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I have seen negative feedback in posts from people who generally "feel" like the stock lower is not worth porting. I’ve seen the same comments about porting the Ford GT40/Cobra/Explorer intakes for heads that flow ~250cfm. I want to make my case and points contrary to that line of thinking. I will be limiting this discussion to the intake and not addressing heads or cam, which also play a very big part in how an engine performs with any manifold combination.
In the case of the stock intake has runner cross sections (square inches = cross section) that vary through the runner length. The upper intake runners have a cross section of ~1.7 square inches. The runner is an extended oval shape that can be measured by converting the area to one hole (one half at each end of the rectangle) and one rectangle and calculating the square inches of area. The area of a circle is pi (3.14) x radius squared. The radius = ½ circle diameter. For example, the Cobra has a 1.625" round hole for the upper runner. Its area is 1.625"/2 = .8125" radius, so .8125" x .8125" x 3.14 = 2.07 square inches. For a rectangle just multiply the length times width. So, a stock upper intake has a cross section 21% smaller than the Cobra. Not a HUGE difference but significant for rpms above 6,000.
The stock lower transitions that oval upper runner shape to a rectangular shape just below the interface of the upper-to-lower intake. There it becomes an area of very close to or at 2.0 square inches. The catch is, it narrows down to a cross section of 1.2 square inches where the lower intake runner opening meets the head intake runner opening. That cross section is only 71% of the stock upper and 58% of the Cobra intake. That IS a significant difference and is the one of the primary reasons the stock engine stops making power over 5,000 rpm (assuming the valve springs are capable of those rpms). The stock lower small cross section sets up an "inertial block" at about the 5,000 rpm point for a 302 engine. The inertia block varies by cross section, runner length and cubic inch displacement. I have another document, originally written by Neil Erickson (aka MaxFlow) to evaluate heads that is edited by me to demonstrate how the same formula can be used to evaluate a head or intake runner for the inertia block point and calculates those inertia block rpm points for some heads and intakes. See the tech section of this site. Below are photos comparing stock runners to ported runners.
Ported (top) VS Stock (bottom)
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Ported (top) VS Stock (bottom)
(Click on Image for Larger View) | |
Porting the stock lower to a 1.9-2.0 square inch size while reshaping the runner short side radius (just as important as enlarging the intake-to-head opening cross section) can provide substantial increases in the ability of the stock intake to make power. Essentially you are removing the inertia block of the lower intake and making the 1.7 square inch cross section of the upper intake the limiting inertia block. This moves the critical upper usable rpm range from 5,000 rpm up to about 5,400-5,600 rpm and pushes the inertia block up to ~6,000 rpm (near the stock rev limiter).
The stock lower intake flows in the range of 140-150 cfm and can be ported to flow in the 200-220 cfm range with excellent runner design. One of my lowers with a 1.05" x 1.90" runner opening at the head flange was flow tested at 215 average cfm (corrected) by XSR Cylinder Heads on a Super Flow 600 flow bench. The flow restriction in the stock system then becomes the stock E7 heads. Power of engines with stock E7 and ported E7 heads have resulted in power in the 230-260RWHP band and 290-311RWTQ band. With select stock cores and porting of the upper and lower with a custom cam, power levels of 280-290RWHP and 325-330 RWTQ can be achieved.
The Ford GT40 family of intakes as well as the Edelbrock Performer and RPM I intakes share the same runner design limiting characteristics as the stock lower but to a lesser degree. The stock GT40 Ford intake family flow ~205-210 cfm stock and can be ported to flow 250+cfm. This will support well over 300RWHP with the right engine combination. The Cobra cross section will carry flow to 5,850 rpm in stock form but ~7,500 rpm before hitting the inertia block for a 302 engine when ported. A word of caution on porting a Ford intake to AFR and other aftermarket heads. The head runner opening has a slight bias to one side when compared to the Ford intakes. Be sure to make and use a template to port a GT40 family intake. Below is a picture of the bias on an intake marked up for porting.
Cobra Intake Scribed for AFR 165 Heads
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When adding Twisted Wedge, AFR, Edelbrock, or any head that flows more than the GT40 Family of intakes is designed to supply, I often see people claim that a larger runner cross section intake like the TF Track Heat, Edelbrock RPMII, Holley, or other similar intake is the only way to go. This is simply not true as a "rule of thumb". I had ported an Explorer for a customer that had friends tell him to try an Edelbrock RPM II intake to replace the Explorer I ported to make more power. Matching the flow characteristics of the engine combination is more important than slapping a bigger cross section intake on. Here is what he sent me in an email...
From: "Robert Pearson" To: "Thomas Moss" <TMoss@TMossPorting.com> Subject: Re: hey this is the dyno graph for the explorer Date: Sat, 6 Nov 2004 22:18:06 -0500
hey man i gotta get u the dyno graph of the rpm 2 compared to the explorer with your lower ported, all i can say is your intake kills the rpm 2 I lost 24 ftlbs of torque and 7 hp and under the curve the performer rpm2 could not compete on horsepower or tq the intake is coming off u will see more when i get the graphs on the same chart, oh and the explorer is going back on u should have good info to post and this was with nothing else changed - i'll definitely recommend your porting to people thanks |
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All of the people I have ported stock lower intakes for have noticed a significant difference in the usable power band their engines make. 10-15+ horsepower and 20-25 lb-ft is common in a ported stock lower. However, this is NOT a race intake modification. A properly ported stock intake will make more power than a Cobra up to about 4,200-4,500 rpm where its flow potential will take over and it will make more power. HOWEVER, if you look at average power (what matters most) over a usable rpm band, you will find that a properly ported stock intake will make the same average power over a usable stock short block rpm range (1,500-5,600 rpm). Below is a comparison of a ported stock lower vs. a stock Cobra on the same car with no other changes. If the Cobra test had started at 1,600 rpm, you would have a better idea of how much stronger a ported stock lower is than the Cobra in the 1,600 – 3,000 rpm band and the average power of the ported lower would likely have been slightly higher than the Cobra intake over a 1,600-5,600 rpm band.
(Click on Image for Larger View) |
A properly ported stock lower intake will work very well with stock or mildly ported E7, Gt40 or GT40P, Windsor Jr, or any head that flows up to 200cfm on the intake side and engine speeds up to 6,000 rpm. You will experience very good torque through the usable rpm, good power and it will be a FUN car to drive on the street. Power should be somewhere between 260-310HP at the flywheel (230-280 RWHP) depending on your head/cam combination and maybe a little more if you get a custom ground cam supplier to do a custom grind cam for you. With good transmission, stall converter, gears, chassis, and 60’ times you’ll run 13s or 12s depending on weight and driving skill. For the money, it’s hard to beat. If you can find an Explorer or Cobra intake for $200 – then go that route, but if you can’t – do this, you’ll be glad you did, ESPECIALLY if you have a heavier Fox body or T-Bird car.
If you have a Ford GT40 family intake, Performer, RPMI, or similar intake and have upgraded to Edelbrock, AFR, or Twisted Wedge heads that flow 250+cfm and need to upgrade intakes, porting the lower of those intakes is a very cost effective method of getting there.
Follow my guide on porting your own stock lower intake (see the tech section of the website) or let me do it and get rid of that factory power limiting intake port design.
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