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Slant four - Jeff Smith Technical Questions & Answers - Ask Anything

In with the Good, Out with the Bad

Don Mueller; Brookfield, WI: I have a 2,500-pound car that has its original ’69 Ford J-code engine with 48,000 miles on it. It still has the factory cast HiPo exhaust manifolds. I’ve installed an Edelbrock RPM intake, a Holley 670-cfm Street Avenger with vacuum secondaries, a Comp Cams Xtreme Energy 262H cam, a PerTronix Ignitor, a TR3550 trans, a 3.80:1 rear gear, and 205/60R15 tires. I have a set of AFR 1402 heads that I would like to install along with a Comp Cams XE274H cam with a 351W firing order and a 21/4-inch dual exhaust with a cross-pipe.

The car is used for show and shine, cruising, taking my son to school and practices, the occasional stoplight GP, and just plain driving enjoyment, always with the top down and pipes roaring. Its weak links are the original 5/16-inch rod bolts. These prevent me from running more than 5,500 rpm. What is the redline on these bolts? I’ve researched online the question of whether they will go 6,000 rpm, but the only answer I got was “at least once.”

CC Quickies

We’ll spare you the guessing as to what this is. Just imagine spoiled milk, a cupholder, a warm car, and plenty of time to ferment. Channeling Gordon Ramsay, the Meguiars guys did their own trash-can barf routine after pulling this out of one of our cars. Details about this calamity will also be in December’s issue.

Of course, money is tight. Can I replace the rod bolts with 3/8-inch bolts when I change the heads, without having to totally tear everything down to have the whole rotating assembly rebalanced? If I am stuck with a 5,500-rpm limit, will the XE274H cam be too big? Your articles say big heads need big cams. I don’t need a lot of low-end torque because the rear trailing arms limit the rear tire size to 225. When I look at the dyno charts for the XE262H and XE274H, from 2,500 rpm to say 3,500, they look pretty much the same to me. The XE274H has about a 20hp and 50 lb-ft advantage at 5,000 rpm.

Finally, I’m told that the cast HiPo manifolds are my power restrictors. My frame prevents the use of standard, equal-length or block-hugger–style headers.

Jeff Smith: There are a bunch of things going on here that all relate to engine power. The heads and the camshaft should work well together, but the exhaust manifolds are certainly a restriction. Based on the fact that you really don’t want to spend the money to upgrade the rods (better rods would be stronger and cost about the same as new ARP bolts and resizing the big end of the stock rods), you’re limited in several areas. Engine durability should limit the rpm to certainly no more than 6,000, and your self-imposed 5,500 rpm is very safe. It’s not horsepower or cylinder pressure that kills connecting rods—it’s rpm. Weak rod bolts with heavy pistons are a poor combination. What happens is that as rpm increases, the rod bolt is subjected to higher g-loads that try to pull the rod cap off the rod as the piston travels across top dead center (TDC). Heavy piston and more rpm combine to create increasing g-forces that will eventually stretch the rod bolts. Soon after, the motor comes apart.

Since you have both a 3.80:1 rear gear and a good Tremec five-speed, you have plenty of gear to accelerate the car, so you can go with a bigger camshaft without giving up too much. Acceleration and throttle response below 3,000 rpm won’t be stellar, but the deep rear gear and the fact that your car only weighs 2,700 pounds are reasons you can make this work. But the big limitations are the exhaust manifolds. Good cylinder heads like your AFR 165cc castings not only have high flow intake ports, but they also match the intake with excellent exhaust flow. One way to look at this is from a comparative-flow standpoint (i.e., the relationship of the exhaust flow to the intake). If the exhaust is around 70 percent of the intake at the same valve-lift points, you have a good head. Your AFR 165cc heads calculate out at 77 percent at 0.550-inch lift, which is excellent. The problem is that the exhaust manifolds restrict the flow from the ports. This creates backpressure in the manifolds, requiring the engine to use horsepower to push the exhaust out of the cylinder. Worse, a direct result of this backpressure is a larger percentage of exhaust gas that remains in the cylinder after exhaust stroke is complete. This mixes with the fresh, incoming air and fuel, displacing and diluting them and ultimately reducing cylinder pressure because the exhaust gas won’t burn a second time. If the backpressure stack-up in the cylinder is severe enough, the exhaust gas will even travel up into the intake manifold. If you’ve ever seen an intake manifold in which the inside of the plenum area is black, that is direct evidence of exhaust carbon residue coating the inside of the intake manifold. The more overlap and duration the camshaft employs, the worse this situation will become. It’s a guarantee that there will a lot more overlap with that XE274 cam than with the smaller XE262.

CC Quickies

Did you like the color of IMM’s 360 Stroker? Get the same look for your engine with a can of DupliColor’s Low Gloss Black. “It looks like the Batmobile,” Brian Hafliger told us.

If you are not willing to add headers because of clearance problems, it will be difficult to make any kind of decent power by adding the AFR heads and bigger camshaft. What will happen is that in the midrange rpm (under 4,000 rpm), the engine will probably make decent power. But above 4,000, exhaust gas backpressure will begin to cause cylinder pressure to decrease, and that means less horsepower. So at the exact rpm that those heads and larger cam should really be working, the engine lays down because of a restricted exhaust. The net result is it’s entirely possible that this combination will make less power everywhere. We tend to take headers for granted, but they are an essential component of the engine performance package. All the systems have to work together for the engine to make decent power.

So it appears you have several choices. The first would be to remain with the smaller XE262 cam but go ahead and bolt on the better AFR heads. Better heads will still make a little more torque with the small cam and perhaps a little bit better peak power. It all depends on how much flow restriction is present with the manifolds and exhaust system. You could go this route, but absolutely add the 21/2-inch mandrel-bent exhaust and cross-pipe. One way to measure exhaust gas backpressure is to weld an 18mm oxygen sensor bung into the exhaust pipe just downstream of the exhaust manifold. Adapt a copper 1/8-inch line to a 10- or 15-psi pressure gauge and run the engine at WOT in Third gear to see what the pressure gauge reads. If you see more than a consistent 2 to 3 psi, the system is restricted. The real test would be to place a pressure sensor in the exhaust manifold right near the exhaust port, but that’s not easily accomplished. Other ideas are to modify a set of full-length headers to clear the chassis interference problems. I’ve run into this situation myself and find that often the solution isn’t that difficult to achieve. It may require changing the location of one or more tubes to clear the obstruction. The cost is reasonable, and the difference in power—even with your current combination—would be noticeable. This is the best way to go.

CC Quickies
Mechanical Choke

“Imagine how much power we could make without the rag in the intake.” Don’t worry, all the dyno numbers in the 360 Stroker article are rag free. Brian Hafliger just used it to cover the intake opening while changing carburetors.

I have a last suggestion. Once the header question has been resolved, why not consider retaining the stock engine and building a whole second engine? Start with an affordable 5.0L engine that will have better seals and would allow you to run a hydraulic roller cam with the stock lifters. Make it a 4.030 bore and 3.25-inch stroke crank from Scat along with good I-beam rods, and you’d have a 331ci. Our buddy Tim Moore built an admittedly stout version of this bore/stroke combo back in the Oct. ’08 issue (“495hp at 7,400 rpm,” pg. 44), and it rocked. He used CNC-ported AFR 205s, but your smaller heads would make more torque and a little less horsepower than Tim’s version. Bolt on your AFR heads with about 10.5:1 compression and an XE274 hydraulic roller for a 5.0L engine. This cam delivers less duration (224/232 degrees at 0.050, 6 degrees shorter on the intake and 4 degrees shorter on the exhaust) and 0.555/0.565-inch lift, roughly a 0.030-inch increase, and it will allow you to run those AFR heads right up to their max lift-flow potential. Now you have a thumpin’ small-block that will impress the hell out of your son when you take him to school. Just don’t tell Mom!

More Info

Comp Cams; Memphis, TN; 800/999-0853;
Scat Enterprises; Redondo Beach, CA; 310/370-5501;

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