Snap, crackel & pop

[Psycho man]

Now I like Bar-b-queing hamsters, cats and old ladies at bus stops as much as any one else, but I think that I'm running the risk of frazzling most life in London. Yes an X-flow user, the question is, just what should be the limit to popping, crackling and backfiring from the exhaust when easing of the accelerator at London speeds? Does it effect the long term health and performance of the engine?

 

 

 

Edited by - Jace on 18 Jul 2001 11:34:40

[EFA]

Jace,

 

You are either firing early (advance your ignition slightly) or overfueling (adjust or rejet carbs).

 

Unless really excessive (undriveable)it won't harm your engine.

 

Roger????

 

Arnie Webb

So fat the chassis snapped....wink.gif

 

See Fat Arnies 2 piece Seven here

See the Le Mans Trip Website here

See the Lotus Seven Club North Kent Website here

[Red SLR]

After getting my K series TBs put on my car started doing this. When ever I lift off it pops, the guy who did the ECU said it he has set it up to do this as it helps throttle response on the track when changing up.

 

Is this true?

 

X777CAT

[oilyhands]

For the exhaust to pop you need hydrocarbons and oxygen, it may be that your exhaust has a small air leak, or your advance at closed throttle isnt enough.Check your idle jets arent oversized and that you idle fuelling is correct, also check your static timing.

 

On the K series side, many ECUs provide a fuel cut-off on overrun which prevents fuel from being injected when the engine is being driven by the cars momentum, this eliminates exhaust popping to a great extent, another way is to reduce the fuel numbers on the 'overrun' side of the map. The transition from no fuel (fuel cut-off) to normal map fuelling takes place in less than a single engine cy le when the throttle is opened so the affect on transient throttle repsonse must be minimal.

 

Oily

 

[Mike Bees]

Possibly... He may have set the fuelling extremely lean when the throttle is closed at high rpm, this might encourage the revs to drop a bit faster when you're changing gear although I doubt you'll notice the difference. It might also be because there isn't enough ignition advance at small throttle openings & high rpm - adding more advance here usually clears up the overrun popping & banging.

 

Popping & banging on the overrun is usually caused by not having enough ignition advance in that area. On the overrun you get poor cylinder filling which needs a long time to fully burn, hence if you don't set light to it soon enough then some unburnt mixture gets chucked out of the exhaust.

 

Mike

[Mike Bees]

Oh - my 'possibly' was in response to Simon's question - Oily must have been typing his reply at the same time as me.

 

Mike

[Red SLR]

So is it a problem?

 

X777CAT

[Mike Bees]

Only in that it will burn the stuffing out of the exhaust a lot quicker. As Oily suggests it's easy to turn on the fuel cut on overrun feature on the GEMS ECU. In the GEMS software you set "DFCO Speed" to be the rpm above which you want the cut-off to operate (e.g. 3000), and "DFCO load" to be the throttle position below which it will operate - a small number like 3 works here.

 

Mike

[Graham Perry]

You can make my cross-flow spit,pop and bang, easily by leaning out the idle mixture screws. On the over-run in the dark you can get really quite effective pyrotechnics this way.

[millsn]

I used to find that when my XF carbs were out of balance I got huge flames from the exhaust - have you checked this? I always assumed I was just throwing in too much fuel on one cylinder.

 

Nigel Mills - 2.0 Zetec carbs

[russb]

Make sure both your throttles are fully shut when you lift off, if one is even fractionally open it will draw fuel and air and couse the popping

[domster]

Hmmm, I always liked the popping sound and fireworks - especially at night. Just assumed it was the Webers doing their thang and perfectly normal.

 

Does anoyone know what the 'worst' (or best!!!) engines are for popping on the overrun. I've had a couple of tuned xflow 1700s (one comp exhaust, one normal, both popped, even after Roger King tune).

 

Cheers

Domster

 

 

Edited by - domster on 19 Jul 2001 13:05:21

[Simos]

Listened to a simply awesome display of pops and bangs on overun last w/e coming down the lane past our house.

 

When it eventually hove into view it was a VW camper van of about 1970 vintage. I swear it sounded awesome !

 

Reminds me of a visit to '99 Grandprix, inbetween practices it was pretty quite when a vehicle hooned down the straight making a great noise. Again couldn't at first see it, when we did it was a landrover rescue truck and the noise was the nobbly tyres. Again I swear it sounded great ! (and have a friend as witness)

 

It just goes to show, although I'm not sure what...

[Roger King]

The better set up the engine (ignition timing, carb balance, mixture, etc) the less the 'problem' will be. We've had plenty of requests to make it worse believe me!

 

Basically, all I'm saying is what everyone else has, but a few points:-

 

The 4 into 1 side exit exhaust has an engineered in air leak at the collector. This is inevitable when the pipes are a push fit and it also provides a degree of flexibility to the system. Rear exit systems are generally less prone (but not immune) to overrun pops and bangs. Look upon it as being environmentally friendly - a catalytic converter is there to complete the combustion of unburnt/part burnt fuel. You of course are achieving the same thing more spectactularly! (like hell).

 

Anything that reduces efficiency of combustion in the engine will make the problem worse by chucking part used fuel into the exhaust ready for the air leak to provide oxygen to. Too rich a mixture for example will guarantee excess fuel in the exhaust and too weak will slow down the speed of combustion leaving incompletely burnt products to go into the exhaust. Likewise, retarded ignition (which you will automatically get in the absence of 3D mapping) will not ignite the fuel early enough, again leaving unburnt products to go into the exhaust. It is quite noticeable when monitoring exhaust temperature data on the dyno, that exhaust temperature drops with increased ignition advance.

 

One of the main reasons that it happens on overrun is that when you lift off, the pressure in the exhaust drops to the point that air is sucked into a leak rather than exhaust gas blown out.

[Psycho man]

Thank you for your replies, I'm now extracting my X info from the K info. One last question is the type of petrol used critical to this, I switch between Super Unleaded (green) and LRP (red).

 

 

[John Whyman]

Unleaded petrol is made to a standard, (BS 4040)and is therefore more consistant in it's make up. LRP is not made to any required standards, and the ingredients can vary from manuf to manuf. I've had some which has caused very noticeable change on my X/F immediately. I therefore use an additive, (Millers VSP usually)which gives a good boost.

I get only the very occasional slight pop on my 1700ss with 4-to-1 exhaust system. It is worth getting a professional to set the carbs and timing. They don't go out of tune very often.

 

 

 

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