Stock ECU & 10 psi........... Myth??????
 

For a long time, I have been hearing that you must limit yourself to 10 psi on the stock ECU. That you can do whatever mods you want, as long as you don't exceed 10 psi. I don't understand.

We know that the stock ECU can only supply a certain volume of fuel in a given time period. A standard measurement would be CFM (cubic feet per minute). Lets say, for example, that the stock ECU will supply enough fuel to support 500 CFM, and a bone stock FD will flow 450 CFM at 10psi. I think everyone will agree that adding a catback to a stock FD is safe. With a free flowing catback, you will now be nearing 11 psi and maybe 470 CFM on the stock ECU. Now we add a DP. We are now at around 12 psi, and maybe 490 CFM. In the distant past, we had the three mod rule, and this setup was OK. Some people still think it is safe. Certainly, it is pushing the limits. So, every time that we add another mod to our intake/exhaust system, we increase CFM, given the same boost level. That's why a modded car at 10 psi is so much faster than an unmodded car.

So now we have a car that has a catback and DP flowing 12 psi, approaching a dangerous level. And yet we are told that a car with the following mods at 10 psi is safe:

Intake
Hard pipes
Efini y-pipe
Big intercooler
Dp
High flow Cat
High flow CB
High flow fuel pump
And of course, loud polished BOV's :)

It seems to me that this configuration should be flowing more CFM at 10 psi than the car with 2 mods at 12 psi. Yet this list of mods is considered safe???

Discussion time :)

Wade's findings where such that at 10 PSI, the ECU was providing enough fuel to support the increased CFM and provide a safe A/F ratio.

OK, who is Wade, and what exactly did he test? Where is the data?

Ah, you disappoint me! j/k'ing ;)

Wade is "da man" when it comes to the fuel stuff. ;)

Here's the site:

http://www.newwave.net/~flanham/wlan.../3modrule.html

:)

IIRC Wade's findings correctly the stock ecu runs so rich that it can support a few mods. The stock ecu by no means adjusts itself for the added CFM of the mods, but it is so rich to start with that you can add quite a few things before getting AFR's that are too lean.

Pesonally I am not one of the "you can bolt on whatever you like as long as you stay at 10 psi" believers. If you're not checking AFR's with a wideband you have no idea what the engine is really doing. As adam c says psi does not equal CFM and it's the CFM that really determines the AFR.

I run a downpipe and catback with a stock ecu and I run my car so hard so often that I err on the side of caution. I have no interest whatsoever in adding a highflow cat or an intake to my engine mods without putting it on a dyno and being absolutely certain.

The "mod limit" rule is gonna be diffrent for every car. Ive read threads where people were able to add more than 3 mods, some including a mp and were still ok. (how some people add a mp w/ stock ecu i dont know) others have poped their motors with less than 3 mods. I guess it depends on how stong your motor is.

I was running K&N Filters, Y-pipe, stock IC, removed double throtle, downpipe, hf-cat, and N1 Duals. I put on the techedge WBO2 sensor, I got 10.5 AFR straight on a third gear run from 2500rpm to 7500rpm with 10-8-10 boost. Granted, I have HD Boost controller to keep it 10-8-10 at the time, otherwise it will boost higher than 10psi.

I think the main problem is the fuel cut limit from around 10.5psi to 12psi.
If this fuel cut somehow eliminated, I think it can safely dump fuel up to 14psi.
Why 14psi? It seems that if the turbo ran uncontrolled(had a hose pop off), it will built up to about 14psi.

Does the fuel/ignition mapping on the stock ECU even extend beyond the "fuel cut"?

Thanks for the info. I looked at the data on the chart. It looks like the only car with significant mods, and a stock ECU, is the last car. The first couple have exceeded the meter, which (I think) means that the meter only goes to 10.0:1 . The last car is 11.X:1, which may be approaching safe limits, depending on where it falls in that 11ish category. Did Wade do more? It appears that this was just a start to his testing.

Thanks

I am pretty sure that is the case. Because the fuel cut based on boost is not a constant number. it ranges from 10.5 psi to 12 psi.
I think around 3000-5000rpm its in the 12-psi range...I maybe wrong, have to check the lightning rx-7 website..

Right, MAX on the gauge means "richer" so anything above 10:1 (in number terms would be 9:1 ect) would be even richer:

http://www.rx7store.net/ProductImage...efiafgauge.jpg

So, at 10:1 or 11:1, that is showing signs of a safe A/F ratio. Now, the one that's getting into the 12:1 area is getting a little scary but that's at about 14-15 PSI with no fuel added. ;)

I was under the impression it's based upon a certain time period at a particular boost level, since you can spike up to 13-15 psi without a problem, but a second or two above a certain level above 10 psi, and the ECU initiates the fuel/ignition cut. Thus I'm wondering if there are even any maps up to the 14 psi you were referring to.

Wade's findings aren't clear about whether or not he tested on the street with a wideband or on the dyno.

I've routinely seen that cars run about .5 points leaner on the street than the dyno. With the boosted Hondas that I've tuned, I've always aimed for 12:1 or 11.5:1 on the dyno because that'll put them at about 12-12.5:1 on the street. The rotaries seem to like to run about 1 full point richer.

Once my car is running again (if that ever happens :p: ), I'm gonna dyno it with the stock ECU and with the M2 ECU at several boost levels and see what happens. :)

Sonny

Me too...I was wondering about that.
Maybe there are some delay in the MAP sensor. It probably smooth out the output.
Here is the link of the fuel cut:
http://www.scuderiaciriani.com/rx7/fuel_cut_info.html

RPM ABS. CUT OFF(PSI) SEA LEVEL CUT OFF(PSI)

1000 26.5 11.8
1500 26.5 11.8
2000 26.5 11.8
2500 26.5 11.8
3000 26.9 12.2
3500 27.1 12.4
4000 27.5 12.8
4500 28.0 13.3
5000 26.9 12.2
5500 26.0 11.3
6000 25.8 11.1
6500 25.4 10.7
7000 25.4 10.7

You can see that you can go up to 13.3 psi. This is pretty close to 14psi on regular boost gauge.
I am sure the map handles that high, but the fuel cut is what need to be removed.

Why that is what saves the engine.

I would guess the 13.3 is to compensate for the spike at transistion.

Some data I've "gathered" is the main cat/hi-flow gives enough resistriction to run leaner maps.

I have a dyno chart with a 12.0:1 map with a cat, no problems.

I have a dyno chart with as high as 13.7:1 @ 6700rpms @ 13psi. with a cat...(accident, not that I was aiming for it to be that high, 282 ft-lbs of torque, however...leaner is meaner :D )

Then my motor lost a seal with a 11.85ish:1 with a mp @ 6.1k with 13.5psi. all runs with PFS PMS.

I no longer run the PMS, although I passed emissions with a street port and all bolt-ons (300rwhp @ 10psi). So if you want to pass emissions and make power, this unit worked well.

With the stock ECU and the mods in my sig I dynoed last weekend and have some AFR data:

~8PSI (.55 bar) - Between 11:1 - 12:1 from 3800 to 5450 or so where it dropped below 11:1 and stayed there through redline

~11PSI(.75 bar) - Between 11:1 - 12:1 from 3400 to 3700 where it dropped and stayed below 11:1 until fuel/ignition cut hit at 6600 (on the dyno chart so probably the 10.7PSI cut at 6.5k that killed it).

This was tested with W02 on a dyno inside of a cement oven (err building) which was probably around 80F at the time of my run.

See my mods below. I've been running the stock ECU for about a year now at 7psi. No problems. It's actually running pretty damn rich. I have been meaning to dyno it but i'm too scared to pop the motor.

I had the AEM ECU but was forced to sell the $1200 paperwieght. Waiting on the cash for the Power FC now.

How the hell (and why?) are you running 850cc primary injectors on the stock ecu?

They were swapped back when I went back to the stock computer, just forgot to change it. They will go back in soon.

i got my car dynoed 3 days ago and it is com,pletely stock...at one time the boost was up to 11.8 PSI

This is the answer to original post .... stock ecu will hit fuel cut if you attempt to hold 12 psi for high cfms at high rpm.

3-mod is usually conservative. I've had 5 or 6 mods for years. Stock ecu is rich, and programmed richness increases with sensed manifold air temperature. What can bite u at 10 psi is lots of efficiency mods, big well chilled IC, and cold weather.

my stock ecu rule is stay below fuel cut, and do wide band testing if u do a street port, use a a mid pipe, or want to push a big IC in cold weather.

What seems to be missing here is intake temps. As pointed out, fuel cut is based on psi, which does not necessarily equal CFM, and a lean out could occur if the CFM was radically higher than that predicted by the original engineering staff for a given rpm. But CFM is only part of the story. PV=nRT dictates that the pressure multiplied by the volume will equal the overall number of air molecules in the engine, but CFM is modified directly by temperature in this equation, i.e. as temp goes up, the volume inducted remains the same, but the amount of air (read this as density, of course) goes down (leading to a rich condition). However, temp must be controlled because too hot will pre-ignite just as easily as lean. Where I think a lot of the blown engines on more than 3 mods on the ECU comes from is improper management of the fuel with respect to fuel for a given intake temp. Discussions of the accuracy and speed of response of the stock intake temp sensor can be brought up, but also the mapping of the temp vs rpm vs intake pressure, in that they use these three to calculate the CFM, but faster/wider gas management has changed the actual CFM into the engine. So CFM ultimately DOES effect the blow-ability on the stock ECU, but as modified by temp, which I've not seen a mapping of from the stock ECU.

Anyway, my muddlings above are a way of saying that the 3 mod rule probably holds provided the intake temps are within the design specs. As the velocity AND temp fall out of range, I suspect the fuel maps are incorrect, but then what the hell do I know?

Yes, but this is case with any tuning whether on the stock ECU, PFC or whatever. ;)

I think a final thing thats also important is how healthy are your turbo's. I bet people with new turbo's or very healthy ones are probably the ones in more danger. Since their pushing more air and slightly more efficiently.

All this talk of AFRs is absolutely pointless without knowing the timing maps on the stock ECU. If you have too much timing at peak torque (usually where the spike is on a sequential setup), it hardly matters if you're running 10.x AFRs, you'll still ping. I'd rather run 12.0 AFRs with conservative timing than 10.0 with aggressive timing.