I've done alot of searching and just about everybody agrees that exceeding stock boost levels = running lean.

My FD is getting dyno'd soon so I'll be able to post the actual AFR at 10psi and 12psi.

In the meantime, is there a way of calculating the approx AFR?

I've read lots of threads stating how the stock ECU runs rich, even as rich as 10/1 at 7000 rpm / 10psi boost.

If that's the case, wouldn't raising the boost by 2 psi still give a rich AFR?

14.7 ATM + 10 psi boost = 24.7 ABS.

14.7 ATM + 12 psi boost = 26.7 ABS.

Thus 12 psi = approx 8% leaner AFR per combustion stroke = AFR 10.8 /1.

Add to that a marginally higher CFM turnover of air, as long as the stock fuel pump can push the fuel - and it can or 12 psi wouldn't be safe with a PFC mapped even richer, and slightly higher intake temps and the AFR would still be no worse than about 11 /1?
Please feel free to correct me, I'm learning with this stuff

Also, lots of posts state that an intake, downpipe and midpipe mod needs a new ECU even at stock boost.
But isn't 10 psi 10 psi? Same size bang per stroke but lots more bangs per minute due to free flowing midpipe = higher CFM of air but nothing the fuel pump cant handle.

I'm currently running intake, Jap downpipe, midpipe and cat-back on the stock fuel system and ECU, at 12psi. 8k engine, good compression, on 98RON. I guess I'm getting near to maxing the pump/injectors but still safe at <12 /1 , but I dont usually go over 6.5 / 7k rpm.
I'll post back my AFR after the dyno.

 

US stock ECUs enact fuel cut over 10.7 psi above 6000 rpm. There is no mapping above this boost level on the stock US ecu. Perhaps European or JDM ECUs are different.

 

If you've looked around thne you found the old school thought on 3 mods that affect air flow = replace stock ECU. New scchool of thought is stay at stock boost levels = stock ECU is ok to use.
Your school of thought from what you've read is 12 psi is with in AFR of what stock ECU should handle = we'll be reading your new thread on "well I poped my motor" idea's suggestions ? ? ?

 

Thanks for the reply, could you elaborate on that please?

I explained that I think 12 psi boost still gives a safe AFR of 11/1 , if you're starting from 10/1, and why.

Very interested in hearing your reply as to why you think I'm wrong, don't just tell me I'm wrong! Facts please

 

With less backpressure, the engine flows more, even at the same boost level.

I'm not precisely sure how to estimate the change in mixture with boost level, but your appraoch seems reasonable.

The stock ECU (US models anyway) will do a fuel cut -- it won't let you run a sustained 12 psi. That's the main problem with trying to run the stock ECU at 12 psi, as rynberg pointed out.

AFRs vary over a run, so if you can get a graph that would be the best way to post the AFR. Boost levels vary, too, but we often imperfectly report that we're running "12 psi", even though it varies between 8 and 13 psi (for example) as you rev out. Graphs are better.

-Max

 

How can this be the case if the engine chamber hasn't changed size, and the air is still being pumped in at the same pressure? Lower intake temps?

(And the stock ECU does richen some to compensate for colder intake temps)

Dave

 

Not an engine performance expert here, but here is my 2 cents

AFR is the ratio for fuel and fresh/flammable oxygen in the combustion chamber.

when the exhaust port open, NOT ALL the exhaust gas is push out of the combustion chamber. The amount of exhaust gas left in the chamber will take up space that could have otherwise be use for fresh air (which can be burn when is time for combustion).

There are a few factors I know of that can effect this prcoess( pushing out the exhaust gas), but I am sure there is more to what little that i know. The factors that I am aware of are, exhaust port and runner size, Back Pressure(which is restriction in flow i.e. cat, Muffler, and our turbo or turbos).

These bolt on mod that we use on our RX7, i.e midpipe, Catback, etc. are there to reduce the restricion/ back pressure. In turn, it changes the amount of exhaust gas left in the combustion chamber. So even with the same boost, You will put more fresh air into the combustion chamber. This will also change your AFR.

This is why Ported Motor makes more power even with the same boost.

But Like I said i am not an engine performance expert, so i don't know the exact AFR change with mod. ( that is why Greddy and HKS makes all the money, because they know these specs.) I just like to read about engine performance and read the How Stuff works website. hehe.

 

That makes plenty of sense, thanks. And it would apply on the intake side as well - some restriction is caused at the higher flow rates that prevent the full manifold pressure from filling the intake chamber. I knew there was a reason - I couldn't think of it myself.

Dave

 

The stock ECU (US at least) does run rich.

Here's the fuel cut info, but it's from the US market stock ECU...

http://www.fd3s.net/fuel_cut_info.html

 

I don't believe that you can accurately calculate/estimate A/F ratios. They must be measured with an accurate measuring device. A calculation is just a guess of what the numbers "should" look like. That may be very different than actual numbers.

 

Great point! Thanks! It's so obvious when someone's explained it. That theory has to be correct as it's perfectly logical.

I can't imagine how to do a calculation to determine how much less exhaust is left in the combustion chamber after fitting a mid pipe, but I would guess at a possible 10% increase (generous maximum) in air intake on each induction stroke.

That would mean 10psi with a midpipe mod is now 10% approx leaner than stock exhaust, without including the more bangs per minute cfm.

Hmmm, having said that , the cylinder capacity (or rotor capacity) induction is still 100%, whether it be made up of 90% fresh air + 10% old recycled exhaust air, or 100% fresh air if old exhaust air is totaly expelled. Or maybe exhaust air can't be combusted twice?

So 10 psi with mid pipe would now be 11/1 AFR , 12psi maybe 12.1 /1.

I'll ask the question in the UK rotary club whether JDM ECU's are supposed to have the same level fuel cut. If they are mine deffo hasn't lol.

 

I wrote a web page a while back with a little JavaScript model that is intended to illustrate why the mixture and HP can change even at the same boost level (note that the numbers are for illustration purposes only and should not be used in situations where getting precisely correct numbers matters):
http://maxcooper.com/rx7/how-to/fuel_system/why.html

Also note that I wrote it before the "3 mod rule" was debunked with real data. But keep in mind that the "3 mod rule" proved false because the stock ECU runs so rich that it can keep mixtures in check even after you upgrade some parts -- not because changing the parts didn't change the mixture.

-Max

 

Famous last words!

 

Also, someone correct me if I'm wrong please (or if this has been posted and I missed it) But, one's elevation will make a difference in the air/fuel ratio as well because the air is thinner as altitude increases, correct? And if so, then wouldn't the only "correct" way to calculate would be "at sea-level" and adjust for the difference in altitude between the altitude of where you live (i.e. where the car will be driven) and sea level?

 

Thanks for the link Max - good stuff

Quicksliver, yeah I think at 3000ft you'd lose about 1.5 psi, so run a bit richer.
Guess that's better than tuning for 3000ft then running lean when you come back down.

I dont have that prob here, I live at a place called Burnham on sea - at sea level.
We have the second highest rise and fall of tide in the world... just a bit of useless info

 

After Bande Aceh!

Sorry, couldnt resist!