Heads, intakes, and camshafts--- THE IMPORTANCE OF THE INTAKE MANIFOLD
May 1, 2014
While this blog entry is about the importance of the intake manifold, I must mention the cylinder head and get it out of the way...
The cylinder head is without a doubt, the most important power producing component in the engine as EVERY horsepower produced is done so in the combustion chamber. It must get there AFTER it makes the turn at the short turn radius just above the valve seat and before that, it has to get through the pushrod pinch. If the air/fuel charge can navigate through those areas with as little separation as possible and enough quality flow for the power desired, then you are bound for success. IF you are at all familiar with aftermarket cylinder head offerings for Pontiacs, then you may be aware of the wide-port offerings from various vendors; ANY non-wide port offering will have more than enough velocity for excellent street performance up to about 650hp... as long as the other two components are appropriate for the combination.
The intake manifold communicates the cylinder demand to the carburetor and if the pathway is long and torturous, high rpm power will be hard to come by. Longer runners dictate a lower rpm level as the length determines the rpm at which the intake and exhaust paths "tune". Of course, header primary diameter and length make a difference as well. Dual plane intakes cut the available carburetion for a given cylinder in half. Larger volume runners can extend the rpm at which the intake becomes a "choke", that's one reason the Performer RPM runs so well on engines running below 6,500 rpm. Therefore, a small runnered dual plane intake is purely a low rpm intake for low- to medium performance engines.
The use of a stock intake manifold or a small runnered Performer intake will make around 400hp, possibly more with a longer cam at the expense of a broad torque curve. If the use of a more restrictive intake manifold actually IMPROVES performance on an engine with aftermarket heads then it would be tempting to say the heads are too large as the smaller intake undoubtedly increases velocity and increases low speed throttle response. BUT, we know big block Chevies, Fords, and Mopars are running SIGNIFICANTLY more port volume than our Pontiacs, so what gives?
The camshaft MUST be matched to the combination. Camshaft duration, lift, and overlap conspire to make or break a combination. Most enthusiasts will select a camshaft based on duration and LOBE SEPARATION and there is simply SO MJCH MORE to camshaft timing and its relationship to a given engine's performance. David Vizard wrote a very informative article about cam selection where he states a number of overlap ranges for given performance levels and rpm ranges. It is my opinion that lobe separation can somewhat confuse the issue because it is actually the OVERLAP period the intake tract experiences that can make or break performance. You can still have a broad torque curve with a tight LSA but the engine will be more sensitive to an overcammed situation. Basically speaking, more duration moves the rpm range up the scale; more lift increases torque and power throughout the rpm range and actually INCREASES THE RPM RANGE, while more overlap allows the intake and exhaust tracts to tune, focusing the torque curve in a narrow range. Wide LSA cams don't truly tune, so peak torque and V/E are less, but work over a wider range allowing you to overcam the engine.
Getting back to the small-runner intake on aftermarket heads scenario; IF that worked for you, then you may have picked your cam based on others' opinions or based on duration without considering the compression ratio, rpm range, vehicle weight, gearing, performance expectations, etc., regardless... something is wrong with the combo. Pick the heads first considering the power output, rpm range and compression ratio. Then pick the intake manifold based on driveability and rpm range in which you wish to ENHANCE performance. THEN... pick the camshaft.
When done correctly (not overcammed), the increased volume added to the intake tract WILL aid performance throughout the rpm range and allow you to do more with LESS camshaft... adding driveability and idle quality to a given combination. You may find you don't need the most expensive cylinder heads or camshaft to make 500hp after all!
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