[eggersn]

What causes an exhaust backfire when you let off the pedal....the crakling and so on....
reson is that i have a 355 with sportsman II's and XE268 i will be running Hedman 1 5/8 primary headers to a 3" collector
I want to run a dual 3" system with and X-pipe, flowmaster 40's and turndowns at the axle.
I want to know if this will make me crackle and backfire on deceleration.....if reduced backpressure is the cause or would a 2 1/2" system work better for my application
Desktop Dyno says it will hve 395 hp and 415ft/lbs...if this helps.
thanks

[BreathWeapon]

Both my motorcycle and my car do that crackle. I think it is just from a free flowing exhaust like I have on them. I say this because until I switched pipes on the bike, and exhaust on the car, that crackle was never there.

[Eric68]

Usually unspent gas in the exhaust will cause a backfire (like from a cylinder missing). Not sure what you mean by a "crackle" - could be normal. Retarded timing might be something else to check for.

ps. IMO 3" exhaust is too big for your combo - you just don't need it. It will kill some torque and is a little bit of a mismatch for your cam and RPM range. I only run 2-1/2" with mine.

[This message has been edited by Eric68 (edited 05-07-2002).]

[4U 2 NV]

When a car backfires it usually has to do with the timing. We had the same problem last week. We bought a new Distributor and about a week of using it made it go bad. The car would run for a couple of seconds and backfire when it turned off. We swithed the simple stuff first. Coil,Cap,Rotor. Then after no improvement put the old distributor back in and wala it worked. We went later to get another one and seems to be working fine.

[BreathWeapon]

I don't think he is referring to a true "backfire", but rather the crackle that is heard sometimes when you let off the accelerator from around 40 mph. You know, that snapping wind down of the engine as the car slows down? It is usually associated with a high-flow exhaust. The next Harley you see, pay attention to the sound when the guy approaches a stop. You will hear it then no doubt. Some cars do it too, mine inclusive.

[Eric68]

[QUOTE]Originally posted by 4U 2 NV:
[B]When a car backfires it usually has to do with the timing.

I don't agree. I always would test and confirm a problem before replacing parts, especially something major like a distributor. Look at both base timing and total timing in addition to the other suggestions. Again, a cackle on sharp deceleration may be perfectly normal.


[This message has been edited by Eric68 (edited 05-09-2002).]

[eggersn]

thanks for all the help guys
i decided to go with the 2.5 inch system
i was just unsure if the "crackle" was unhealthy or not
i hear some engines on stock pickups camaros and such that have what sounds to be glass packs and they crackle really bad at a shril and almost high pitched crackle (like a big rig hitting the jake brake) and i dont like the sound. I was hoping that it was just the glasspacks and that the rumble of the 40's would be better than that
thanks
nate

[travis]

I too have always been curious to know what causes that deceleration crackle. I too have noticed that the vehicles that do it best (worst?) are mid 70's and up stock V-8's. I don't think the exhaust system has a lot to do with it...I think that the loudness of the glasspacks just amplifies the crackling. My theory is that most of the cars and trucks that you hear doing this are running lean calibrated stock carbs. When you back off the gas, the engine naturally goes lean anyway...and a lean mixture will burn slow. With a lean smog calibrated carb and a semi-free flowing exhaust (stock engine with duals and glasspacks), I think that they go WAY lean and what you are hearing is the lean mixture burning in the exhaust. I have built a lot of engine and run glasspacks on them and have never experienced the crackling deceleration on anything I have ever built...with any type muffler. I don't think even cam timing has much to with it because a friend in high school had a 355 in a truck with a crane 272HMV camand duals with glasspacks and it still crackled like crazy when he decellerated....and he ran the stock '84 305 q-jet.

[davidpozzi]

Travis, how does your friend like the HMV-272?
I put one in our truck and it seems like too much cam, no low end.

If you have an air leak like a loose header bolt, you will get lots of popping and banging on decel. Extra fuel is pulled through the idle circut on deceleration due to very high vaccumn, all you need is air to make it burn in the exhaust system. Just my theory on it.
with injection, the fuel is shut off, - completely, I think.
David

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Check my web page for First Gen Camaro suspension info:
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67 RS 327 original owner. 69 Camaro Vintage Racer, 65 Lola T-70 Chev SB Can-Am Vintage Racer

[travis]

I was definately not impressed by the 272HMV. Granted, his setup was doomed from the start. Stock replacement dished pistons, 76cc 1.72/1.50 truck heads, iron intake, stock 305 q-jet, stock 305 exhaust manifolds, dual 2" pipes with 16" glasspacks, stock HEI, etc. All in a '84 SWB chevy truck with a 2.73 peg leg rear and 255/70-15's. He thought it was great (compared to his old 305, I guess it was), I thought it was a pig. I tried to talk him into a considerably smaller cam but he knew better Now, I knew another guy with a mid '70's nova like mine with a 10.5 compression 327 with 2.02 camel hump's and the 272HMV cam and a 4 speed backed with a 3.42 gear. His car was very strong...it ran 13.5's and was quite tame. A better combo for sure.

David, aren't you the one I suggested the comp cams 260H to for his truck? You should have listened j/k. IMO the 260 high energy is the perfect low compression 350 truck/heavy car cam.

[This message has been edited by travis (edited 05-10-2002).]

[Silver69Camaro]

David Pozzi is right about the backfire. The system becomes too rich due to the vacuum.
Some cars will have a valve that will open up, leaning out the mixture, when high vacuum situations occur (like deacceleration). My '84 Buick has that valve, but it will still 'crackle' just a little. BTW, the 'crackle' he is referring to is a whole bunch of small backfires, so it makes a crackling noise. It's normal.

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1969 Base Camaro
Vortec 355, Perf. RPM, Demon Carb., TH-400
All sheetmetal is NOS GM
See my NEW webpage at: http://www.geocities.com/compuboy007/

[Milan]

The cackle is due to cylinder scavenging and a rich decell mixture mixture.

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67 prostreet BB Camaro
71 BB SS Chevelle (clone)
2002 35th anniv SS Z28 conv.

[stingr69]

The diverter valve on the factory AIR smog system is designed to cut off the flow of air to the exhaust manifolds during high manifold vacuum situations to avoid afterfiring in the exhaust. When they go bad (and they always do) the car will afterfire in the exhaust between gears or on decelleration. The same thing can happen without the smog system as unburned fuel is in the exhaust and all it needs is a little air and it will burn causing the afterfiring.

-Mark

[pdq67]

Go talk to a smog inspector that is a good guy b/c he can tell you the straight skinny on this.

Apparently, a lean mis-fire on deceleration causes the car to miss which in turn causes just enough unburned fuel to pass through the engine. This causes the H/C reading to go rich, adding fuel for the little back-fires or cackles, (I think).

I personally like the cackle along with them "Cop-calling" glasspacks! But you need a full rear exit exhaust system to get the noise back far enough so that it doesn't slowly drive you banana's.

But glasspacks just aren't "in" nowadays unless they are on a roadster and are all stainless steel and cost a bunch. pdq67

[cavemanmoron]

i have the Flowmaster American thunder duals on my 427,running the edelbrock performer intake and Cam,hooker headers,and 850 holley d/p,hei to fire it,
i get the crackle on slowing down in gear,and with the full length exhaust it doesn't bother me,
i assume i have an exhaust leak,either where the headers bolt to the engine,or where the collectors/reducers bolt on...

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'67 RS/SS 427,4 Speed
Disc brakes,console, fold down rear seat,heater,
Radio Delete, Cowl hood,front,rear spoilers,
Black/black white bumblebee stripe