What would make the BLMs be different for the right side vs the left side?

I was dyno tuning my car last night and I noticed, or at least I think I noticed that my left side BLM's were higher than the right side. Of course under WOT they went to 128 but for part throttle on the dyno and for the trip home the BLM's were higher, like 150 left, 108 right. Now my car is a 92 so it doesn't have the crossover on the front of the fuel rail, perhaps this is a pressure differential thing? Maybe one of the O2's isn't working like it should?

Any ideas would be appreciated. The car seems to drive just fine and makes great power on the dyno, the data log just caught my eye. It was way too late by the time I got home last night to really investigate, I was just curious if any of you had encountered this before.

Other possibly important information. When I last dyno'd I was running a set of 24# per hour SVO injectors. I have since changed to a set of 36# injectors to lower my pulse width somewhat. At first all I had done was to simply change the injector constant in the computer, that made the car run. I was tuning last night (and early this morning) mainly to get the A/F ratio nailed back down.

For starters I had programmed the injector constant to 38, because these are SVO injectors. The first dyno run had the A/F pretty lean, it fluctuated around 14:1. I knocked the injector constant back down to 36 and it ran closer to 13:1 but not quite there. I then started tinkering with the power enrichment vs RPM tables to straighten the A/F curve. The end result was a very straight A/F curve hanging right at 13:1 from about 3000-6500 RPM's.

Hmm... my lunch ride is here, to be continued.


[Modified by Nathan Plemons, 12:01 PM 7/3/2003]

 

Nathan, you IMed me and asked for my opinion. I will post it in here as I HATE This IM system LOL

First off. There are several items that could cause this. The first is the 02 sensors you mentioned. You have headers, are they heated O2s. If not get some. What can you post the logs data here. Mainly the voltage from the o2s. I can usually tell if they are lazy from that.

Seond is the FP you are running at. I have been harping on FP regualtor and them doing their job correctly. They have to be able to keep a CONSTANT pressure on the injectors or you will have problems. The basics are you need to get a vac reading from your car at idle. Typically each

20" of vac =10psi

So at ~15" vac you should see when you rehook up your vac hose to the AFPR a 7.5 psi drop in pressure. So your base pressure is 45psi with vac hose off, and when reconnected to the manifold it should read 37.5psi. If you don't get the correct pressure drop them your regulator can't regulate properly. Raise the pressure a few PSI and restart the testing over till you get the correct pressure drop.

3rd. You should never WOT test then go back to check part throttle tune. :nono:

You need to get your part throttle tune as close to 128/128 as possible. Then only go to the dyno and adjust the PE AFR vs RPM for your final commanded AFR.

Otherwise you will be chasing ghosts.

You will find, and I remember us debating larger injectors in the past. Why I suggested not that large of injectors. The reason is they, because of the Pw vs Fuel flow characteristics are a nightmare to tune. You do have an advantage with the SD and the VE tables, but with that large of injectors, you will have your work cut out for you.

I would start with a base bin from scratch and put the injector constant to 36, what your injectors are. 43psi, so long as your FP does as I describe above, and then start tuning the VE tables to get 128 BLM. Once that is done, then you have to do WOT. You will find the WOT tuning much more responsive once your close to 128 across all cells.

But lets first find why your not getting accurate BLM readings side to side.

:cheers:

 

I can post the data logs but I don't have the info in front of me right now, will probably be sometime tomorrow before I can post them.

Let me explain a few things about my actions though. I initially went out there to do some driveability tuning and such but it's a dynojet so it doesn't lend itself to a whole lot more than WOT. In any case after I made my baseline pull and saw that the AF ratio was leaner than normal I decided to try the WOT thing to try and get a feel for it. At least now I know what I'm doing should I need to go back and start all over. Honestly I was pretty much there last night to play around with my software and get an idea what I needed to do. The dyno didn't cost me anything so what would it hurt? If all else fails I've still got the chip I drove in on :)

With that being said I will be back at some point in the future after I get my part throttle BLM's and such resolved. I have no doubt that I will need to change the WOT tune when I'm done but at least now I know how to go about doing that.

So to answer some of your questions:

The O2 sensors are fairly new and appeared to be switching properly on the last data log I remember. I'll pull up my drive home that was showing the split BLM's and look at the O2 behavior.

My fuel pressure regulator is stock but it appears to be working perfectly. I've got an autometer fuel pressure gauge in my pod and the fuel pressure has always behaved exactly like you would expect. During WOT dyno runs it would reach it's maximum pressure and hold it very consistently, there was no fluctuation at all. Max fuel pressure was always the same for every run so I have no reason to believe it wouldn't be working correctly.

Like you say there are some things that could be done differently and I will certainly do differently. Right now I can't afford to spend any money on the car so if I want to continue to experiment I'm gonna have to do it in the tune since that doesn't cost me anything. The split BLMs just caught my eye and I thought they should be resolved, from there I'll do a lot of data logging and tweaking on the street. Then eventually I'll head back to the dyno.

As a side note, although my BLM's might not be perfect at 128 yet the driveability of the car is very good and I was able to tune myself back about 20 horses last night so it wasn't a complete waste.

Again thanks for the info and I'm sure I'll have many more discussions with you in the future.

BTW, the injectors probably are still a little big. I honestly think 30's are what I need, I just didn't feel safe with the 24's. I'm trying to work out a deal to get a set. I just got a great deal on a set of 42's and I find it easier to trade down than to trade up. Essentially I'm going to end up swapping my 42's for some 30's but the price is still very reasonable. I may have to retune it a few times but I hope to learn a lot in the process.


[Modified by Nathan Plemons, 1:42 PM 7/3/2003]

 

Just a side note on injectors...

The Ford (and I believe this applies to their SVO brand) injectors are rated at 39.5psi vs the GM LT1/4 injectors which are rated at 43.5psi. This means your injector constant is larger than 36 if you're running stock fuel pressure.

Besides the injector constant, you'll also need to change the injector offset values. The Ford offsets are different than GM.

Assuming you have heated o2s, significant splits in BLMs during part throttle cruise are usually the result of an exhaust leak that is upstream of the o2 on the "lean" side. Aftermarket throttle bodies can cause a split, but it's usually not very severe and usually only occurs at idle.

 

Yes, all of the above are true. A vacuum leak can also cause split BLM's. It could also be a faulty injector. I hope that you can find your problem, I haven't been able to fix mine. Let us know how it goes!!!

 

I agree with all that has been said. If your split BLMs are at cruise, look for exhaust leaks, or other mechanical factors.

If at idle you should make certain that your IAC counts are in the 20-50 range at idle with the TB blades fully closed. Get all your idle air through the IAC passage. With a built motor this generally requires drilling the IAC passage bypass a bit larger. (See Jim Mason's Grand Sport TB page; do a google search).

And you can't begin to tune a car that is 150 blm on one side and 120 on the other....at least until the mechanical problem is located and fixed.

If the split is at idle, the IAC fix should make it closer, but remember big cams will actually get reversion from thier increased duration and may make it very difficult to bring your blms together at idle. I am in the 120/130 range on my split at idle and have my IAC counts down in the 30-40s. I am still chasing it but probably am getting kinda **** about that little of a difference.

The part throttle tuning before PE tuning order is important for us in the Speed Density world since the 75-100 kPa VE cells are controlling PE as much as the PE tables. In fact I will do my PE tuning in those cells first and than smooth it out with the PE v rpm table. It is a HUGE advantage to have your own wide band. Particularly in the world of speed density tuning.

Perry

 

Alrighty, I've got the data logs up online, now this is a 92 so it uses datamaster DA for anybody who wants to look at them.

The logs can be found here
http://www.wku.edu/~nathan.plemons/datamaster

Scottsville and BG were from some time ago, the other two were from dyno night.

As an interesting side note I've been looking at some older data logs and it seems like my 24 pound injectors didn't have the split. To narrow down the problem I'm gonna go toss my 42's in and see if the split is still there. If the split goes away then I know I've got a bad 36.

 

The split seems to be worst at idle or very low throttle operation.

I put my 42's back in and programmed the injector constant accordingly, here is the log.
http://www.wku.edu/~nathan.plemons/d...r/firstrun.uni

Now the BLM split seems to have gotten better by the end of the log, there doesn't seem to be more than a 6 point split worst case, at times they are equal.

Now when I parked the car and returned to idle the split is still there. My question is though how long term are these values? I didn't reset the computer and I didn't idle very long in closed loop at all, I'm thinking it could just be remembering values from before.

Any thoughts?

 

I would run a few more logs. On my '96 it seems I usually need to take a couple 20 minute drives or varying rpms/loads/speeds before I my fuel trim info settle down. Gotta hit all or most of the cells in the fuel map :)

If the split is worse at idle and you have an aftermarket throttle body, you may consider modding it as suggested above.

Next would be to look for vacuum leaks and exhaust leaks.

If your idle speed is low, increasing it a bit will help .

FWIW, even stock cars aren't dead on. A split of 2-4 pts at idle isn't uncommon.

Lastly, if you're not already, you really need to look at the data using a spreadsheet package like Excel. In Excel you can use Pivot Tables to group and average thousands of data points into something that can be interpreted in seconds. You'll always have spurious data points, so only looking at a log file can be misleading.

 

Big time vacuum leak or you are running a MONDO cam. Which is it?

I have seen this kinda MAP at idle but it was on a 230/236 with better than 0.600 of lift. Also I was able to make it idle at 50 kPa by adding a bunch of static timing at idle.

I run a couple of CC305s in 383s that idle at 40-41 kPa. I run a TR 224/224 with .563 lift that idles at 40 KPa. You are at 60 kPa. Find the vacuum leak or tell us more about the cam.

Perry

 

Also get the idle up to 850 or so.

PK

 

I've played with the idle a little bit before, it was 850, I kicked it down just for fun when we were playing on the dyno. I set it back now.

I'm not running what you would consider a huge cam. It's an LT4 Hot Cam but I've got 1.7 rockers so that kicks the lift up to the .550 range.

218/228 .525/.525 112 with 1.6 rockers

I don't believe I've got any vacuum leaks, I never have before. I haven't checked lately but it's always pulled a very healthy vacuum, I'll double check that tomorrow. The heads flow very well too so that might be helping the cam effect.

 

Trust me you have a vacuum leak. Also the 42 psi injectors are way to big for that cam and heads unless you also have a power adder.

NA, 24s are on the edge but 30's are fine......Now if you have a SC or NOS than the 42s can be OK.

PK

 

Does nobody ever listen to anything? I know that 42's are too big already, I only put them in there to see if the BLM split would go away.

 

A CC305 is very similar to the Hot Cam, and a 383 would tend to pull more vacuum than a 350 would for any given cam. On my very first dyno run I hadn't yet changed the idle speed, it was commanded 850 and MAP was right at 52. That might be a little higher than optimal but with the heads flowing like they do it's isn't hard to get air into this motor.

On an interesting note with that the IAC counts are at 78 for that idle. Seems like the IAC wouldn't have to open as far if it was sucking air from somewhere else. I know I'm soundling like a broken record here but I really don't feel like a vacuum leak is the problem. All my vacuum accessories work fine and fuel pressure does it's thing just fine as well.

Oh, one more thing. I just reviewed a datalog for another friend of mine who is running a Hot Cam, his is about 55 at idle with a commanded IAC of 93!

 

Hey nathan,

I have the same heads and cam as you and my MAP KPa is in 48-50 range and IAC counts are in the 48-53 range at idle with engine temp at 180.

 

You can increase or decrease your IAC counts by adjusting the minimum idle air screw in the throttle body. In many cases, adjusting this will also address the BLM split at idle and *may* (speculation on my part) help the split BLMs during small throttle angle (<5%) cruise conditions.