Capn' Wankel

I read in someone's post recently that doing too many mods to a stock ported NA will create a lean condition? I have the mods in my sig which include header, straight through exhaust (no cats, one straight through muffler), cone filter with a very effective cold air box.

What I read was that the air flow meter on the series 4 (86-88) NAs can only compensate or change the fuel delivery to a point and that even with just the "breathing" mods that I have I am going to run lean.

Is this true? I thought that our cars were supposed to run really really rich stock and that a few breathing mods would be just fine and the fuel delivery would still be adequate for a stock ported engine.

Recently I changed my spark plugs and then took them out a couple days later to see what color they were to see whether I was running lean or rich and they had a touch of dark tan on them and a little bit of light tan.

Should I be worried? Is the air flow meter really limited in it's ability to change fuel delivery? If so, would I see a large gain in hp by purchasing an s-afc or standalone ecu system? If going standalone or s-afc and richening up my ratio would getting 550cc secondaries help a lot due to a lower duty cycle at higher rpms(making the injectors more efficient at higher rpms)?

Thanks for the help guys...the info that I had read really got me thinkin hehe.

-Lee

J-Rat

iTrader: (2)

Plugs that color are fine..

Jarrett

mazdaspeed7

Dont be worried. Besides, the running lean thing is a myth. A 6 port motor bridged on all 6 ports didnt max out a S4 n/a AFM. S5's are another story, the n/a afm cant really support more than a small to moderate SP with all the bolt-ons. But you cant max out the stock afm on a stock port motor without forced induction.

Capn' Wankel

Thanks mazdaspeed7.

SureShot

I had assumed the S5 NA & turbo used the same AFM, which left some headroom for the NA?

Who knows for sure?

mazdaspeed7

Wrong. The S5 TII AFM has a stiffer spring, and a different shaped cone. It is NOT interchangable with a S5 n/a afm, or vice versa.

JerryLH3

iTrader: (11)

Pretty rich. On my stock (stock!) dyno run last year:

3k-6k RPMs A/F hovered around 13:1
By 6.5k RPMs it was 12:1
At 8k RPMs it was 11:1

The stock computer and injectors can handle just about anything besides forced induction and engine porting.

I have heard of one case where an FC was getting lean with just bolt ons, but I think that was an isolated incident.

Evil Aviator

The AFM itself is not the problem, but rather the stock ECU maps which have less accurate correlation as the engine's volumetric efficiency changes from stock levels, and especially when the vacuum signal changes with porting. This Bonneville record-setter had about 350bhp on the stock S4 TII AFM with adjusted spring tension and various piggyback add-ons.
http://www.geocities.com/evilaviator/bonneville

The spark plug reading isn't very accurate unless you check the plugs immediately after running under load, and even then it's rather subjective. Basically, light brown = normal, chalky white or satin sheen or blue = lean, pepper specks = detonation, dark gray or black with black soot = rich.

You can get a narrow-band O2 AFR meter for $50-100. Also, you can get your car dynoed with wideband O2 datalogging for about $20-50. Both options are much cheaper than assuming that you are running lean, and going out and buying all new EMS and fuel system components just because of something you read on the internet.
http://www.alltrac.net/tuning/afgauge.html
http://www.markvanderkwaak.com/dbbp/tech-tips/mm.html
http://techedge.com.au/vehicle/wbo2/default.htm

PPC-Racing

.. on my S5 NA with the mazda trix SP, I've set the Super-AFR to take off almost 20% pulse width 1000 rpm to 4500 and 10% to 7500 from OEM settings..