Popping noise from exhaust

[SMYUEN]

Shaun,

What about the smell from the exhuast after hard revving???? Like rotten sulphur.. is that normal??

[Shaun]

hmm not sure about that leh.. i have run catted rich, catless rich, catless lean, but have never experienced a rotton sulphur smell. maybe I have but don't know it because I don't exactly know what rotten sulphur smells like?

catless or dead cat, you can smell it, but am not sure if that is what you are experiencing. i don't think your cat can be dead at such a low mileage. Usually it is cats run in excess of 100K miles that lose efficiency.

[Shaun]

Question for all the BMW gurus..

which models have the new valvetronic that has the throttleLESS? the one where they can have both the intake and exhaust valves stay completely shut [zero lift] even as the engine cycles? IIRC it is only the new 5 series that has it right?

Anyway.. the throttleless system will never see low pressure AF mix seeing hot, atmospheric pressure in the exhaust system simply because it has nearly zero pumping losses under overrun conditions. This engine will never experience this type of backfire that the Z4 experiences. The Z4 runs a normal DBW throttle and so has pumping losses and low pressure AF mixes going off and popping in the exhaust system.

[Tanzy]

Shaun,

Thanks for the mroe concise explaination. To check my understanding, the sound is due to not totally combusted fuel/air mixture from the cyclinders entering the exhaust during closed throttle.

Since you mentioned vacuum gauges. I've seen them on some cars, think it's made my Defi. Looks really cool but what is the point of monitoring the vacuum pressure, even on the track? What about exhaust temperature?

[Shaun]

Originally Posted by Tanzy

Shaun,
Thanks for the mroe concise explaination. To check my understanding, the sound is due to not totally combusted fuel/air mixture from the cyclinders entering the exhaust during closed throttle.

Yup! You got it. Short version is that throttle closed at sufficient RPM will generate sufficiently low cylinder pressures to significantly slow down burn speed of the AF mix.

Since you mentioned vacuum gauges. I've seen them on some cars, think it's made my Defi.

Yah there are much cheaper ones by autometer and other taiwanese brands.. good enough for checking out option 2 as mentioned in last post to you.

Looks really cool but what is the point of monitoring the vacuum pressure, even on the track?

it is actually really useless esp on modern engines.. ESP on low mileage, un-heavily modded modern engines. On old stuff, it can be used to check for worn valve guides, damaged valves, clogged exhaust system, worn rings, etc. anything to do with the rings and valves nowadays is better checked via leakdown and/or compression testing. as for clogged exhast.. it doesn't happen very much at all unless a rag was left in the system after it had been worked on, or the cat has melted, etc. all these problems deteremined by watching when and how the vacuum level changes.

it is good to see how much vacuum you're pulling at idle or low RPM to see if a cam duration or phasing allows for the minimum required vacuum that allows vacuum accesories like power assisted brakes properly. this vacuum reading is often the limiting factor as to how aggressive a cam setup can get on a street driven car with assisted brakes.

but that's just about all a vacuum gauge is good for.

What about exhaust temperature?

EGT reading being a product of AF ratio, ignition timing, probe location, and exhaust design, is only as good as the user. The user has to understand engines to beyond what 95++% of enthusiasts do. Even on planes where EGT is a big deal, it is always referenced to cylinder head temperatures, and used for AF mixture adjustment on the fly - maximum fuel economy within limits of what the exhaust port, system and valve can take [fixed value] . The engines have also been heavily tested to very accurately determine safe limits of EGT at mapped ignition settings.

If anything EGT on street cars or track cars will give an indication of knock sensor activity and ignition retard will show up as high temperatures. Determining how safe it is to deviate from baseline stock EGTs is not something that can be done without manufacturer's knowledge/test result OR your own heavy and expensive testing. So EGT is only an indication and an expensive one at that.

Much more simple and useful for the average enthusiast is a wideband AF ratio sensor and gauge.

For the average track nut enthusaist here is rough order (essential to less essential) of useful gauges on light-medium modified NA car..

Oil temp
Coolant temp (stock gauge resolution is not good enough)
Oil pressure
Wideband AFR
Inlet air temp (preferably reading off intake manifold)
EGT

[Tanzy]

I think I over did the gauges in my car.
Darn it! Kind of cluttered now...

[Arsony]

Tanzy,

Wah seh... you garang bro!!! Lambo-styled doors some more!!!
You win... you win...

Arsony
Topaz Blue NOT lambo-ed....