FMU question
02-04-2005, 12:26 PM
#1
FMU question
98 GXE
Right now I am running a 4:1 disc. I have 370s/walbro/AEM FPR. I use my SAFC-II (currently it is maxed at -50%) for tuning. I am on the stock ecu, and US Manifold with at PF TB. I am having problems tuning the car. At 3.x RPM my air fuel goes off the chart below 10:1. My question is, will a 3:1 recalibration kit help this problem, or am I too rich to correct this with a 3:1 fmu disc? Also is there any FMU disc with a smaller ratio than 3:1? Vortech lists 3:1 as the lowest ratio. Thanks for any help.
-Carson
Right now I am running a 4:1 disc. I have 370s/walbro/AEM FPR. I use my SAFC-II (currently it is maxed at -50%) for tuning. I am on the stock ecu, and US Manifold with at PF TB. I am having problems tuning the car. At 3.x RPM my air fuel goes off the chart below 10:1. My question is, will a 3:1 recalibration kit help this problem, or am I too rich to correct this with a 3:1 fmu disc? Also is there any FMU disc with a smaller ratio than 3:1? Vortech lists 3:1 as the lowest ratio. Thanks for any help.
-Carson
02-04-2005, 12:40 PM
#3
I have a question about the FMU also... with say the 3:1 disc does that increase your fuel pressure 3 lbs. for every pound of boost or does it increase your fuel pressure 1 lb. for every 3 lbs. of boost?
02-04-2005, 12:46 PM
#4
Moderator who thinks he is better than us with his I30
iTrader: (8)
Join Date: May 2002
Posts: 9,335
Originally Posted by mtcookson
I have a question about the FMU also... with say the 3:1 disc does that increase your fuel pressure 3 lbs. for every pound of boost or does it increase your fuel pressure 1 lb. for every 3 lbs. of boost?
here is a copy and paste of how Ice explained it to me
Originally Posted by IceY2k1
1)Basically, yes the equation you have is correct. Whatever your base fuel pressure is, just take half. Stock is 34psi, but with a walbro or AFPR it can be higher/lower. I believe the explaination I heard on why you use 1/2xbase fuel pressure is since there is a FPR and FMU installed in parallel it is divided between them. Not 100% sure really, but it's probably just how Vortech decided to design it for multiple applications.
2)This is based on the stock FPR controlling fuel pressure until the FMU increases fuel pressure more.
So in other words if the stock FPR is controlling 43psi at WOT, then until the equation 1/2(base)+disc ratioxPSI surpasses 43psi, you'll still have 43psi, ie no less, until the FMU creates 44psi, 45psi, 46psi, etc.. For example, at WOT with 34psi base fuel pressure and 5psi of boost:
At 5psi:
8:1 disc means....
1/2(34)+8*5 = 57psi<~FMU added 14psi more fuel
6:1 disc means....
1/2(34)+6*5 = 47psi<~~FMU added 4psi more fuel
4:1 disc means
1/2(34)+4*5 = 37psi<~~FPR will override with 43psi, since this is lower.
So, you can see that if you only hit 5psi, a 4:1 wouldn't do anything over the stock FPR and the 6:1 would just barely add anymore fuel. So, you'd want 8:1 or higher *IF* you need more fuel, which depends on your injectors capacity and hp level at Xpsi of boost.
2)This is based on the stock FPR controlling fuel pressure until the FMU increases fuel pressure more.
So in other words if the stock FPR is controlling 43psi at WOT, then until the equation 1/2(base)+disc ratioxPSI surpasses 43psi, you'll still have 43psi, ie no less, until the FMU creates 44psi, 45psi, 46psi, etc.. For example, at WOT with 34psi base fuel pressure and 5psi of boost:
At 5psi:
8:1 disc means....
1/2(34)+8*5 = 57psi<~FMU added 14psi more fuel
6:1 disc means....
1/2(34)+6*5 = 47psi<~~FMU added 4psi more fuel
4:1 disc means
1/2(34)+4*5 = 37psi<~~FPR will override with 43psi, since this is lower.
So, you can see that if you only hit 5psi, a 4:1 wouldn't do anything over the stock FPR and the 6:1 would just barely add anymore fuel. So, you'd want 8:1 or higher *IF* you need more fuel, which depends on your injectors capacity and hp level at Xpsi of boost.
02-04-2005, 12:49 PM
#5
That was the other thing that I was leaning towards....is there are rule of thumb how much fuel you should pull with your AFC? Because when I was only pulling ~-35 I was fairly tuned at about 11:1 A/F. I did a few more mods and wanted to re dyno and lean out a little more. The first pull I did the A/F went off the chart again (I was thinking I was already running 11:1 A/F)....so I went ahead and pulled more and more until I maxed out the AFC and nothing really changed the A/F kept going off the chart at ~3K. I have verified that my SAFC-II is working. Should I try the 3:1 and start from scrach on tuning the AFC? Does any one have experience tuning a set up similar to mine? So many people have the JWT ecu and the MIVI that its hard to get experienced advice on my set up.
Originally Posted by I30tMikeD
Having your SAFC taking out that much fuel is really messing with your timing.
02-04-2005, 01:03 PM
#6
Moderator who thinks he is better than us with his I30
iTrader: (8)
Join Date: May 2002
Posts: 9,335
Originally Posted by cardana24
That was the other thing that I was leaning towards....is there are rule of thumb how much fuel you should pull with your AFC? Because when I was only pulling ~-35 I was fairly tuned at about 11:1 A/F. I did a few more mods and wanted to re dyno and lean out a little more. The first pull I did the A/F went off the chart again (I was thinking I was already running 11:1 A/F)....so I went ahead and pulled more and more until I maxed out the AFC and nothing really changed the A/F kept going off the chart at ~3K. I have verified that my SAFC-II is working. Should I try the 3:1 and start from scrach on tuning the AFC? Does any one have experience tuning a set up similar to mine? So many people have the JWT ecu and the MIVI that its hard to get experienced advice on my set up.
Where did the dyno shop get the AF ratio reading from? The muffler tip or did they scew it into your o2 bung?
02-04-2005, 01:48 PM
#8
the a/f reading was taken in my tail pipe (on the exhaust tip), and when my car boosts it boosts smoothly, it does not feel like the timing is skiping around. Does anyone think that with my set up I could benift from a 3:1 FMU or am I just going to run into the same issues?
Originally Posted by I30tMikeD
I don't think anybody know for sure exactly how much the timing is effected when adjusting large amounts with the AFC, StephenMax or another member would know more than me. But IIRC adjusting more than 10-15 on the AFC can start to mess with timing. Taking out too much will advance it and adding too much will retard it. Maxing it out at +/- 50% will most likely cause bad timing issues.
Where did the dyno shop get the AF ratio reading from? The muffler tip or did they scew it into your o2 bung?
Where did the dyno shop get the AF ratio reading from? The muffler tip or did they scew it into your o2 bung?
02-04-2005, 03:17 PM
#9
Supporting Maxima.org Member
Join Date: Mar 2004
Posts: 275
I could be wrong, but this is my understanding: The A/F sensor is a wideband O2 sensor that monitors the amount of oxygen left over after combustion. If the leftover amount is too low, you are running rich, and too much you are running lean.
The catalytic reactions in the pre-cats and main cat will remove almost all oxygen, giving a tailpipe measurement that will always look like you are running rich, even though you might actually be lean.
That's why the A/F must be measured before, or without the cat. Tailpipe is good for measuring CO, CO2, and NO emissions. Try measuring A/F using test pipe in place instead of cat. www.budgetexhaust.com sells a test pipe with sensor bung for $25 CAN, or about $20 USD.
The catalytic reactions in the pre-cats and main cat will remove almost all oxygen, giving a tailpipe measurement that will always look like you are running rich, even though you might actually be lean.
That's why the A/F must be measured before, or without the cat. Tailpipe is good for measuring CO, CO2, and NO emissions. Try measuring A/F using test pipe in place instead of cat. www.budgetexhaust.com sells a test pipe with sensor bung for $25 CAN, or about $20 USD.
02-07-2005, 12:29 PM
#13
Originally Posted by cardana24
so should I try the 3:1 FMU? Anyone with input?
How much power are you making? What size pulley?
02-07-2005, 12:36 PM
#15
288hp
-I am running the 3.125 pulley. I know the 3:1 is the next logical step but my A/F is so rich I am wondering if you guys think the 3:1 would be a waste of time.
-I am ~25psi at idle and ~42psi at full boost
02-07-2005, 12:45 PM
#16
You could try a boost bleeder valve, which is what Mardigrasmax used along with a 3:1 disk when he first converted to the 370 cc/min injectors. The bleeder valve is only about $15 and along with a check valve, essentially turns your fmu into an adjustable one. If you use a bleeder valve, make sure you don't tee into your boost gauge vacuum line, or else it will make your boost gauge read low.
02-07-2005, 01:28 PM
#18
Originally Posted by Stephen Max
You could try a boost bleeder valve, which is what Mardigrasmax used along with a 3:1 disk when he first converted to the 370 cc/min injectors. The bleeder valve is only about $15 and along with a check valve, essentially turns your fmu into an adjustable one. If you use a bleeder valve, make sure you don't tee into your boost gauge vacuum line, or else it will make your boost gauge read low.
just put it in the line that feeds the fmu. it will bleed the boost off.
Thread
Thread Starter
Forum
Replies
Last Post
Unclejunebug
5th Generation Maxima (2000-2003)
10
04-02-2016 05:42 AM
Stagnet04
4th Generation Classifieds (1995-1999)
2
10-11-2015 08:16 PM