Anyone run high-flow Cats to reduce hydrocarbon exhaust smell?

[jbc426]

Now that makes sense, if its misfiring, then you would get the extra oxygen to the o2 and the extra fuel for your cat............. (or nose without it.) Your misfire counter is only counting how many time your spark plug did not fire. Not how many time you're cyl didn't make a BANG. SPARK! NO BANG, extra oxygen in the exhaust.

Your assumption on how the ECM detects misfires is incorrect. It does NOT just count the number of times the spark plug does not fire. It is much more sophisticated than that and vastly more accurate at detecting actual misfires/ incomplete combustion events.

Reference the following

 

[magnummopar]

running an open loop EFI,

If you are running open loop, the pcm ignores O2 readings and runs off programmed parameters instead. You want closed loop operation so O2 has control over fuel trim.

 

[jbc426]

If you are running open loop, the pcm ignores O2 readings and runs off programmed parameters instead. You want closed loop operation so O2 has control over fuel trim.

My bad. I got it backwards. Thanks It is running with O2 feedback.

 

[magnummopar]

Thats good!!!

 

[Cudafever]

Is it reading timing off of a distributor or a crank sensor? If it a crank sensor then....ok
If it is reading it off the pick up of the distributor, it's not capable of that kind of accuracy.

Not trying to argue with you just trying to figure it out in my own mind how we get all the extra oxygen in at idle with a high duration cam.
I have said the same as you, that the overlap is why. But now i just don't agree. it's not going from the intake thru the exh at idle, it just not. So now i'm trying to theorise how how it's all happening.

 

[MopaR&D]

Your assumption on how the ECM detects misfires is incorrect. It does NOT just count the number of times the spark plug does not fire. It is much more sophisticated than that and vastly more accurate at detecting actual misfires/ incomplete combustion events.

Reference the following

View attachment 1714969838

Dang that's sophisticated! Well there goes that idea...

 

[magnummopar]

Factory magnum MPFI uses a sensor in distributor and a crank sensor to determine both crank speed, cylinder firing and when to time injector firing, the PCM figures out timing as the distributor has neither centrifical or mechanical advance....if you turn distributor it doesnt change the timing, just cam sync.

 

[jbc426]

Is it reading timing off of a distributor or a crank sensor? If it a crank sensor then....ok
If it is reading it off the pick up of the distributor, it's not capable of that kind of accuracy.

Not trying to argue with you just trying to figure it out in my own mind how we get all the extra oxygen in at idle with a high duration cam.
I have said the same as you, that the overlap is why. But now i just don't agree. it's not going from the intake thru the exh at idle, it just not. So now i'm trying to theorise how how it's all happening.

I understand that you think there is a high pressure at the exhaust valve during overlap, but there is actually the opposite. It may help to visualize the exhaust scavenging that is going on as the exhaust valve is on its closing ramp. The high pressure spike has pulsed down the header tube creating a low pressure area behind it. The old objects in motion tend to stay in motion theory helps draw the intake charge into the cylinder, and a small portion of the intake charge goes out with the tail end of the exhaust charge. The exhaust is always drawing small amounts of the intake charge down the header tube just as the exhaust valve closes on high overlap cams.

 

[jbc426]

Factory magnum MPFI uses a sensor in distributor and a crank sensor to determine both crank speed, cylinder firing and when to time injector firing, the PCM figures out timing as the distributor has neither centrifical or mechanical advance....if you turn distributor it doesnt change the timing, just cam sync.

And, the cam synch is what tells the injectors when to fire in relation to the valve opening event.

 

[T-N-T]

And, the cam synch is what tells the injectors when to fire in relation to the valve opening event.

Actually it only tells the PCM/ECM the camshaft position in relation to the crankshaft. The PCM/ECM utilizes this reading along with other sensor readings to determine spark and fuel control.

 

[magnummopar]

and when to time injector firing,

 

[Cudafever]

I understand that you think there is a high pressure at the exhaust valve during overlap, but there is actually the opposite. It may help to visualize the exhaust scavenging that is going on as the exhaust valve is on its closing ramp. The high pressure spike has pulsed down the header tube creating a low pressure area behind it. The old objects in motion tend to stay in motion theory helps draw the intake charge into the cylinder, and a small portion of the intake charge goes out with the tail end of the exhaust charge. The exhaust is always drawing small amounts of the intake charge down the header tube just as the exhaust valve closes on high overlap cams.

If that's how it gets there them that make sense. But i just don't buy it. Now at High speed, yes! Not only is there enough velocity to pull some of the fuel across but there is intake pressure pulses to help push as well. At idle i just don't think so.

 

[jbc426]

Actually it only tells the PCM/ECM the camshaft position in relation to the crankshaft. The PCM/ECM utilizes this reading along with other sensor readings to determine spark and fuel control.

That's correct, but by turning the distributor one way or the other you can alter the fuel synch setting. That's another way of saying adjusting the timing of the injector pulse in relation to the cam position. Cams that are ground for higher than stock rpm can benefit from increasing the fuel synch reading up to double the factory fuel synch setting. It's a well documented performance tweek. However as stated earlier, moving the distributor on these set-up does nothing to affect the timing or fuel volume per injector pulse.

 

[jbc426]

If that's how it gets there them that make sense. But i just don't buy it. Now at High speed, yes! Not only is there enough velocity to pull some of the fuel across but there is intake pressure pulses to help push as well. At idle i just don't think so.

Even at a 750 rpm idle the piston is going up and down 12 1/2 times a second. That's a high enough speed to introduce a bit of intake charge into the exhaust with a high overlap cam, but it's nothing compared to the amount of times the piston cycles up and down at 6700 rpm, which equals 111 2/3 times a second.

Of course the event we are talking about only occurs at half that rate. It is a four cycle after all. My point is that there are violent forces at work even at idle.

 

[MopaR&D]

Even at a 750 rpm idle the piston is going up and down 12 1/2 times a second. That's a high enough speed to introduce a bit of intake charge into the exhaust with a high overlap cam, but it's nothing compared to the amount of times the piston cycles up and down at 6700 rpm, which equals 111 2/3 times a second.

Of course the event we are talking about only occurs at half that rate. It is a four cycle after all. My point is that there are violent forces at work even at idle.

I'm not sure I agree with that, the fact that the engine is throttled at idle and there is vacuum on the intake side causes reversion of the intake charge which most everyone is familiar with... it's a common occurrence with engines that have high-overlap cams to get carbon buildup in the intake ports from burned gases escaping back through the intake valve before it shuts. The absolute speed of the piston is only important with direct relation to the cam timing events; 12 1/2 times a second seems fast but not if you have an ICA of 70+ degrees past BDC, the gas charge is essentially bouncing back and forth instead of flowing completely from intake to exhaust.

 

[magnummopar]

We often set sync at 5+ degrees, which is 5x factory...

 

[T-N-T]

That's correct, but by turning the distributor one way or the other you can alter the fuel synch setting. That's another way of saying adjusting the timing of the injector pulse in relation to the cam position. Cams that are ground for higher than stock rpm can benefit from increasing the fuel synch reading up to double the factory fuel synch setting. It's a well documented performance tweek. However as stated earlier, moving the distributor on these set-up does nothing to affect the timing or fuel volume per injector pulse.

I agree. It is common to set it at about +4-5 degrees. I did this quite often to stock trucks. It has a nice effect on power especially on engines that have a larger cam. I used to check it and fine tune it if needed when I did a tune up or driveability work. They even recommended advancing it in factory training.

 

[jbc426]

I'm not sure I agree with that, the fact that the engine is throttled at idle and there is vacuum on the intake side causes reversion of the intake charge which most everyone is familiar with... it's a common occurrence with engines that have high-overlap cams to get carbon buildup in the intake ports from burned gases escaping back through the intake valve before it shuts. The absolute speed of the piston is only important with direct relation to the cam timing events; 12 1/2 times a second seems fast but not if you have an ICA of 70+ degrees past BDC, the gas charge is essentially bouncing back and forth instead of flowing completely from intake to exhaust.

The vacuum is created in the cylinder and passed into the intake tract below the throttle blades when the intake valve is open. Vacuum does not cause reversion. Even sound waves have an effect on fuel air flow, but we are getting out in left field. It is what it was is. Maybe Andy F, Dwayne Porter, or Dave Hughes could explain it better, but I'm satisfied that the cats I installed have eliminated the hydrocarbon smell at idle that resulted from my camshaft's design specs. It is not the result of misfire, too rich an A/F ratio or even reversion; it's the overlap. Period.

 

[Cudafever]

I guess we'll just have to agree to disagree.
You are correct! The cat installation fixed your exhaust smell, which has to do with rich unburned/ partially burned fuel, that you cat is currently burning for you. Not the extra Oxygen in the exhaust stream......ok it's using some of that too.

 

[AJ/FormS]

FWIW,
I ran the 292/508/108 cam in my 367 at 11.3Scr.With 1.6 arms the lift was closer to .540. This cam in this small cuber did not like to idle very low with a 750DP-AG and TTIs. But with the correct or reasonable advance, it would free idle at 700, and I could pull it down to a bit under 500 with the clutch, and it would pull itself around a flat level parking lot with a 10.97 starter gear. She produced none of that eye-burning exhaust stink at all with the 76* of overlap.And no MSD either.And on 87E10.
The cure With these 250@050 and smaller cams is almost always in the T-port sync/timing.With maybe a little bit of bleed work. My cylinder pressure was around 175. But I run a minimum water temp of 205*.My squish was .022 +/-.002. I ignore the AFRs at idle giving the engine what she wants/needs to make her happy.
-I have come across this eye-burning stink in many engines when the T-port was out to lunch because the owner was scared to crank in some timing. About 5 minutes later it would all be gone. Unless the pistons were too far down in the hole. Then the lack of squish wants a bit more rpm. And if the TC is too tight, it comes back when put in gear.
-Sometimes if I don't start the fire early enough, the charge is still burning when it leaves the chamber.If it finds fresh air there it messes up the O2 reading. If you start the fire too early it works against the crank,and the idle has to be raised to compensate, and maybe not all the burned charge wants to leave. This is where an old fashioned dizzy comes in handy; you can just tug it this way and that and let the engine tell you what it wants. But you have to be careful cuz occasionally they will try to fool you.
-A lot of times the TCs are just too tight. Jack up the rear, put it into gear and ride the brakes until the stink comes up. There's your proof. More timing and less throttle may help. But it can initiate a hesitation, so then you work on pumpshot, and transitions. In the end a looser TC is probably a better solution.
-I wasn't paying attention to what heads you are using, but the lack of swirl in the factory LA open chamber heads seems to aggravate this stink
-The problem is not the late closing intake. Nor the overlap. Unless you are trying to idle it way too slowly, and your header pipes are pulling unbelievably hard, which I can't see with a full exhaust (I have full 3 inch duals with Dynomaxers)..You are probably idling a little lean, too, as others have mentioned.
-I work exclusively with carbed engines, so, I always attack the T-port sync first. Smaller cams seem to have less of a problem with this. I always figure it's a rich idle;possibly due to lack of T-port sync, and/or lack of timing; and that's how I get rid of it.Smaller cams don't need as much correction as the 250s.
--With EFI, and quench-heads,it should be a breeze to get rid of it. Not sure what could be bothering your combo, except maybe the cylinder pressure.or the chamber temperature that goes with that hi cylinder pressure.or the chamber shape.
Sorry I couldn't be more helpful