Big cams giving a false lean condition at idle?

[heytonyman]

I'm running Comp Stg 3 blower cams with the idle set around 850rpm's. Tuning with the PRP software on 93 octane. SCT2400 meter, 60's, triple returnless pumps. Motor is a 304ci 4V with race ported heads, Kooks LT's. About 9.3:1 CR. Vortech T-Trim at 10psi until I get a handle on the tuning. Checked and double checked for no vacuum or plumbing leaks. Nothing is getting in without going past the meter.

Been fighting with idle A/F ratio on my setup and wondering if other guys have seen the same. The o2's are fresh Bosch sensors. Adaptive is off. Initial STFT's are around .79-.83 at idle. Wideband says it was around 15-16 fluctuating. If I increase the MAF curve to make the o2's happy the wideband is in the high 12's and it certainly smells pig rich and sounds flat. Are the lumpy cams to blame for this? From 2000rpm's on up the STFT's are surprisingly close to 1.0.

This is my first adventure with the PRP package so I hope I'm just missing something in the tune. Started with the SCT 04 Cobra tune, plugged in value files for the SCT MAF, 60lb injectors, 99 cobra IAC and intake volume, and multiplied the displacement for my 304ci. Added 15% o2 delay for the LT headers.

 

[01yellercobra]

Couple things off the top of my head... You might want to ditch the Bosch sensors for Ford replacements. I've heard of issues with aftermarket sensors. I've never used them personally. I've always stuck with OEM stuff. Don't forget you also have to add to the O2 heater duty cycle. I usually add around 15%. The only other thing I can think of is the injector delay. I know that needs to be adjusted for aftermarket cams, but it's not something I've personally messed with.

 

[cj428mach]

I think when you start dealing with cams people run open loop at idle. Then you add/remove fuel to reach maximum vacuum.

 

[03Steve]

Open loop here. Screw the narrowbands.

 

[jpplaw]

Are you bumping up the #/min all across the curve to get more fuel or just for the A/D count range (or voltage range) at idle? Force open loop with adaptive off and find the count range where the AFR seems to be good, then bump up the#/min in the count range below where it seemed good. Sounds like your high count range may be alright.

Can you change your WB to read lambda? That makes the AFR a little easier to gauge and adjust. Using lambda, for example, if it reads 1.2 (which is lean) in a certain count range, you can bump the #/min in that range up 20% and it would get you in the ballpark - because the engine will think its getting 20% more air, the pcm will add 20% more fuel.

 

[heytonyman]

Thanks for the input guys. Adding o2 heater duty cycle is one thing I did not do. Adaptive is turned off. Maybe my understanding of adaptive is incorrect. I thought turning off adaptive would prevent it from registering LTFT's into the trim equation. Will this also prevent short term fuel corrections?

I am logging the idle A/D counts in the lower RPM's along with STFT's and bumping up the voltage #/min in those ranges to add fuel. Looking back thru my logs it may very well be a heater duty cycle issue. After a pull and coming to idle the STFT is close, around .9-1.0 then it slowly drifts down to .7x and flatlines.

I can change the wideband to lambda. It also has a 0-5v output that I'm looking into wiring into the Xcal so I can overlay the wideband with the rest of the data. The wideband is located in the X-Pipe where the downstream o2's were (no cats on my setup) and sampling off of bank 2 only so I'm not certain if I can always rely on it for the low speed tuning. Should I put the wideband in the collector also or will it's reading 2-3 feet further away be accurate enough?

I never thought about playing with the A/F ratio to obtain max vacuum. I'll try toying with that also.

I read the section in the Pro-Racer manual about adding injection delay with large cams. Unfortunately it doesn't give any insight as to what corrections to make. Anyone have experience modifying this table or is it a non-issue with Blower (ie small LSA) cams?

Thanks again for the help guys.

 

[cj428mach]

If you have a big cam with enough overlap that will throw off your AFR at idle. You can keep throwing fuel at it but it will show leaner than it truely is causing you to keep fattening it up. Forcing open loop at idle will keep the pcm from trying to do the same thing.