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Scratchbuilt L6 EFI Intake Manifold

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Saturday, Braap, myself, and the owner (Clint Barnts) met at TJ Wongs Performance in Vancouver, WA for some dyno tuning (Mustang Dyno). After putting in a fair amount of windshield time in this car, I was a bit surprised by the numbers. Car runs exceptionally well... a bit contradictory to the peak numbers.


Cutting through the crap, peak wheel hp was 172@6400rpm with peak torque 150@5400 rpm....




Looking at the average power, it torque peak's at 5400rpm and still makes far more low end grunt than stock. That partly reflects the cars broad powerband and demeanor. Broad, streetable power was at the top of the list.


You'll notice a couple dips in the torque curve around 2800 and 4600rpm. The AFR's went a bit rich in the same regions. This implies that airflow is down at these points. In a nutshell, I believe this is a function of mismatched parts. Let me clarify... the headwork is pretty extensive (built by Braap), with little left for improvement, outside welding up the chambers. The intake manifold is also very aggressively sized, with liberal runners. Prior to getting it up and running, I had calculated the manifold with a torque peak around 5500rpm (which is almost exactly what we got). The cam, an Electramotive L7 grind (265@.465",) is far more docile. The headers are the larger 1 5/8" primaries, enough to support well over 300hp... attached to a smallish 2.25" exhaust system. These compromises were made to keep the car tractable enough for an everyday commute, but produce enough power to be fun when the 'itch needs scratching'. From that perspective, it was successful. However, I am confident the right cam and exhaust would amplify the peak number's substantially. The sacrifice would be drivability.


Some side notes...


Some things came to light from this session, particularly regarding heat. Not surprising is the location of the air filter, next to the radiator. It was a warm day to begin with, around 85 degrees. I was recording 160 deg. intake temps. Ouch. My best guess is that this cost us close to 7 or 8 hp. If I'm right, we would have seen ~180whp. Not bad, but still about 15 or 20hp less than I was betting on. The owner was a contributing factor in the location of the air filter. After seeing the results empirically.... lets just say, before day's end, a resolution was in the works.


The one that did surprised me a little was water temps, compression ratio, and detonation. First, the compression ratio of this engine is modest, at about 8.1 to 8.2. Never expected any detonation. Running a 160 deg. thermostat, we never got any when the water temps were 'normal'. However, even with premium fuel and low compression, we experienced SEVERE detonation once the water temps reached about 225 deg. Obviously, dialing back the timing, based on temp, 'solves' the problem, but that's not really the point. Backing into it.... we got the best numbers at 160 deg. Every consecutive run, brought temps up... and power down. Let the car cool down and the power reappeared. When we pushed much beyond 210 deg., we would get detonation, progressively worse with temp.


At 160 degrees, the car made the same power with total timing anywhere between 37 and 43 degrees. Running less than 37 cost power. With temps above 200, 40 degrees caused a power reduction.


This, in itself, is not all that surprising... the surprise is the knock propensity at such modest comp. ratios. How about all those L6's, running 10+ CR, with lackluster cooling systems? My point is this... don't discount a cool T-stat and a cooling system to back it up.... the car is receiving a Ron Davis radiator and an upgraded fan.


The other related conclusion is that this is inspiring me to install a cylinder head temp sensor near cylinder 5, as this appears to be directly related to chamber temperature. Wolf will allow additional sensors to manipulate the mapping... I will be taking advantage of that, even if for no other purpose than data logging... I think it will be interesting to see the correlation between cyl. head and water temp sensors.


In the end, this car is proof that a dyno sheet doesn't tell the whole story.


The question for me at this point is runner size. I'm of the opinion that a slightly smaller runner, say 1.4" to 1.450", wouldn't have cost us any peak power, but could in fact, bolster the low end (and quite possibly smooth out the torque curve). I need to make a decision shortly.

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Monzter was kind enough to run a CFD analysis on the proposed plenum revision. The goal was to improve three areas... entry into #1, 3 and 4, while reducing the swirling mid-plenum. It seams reducing the swirling was the only item that became 'reality'... it was also of my least concern.



Velocity chart at 20psi...







Thanks again Monzter!

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Monzter was able to calculate individual runner CFM.


1. 129

2. 170

3. 156

4. 141

5. 136

6. 138


These correlate well with the pictorial view in the previous post.


A runner-runner disparity of nearly 32%. Ouch.


Here's the crux... these are static flow numbers and an intake is anything but static. Valve events and acoustics play a significant role in manifold function.


For example, a friend of mine was relaying a story of intercooler flow testing, using an inlet 90 degrees to the core such as below....





They found that the air passing by the first row's actually created enough negative to cause backflow through the core. The CFD on my manifold shows a tendency to do this as well.


However, a large number of 'successful' manifolds are built with a 90deg. entry, both OE and aftermarket (Ferrari, Porsche, Lingenfelter, etc, etc). That, in itself, isn't definitive proof, but it does build suspicion that static flow is only a small part of the picture... in fact, it may be so skewed as to be nearly useless.


Maybe some testing would shed some light...

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After a heap of procrastination, I decided to have a go at a carbon fiber plenum...






CF has a number of attractive qualities that make it a nearly ideal candidate for manifold use. Aside from the obvious weight and thermal advantages, it has a nearly unlimited fatigue life. Fatigue resistance is one of my primary motivations as a boosted plenum is subject to significant stress. A well built CF plenum should survive many lifetimes. Add in nearly infinite shaping possibilities and an audible resonance that can't be duplicated by any other material.


Of course CF comes with its own drawbacks, mainly cost, manufacturing complexities, and a limited heat range.


Anyhow, the first step is to build a plug that mimics the final shape. A mold resembling the 'silhouette' was constructed, filled with 8lb expanding foam, and the plug extracted...

















Next up, hand shaping...

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One of the critical elements of the plug is the TB interface. A bonded aluminum flange is the plan (a proper bond will shred the CF before the adhesive fails). Dimensionally, I have some wiggle room in machining the flange to final diameter. But, the plenum needs to be ROUND for the adhesive to work correctly. An easy way to get it close was to remove the backside of a hole-saw, and power by hand...









Its not *exact*, but its close enough to finalize during the finishing steps.





Blended in the new inlet...












About another hour or two of roughing in and it will be time to start the finishing stages.

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Fortunately, I came to my senses and realized I was making this harder than need be. New strategy...






As round as round gets.


Where the aluminum 'cap' begins is where the CF will visually end and the TB flange begins. This will give me a short, perfectly round neck to bond to.

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Was it scrapped for a different design?


Yeah, it was scrapped for a BMW manifold...














The CF plug is sitting high on a shelf. When I'm feeling nostalgic I may pull it down have another go at it. The BMW conversion comes first, as well as one other large project, so it may be a few decades :wink:

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