Best A/F Ratio

[proggod]

Whats the best A/F ratio for a supercharged 2005 viper. I have heard different things, from 12.0 to 12.8...

 

[HP]

at WOT 12.0 would be safer

 

[Joseph Dell]

every car is different but 12.0 is the LEANEST you'd want a supercharged viper to run when on a dyno. 12.8 is pushing it. a lot.

11.5 is usually the target for a SC car. much, much safer.

JD

 

[plumcrazy]

11.5 is what my tuner told me to shoot for....

 

[Viper X]

Good question.

This one really depends on the car, your boost, how you will use it and the climate you will use it in. You can also help a bunch by installing a larger capacity "intercooler" with a larger holding tank and pump. Higher boost and more hp will cause additional cooling issues and will usually require a richer mixture.

I've gone through two engines now due to poor tuning and resulting detonation. Lots of trial and error. Too much error.

I can tell you that intake air temps climb progressively the longer you run the car at WOT under boost. If you run the 1/4 in cool weather with a basic Paxton unit on a stock bottom end and cool the car between runs, you have very little to worry about. You could get away with 12.0 to 1. I wouldn't do it, but you could.

If you run the 1/2 mile or mile, run higher boost, on a warm day, don't cool the car properly between runs, etc., you'd better be at 11.0 or even high 10s if you want your engine to survive and I'm talking an all forged engine.

Vipers don't seem to mind an Air/Fuel mixture that's a bit rich. They don't like lean. Not at all.

Lots of variables.

Dan

 

[Jack B]

Good question.

This one really depends on the car, your boost, how you will use it and the climate you will use it in. You can also help a bunch by installing a larger capacity "intercooler" with a larger holding tank and pump. Higher boost and more hp will cause additional cooling issues and will usually require a richer mixture.

I've gone through two engines now due to poor tuning and resulting detonation. Lots of trial and error. Too much error.

I can tell you that intake air temps climb progressively the longer you run the car at WOT under boost. If you run the 1/4 in cool weather with a basic Paxton unit on a stock bottom end and cool the car between runs, you have very little to worry about. You could get away with 12.0 to 1. I wouldn't do it, but you could.

If you run the 1/2 mile or mile, run higher boost, on a warm day, don't cool the car properly between runs, etc., you'd better be at 11.0 or even high 10s if you want your engine to survive and I'm talking an all forged engine.

Vipers don't seem to mind an Air/Fuel mixture that's a bit rich. They don't like lean. Not at all.

Lots of variables.

Dan

Just to reinforce your point, that is why the viper pcm adds fuel when the IAT goes up - if you think about it it should be pulling fuel, but, they are protecting the engine and disregarding the correct a/f.

 

[proggod]

i think I am going to shoot for 11.7, after talking to a local tuner here, and the input from the board I think this should be a good safe place to be.

I'll have to keep an eye on the engine to make sure there are no problems.


Anyone have any experience with using the split second computer to tune the fuel mixture who wants to make a couple of bucks helping me out over the net/phone?

I just hade widebands installed and have a pretty good idea how to use the software, but wouldn't mind someone looking over my shoulders before I write to the computer..

 

[mjorgensen Woodhouse]

Just to reinforce your point, that is why the viper pcm adds fuel when the IAT goes up - if you think about it it should be pulling fuel, but, they are protecting the engine and disregarding the correct a/f.

Actually when the AIT increases the fuel is added to protect the "cats" and yes the fuel should cool them but when the flames start shooting you would think the opposite would be true!

We tune along with DC Performance fr an 11:3 a/f through the midrange and then to about 12:0 at 6000 rpm. This actually may help the amount of fuel actually dumped in this senario. Good luck.

 

[Steve 00RT/10]

Actually when the AIT increases the fuel is added to protect the "cats" and yes the fuel should cool them but when the flames start shooting you would think the opposite would be true!

We tune along with DC Performance fr an 11:3 a/f through the midrange and then to about 12:0 at 6000 rpm. This actually may help the amount of fuel actually dumped in this senario. Good luck.

.......always looking to learn a little more. What is the theory for tuning to 11.3 through the midrange and removing some fuel as the RPMs go up? ......as opposed to tuning for one AFR through the entire range? I am a no water/****, 5 pound, cast piston Roe guy. Is the scenario different for a intercooled Paxton?

thanks--Steve

 

[mjorgensen Woodhouse]

I believe it is a bit different but since I'm no expert on the Roe I will decline from comment. We seem to make more power letting it go to 12:0 at the 6000 range and since you don't spend much time there anyway it is still very safe. The midrange needs the help the most though that is why we use 11:3 - 11:5. I would suspect the Roe needs to be richer in the mid range becaus it is pressurizing your motor sooner and more often than the Paxton does, thus creating more heat and potenial for detonation.

Dang I guess I did need to comment.

 

[Steve 00RT/10]

I believe it is a bit different but since I'm no expert on the Roe I will decline from comment. We seem to make more power letting it go to 12:0 at the 6000 range and since you don't spend much time there anyway it is still very safe. The midrange needs the help the most though that is why we use 11:3 - 11:5. I would suspect the Roe needs to be richer in the mid range becaus it is pressurizing your motor sooner and more often than the Paxton does, thus creating more heat and potenial for detonation.

Dang I guess I did need to comment.

Thanks for the input. I sense a little friendly bias For the record, we have 2 Roe blower cars, tuned by me to around 11.6 or so across the entire range, (no dyno in these here parts), and over 22,000 SC miles (17,000 in the last 12 months)....including some road race track events, several autocrosses, and a couple sets of tires up in smoke. I have no regrets for going with the Roe. The 00 car is pushing 70,000 miles.

Steve

 

[Viper X]

In my limited experience, most issues with forced induction Paxton engines, like detonation, seem to occur around peak torque (about 5000 rpm). If you ever get a chance to look at a fuel or timing curve, you'll see that the a/f ratio is "fattest" like 11.0 at this engine speed and then leans out towards the top of the rpm band (6000 rpm), back up to 11.5 or 12.0.

Having run a few 1/2 mile races and longer, I tend to want more fuel at the top than 12.0 to 1. Here's why; when you run through 4 or 5 gears one right after the other at WOT, as soon as you're in the next gear, you're making big torque, pulling like crazy and consuming large amounts of fuel. The kicker is that you're not in these gears very long. Take 2nd to 3rd gear for example. I want as much fuel in the system as I can get (within reason) so that I don't have any lean issues going from 2nd to 3rd or 3rd to 4th.

Also, the engine timing is at it's lowest advance around peak torque rpm, again to try and avoid detonation.

Dan

 

[proggod]

Got my widebands working today, I am running at 9.5. Houston I think I found my problem

Guess I better get dc perf on the phone and ship my car to them.

 

[Dyno Dave]

I've just retired from the SRT group after doing Viper calibration work for the last two series of engines and here is how it is done;

The best A/F ratio to use will be the one that makes the most power, typically between 12.5 - 12.8, WHILE keeping the EGT's under 1600-1650 degrees at the exhaust port or 1725 degrees in the catalyst and having no knock AT STEADY STATE throttle and speed. The typical method is to hold the engine speed steady and hold the throttle wide open until the EGT's stabilize at their peak temperature. This will cover all conditions that the car may see. Is it optimal? No. At the track engine speeds vary constantly and it will be possible to lean the engine down if the EGT's and knock are under control. But from the big picture I don't know if the car will be run at Bonneville or the 1/4 mile so I must calibrate for the worst case.

Knock will tend to be the worst at peak torque because that is when the cylinder pressures are the highest. Elevated intake temperatures and coolant will make the engine knock sooner and more severe. To compensate for this spark timing is pulled back which increases EGT's. Additional fuel is added to lower the exhaust temperatures. The additional fuel for cooling does not raise the exhaust temperature because there is no oxygen left in the exhaust. If there is an air leak due to a cracked exhaust manifold or poor fitting joint then the temps will go even higher.

Peak torque is also when cylinder filling is the highest and from there on up it drops so the engine becomes less knock sensitive at high speeds and can tolerate a leaner A/F.

Reducing exhaust backpressue will often allow leaner A/F's because the heat is carried away quicker and the EGT's don't climb as high.

 

[proggod]

Man dyno dave I wish you were in phoenix, need someone who can tune my car. Should I install a VEC3 in my car to make it easier for the tuners here that can write to the ecm? Can the split second do a decient job of tuning?

I am thinking about sending my car to DC performance, but having to ship it there everytime I get new mods will be a pain.

 

[Dyno Dave]

I can't speak positively or negatively about the aftermarket units. I've never had the need to use one.

 

[GR8_ASP]

Hey Dave. Good to see you posting again. You have a wealth of Viper knowledge and it is good to see it shared.

 

[Viper X]

Hey Dave, I second that.

Proggod, I'd call DC and talk with them about your engine's specifics. I'll bet they can send you a computer with a tune in it and get you close just knowing your mods.

As long as you are using the stock injectors, I think DC can likely tune the stock PCM. You will want them to "custom tune" your car individually at some point. You could also likely have Chris fly out to your location for tuning.

Dan

 

[proggod]

Man they didn't mention the flying out to my location for tuning. Chris is with DC?

I'd do that in a second. I was getting ready to ship my car to them wednesday. But its going to be ashame to be without my car for a couple of weeks. My vette is in the shop getting turned into a race car as well.. :|

 

[Viper X]

Chris is their chief tuner. Very experienced with Paxtons and very knowledgeable along with being a nice guy.

It's worth a try.

Dan

 

[proggod]

Its ok, i have a larger intercooler i want them to install, biger water pump, new oil pan, and a few other things. I guess I'll just ship it down there tommorow and let them have their way with her