Coyote cross-over header design

[Joel Heinke]

I'm at the point of designing the headers for my Coyote powered Miura. The engine sits transverse but that really just means I need to build custom headers because there's no chance someone will have already built headers for this application due to zero other cars having this engine and engine orientation combination. So I'd really like to have the "even firing" exhaust note that the original V12 Miuras have and went with a V8 because a V12 would never fit in the chassis I used for this car. If I understand the theory of cross-over header design, the even firing exhaust note is possible on a V8 with the proper header design. In fact, it's fairly common on GT40s with the "bundle of snakes" header design.

The Coyote firing order is 1-5-4-8-6-3-7-2. Same cylinder bank sequential firing happens between 8-6 and 2-1 on the Coyote. So it appears on paper, that if I route the header tubes for cylinders 2 & 3 to a shared collector with 5 & 8 and cylinders 6 & 7 to a shared collector with 1 & 4 then I should eliminate the sequential entry of exhaust into the collector. In other words 1-4-6-7 for one collector and 5-8-3-2 for the other.

Question 1: has anyone built cross-over headers for a Coyote and does it really make a difference in the exhaust sound for the combination of a mid-engine car with very short exhaust pipes and a 4 valve per cylinder motor?

Question 2: did I get the cross-over cylinders right on my paper design?

Question 3: if I can't get all the primary tubes equal length, will the whole cross-over exhaust attempt be futile in the end? Given the transverse engine orientation, the engine bay is very tight and thus very little space for routing a bunch of hot exhaust pipes around. I plan to mockup the headers with PVC pipe to see how close to equal length I can get. I plan to use merged collectors so each header tube can be installed individually but this doesn't really help much if the tubes have to spiral around one another to make them equal length. I guess this question boils down to is close to equal length good enough with cross-over header design.

I'm really hoping there are some people on this forum that are well versed in this topic that can pitch in with real world experience here.

 

[Andrew Robertson]

Hi Joel,

looking at the photos in post #33 in your main build thread illustrates your point about lack of space. I'm not sure if it will give you the sound you want but from a packaging and heat management perspective it looks like you could effectively bring all 8 together somewhere above the area where the gearbox meets bellhousing. That could be achieved in a couple of ways.

Firstly by pairing cylinders in each bank that are not adjacent in the firing order tri-Y style, giving you two secondaries for each bank, then into a single 4 into 1 collector with a large single running into the muffler/aesthetic options you desire from there. 1-3 and 4-2 would be paired into secondaries on one bank, and 5-6 and 7-8 on the other. The concept on each bank is illustrated in the attached pic of an LS tri-y header although obviously your design and routing would be different.

Secondly, you could simplify further by using conventional shorty up and forward turbo headers ( https://www.summitracing.com/parts/big-12152flt ) and bringing the two main pipes together above that same area as they run toward the back of the car.

Just some ideas. I'm not sure how a bundle of snakes would fit in there!

Cheers, Andrew

 

[Julian]

The main benefit of the crossover is pulsed scavenging, thus I would imagine equal length primary tubes are quite important. When the topic came up previously I seem to recall that the limitation was that the Coyote ECU requires Lambda input from each bank of the engine, which becomes difficult if they headers are crossed over and gases mixed.

 

[Jac Mac]

When I built headers for J Ondracks TVR I made them conventional 4 into 1 for each bank, collectors for each bank was flat 4 into 1 and then 3'' secondary for each bank into a flowmaster style muffler with two 3'' inlets into the front chamber of the muffler . Other bank was same feeding into the remaining 3'' inlet of the muffler. Muffler had single 4'' outlet and this tailpipe was flattened in section to give the reqd tailpipe area for its length. Sound is more even than a V8 and many people thought it had a flat plane crank, develops into a howl at the upper RPM. Just under the 90 or 95 db track limit we have to comply with. ( Sorry, cant remember what the limit was at the time.) Room for the headers was at a premium & in case you think there was plenty they went down thru the chassis and were stepped primaries as well as equal length.

 

[Joel Heinke]

Hi Joel,

looking at the photos in post #33 in your main build thread illustrates your point about lack of space. I'm not sure if it will give you the sound you want but from a packaging and heat management perspective it looks like you could effectively bring all 8 together somewhere above the area where the gearbox meets bellhousing. That could be achieved in a couple of ways.

Firstly by pairing cylinders in each bank that are not adjacent in the firing order tri-Y style, giving you two secondaries for each bank, then into a single 4 into 1 collector with a large single running into the muffler/aesthetic options you desire from there. 1-3 and 4-2 would be paired into secondaries on one bank, and 5-6 and 7-8 on the other. The concept on each bank is illustrated in the attached pic of an LS tri-y header although obviously your design and routing would be different.

Secondly, you could simplify further by using conventional shorty up and forward turbo headers ( https://www.summitracing.com/parts/big-12152flt ) and bringing the two main pipes together above that same area as they run toward the back of the car.

Just some ideas. I'm not sure how a bundle of snakes would fit in there!

Cheers, Andrew

Andrew: very creative solution! If there is simultaneous arrival of an exhaust pulse, it would only occur at the 4 into 1 collector at which point it would hopefully be less impactful than occurring closer to the exhaust port. It should also simplify the header tube routing as you get the pair of headers down to 4 tubes fairly quickly. My guess is that for a tight engine compartment, you'd need to use slip-in style merged collectors so the headers could be assembled piece by piece around the engine and transaxle.

So the 2 into 1/4 into 1 approach is attractive, at least in theory. Have you seen such a header system actually built? If so, how did it sound?

 

[Joel Heinke]

The main benefit of the crossover is pulsed scavenging, thus I would imagine equal length primary tubes are quite important. When the topic came up previously I seem to recall that the limitation was that the Coyote ECU requires Lambda input from each bank of the engine, which becomes difficult if they headers are crossed over and gases mixed.

Julian: thanks for the response! I think you're right about equal length primary tubes being important to getting the benefits. In looking the engine and engine compartment over further today, it's clear to me there is no chance at getting equal length primaries for this application. I say this because all primaries would need to be same length as the longest primary, there will be a very long primary with a V style transverse engine, and thus I'd run out of space way before all the primaries could be routed.

Now that you've mentioned ECU and O2 sensors, that adds another dynamic into this header design exercise that I had yet to consider. I'll be using an 8 stack Weber style EFI in this car so it won't be the factory Coyote ECU being used. It will be the Holley Terminator-X ECU because I need an ECU to control both ITB EFI and variable cam timing. I believe for most EFI systems the O2 sensors should ideally be both very close to exhaust ports (to minimize sampling delay back to ECU) and sampling gases from all cylinders (in case there's a rich/lean difference between cylinders). With long tube headers in this transverse V8 application doing both of these may be hard if not impossible to do. I'll need to check into the O2 sensor constraints specific to Terminator-X ECU to make sure I don't do something non-optimal or just plain stupid.

 

[Joel Heinke]

When I built headers for J Ondracks TVR I made them conventional 4 into 1 for each bank, collectors for each bank was flat 4 into 1 and then 3'' secondary for each bank into a flowmaster style muffler with two 3'' inlets into the front chamber of the muffler . Other bank was same feeding into the remaining 3'' inlet of the muffler. Muffler had single 4'' outlet and this tailpipe was flattened in section to give the reqd tailpipe area for its length. Sound is more even than a V8 and many people thought it had a flat plane crank, develops into a howl at the upper RPM. Just under the 90 or 95 db track limit we have to comply with. ( Sorry, cant remember what the limit was at the time.) Room for the headers was at a premium & in case you think there was plenty they went down thru the chassis and were stepped primaries as well as equal length.

Jac Mac: a great suggestion! Just out of curiosity, did the car you mention have a Coyote in it or was it a push rod 2 valve motor?

So in essence what you did is have all 8 exhaust pulses intermixed within a single muffler to even out the exhaust sound resulting in the same sound attributes as an even firing engine. I'm guessing the same result could also be achieved with a 2 in 2 out style muffler if there's intermixing of the exhaust pulses within the muffler. I'm not opposed to using a 2 in 1 out muffler but the Miura had 2 very visible exhaust tips and I'd like to retain that look. So would likely need to Y out to 2 pipes if a 2 in 1 out muffler were used.

 

[Andrew Robertson]

I believe for most EFI systems the O2 sensors should ideally be both very close to exhaust ports (to minimize sampling delay back to ECU) and sampling gases from all cylinders (in case there's a rich/lean difference between cylinders). With long tube headers in this transverse V8 application doing both of these may be hard if not impossible to do. I'll need to check into the O2 sensor constraints specific to Terminator-X ECU to make sure I don't do something non-optimal or just plain stupid.

Hi Joel, the o2 placement is not that critical, in a collector 1 or even 2 metres from the exhaust port is fine. Many systems do just fine only sampling 1 bank for closed loop control, so getting all 8 is a nice-to-have but is by no means critical either. What could be a design and placement constraint is the collector fit method. If it is a totally sealed connection to the merging secondaries or primaries such as a gasket and bolted flange, no issues, put the o2 in the collector or just downstream of it. However if there is ANY leakage or vacuum pulling oxygen in through a slip-fit, you will read way off. Voice of experience!

Cheers, Andrew

 

[Jac Mac]

Jac Mac: a great suggestion! Just out of curiosity, did the car you mention have a Coyote in it or was it a push rod 2 valve motor?

So in essence what you did is have all 8 exhaust pulses intermixed within a single muffler to even out the exhaust sound resulting in the same sound attributes as an even firing engine. I'm guessing the same result could also be achieved with a 2 in 2 out style muffler if there's intermixing of the exhaust pulses within the muffler. I'm not opposed to using a 2 in 1 out muffler but the Miura had 2 very visible exhaust tips and I'd like to retain that look. So would likely need to Y out to 2 pipes if a 2 in 1 out muffler were used.

Pushrod two valve Ford Small block. You can make the muffler 2 in 2 out, just make sure the two sets of 4 headers go into a common open chamber at front of muffler and make sure you keep the tailpipe area down , that will make the muffler 'work' and help with noise reduction.

 

[JP Verweij]

So in essence what you did is have all 8 exhaust pulses intermixed within a single muffler to even out the exhaust sound resulting in the same sound attributes as an even firing engine. I'm guessing the same result could also be achieved with a 2 in 2 out style muffler if there's intermixing of the exhaust pulses within the muffler. I'm not opposed to using a 2 in 1 out muffler but the Miura had 2 very visible exhaust tips and I'd like to retain that look. So would likely need to Y out to 2 pipes if a 2 in 1 out muffler were used.

Think outside the box,
Hooker Aerochamber muffler. 21720HKR. Single inlet, dual outlet. Single inlet can be simple converted to dual inlet by welding a connection pipe on the oposite side.

Its the same muffler as the 21710HKR I am using that has dual inlet, dual outlet but that one seems to be sold out everywhere.

This muffler has a collector inside & an expansionchamber as secondairy chamber.

But any Aerochamber muffler can be converted to dual inlet, dual outlet. I am using these mufflers on all my toys.

This is my outside the box thinking creation

Basicaly what Jac Mac describes, 4 into one, 2" narrow pipes to speed up exhaust gasses, the collector chamber will act as a balancingpipe. The second expansion chamber will help scavenging. I added two titanium slip on akropovic mufflers as insurance for DB regulations.
Engine did not run yet as I am not finished.

 

[Jac Mac]

I should point out that I built my own mufflers, that way you can tailor the size etc to the space you have available. Be careful looking for mufflers off other cars that might fit the space ,but are designed for 1 in /1 out on each end from some front wheel drive applications.

 

[Mesa]

There's a Ducati 916 missing it's mufflers somewhere in JP's neighborhood !