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

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.

I did a search on this site and saw a single reference saying that "double slip" collectors should be used with EFI but it didn't have much background info. 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.
 
Joel
I am anxious to see if you get any responses.

I have an EFI car running a single slip connection on the pipes and it seems to work fine. I have had some decel popping issues and not able to tell if it is the slip joint, reversion from the side pipe or something else. System is a Ford FE running a Borla EightStack system into 2" exhaust headers, thru the slip joint area and then thru collector and 3-1/2" side pipe on a replica cobra.

No issues while cruising or accelerating. Issues seem to appear only on decel which makes me think it is more an ECU setting issue or reversion than it is a leak to the O2 sensor thru the slip joint.

Hoping to learn along with you - I have a second build I am getting ready to tune and it has a very similar system and don't know if I should do anything special to it. Too late to go to dual slip joints.

Paul
 
Hi Joel,

Obviously every case is different but in my case, using the "single" slip fit collector in the picture below, I had to abandon the use of the O2 bungs you can see in the collectors, and install a new bung in one of the primary pipes about 12 inches from the end to avoid air sucked in by reversion (see top inner primary pipe left-hand side). My O2 readings were so far out as to be useless when taken from the collector. While I did have visible signs of leakage, I think that is hard to avoid with single slip-fit designs - I would therefore say you have a high chance that you will need to do something similar. That's no big deal with modern EFI and good mixture distribution cylinder to cylinder.
P1010272.JPG


Cheers, Andrew
 

Paul Hendrickx

Supporter
Hi,
just my thoughts....... see your engine as a air pump.... depending on the valve overlap one moment you have intake and exhaust connected together ..... when you come off throttle the intake is closed except for the idle function...(little air gets in). so when decelerating you create hugh vacuum in the cylinder.... while the intake throttle is closed down , the exhaust is not restricted , the cylinder will pull fresh air in through the (often) big short exhaust pipe, so you getting fresh oxygen with the unburnt left overs , burning in the exhaust pipe... popping , you have 1 bar atmospheric pressure on the exhaust side ...
if the injection ecu is not good programmed and does not cut fuel off completely while decelerating, fuel will still get inside de cylinder , and mixing with the fresh oxygen in the hot exhaust manifold ...
Ps: on later standard engines to minimise pollution often the intake valve opes AFTER tdc et the exhaust closes BEFORE tdc ... so no valve overlap and non unburnt fuel reaches exhaust
Paul
 

D. Nye

Lifetime Supporter
Joel, I can only give you my experance with single slip on headers and EFI. I originally ran dual Holley's with no problems other than starting issues I had with an auto electric choke. I made the commitment to go with the Holley Sniper dual four EFI setup and placed my O2 sensor after the collector, see attached picture. The Snipper setup has a learn mode to set the fuel map however it was always running to rich. What I found was I had small air leaks at the collector slip joint and like you had invested a lot of money in the bundle of snakes. My solution was to seal the slip joints with a product called liquid steel or something like that. It's made to seal small exhaust leaks and in my case worked. I placed my shop vac on the end of the exhaust pipe and let the vac pull the liquid into the joint. It fixed my problem and the car now runs great. You need to do your research because any air leak before the O2 sensor is going to be a problem.
Air Box 4.jpg
 

Terry Oxandale

Skinny Man
I use Slip-on collectors that have never been swapped...no EFI mixture problems with O2 on the collector. BUT, I utilize a V-band connector at the end of the collector, and that is where I swap out the track exhaust/street muffler changes. The slip-on has discoloring (carbon) on one of the visible areas on a top tube, but that is after years of use, and I'm pretty sure the carbon has reduced (filled in/clogged) any leak that has an impact to the sensor's accuracy.
 

Randy V

Admin
Lifetime Supporter
I’ve used Permatex Copper or Red High-temp RTV With great success in sealing exhaust systems. So much so, that many years ago, I stopped using header gaskets as well, using a bead of RTV that was allowed to “skin” prior to installing. I can report that I had no incidents of leaks in the numerous racing as well as street driven vehicles I did this on.
 
Hi Joel,

Obviously every case is different but in my case, using the "single" slip fit collector in the picture below, I had to abandon the use of the O2 bungs you can see in the collectors, and install a new bung in one of the primary pipes about 12 inches from the end to avoid air sucked in by reversion (see top inner primary pipe left-hand side). My O2 readings were so far out as to be useless when taken from the collector. While I did have visible signs of leakage, I think that is hard to avoid with single slip-fit designs - I would therefore say you have a high chance that you will need to do something similar. That's no big deal with modern EFI and good mixture distribution cylinder to cylinder.View attachment 116299

Cheers, Andrew
Andrew: it's a bit hard to tell from the picture, but does your collectors only have a single lug holding them on? Are the lugs welded on top of the slip area? I've been told to weld the lugs after the slip area as welding on the slip area does cause warpage that can lead to a leak. I'm somewhat concerned about putting the O2 sensor on a primary tube in my case as the EFI is an ITB/8 stack setup so no mixing of fuel loads across cylinders.
 

Markus

SPRF40
Lifetime Supporter
This topic was discussed here a few times.... do a search for "double slip" +"efi" / "8- stack" or any combination.
This definitly a problem - connection needs to be sealed....
 
Andrew: it's a bit hard to tell from the picture, but does your collectors only have a single lug holding them on? Are the lugs welded on top of the slip area? I've been told to weld the lugs after the slip area as welding on the slip area does cause warpage that can lead to a leak. I'm somewhat concerned about putting the O2 sensor on a primary tube in my case as the EFI is an ITB/8 stack setup so no mixing of fuel loads across cylinders.
Hi Joel,

My joint area is definitely pretty leaky as you can see - the lugs on the collectors are OEM but are not used as both the engine and the back section of the exhaust (mufflers etc) are completely solid mounted to the frame. I should also have clarified that I run in open loop until around 2500 which in exhaust pulse terms is about equivalent to idle conditions in a 4 into 1 collector. At that point I get stable and good readings from the O2 sensor and I go to closed loop for cruise and moderate engine load. I run open loop at full throttle, and I use datalogging off that single primary to check my fuel curve at wide open is OK at each track.

You should have good fuel metering cylinder to cylinder with multi-point EFI, the issue with 8-stack is primarily low throttle angle airflow variations cylinder to cylinder. A collector O2 sensor won't fix that but it will shift fuel in all cylinders up and down to a least worst position given the air metering variations cylinder to cylinder in that bank.

Cheers, Andrew
 

Terry Oxandale

Skinny Man
... the issue with 8-stack is primarily low throttle angle airflow variations cylinder to cylinder. A collector O2 sensor won't fix that but it will shift fuel in all cylinders up and down to a least worst position given the air metering variations cylinder to cylinder in that bank.

Cheers, Andrew
Great point Andrew. I had large variations in primary tube temperatures, which was narrowed down to very slight variations in the idle-position throttle plate angles. Once I found a way to true-up all 4 plates (per bank), the idle temperatures all evened out.
 
Whilst I love copper silicone for sealing exhaust systems, using that upstream of a lambda probe will poison the probe anyway.
I've used Heldite with no issue before though.
 

Terry Oxandale

Skinny Man
Terry, can you expand on this - how did you get them to match

Paul
I loosened the shaft clamps for the ITBs, closed each plate solidly against their bores, and then re-tightened the shaft clamps. I then set the throttle for a small off-idle positioning angle that allowed a small diameter rod (TIG electrode which is very hard), to snuggly fit between the throttle plates and bores with consistent "snugness" or sliding friction from one ITB to the other. This second check on synchrony validated the initial, against-the-bore setting. It was immediately clear on start-up that my primary tube temperatures had leveled out from one tube to the other. One of the other drivers for me doing this was I was consistently fouling plugs on specific cylinders. My assumption was a mechanical issue with rings or bore, and it would clear up once I operated the engine off idle for a bit, or normal driving (sometimes). It was quite aggravating, but since I performed this synchronization, I no long have fouling of any plugs during warmup.
 

Randy V

Admin
Lifetime Supporter
Whilst I love copper silicone for sealing exhaust systems, using that upstream of a lambda probe will poison the probe anyway.
I've used Heldite with no issue before though.
Negative... No issues here and have used upstream of many O2 sensors both heated and non heated...
 

Attachments

Thanks for all the good information and feedback on this topic! My impression based on the feedback from this forum and ClubCobra.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. 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.

Thanks again for the feedback!!
 
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