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Is Do-It-Yourself as good as Off-The-Shelf?

theNomad

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Intake Manifold DIY

I'm frugal. It is more accurate than what my wife calls me: "cheap". It means that I don't like to spend money, but when I do I want a good return on my investment.

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I went ahead and purchased a used Weber 38/38 carburetor as an upgrade. While I have a stock cam and compression I may upgrade in the future. The Weber has larger throttle plates so it will not fit on my stock two-hole intake manifold. What follows is my "Frugal" DIY way of modifying the stock manifold work. While not "machine shop pretty" I believe that its a good attempt with minimal downsides and possibly great benefits.

After reading several threads and several debates regarding fitment on intake manifolds (on bmw2002faq.com) I knew I only had a few several options:

1. Buy a different model "peanut" style stock manifold that has a larger, single opening at the top.

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2. Buy an aftermarket Canon brand manifold that has a large Oval machined out of the carb mount. Commonly called the "NASCAR oval" due to the oval design's use on NASCAR V8 engines.

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3. Modify the existing stock two hole manifold to allow fitment of the larger carb's throttle plates.

I'm crazy enough to try my hand at most things. The way I see it, the downsides of modifying the intake myself seemed fairy small. It's not an expensive proposition to modify the existing intake manifold if you already own a die grinder or a Dremel. If all went wrong I'd just buy an aftermarket replacement. If the DIY works I'd be saving $100-200.

Ultimately, the consensus is that on a street driven car most intake manifold differences are so small as to be inconsequential.

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Me being "frugal" I decided to give it a go.

Tools:

Dremel

Carbide Burr assortment (amazon.com)

WD40

Carb gasket

I used the carb gasket to scribe the wider area that needed to be removed. I decided on a two hole system as opposed to the large oval after some research on intake and plenum design.

In general:

The wide open "NASCAR ovals" allow uninhibited flow and work best at wide open throttle for higher horsepower at high RPM.

The two-hole system works best for part throttle situations where the partially open throttle plates disrupt the fuel-air path and the extended walls help redirect it before it enters the plenum.

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The carbide burrs did a good job of cutting through the aluminum. I sprayed WD-40 occasionally to keep the burr from packing up with aluminum and to keep the dust down.

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The burrs worked well. They are not super fast but with patience they got through the material just fine.

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After I got the diameter needed, I widened the inner walls and carried the angle inside as far as I could. The hardest thing for fuel-air suspension to do it turn a 90 degree corner from the carb into the plenum and the intake runner. The smoother transition from carb to plenum area should help the fuel-air transition better.

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Inside the plenum looking at the modified intake hole with flared inner sides. I added "dimples" to break up laminar flow on the walls but I'm sure it's fairly insignificant (ie: couldn't hurt).

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Finished product under the old gasket. I will take off just a little more from the top hole. It's extremely hard to get a perfectly straight circle by hand but its close enough to do its job.

I then turned my attention to the intake runners. I gasket matched the ends of the intake runners and took off any casting marks or sharp edges. The Dremel can only reach inside about 1.5" so there is definitely a limit to the effectiveness of the port match. The smallest cross section in the runners seem to be halfway up.

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After reading some interesting home flow bench work done by FAQ member "PatAllen" seen here I decided to lightly deburr cylinders 2-3-4 and turn most of my attention to cylinder #1. According to his tests that cylinder has the worst flow characteristics, possibly causing an imbalance in air-fuel-ratio to the #1 cylinder. As you can see there is a big difference in available port size. The head ports are nearly at the gasket opening. Time to grind!

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The finished product! (yes, shaky hand at times and I zipped the gasket surface but I'll fix that)

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Feel free to tell me how I irreversibly messed up or somehow lost all my torque potential, or how I did just fine. Minimal gains if any but it WILL fit a Weber 38/38 synchronous carburetor so as far as I'm concerned...Mission Accomplished.

So what do you think? Waste of my time or a great way to save cash?



1 Comment


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I'm no expert...number one...great job at trying it!  "Built not bought" is my motto.  I have a 38 and got the Canon intake for a few reasons.  My mechanic at the time said it would make installations ALOT easier as well as tuning.  How correct is this?  I dunno...but I do know that the weber 38 is notorious for being hard to "dial in" and I would bet that maybe this is due to the intake...Try it and report back.  

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