TweakGames

iTrader: (3)

Hi all, I got my car dynoed a while ago and a few people are commenting that I am way to lean. I was wondering what you guys think. S4 NA-turbo 4 psi.
t2 ecu, injectors, and fuel pump.

Thanks.

Also if I am to lean.... other than safc and things like that, any way to turn up the fuel?

BLKFC3S

you start out alittle high then end up where you need to be in the mid 11's. 12.0-13.0 is to much gas try adding some air to it to richen it up down low and
it should be fine.

Aaron Cake

iTrader: (1)

At 4 PSI, 11:1 is way too rich.

12.5:1 is more then safe. If you are not making boost at 2.5-3.5K, then those ratios are fine as well.

TweakGames

iTrader: (3)

Hmmm.. ok I know I am @ full boost in the 2.5-3.5k range. I Will have to take it easy down there. Thanks.

sharingan 19

iTrader: (16)

I guess I'm the only one wondering how you're making almost as much to the whp as a stock TII makes at the crank, and at only 4 psi nonetheless?

Either you've got a deadly efficient drive train or maybe I need get my A/F to look more like yours

Aaron Cake

iTrader: (1)

Add a bit more fuel to that area and you''ll be fine. I'd be comfortable with mid 13s at such low boost.

TweakGames

iTrader: (3)

https://www.rx7club.com/time-slips-dyno-128/184-5-whp-s4-na-t-6-port-stock-4-psi-677566/

sharingan 19

iTrader: (16)

So would you say 10 psi is safe in the 12's ?

and while we're on the topic...

*Disclaimer* I am not, nor do I claim to be any type of tuner

But I've been trying to figure out why so many seem to be so fixated on running at or near 11 afr. Considering max power/ efficiency is at 14.7, that seems a tad rich (yes I know you can't really expect to run in the 14's). Seems like there's more power to be had in the 12's maybe low 13's if the map is well tuned.

I've seen some charts where the AFR stays within a .5 range the whole run, but that range was like 11.5 - 12.0 If its possible to keep afr's steady like that why not bump it up to 12 or even 12.5 or 13 if you're feeling cocky ?

I know about spikes and equipment failure and all that, but honestly speaking, if your fuel pump dies while you're in full boost, is a 11.0 afr really gonna save you?

Spectator

iTrader: (1)

A tII at 8 PSI shpuld run in what? Mid 13s?

Aaron Cake

iTrader: (1)

And the answer to that is a definite maybe. Low 12s for sure, but not mid 12s. The stock ECU hammers the engine with fuel under boost well into the 10s. That's just crazy. High 11s is more then safe.

But just saying that this ratio or that ratio is going to be safe is a hard call to make because it depends on the exact setup. Even stock for stock there is a wide variance.

14.7 is the stoichiometric ratio at which all fuel is burned in the available air. However that does not mean that it's the ratio which the most power is produced. NAs will make most power anywhere from 12.5 (again, usually...) to mid to low 13s.

Turbo cars need more fuel to keep combustion temps down. Running 13s under boost is crazy because you'll detonate the engine to hell, hence turbo ratios are somewhat richer. Under low boost, 12s are fine. But as the boost increases (or airflow increases from a bigger turbo) you'll need to be in the 11s to stay safe. Once you get into crazy amounts of airflow and/or boost, you'll need to be in the 10s using pump gas.

And of course this doesn't cover light load ratios. If you are crusing around at 14.7 you're just wasting fuel. Get those ratios up into the 16s...

You tune for the A/F ratio which produces the best power and is still safe.

That's too lean. Low 12s is much safer.

Roen

iTrader: (2)

16's under cruise? Isn't the engine running less efficiently than the 14.7 (1:1 Lambda I should say, since some people could be using E10) at stoich?

Aaron Cake

iTrader: (1)

14.7 is way too rich under light load. You are just burning fuel unnecessarily. The engine will function fine under cruise with far leaner ratios. Then when you step on the fun pedal, the A/F ratio drops...

Roen

iTrader: (2)

I'm just not getting the connection between stoichiometric efficiency and being too rich under light load. Please educate me. I'm very interested since my car is going on the dyno to be tuned tomorrow, and while my tuner is more than capable, I'd like to understand exactly what he's doing, and on the odd possibility that his knowledge is a bit outdated, submit some tips to him and see what he says. If it helps at all, my car is an NA, stock ports.

Are low 16's (Range: 16.0 - 16.3? Correct me if I'm wrong) the A/F you arrived at using empirical testing of seeing which A/F gives you the best gas mileage?
Would things like porting, boost, anything that causes more load and/or airflow affect this number?
Is this A/F determined by fuel type or is this the lambda ratio that is the best for mileage regardless of fuel type?
I ask this because NY mandates E10 and all of it's pumps, but some NJ/PA/CT pumps are 100% gasoline or possibly gasoline with MTBE, so if that's the magic number across fuels, then I'm set.

Aaron Cake

iTrader: (1)

14.7:1 is the ratio at which all air and fuel is consumed in the reaction. While it is the most efficient way of burning fuel, it is not necessarily the most efficient way of running an engine.

Most gasoline engines are tuned to run somewhat lean under light load, to give better mileage. Very few of them try to maintain exactly 14.7:1 unless they have a catalyst system that calls for it (like the old Mazda thermal reactor system). If you measure any modern car with a wideband you'll see ratios in the 16s or 17s under light load and it will go offscale lean in no-load conditions.

A/F ratios are determined by a load of factors. Porting, volumetric efficiency (being able to pull in more air), air and water temperature, emissions, etc. There is no magic ratio that is good for all cars under the same conditions. My engine cruises quite happily at slightly leaner then 16:1. Any leaner and it begins to buck and misfire. Stock port engines will handle leaner ratios then that but you get into issues with driving EGTs up (which will do things like melting turbos and flattening apex seal springs...). The ratio you tune for under cruise/light load will be the leanest ratio at which the engine is happy and the car performs acceptably.

If your tuner is recommending anything richer then 15.5 at light load then you are wasting fuel (assuming your engine is happy at those ratios).

Under load, safety and power (generally in that order for a street car, though those can reverse for a race car!) will determine A/F ratio.

Stoich ratios are different for various fuels. Here's a page that has a chart:
http://www.marshallinstruments.com/air_fuel_gauge.cfm

Your NA stock port car will be an easy one. You will likely end up with mid 13s under WOT, and should end up fairly lean at cruise (15+). Under idle you should be able to deal with mid to low 13s as well, though if the idle drops when a load is applied you need to go slightly richer.

Roen

iTrader: (2)

idle should be mid to low 13's? Isn't that a bit rich as it is? I thought idle was one of the places where you can get away running really lean.

I understand stoich ratios are different for various fuels, that's why I quoted it in lambda. I was wondering if the 1.1:1 Lambda range is sufficient across all fuels, or if the Lambda changes depending on fuel.

So just lower the fuel until the car bucks and then back it up until it's nice and happy under cruise?

Though, I've been reading that these days stock port NA's are making their best power in the 14's, but the EGTs are shooting up way too high as a result.

Roen

iTrader: (2)

Also, I had remembered that before I brought my car to get tuned, I was averaging 22 mpg highway. When I brought it in, they said the wideband was reading 17's at cruise, so they added fuel to get me to the 15's. I then averaged 24 mpg at +5% fuel correction. Now, I had no problems running that lean on stock, no bucks, misfires, etc...., so why is it that I ended up spending less gas per distance by adding more fuel at cruise?

Aaron Cake

iTrader: (1)

As I've said, there is no magic AFR/Lambda. It totally depends on what the engine wants to see.

It is possible that at 17s you were harder onto the throttle and thus burning more fuel, even though the ratio was leaner (the engine was more highly loaded). 17s is pretty damn lean for constant cruise anyway.

sharingan 19

iTrader: (16)

I've been using the A/F function on my TT to "monitor" ratios as I drive around. I figured it wasn't too accurate because from what i understand it just references a guessing chart (for lack of a better term) to estimate A/F outside of 14.7 +/- 1. Even still, when I get in boost, it drops to high 10's then stabalizes in the low 11's so maybe it's not that far off after all.

That makes sense, because things much less significant than porting/forced induction still have an impact on VE, and thus how much air is available for combustion.

brain fart on my part, you mean our engines aren't 100% efficient !?!

Combustion temps! forgot about those, I got carried away worrying about the big picture and the details slipped my mind. Wouldn't running water/alki injection would allow you to run higher A/F under the same boost, all other things being equal?

According to my "geuss-o-gauge" A/F is between 15- 20(yes 20) under light load. But I guess I shouldn't read too much into that till I get the proper equipment...



Thanks for providing some useful information to all the would-be tuners out there!