Understanding on fuel cut

[bountykilla0118]

I am going to take a stab at trying to explain fuel cut in a very simple way … So if I am misstating or misinforming feel free to re-educate me and everyone else that might be misinformed also.

Fuel cut has a lot of variable but understanding it is easy. Fuel cut is there to save your motor, the stock AFM and lex AFM measures the air entering the system in HZ and I believe fuel cut happens around 1600hz in second and third gear, but I don’t know for sure the exact numbers because they change when fuel cut happens depending on so many things so don’t quote on that. I used my eyes to keep up with numbers that were changing pretty damn fast lol. Moving on fuel cut has to do with the flow of air being measured and with that said, you should already know why the lex afm and 550’s are a very good and simple upgrade. In short the lex afm lets you bring in roughly 25% more air before fuel cut and 550’s spray roughly 25% more fuel to compensate for the extra air coming while operating use the stock signals. A 66mm turbo will hit fuel cut well before a stock ct26, because the amount of air that a 66mm turbo will flow is much more that the amount a stock ct26 can flow at the same PSI. That’s why some people with bigger turbos complain about hitting fuel cut at 10psi when they used to hit fuel cut at 15psi with the same setup just different turbos.

Get the point?

 

[giterboosted]

So if I swapped to maft pro I could elimiate my neo, but would it eliminate my BC and whatnot as well, I'm thinking about doing this for tuning and boosting purposes, I'm not really sure what all other than tuning this pro unit does

 

[bountykilla0118]

Yea it would eliminate fuel cut if you wanted to ...... but again fuel cut is a good thing. How much power are you trying to make? PSI does not equal power.... 18psi with an upgraded ct26 will make a 1/3 of the power that a gt35r will make ..... get where i am going?

 

[Kckazdude]

So if I swapped to maft pro I could elimiate my neo, but would it eliminate my BC and whatnot as well, I'm thinking about doing this for tuning and boosting purposes, I'm not really sure what all other than tuning this pro unit does

Yes the MAFT Pro will eliminate the need for serperate units. The feature list for this unit at this price point is unheard of. Here is an old but comprehensive list of features and things have improved even more since this was written:
http://www.mkiiitech.com/forum/showthread.php?t=622

 

[giterboosted]

well im not really into making rediculous power, what do you think i would be better off doing, "super tuning" the living crap out of my lex/550s 57 trim setup and later down the rd when i can afford a bigger better turbo setup convert to maft... that sound about rite? my total max im looking for, even in the ong run is maybe 4-450

 

[Fubar231]

I remember reading a quote in someones sig

Boost = resistant to flow
flow = power

Dont think of it as high boost

That was the quote, aka just because hes boosting 14psi and hitting fuel cut doesnt mean anything, hes still flowing just as much air through the engine as those who are hitting it at 17-20psi its just his air flows with less resistance giving him "less boost".

If im wrong, go ahead and slap me lol

 

[bountykilla0118]

I think I get what you are trying to say ...... If you mean 14psi with a bigger turbo flows more air than a smaller turbo at 17psi. Pressure is the measure of resistance with flow and flow equals power is saying that if you are making 400rhwp at 22psi there is a lot of work that can be done to increase the flow in the engine as in making the air ways bigger and smoother like a good porting, polishing, bigger valves, etc. and it will help make 400rwhp at 15psi instead 22ps with the same setup because the air is moving way more efficiently.

 

[jdub]

It's not just the turbo that affects flow

 

[bountykilla0118]

That is true ..... the stock exhaust, stock intake, IC and IC pipes and the turbo would be the first few places to start because they are a lot of gain with these mods alone. Then when I would start to do the more expensive stuff but I would work with the next thing that has the most performance for the dollar which might be a cam, then some head work or shit do the head work at the same time while you have the head off.

 

[jdub]

It's the volume of air past the AFM that causes FC - higher flow from any mod will lower the psi that you see FC occur.
Besides environmental differences (OAT, etc), this is the reason you will see some cars hit it earlier vs later...even stock. A factory IC that just so happens to flow better will do it. PSI does not = power.

 

[Nick M]

You no longer need the neo hooked up at all. The maft pro is a piggyback just like the neo but it uses a speed density sensor which is better than a maf.

This is a tangent, but no it isn't. Speed density is a guess, mass air flow measures actual mass. And we don't have a MAF sensor.

 

[giterboosted]

Yea yea we have karman vortex we know lol, so my next step I think is going to be intercooler and piping, as I already have the aftermarket exhaust

 

[bountykilla0118]

It's the volume of air past the AFM that causes FC - higher flow from any mod will lower the psi that you see FC occur.
Besides environmental differences (OAT, etc), this is the reason you will see some cars hit it earlier vs later...even stock. A factory IC that just so happens to flow better will do it. PSI does not = power.

Jdub we are saying the same things just differently .... hence my post about the AFM and the area around 1600hz (give or take) in 2nd and 3rd gear that fuel cut happens around. The reason I was why I stuck to the angle of “flow and turbo” is because it supposed to be understood that the turbo pulls air through the AFM first and then moves air through the intake track. You just left off the background stuff

I am trying to give him some back ground info and at the same time make sure I am staying on top of my knowledge base around the system and how to think about it and not what to think. Its like give people the answers but never telling them how to actually find correct answer for the equation (the problem). In understanding how to think about a problem you learn what to think, but if you only know what to think you will never know how to think ….. So by all means Jdub unload some of that info man about making more power with less boost and the actual things to do while keeping the stock compression ratio even though I am thinking of bumping up my compression

 

[ohiosupramst]

This is a tangent, but no it isn't. Speed density is a guess, mass air flow measures actual mass. And we don't have a MAF sensor.

Wow I'm totally owned.

 

[jdub]

Bounty - Not disputing what you said...just making it clear that the turbo is not the only part of the system that has an affect on flow. What matters is the the cubic feet of air delivered to the cylinders and how fast. A system with 3" piping and a unrestricted IC will deliver more air at a lower PSI compared to a more restrictive system with the same turbo - more air, more fuel = more power. It's not the boost pressure (PSI) that is making the power, it's the greater volume of air combined with more fuel (of course). Where guys make a big mistake is slapping on a big turbo then not sizing the piping and IC to handle it. You'll get crazy high PSI and not what you expect for power...that's because the piping and IC are not capable of delivering the volume produced by a big turbo to the cylinders. The entire thing from the intake to the plenum needs to be treated as a system to achieve optimum results.

Keeping this in the FC context, the AFM will sense the the greater amount of air flowing past it and you will hit FC at a lower PSI.

 

[giterboosted]

Well then next step better intercooler and piping

 

[grimreaper]

I dont hit fc until 1800hz or so... I could never get more then 16 psi out of my sp61gt with out hitting fc on the lex afm setup (no piggyback).

 

[TurboStreetCar]

I am going to take a stab at trying to explain fuel cut in a very simple way … So if I am misstating or misinforming feel free to re-educate me and everyone else that might be misinformed also.

Fuel cut has a lot of variable but understanding it is easy. Fuel cut is there to save your motor, the stock AFM and lex AFM measures the air entering the system in HZ and I believe fuel cut happens around 1600hz in second and third gear, but I don’t know for sure the exact numbers because they change when fuel cut happens depending on so many things so don’t quote on that. I used my eyes to keep up with numbers that were changing pretty damn fast lol. Moving on fuel cut has to do with the flow of air being measured and with that said, you should already know why the lex afm and 550’s are a very good and simple upgrade. In short the lex afm lets you bring in roughly 25% more air before fuel cut and 550’s spray roughly 25% more fuel to compensate for the extra air coming while operating use the stock signals. A 66mm turbo will hit fuel cut well before a stock ct26, because the amount of air that a 66mm turbo will flow is much more that the amount a stock ct26 can flow at the same PSI. That’s why some people with bigger turbos complain about hitting fuel cut at 10psi when they used to hit fuel cut at 15psi with the same setup just different turbos.

Get the point?

Couple technical problems with this. The AFM measures two things, speed and temperature, NOT actual Mass air flow. CFM Vs. LBS/MIN is a better way to describe it. The HZ output is the speed only.

Fuel cut is based on many things the biggest "factor" is airspeed as the other factors such as temperature are just correction factors that generally dont change much. 50 CFM @ 50F-70F is relatively close for correction where as 20CFM and 50CFM are much different. Think of CFM as the HZ output. The temperature is the correction factor used in calculation of mass airflow or LBS/MIN.

Fuel cut is based on the calculated Mass Air Flow (LBS/min) based on Air Speed (CFM), temp, displacement and RPM. Those are computed to in essence guess the actual Mass Air Flow (LBS/Min). The end result of mass airflow (LBS/MIN) is what fuel cut is based on.

Other then that exactly what you said about fuel cut changing with turbo and intercooler selection is correct.

 

[bountykilla0118]

I dont hit fc until 1800hz or so... I could never get more then 16 psi out of my sp61gt with out hitting fc on the lex afm setup (no piggyback).

I would think thats pretty good bc if you had bumped up the fuel pressure and with a piggyback you had scaled back you would be in the 20+psi. I personally hit 22psi with an older boss stage 5 I want to say it was a 61mm or 66mm .... cant remember

Couple technical problems with this. The AFM measures two things, speed and temperature, NOT actual Mass air flow. CFM Vs. LBS/MIN is a better way to describe it. The HZ output is the speed only.

Fuel cut is based on many things the biggest "factor" is airspeed as the other factors such as temperature are just correction factors that generally dont change much. 50 CFM @ 50F-70F is relatively close for correction where as 20CFM and 50CFM are much different. Think of CFM as the HZ output. The temperature is the correction factor used in calculation of mass airflow or LBS/MIN.

Fuel cut is based on the calculated Mass Air Flow (LBS/min) based on Air Speed (CFM), temp, displacement and RPM. Those are computed to in essence guess the actual Mass Air Flow (LBS/Min). The end result of mass airflow (LBS/MIN) is what fuel cut is based on.

Other then that exactly what you said about fuel cut changing with turbo and intercooler selection is correct.

Chunks good shit man .... that’s the technical stuff I wanted some feedback on. I asked so many different people about the Karman Vortex and most don’t know anything about it at all, except if it doesn’t work get another one. I have been trying to explain to people that its not really a like a MAF because the way it functions is very different and you cleared it up for me. I guess that’s what makes the system so accurate compared to the rest.

 

[jetjock]

Good grief. There's a lot of BS in this thread...