Air leaks with slip fit collectors and EFI causing rich A/F?

[C5GTO]

I am in the middle of fabricating headers for my Miura project car and was warned by someone that single slip merge collectors can leak air in causing a faulty O2 sensor reading that leads to a rich A/F condition. So my question out to others that have a similar setup (EFI and equal length headers with single slip collectors) is if you've experienced an issue with A/F mixture rich condition. If so, were you able to permanently resolve/mitigate the issue? If so, how? If you have this type setup and haven't had an issue, I'd like to get that feedback as well.

My quandary is that I already have had single slip collectors made and was told by the very reputable shop that built them that they should not leak if the primary pipes going into them are round, properly sized and properly secured with lugs/bolts. So I have professional header people telling me what I have should be OK and another person that's very adamant that I'm sure to run into an issue. On one hand, I don't want to go to a bunch of time and expense to solve for something that has a low probability to be an issue and on the other hand I'd hate to proceed forward only to find out later that I need to re-engineer and re-build the headers.

I'd like to get a larger data set of people's experience if I can. Please pitch in if you have experience with this type setup.

[eschaider]

You have a virtual certainty of an air leak, that an O2 sensor will pick up, with slip fit headers. Everything on the exhaust side of an EFI fueled engine has to be locked up tighter than a bug in a rug. That means welded joints, thick flanges, gasket surfaces that are machined flat and good gaskets wherever a flange is used.

Of course you don't have to do this. You could use slip fit components and chase your O2 tail until you surrender and then weld up everything like you should have done the first time, You will save yourself a lot of anguish, work and pain by just welding it all up out of the gate.

There is a reason most exhaust shops are not good tuning information sources. It has a lot to do with the fact they don't tune, they build exhaust headers. The guys who build a lot of exhaust headers for EFI fueled engines will know not to use slip fit components if for no other reason than from customer complaints.


Ed

[Tom Wells]

C5,

I have flanged headers and EFI. One way I can tell I've got a flange leak (sensor is only an inch or two away from the flanges) is when the car starts running pig rich. Then it's time to replace that collector gasket.

Another way to tell is to run at highway speed a few minutes, then back off the throttle suddenly - if there's a leak I'll get a big bang (backfire) out of the exhaust on the side where the leak is located. Replace the collector gasket.

I've tried lots of different flange gaskets and they each seem to develop leaks sooner or later. Some are better than others but so far I haven't found one that is immune.

The only header attachment I've had no problem with is the V-band clamp. It just works. The clamp won't work where the individual headpipes are located - only where the 3" collector can be attached this way.

You pays your money and you takes yer cherce!

Tom

[Luce]

I don't run my efi closed loop for this reason.

The most miniscule air leak will considerably skew the wide band O2 sensors readings way lean, and closed loop operation will try to correct by adding fuel.

[C5GTO]

Thanks for all the good information and feedback on this topic! My impression based on the feedback from this forum and GT40s.com is that the majority of EFI outfitted engines with single slip collectors have an issue with air leaks sometime in their service life. Some of these air leaks have been successfully resolved with sealants and some have not. For some, it's an ongoing maintenance issue where the sealant (or gasket in a flange situation) needs to be replaced to repair a recurring leak.


The goal I have for my Miura's headers and exhaust system is to build it right the first time such that I don't need to re-engineer/re-build it later. I plan to have it ceramic coated and this makes welding on it later a real pain. So I relayed the information provided by you all back to Vince at Burns Stainless. He then offered to swap out the single slip stacked collector for a double slip version. I can't weld the collector on as the header installation can only be done without the collector in place. The Holley Terminator X EFI system that will be used only has a single wide band O2 sensor so I only need to make sure that the collector where it is mounted is leak free. I will leave the other "flat" collector as a single slip because it would have to be much wider if double slip and getting it to fit under engine oil pan would be much harder. If the flat collector leaks some air in, I don't think it will cause any issues as there will be 30" of secondary pipe downstream of the collectors prior to all the exhaust being merged together with a Y pipe.


I now have a revised plan and just need to get all the U bends, replacement double slip collector and Y pipe to fabricate it all up. If you're interested in seeing this Miura project car, I maintain a build thread here: https://www.gt40s.com/threads/modern-day-miura.54786/


Thanks again for the feedback!!