the injectors in a mode where they won't last as long.

19#/hr injectors are, IMO, too small on a ZZ3/ZZ4 350.... even on a TPI 350. You need larger injectors on a ZZ3/ZZ4 engine like the one you have. And if you aren't running an increased FP, then you are still running lean. But it's your engine, not mine...

And the stock ECM isn't appropriate with stock programming. Short of reprogramming it, you need an increase in the FP. Do you know what FP you have right now? I still wouldn't trust the O2 data because you don't have a wideband sensor. If you really want this ironed out, you need an a:f sensor in the exhaust and that would be best done on a chassis dyno when you can do an acceleration run.

Sorry for the factor-of-two error. I knew something didn't look right. Batch mode was the problem and I forgot that. - Ken

86ZZ4

Kdrolt...... "its your engine ,not mine"?OK......................Fuel pressure is at 53psi. Once again thanks for the info Glen,when you get my age you start to forget stuff LOL 17 years of being a dealer tech,lots of specs start to run together

Grim Reaper

86, I'm 46, so I know what you mean. And yes, some of these "specs" do seem to "blur" into one car from another; even though there can be some drastic differences just from one year to another.

Once you start digging into the eprom, you'd be stunned by all the subtle differences from year to year. One year, a particlular table works this way, and the next year, it works differently again. A good example is just between how the Main Spark Tables between SD and MAF works. On SD, you must add the "base" to get your total spark advance (which really should be called "desired total spark advance" when you consider the "Retard Routines"); and on MAF, the Base is already included.

I look forward to the day the LT-1s 8051 ECM can be incorporated onto our 3rd Gens. That ECM is LIGHT YEARS ahead ANYTHING that ever came in a 3rd Gen. Just have to figure out how to "graph" the spark routines from SD (or MAF) cars on to the 8051, and I will be laughing. The ability to "trim" individual injectors, sequential fuel injection, controlling your "shift points" for your tranny and BOTH MAF and SD...it is the only way to go.

86ZZ4

so true Glen, 39 here.Well i put an 85 prom from a vette in it this morn,we drive it with the tech 1 on it ,10.5 pulse width and 950 mv on o2.Still a litle high,but im kinda comnfused,at WOT car goes open loop and runs off set fuel tables,so why is pulse width still high??

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86 z28 zz4 ssm lift bars ,3.73 simesed runners and a traction deficient 13.707@101.02,on a 2.16 60 ft on 295/50/15 junk street rubber ,best MPH run 13.82@102.7,69 AMX,390 4 speed sticky tires and other stuff..12.94@102.3

kdrolt

19 lb/hr (rated at an assumed 40 psi) but now used with 53 psi:

19 #/hr * sqrt(53/40) = 19 * 1.15 = 22 #/hr

(using, for example, the math shown at http://www.pro-flow.com/mustmath.htm )

Now calculating the potential hp:

(22 #fuel/hr)*0.90*8/.50 = 317 flywheel hp

where I am again assuming a 90% volumetric efficiency and a BFSC of 0.50 lb fuel/(hp hr). This calc is not meant to be perfect, it's only meant as a sanity check to see where you might be.

317 fwhp is, IMO, still too low for a 350 with the ZZ3/ZZ4 cam, even with the TPI as the weak induction link. If the TPI were NOT restrictive (and yours might be ok since you siamesed the runners), then it would be fairly easy to get 350 fwhp. In your case, with the TPI and siamesed runners, and an unknown set of heads (didn't see that specified), you might not get 350 out of it but I do expect that you should get more than 317. So that still suggests that you aren't making the power you should, and that points to fuel given that you are so close to maxing out the 19# injectors. IMO.

You would be better off using a lower fuel pressure and the 24#/hr injectors you said you had. The 19# injectors are, IMO, still too small even with 53 psi in the fuel rails.

I saw yopur comment on the low ET due to the street tires, so that's helpful because you should be able to run quicker even with the combination you have right now. I was using the LT1 and LS1 Fcars as a reference point, because your car should run on-par with (at least) a stock LT1 and close to an LS1 car, based on what the 350 should be doing. You have a ZZ4 engine with ZZ4 cam, plus other mods, so I would expect power greater than the 285 hp that the best LT1 Fcar got, and near equal the 305 to 320 that the LS1 cars got (and are getting). You should be running as quick as those cars, quicker than LT1 and damn close to stock LS1. If you aren't, then you have a tuning issue and it still looks in part like a fuel issue.

Running too lean at high rpm and WOT is a quick way to kill an engine.... that's why I cautioned you that it's 'your engine'(not mine). I wasn't trying to be offensive and I'd prefer to NOT see you burn a piston from running too lean.

FYI, HTH. - Ken

Grim Reaper

Ken, that formula is just a rough guide, you REALLY need to analyze your injector pulse widths at various RPMs to see what if you are actually approaching your max Duty Cycle.

I like the "formula" to do an initial sizing for determining what size injectors I am most likely to need based on my engine combo. After that, I specifically review my Injector Pulse Widths at various RPMS vs what 80% Duty Cycle should be by this formula:

10ms x 80% x 6,000 / RPM

I compare it to my actual Injector Pulse Width. You could find situations where everything is fine EXCEPT for at a specific RPM level.

kdrolt

I agree. I used it to show that 53 psi into 19# injectors probably won't work well enough. I think it's pushing the injector too far, and it really isn't providing enough to a 350. Larger injectors are needed, IMO.

I agree.

To do this really right you'd want to look at both injector PW and the air:fuel ratio for any given rpm during a loaded run. That would expose the VE, and allow you to tweak what the fuel delivery should be for those conditions. A data logging tool storing the injector PW plus the output from a wideband O2 sensor would allow you do do that directly on the track (as opposed to doing it on a dyno, which isn't as good).

One of the GM DIY groups is working on a cheap wideband O2 sensor for the DIYer, that uses relatively cheap Honda sensor parts. I know this because a neighbor is one of the hackers working it. - Ken


[This message has been edited by kdrolt (edited October 30, 2001).]