Okay I've searched and have not found what I need to know..maybe I'm not looking in the right place either.

Once and for all is there a "magic" A/F ratio all us LSX guys are looking for and does it matter if its a N/A motor, FI motor or a motor built for nitrous?

Here's my dilema...every LSX motor I've run, weather it was a head/cam 346 or an MTI 422 stroker they have all seemed to run rich. A/F on most tunes around 11-11.5:1. Very safe in my book...

Why exactly, looking at our plugs and combustion chambers, does it show this A/F ratio being so very rich? Is that what is "normal" for these LSX motors, a very sooty combustion chamber and not a nice clean, tan colored chamber and plug??

In order to get that I believe we would have to run 12-12.3:1 A/F...I've heard of a few other FI motors, not LSX, running a 12:1 A/F with no problems...

So what "exactly" would be the proper A/F ratio to reach peak performance and still be a safe tune? And like I mentioned earlier does it really matter if its N/A, FI or NOS...?

I'm at around 11-11.2 and the combustion chamber looks sooty as hell. The plugs look so-so a little on the rich side but not pig rich...

 

Lean is mean! 12.1 is pig rich in my book.

 

No, each motor combo will be different. I have monitored a few engines
using a wideband LM-1 and some motors will make more power closer to 13:1
and some are more at home around 12.2:1

LSx motors tend to like a leaner ratio because of the efficient chamber design.
They don't require as much fuel as compared to a gen 1 iron SB to make
the same power. BSFC numbers are fairly good with a well built engine combo.

The safest answer is, the ratio that gives the best performance values
(dyno/track tune). I truly believe a motor that continues to make power
and is built to handle the RPM is safe. I also believe most motor failures are
due to incorrect timing, motor assembly, clearances, etc. - not because of a tune which yields the most power.

 

from what ive seen 12.5ish for a N/A car
11.1-11.5 FI

also from what ive seen is a car may make more power at 13.1 on the dyno but its slower on the street than a car thats a little fatter

 

Thanks for all the info...!!

 

11.7 is the max I would set your monster at!

 

Forced induction cars are better off being rich. Any n/a car will do fine around 12.5 usually...Most turbo or blower cars seem to do good around 11-11.5...

I'd rather have my car make a little less power and run richer than make more power and run lean.

 

Thats right around where I'll be...I'm looking at 11.5-11.7. I thought 11.0-11.2 was a little on the rich side.

We can mess motor up being overly rich just as well as being lean so...I'll be doing more "plug" tuning than I've been doing lately.

 

Dyno testing is one thing. But, on the street, the A/F ratio will shift. This is based on the load, a non-load dyno will generate vs what you see when the car is actually on the ground.

Too often folks get wrapped up in thinking about an Ls1 like an old school Gen I motor. This is why you see folks make the mistake of trying to put in too much spark lead, etc...

Some of the things folks neglect to check are EGT and BSFC along with AFR. If you take a older non-efficent motor, you often have to run 12:1 to keep the motor alive. While a very efficient motor can run 14:1. Most engines can't run this lean and make their best power curve, but engines that are highly efficient, without any dead areas or fuel air separation spots in cylinder heads, will like to run this lean .

There are no hard fast rules, the motor will tell you what it likes. There is one caveat to this. You can loose power but keep the motor alive with a safe fat tune. You simply leave power on the table.

One thing I also see no one doing is to make their tune fatter at TQ peak, and to roll the A/F leaner as they roll towards HP peak. As an example on an N/A motor going from 12.2 - 12.5 at peak Tq to say 13.5 at peak Hp. You motor needs fuel at peak Tq, but it doesn't need the same ammount of fuel at Peak HP.

Ok, this is a gross generalization, but a nice "safe" number is 11.8-12.2 AFR for a boosted motor. Can you go leaner? Most certainly. You can creep up on leaning the motor down. When you stop seeing power, then back up a few steps. Same with timing. Add timing till you stop seeing gains. As an example, on a stock Z06, there was no HP gain on a dyno from going from 22 to 28 degrees. So, why add timing? All it is is negative work?

You can add timing and lower BSFC.
You can add fuel and lower BSFC.
You can add fuel and raise EGT temps.
You can take away fuel and Lower EGT temps.


See this article:
http://www.maxracesoftware.com/egt_probes.htm

 

on my dyno for my 00 Z28, my A/F was around 12.5 - almost 13.0 ...should i have it set to around 11.7 - 12.0 for a good power gain? only a full bolt on car and a tune.

 

That is the best bit of information in this thread. The final AFR is a product of
what the motor wants, not a specific number that a tuner is shooting for.

 

Jarrod, I need to show you some of my dyno sheets. We've done a lot of tuning recently on a local Mustang dyno and have had great success by giving the motor around 12.8:1 from 2000-4000 rpm, then richening it up to 12.5:1 at peak torque (4500-5400 area) than gradually leaning it out to 13.0-13.2:1 A/F ratio. We've done the same with timing. A little more in the 3500-4500 range, less in the 4500-5400 range, then more toward peak power. For us, it's all about running the most cylinder pressure we can on pump gas and living. More tuning is in order as I want my car to live for those long 1 mile blasts during the upcoming Texas Mile in October. For this kind of tuning, the Mustang dyno is sooooo much better than the Dynojet. WOT pulls take about 8 seconds on the DJ and around 19 seconds on the Mustang. The MD replicates the street a lot better than the DJ.

One final note: The wide band O2 sensor reads much more accurately when placed in a bung just past the header collector. When shoved up the tailpipe, I've seen them read more than a point leaner than they really are. As a result, people that are tuning for 12.5:1 A/F ratio at the tailpipe are actually giving the motor 11.5:1 when read at the header collector. Something to think about.

 

With his car being in SD and not a MAF tune I reconmend tuning it on the street. There no better way to do it IMHO. Ive use a dynojet and I have seen AFR's leaner on the street under load than on the dyno on a WOT. As long as you have a wideband installed in the car tune it on the street. Im using HP tuners with the RTT and the output from the wideband into the scanner

 

I would have to agree with this completely. Different head/cam/carb combo's can result in wide variances in AFR due to efficiency changes(mechanically speaking), weather changes, etc in order to make power. A motor with a 9:1 cr will always run different than a motor with say an 11:1 ratio and a cam change will change it again on top of that. From experience I'd say start out with an AFR of around 12-12.5 and go lean from there until the EGT gets too high or it drops power. There's a fine line between fat and lean. Too fat you lose power(or in the extreme wash the rings), too lean and once again you lose power(and/or burn holes in parts like a torch) If I'm not spraying a car I'll generally run in the neighborhood of 13.1 or so. If it's sprayed hard I like to go a little fatter so I don't go lean and hurt parts. I think the only time I've gone below 12 is on a blower motor though. If a blower motor burps a little black smoke when you first hammer it the thing is happy!!

 

My car is SD tuned as well, but I don't like doing long 4th gear pulls on the street. 7000 rpm in 4th gear for me is over 150mph! 2nd or 3rd gear pulls on the street don't load the motor as well as 4th gear pulls. That's why I use the Mustang dyno. 4th gear pulls on the MD take as long as 4th gear pulls on the street, I can make instant, real-time timing and fuel changes with my EFI Live Roadrunner Emulator, and I never have to worry about speeding tickets or accidents. In my experience, there is no change in A/F ratio going from the Mustang Dyno to the street.

 

fatter at peak torque, lean out until you approach max RPM/max HP then fatten up again. thats how I do it, and with FI setups I aim for normalized 11-11.5, to be safe. theres more power to be had...but not worth the risk on anything but an all out race car.

as people have already mentioned, with N/A you can go leaner in general with no worries. 12-12.5

I also like to run a little more timing when its a little rich. the extra fuel helps prevent detonation, and the extra timing makes up for the loss of power from the slightly rich ratio. I run 23deg with a cam and forced induction, and get no knock.

 

You bet, and also make sure it's before the catalytic.

Some cat systems (3-way cats) inject air into the exhaust to reduce NOx.
Any reading taken after a cat (especially a 3 way is skewed).

 

Great info guys! And just when you think you know it all..Jarrod as usual you've given me a lesson in these new Gen Motor Engineering...

BTW we did the intial tuning on the street...Thank god I live in an area I can do that. 4th gear roll ons need some room for sure. This did cut down on the dyno tuning and time involved.

We did the final tuning on a Mustang Dyno at Conleys. I've always felt the car was too rich and apparently I was correct. Damn I hate to see more HP still waiting to be tapped...

 

Well you can imagin with 600+ in 4th gear

 

Man. Great info here. I never thought about tunning the A/F ratio vs the curve.