F.A.S.T Decelleration question

[foxspy]

Hey guys,

I have a question about my F.A.S.T computer and decelleration. I have the a/f ratio set at 14.5 when I get off the gas, and my VE numbers are around 50, but my actual air fuel is way lean at 15.9, but if I actually add VE numbers to match the 14.5:1 air fuel ratio, the numbers are huge, like 7x.

Also, in neutral, I give it a quick rev and let off, and every now and then it'll make an airy pop, is this due to the lean condition? What should the timing look like on decelleration, more advance, or less? What should I do about the lean VE numbers, should I just correct them even though the numbers match my WOT VE numbers?

Any help is appreciated,

Thanks.

 

[foxspy]

Any suggestions??

 

[bobc455]

General consensus is to not bother spending too much effort tuning the deceleration areas of the map.

Deceleration can be very dynamic- you have high RPM, but barely any exhaust flow, which gives you tons of turbulance. Therefore the O2 sensor doesn't give a very accurate reading.

Basically, just tune it so you don't get any popping or backfiring. 15.94 is fine for an actual A/F reading, even though that's probably not what you are getting.

I don't know what an "airy" pop is, but I guess that you don't have quite enough AE enrichment. I would bump up the curve for AE vs. TPS.

-Bob Cunningham

 

[foxspy]

It's only making the "airy" pop on the rev back down, not on the intial stab of the throttle. Would I still adjust the AE fuel vs TPS? I thought that this table is only for the first intial load on the motor right when you give it gas.

What about the timing, you think not enough, or too much timing could cause the airy pop? It's at 32* right now where it's making the pop, and it doesn't do it all the time.

 

[bobc455]

Where is the pop coming from- under the hood or out the tailpipe?

Describe a bit more about when this popping noise occurs - RPM range, engine load (neutral or coasting after a 1/4 mile run etc.). Still not sure what an "airy" pop is.

-Bob Cunningham

 

[foxspy]

Hello,

The "airy" pop is coming out the tailpipe, coming down from 2200 to 1500 rpm while the engine is in neutral, 26-32 kpa. The engine makes occasional pops while in drive when I'm going about 3500rpm and let off, it makes slight pops while it's coming back down the rpm range.

I've tried drastic changes in the VE numbers, from 25-75, and I still get the same results. Maybe I just need to add more fuel?? Maybe it's a timing issue?? My timing is at 33* when it's making the pops. I'm not real familiar on what the timing should be on decelleration, more advanced than WOT, or what. I have a lot to learn.

You guys have been a huge help so far, I thank you guys greatly. This is a great site, and has very knowledgable people.

 

[Rickerbucks]

I have EXACTLY the same condition and I have not been able to get rid of it. I hate it and I'm looking for a solution. I have the VE numbers at 75 on row 2 and row 3 - I have a 2 bar Map, basically 48 kpa and lower. My AFR says about 12 - 13 - although Bob said that these results are unreliable.

My worst situation is at 1700 rpms in 4, and 5 where I'm just taking up the slack in the throttle- not accel nor decel - and it pops from the tail pipe. My timing is at 36 degrees. If I'm at about 1300 - 1500 rpms its fine.

I too am seeking out advice and ideas.

 

[bobc455]

Well it's just a guess, but it seems like you are still combusting something when the exhaust valve opens. This could be either because of late timing (doesn't sound like this is the problem) or too rich (I guess this is the problem).

I think my VE numbers for deceleration are in the high 20's and low 30's.

I think the best way to tell this would be to look at your injector PW (ms)- should be about the same as when you idle. The duty cycle % will be slightly higher because of the higher RPMs, so I would look at injector PW instead.

-Bob Cunningham

 

[foxspy]

Thanks,

I'll give that a shot.

 

[Craig Smith]

Two things:

One is that when the throttle is closed, there is very little air going through the engine. From this fact we can say that the fuel requirements are extremely small at best. I have been able to cure "popping on decel" problems many times in the past by lowering the fuel delivery in the affected area. This can be done in either the VE table or the target a/f table.

Another thing that frequently helps is to advance timing in the affected area as well. It helps to complete the burn before the exhaust valve opens. Decel conditions are generally very tolerant of lots of timing.

 

[Rickerbucks]

TY Craig. I must say that when I reduced the fuel to about 10 - 25 in the affected areas, the popping was positively worse. BUT, I did not adjust the timing so maybe.......

BUT........My 6 year old DELL laptop just had the backlight blow yesterday, so now ANOTHER expense. Probably buy a new one.