VE coming out way over 100

[Mastiff]

I'm really bummed. My engine would not tune and had a terrible off-idle hesitation. I discovered fluctuating fuel pressure and air getting into the fuel lines with the in-line pump. Since the tank had to be replaced anyway for another reason, I went to a bone stock in-tank fuel pump setup (copying a 1991 Blazer). This is for a GM TBI 7747. Fired it up today and started tuning. First of all, the hesitation is not gone. But, maybe it just needs a tune. Thing is, the first time out I came back to adjust the VE and I have values as high as 112. And this is with the BPW already bumped up from 135 to 150. The BIN I'm starting with is ARJT, which I adjusted so that the VE2 table is zeroed out - just to make the tuning easier.

One thing is that the 135 BPW seems to come from a nominal fuel pressure of 12 PSI. My fuel pressure is steady at 9-10 PSI. Just working the math, this would seem to want to push the BPW up to about 180, all else equal. The only major mod to my engine is headers, but that might push the efficiency up a little. I don't mind trying the 180, but it makes me wonder how this ever worked in the factory. The VE table doesn't seem to have enough range to soak up any fuel pressure variation at all. I've read that any VE over 100 will just limit to 100, even if it's the combination of VE1 and VE2.

[EagleMark]

I've tuned a lot of 1227747 and if it doesn't have 13 PSI it'll never run right. Yes GM says between 9 and 13 psi and it will run there. But if you want it to run right you need 13 PSI. Injectors are also an issue at this age. Last set I had serviced gained 9 and 13 persent to come out almost a perfect match.

Battery voltage also plays a role in fuel pressure as well as ECM function. I know this is a conversion on your truck so you have an older altenater that usually does not keep voltage in the 13.5 to 14.2 range at idle, I've seen some creep down past the 12 volt mark at idle which causes all sorts of issues, so give that a check and rev engine, watch voltage and see what kind of fuel pressure you get when it's charging. Fuel pump ground can also be an issue with low pressure, ECM grounding to block, block to frame where fuel pump ground is usually located.

I've heard a lot of people 0 out VE2 and add it to VE1 but you need to leave the number in 0 RPM in VE2. That said, I have never done this. I always leave VE2 alone and tune VE1 and I get results in tables that add over 100. Why? I don't know but it works. Although I have been told by the most intellegent EFI person I know that this was a GM mistake. But it works for me?

There is also and issue with BPW over 150... I wish I could remember what it is but it's been so long I forget? I never try to go above that, if I need more I raise fuel pressure.

HTH!

[dave w]

When I have a VE cell of 100 or more, I increase the BPW. I'm in the camp of tuners that will add VE1 and VE2, which is a personal preference.

dave w

[EagleMark]

We talked about this before, what about 1227747 that VE1 and VE2 already add up to over 100 on stock chip?

I agree with you on the 100 Dave! But factory is already beyound that and I have got more out of it. Why?

But his motor is stock and already can't get enough VE but with 9 - 10 PSI I think fuel pressure is his issue...

[PJG1173]

I know when I first started messing with truck my fuel pressure was 9psi. after putting a TBI rebuild kit in with new diapham for the FPR I gained almost 3 psi before I made it adjustable.

[Mastiff]

I think we're just going on comments from thirdgen that VE1+VE2 greater than 100 gets clipped at 100 within the computer. I'm not sure why GM developers would go to the trouble to limit system flexibility like that though, unless it was some sort of safety thing for runaway BLM. Tunerpro definitely cuts off VE at 100 within any one table.

I like the voltage idea. I'm going to check that next. I do seem to recall my voltage dropping down at idle sometimes. This could explain a lot, but doesn't the ECM measure the voltage and compensate for it in the sensors? The main thing would be fluctuating pressure right from the pump at lower voltages I guess, but if the regulator can't get what it wants even at idle that seems weird...

[rokcrawlin85]

just thinking here, how far off are your BLM's with the latest tune you've done to the VE tables?

[Mastiff]

just thinking here, how far off are your BLM's with the latest tune you've done to the VE tables?

Well, I didn't actually adjust them when I saw a bunch of the values saturating at 100%. The BLM's for the first run were 150-160 type range mostly. that was with BPW of 150. If I did my math right, that's about 35% above nominal (128 BLM and 135 BPW). I guess that's not unreasonable since just the fuel pressure alone is 33% (9/12).

[1project2many]

I think we're just going on comments from thirdgen that VE1+VE2 greater than 100 gets clipped at 100 within the computer. I'm not sure why GM developers would go to the trouble to limit system flexibility like that though, unless it was some sort of safety thing for runaway BLM. Tunerpro definitely cuts off VE at 100 within any one table.

Why would they do this? They're not limiting system flexibility. They're engineers and they know VE isn't going to exceed 100% on a stock N/A engine. If you're trying add fuel so you have greater than 100% then you've got the wrong injectors, or wrong fuel pressure, or wrong BPW constant. If you're over 100% then you're *at* 100%. As long as the injector duty cycle isn't exceeding 80% you can change the BPW, scale your tables down, and commence further tuning.

[dave w]

We talked about this before, what about 1227747 that VE1 and VE2 already add up to over 100 on stock chip?

I agree with you on the 100 Dave! But factory is already beyound that and I have got more out of it. Why?

But his motor is stock and already can't get enough VE but with 9 - 10 PSI I think fuel pressure is his issue...

Very seldom do I tune a "Just Stock Engine". I really don't know what is actually happening in the 1227747 ECM when the VE values exceed 100 (with VE1 added with VE2). I base my input on real world tuning experiences, others usually experience similar results when using same tuning practices. A very long time ago, back in the 70's, Hot Rod Magaizine had a tech section called, "It worked for Me". Keeping the VE tables values (with VE1 added with VE2) of a 122774 between 40 ~ 95 has always worked for me.

The basics of tuning must be verified; good sound mechanical condition of the engine, good sensors, good fuel, and good fuel pressure are a must before attempting to tune a chip.

dave w

[rokcrawlin85]

The basics of tuning must be verified; good sound mechanical condition of the engine, good sensors, good fuel, and good fuel pressure are a must before attempting to tune a chip.

dave w

spot on i'd say... make sure your connections are solid and that the voltage is good. If still no change, raise the fuel pressure to 15 psi. if your BLM's are still not dropping, i'd lean toward that your injectors are fully maxed out or they're going bad/faulty. Like project said i'd try to keep your duty cycle around the 80% mark at its max, not by just changing the BPW, if your injectors have no more in them, you can't get anymore out of them.

[Mastiff]

How can you know what the duty cycle is?

[EagleMark]

It's in the data stream on many 8192 baud rate systems, but I don't recall seeing it in any 160 Baud data stream like the 1227747...

[EagleMark]

They're engineers and they know VE isn't going to exceed 100% on a stock N/A engine.

Well they sure wern't paying attention when they did the 1227747 $42 bins...

Unless there is something else in the bin that is calculating this differantly?

The 1228746 $61 has VE1 and VE2 adder and it does not go over 100, high ones stop at 90ish

[RobertISaar]

duty cycle isn't directly reported by any ECM, it's created in tunerpro taking the BPW and RPM variables to make a DC.