FAST, Exhaust leaks, 02 Sensor, Rough Running

I'm running a Roush 427IR with eight stack injection and megaphones. Thought I'd share something that others might be experiencing...

Ever since I got my car on the road last year, I felt it was a bit of a challenge to drive - mostly in a good way. But there has always been a engine lugging, chugging, pulsing, and general unhappiness below 2200RPMs. I lived with it for a while because I thought it was just the nature of the beast. After driving it for a year I finally decided that a 568hp engine with 540 ftlbs torque in a 2400 lb car simply should not lug/chug/pulse as much as mine did. I'm not the world's greatest driver, but this car should simply not be that hard to drive well...

Called up Roush. They had Chad, their FAST expert in town last week and asked me to bring the car over (the advantage of going with a local engine builder!). Chad hooked up his laptop to my car and off we went for a drive. Through his wizardry, he was able to smooth out the chugging/lugging live as it was happening. It was amusing driving around Livonia at 20MPH intentionally trying to get my car to lug. Apologies to all the drivers behind me that thought me an idiot. (No hazard flashers on a GT40 that I could find). After about 20 minutes of doing everything I could to drive badly, Chad had all the rough spots smoothed out. How he managed to sit sideways, work intently on his laptop in a closed cramped cabin and not spew cookies as a result of my intentional bad driving is a talent equal to his skill with the FAST system.

Apparently the design of our collectors and my megaphones are the culprit. The leaky fittings allow fresh air to hit the O2 sensor at lower RPMs. When the O2 sensor smells fresh air, it apparently responds by dumping a ton of fuel into the system. Above 2200RPMs the engine is pushing out enough exhaust to keep the fresh air out of the system, but below 2200RPMs was the problem. Chad managed to override/change the FAST system's reaction to fresh air below 2200RPMs and made a few other minor tweaks.

I was also getting occasional stalls when transitioning quickly from higher RPMs to zero throttle when approaching a stop sign or turn. A quick tweak of the throttle plates corrected that problem. Apparently the throttle plates are set somewhere between 20-25 degrees. Opening them a touch solves the stall problem. Apparently more of an issue with cars that are always running the AC.

The result is a gloriously more livable GT40! The only downsides are it does not snap/pop/backfire on downshifts like is used to. The rough running below 2200RPMs also made the motor sound a bit hairier than it really is :) It was unexpected when I first drove my car to hear the occasional backfires on downshifts - did not think a EFI car would do that. Apparently now I know why it did. I wonder if I can get Chad to put that backfire back into the program while preserving driveability :)

A thank you to Mark, John, and Chad at Roush for getting things sorted so quickly and easily!

- Jeff
I had the same problem with my FAST system on my 408 Windsor with 8 stack. I found Bob Kurgan of Kurgan Motorsports. He just moved south to Gainesville from Chicago. He also is a wizard with the FAST system. Mine was surging something awful above 2000 rpm unless I was accelerating. Bob is not the most trim of guys and getting him into my 40 was a bit of a squeeze. After 30 minutes of driving in second and third gear at 2200 RPMs he got it singing like a bird. He told me that my backfiring was due to exhaust leaks which I can fix. If you liked the performance of the tune back when it was surging, all you have to do is go into the FAST software and select the tune just before the last one on the page. You will have the old performance back(if you want). When he tuned mine the outside temp was 20 degrees F. He still owes me a final tune and a dyno session. Occasional spark knocking. Wanted to wait til the temps were where I would normally be driving. Minor hiccups with the car other than tune have held me up.

I need a button on the shifter to induce backfire and a hairy rough idle on demand :) I'd call it the Cars and Coffee button.

- Jeff
It sounds like your headers and collectors were designed for a carbureted system. For EFI, use the double-slip joint collectors to seal the header/collector interface. Depending on where you mount your sensor, the reversion issue may still be a factor, but may give you the effect you're looking for after all...

Megasquirt ECU's and I'm sure others allow you to change the tune files on the fly, i.e. pump gas/race gas/E85 and in your case, Cars and Coffee (Really??)...

Yeah, EFI can tame the most gnarly of engines. Mine had a radical sounding idle, till the adaptive learning phase took over...
Forgot to mention that I also use a V-band clamp between the collector and megaphone. I had Stainless Headers, when they were a supporting vendor here, fabricate mine and I'm very pleased with the quality, look, and sound of this system.

From their cover page, as an example:


The recalibration work by Roush does not seem to have completely solved the lugging/pulsing I get likely as a result of the leaking exhaust fittings. I took apart the slip fits and put it all back together using Permatex Copper gasket sealer. That seems to have done the trick. Question is how long will the solution last. And I wonder if there is even more to gain by even better sealing. I've got some inquiries into Roush to see what else we can do. Maybe further disable O2 sensor sensitivity below a certain RPM range, or maybe relocate the O2 sensor to one of the header tubes (of course not ideal to just be sniffing one cylinder instead of four). I spent $$$$$ on my current polished exhaust set up, so would like to try to make it work. I also wonder if the Megaphone set up is so free flowing that this will be more of a problem. Certainly with my car it became more of an issue after I put them on. Likely the stock exhaust had just enough back pressure to keep fresh air out of the system. Input welcome!

- Jeff
Some things should be noted:

With larger cams that run choppy at lower RPM's, you can actually get some raw fuel hitting the O2 sensor at those lower RPM's. You would think it would read rich in this situation, but it actually reads lean. This is then compounded if the computer is in 'closed loop' which then tries to compensate by adding even more fuel making the condition worse. I actually dont use the o2 sensor readings when i idle my engine because its simply not accurate with the cam i use. Only when i pass about 2k rpm do I start taking the o2 readings into account.

Its not hard to induce afterfires (the crackle and pops) in the exhaust with an EFI system. But it sure wastes a lot of fuel and makes your exhaust really hot. In the cells that are used off throttle, (the highest vacuum cells) you simply retard the timing to something like 10° or 15° ATDC and set your pulse widths to dump fuel in. On my car, which has an HEI ignition, i always worried about how much retard I can give it before it ignites the wrong post in the dizzy. But even at 0° i was getting the effect, but it was making my RPM's come down slowly because it was 'driving' the engine. Less timing should reduce this effect. Currently, I just cut the fuel completely off throttle, this makes the RPM's fall like a bomb and increases how quickly I can shift. This is because the RPM's of the engine are closer to what the trans is requiring for the next gear.
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That shutting the fuel might work on the track, but I don't shift that fast when not on the track. I would hate to think of what that does to the drive train on the street. The crackling and popping of the exhaust "usually" comes from a leaky exhaust(what I was told by my tuner). The way I see it, with closed butterflys and higher than idle rpm, there would be a decreased atmospheric pressure in the system, which would imply that there is air getting sucked into the exhaust(the leaks) and giving the ECU a drop in AFR. The ECU tries to make it up by dumping more fuel into the cylinders, with closed throttle the timing would drop from 30 or so degrees to about 18 or so, probably getting some of the raw fuel into the exhaust as it doesn't have time to ignite it all in the cylinder. The exhaust already hot from the increased rpms will surely ignite that fuel(my exhaust starts to glow above 2K rpm). That is the way I see it. Right or wrong??? I have the slip fits you are talking abut on my stainless exhaust. I try to make sure the facing flanges and the slip fit itself has plenty of the copper gasket sealer. Putting the pipes together by myself is not an easy task.
I am having a problem seeing how raw fuel on the sensor gives a lean number to the ECU. Help me out here. Is it because the sensor can't find any O2 with the fuel on the sensor?? Given that is so, it would tell me that the ECU would cut the fuel load because the AFR would be very high??? That is how my anesthesia machine works anyway.

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Bill, I don't want to be offensive or a to be a jerk, but your tuner is wrong. The after fires still happen with no leaks on the exhaust system, even if you cut the fuel completely like what I do. I do not have a very high opinion of tuners and i have yet to meet one thats not a flake. I'm not saying their aren't good guys out there, just that I haven't met one. But the reason for after fires in this situation is because some fuel still lingers in the intake on a quick transition from on to off throttle, and there is enough air that gets past the idle valve or there is already enough in the system to cause the after fire. I believe this is more pronounced on engines with a wet flow intake manifold.

With most cars that have a lot of drama out the exhaust, this is done totally through tuning. If I recall correctly, the 458 ferrari recently had a new software update. A lot of the owners were disappointed because the new tune reduced the "drama". They did not fix any leaks.

From my understanding, it's not that the o2 sensor reads raw fuel, it's that it doesn't take into account that raw fuel is even present. Let's say you have a cylinder that goes out and a ton of raw fuel enters the exhaust. Well, there was also a lot of air that entered the exhaust that was supposed to burn that fuel, but it didn't. The o2 sensor doesn't read the raw fuel, but it does read all that fresh air. The result is that your o2 gauge spikes lean and the computer compensates by adding more fuel... which probably isn't helping at that point.

With a big cam that has a lot of overlap, there is so much turbulence in the intake and incomplete combustion going on at idle that the o2 sensor just doesn't get a consistant sample of exhaust. Not only does this cause a jumpy o2 reading, but the readings you do get are just not accurate. The engine might read lean because it can't combust the fuel due to conditions at idle not being quite right. Most people see this and then add even more fuel which hurts combustion further resulting in an even more lean reading. It's a vicious cycle...