Kenny H

A guy I know got his '05 GTO tuned and he is thrilled with his dyno numbers. The car picked up a solid 20 RWHP throughout the RPM band and now makes ~ 370 RWHP.

But when I looked at the AF ratios, below 3k the car is 14:1 and then gradually it richens up to 13.5 : 1 right up to fuel shut off. The car has the usual bolt-ons, stock internals. I think thats a bit too lean, what do you guys think?

foff667

iTrader: (2)

was the reading a tailpipe sniffer through cats? I assume the car is tuned?

Rob

iTrader: (6)

If its a tailpipe wide band, sounds perfectly fine.

Zick

iTrader: (18)

Aren't the tail pipe sniffers calibrated so they show the correct AFR?

ringram

Impossible as it would depend on exhaust diameter, gas velocity, rpm, etc.
It reads leaner due to atmospheric O2 reversion into the pipe.

Ive been told its a rough 0.5 AFR, though it will depends as discussed.

Zick

iTrader: (18)

Hmmm, that sounds about right then. When I got my truck dyno'd, the dyno sniffer was reading about .5-.8 leaner than what my LC-1 was reading. I figured I just didn't have the WB calibrated correctly but maybe this is why.

KLHAMMETT

iTrader: (7)

I find the same thing tuning car with F.A.S.T systems,My 02 is usually .5-.7 richer than the dyno shows,So if its naturally asperated id say its fine

WS6FirebirdTA00

iTrader: (33)

yeah in the tail pipe is no good, if it wasnt in the tail pipe its a little too lean i think to be safe. i run mine in the low 13's

YellowToy/A

iTrader: (1)

Lean is hot. There was a thread not to long ago that talked about ARF. I know i will make mine mid to high 12. I do not think that the 5 HP is worth a broke engine.

SSnakekiller

iTrader: (8)

13.5-13 is a little on the lean side and may introduce unwanted knock retard during actual driving conditions. I say make it at least 13.2 across the board, and add some spark (no more than 29* MAX).

WS6FirebirdTA00

iTrader: (33)

i wouldnt just add spark, that is something that needs to be determined and not just added, lots of guys run only 26*, if u are gonna run higher you run into the same problems as running it lean

Chris81

iTrader: (1)

Mine runs 13.2 - 13.5 WOT on the street/track with no problems, ever. I've also seen other cars that high as well with no issues..

Actually, I was reading the EFI book by Ben Strader (founder of EFI University) the other day (I have it) and he was even talking about N/A motors liking their AFR at 13.5 as well.

Alvin

iTrader: (2)

Check the graph again.. Its from 12-14. The car was about 13.2 from 3500rpm to 4800 where it went to about 13.0.


Theres nothing wrong at all with a tail pipe sniffer. We had the privlage to visit Hendrick racing's dyno and they are using the same tail pipe setup with the same winpep 7 software. I've even had cars with cutouts run with the cut out closed and open with the sniffer still in the tail pipe and you get the same reading at wide open. part throttle is skewed bad but once you get some load across the engine it falls right back inline. I can offer up dynosheets and runviewer files to support this claim if anyone is interested.


I think you would be surpized if you tested a LC1 against a dynojet wideband. If I have a innovative wideband I have to shoot about .8 fatter to get what I'm really shooting for on a dynojet wideband.. I have a twin tech, tech edge, and PLX widebands and I've also compared the AEM while on the dyno and I have found that the twin tech and tech edge to be more consistant with what the dynojet is reporting. The AEM was pretty damn close but absoulutely terrible at part throttle/ no load (sensor was in the downpipe of a turbo LS1). The innovative seems opposite.. pretty close at part throttle/low load and almost a point off at wide open.

So who is the winner? Who is more accurate?

Well, are you tuning for a number or are you tuning for the most power and torque. Considering how many cars i've had on our dynojet dyno and how consistant my power is falling inline with AFRs between 12.9 and 13.3 which in reality isn't that big of a gap.... I trust it. After I find the timing the car likes i know that when I get into that AFR range i'm about 3-4 hp of being done.


Too many guys get hung up on a number. What are you tuning for? 28 degrees of timing and 13.0 AFR or are you tuning for the best performance? Are you tuning for a number or a result?

Alvin

iTrader: (2)

We notice the same thing.. I have a venturi style tail pipe sniffer for my LC1 and in fact I was on the dyno in LA with a innovative dealer who flew me in to help tune a monster of a car for him.. He was praising how nice and everything they where. When it came down to tuning his car which had a innovative mounted in the header we found the same typical thing... .5-.8 richer which was a surprize to him but not me.

WS6FirebirdTA00

iTrader: (33)

i wish more people would listen to this though, i know a lot of people that dont take into account the difference, esp if you have an exhaust leak, the number car be even more off

Alvin

iTrader: (2)

Exhaust leaks don't show that much (if any) at wide open. How do I know? Put a dynojet wideband in the tail pipe of a car.. run it with the cutout open then with it closed.. The only difference you'll see is the part throttle stuff is way off.. as soon as you floor it it comes right inline with what its supposed to.

What can happen is the exhaust leaks before the NB's can drive the part throttle fuel trims hard in one way (lean) and trims carry over to the WOT stuff.. Remeber that lean fuel trims can carry over (pcm can add fuel during wot) there are no rich fuel trims at wide open.

WS6FirebirdTA00

iTrader: (33)

i just assumed leaks woudl cause that cause my tail pipe readings were way off and i had a leak, but i think that was at idle and part throttle now that i think of it

Chris81

iTrader: (1)

On my car, the dynojet wideband was within .2 (lean) of my LC1 through my cutout.. once the cutout was closed and the wideband was put in the tailpipe.. the AFR on my LC1 stayed the same but the dynojet wideband jumped to .8 to 1.00 lean.

Why would this happen?

Zick

iTrader: (18)

Do you still have CATs? If so, then once you closed your cutouts the exhaust was going through the CATs and burning off unburt fuel and now the tailpipe sniffer is reading leaner.

Chris81

iTrader: (1)

No cats.. just runs though the Hooker muffler and out the tail pipes.

My LC1 reads identical, the DynoJet WB reads way lean.