A/F ratio different depending upon exhaust pipe

[FresnoBob]

I recently installed a rebuilt 250 engine in my 230 SL.  I'm trying to get the tuning perfect and struggling a bit.  I have an Innovative air/fuel meter, which reads 13.8-14.5 at idle with the inboard exhaust pipe, but 23-24 on the outboard pipe.  My fuel injection pump was fully rebuilt by Gus at Pacific Fuel Injection and the car runs quite well.  I have new MB original mufflers on the car. 

Are the exhaust pipes dedicated to a bank of cylinders?  Or is there a higher flow side that isn't used until higher rpms? 

[Pawel66]

I have just posted a question on exactly that topic...

[mrfatboy]

I can't take credit for this but here is what an expert says 


They don’t necessarily get “mixed up” but they do cross paths. A hole anywhere in the exhaust will also skew readings.

And make sure the probe is installed in the correct 12 o-clock orientation. Otherwise the vent hole wont work correctly and condensation builds up and affects reading. The inboard pipe is hard to get the probe installed in correct orientation because the bumper gets in the way of the thumbscrew.

Just tell both of them to add 12 inches of pipe onto the probe. Scavenging between the pipes can skew measurements.


from the Easter Pagoda Bunny! 

[Pawel66]

Well, thank you.

The readings were 3.2 vs. 3.7, pretty clear difference...

I still do not know if they mix...

I was sure that it would be mentioned by someone doing the CO, this would be a way to diagnose things... Never seen any of that. So I thought streams of fumes are same/similar/mixed, but they may not. If you blow air into the 4-6 exhaust manifold - would get solid stream of air in the left pipe and just small in the right, just because of the cross-connections in the front?

[Shvegel]

I used to run with a sock in one pipe to limit the noise so at least on my car they definitely mix.  If you hold your hand over one pipe and it still runs it mixes.

[MikeSimon]

The way mufflers are designed, they should mix. The two separate pipes do not continue separately inside the mufflers.

[stickandrudderman]

I think the OP is referring to A/F ratio, that is stochiometric ratio, rather than CO measured at the outlet.

[Pawel66]

Thank you for all the answers from my side.

Shvegel, the topic was different, that is why I did not delete my post, but I this one is referring to a similar question of exhaust gases mixing or not mixing inside the mufflers...

[FresnoBob]

I've now got my dwell set at 38º and timing at 36º at 3,000 rpm, so the car is running better.  My A/F ratio is closer for the two exhaust pipes, but there is definitely a difference in temperature.  As my engine is brand new, i assume this is due to differences in valve adjustments now that the car has 300 km on this new engine.  Is my temperature difference hypothesis reasonable? 

[Benz Dr.]

I would add a couple more degrees to your timing. I like a minimum of 38 degrees BTDC at 3,000 RPM   I've even run as high a 40 degrees without any side effects.