engine and drivetrain opening up an intake manifold- good/bad/pointless?

[jerry]

last night i was on the verge of 'hogging' out the opening of a stock two-barrel intake manifold as used with the 72-76 vintage 02's. i happened to be in the company of a muscle-car co-worker (owns an authentic Boss 302 Mustang, btw) who suggested that the existing holes *may* in fact help distribute the fuel/air mixture more evenly amonst the 1&4, and 2&3 cylinders. looking at the casting and the apparent flow lines of the intake manifold, it made me wonder if he had a point. furthermore, he suggested that i would be second-guessing the original design engineers who probably would have made such a modification if deemed appropriate.

so,..... i'm wondering about the collective wisdom of this modification. i just took it for granted that it was a quick and easy 'upgrade'. also note i'm intending this for a Weber 32/36. i'm not familiar with the Weber 38/38, or whether this modification is more appropriate for that carb.

[Mike Self]

indicates that porting/polishing both intake and exhaust manifolds (as well as the carb mount) is most effective when there's a mismatch in the joint between the opening in the manifold and the corresponding opening in the head. You want this transition to be smooth. You can make a paper template to determine if manifold and head openings align; if they don't usually a few minutes work with a Dremel tool will radius out and eliminate the "step" between the two.

Same goes with the joint between the carb base and its mount on the manifold. Most 2 bbl 2002 manifolds need a bit of tweaking to make a Weber 32/36 fit nicely. Again a paper carb base gasket makes a good template. If you have a Weber 38/38, you will definitely need to do a bit of grinding. I sucessfully did this on the car by stuffing an oil-soaked rag down in the manifold before I started grinding, then removing the rag and carefully wiping out the manifold when I finished. That was in 1979 and the engine suffered no ill effects.

If you're rebuilding and have an old cast iron exhaust manifold, a good soaking in a solvent tank or some serious sandblasting on the manifold's insides will remove an amazing amount of carbon...I did it the hard way with an old Dremel tool and emery wheel--that would reach surprisingly far into the manifold.

mike

[jimk]

pointless! It causes a "sudden expansion pressure loss effect". Many have done it but never have any numbers (relative to pressure loss) to convince themselves of the mistake in doing it.

Reference any gas flow pressure drop texts and you will see the k factor (loss factor) is much greater than a tapered expansion. My current project at work, I am responsible for a system that moves over 900,000 cfm of gas and that type of configuration would ba avoided like the plague. An 18deg taper from the top of the manifold to the hole outlet to the plenum is the most ideal duct configuration. The step at the bottom of the hole can't be avoided.

[jerry]

An 18deg taper from the top of the manifold to the hole outlet to the plenum is the most ideal duct configuration. The step at the bottom of the hole can't be avoided.

thanks for the insight. is your taper narrowing downward (entering the head), or upward (towards the carb base)?

any thoughts as to benefit of biasing the taper towards the direction of the 1&4, and 2&3 cylinder, respectively?

just too much idle free time, i suppose....

[Guest]

I port matched the manifold to the head, smoothed the short radi in the long runners,and opened up the manifold to 38/38 cutting groves with a left hand twist in the base for the carb.

I followed what I had seen at an 80's prostock champdrag bike shop.

Made a real difference in the seat of the pants on an Auotmatic and a 5 speed car with 390's

ask Les

[jimk]

Match the hole size at the base of the carb and taper it out as you go down. The step at the end is unavoidable. Larger taper angles cause eddies along the wall, just like you see in water in an expanding flume with too large an angle.