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NA 3.1L=>head & camshaft questions. No shortcuts, max


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I'm not a good case study of how to spend money on a head. Jim made three custom cam grinds doing the R&D on my N42 cylinder head. As an example, I'm running valve springs that have 25% less sea

And final assembly drew near. I literally started at 9pm that night, and in these pics it was about 9am the next morning. I was leaving for the airport at 11am and had to wrap this up, get it running

Sounds yucky.

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Anytime!

 

Regarding the 40-41mm venturi, I don't expect much difference either. I still reference this Weber chart, particularly the note about aux. venturi performance.  (EDIT -- comment about carb body's exit removed, as I was mixing up the numbers with the 40mm carb body.)

 

I have no idea how accurate my "209 cfm per cylinder" flowbench measurement from yesteryear is. A 5-10% variance between that and the chart would translate to a + 2mm venturi size accuracy.

 

That's a blind guess, so it renders this chart conceptual. 

 

It's still nice to know that, in theory, around 39mm you're starting to get all that you can out of the 45mm carb body. Road testing will quickly determine which > 39mm venturi offers the most mid-range drivability, no doubt.

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Edited by zredbaron
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Nope. The image came from the internet, but I own the book as well and can confirm that's all we get. There's one other potentially useful graph, which bases venturi selection on cylinder displacement and peak horsepower RPM. The numbers on the chart are lower than what road race engines use, so it too is more interesting than useful for our purposes. 

 

I feel the book(s) could provide a LOT more charts, to include distinction between "street" charts and "maximum performance" charts. In my experience those that tinker on carbs do not value books very much past initial familiarization, because, well, that's all they're useful for. (EDIT: In practice, what matters is what works, not why. "Why" fosters improvement; knowing what works fosters repetition.) Perhaps this is part of why the carburetor books "lack" tangible, scientific data relative to other industries in the information age? (EDIT: Yes, fuel injection, I know. Carbs are still purchased new!) Carburetor "theory" without solid carburetor "data" decades after the theory was written? Hard to improve if we never circle back with upgraded, modern perspectives. Doesn't seem like the industry is interested in such a thing, either.

 

Sounds like our government! I digress...

Edited by zredbaron
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Glad to see this build still going. Really looking forward to seeing dyno results between the 40 and 45's, along with the change in venturis.

 

Maybe this is already widely known...but one thing I found was that there is an ebay seller that doesnt say it's Pierce Manifolds out of California, but I'm pretty sure they are based on the shipping label. The price difference between the pierce manifolds website and the ebay store were close to 30-40% less. Havent checked ebay lately, but might be worth poking around if you want to buy a couple of different main venturi sizes. At ~$25/per, it gets expensive!

Edited by T-Bone028
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In my build, I started with 36mm with the 40DCOE and settled on 32 after working my way down. High compression stroker build (12:1 with e31, electromotive hpx ignition, etc.) - I didn't go with nearly as much cam as OP. I'm also running fairly retarded with 26 degrees all in so I can run on pump gas most of the time and adjust for 104. I found the 32's far more drivable for around town and plenty fun with auto-x. On pump gas I still got decent dyno results as well, 222/220 rwhp/rwtq. If I were down at VIR with it, sure, I'd go bigger. But honestly for around town and entrainment on country roads the 40/32 combo has been pretty satisfying compared to the 34s or 36s.

 

Yea, that sounds familiar to me as well.

Regarding the graph, I'm running 36s in my DCOE 40s and think I have some 34s round here. I'll have to put those in and see the difference ...see if power remains the same but response quickens up.

Edited by bajcsi
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In my build, I started with 36mm with the 40DCOE and settled on 32 after working my way down. High compression stroker build (12:1 with e31, electromotive hpx ignition, etc.) - I didn't go with nearly as much cam as OP. I'm also running fairly retarded with 26 degrees all in so I can run on pump gas most of the time and adjust for 104. I found the 32's far more drivable for around town and plenty fun with auto-x. On pump gas I still got decent dyno results as well, 222/220 rwhp/rwtq. If I were down at VIR with it, sure, I'd go bigger. But honestly for around town and entrainment on country roads the 40/32 combo has been pretty satisfying compared to the 34s or 36s.

 

Good numbers on 26 degrees, I'll try my 34s or 32s. Its one of the two I have on hand.

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Thanks for sharing your experiences! I also experimented with 34s and 36s with my 40 DCOEs.

 

I thoroughly expected to find the 34mm to be the overall winner. For my engine, though the 34mm did offer some bonus drivability, the 36mm wasn't different enough down low. Both performed and sounded gargly down low. Top end was more impactful, so I went with 36mm.

 

Volumetric Efficiency... Header selection, port work and cam selection probably accounts for the bulk of the difference between our experiences. I also run race gas, not pump gas. Possibly a factor? Not a chemist.

 

In theory, early next week Joe will be ready for me to schedule our time on the dyno. Just sent him the motor mounts.

Edited by zredbaron
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Final assembly pics before engine dyno. Getting close!

 

The second pic shows that Joe "pinned" the intake manifold to the head to ensure the ports are aligned. (The purple marking.) Pretty slick. Perhaps that's a technique commonly found in comparable engines... but it's the first I've seen it?

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Edited by zredbaron
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Final assembly pics before engine dyno. Getting close!

 

The second pic shows that Joe "pinned" the intake manifold to the head to ensure the ports are aligned. (The purple marking.) Pretty slick. Perhaps that's a technique commonly found in comparable engines... but it's the first I've seen it?

 

Looking good, is that a Mikuni manifold? Rebello pinned my intake as well, it's pretty common in the race engine world.

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