Fuel Enrichment with supercharging

[JBrady]

A Lextreme member has supercharged his 97 SC400 (Soarer for our down under friends) and believes the stock fuel system and ECU will compensate for up to 7psi of boost. I am highly sceptical of this. I believe he would need at least a boost referenced rising rate fuel pressure regulator (FMU) to provide a safe A/F ratio with really any boost.

Opinions please.

 

[Lextreme II]

I am not sure what people are thinking and giving ill advice in CL. The questions I have would be:

1. How does the ecu know when and how much to add fuel?

In order for the ecu to compensate, it has to be learned and it will take time. However, I still dont think it can compensate that much. The stock regulator would not able to increase fuel that much due to the ratio. Therefore a FMU is a must to have a safe running system.

 

[xirforever]

Im not really sure how much fuel should be raised per psi, but everytime i dyno my car i see as low as 10:1, which as we all know is really rich. With boost, would it still be at an acceptable level? I mean all that extra fuel could compensate for boost, right?

i have a 97 sc400 btw and actually just got it dynoed today and once before a couple months ago

 

[JBrady]

Yes, as I said, I believe SOMETHING needs to be done to provide safe fuel ratios under boost.

The specific member is both on Lextreme and CL and has a beautiful 97 SC400 with an Eaton 90 SC.

Here is his post on Lextreme

http://lextreme.com/forums/showthread.php?t=7720

Here is his post on CL

http://www.clublexus.com/forums/showthread.php?t=240378

I just DO NOT WANT this guy to hurt his engine or influence others to do the same.

He says he got his advice HERE at Lextreme...

I maintain that the stock fuel system can compensate for a small amount of modification but not any appreciable boost.

Hopefully our Australian friends with supercharged/turbocharged 1UZFEs can clear this up.

 

[Lextreme II]

He says he got his advice HERE at Lextreme...

Its hard to understand that getting advice from Me and getting advice from Lextreme is two different sources. I think he got advice from this site not spefically from ME. I highly recommend this member to use a FMU for safer air fuel.

 

[SCV8]

...or at least a linear rate, adjustable regulator.

 

[Edwin_S]

A good FPR should be enough I would say. The ECU never sees the boost, just the airflow at the AFM. As long as he's not maxxing out the AFM, he should be okay.

 

[jibbby]

From my reads 6psi on the 1uz-fe's should be the max you should push the M90's when using the stock fuel system...However, I would still be weary using the stock fuel pump and would recommend a walbro just incase...

However, it appears this guy pulled it off at 7psi, go figure?

 

[stevechumo]

I have the same thought as all of you and I told him that. However, he said he has successfully run it using the stock fuel system. He also said his A/R gauge showed he's running rich. His thread is on the CL. What else can I say?

 

[Zuffen]

Ah. So it was on the Net. It must be correct!

Think about one person can do it but no one else???

 

[JustenGT8]

It's not that far fetched if you understand what's going on.

1st of the stock toyota fuel reg is perfectly capable of handling boost and will increase fuel pressure directly proportional to boost pressure which is all you need. Mine was good up to 265rwkw!!

The AFM just sees mass airflow so as long as the airflow of the SC doesn't exceed the 'range' the AFM can handle there's no real problem there.

Same goes for the injectors...the ECU will just crank up the injector duty to suit the AFM signal so if they are big enough there's no prob there either.

If boosting exceeds the capacity of any of these parts though you will have trouble (same for fuel pump).

Could be all his gear is in very good nick and he's just sneaking by with everfything maxed at 100%?

Not the wau i'd do it but theoretically feasible and a great budget approach if you aren't looking for more than a mild upgrade.

 

[jibbby]

I don't have a problem with the ECU and MAF air flow reads...It's the fuel feed that is in question for me...

It's just hard to believe the stock fuel pump can keep up with 7psi of boost....The fuel pump at stock is not all that, and should peg out way before the injectors peg....

I had to go with the Electric high flowing Walbro fuel pump before the injectors were even considered to be pegged.... At 7psi I am almost sure you would run lean and starve the motor of fuel feed with the weak stock fuel pump... This is what is hard to believe for me..

The stock injectors can handle what approx. 330whp before being pegged? I think RMS stated that fact and said he went lean at around 330whp with the stock injectors...

Hmmmmmm? One guys word could be mush....

 

[rms]

Dont have precise figures for Lexus injectors or pump.
Injectors quoted at 251cc/min but at what duty cycle?

Commonly accepted injector flow rate is 500cc/min/cyl at 100 flywheel bhp/cyl MINIMUM.
At 251cc/min * 8 cyls = 50bhp/cyl *8 = 400 flywheel bhp.
Total injector flow is 2.008 ltrs/min = 120 ltrs/hour at 3 bar (std regulator)
A commonly used pump in Oz is the Bosch 008 which is supposed to flow 190 ltrs/hour at 3 bar pressure.

With this setup both the injectors and pump are MARGINAL at best.
There is NO safety margin at 400 bhp.

Our race engine uses a Bosch 413 pump rated at 200ltrs/hour at 8 bar pressure. At the 3 bar pressure we are running the pump will supply well over 200 ltrs/hr.
With 650cc/min injectors we have 50% duty cycle at 8000 rpm.

The other problem I see is what does the ECU do to fuel and ignition when the mass flow from the AFM is outside programed parameters ?

 

[jibbby]

RMS I confused you with another member whom actually tested the stock injectors on his twin SC400...He ran the stock injectors to around the range of 330whp and then went lean as stated in one of his posts... He's the member with the white turbo'd SC400..Forgot his screen name, as he hasn't been an active member of late.... I also read other guys getting weary around the 300whp mark with the stock injectors.....

 

[SCV8]

So lets see, the three fuel variables are 1) Injector flow rating, 2) Fuel pressure, and 3) Duty cycle.
If the injector duty cycle is safely maxxed out at around 85%, using stock injectors, what is the addition of X more pressure going to do to the available hp?

I've played around with this, while still remaining in open loop, passing smog and all the good stuff, and it's not that easy to do, but still possible with the stock fuel pressure regulator(modified).

I'm wondering if the same thing would work as smoothly on a mildly boosted car, by using boost pressure on one side of the fuel pressure regulator daiphram, as mentioned above, or if it would just toggle in and out of open loop/hiccup?

I wonder about this especially since the air measurement will be skewed, since air will have different volume/properties while under pressure(vastly different fuel curve). This is the same reason carbs don't work so great under pressure.

 

[rms]

The flow rate of the injector v fuel pressure uses the following fomula

%inc/dec = (1-(sqrt(old P/new P)))+100

 

[scotturnot]

Hey guys, Its me! I'm the guy with the supercharger. It is true I am running 7lbs boost on the stock injectors but thats not my long term plan. I have a set of 315cc injectors and a aeromotive 1/1 regulator that is going in along with a 10lb pulley. I do have a walbro pump. Also installed is a Perfect power SMT6 running transparent for now.

Currently I have 10.5-11 on my AEM wideband at WOT. One thing I have not done is stand on the throttle thru a whole 1/4 mile. I plan On a dyno tune before I do that.

 

[cribbj]

Scott, how do your plugs look? While your overall AFR may be rich, it's possible to have a cylinder or two running lean due to dirty injectors, etc.

Also, do you have anything to measure the duty cycle of your injectors?

In Supraland, for years we thought it was impossible to fry a motor at BPU level, which is basically with the stock twin turbochargers, running slightly higher boost than stock. We were wrong, and it was done by a knowledgeable owner, and to this day it's probably still a mystery why it happened. We don't know if one injector was fouled and that cylinder ran lean, or what. All we know is that the guy holed a piston in his 2JZ-GTE, on the stock ECU, with the stock twin turbos, at relatively low boost.

 

[scotturnot]

I have not pulled the plugs but i will do it this weekend and take a look.

I dont have any way of measuring the duty cycle on the injectors. To be honest I am new to the world of tuning injected cars. My background is with carburated FI engines. This is new to me. Every bit of information helps.

 

[cribbj]

Scott, probably the best way to measure duty cycle is with a DVM that has that capability like a Fluke 189. But that's a $350 DVM.

Several piggy back fuel/boost controllers (the Apex'I AVC-R, HKS, etc.) also have this capability.

And if you're good with a 'scope, you can do it too.

To me, duty cycle of the injectors at peak torque & HP is one of the handiest parameters available to know whether your fuel system is marginal or not.

That, and following the fuel pressure delivered to the rail at peak torque & HP. That will let you know if your fuel delivery system is up to snuff.