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Discussion Starter #21
Phil, I've seen those fuel pressure graphs of yours before and this is mainly where I found the 47psi value from. I'm not sure why Jermaine recommended 58-60psi then. I'll wait for his confirmation. If the Walbro 255 normally outputs ~53psi, then would it even be able to produce 58-60psi at the surge tank regulator?

The AFR chart that I posted is measuring from the tail-pipe since I do not have an extra 3rd bung welded yet. Will do that this week. I'm not 100% sure if those AFR values from the tailpipe are accurate. It was measuring at the left tail-pipe of my stage 2 muffler tips.

The MAF sensor is stock, but piping is not. The intake pipe and filter are provided with the VF2 kit. Diameter is same as stock as far as I can tell and others w/ VF2 do not have my problem it seems. I guess I'm special. I don't wanna be special.

I will look into buying an OBD logger for my iphone4 as well as install the AEM wideband when it arrives this week. Keep the good information coming guys. This is good info for others who may have similar symptoms.
 

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Plug Whisperer
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Not sure what he was thinking either. THe 255 pump can make 100 psi in theory. That's not the limiting factor. The pressure regulator for a stock setup is in the fuel tank and that's what drives the 47 or 53 psi. It's not adjustable and just "is what it is".

You can adjust your aeromotive regulator on the surge tank to whatever you want, but 95% of those that have one, adjust it to 47-50psi to be similar to stock, if not just wee bit richer than a stock fuel supply would be without going outside of the fuel trim limits.

Bottom line, there's no way the flash you have from VF was designed to be ran at anything other than stock FP, which it sounds like you are running...

One place to lose accuracy is the dual tip configuration. You're not likely to get accurate readings in the way that you have the sensor... With a stage 2, you need to mount the sensor where the 2nd 02 sensor is for monitoring...

Now that you mention that tail pipe sniffer and the horsepower numbers you referenced, I'm not longer thinking you're running as lean as you think. Our cars make NO power at 15:1. It would not likely have made 240whp on a dyno jet at true 15:1. The dynojet plat I have a VF kits makes something around 245whp as I recall, so you may be closer than you think... I really think you had a lot of atmosphere "leaking" into the exhaust stream with that sniffer setup. Please the WB02 in the secondary 02 hole for the time being and retest...

My 2 cents,

Phil
 

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Yea your dyno operator should have known better. You will not get a very accurate number when measuring at the tail pipe like that. Even worse if you have a cat (not sure if you still have one or not)

FWIW the dynoshop I work with will either use a secondary bung if one is available, or weld in a new bung right after the headers. No question.
 

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+1 to DSM...

Here's a VF dynojet plot. Does yours look similar? If so, you may not be having issues...


@holmz- He had a fuel starve problem before due to a bad seal at the in-tank pump. Now he has a new fuel system, surge tank, etc--- should have fixed the issue that lunched the first installation. So the setup is different.
 

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Discussion Starter #26
As fast as it still feels, I too doubt it is running as lean as 15:1. I'm staying below 4k rpm until I see safer values (10-12:1) on a correctly installed new AFR. I have a Larini de-cat installed since the rebuild last week. I figured it should be more accurate w/ de-cat but was unsure about the exposed 2nd stage2 tailpipe.

The VF2, I believe, was designed to run off of Walbro 255 and stock regulator at the tank so 50-53psi w/ pressure-regulated surge tank @ idle makes sense right now.

I'll either mount the brand new AFR to the 2nd port or have a 3rd bung added permanently, whichever makes sense for now to get a better reading.

A couple days before the engine blew, it was a tick faster than a few base Lamborghini Gallardos from highway speeds :shift:. And of course, we Loti passed the inexperienced Gallardo drivers when reaching the twisties. :wave:
 

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Scary

There is some interesting data here...

I find it kind of scary:

1. Pump pressure fluctuating as RPM and power is going up! How can you create a tune? The un-linear pressure drop would make it impossible to create a good tune! I am used to FP being either flat or rise with boost, but otherwise rock solid.

2. 250whp on 440's?!? With the same funky FP curve? On whose dyno? I know high numbers make us all feel good :) ... me too. :) 250whp = 300 crank hp/ Plug in the numbers, below and se the size of injector you would need:

Fuel Injector Calculator from WitchHunter Performance

at 300 real crank hp at 60 psi a 440cc injector on a supercharged engine would run on the ragged edge at full power.... Too close for comfort, and then with an unstable and patched up fuel system....

Runnig a Walboro pump at high pressure is a good idea. It works. See www.kinsler.com for pump flow rates. For 300hp, you need about 200lbs/hrto be safe. Little Walboro pumps are quite good compared to expensive Bosch pumps.

So if in-tank Walboro pump is OK, why not switch-out the internal FPR with a good aftermarket unit and use BOE can to house the return line? You should immeditely get good, stable, bost referenced FP.

Anton
 

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...

@holmz- He had a fuel starve problem before due to a bad seal at the in-tank pump. Now he has a new fuel system, surge tank, etc--- should have fixed the issue that lunched the first installation. So the setup is different.

I am not reading it that way...

Thanks all. I did find a torn rubber seal at the outlet of the in-tank Walbro pump when the supercharger was first installed. I replaced the seal, trimmed the pump assembly a little more, then checked the seal days later to make sure it was still intact. This was all before the engine went kaput.



There is some interesting data here...
I find it kind of scary:
...
Anton
All true - but that data you saw was from Phil, so it may not be the same car as the OP, and just a representative example.

But nothing beats some measurements to understand what is happening.


As fast as it still feels, I too doubt it is running as lean as 15:1. I'm staying below 4k rpm until I see safer values (10-12:1) on a correctly installed new AFR. ...

...

A couple days before the engine blew, it was a tick faster than a few base Lamborghini Gallardos from highway speeds :shift:. And of course, we Loti passed the inexperienced Gallardo drivers when reaching the twisties. :wave:
1) I would not be using an ASSelerometer (the seat of the pants method) for quantifying the A:F .

2) I think that there is a correlation between A:F and power. If you had power versus lambda, and EGT versus lambda then you might be able to plot power (torque) versus EGT. The highest power happens around 50 degrees cooler than the highest EGT - assuming you are keeping timing (etc) fixed.
It is not uncommon to run engines on the rich side of peak EGT (Rich Of Peak or ROP - which by the way is at lambda=1). It is also not uncommon to have the A:F richen up after some seconds of WOT to aid in cooling the pistons and valves and to prevent detonation.
The power you make at 15:1 A:F is not a lot lower than at lambda=0.85, which is why seat of the pants does not work... However the detonation is much easier at lambda=1.0 and the engine runs hotter.
 

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I think we've determined that the AFR data is likely wrong based on the placement of the sniffer.
 

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I think we've determined that the AFR data is likely wrong based on the placement of the sniffer.
Many airplane crashes are the result the pilots saying something to effect of; "There is no way that can be correct - I'll bet that gauge must be broken".
 

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Many airplane crashes are the result the pilots saying something to effect of; "There is no way that can be correct - I'll bet that gauge must be broken".
Point is the data is suspect due to the sensor not being installed where it should be. For now it sounds like the op is taking it easy and is going to put a wideband in the correct location along with some obd data which will help correctly diagnose the issue.
 

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It took me a long time to work through the lean condition on my Lotus after the supercharger install. I have a pre cat on board wideband and I would see A/F in the 15's at redline. And this was confirmed by two other wideband sensors/gauges. Power was smooth and strong. The only audible detonation was in high gears at low rpms.

johnnypolex - If you have the wideband gauge you can temporarily hook it up by putting it in the 2nd O2 sensor location. Just wire the stock O2 to the frame so it does not get banged around. It is fine to drive like this, it will eventually through a CEL for a bad cat.

Once you have the wideband hooked up get a sense of the A/F then pull the battery cables and reset the ECU. Take it for another spin and see if the A/F is different. When I did this my A/F would look very good, but over 50-100 miles would work its self lean by 2-3 points.

I checked for air leaks, exhaust leaks, fuel pressure and installed a new MAF. Nothing changed the A/F. I eventually replaced my Cup Air Box and K&N with the stock air box and filter. This helped bring down my A/F to an acceptable level.
 

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ERP makes an interesting observation. When checking OBDII data, look for the short and long term fuel trim numbers. If either are more than a few percent, then I'd be concerned that the open loop scalars may be modified.

I have never had anyone definitively confirm this, but the cars act this way:

During closed loop operation, the computer trims the injectors to achieve stoichiometric. This change is in turn used to set an overall scalar for the injectors. E.G. if the computer is leaning the injectors by 20% in closed loop, it assumes the injecotrs flow that much more all the time. So in OPEN loop (generally WOT), it ALSO leans the injectors 20%.

I feel pretty sure some folks running piggybacks back in the day messed up their engines by adding fuel in clsoed loop via the piggy and the computer took it away via learning, so that when open loop time at WOT came, the car went lean.

A good tune should never put long term fuel trim outside about a -/+ 5% window (smaller the better). Worth a look while you are at it.
 

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well he said his car has a decat, so it really shouldn't matter where the sensor is placed.
This is not true.

Placing a sensor at the exhaust tip exposes it to lots of free air and can screw up the readings. It will be very bad at idle and low load, but even at wot the reading can be skewed.

The correct way to do it is to place the oxygen sensor away from the tip and any free air... A good place is something like a header.
 

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well he said his car has a decat, so it really shouldn't matter where the sensor is placed.

:wallbang:

This is not true.

Placing a sensor at the exhaust tip exposes it to lots of free air and can screw up the readings. It will be very bad at idle and low load, but even at wot the reading can be skewed.

The correct way to do it is to place the oxygen sensor away from the tip and any free air... A good place is something like a header.
+1 and Thank you. :up: Not to forget that the 02 was put into 1 side of a twin tip tail pipe... "can be skewed" is putting it lightly... "just say'n" :D
 

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well measuring A/F on dynojet the sensor is placed in the exhaust tip. This sensor is like a microphone (i'm sure you've seen it) so it goes several inches inside the tip. At WOT not sure (especially a decat, with how fast the gases fly out of the tip) how outside air will mix in.

Dynapack's sensor gets placed inside one of the bungs (for decat).

Having said this, I'd prefer the bung sensor. Though is more work to prepare (if you have the grille on).
 

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I'm just speaking from experience.

I've seen lots of widebands (and narrowbands) act up in the presence of free air... Many times it doesnt even appear to be free air at all.

Case in point about a month ago we were tuning an LS1 car. One bank just constantly showed long term fuel trims about 20% (!) higher than the other. After swapping sensors, then wiring bank to bank it just kept following that particular physical bank. We ended up pulling the cat back and found some pinholes at the top of the collector flange causing a very small exhaust leak... One we couldnt even hear. After welding it up and strapping it all back together, fired it back up and the trims were within 1%...

Have a 2jz 240 I'm helping a guy out with. At idle its reading 16ish AFRs if I add 20% fuel it hardly moves. That's with an open downpipe with the sensor located about 2 FEET from the outlet. I will admit the readings get much more reasonable at a cruise or wot, but the point still exists even 2 feet from the exit and it skews the results.

Having a sensor located at the tip on a twin exhaust 4 cylinder car is just not a good way to measure AFRs. It most certainly can change the reading. When trying to diagnose a problem like this you really have to be sure you are getting an accurate reading or else you are simply chasing a red herring.

Good afr data along with some OBD data will go a long way in finding out what is going on.
 

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EGT might provide insight as well.

Until the A:F is rechecked, one should probably assume that the readings are correct and treat the engine accordingly, as there is no evidence that the A:F is correct and with one engine having been fried... that suggests that not all is correct.
 

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Hint: the Lotus ECU learns and adjusts the long term fuel trims constantly. If the car runs too rich or lean at part throttle the ECU will trim for it. This takes a while. This DOES change how the car runs at WOT. The car might run great after the ECU is reset then too rich or lean at WOT after it learns for a while. Part throttle tuning is very important.
 
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