Greetings!

Has anyone else noticed that at low kpa values, they need to remove timing to stop misfires. For example I am finding that I need to just about totally remove vacuum advance between 2000 and 3000rpm and any map reading below 40kpa.

Originally I thought my afr must just be too lean to fire. But I got to the point where really light cruise was around 12.8:1 AFR with the engine fully warmed, and it would still misfire. I started removing timing at first but noticed no improvement until I removed a whole lot of timing. I've got it pretty good now, but I was worried the lack of timing would cause poor economy, I know I can probably reduce the AFR back to around the 13.8:1 sorta region. I don't want to take it too lean at cruise like that because my injectors are far from the engine and I'm pretty sure it will end up with lots of stalls when clutched.

I know the standard mazda distributors get the vacuum advance from a ported vacuum source, but I thought the leading vac advance was about 3deg, and the trailing was about 12deg, to bring the timing in closer on faster cruise speeds, with little vac advance with slow cruise speeds.

I wasn't sure if I just have a TERRIBLE chamber fill or airspeed or something because I'm using downdraft (not staged at all) twin 50mm throttle bodies, and the type of cruise I'm talking about is probably 3% throttle position.

 

with stock S4 TII manifolds, i havent noticed any problems like that. I do have it running pretty rich there so that it catches right away after coming back from overrun, but the timing is still in the 30* BTDC region

 

yeah i was thinking it must be the case with the TII because all the maps I find have lots more timing there than I can use with the 50mm downdraft throttles.

My TII intake has too many vac leaks to even get the tune to the state it is in right now.

The real difference I can see between the two setups is that the stock throttles are staged, so no air should be entering the engine through the secondary ports. Its gotta be that the airspeed is ridiculously low. I was expecting timing to be in the 30degrees ballpark there, but I'm probably using 12-18 from 2000 to 3000rpm

 

well, even if the airspeed was very low, and the secondary ports are open at very low load, that would mean there would be less air and fuel in the engine, and that you'd need more advanced timing to get CA50 at about 10* ATDC.

Can you try plugging the secondary runners (and configuring MS to just fire the injectors in the open runners), and see if it responds normally?

 

What do you mean by CA50 at about 10*ATDC? My timing is probably between 10BTDC and 15BTDC.

I did think about plugging the runners actually but that is so much effort to go through with. The other thing is the length of my intake is now also much shorter than the TII intake, so the tuning of that would be for higher rpms.

I was thinking that more timing would make sense with less AF mix in the engine, but I don't know....

I just need to take a little bit more out and it will be driving perfectly.

 

CA50 is the crank angle at which 50% of the mass of the intake charge has burned. In general, for maximum torque at any point and most stable combustion, you want CA50 to occur at 10* ATDC.

We're on the same page concerning what we expect vs what you're seeing tho, unless at extremely low loads and that speed you're getting a very weird resonance and the chamber is filling exceptionally well. What VE do you have at say 20 kPa at those speeds, compared to your VE at 30 and 40 kPa?

 

I have a single throttle body on high overlap ports, thus air velocity on my setup is very low at low RPMs as well. But I've never had the same issue. I run 30 degrees or so between 2000 and 4000 RPM at low vacuum at about 15:5 AFRs. Then again, I have a large plenum which might make up the difference.

The last ITB car I tuned was pretty happy running high 20s for timing at low load so I don't know what to say...

 

The zone I'm more talking about with the annoying miss would be say, 30-45kpa.

I will pull out the plot to show you later on.