Best A/F Ratio for NA Engines?

[DrumrBoy]

What's the target A/F for a higher HP but normally aspirated engine?


There are a bunch of posts about A/F in forced induction applications and the consensus there is somewhere between 11.5 and 12.5 to 1 is appropriate (most suggesting 12 or below).

The tuning just completed on mine resulted in good power (555/585) but the A/F is 13:1.
The car has heads/cam and is tuned with a VEC 2.

Lean is powerful but too lean is, well, bad. Thoughts on the best A/F for a mostly street and sometimes road-course application?

 

[DrumrBoy]

Dyno chart if it helps:

 

[britospeed]

I'd rather be closer to 12.8-9. Add a little more fuel and if you lose power you might be able to add a little more timing. If your able to add more timing it'll lean you out, and intern add fuel back untill your about 12.8-12.9 and at the same power as before. Also based on your graph you can lean your map from 5500-6000 a bit. Throwing that much fuel in at those rpm's can cause your egt's to rise.

 

[MTGTS]

11.5 is for boosted apps.

 

[Qualitywires.com]

I would say around 14.4:1 to 16:1 is fine.
There has to be a balance. Too rich can cause detonation with carbon build up and too lean can over heat your engine.

 

[SSSSE YA]

Whos head and cam package?? nice numbers by the way..

 

[GR8_ASP]

The recommendation is the same NA or with boost, as long as you are taking into consideration the IMEP (internal power) of the engine. A 700 hp supercharged engine is probably around the equivalent of a 800 hp NA engine for IMEP. Supercharged and turbocharged engines also need to deal with higher intake temperatures.

I would say in general 12.5:1 maximum. I would not venture above 13:1 unless you are monitioring combustion and can retard spark appropriately. Of course if you can do that you can dial up MBT at each fuel level and find the best power point for both fuel and spark and back spark off from there to properly protect for detonation.

By the way 14.7:1 is stoichometric. Below 14.7 is rich. The comment above about 14 - 16:1 is fodder for the engine morgue.

 

[Russ M]

When my car was being tuned on the dyno we tested all sorts of a/f ratio's and there was very little power gain between 11.9:1 and 13.0:1 we are talking 1-2 hp. The tuner and my self decided that the risk was no where near enough reward to have it lean.

Perhaps if you have a very good set of headers(scavenging) or serious port work done to the heads to justify rewards for a leaner tune then it would be a different case. But far as I know there are no good viper headers out there, at least nothing like LG's for vettes and such.

 

[Joseph Dell]

Everyone is a little correct. A/F requirements are different based on RPM, vaccum/boost (or just vacume in this case), and timing. so it isn't a universal 12.2-12.8 as the books would like you to believe. what about accel vs. decel? 18.0+ is normal on decel. part throttle? anything that doesn't buck (closer to 13.5-14.0). WOT - different. no-load cruise: different.

so it isn't as simple as black and white on this one. sorry to say...

JD

 

[Craig 201 MPH]

The recommendation is the same NA or with boost, as long as you are taking into consideration the IMEP (internal power) of the engine. A 700 hp supercharged engine is probably around the equivalent of a 800 hp NA engine for IMEP. Supercharged and turbocharged engines also need to deal with higher intake temperatures.

I would say in general 12.5:1 maximum. I would not venture above 13:1 unless you are monitioring combustion and can retard spark appropriately. Of course if you can do that you can dial up MBT at each fuel level and find the best power point for both fuel and spark and back spark off from there to properly protect for detonation.

By the way 14.7:1 is stoichometric. Below 14.7 is rich. The comment above about 14 - 16:1 is fodder for the engine morgue.

Great post. AB.... what you talkin about man?? 16:1???? KABLAMMO.

 

[GR8_ASP]

Joseph, I believe all of the comments were in regards to WOT. Part throttle you let the closed loop system do its thing, which is at or near stoich, unless during an extended decel.

 

[Russ M]

I would say around 14.4:1 to 16:1 is fine.
There has to be a balance. Too rich can cause detonation with carbon build up and too lean can over heat your engine.

Lets hope for everyone's sake you don't tune cars in any way shape or form.

 

[F4PHANTOM]

What's the target A/F for a higher HP but normally aspirated engine?


There are a bunch of posts about A/F in forced induction applications and the consensus there is somewhere between 11.5 and 12.5 to 1 is appropriate (most suggesting 12 or below).

The tuning just completed on mine resulted in good power (555/585) but the A/F is 13:1.
The car has heads/cam and is tuned with a VEC 2.

Lean is powerful but too lean is, well, bad. Thoughts on the best A/F for a mostly street and sometimes road-course application?

DrumrBoy,
I have a '98 Venom 650R w/ a VEC 2 also with similar dyno numbers. I'm running a 12:1 A/F and not worrying at all about running as hard as I want for as long as I want. Nothing on the road to worry about especially since I never even see a single stock Viper on the road let alone an S/C or TT.

 

[F4PHANTOM]

I would say around 14.4:1 to 16:1 is fine.
There has to be a balance. Too rich can cause detonation with carbon build up and too lean can over heat your engine.

Gee ab...just because you wrecked your Viper doesn't mean you have to try and get everyone else to blow theirs up. I wouldn't even run those A/Fs in a stock Viper let alone a 650 HP NA one. Stick to spark plug wires and let others with the knowledge to tune correctly set the A/Fs.

 

[Qualitywires.com]

At "idle", my car which was SC ran around 14.7 -15.5:1, under boost it went to around 11.5 to 1. You run rich, you will get carbon build up which can act like charcoal and cause detontation. Maybe I misunderstood the question. I thought the question was about idle.

F4phantom what have you done to make you such the expert? No need to smart off to me.

 

[DrumrBoy]

Thanks for the input folks. Yes its a dynamic situation and one-number-doesn't-fit-all, but on balance it seems like keeping things below 13 is prudent. I'm going to shoot for 12.7 or so and see how that goes.

This weekend I've had a few codes thrown at me, both lean and rich which I assume are thrown at different times under different load situations. I may be too rich at idle and too lean up high at WOT.

I appreciate the input and will report back any further findings.

(the heads/cam are : Lauthan (reportedly flow 290) with a Heffner spec'd Comp Cam). It makes good power, now working on driveability.....

 

[DrumrBoy]

Thanks for the comments folks, I appreciate the input.

I was indeed talking about the WOT part, though my no-load cruise is very balky still. Got some more work to do on driveability but certainly want to ensure no kablammo's on the track!

I think the next step is to get things under 13 (say 12.7 or so) and see how it responds.




BTW - Lauthan did the heads right (one firm - not John H - did them very wrong) and the cam is a Heffner spec Comp Cam.

 

[DrumrBoy]

I know, I know....check "page 2" before assuming what you wrote didn't go through.

I'm a bonehead.